In-Situ Surface Passivation Techniques for Quantum Dots: Strategies for Enhanced Performance and Stability

Abstract

This article provides a comprehensive analysis of in-situ surface passivation techniques for quantum dots (QDs), a critical technology for enhancing the optical and electronic properties of semiconductor nanocrystals. Targeting researchers, scientists, and drug development professionals, we explore the fundamental principles of surface states and their detrimental effects on QD performance. The review systematically covers advanced methodological approaches, including precursor engineering, ligand exchange, and epitaxial shell growth, with specific applications in photovoltaics, biosensing, and medical imaging. We further address common troubleshooting challenges and optimization strategies, supported by comparative validation of techniques through performance metrics and stability assessments. By synthesizing the latest research, this work serves as a definitive guide for selecting and implementing optimal in-situ passivation strategies to develop high-performance, stable QD systems for biomedical and clinical applications.

Understanding Surface States and the Critical Need for In-Situ Passivation

Surface states are inherent, localized electronic states found at the interface between a quantum dot (QD) and its surrounding environment. These states arise from the abrupt termination of the crystal lattice, leading to "dangling bonds" and unsaturated chemical bonds at the surface. In the context of a broader thesis on in-situ surface passivation techniques for quantum dots, understanding these surface states is paramount. Surface states act as trapping centers for charge carriers, non-radiative recombination pathways, and sources of spectral diffusion and blinking, collectively degrading the optical and electronic performance that makes QDs promising for applications from quantum light sources to photovoltaics.

The degradation caused by surface states is particularly detrimental for near-surface semiconductor quantum dots, where the active region is closer to the material interface [1]. For modern, compact device structures—such as QDs coupled with photonic crystal or circular Bragg grating cavities where the emitter may be located as close as 60 nm from the surface—the impact of these states becomes a critical limiting factor [1]. They introduce significant linewidth broadening and can completely quench the desired luminescence, posing a major challenge for developing optimal QD-based quantum light sources and other quantum devices.

Mechanisms of Performance Degradation

Surface states degrade QD performance through several interconnected physical mechanisms, each negatively impacting key operational metrics.

Charge Trapping and Fluorescence Interference

Surface defects create localized energy levels within the bandgap of the semiconductor. These states can trap charge carriers (electrons and holes) that would otherwise contribute to radiative recombination. The trapping and subsequent random untrapping of these charges causes fluctuating local electric fields, a phenomenon known as spectral diffusion, which manifests as broadening of the emission linewidth and increased noise in the output signal [1]. Under resonant excitation, which is critical for on-demand single-photon generation, these processes are particularly disruptive, as they can cause the RF signal to vanish entirely.

Non-Radiative Recombination Pathways

Perhaps the most direct impact of surface states is the introduction of non-radiative recombination channels. When an electron-hole pair is generated, instead of recombining to emit a photon (radiative recombination), the energy can be transferred to a surface state. This energy is then dissipated as heat through lattice vibrations (phonons). This process directly competes with radiative recombination, lowering the photoluminescence quantum yield (PLQY) and reducing the overall efficiency of the QD [1] [2]. In photovoltaic applications, this translates to significant losses in power conversion efficiency.

Experimental Evidence of Surface State Effects

Recent studies provide quantitative evidence of how surface states degrade performance and how passivation can mitigate these effects.

Table 1: Quantitative Impact of Surface States on Quantum Dot Optical Properties

Performance Metric	Unpassivated QDs	Passivated QDs	Improvement	Measurement Context
Resonance Fluorescence (RF) Linewidth	43.23 ± 22.53 GHz	19.68 ± 6.48 GHz	54.5% reduction [1]	Single QD, resonant excitation
Non-Resonant PL Linewidth	21.32 ± 5.48 GHz	16.49 ± 2.03 GHz	22.7% reduction [1]	Single QD, non-resonant excitation
Photon Noise Level (Variance)	0.2749	0.1587	42.3% reduction [1]	Single QD, resonant excitation
Pulsed-RF Signal	Non-existent in many QDs	Revived and measurable	Enabled on-demand operation [1]	Two-color excitation on single QD

Table 2: Impact of Defect Type and Density on Graphene Quantum Dot (GQD) Properties

Defect Characteristic	Impact on System Energy	Impact on HOMO-LUMO Gap	Consequence for Physicochemical Properties
Single Carbon Vacancy	Increase from -75 kcal/mol to -22 kcal/mol [3]	Modifies the gap from 0 eV (ideal graphene)	Disrupts electron distribution, creates electron-rich/poor regions [3]
Multiple Vacancies (1-6 C atoms)	Energy plateaus around -20 kcal/mol [3]	Variable changes	Alters layout polarity and flatness, reduces electron transport in percolation structures [3]
Increased Form Factor (l/d ratio)	Energy rises to +2 kcal/mol (l/d=1.95) [3]	Not specified	Elongated, flawed structures are thermodynamically less preferred [3]
Surface Functional Groups (-OH, -COOH)	Alters system polarity	Values in the 1.6–3.0 eV range [3]	Affects adhesion to substrates and interactions in sensory/drug delivery systems [3]

Surface Passivation Experimental Protocol

The following section details a validated, optimized protocol for passivating the surface of near-surface InAs/GaAs quantum dots to suppress surface states and recover performance. This protocol is designed for in-situ application within thin-film quantum devices.

Materials and Equipment

• Sample: Semiconductor wafer containing near-surface QDs (e.g., InAs/GaAs QDs in a GaAs layer, with dot-to-surface distance < 40 nm).
• Passivation Chemical: 20% Ammonium Sulfide ((NH4)2S) aqueous solution.
• Filter: 0.02-μm syringe filters.
• Deposition System: Atomic Layer Deposition (ALD) system capable of depositing Al2O3.
• Inert Atmosphere: Glove box with controlled atmosphere (H2O and O2 < 1 ppm).
• Solvents: Isopropanol, deionized water for cleaning.

Step-by-Step Passivation Procedure

• Sample Preparation: Etch the sample surface to achieve the desired dot-to-surface distance (e.g., < 40 nm). Clean the sample sequentially in solvents to remove organic and particulate contaminants.
• Solution Filtration: Inside the inert atmosphere of the glove box, filter the 20% (NH4)2S aqueous solution using a 0.02-μm syringe filter to remove polysulfide particles that can lead to non-uniform passivation.
• Sulfur Immersion: Immerse the sample in the filtered 20% (NH4)2S solution for 10 minutes. This step directly eliminates surface dangling bonds by forming sulfide bonds.
• Inert Transfer: Directly transfer the sample from the glove box to the load-lock chamber of the ALD system. This critical step prevents reoxidation of the freshly applied sulfur layer by atmospheric oxygen and moisture.
• Al2O3 Capping: Deposit a 10-nm-thick capping layer of Al2O3 using ALD at a substrate temperature of 150 °C. This layer permanently protects the passivated surface from subsequent degradation and environmental exposure.

The entire workflow, from sample preparation to final characterization, is designed to be a seamless and controlled process, as visualized below.

Performance Characterization Methods

• Resonance Fluorescence (RF): Use a resonant laser to excite the QD and collect the emitted light through a spectrometer. For high-resolution linewidth measurements, employ a Fabry–Pérot interferometer.
• Non-Resonant Photoluminescence (PL): Use an above-bandgap laser excitation to measure the PL spectrum and linewidth.
• Noise Analysis: Record the photon count statistics over time to calculate the variance in photon number fluctuations.
• Surface Analysis: Utilize X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy to confirm the reduction in surface state density and electric field.

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Materials and Reagents for Surface Passivation Experiments

Reagent/Material	Function/Application	Key Consideration
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Sulfur-based precursor for chemical passivation; eliminates dangling bonds on III-V surfaces.	Solution purity and freshness are critical; filtration is required to remove polysulfides [1].
ALD Al₂O₃ Precursors (e.g., TMA)	Forms a uniform, dense, and protective capping layer to stabilize the passivated surface.	Low-temperature deposition (e.g., 150°C) helps preserve the underlying sulfur passivation layer [1].
Graphene Quantum Dots (GQDs)	Act as bulk and surface passivation agents for perovskite films in solar cells.	The structure (defect amount/type) dictates the HOMO-LUMO gap and passivation efficacy [2] [3].
Inert Atmosphere Glove Box	Provides oxygen- and moisture-free environment (Hâ‚‚O, Oâ‚‚ < 1 ppm) for passivation and transfer.	Prevents reoxidation of the sensitive, freshly passivated surface before capping [1].
1,2-dimethoxy-4-(2-nitroethenyl)benzene	1,2-dimethoxy-4-(2-nitroethenyl)benzene, MF:C10H11NO4, MW:209.2 g/mol	Chemical Reagent
Methyl 2,6-Diamino-5-chloronicotinate	Methyl 2,6-Diamino-5-chloronicotinate|CAS 519147-85-4

Visualization of Degradation and Passivation Mechanisms

The following diagram illustrates the core problem of surface state degradation and the restorative mechanism of surface passivation at the quantum dot level.

The Fundamental Principles of In-Situ vs. Ex-Situ Passivation Approaches

Surface passivation is a critical technological process in quantum dot (QD) research that aims to mitigate performance limitations caused by surface defects. Unpassivated surfaces of quantum dots contain dangling bonds and surface states that act as traps for charge carriers, leading to non-radiative recombination and degradation of optoelectronic properties. The strategic application of passivation techniques directly addresses these challenges, with two fundamental methodologies emerging: in-situ and ex-situ passivation.

In-situ passivation refers to defect mitigation strategies integrated directly during the synthesis or film formation process of quantum dots or quantum dot-embedded devices. This approach creates a unified, epitaxially compatible interface between the passivation layer and the QD core or host matrix. Ex-situ passivation, in contrast, involves applying passivation layers to pre-formed quantum dots or completed device structures as a subsequent processing step. The distinction between these approaches significantly impacts the effectiveness, stability, and scalability of quantum dot technologies across applications from photovoltaics to quantum light sources.

This application note examines the fundamental principles, comparative advantages, and practical implementation of both passivation strategies within the context of advanced quantum dot research, providing detailed protocols for researchers pursuing enhanced performance and stability in quantum dot-based systems.

Fundamental Principles and Comparative Analysis

Core Mechanistic Principles

The operational mechanisms governing in-situ and ex-situ passivation differ fundamentally in their temporal application and resulting interfacial properties:

In-situ passivation operates on principles of epitaxial compatibility and simultaneous integration. When passivating agents are introduced during QD synthesis or host matrix crystallization, they become incorporated at critical defect-prone regions such as grain boundaries and interfaces through processes of lattice matching and strong interfacial bonding. Research on perovskite quantum dots (PQDs) demonstrates that in-situ integration during antisolvent-assisted crystallization enables spontaneous embedding of core-shell PQDs at grain boundaries, creating chemically bonded interfaces that effectively suppress non-radiative recombination pathways [4] [5].

Ex-situ passivation functions through principles of surface coordination and post-synthesis modification. This approach typically involves applying coordinating ligands or shell materials to pre-synthesized QDs through ionic exchange, chemisorption, or layer-by-layer deposition. The effectiveness depends on the binding affinity of passivants to pre-existing surface sites and their ability to uniformly coat complex topographies. Studies on sulfur-based passivation of pre-formed InAs/GaAs QDs demonstrate how ex-situ treatments can reduce surface state density and electric field fluctuations, reviving resonance fluorescence in previously non-emissive dots [1].

Quantitative Performance Comparison

The table below summarizes key performance metrics achieved through both passivation approaches across various quantum dot systems:

Table 1: Performance Comparison of In-Situ vs. Ex-Situ Passivation Approaches

Performance Parameter	In-Situ Passivation	Ex-Situ Passivation	Measurement Context
Power Conversion Efficiency (PCE)	22.85% (from 19.2% baseline) [4]	17.04% (from 14.85% baseline) [6]	Perovskite solar cells
Open-Circuit Voltage (Voc)	1.137 V (from 1.120 V baseline) [4]	Not specified	Perovskite solar cells
Short-Circuit Current Density (Jsc)	26.1 mA/cm² (from 24.5 mA/cm² baseline) [4]	Not specified	Perovskite solar cells
Fill Factor (FF)	77% (from 70.1% baseline) [4]	Not specified	Perovskite solar cells
RF Linewidth Reduction	Not applicable	43.23±22.53 GHz to 19.68±6.48 GHz [1]	Near-surface InAs/GaAs QDs
PL Linewidth Reduction	Not applicable	21.32±5.48 GHz to 16.49±2.03 GHz [1]	Near-surface InAs/GaAs QDs
Stability Retention	>92% after 900 h [4]	Improved stability vs. control [6]	Ambient conditions

Strategic Advantages and Limitations

Each passivation methodology presents distinctive advantages and constraints that dictate their appropriate application:

In-situ passivation advantages include: (1) Enhanced interfacial stability through epitaxial matching and chemical bonding with the host matrix; (2) Prevention of aggregate formation by integrating passivants during crystallization rather than applying to pre-formed surfaces; (3) Superior defect mitigation at critical grain boundaries where they naturally form during synthesis; and (4) Improved long-term stability against environmental stressors such as moisture and oxygen [4] [5]. The core-shell MAPbBr₃@tetra-OAPbBr₃ structure developed for perovskite solar cells exemplifies these advantages, demonstrating both high efficiency and exceptional ambient stability [4].

In-situ limitations primarily concern: (1) Process complexity in optimizing simultaneous crystallization and passivation; (2) Scalability challenges for large-area deposition techniques; and (3) Reduced flexibility for post-processing correction of passivation inadequacies.

Ex-situ passivation advantages include: (1) Process modularity allowing separate optimization of QD synthesis and passivation; (2) Broad material compatibility with diverse QD systems through various ligand chemistries; (3) Accessibility for laboratories without advanced synthesis capabilities; and (4) Correction capability for imperfectly synthesized QDs. The revival of resonance fluorescence in previously non-emissive QDs through sulfur-based passivation demonstrates this corrective potential [1].

Ex-situ limitations involve: (1) Incomplete surface coverage due to steric hindrance or limited diffusion; (2) Weaker interfacial adhesion without epitaxial matching; (3) Potential introduction of impurities during post-processing steps; and (4) Limited access to buried interfaces in completed device structures.

Experimental Protocols

Protocol 1: In-Situ Epitaxial Passivation of Perovskite Solar Cells

This protocol details the in-situ integration of core-shell perovskite quantum dots (MAPbBr₃@tetra-OAPbBr₃ PQDs) during the antisolvent-assisted crystallization of perovskite solar cells, achieving PCE improvements from 19.2% to 22.85% [4] [5].

Preparation of Core-Shell Perovskite Quantum Dots

• Core Precursor Solution: Dissolve 0.16 mmol methylammonium bromide (MABr, 80 wt%) and 0.2 mmol lead(II) bromide (PbBrâ‚‚) in 5 mL dimethylformamide (DMF) under continuous stirring.
• Surface Ligand Addition: Add 50 µL oleylamine and 0.5 mL oleic acid to the precursor solution to form the final core precursor solution.
• Shell Precursor Solution: In a separate vial, dissolve 0.16 mmol tetraoctylammonium bromide (t-OABr, 20 wt%) following the same protocol as the core precursor.
• Nanoparticle Synthesis: Heat 5 mL toluene to 60°C in an oil bath with continuous stirring. Rapidly inject 250 µL of core precursor solution into heated toluene, initiating MAPbBr₃ nanoparticle formation.
• Shell Formation: Inject controlled amount of t-OABr-PbBr₃ precursor solution into reaction mixture, indicated by emergence of green color, signaling core-shell nanoparticle development.
• Purification: Allow reaction to proceed for 5 min. Transfer solution to centrifuge tube; centrifuge at 6000 rpm for 10 min. Discard precipitate, collect supernatant.
• Further Refinement: Centrifuge supernatant with isopropanol at 15,000 rpm for 10 min. Redisperse final precipitate in chlorobenzene for subsequent applications [4] [5].

Solar Cell Fabrication with In-Situ Passivation

• Substrate Preparation: Clean FTO substrates sequentially in soap solution, distilled water, ethanol, and acetone via ultrasonication. Treat with UV-ozone for 15 min, then preheat at 450°C for 30 min.
• Electron Transport Layer: Deposit compact TiOâ‚‚ via spray pyrolysis; maintain at 450°C for 30 min. Apply mesoporous TiOâ‚‚ layer by spin-coating colloidal TiOâ‚‚ paste (18NRT in ethanol, 1:6 ratio) at 4000 rpm for 30 s; anneal at 450°C for 30 min.
• Perovskite Precursor: Dissolve 1.6 M PbIâ‚‚, 1.51 M FAI, 0.04 M PbBrâ‚‚, 0.33 M MACl, and 0.04 M MABr in 1 mL DMF:DMSO (8:1 volume ratio).
• Film Deposition with In-Situ Passivation: Employ two-step spin-coating process (2000 rpm for 10 s, then 6000 rpm for 30 s). During final 18 s, introduce 200 µL of PQDs in chlorobenzene (optimal concentration: 15 mg/mL) as antisolvent.
• Crystallization: Anneal films at 100°C for 10 min followed by 150°C for 10 min in dry air atmosphere.
• Completion: Deposit Spiro-OMeTAD hole transport layer and metal electrodes to complete device architecture [4] [5].

Protocol 2: Ex-Situ Passivation of Near-Surface Quantum Dots

This protocol describes an optimized sulfur-based ex-situ passivation technique for near-surface InAs/GaAs quantum dots, demonstrating resonance fluorescence revival and linewidth reduction [1].

Customized Passivation System Setup

• Inert Atmosphere Configuration: Connect glove box to atomic layer deposition (ALD) system, maintaining Hâ‚‚O and Oâ‚‚ levels below 1 ppm to prevent reoxidation of sulfur layer before ALD deposition.
• Solution Filtration: Filter (NHâ‚„)â‚‚S aqueous solution with 0.02-μm syringe filters inside glove box to remove polysulfide particles.
• Sample Immersion: Immerse sample in 20% (NHâ‚„)â‚‚S solution for 10 min within glove box environment.
• Transfer Protocol: Transfer sample to load-lock chamber of ALD system under continuous inert atmosphere.
• Protective Coating: Deposit 10 nm Alâ‚‚O₃ at 150°C via ALD to encapsulate and stabilize passivated surface [1].

Optical Characterization and Validation

• Non-Resonant PL Assessment: Randomly select 25 QDs from same sample before and after passivation. Compare PL linewidth distribution, typically showing reduction from 21.32±5.48 GHz to 16.49±2.03 GHz.
• Resonance Fluorescence Validation: Randomly select 9 QDs for RF linewidth comparison. Measure average RF linewidth reduction from 43.23±22.53 GHz to 19.68±6.48 GHz.
• Dot-to-Dot Comparison: Perform quantitative pulsed-RF measurements on 8 individual QDs before and after passivation using Fabry-Pérot cavity for resolution.
• RF Revival Assessment: Identify QDs lacking RF signals before passivation; recheck after passivation to confirm signal revival in previously non-emissive dots [1].

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Research Reagents for Quantum Dot Passivation Studies

Reagent/Material	Function/Application	Representative Examples
Methylammonium Bromide (MABr)	Organic precursor for perovskite quantum dot cores	MAPbBr₃ core synthesis [4]
Tetraoctylammonium Bromide (TOABr)	Shell precursor for core-shell QD structures	Tetra-OAPbBr₃ shell formation [4]
Lead Bromide (PbBr₂)	Lead source for perovskite lattice formation	MAPbBr₃ and shell precursor [4]
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Sulfur-based surface passivant for III-V QDs	Ex-situ passivation of InAs/GaAs QDs [1]
Al₂O₃ (ALD precursor)	Protective encapsulation layer	Stabilization of passivated surfaces [1]
Oleylamine/Oleic Acid	Surface ligands for colloidal stability	QD synthesis and dispersion [4] [6]
Cesium Carbonate (Cs₂CO₃)	Cesium source for inorganic perovskite QDs	CsPbI₃ QD synthesis [6]
Tri-n-octylphosphine (TOP)	Ligand for nucleation control in InP QDs	In-situ etching and passivation [7]
Zinc Fluoride (ZnFâ‚‚)	Etchant for defect passivation in InP clusters	Dual-stage in-situ etching [7]
Cyclobutyl(piperazin-1-yl)methanone	Cyclobutyl(piperazin-1-yl)methanone|CAS 64579-67-5	Research-grade Cyclobutyl(piperazin-1-yl)methanone for scientific use. A key piperazine building block for medicinal chemistry. For Research Use Only. Not for human consumption.
Benzoic acid, 4-[2-(2-propenyloxy)ethoxy]-	Benzoic acid, 4-[2-(2-propenyloxy)ethoxy]- Supplier	High-purity Benzoic acid, 4-[2-(2-propenyloxy)ethoxy]- for research (RUO). Explore its applications in liquid crystal polymer and advanced material synthesis. For Research Use Only.

Workflow Visualization

In-Situ Passivation Methodology

Ex-Situ Passivation Methodology

Application-Specific Implementation

Photovoltaics Implementation

In perovskite solar cells, in-situ passivation with core-shell PQDs demonstrates comprehensive performance enhancement through simultaneous defect mitigation and stability improvement. The epitaxial compatibility between MAPbBr₃ cores and the host perovskite matrix enables effective passivation of grain boundaries and surface defects, suppressing non-radiative recombination while facilitating efficient charge transport [4]. The optimal PQD concentration of 15 mg/mL delivers enhanced photovoltaic parameters across Voc, Jsc, and FF, contributing to the overall PCE improvement from 19.2% to 22.85%. Long-term stability assessments confirm retention of >92% initial PCE after 900 hours under ambient conditions, significantly outperforming control devices at ~80% retention [4] [5].

Alternative approaches incorporating CsPbI₃ QDs into MAPbI�3 perovskite precursor solutions demonstrate how partially dissociated QDs can fill vacancy defects in the active layer while serving as nucleation centers to enhance film compactness. This strategy improves exciton recombination reduction and carrier transfer, ultimately increasing PCE from 14.85% to 17.04% while providing superior environmental stability [6].

For quantum information applications, ex-situ passivation proves particularly valuable for near-surface quantum dots in photonic structures. The optimized sulfur-based passivation reduces surface state density and associated electric field fluctuations, directly improving resonance fluorescence characteristics essential for quantum light sources [1]. Quantitative measurements demonstrate passivation reduces average RF linewidth from 43.23±22.53 GHz to 19.68±6.48 GHz while simultaneously reducing noise levels in pulsed-RF signals.

Critically, ex-situ passivation can revive previously vanished RF signals in QDs rendered non-emissive by surface state domination. This revival capability, combined with linewidth narrowing, enables practical implementation of QDs as on-demand quantum light sources in scalable quantum photonic circuits where thin device architectures bring QDs necessarily close to surfaces [1].

Emerging Materials Systems

Recent advances in indium phosphide (InP) QD synthesis demonstrate innovative hybrid approaches combining in-situ etching with interfacial engineering. The dual-stage in-situ etching strategy employing ZnFâ‚‚ during nucleation and shelling stages achieves atomic-level defect passivation while preserving crystallographic integrity. Integration of tri-n-octylphosphine ligands during nucleation controls the process while ZnSe interfacial layers in ZnSeS/ZnS shell growth suppress excessive etching, yielding green-emission QDs with exceptional photoluminescence quantum yield (93%) and narrow emission linewidth (36 nm) [7].

The strategic selection between in-situ and ex-situ passivation approaches represents a critical decision point in quantum dot research and development. In-situ methodologies offer superior interfacial stability and integrated defect mitigation for applications demanding long-term operational reliability, particularly in photovoltaic and display technologies. Ex-situ techniques provide valuable modularity and corrective capability for specialized applications including quantum light sources and sensor platforms.

Future developments will likely focus on hybrid approaches combining the advantages of both methodologies, such as initial in-situ integration followed by complementary ex-situ treatment for comprehensive surface optimization. Advanced characterization techniques will further elucidate the atomic-scale mechanisms of passivation, enabling more targeted design of passivation strategies for specific defect types. As quantum dot technologies mature toward widespread commercialization, scalable and reproducible passivation protocols will become increasingly critical for achieving consistent performance and reliability across application domains.

Surface states are inherent properties of quantum dots (QDs) that significantly dictate their performance and commercial viability. The high surface-to-volume ratio of these nanocrystals creates numerous dangling bonds and surface defects, which act as non-radiative recombination centers and degradation initiation sites [8]. These surface imperfections manifest as three critical limitations: reduced quantum yield (QY) due to non-radiative decay pathways, broadened emission linewidth from spectral diffusion, and compromised photostability under operational conditions [8]. The instability is particularly problematic for lead-halide perovskites, which suffer from defect-rich surfaces arising from intrinsically soft lattices and low defect formation energies [9]. This application note, framed within broader thesis research on in-situ surface passivation techniques, quantitatively characterizes how advanced passivation strategies directly mitigate these challenges by neutralizing surface trap states, protecting core materials from environmental factors, and maintaining superior optical properties over extended periods.

Quantitative Impact of Surface States on Key Metrics

Surface states profoundly degrade quantum dot performance by introducing non-radiative recombination pathways, spectral diffusion, and sensitivity to environmental factors. The following data, synthesized from recent studies, quantifies these effects and the improvements achievable through passivation.

Table 1: Impact of Surface Passivation on Quantum Dot Performance Metrics

Quantum Dot System	Passivation Strategy	Quantum Yield (QY) / Change	Emission Linewidth / Change	Photostability Improvement	Citation
InAs/GaAs QDs	Optimized (NH(4))(2)S + Al(2)O(3)	Information Missing	RF linewidth reduced from 43.2±22.5 GHz to 19.7±6.5 GHz (avg); Single QD: 14.2→7.8 GHz	Noise level reduction (42.3%); Revival of vanishing RF signals	[1]
Carbon QDs (ACQDs-plasma)	Plasma-amino functionalization	Increased to 54.6% (2.1-fold enhancement)	Information Missing	PL degradation: 14.9% after 10 mo (vs. 87.7% for pristine CQDs); Broad pH tolerance (4-11)	[10]
CdSe/ZnS QDs (RGB)	Triphenylphosphine (TPP) treatment	Blue: 75.6%→90.0%; Green: 81.0%→94.9%; Red: 80.2%→96.1%	Information Missing	Enabled direct photopatterning in air (oxidation protection)	[11]
CsPbBr(_3) Nanoplates	PPA(2)SO(4) organic sulfate	Increased to 96%	Information Missing	Enhanced stability against moisture, heat	[12]
CsPbBr(_3) QDs in Silica	Sulfonic surfactant (SB3-18) + matrix	PLQY of 58.27% for composite	Information Missing	Retained 95.1% PL after water test; 92.9% PL after light aging	[9]

Table 2: Impact of Passivation on Non-Radiative Recombination and Carrier Dynamics

Quantum Dot System	Passivation Strategy	PL Lifetime (Before → After)	Interpretation	Citation
Blue CdSe/ZnS QDs	Triphenylphosphine (TPP)	14.8 ns → 18.3 ns	Reduced non-radiative recombination; better surface passivation	[11]
Green CdSe/ZnS QDs	Triphenylphosphine (TPP)	20.0 ns → 23.0 ns	Reduced non-radiative recombination; better surface passivation	[11]
Red CdSe/ZnS QDs	Triphenylphosphine (TPP)	26.8 ns → 28.4 ns	Reduced non-radiative recombination; better surface passivation	[11]

The data demonstrates that surface passivation consistently enhances performance across diverse quantum dot materials. The universal increase in photoluminescence lifetime after passivation indicates a fundamental reduction in surface trap states that cause non-radiative recombination [11]. This directly translates to the observed increases in quantum yield. Furthermore, specialized passivation techniques enable functionality in demanding environments, such as ambient patterning [11] or aqueous stability [10] [9], by protecting the vulnerable surface from reactive species.

Experimental Protocols for Surface Passivation

Optimized Sulfur-Based Passivation for Near-Surface QDs

This protocol describes a two-step, optimized sulfur passivation process for near-surface semiconductor QDs (e.g., InAs/GaAs) to improve resonance fluorescence signals by reducing surface state density and electric field noise [1].

Materials:

• (NH(4))(2)S aqueous solution (20%)
• 0.02-μm syringe filters
• Atomic Layer Deposition (ALD) system capable of depositing Al(2)O(3)
• Glove box with inert atmosphere (H(2)O and O(2) < 1 ppm), connected to the ALD load-lock chamber

Procedure:

• Sample Preparation: Etch the sample surface to achieve a dot-to-surface distance of less than 40 nm to accentuate surface state effects [1].
• Solution Filtration: Inside the glove box, filter the (NH(4))(2)S aqueous solution using a 0.02-μm syringe filter to remove polysulfide particles [1].
• Sulfur Treatment: Immerse the sample in the filtered 20% (NH(4))(2)S solution for 10 minutes [1].
• Inert Transfer: Immediately transfer the sample from the glove box to the load-lock chamber of the ALD system. This step is critical to prevent reoxidation of the sulfur layer [1].
• ALD Capping: Deposit a 10-nm-thick Al(2)O(3) capping layer at 150°C via ALD. This layer seals and protects the passivated surface [1].

Validation Metrics:

• Compare resonance fluorescence (RF) linewidth before and after passivation using a Fabry–Pérot interferometer for high-resolution measurements. A successful passivation shows significant linewidth narrowing [1].
• Perform noise analysis by measuring the variance of photon number fluctuations. A >40% reduction in noise level indicates effective suppression of charge noise [1].

Plasma-Controlled Defect Modulation for Carbon QDs

This protocol outlines a plasma-assisted surface defect modulation and passivation strategy to dramatically enhance the photostability and quantum yield of carbon quantum dots (CQDs) [10].

Materials:

• Pristine CQDs (e.g., synthesized from gelatin via hydrothermal method)
• Aniline
• Plasma surface treatment system

Procedure:

• Synthesis of Amino-Functionalized CQDs (ACQDs-plasma):
  • Design the amino-functionalized quantum dots (ACQDs-plasma).
  • Systematically optimize the plasma discharge conditions and the mass ratio of CQDs to aniline [10].
• Plasma-Induced Radical Generation: Subject the CQDs to plasma treatment. The high-energy reactive species (electrons, ions) in the plasma generate active radicals on the CQD surface [10].
• Amino-Group Passivation: Employ precise grafting techniques to functionalize the radical-activated CQD surfaces with amino groups from aniline. This two-stage mechanism passivates the generated radicals, stabilizing defect structures [10].

Validation Metrics:

• Quantum Yield: Measure using an integrating sphere. The target is a high QY (e.g., 54.6%) representing a significant (e.g., 2.1-fold) enhancement over pristine CQDs [10].
• Photostability Assessment:
  • Long-term Storage: Monitor PL intensity after storage in ambient air for extended periods (e.g., 10 months). Successful passivation results in minimal PL degradation (e.g., <15%) [10].
  • pH Stability: Assess PL intensity across a pH range of 4-11. Passivated CQDs should exhibit broad tolerance with minimal quenching [10].

Multifunctional Ligand Passivation for Ambient Patterning

This protocol utilizes triphenylphosphine (TPP) as a multifunctional ligand for QDs, enabling high-resolution direct optical patterning in ambient air while simultaneously improving optical properties [11].

Materials:

• Core-shell QDs (e.g., CdSe/ZnS) in non-polar solvent
• Triphenylphosphine (TPP)
• Non-polar solvent (e.g., hexane)

Procedure:

• Photosensitive Ink Preparation: Add TPP directly to the QD solution in a non-polar solvent. Ensure the mass fraction of TPP is at least 5% for optimal performance. The resulting QDs-TPP ink should exhibit good dispersion and colloidal stability [11].
• Film Processing and Patterning:
  • Deposit the QDs-TPP ink onto a substrate via spin-coating or other suitable methods.
  • Expose the film to UV light through a photomask in ambient air. The TPP ligand undergoes an oxygen-mediated photo-reaction, inducing a solubility change in the exposed regions [11].
  • Develop the pattern by rinsing with a suitable solvent to remove unexposed regions, leaving a high-resolution QD pattern [11].

Validation Metrics:

• Photoluminescence Quantum Yield (PLQY): Use an absolute PLQY measurement system. Successful passivation is indicated by a significant increase in PLQY for RGB QDs (e.g., >90% for blue, >94% for green, >96% for red) [11].
• Patterning Resolution: Characterize the patterned features using microscopy. The process should achieve high resolution, up to 9534 dots per inch (dpi) [11].
• Device Performance: Fabricate QLEDs with the patterned QDs. Target external quantum efficiencies (EQE) exceeding 20% for blue, green, and red devices [11].

Signaling Pathways and Workflow Diagrams

The following diagrams illustrate the mechanistic pathways and experimental workflows for key surface passivation techniques.

This diagram illustrates the logical relationship between surface state-induced degradation mechanisms and the corresponding passivation strategies that mitigate them to improve key performance metrics.

This workflow details the sequential two-step mechanism for plasma-controlled surface defect modulation of carbon quantum dots, involving initial radical generation followed by chemical passivation.

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Reagents for Advanced Surface Passivation

Reagent / Material	Function / Role in Passivation	Key Application / Outcome
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Precursor for sulfur-based passivation; eliminates surface dangling bonds on III-V semiconductors.	Reduces surface state density and charge noise in near-surface QDs; narrows resonance fluorescence linewidth [1].
Triphenylphosphine (TPP)	Multifunctional ligand: surface passivator, photoinitiator, and oxidation protector.	Enables direct high-resolution photopatterning of QDs in air; significantly boosts PLQY of RGB QDs [11].
Organic Sulfates (e.g., PPA₂SO₄)	Passivates surface defects on perovskite nanocrystals via strong coordination of SO₄²⁻ with Pb²⁺ ions.	Achieves near-unity PLQY (96%) for blue-emitting CsPbBr₃ nanoplates; enhances film stability [12].
Sulfonic Acid Surfactants (e.g., SB3-18)	Passivator for perovskite QDs in composites; SO₃- group coordinates with Pb²⁺ to reduce trap density.	Used in synergistic strategy with silica matrix encapsulation; enhances PLQY and water/light stability [9].
Plasma Treatment System	Enables dry, solvent-free surface engineering; generates active radicals for subsequent functionalization.	Core to activation-passivation cascade strategy for carbon QDs; enables precise defect modulation [10].
Atomic Layer Deposition (ALD)	Deposits uniform, pinhole-free inorganic capping layers (e.g., Al₂O₃) at low temperatures.	Protects passivated surfaces from re-degradation; essential for long-term stability of passivated QDs [1].
Mesoporous Silica (MS) Matrix	Provides rigid, hermetic encapsulation for perovskite QDs upon high-temperature pore collapse.	Physically blocks ingress of water and oxygen; dramatically improves operational stability [9].
4-Methoxy-3,5-dimethylaniline HCl	4-Methoxy-3,5-dimethylaniline HCl, CAS:158400-44-3, MF:C9H14ClNO, MW:187.66 g/mol	Chemical Reagent
2,2,6,6-Tetramethyloctane-3,5-dione	2,2,6,6-Tetramethyloctane-3,5-dione (TMOD)	High-purity 2,2,6,6-Tetramethyloctane-3,5-dione, a sterically hindered β-diketone ligand for advanced materials research. For Research Use Only. Not for human or veterinary use.

The Role of Surface Chemistry in Non-Radiative Recombination and Charge Trapping

Surface chemistry plays a decisive role in determining the optoelectronic performance of quantum dots (QDs). Non-radiative recombination and charge trapping at surface states represent the most significant challenges limiting the efficiency and stability of QD-based devices. These surface defects, arising from under-coordinated atoms, surface stoichiometry imbalances, and dangling bonds, create midgap states that provide efficient pathways for non-radiative recombination, ultimately quenching photoluminescence and degrading device performance [13]. The inherent vulnerability of QDs to surface effects stems from their exceptionally high surface-to-volume ratio, where a substantial fraction of atoms resides on the surface [13]. This application note examines recent advances in in-situ surface passivation techniques that directly address these challenges, with particular focus on their efficacy in suppressing non-radiative recombination pathways and mitigating charge trapping phenomena. Through detailed protocols and quantitative analysis, we provide researchers with practical methodologies for implementing these strategies across various QD material systems.

Quantitative Analysis of Passivation Efficacy

Performance Metrics of Passivated Quantum Dot Systems

Table 1: Comparative analysis of quantum dot performance metrics before and after surface passivation

Material System	Passivation Strategy	Key Performance Indicator	Pre-Passivation	Post-Passivation	Improvement	Reference
InAs/GaAs QDs	Optimized sulfur-based passivation + Al₂O₃ capping	Resonance fluorescence linewidth (GHz)	43.23 ± 22.53	19.68 ± 6.48	54.5% reduction	[1]
InAs/GaAs QDs	Optimized sulfur-based passivation + Al₂O₃ capping	Non-resonant PL linewidth (GHz)	21.32 ± 5.48	16.49 ± 2.03	22.7% reduction	[1]
InP QDs	Dual-stage in-situ etching + ZnSe/ZnS shelling	Photoluminescence quantum yield (%)	Unspecified baseline	93%	Significant improvement	[7]
InP QDs	Dual-stage in-situ etching + ZnSe/ZnS shelling	Emission linewidth (nm)	Unspecified baseline	36	Significant narrowing	[7]
MAPbBr₃@tetra-OAPbBr₃ PQDs	Core-shell structure integrated in antisolvent	Power conversion efficiency (%)	19.2	22.85	19.0% improvement	[5]
MAPbBr₃@tetra-OAPbBr₃ PQDs	Core-shell structure integrated in antisolvent	Open-circuit voltage (V)	1.120	1.137	1.5% improvement	[5]

Defect Density and Stability Metrics

Table 2: Impact of passivation on defect density, charge trapping, and operational stability

Material System	Passivation Strategy	Trap State Reduction	Noise Level Reduction	Stability Retention	Duration	Reference
InAs/GaAs QDs	Optimized sulfur-based passivation	Not quantified	42.27% (photon number fluctuation)	Not specified	Not specified	[1]
Perovskite QDs	QD-delivered dopants & surface ligands	Significant trap-state density reduction	Not specified	Enhanced thermal and light stability	Not specified	[2]
MAPbBr₃@tetra-OAPbBr₃ PQDs	Core-shell structure	Non-radiative recombination suppression	Not specified	>92% PCE retention	900 hours	[5]
Control PSCs	None (baseline)	Not applicable	Not specified	~80% PCE retention	900 hours	[5]
CdSe/ZnS QDs in PFO polymer	Not applied (characterization only)	Acting as charge trapping centers	Not specified	Not specified	Not specified	[14]

Experimental Protocols

Protocol: Optimized Sulfur-Based Passivation for Near-Surface Quantum Dots

This protocol describes the optimized sulfur passivation technique for near-surface InAs/GaAs QDs, which demonstrated significant reduction in resonance fluorescence linewidth and revival of previously vanishing RF signals [1].

Materials and Equipment

• Customized passivation system consisting of glove box connected to atomic layer deposition (ALD) system
• (NHâ‚„)â‚‚S aqueous solution (20%)
• 0.02-μm syringe filters
• Atomic layer deposition system capable of 150°C operation
• Alâ‚‚O₃ precursor (trimethylaluminum or equivalent)
• Inert atmosphere enclosure (Hâ‚‚O and Oâ‚‚ < 1 ppm)

Procedure

• Sample Preparation:

  • Etch the sample surface to achieve dot-to-surface distance of less than 40 nm to enhance surface state effects.
  • Transfer sample to inert atmosphere glove box maintaining Hâ‚‚O and Oâ‚‚ levels below 1 ppm.

• Solution Filtration:

  • Filter the (NHâ‚„)â‚‚S aqueous solution through 0.02-μm syringe filters within the glove box to remove polysulfide particles.

• Surface Treatment:

  • Immerse the sample in 20% (NHâ‚„)â‚‚S solution for 10 minutes at room temperature.
  • Ensure complete surface coverage during immersion.

• Transfer and Capping:

  • Transfer the passivated sample directly to the load-lock chamber of the ALD system under inert atmosphere.
  • Deposit 10 nm Alâ‚‚O₃ capping layer at 150°C to protect the passivated surface from re-degradation.

Validation Metrics

• Resonance fluorescence linewidth reduction from 43.23 ± 22.53 GHz to 19.68 ± 6.48 GHz
• Non-resonant PL linewidth reduction from 21.32 ± 5.48 GHz to 16.49 ± 2.03 GHz
• Revival of pulsed-RF signals in previously non-emitting QDs
• Reduction in photon number fluctuation variance by 42.27%

Protocol: In-Situ Epitaxial Core-Shell Perovskite Quantum Dot Passivation

This protocol details the in-situ integration of core-shell perovskite quantum dots for simultaneous bulk and surface passivation in perovskite solar cells, demonstrating enhanced PCE and long-term stability [5].

Materials and Equipment

• Methylammonium bromide (MABr, 80 wt%)
• Lead(II) bromide (PbBrâ‚‚)
• Tetraoctylammonium bromide (t-OABr, 20 wt%)
• Dimethylformamide (DMF), toluene, chlorobenzene
• Oleylamine and oleic acid
• Centrifuge capable of 15,000 rpm
• Perovskite precursor solution: 1.6 M PbIâ‚‚, 1.51 M FAI, 0.04 M PbBrâ‚‚, 0.33 M MACl, 0.04 M MABr in DMF:DMSO (8:1 v/v)

PQD Synthesis Procedure

• Core Precursor Preparation:

  • Dissolve 0.16 mmol MABr and 0.2 mmol PbBrâ‚‚ in 5 mL DMF under continuous stirring.
  • Add 50 μL oleylamine and 0.5 mL oleic acid to form the final core precursor solution.

• Shell Precursor Preparation:

  • Dissolve 0.16 mmol t-OABr in 5 mL DMF following the same protocol as the core precursor.

• Nanoparticle Growth:

  • Heat 5 mL toluene to 60°C in an oil bath under continuous stirring.
  • Rapidly inject 250 μL core precursor solution into heated toluene to form MAPbBr₃ nanoparticles.
  • Inject controlled amount of t-OABr-PbBr₃ precursor solution to form core-shell structure.

• Purification:

  • Allow reaction to proceed for 5 minutes before centrifugation at 6,000 rpm for 10 minutes.
  • Discard precipitate and collect supernatant.
  • Perform additional centrifugation with isopropanol at 15,000 rpm for 10 minutes.
  • Redisperse final precipitate in chlorobenzene for stability.

Device Integration

• Solar Cell Fabrication:

  • Deposit compact TiOâ‚‚ layer via spray pyrolysis on cleaned FTO substrates.
  • Apply mesoporous TiOâ‚‚ layer by spin-coating colloidal TiOâ‚‚ paste at 4,000 rpm for 30s.

• Perovskite Film Deposition with PQDs:

  • Deposit perovskite film using two-step spin-coating: 2,000 rpm for 10s followed by 6,000 rpm for 30s.
  • During final 18s of spinning, introduce 200 μL of PQDs in chlorobenzene (optimal concentration: 15 mg/mL) as antisolvent.
  • Anneal films at 100°C for 10min followed by 150°C for 10min in dry air.

Validation Metrics

• Power conversion efficiency increase from 19.2% to 22.85%
• Open-circuit voltage improvement from 1.120V to 1.137V
• Retention of >92% initial PCE after 900 hours under ambient conditions
• Enhanced photoresponse in 400-750nm wavelength range

Visualization of Passivation Mechanisms

Surface Passivation Mechanisms in Quantum Dot Systems

Surface Passivation Mechanisms Diagram

This diagram illustrates the fundamental mechanisms through which surface passivation strategies address the challenges of non-radiative recombination and charge trapping in quantum dots. Surface states act as midgap recombination centers that facilitate non-radiative pathways and charge carrier trapping, leading to photoluminescence quenching [13]. Optimized sulfur-based passivation directly reduces surface state density, while core-shell architectures provide physical separation of charge carriers from surface defects [1] [5]. In-situ epitaxial integration suppresses band bending effects, and ligand engineering promotes defect healing through ion exchange and coordination bonding [5] [2]. These mechanisms collectively contribute to improved emission efficiency, enhanced operational stability, and reduced noise levels as demonstrated in the quantitative metrics.

Experimental Workflow for Optimized Quantum Dot Passivation

Passivation Experimental Workflow

The experimental workflow diagram outlines two complementary approaches for quantum dot passivation. The upper path depicts the optimized sulfur-based passivation protocol for near-surface InAs/GaAs QDs, highlighting critical steps including sample preparation with precise dot-to-surface distance control, maintenance of inert atmosphere throughout the process, and final capping layer deposition [1]. The lower path illustrates the in-situ epitaxial core-shell perovskite quantum dot integration method, showing key stages from PQD synthesis through purification to device integration via antisolvent application during spin-coating [5]. Both pathways converge on performance validation using standardized metrics including resonance fluorescence linewidth, photoluminescence quantum yield, and operational stability measurements.

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key research reagents and materials for quantum dot surface passivation

Reagent/Material	Function/Application	Specific Usage Example	Key Consideration	Reference
(NHâ‚„)â‚‚S aqueous solution	Sulfur-based surface passivant	20% solution for eliminating surface dangling bonds	Requires filtration to remove polysulfide particles	[1]
Al₂O₃ precursor	Capping layer deposition	10 nm protective layer via ALD at 150°C	Prevents re-degradation of passivated surface	[1]
Tri-n-octylphosphine (TOP)	Ligand for nucleation control	In-situ etching synthesis of InP QDs	Suppresses excessive etching during synthesis	[7]
ZnFâ‚‚	Etchant for defect passivation	Dual-stage etching of magic-sized InP clusters	Achieves atomic-level defect passivation	[7]
Methylammonium lead bromide (MAPbBr₃)	Core material for PQDs	Green-emitting perovskite nanocrystals	Compatible with shell growth for core-shell structures	[5]
Tetraoctylammonium lead bromide	Shell precursor for PQDs	Epitaxial shell growth on MAPbBr₃ cores	Provides lattice matching with host perovskite	[5]
Carboxylic acid-thiol bifunctional ligands	Surface modification	Enhanced charge transport in InP QDs	Improves carrier mobility in QD films	[7]
ZnSe/ZnS precursors	Shell growth for InP QDs	Interface passivation in core-shell QDs	Reduces interfacial defects and non-radiative recombination	[7]
2,3'-Dichloro-4'-fluorobenzophenone	2,3'-Dichloro-4'-fluorobenzophenone	2,3'-Dichloro-4'-fluorobenzophenone is a high-purity benzophenone derivative for research use only (RUO). Not for human or veterinary use.	Bench Chemicals
N-Methyl-3-(p-tolyloxy)propan-1-amine	N-Methyl-3-(p-tolyloxy)propan-1-amine, CAS:915923-08-9, MF:C11H17NO, MW:179.26 g/mol	Chemical Reagent	Bench Chemicals

The precise engineering of quantum dot surface chemistry through advanced in-situ passivation techniques represents a critical pathway toward suppressing non-radiative recombination and mitigating charge trapping phenomena. The experimental protocols and quantitative data presented herein demonstrate that optimized sulfur-based passivation, core-shell architectures, and in-situ epitaxial integration can significantly enhance optical properties, improve quantum efficiencies, and extend operational stability across diverse QD material systems. The resurgence of pulsed resonance fluorescence in previously non-emitting quantum dots following passivation [1] provides particularly compelling evidence for the critical role of surface chemistry in determining quantum dot performance. As research in this field advances, the continued refinement of these passivation strategies will undoubtedly accelerate the development of high-performance QD-based technologies for photonic, electronic, and quantum information applications.

Exploring the Link Between Surface States and Device Long-Term Stability

Surface states in quantum dots (QDs) represent a critical frontier in nanomaterials research, acting as primary determinants of both performance and operational lifetime. These states, arising from dangling bonds and surface defects, introduce trap levels within the bandgap that promote non-radiative recombination, quench photoluminescence, and accelerate degradation under environmental stressors. For QD-based devices to transition from laboratory demonstrations to commercial applications, stabilizing these surfaces is paramount. This Application Note examines the fundamental link between surface states and long-term device stability, framing the discussion within the context of advanced in-situ surface passivation techniques. We present a consolidated analysis of recent pioneering studies, structured quantitative data, detailed experimental protocols, and practical reagent toolkits to guide researchers in developing robust, high-performance QD systems for optoelectronics, photovoltaics, and quantum technologies.

Surface States: The Primary Instability Driver

The high surface-to-volume ratio of quantum dots makes their optical and electronic properties exceptionally susceptible to their surface chemistry. Surface defects, such as unsaturated bonds (e.g., Pb dangling bonds in perovskite QDs) or surface disorder, create electronic trap states that:

• Quench Luminescence: Trap states provide non-radiative pathways for exciton recombination, significantly reducing photoluminescence quantum yield (PLQY) [15].
• Broaden Emission Linewidth: Spectral diffusion and charge fluctuation at the surface lead to significant inhomogeneous broadening, which is particularly detrimental for quantum light sources [16].
• Accelerate Environmental Degradation: Unpassivated surfaces are highly reactive with ambient oxygen and moisture, leading to irreversible chemical degradation and rapid device failure [15] [17].

Passivation strategies aim to chemically saturate these dangling bonds, thereby eliminating mid-gap trap states and forming a protective barrier against the environment. The following sections explore how modern in-situ techniques achieve this with remarkable efficacy.

Quantitative Analysis of Passivation Efficacy

Recent studies demonstrate that sophisticated passivation strategies can simultaneously enhance both the efficiency and lifetime of QD devices. The table below summarizes key performance metrics from cutting-edge research, providing a benchmark for the field.

Table 1: Performance Comparison of Quantum Dot Systems with Advanced Passivation

QD System & Passivation Method	Key Performance Metrics Post-Passivation	Long-Term Stability Performance	Reference
InP/ZnSe/ZnS QDs (Photochemical passivation & ligand exchange)	Retained 92.5% of initial Quantum Yield (QY)	92.5% QY retention after 2 months in water dispersion vs. 47.3% in chloroform control.	[18]
CsPbBr₃ PQD Glass (Air-induced passive surface passivation)	PLQY increased from 20% to 93%.	Enhanced PLQY maintained over 4 years of air exposure.	[15]
MAPbBr₃@tetra-OAPbBr₃ Core-Shell QDs (In-situ epitaxial passivation in PSCs)	PCE increased from 19.2% to 22.85% (Voc: 1.120V to 1.137V, Jsc: 24.5 to 26.1 mA/cm²).	>92% initial PCE retained after 900 hours under ambient conditions.	[4]
InAs/GaAs QDs (Optimized sulfur-based passivation + Al₂O₃ capping)	Resonance fluorescence (RF) linewidth reduced from 43.23 ± 22.53 GHz to 19.68 ± 6.48 GHz.	Pulsed-RF signals revived in previously non-emissive QDs; reduced noise level.	[16]
CsPbBr₃@UiO-66 (MOF) (Encapsulation in metal-organic framework)	Strong exciton-polariton coupling demonstrated in a microcavity.	Luminescence maintained over 30 months in ambient air and several hours underwater.	[17]
MAPbBr₃@S-COF (Encapsulation in thiomethyl-functionalized COF)	N/A	Exceptional water stability for more than 1 year.	[19]

The data unequivocally confirms that targeted surface passivation directly correlates with enhanced device longevity. The most successful strategies often combine multiple mechanisms, such as chemical saturation of dangling bonds and physical encapsulation.

Detailed Experimental Protocols

To facilitate the adoption of these techniques, this section provides detailed methodologies for two representative and highly effective passivation protocols.

Protocol: In-Situ Epitaxial Core-Shell QD Passivation for Perovskite Solar Cells

This protocol, adapted from a study that boosted PCE to 22.85%, describes the integration of core-shell perovskite QDs during the film fabrication process [4].

Workflow Overview:

Materials:

• Fluorine-doped Tin Oxide (FTO) Substrates
• TiOâ‚‚ Paste (e.g., 18NRT, Dyesol)
• Precursor Salts: PbIâ‚‚, FAI, PbBrâ‚‚, MACl, MABr
• Solvents: Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), Chlorobenzene, Toluene, Isopropanol
• Core-Shell PQDs: MAPbBr₃@tetra-OAPbBr₃ (synthesized separately as per Section 4.2)

Step-by-Step Procedure:

• Substrate Preparation: Clean FTO substrates by sequential sonication in soap solution, distilled water, ethanol, and acetone. Treat in a UV-ozone cleaner for 15 minutes.
• Electron Transport Layer (ETL) Deposition:
  • Deposit a compact TiOâ‚‚ (c-TiOâ‚‚) layer via spray pyrolysis onto the preheated (450°C) FTO substrate. Anneal at 450°C for 30 minutes.
  • Spin-coat a mesoporous TiOâ‚‚ (mp-TiOâ‚‚) layer from a colloidal TiOâ‚‚ paste diluted in ethanol (1:6 weight ratio) at 4000 rpm for 30 seconds. Anneal again at 450°C for 30 minutes.
• Perovskite Precursor Solution Preparation: Dissolve 1.6 M PbIâ‚‚, 1.51 M FAI, 0.04 M PbBrâ‚‚, 0.33 M MACl, and 0.04 M MABr in 1 mL of a DMF:DMSO solvent mixture (8:1 volume ratio).
• QD Integration and Perovskite Film Deposition:
  • Re-disperse the synthesized core-shell PQDs in chlorobenzene at a concentration of 15 mg/mL. This solution serves as the antisolvent.
  • Spin-coat the perovskite precursor solution onto the ETRL. During the spin-coating process, at the appropriate second (typically 10-15 seconds before the end), dynamically drop-cast 1 mL of the PQD-containing antisolvent.
  • Complete the spin-coating cycle. The antisolvent rapidly extracts the host solvent, triggering the crystallization of the perovskite film while simultaneously incorporating the core-shell PQDs at grain boundaries and interfaces.
• Annealing: Transfer the film to a hotplate and anneal at 100-150°C for 10-30 minutes to remove residual solvents and improve crystallinity.
• Device Completion: Complete the solar cell by sequentially depositing a hole transport layer (HTL, e.g., Spiro-OMeTAD) and a metal electrode (e.g., Au or Ag) by thermal evaporation.

Protocol: Optimized Sulfur Passivation for Near-Surface Quantum Dots

This protocol details a two-step, optimized sulfur passivation process for near-surface III-V QDs, crucial for recovering and improving resonance fluorescence for quantum light sources [16].

Workflow Overview:

Materials:

• Custom Passivation System: A glove box (Hâ‚‚O and Oâ‚‚ < 1 ppm) connected directly to an Atomic Layer Deposition (ALD) load-lock chamber.
• Ammonium Sulfide Solution: (NHâ‚„)â‚‚S, 20% in water.
• Syringe Filters: 0.02 µm pore size.
• ALD Precursors: e.g., Trimethylaluminum (TMA) and Hâ‚‚O for Alâ‚‚O₃ deposition.

Step-by-Step Procedure:

• Sample Preparation: Fabricate the near-surface QD sample (e.g., with a dot-to-surface distance < 40 nm) within a DBR-CBG structure to enhance collection efficiency [16].
• Inert Atmosphere Transfer: Place the sample inside the glove box to prevent premature oxidation before passivation.
• Solution Filtration: Filter the (NHâ‚„)â‚‚S aqueous solution through a 0.02 µm syringe filter inside the glove box to remove polysulfide particles that can cause non-uniform passivation.
• Sulfur Passivation: Immerse the sample in the filtered 20% (NHâ‚„)â‚‚S solution for 10 minutes. This chemical treatment saturates surface dangling bonds with sulfur atoms.
• Inert Transfer to ALD: Without exposing the sample to ambient air, transfer it directly from the glove box into the load-lock chamber of the ALD system.
• Oxide Encapsulation: Deposit a 10 nm thick Alâ‚‚O₃ film by ALD at a substrate temperature of 150°C. This layer serves as a dense, permanent barrier that protects the sulfur-passivated surface from re-degradation.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents and Materials for In-Situ Surface Passivation

Reagent/Material	Function/Application	Example Use Case
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Chemical passivator for III-V semiconductors; saturates surface dangling bonds.	Sulfur passivation of InAs/GaAs QDs [16].
Atomic Layer Deposition (ALD) Al₂O₃	Ultra-thin, conformal oxide encapsulation layer; provides a hermetic seal.	Protective capping layer after sulfur passivation [16].
Tetraoctylammonium Bromide (t-OABr)	Bulky organic ligand used to form a wide-bandgap shell on PQDs.	Shell precursor in MAPbBr₃@tetra-OAPbBr₃ core-shell QDs [4].
Metal-Organic Frameworks (UiO-66)	Microporous inorganic-organic hybrid material providing nanoscale spatial confinement.	Host matrix for CsPbBr₃ QDs, enabling >30 month stability [17].
Covalent Organic Frameworks (S-COF)	Porous organic polymer with functionalizable pores for synergistic stabilization.	Thiomethyl-functionalized COF for confining MAPbBr₃ QDs [19].
3-Mercaptopropionic Acid (3-MPA)	Hydrophilic ligand for phase transfer of QDs from organic solvents to water.	Ligand exchange for water-dispersion of InP/ZnSe/ZnS QDs [18].
3-[3-(Trifluoromethyl)phenoxy]aniline	3-[3-(Trifluoromethyl)phenoxy]aniline|CAS 625106-28-7
3,4-(Ethylenedioxy)-4'-iodobenzophenone	3,4-(Ethylenedioxy)-4'-iodobenzophenone	3,4-(Ethylenedioxy)-4'-iodobenzophenone (97% purity) for research. CAS 878969-65-4. Molecular Formula: C15H11IO3. For Research Use Only. Not for human or veterinary use.

The direct correlation between surface state density and the long-term stability of quantum dot devices is irrefutable. The experimental data and protocols outlined in this Application Note demonstrate that modern in-situ and post-synthesis passivation strategies—including epitaxial core-shell growth, optimized chemical treatment, and nano-encapsulation within porous matrices—can dramatically enhance both performance and device lifetime by orders of magnitude. Moving beyond simple ligand exchange to these multi-faceted, engineered approaches is essential for the future commercialization of QD technologies. Researchers are encouraged to adopt and further refine these protocols, tailoring the chemical and structural parameters to their specific material systems and target applications.

Advanced Passivation Methods and Their Biomedical Applications

The optical and electronic properties of colloidal quantum dots (QDs) are critically dependent on their surface chemistry. In-situ ligand capping is an advanced precursor engineering strategy that involves the introduction of passivating ligands during the synthesis reaction itself, rather than in a separate post-synthetic step. This approach allows for the immediate coordination of ligands with nascent nanocrystal surfaces, leading to more effective suppression of surface defects and a significant enhancement in photoluminescence quantum yield (PLQY) [20]. This application note details a protocol for the synthesis of high-efficiency, NIR-I emitting core-only InP QDs via a multi-ligand synergistic system, providing a robust methodology for researchers focused on in-situ surface passivation.

Experimental Design & Workflow

The following diagram illustrates the integrated one-pot synthesis and in-situ ligand capping process.

Synthesis and In-Situ Capping Workflow

Research Reagent Solutions

The table below lists the essential materials and their specific functions in the synthetic protocol.

Reagent / Material	Function / Role in Synthesis
Indium(I) Chloride (InCl)	Primary indium precursor; acts as both metal source and reductant for aminophosphine [20].
Tris(dimethylamino)phosphine ((DMA)3P)	Phosphorus precursor for InP core formation [20].
Zinc Chloride (ZnCl₂)	Co-passivating agent; provides Zn²⁺ and Cl⁻ ions for coordinated passivation of surface dangling bonds, critically enhancing PLQY [20].
Ammonium Hexafluorophosphate (NH₄PF₆)	Co-passivating agent; PF₆⁻ ions selectively etch and remove surface oxides (e.g., In₂O₃, In(PO₃)ₓ) that act as luminescence quenchers [20].
Oleylamine (OLA)	Primary solvent and ligand; coordinates with metal precursors and facilitates nanocrystal growth [20].
1-Octadecene (ODE)	Non-coordinating solvent; provides a high-temperature reaction medium [20].
Trioctylphosphine (TOP)	Reaction initiator and secondary ligand; injection triggers rapid nucleation [20].

Quantitative Synthesis Data & Outcomes

Key reaction parameters and the resulting optical properties of the synthesized core-only InP QDs are summarized below.

Table 1: Synthesis Parameters and Optical Properties

Growth Temperature (°C)	NH₄PF₆ (mmol)	ZnCl₂ (mmol)	Reaction Time (min)	Emission Wavelength (nm)	FWHM (nm)	PLQY (%)
220	0.4	0.4	5	683	65	52
260	0.4	0.4	30	710	68	65
300	0.4	0.4	60	742	71	74
260	0.0	0.4	60	~700	N/R	<10
260	0.4	0.0	60	~700	N/R	~30

FWHM: Full Width at Half Maximum; PLQY: Photoluminescence Quantum Yield; N/R: Not Reported in source material [20]

Detailed Experimental Protocol

Precursor and Ligand Solution Preparation

• InCl Stock Solution: Load 0.4 mmol (46.4 mg) of InCl and 4 mL of OLA into a 25 mL three-neck flask. Heat to 120°C under vacuum for 30 minutes with stirring until a clear, colorless solution is obtained. Maintain under a nitrogen atmosphere thereafter.
• Ligand Master Mix: In an inert atmosphere glovebox, prepare a solid mixture of 0.4 mmol (106.8 mg) of NHâ‚„PF₆ and 0.4 mmol (54.4 mg) of ZnClâ‚‚. This mixture is used as the synergistic co-ligand system.

One-Pot Synthesis and In-Situ Capping Procedure

• Reaction Setup: To the flask containing the clear InCl solution, add 0.4 mmol (100 µL) of (DMA)3P and the prepared solid ligand master mix (NHâ‚„PF₆ + ZnClâ‚‚).
• Nucleation Initiation: Rapidly inject 0.5 mL of TOP into the reaction flask to initiate nucleation.
• QD Growth and Passivation: Immediately heat the reaction mixture to the target growth temperature (between 260°C and 300°C). Monitor the growth by observing the development of color.
• Aliquot Sampling: At regular time intervals (e.g., 5, 30, 60 minutes), withdraw 0.5 mL aliquots using a syringe for subsequent optical characterization.
• Reaction Termination: After the desired reaction time, remove the heating mantle and allow the reaction flask to cool to below 80°C.

Purification and Isolation

• Precipitation: Add a 2:1 volume ratio of hexane to the cooled reaction mixture, followed by the addition of excess ethanol (approximately 20 mL) to precipitate the QDs.
• Centrifugation: Centrifuge the mixture at 8000 rpm for 5 minutes. A solid pellet of QDs will form.
• Washing: Decant the supernatant. Re-disperse the pellet in 5 mL of hexane and re-precipitate with ethanol. Repeat this washing cycle twice.
• Final Dispersion: After the final centrifugation step, disperse the purified QD pellet in 5 mL of an anhydrous non-polar solvent (e.g., hexane, toluene) for storage and further characterization. Store in an inert atmosphere at 4°C.

Ligand Coordination Mechanism

The synergistic effect of the multi-ligand system is crucial for achieving high performance in core-only QDs.

Ligand Synergy Mechanism

Characterization and Validation

• Optical Spectroscopy: Measure UV-Vis absorption and photoluminescence spectra to determine the emission wavelength, FWHM, and Stokes shift.
• Photoluminescence Quantum Yield (PLQY): Determine the absolute PLQY using an integrating sphere, calibrated against a standard with known quantum yield [20]. The protocol achieves PLQY up to 74%.
• Structural Analysis: Perform Transmission Electron Microscopy (TEM) to analyze the size, size distribution, and morphology of the QDs. X-ray Diffraction (XRD) can confirm the crystal phase.
• Surface Analysis: Techniques like Fourier-Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) can be used to verify the presence and binding of the ligands on the QD surface.

In quantum dot (QD) research, surface ligands are indispensable for governing nanocrystal stability, solubility, and electronic properties. These molecules, typically comprising a surface-binding headgroup and an organic tail, dynamically coordinate with the QD surface, creating a complex equilibrium between bound and free states [21]. The process of ligand exchange—replacing native ligands with custom-designed molecules—is a fundamental technique for tailoring QDs for specific applications, from photovoltaics to biosensing. Effective ligand exchange is particularly critical within the broader context of in-situ surface passivation, a methodology aimed at mitigating surface defects that degrade optical and electronic performance. This document provides detailed application notes and protocols for executing and characterizing ligand exchange, serving as a practical guide for researchers developing advanced QD-based materials and devices.

The Role and Dynamics of Organic Capping Ligands

Ligand Binding Motifs and Classification

Ligands bound to QD surfaces are systematically classified using Green's covalent bond classification:

• X-type ligands: Anionic species (e.g., carboxylates like oleate, thiolates) that donate one electron to a surface metal cation, compensating for excess positive charge [21].
• L-type ligands: Neutral two-electron donors (e.g., amines, phosphines, carboxylic acids, thiols) that typically do not alter the net charge of the QD [21].
• Z-type ligands: Neutral two-electron acceptors (e.g., metal complexes like Pb(oleate)2) that coordinate to surface chalcogen anions [21].

The binding is highly facet-dependent. For example, on PbS QDs, oleic acid (OAH) can coordinate weakly as an L-type ligand on (100) facets through its -COOH headgroup, while its anionic form, oleate (OA), binds strongly as an X-type ligand on (111) facets [22] [21]. This diversity in binding environments directly influences the efficiency and outcome of subsequent ligand exchange processes.

Evidence of Ligand Ordering and Dynamics

The organic capping shell is not a static, disordered layer. Powder X-ray diffraction studies have directly observed ordered organic capping ligands on semiconducting QDs, with a characteristic diffraction peak at approximately (q=1.4{\AA}^{-1}) assigned to well-ordered aliphatic chains [23]. This ordering is sensitive to ligand length, geometry, and temperature. Furthermore, multimodal nuclear magnetic resonance (NMR) studies reveal a dynamic binding equilibrium, identifying at least three distinct ligand populations: strongly bound (chemisorbed), weakly bound, and free ligands [22] [21]. The exchange between weakly bound and free states can occur rapidly, on the order of 0.09 to 2 milliseconds [22] [21].

Quantitative Analysis of Ligand Exchange

Table 1: Key Parameters from Quantified Ligand Exchange Studies

QD System	Exchange Ligand	Analytical Technique	Key Quantitative Finding	Reference
PbS QDs	Oleic Acid (OAH)	Multimodal NMR & Diffusometry	Bound ligands exist in two sub-populations: strongly bound on (111) facets and weakly bound on (100) facets.	[22] [21]
PbS QDs	Ethanedithiol (EDT)	AFM-IR Nanospectroscopy	~90% ligand exchange achieved in ~60 seconds for structures up to ~800 nm thick.	[24]
PbS QDs	Halides + TPPO	Device Performance / DFT	TPPO "patch-passivation" increased IR solar cell PCE; adsorption energy of TPPO on (100) facet = -1.26 eV.	[25]
CdSe QDs	Butyl Amine	FTIR / Photoluminescence	Higher fraction of accessible fluorophores ((f_a)) for TOPO-capped vs. oleic acid-capped CdSe.	[26]

Table 2: Reagent Solutions for Ligand Exchange and Passivation

Research Reagent	Chemical Classification	Primary Function in Experiment
Oleic Acid (OAH) / Oleate (OA)	Carboxylic Acid / Carboxylate (X-type/L-type)	Native surface ligand; model system for studying acid-base exchange dynamics.
Ethanedithiol (EDT)	Dithiol (X-type)	Short-chain ligand for cross-linking QD solids and enhancing charge transport.
Triphenylphosphine Oxide (TPPO)	Lewis Base (L-type)	"Patch-ligand" for passivating uncoordinated Pb²⁺ atoms after initial halide exchange.
Butyl Amine	Amine (L-type)	Short-chain ligand for rendering CdSe QDs water-soluble via ligand exchange.
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Inorganic Salt	Source of sulfur for inorganic surface passivation to reduce surface state density.

Experimental Protocols for Ligand Exchange

Protocol: Acid-Base Ligand Exchange on PbS QDs

This protocol quantifies the populations and kinetics of oleic acid (OAH) ligand binding, suitable for NMR studies [21].

Materials:

• Purified OA-capped PbS QDs in non-polar solvent (e.g., toluene, chloroform).
• Anhydrous oleic acid (OAH).
• Deuterated solvent for NMR (e.g., CDCl₃).
• Inert atmosphere glove box ([Hâ‚‚O] and [Oâ‚‚] < 1 ppm).

Procedure:

• Purification: Purify the synthesized OA-capped PbS QDs through three cycles of precipitation and centrifugation to remove unbound and weakly bound OA species. Confirm purification via ¹H NMR (free OA concentration < 1 µM) [21].
• Sample Preparation: In a glove box, prepare a stock solution of purified PbS QDs (e.g., 24 mM in bound OA) in CDCl₃ with a known concentration of an internal standard (e.g., ferrocene).
• Titration: Titrate known concentrations of excess anhydrous OAH into the QD stock solution. For each titration point, transfer the sample to an NMR tube and seal it.
• NMR Data Acquisition:
  • Acquire ¹H NMR spectra to distinguish the alkene resonances of bound OA, weakly bound OAH, and free OAH based on their chemical shifts and line shapes.
  • Perform Diffusion-Ordered Spectroscopy (DOSY) to separate species by their diffusion coefficients, quantifying the population fractions in each state (strongly bound, weakly bound, free).
  • For kinetic studies, use dynamic NMR spectroscopy (variable-temperature ¹H NMR) and perform line shape analysis to quantify exchange rates between states.

Protocol: Rapid Thiol-Based Ligand Exchange and Nanoscale Analysis

This protocol describes a rapid, efficient ligand exchange with ethanedithiol (EDT) for PbS QD solids, with quantification via AFM-IR [24].

Materials:

• PbS QDs capped with native ligands (e.g., oleate).
• Acetonitrile (anhydrous).
• Ethanedithiol (EDT).
• Substrate for QD deposition (e.g., Au, Si).
• Electro-hydrodynamic (EHD) printing apparatus (optional).

Procedure:

• Fabrication of QD Microstructures: Deposit a film of PbS QDs onto a substrate using a technique such as spin-coating or electro-hydrodynamic (EHD) printing to create defined microstructures [24].
• Ligand Exchange Solution: Prepare a 1% (v/v) solution of EDT in anhydrous acetonitrile.
• Exchange Reaction: Immerse the QD-patterned substrate in the EDT solution for 60 seconds at room temperature.
• Rinsing and Drying: Gently rinse the substrate with fresh acetonitrile to terminate the reaction and remove any liberated ligands, then dry under a stream of nitrogen.
• Nanoscale Analysis via AFM-IR:
  • Use Atomic Force Microscopy combined with Infrared (AFM-IR) spectroscopy to map the topography and chemical composition of the microstructures.
  • Quantify the exchange efficiency (~90%) by comparing the intensity of infrared vibrational signals (e.g., C-H stretches from oleate) before and after exchange, correlating the signal loss with the fraction of ligands replaced [24].

Protocol: Two-Step In-Situ Passivation for PbS CQD Solar Cells

This protocol employs a halide exchange followed by a Lewis base "patch-passivation" to enhance the performance of PbS CQD infrared solar cells [25].

Materials:

• Synthesized PbS CQDs.
• Lead iodide (PbIâ‚‚) and Lead bromide (PbBrâ‚‚).
• (N,N)-Dimethylformamide (DMF), anhydrous.
• Triphenylphosphine Oxide (TPPO).
• 1,2-ethanedithiol (EDT) in acetonitrile (for solid-state cross-linking).
• Acetonitrile, n-octane.

Procedure:

• Halide Ligand Exchange (Solution-Phase):
  • Disperse the native PbS CQDs in a mixture of n-octane and DMF containing PbIâ‚‚ and PbBrâ‚‚.
  • Stir the mixture vigorously to facilitate the exchange of native organic ligands with I⁻ and Br⁻ ions.
  • Separate the halide-capped CQDs and purify them by precipitation and centrifugation [25].
• Patch-Passivation with TPPO:
  • Re-disperse the halide-capped CQDs in an anhydrous solvent.
  • Add a controlled amount of TPPO (e.g., 1.5 mg/mL) and stir. The electron-rich oxygen atom of the P=O group will coordinate with uncoordinated Pb²⁺ sites on both (100) and (111) facets without displacing the halide ligands.
• Device Fabrication:
  • Deposit the passivated CQDs onto a substrate via layer-by-layer spin-coating.
  • After each layer deposition, perform a solid-state ligand exchange by dipping the film in a solution of EDT in acetonitrile to cross-link the CQDs and improve charge transport.
• Characterization:
  • Use photoluminescence (PL) spectroscopy and transient absorption (TA) spectroscopy to confirm suppressed non-radiative recombination.
  • Fabricate solar cells and measure current-voltage characteristics to determine the power conversion efficiency (PCE), particularly the IR PCE under silicon-filtered AM 1.5G illumination [25].

Ligand Exchange Workflow and Signaling Pathways

Ligand Exchange Mechanism Workflow

Diagram 1: The workflow illustrates the three-state ligand binding equilibrium on a QD surface following the titration of excess oleic acid (OAH), as quantified by multimodal NMR. Strongly bound X-type oleate (OA) ligands coordinate to (111) facets, while weakly bound L-type OAH ligands coordinate to (100) facets, existing in dynamic equilibrium with free ligands in solution [22] [21].

Experimental Protocol Workflow

Diagram 2: This protocol outlines a multi-step, in-situ passivation strategy for creating high-performance QD solids. The process begins with solution-phase exchanges to install initial functional ligands, proceeds with a patch-passivation step to remedy specific surface defects, and culminates in solid-state fabrication and cross-linking [24] [25].

Surface states are inherent limiting factors that degrade the performance of solid-state semiconductor devices, including both classical and quantum systems. This is particularly crucial for quantum devices such as semiconductor quantum dots (QDs), where the source regions are often located closer to the surface (e.g., ~60 nm away when coupled with photonic crystal and circular Bragg grating cavities), making them more vulnerable to surface effects. Surface states introduce significant linewidth broadening for near-surface QDs, which directly impacts their performance as quantum light sources. To address this challenge, surface passivation techniques have been developed and become increasingly significant in modern semiconductor technology.

Sulfur-based passivation, particularly using (NHâ‚„)â‚‚S solutions, has emerged as a promising approach to mitigate surface state effects on near-surface quantum dots. This application note details optimized protocols for (NHâ‚„)â‚‚S passivation combined with atomic layer deposition (ALD) encapsulation, specifically tailored for quantum dot applications requiring high-quality resonance fluorescence signals. The techniques described herein have demonstrated significant improvements in reducing surface state density and electric field fluctuations, leading to enhanced optical properties of individual quantum dots.

Principles of Surface Passivation

The Surface State Problem in Quantum Dots

In semiconductor quantum dots, surface states originate from dangling bonds and surface defects that create electronic states within the bandgap. These states act as trap centers for charge carriers, leading to non-radiative recombination pathways that degrade optical properties. For near-surface QDs (typically less than 100 nm from the surface), these effects are particularly pronounced due to their proximity to the interface. Surface states cause two primary detrimental effects: linewidth broadening of emission spectra and instability of resonance fluorescence signals under pulsed excitation.

The presence of surface states introduces electric field fluctuations and charge noise in the vicinity of quantum dots. This leads to spectral diffusion and broadening of emission linewidths, ultimately reducing the coherence and quality of single-photon emission. In severe cases, a high density of surface states can bend the energy band sufficiently to allow electrons to tunnel out before radiative recombination occurs, completely quenching resonance fluorescence signals.

Mechanism of Sulfur Passivation

Sulfur-based passivation functions primarily through the elimination of dangling bonds on the semiconductor surface. When a gallium arsenide (GaAs) surface is treated with (NHâ‚„)â‚‚S solution, sulfur atoms replace the native oxides and form stable bonds with surface atoms. This process effectively reduces the density of surface states within the bandgap, leading to improved optical properties. The optimized two-step process described in this document further enhances this effect by adding a protective oxide layer that prevents reoxidation and maintains passivation stability over time.

Optimized Passivation Protocol

Equipment and Reagent Setup

Table 1: Essential Research Reagent Solutions

Item	Specification	Function
(NH₄)₂S aqueous solution	20% concentration, filtered through 0.02-μm syringe filters	Removes polysulfide particles; primary passivation agent
Atomic Layer Deposition (ALD) system	Capable of 150°C deposition temperature	Deposits protective Al₂O₃ encapsulation layer
Glove box	Inert atmosphere (Hâ‚‚O and Oâ‚‚ < 1 ppm)	Prevents reoxidation of sulfur layer before ALD
Al₂O₃ precursor	Trimethylaluminum (TMA) and H₂O	Forms 10-nm protective oxide layer

Two-Step Passivation Procedure

Step 1: Sample Preparation and Surface Etching

• Begin with molecular beam epitaxy (MBE)-grown sample containing self-assembled InAs/GaAs QDs in a λ-thick GaAs layer on 30 pairs of AlAs/GaAs distributed Bragg reflector (DBR)
• Etch the surface to achieve dot-to-surface distance of less than 40 nm to enhance surface effects for study
• Fabricate DBR-CBG (distributed Bragg reflector-circular Bragg grating) hybrid structures to improve collection efficiency (8.81-fold increase to 16.28% at 890 nm)

Step 2: Solution Filtration and Preparation

• Within inert atmosphere glove box (Hâ‚‚O and Oâ‚‚ < 1 ppm), filter the (NHâ‚„)â‚‚S aqueous solution through 0.02-μm syringe filters
• This critical step removes polysulfide particles that can cause inhomogeneous passivation
• Prepare 20% (NHâ‚„)â‚‚S solution in an immersion container

Step 3: Sulfur Immersion Passivation

• Immerse the sample in the filtered 20% (NHâ‚„)â‚‚S solution for exactly 10 minutes
• Ensure complete surface coverage throughout the immersion period
• The sulfur atoms replace native oxides and form stable bonds with surface atoms

Step 4: Inert Atmosphere Transfer

• Transfer the passivated sample directly to the load-lock chamber of the ALD system
• Maintain inert atmosphere throughout transfer (Hâ‚‚O and Oâ‚‚ < 1 ppm)
• This step prevents reoxidation of the fresh sulfur-passivated surface

Step 5: Atomic Layer Deposition Encapsulation

• Deposit 10 nm of Alâ‚‚O₃ at 150°C using trimethylaluminum (TMA) and Hâ‚‚O as precursors
• The Alâ‚‚O₃ layer serves as a protective encapsulation, preventing degradation of the sulfur passivation
• This ensures stable and uniform passivation layers for robust and reproducible results

Customized System Configuration

The entire process is supported by a customized system consisting of a glove box connected directly to an atomic layer deposition (ALD) system. This integrated setup maintains an inert atmosphere throughout the process, which is critical for preventing reoxidation of the sulfur-passivated surface before ALD encapsulation. The system ensures stable and uniform passivation layers, making experimental results robust and reproducible.

Performance Validation and Results

Quantitative Assessment of Passivation Efficacy

Table 2: Passivation Effects on Quantum Dot Optical Properties

Parameter	Before Passivation	After Passivation	Improvement
Non-resonant PL Linewidth	21.32 ± 5.48 GHz	16.49 ± 2.03 GHz	22.7% reduction
Resonance Fluorescence Linewidth	43.23 ± 22.53 GHz	19.68 ± 6.48 GHz	54.5% reduction
Pulsed-RF Linewidth (QD2)	14.23 ± 2.34 GHz	7.84 ± 0.48 GHz	44.9% reduction
Photon Number Fluctuation Variance	0.2749	0.1587	42.3% reduction
Collection Efficiency at 890 nm	Baseline	8.81-fold increase	16.28% absolute

Experimental Validation Protocols

Protocol 4.2.1: Non-resonant Photoluminescence Assessment

• Randomly select 25 QDs from the same sample before and after passivation
• Acquire photoluminescence (PL) spectra under non-resonant excitation conditions
• Measure and compare linewidth distribution across the selected QD ensemble
• Calculate average linewidth and standard deviation for statistical significance

Protocol 4.2.2: Resonance Fluorescence Characterization

• Randomly select 9 QDs from the same sample before and after passivation
• Acquire RF spectra under resonant excitation conditions
• For higher resolution measurements, employ a Fabry-Pérot interferometer
• Record linewidth of pulsed-RF spectra for 8 specific QDs in dot-to-dot comparisons
• Measure signal-to-background ratio (SBR) and photon number fluctuations

Protocol 4.2.3: RF Revival Assessment

• Identify QDs that cannot generate RF signals before passivation (using two-color excitation with non-resonant ancillary lasers)
• After passivation, recheck the same QDs for emergence of RF signals
• Confirm coherent manipulation capability by measuring Rabi oscillations of revived QDs
• Document the percentage of QDs showing revived RF signals

Observed Performance Improvements

The optimized sulfur passivation technique demonstrates three significant improvements in quantum dot performance:

Linewidth Reduction: The protocol consistently reduces both non-resonant PL and resonance fluorescence linewidths. Typical RF linewidth reduction of 46.77% was observed, with one specific QD (QD2) showing reduction from 14.23±2.34 GHz to 7.84±0.48 GHz. This linewidth narrowing indicates reduced spectral diffusion and improved coherence properties.

Noise Level Suppression: The variance of photon number fluctuations decreased from 0.2749 to 0.1587 (42.27% reduction) after passivation, indicating significantly improved stability of the RF signals. This noise reduction stems from suppressed electric field fluctuations in the QD environment.

Signal Revival: For QDs that originally showed no RF signals even with additional non-resonant ancillary laser excitation, the passivation technique revived pulsed-RF signals in two out of five tested QDs. This revival demonstrates the technique's effectiveness in mitigating severe surface state effects that completely quench resonance fluorescence.

Troubleshooting and Technical Considerations

While the optimized passivation technique generally improves optical properties, in rare cases (approximately 10% of QDs), newly generated defects around the QD due to (NHâ‚„)â‚‚S etching can cause increased linewidth. This effect should be considered when interpreting results from specific QDs.

The DBR-CBG structure implementation is critical for achieving sufficient collection efficiency to observe RF signals after surface etching. The hybrid structure combines the advantages of DBR for vertical reflection and CBG for lateral diffraction, with 6 lateral circular grating periods optimized using the BOBYQA gradient algorithm to maximize collection efficiency.

For optimal results, maintain strict control over the inert atmosphere throughout the process, as even brief exposure to ambient conditions can compromise the passivation quality before ALD encapsulation. The 10-minute immersion time in (NH₄)₂S represents an optimized parameter—shorter times may provide incomplete passivation while longer times increase etching risks.

Colloidal quantum dot solar cells (QDSCs) represent a promising third-generation photovoltaic technology due to their solution processability, bandgap tunability, and potential for low-cost manufacturing [27]. Despite theoretical efficiencies reaching 44%, practical implementations have faced challenges in scalability and stability [27] [28]. Recent advances in material science, particularly in the domain of in-situ surface passivation techniques, have enabled significant progress in overcoming these limitations. This application note details the latest protocols and methodologies for enhancing the performance and stability of quantum dot photovoltaics, with specific focus on surface engineering strategies that minimize defect states and improve charge transport in quantum dot solids.

Performance Metrics and Recent Advances

Table 1: Recent Performance Benchmarks for PbS Quantum Dot Solar Cells

Device Architecture/Technique	Certified PCE (%)	Active Area	Key Innovation	Reference
Stable ink engineering	13.40	0.04 cm²	Iodine-rich environment in weakly coordinating solvents	[28]
Single-step deposition (200 mg/mL PbS)	9.08	Not specified	Solution-phase ligand exchange, single-step spin coating	[27]
TPPO patch-passivation	IR PCE: 1.36*	Not specified	Lewis base passivation of uncoordinated Pb²⁺ sites	[25]
Scalable module	10.00	12.60 cm²	Stable ink engineering for large-area fabrication	[28]
Centrifugal casting	5.90	Not specified	Single-step deposition method	[27]

*Under 1100 nm filtered AM 1.5G illumination

The quantum dot solar cell market, valued at $1.24 billion in 2024, is projected to reach $3.10 billion by 2030, growing at a CAGR of 16.60% [29]. This growth is fueled by continuous efficiency improvements, such as the recent record of 18.1% efficiency achieved by researchers at Ulsan National Institute of Science & Technology (UNIST) using organic perovskite quantum dots [29].

In-Situ Surface Passivation Protocols

Triphenylphosphine Oxide (TPPO) Patch-Passivation

Experimental Protocol:

Materials:

• PbS CQDs (synthesized by hot injection method)
• PbIâ‚‚ (>99%) and PbBrâ‚‚ (>99%) for ligand exchange
• Triphenylphosphine oxide (TPPO, purity >98%)
• Anhydrous dimethylformamide (DMF)
• Acetonitrile and n-octane (analytical purity)
• 1,2-ethanedithiol (EDT) for device finishing

Procedure:

• Ligand Exchange: Conduct solution-phase ligand exchange of pristine PbS CQDs using PbIâ‚‚/PbBrâ‚‚ in DMF at approximately 100°C for comprehensive halide passivation [25].
• TPPO Treatment: Prepare TPPO solution in acetonitrile at varying concentrations (0.5-2.0 mg/mL). Add TPPO solution to halide-capped PbS CQD solution with constant stirring at 60°C for 30 minutes [25].
• Purification: Precipitate passivated CQDs using n-octane followed by centrifugation at 7500 rpm for 5 minutes.
• Film Formation: Deposit the passivated CQDs onto TiOâ‚‚ substrates via spin-coating at 2500 rpm for 30 seconds in nitrogen atmosphere.
• Solid-State Ligand Exchange: Perform EDT treatment in acetonitrile (0.02% v/v) for 1 minute to create the hole-transporting layer, followed by rinsing with pure acetonitrile [25].

Mechanistic Insight: Density functional theory calculations confirm that the lone pair electrons of the phosphoryl oxygen (P=O) coordinate strongly with uncoordinated Pb²⁺ sites on both (100) and (111) facets of PbS CQDs. The adsorption energies were calculated to be -1.33 eV and -1.55 eV for (100) and (111) facets, respectively, indicating spontaneous binding without disrupting existing halide ligands [25].

Stable Ink Engineering for Large-Area Fabrication

Experimental Protocol:

Materials:

• PbS CQDs from low-cost direct synthesis
• Weakly coordinating solvents (specifically engineered)
• Iodoplumbate precursors
• Substrates for large-area deposition (e.g., FTO glass)

Procedure:

• Ink Formulation: Create an iodine-rich environment in weakly coordinating solvents to convert iodoplumbates into functional anions that condense into a robust surface shell [28].
• Ink Stabilization: Utilize the fully charged electrostatic surface layer to prevent aggregation and epitaxial fusion of CQDs, yielding stable inks with extended shelf life.
• Film Deposition: Employ blade coating or inkjet printing for large-area deposition under ambient conditions.
• Thermal Treatment: Apply mild thermal annealing (specific temperature optimized for ink composition) to enhance inter-dot coupling while maintaining defect suppression.

Key Advantages: This approach eliminates fusion-induced inter-band states, enabling the printing of compact CQD films with three-dimensional uniformity, flattened energy landscape, and improved carrier transport. The technology has demonstrated scalable fabrication from 0.04 cm² cells to 12.60 cm² modules with minimal efficiency loss [28].

Research Reagent Solutions

Table 2: Essential Materials for QD Solar Cell Fabrication

Reagent	Function	Specification	Optimal Concentration
Lead Sulfide (PbS) CQDs	Light-absorbing material	First exciton peak ~937 nm (≈3.17 nm diameter)	200 mg/mL for single-step deposition [27]
Triphenylphosphine Oxide (TPPO)	Lewis base passivator	Purity >98%, PO-functionalized	0.5-2.0 mg/mL in acetonitrile [25]
Lead Iodide/Bromide (PbIâ‚‚/PbBrâ‚‚)	Halide ligand source	Anhydrous, >99% purity	Balanced ratio for complete surface coverage [25]
N,N-Dimethylformamide (DMF)	Solvent for ligand exchange	Anhydrous, oxygen-free	Solution-phase exchange at 100°C [27] [25]
1,2-ethanedithiol (EDT)	Hole-transport layer formation	>98% purity, reducing agent	0.02% v/v in acetonitrile [25]
Iodoplumbate complexes	Ink stabilization precursors	Iodine-rich environment	Optimized for charge balance [28]

Characterization and Quality Control

Optical and Electrical Validation

Protocol for Performance Assessment:

• UV-Vis-NIR Spectroscopy: Measure the first exciton absorption peak to determine quantum dot size and bandgap. Red shifts from 937 nm (synthesized CQDs) to 965-982 nm (QDSC devices) indicate successful integration into devices [27].

• Photoluminescence (PL) Spectroscopy: Quantify passivation effectiveness through PL intensity enhancement. TPPO-passivated CQDs demonstrate significantly increased PL quantum yield, indicating suppressed non-radiative recombination [25].

• Transient Absorption (TA) Spectroscopy: Monitor energy shift of TA bleaching peaks to assess energy loss reduction after passivation treatments.

• Current-Voltage (J-V) Characterization: Perform under standard AM 1.5G illumination (100 mW/cm²) to determine power conversion efficiency (PCE), open-circuit voltage (VOC), short-circuit current (JSC), and fill factor (FF).

• Capacitance-Voltage (C-V) Spectroscopy: Probe energy level structure and tunneling processes in QD ensembles, particularly useful for understanding frequency-dependent and temperature-dependent behaviors [30].

Stability Testing Protocols

Accelerated Aging Conditions:

• Environmental stability: Maintain at 85°C and 85% relative humidity for specified durations
• Light soaking: Continuous illumination at 1 Sun equivalent intensity
• Thermal cycling: -40°C to +85°C for multiple cycles

Performance Retention Metrics:

• TPPO-passivated devices maintain efficiency for >1,200 hours under normal conditions and >300 hours at 80°C [29]
• Stable ink formulations enable months-long shelf lifetime while maintaining performance characteristics [28]

The implementation of advanced in-situ surface passivation techniques represents a critical pathway toward commercially viable quantum dot photovoltaics. The protocols detailed in this application note—specifically TPPO patch-passivation and stable ink engineering—provide researchers with standardized methodologies for enhancing both efficiency and stability of QD solar cells. The integration of these approaches with scalable deposition techniques such as blade coating and inkjet printing positions QDSC technology for accelerated adoption in both specialized applications and mainstream photovoltaics. Future development directions include the integration of QD technology with IoT-enabled smart energy systems and further refinement of cadmium-free quantum dot compositions for environmentally sustainable manufacturing.

The integration of quantum dots (QDs) into biosensing and bioimaging represents a significant advancement over conventional organic fluorophores, primarily due to their superior optical properties. However, a critical challenge limiting their widespread in vivo and in vitro application is the inherent instability and susceptibility to degradation of unpassivated QDs, which leads to diminished signal fidelity and biocompatibility concerns [31] [4]. This document details the application of in-situ surface passivation techniques as a robust strategy to overcome these limitations. By focusing on core-shell nanostructures, specifically through epitaxial growth compatible with the host matrix, we outline protocols and data demonstrating how such passivation enhances quantum yield, bolsters photostability, and ensures colloidal integrity in biological environments [4]. The content herein is framed within a broader thesis on advancing QD technology for reliable and high-fidelity diagnostic and research tools.

Application Notes

The Rationale for Surface Passivation

The exceptional optical properties of QDs—including their narrow, size-tunable emission, broad absorption spectra, and high resistance to photobleaching—make them ideal probes for multiplexed bioimaging and sensitive biosensing [31]. A direct comparison of their properties against conventional organic dyes is summarized in Table 1. Despite these advantages, the high surface-to-volume ratio of QDs creates numerous surface defect sites that act as traps for charge carriers, facilitating non-radiative recombination pathways [4]. This results in lower quantum yields and unpredictable blinking, which directly compromises signal fidelity in prolonged imaging sessions or quantitative assays. Furthermore, unprotected QD surfaces are chemically active and prone to degradation upon interaction with ionic species in physiological buffers, leading to aggregation and leakage of toxic heavy metal ions (e.g., Cd²⁺, Pb²⁺), thereby raising serious biocompatibility issues [32] [4].

Table 1: Comparison of Organic Fluorophores and Quantum Dots for Bio-applications

Optical Property	Organic Fluorophores	Quantum Dots (Passivated)	Implication for Biosensing/Bioimaging
Molar Extinction Coefficient	~(0.5-1) x 10⁵ M⁻¹cm⁻¹ [31]	~(1-2) x 10⁶ M⁻¹cm⁻¹ [31]	QDs are 1-2 orders of magnitude brighter, enabling higher sensitivity.
Emission Bandwidth (FWHM)	50-100 nm [31]	20-30 nm [31]	Enables simultaneous multiplexing of 5-10 colors without spectral overlap.
Photostability	Low to moderate; rapid photobleaching [31]	High; resistant to photobleaching [31]	Suitable for long-term tracking and time-lapse imaging.
Excited-State Lifetime	~1-5 ns [31]	~20-100 ns [31]	Enables time-gated detection to separate signal from autofluorescence.
Quantum Yield (in buffer)	Variable; can be high	80-90% (with advanced shell) [4]	High quantum yield is maintained post-passivation for a strong signal.
Biocompatibility	Generally good	Requires surface functionalization [32]	Core-shell and polymer coatings are key to reducing toxicity [32] [31].

The implementation of an epitaxial core-shell structure, where a higher-bandgap semiconductor shell is grown directly onto the QD core, has proven to be a transformational passivation strategy [4]. The shell confines excitons to the core, effectively suppressing non-radiative recombination at surface traps and significantly boosting the quantum yield from less than 10% to over 80% [31] [4]. This direct integration, especially when performed in-situ during the formation of the host material, ensures strong interfacial bonding and superior lattice matching, which minimizes defect formation and enhances long-term chemical robustness [4].

Quantitative Performance Data

The efficacy of in-situ passivation is quantitatively demonstrated through enhanced performance metrics in both sensing and device integration. For instance, the integration of passivated perovskite QDs (PQDs) into solar cells has shown remarkable improvements in power conversion efficiency, a proxy for efficient charge transfer and minimized recombination—principles that are directly applicable to photodetectors and electrochemical biosensors [4]. Furthermore, passivated QDs enable advanced bioimaging techniques, such as the Multicolor, Multicycle, Molecular Profiling (M3P) technology, which relies on the exceptional photostability of QDs for sequential staining and imaging [31]. Key quantitative data from recent studies is consolidated in Table 2.

Table 2: Quantitative Performance Enhancement from In-Situ Passivation Strategies

Application / Metric	Control (Unpassivated)	With In-Situ Passivation	Notes / Conditions
Perovskite Solar Cell (PSC) Performance [4]
Power Conversion Efficiency (PCE)	19.2%	22.85%	At optimal PQD concentration of 15 mg/mL.
Open-Circuit Voltage (Voc)	1.120 V	1.137 V
Short-Circuit Current Density (Jsc)	24.5 mA/cm²	26.1 mA/cm²
Fill Factor (FF)	70.1%	77.0%
Stability (PSC) [4]	~80% PCE retention	>92% PCE retention	After 900 hours under ambient conditions.
Quantum Yield (QY) [31] [4]	<10% (core only)	80-90% (core-shell)	Achieved with complex shell structures.
Multiplexing Capacity [31]	2-3 colors (organic dyes)	5-10 colors per cycle (QDs)	M3P method allows 100-plex analysis over 10 cycles.

Experimental Protocols

Protocol 1: Synthesis of Core-Shell Perovskite Quantum Dots for Passivation

This protocol describes the colloidal synthesis of methylammonium lead bromide (MAPbBr₃) core quantum dots encapsulated with a tetraoctylammonium lead bromide (tetra-OAPbBr₃) shell, based on a method validated for enhancing the performance and stability of perovskite solar cells [4].

Research Reagent Solutions

Item	Function / Explanation
Methylammonium bromide (MABr)	Precursor for the perovskite core structure.
Lead(II) bromide (PbBrâ‚‚)	Source of lead and bromide ions for the crystal lattice.
Tetraoctylammonium bromide (t-OABr)	Precursor for the formation of the passivating shell.
Dimethylformamide (DMF)	Polar solvent for dissolving precursor salts.
Oleylamine and Oleic Acid	Surface ligands that coordinate to the QD surface, controlling growth and providing initial solubility in non-polar solvents.
Toluene	Non-polar solvent used as the reaction medium for nanoparticle growth.
Chlorobenzene	Solvent for final redispersion of purified QDs, compatible with device fabrication.

Methodology:

• Core Precursor Preparation: In a glass vial, dissolve 0.16 mmol MABr and 0.2 mmol PbBrâ‚‚ in 5 mL of DMF under continuous stirring. To this solution, add 50 µL of oleylamine and 0.5 mL of oleic acid. This forms the final core precursor solution.
• Shell Precursor Preparation: In a separate glass vial, dissolve 0.16 mmol of t-OABr in 5 mL of DMF using the same protocol as the core precursor.
• Nanoparticle Growth: Heat 5 mL of toluene to 60 °C in an oil bath under continuous stirring.
• Core Formation: Rapidly inject a 250 µL aliquot of the core precursor solution into the heated toluene. This initiates the nucleation and growth of MAPbBr₃ core nanoparticles.
• Shell Formation: Immediately after core formation, inject a controlled amount of the t-OABr-PbBr₃ shell precursor solution into the reaction mixture. The emergence of a green color indicates the successful development of core-shell nanoparticles. Allow the reaction to proceed for 5 minutes.
• Purification: a. Transfer the solution to a centrifuge tube and spin at 6,000 rpm for 10 minutes. b. Discard the precipitate and collect the supernatant. c. Add isopropanol to the supernatant and centrifuge at 15,000 rpm for 10 minutes to precipitate the core-shell PQDs. d. Carefully decant the supernatant and redisperse the final pellet in chlorobenzene to achieve a stable dispersion for subsequent use. The concentration can be adjusted as needed.

Protocol 2: In-Situ Integration of PQDs for Enhanced Bioimaging & Biosensing Films

This protocol outlines the procedure for integrating the synthesized core-shell PQDs during the film fabrication process, enabling in-situ passivation of grain boundaries and defects within a host matrix, thereby improving signal fidelity and stability [4].

Methodology:

• Substrate Preparation: Clean a Fluorine-doped Tin Oxide (FTO) glass substrate sequentially in a soap solution, distilled water, ethanol, and acetone using an ultrasonic bath. Subsequently, treat the substrate in a UV-ozone cleaner for 15 minutes.
• Electron Transport Layer (ETL) Deposition: a. Deposit a compact TiOâ‚‚ layer via spray pyrolysis and anneal at 450°C for 30 minutes. b. Spin-coat a mesoporous TiOâ‚‚ layer (from a colloidal TiOâ‚‚ paste diluted in ethanol) at 4000 rpm for 30 seconds, followed by annealing at 450°C for 30 minutes.
• Perovskite Film Formation with In-Situ PQDs: a. Prepare the host perovskite precursor solution (e.g., 1.6 M PbIâ‚‚, 1.51 M FAI, 0.04 M PbBrâ‚‚, 0.33 M MACl, 0.04 M MABr in 1 mL of DMF:DMSO (8:1 v/v)). b. Spin-coat the perovskite precursor solution onto the prepared ETL. c. During the spin-coating process, at the critical antisolvent crystallization step, administer the core-shell PQD dispersion (in chlorobenzene, e.g., at 15 mg/mL) as the antisolvent. This step simultaneously triggers the crystallization of the host perovskite film and embeds the PQDs at the grain boundaries and interfaces. d. Anneal the film to form a crystalline, PQD-passivated perovskite layer.

The following workflow diagram illustrates the key stages of this integrated synthesis and application process.

Diagram 1: Workflow for synthesis of core-shell PQDs and in-situ integration into a host film.

Visualization of Concepts and Workflows

Architecture of a Passivated Quantum Dot Probe

The superior performance of passivated QDs in bioapplications stems from their multi-layered architecture, which is designed to optimize both optical properties and biocompatibility. The following diagram deconstructs the structure of a functionalized, core-shell QD probe, highlighting the role of each layer.

Diagram 2: Layered architecture of a functionalized, passivated quantum dot probe.

Multiplexed Bioimaging with Passivated QDs

The application of passivated QDs in advanced bioimaging, such as the M3P technology, leverages their photostability and narrow emission. The following diagram outlines the cyclical process of staining, imaging, and destaining that enables highly multiplexed analysis.

Diagram 3: Multicycle molecular profiling workflow using photostable QDs.

Addressing Passivation Challenges and Optimizing Protocol Parameters

Quantum dots (QDs) are semiconductor nanocrystals whose optoelectronic properties are highly dependent on their surface chemistry. While organic capping ligands are essential for stabilizing QDs and passivating surface defects, their long, insulating chains often hinder inter-dot charge transport, creating a significant performance bottleneck in devices. This application note details advanced in-situ techniques designed to overcome ligand-induced insulation. We focus on strategies that replace native insulating ligands with shorter, conductive alternatives or employ innovative shell structures, enabling simultaneous defect passivation and efficient charge carrier transport for next-generation quantum dot applications.

Core Strategies and Quantitative Outcomes

Advanced surface engineering strategies have demonstrated significant improvements in both performance and stability for quantum dot-based devices. The following table summarizes the quantitative outcomes of several key approaches.

Table 1: Performance Outcomes of Advanced In-Situ Passivation Strategies

Strategy	Material System	Key Performance Metric	Control Value	Optimized Value	Reference
Core-Shell PQD Passivation	MAPbBr3@tetra-OAPbBr3 in PSCs	Power Conversion Efficiency (PCE)	19.2%	22.85%	[4]
		Open-Circuit Voltage (VOC)	1.120 V	1.137 V	[4]
		Short-Circuit Current Density (JSC)	24.5 mA/cm²	26.1 mA/cm²	[4]
		Fill Factor (FF)	70.1%	77.0%	[4]
Alkaline-Augmented Ligand Exchange	FA0.47Cs0.53PbI3 PQDs	Certified PCE	-	18.3%	[33]
Polymer-Based Trap Passivation	PMMA in QLEDs	Current Efficiency	~12 cd/A (Pristine)	26.28 cd/A (with 1% PMMA)	[34]

Detailed Experimental Protocols

Protocol: In-Situ Synthesis of Core-Shell Perovskite Quantum Dots

This protocol describes the integration of core-shell structured perovskite quantum dots (MAPbBr₃@tetra-OAPbBr₃) during the antisolvent-assisted crystallization of perovskite solar cells, based on the method yielding a PCE of 22.85% [4].

• Reagents & Materials: Methylammonium bromide (MABr), Lead(II) bromide (PbBr₂), Tetraoctylammonium bromide (t-OABr), Dimethylformamide (DMF), Oleylamine, Oleic acid, Toluene, Chlorobenzene, Perovskite precursor solution (e.g., 1.6 M PbI₂, 1.51 M FAI, 0.04 M PbBr₂, 0.33 M MACl in DMF:DMSO 8:1).
• Equipment: Schlenk line, Oil bath, Centrifuge, Ultrasonic bath, UV-ozone cleaner, Spin coater, Hot plate.

Procedure:

• Core Precursor Preparation: Dissolve 0.16 mmol MABr and 0.2 mmol PbBr₂ in 5 mL DMF. Add 50 µL oleylamine and 0.5 mL oleic acid under continuous stirring.
• Shell Precursor Preparation: Dissolve 0.16 mmol t-OABr in 5 mL DMF using a separate vial, following the same additive protocol.
• Nanoparticle Growth: Heat 5 mL of toluene to 60°C in an oil bath with stirring. Rapidly inject 250 µL of the core precursor solution.
• Shell Formation: Immediately inject a controlled amount of the t-OABr-PbBr₃ shell precursor into the reaction mixture. The emergence of a green color indicates core-shell nanoparticle formation.
• Purification: Allow the reaction to proceed for 5 minutes. Transfer to centrifuge tubes and spin at 6,000 rpm for 10 minutes. Discard the precipitate. Subject the supernatant to a second centrifugation with isopropanol at 15,000 rpm for 10 minutes.
• Dispersion: Collect the final precipitate and redisperse in chlorobenzene at a concentration of 15 mg/mL for device integration.
• Device Integration: During the antisolvent step of perovskite film deposition, introduce the core-shell PQD dispersion in chlorobenzene to enable in-situ epitaxial passivation of grain boundaries [4].

Protocol: Alkaline-Augmented Antisolvent Hydrolysis (AAAH) for Ligand Exchange

This protocol utilizes an alkaline environment to facilitate the hydrolysis of ester-based antisolvents, promoting the replacement of pristine insulating oleate ligands with conductive short-chain ligands on the PQD surface [33].

• Reagents & Materials: FA_0.47Cs_0.53PbI₃ PQD solid film, Methyl benzoate (MeBz), Potassium hydroxide (KOH), 2-pentanol (2-PeOH).
• Equipment: Spin coater, Glovebox, Humidity-controlled environment (~30% RH).

Procedure:

• Antisolvent Modification: Add a carefully regulated concentration of KOH to methyl benzoate antisolvent to create the alkaline environment.
• Interlayer Rinsing: Spin-coat a layer of PQD colloids to form a solid film. Rinse the film immediately with the KOH-modified MeBz antisolvent.
• Ligand Exchange & Hydrolysis: The alkaline environment renders ester hydrolysis thermodynamically spontaneous and lowers the reaction activation energy, facilitating rapid substitution of insulating oleate ligands with conductive hydrolyzed benzoate ligands.
• Film Drying: Allow the antisolvent to evaporate completely after rinsing.
• Layer Buildup: Repeat the spin-coating and alkaline-antisolvent rinsing steps to build the light-absorbing layer to the desired thickness.
• Post-Treatment (Optional): For enhanced performance, a post-treatment with short cationic ligands (e.g., FA⁺ salts dissolved in 2-pentanol) can be applied to substitute long-chain OAm⁺ ligands on the A-site [33].

Protocol: In-Situ Etching and Interfacial Engineering of InP Quantum Dots

This protocol describes a dual-stage in-situ etching strategy for synthesizing high-performance, cadmium-free InP QDs with superior optoelectronic properties [7].

• Reagents & Materials: Indium phosphide (InP) precursors, ZnF₂ (etchant), Tri-n-octylphosphine (TOP) ligands, ZnSe, ZnS shell precursors, Carboxylic acid–thiol bifunctional ligands.
• Equipment: Schlenk line, High-temperature reaction apparatus, Centrifuge.

Procedure:

• Nucleation Control with In-Situ Etching: During the nucleation stage of magic-sized InP clusters, introduce ZnF₂ as an etchant. Integrate TOP ligands to control nucleation and prevent excessive etching.
• Shell Growth with Interfacial Passivation: Grow a multi-layered shell (e.g., ZnSe/ZnS) around the InP core. Employ a second in-situ etching step during shelling using ZnF₂ to achieve atomic-level defect passivation at the core/shell interface.
• Surface Modification: After synthesis, modify the QD surface using carboxylic acid–thiol bifunctional ligands to enhance charge transport properties in the solid film.
• Purification: Purify the resulting QDs via centrifugation to remove unreacted precursors and by-products.

Workflow and Pathway Visualization

The following diagram illustrates the logical workflow for selecting and implementing a strategy to mitigate ligand-induced insulation, integrating the core techniques discussed in this note.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for Mitigating Ligand-Induced Insulation

Reagent / Material	Function / Application	Key Mechanism / Outcome
Tetraoctylammonium Bromide (t-OABr)	Shell precursor for core-shell PQDs [4]	Forms a wider-bandgap shell, enabling epitaxial passivation of host perovskite grain boundaries and suppressing non-radiative recombination.
Methyl Benzoate (MeBz) with KOH	Alkaline-augmented antisolvent for interlayer rinsing [33]	Facilitates rapid hydrolysis and substitution of insulating oleate ligands with conductive short-chain ligands, doubling conductive capping.
ZnF2	Etchant for InP QD synthesis [7]	Enables in-situ etching during nucleation and shelling stages for atomic-level defect passivation while preserving crystallographic integrity.
Poly(methyl methacrylate) (PMMA)	Insulating polymer additive for QD emissive layer [34]	Passivates trap states on QD surfaces, improving charge balance and device efficiency in QLEDs without requiring complex chemical synthesis.
Carboxylic acid–thiol bifunctional ligands	Surface ligands for InP QDs [7]	Enhance charge transport properties in QD solid films through improved inter-dot coupling and surface binding.
Tri-n-octylphosphine (TOP)	Ligand for InP QD nucleation [7]	Controls nucleation and suppresses excessive etching during the in-situ synthesis of InP cores.
6-(2,5-Dichlorophenyl)-6-oxohexanoic acid	6-(2,5-Dichlorophenyl)-6-oxohexanoic acid, CAS:870287-01-7, MF:C12H12Cl2O3, MW:275.12 g/mol	Chemical Reagent
8-(3,5-Dimethoxyphenyl)-8-oxooctanoic acid	8-(3,5-Dimethoxyphenyl)-8-oxooctanoic acid, CAS:898792-57-9, MF:C16H22O5, MW:294.34 g/mol	Chemical Reagent

The conflict between surface passivation and charge transport in quantum dot devices is no longer an insurmountable barrier. The in-situ techniques detailed herein—ranging from advanced ligand exchange and core-shell engineering to innovative etching and polymer blending—provide a robust toolkit for researchers. By moving beyond simple ligand substitution to dynamic, in-situ processes that control surface chemistry at the atomic level, these strategies enable the simultaneous achievement of exceptional defect passivation and efficient charge transport. This paves the way for the development of next-generation, high-performance quantum dot-based optoelectronics, including solar cells, displays, and light-emitting diodes.

Overcoming Lattice Mismatch in Core/Shell Structures to Prevent Strain

Lattice mismatch, the difference in lattice constants between two crystalline materials, is a fundamental challenge in the synthesis of core/shell nanostructures. The resulting interfacial strain can lead to the formation of structural defects, degraded optical properties, and reduced stability, particularly in quantum-confined systems. For quantum dots (QDs), where surface states significantly influence performance, uncontrolled strain exacerbates non-radiative recombination pathways. This Application Note details proven strategies and detailed protocols for overcoming lattice mismatch, directly supporting broader research objectives in in-situ surface passivation for high-performance quantum dot applications. The methods outlined herein—focusing on core size engineering, interfacial alloying, and advanced passivation techniques—enable the synthesis of high-quality, strain-managed core/shell heterostructures.

Key Strategies and Quantitative Data

Recent advances have identified several effective approaches to mitigate lattice mismatch. The quantitative findings from key studies are summarized in the table below.

Table 1: Strategies for Overcoming Lattice Mismatch in Core/Shell Nanostructures

Strategy	Material System	Key Experimental Parameter	Quantitative Outcome	Impact on Strain & Performance
Core Size & Thermal Engineering [35]	NaGdF4 (β-phase) @ CsPbBr3	Use of sub-8 nm LnNP cores; High reaction temperature	Successful heterostructure formation despite different crystal phases (β-core, α-shell)	Enabled direct growth; Enhanced upconversion luminescence via defect passivation
Cationic Interfacial Alloying [36]	InP / Zn(Mg)Se / ZnS	Incorporation of Mg2+ into ZnSe inner shell	FWHM reduced to 36 nm; PLQY increased to 88%	Mg2+ expanded ZnSe lattice, improved lattice match with InP, reducing interfacial defects
Optimized Surface Passivation [1]	InAs/GaAs QDs with Al2O3	(NH4)2S treatment + 10 nm Al2O3 capping	Pulsed-RF linewidth reduced from 43.23 ± 22.53 GHz to 19.68 ± 6.48 GHz	Reduced surface state density and local electric field fluctuations

Experimental Protocols

Protocol A: Seeding with Small Cores for Heterostructure Growth

This protocol, adapted from the synthesis of NaGdF4@CsPbBr3 core-shell structures, leverages small core nanoparticles to overcome phase and lattice mismatch [35].

• Reagents:

  • Core Nanoparticles: Sub-8 nm α-NaYF4 or β-NaGdF4 LnNPs.
  • Shell Precursors: Cesium oleate, Lead(II) bromide (PbBr2).
  • Solvents: 1-Octadecene (ODE), Oleic Acid (OA), Oleylamine (OLA).

• Procedure:

  • Core Dispersion: Disperse 10 mg of synthesized sub-8 nm LnNPs in 10 mL of ODE with 1 mL each of OA and OLA in a 50 mL three-neck flask. Degas under vacuum at 100 °C for 30 minutes.
  • Precursor Preparation: In an inert atmosphere, heat the reaction mixture to a high temperature (>150 °C is critical [35]).
  • Shell Growth: Rapidly inject 0.4 mL of cesium oleate and 0.2 mL of PbBr2 precursor solutions (both pre-dissolved in OA/ODE) simultaneously into the vigorously stirring core dispersion.
  • Reaction and Quenching: Allow the reaction to proceed for 5-15 seconds. Immediately cool the reaction flask using an ice-water bath to terminate shell growth.
  • Purification: Purify the resulting heterostructures by centrifugation with anti-solvents (e.g., acetone/ethanol) and re-disperse in a non-polar solvent like hexane or toluene.

Protocol B: Mg2+ Doping for Lattice Matching in InP/ZnSe/ZnS QDs

This protocol details the incorporation of Mg2+ ions at the core/shell interface to balance the lattice mismatch in cadmium-free QDs [36].

• Reagents:

  • Core Precursors: Indium chloride (InCl3), Tris(dimethylamino)phosphine ((DMA)3P).
  • Shell Precursors: Zinc iodide (ZnI2), Selenium powder (Se), Sulfur powder (S), Magnesium chloride (MgCl2).
  • Ligands and Solvents: Oleylamine (OLA), 1-Octadecene (ODE), Trioctylphosphine (TOP).

• Procedure:

  • InP Core Synthesis: Synthesize InP core QDs via hot-injection of (DMA)3P into a mixture of InCl3 and OLA at 180 °C. Maintain growth at 160 °C for 20 minutes.
  • Zn(Mg)Se Shell Growth:
    • Purify the InP cores and re-disperse in ODE/OLA.
    • In a separate flask, prepare the Zn(Mg)Se precursor by dissolving Se powder in TOP and mixing with a solution of ZnI2 and MgCl2 in OLA. The molar ratio of Mg:Zn should be optimized (e.g., 1:10).
    • Heat the InP core solution to 280 °C under inert gas.
    • Slowly and dropwise inject the Zn(Mg)Se precursor solution into the core solution. Maintain this temperature for 1 hour to allow for homogeneous shell growth.
  • ZnS Outer Shell Growth:
    • Add a solution of S powder in ODE to the reaction mixture.
    • Slowly inject a solution of Zinc stearate in ODE.
    • Maintain the temperature at 240 °C for 1 hour to grow the ZnS outer shell.
  • Purification and Storage: Cool the reaction and precipitate the InP/Zn(Mg)Se/ZnS QDs with ethanol. Collect by centrifugation and re-disperse in hexane for storage.

Protocol C: Optimized Sulfur-Based Surface Passivation for Near-Surface QDs

This protocol describes a two-step passivation process to reduce surface state density in near-surface QDs, reviving resonance fluorescence [1].

• Reagents:

  • Ammonium Sulfide Solution: (NH4)2S (20% in water), filtered with a 0.02 µm syringe filter.
  • Atomic Layer Deposition (ALD) Precursor: Trimethylaluminum (TMA) and H2O.
  • Inert Atmosphere: Nitrogen or Argon glovebox (H2O and O2 < 1 ppm).

• Procedure:

  • Sample Preparation: Etch the sample containing near-surface QDs (dot-to-surface distance < 40 nm) to the desired thickness. Clean the surface with standard solvents.
  • Sulfur Treatment:
    • Transfer the sample into an inert atmosphere glovebox.
    • Immerse the sample in the filtered 20% (NH4)2S aqueous solution for 10 minutes to eliminate surface dangling bonds.
    • Rinse gently with deoxygenated water and dry under an inert gas stream.
  • ALD Capping:
    • Without breaking the inert atmosphere, transfer the sample directly to the load-lock chamber of an ALD system.
    • Deposit a 10 nm thick layer of Al2O3 at 150 °C using TMA and H2O as precursors. This layer protects the sulfur-passivated surface from reoxidation.
  • Post-Processing: The sample is now ready for optical characterization or device fabrication.

The Scientist's Toolkit

Table 2: Essential Research Reagent Solutions

Reagent / Material	Function in Overcoming Lattice Mismatch
Tris(dimethylamino)phosphine ((DMA)3P)	Less hazardous phosphorus precursor for synthesizing the core of cadmium-free InP QDs [36].
Magnesium Chloride (MgCl2)	Source of Mg2+ ions for doping the inner shell (ZnSe) to expand its lattice constant, improving match with the InP core [36].
Ammonium Sulfide ((NH4)2S)	Sulfur-based passivation agent that eliminates surface dangling bonds on etched semiconductors, reducing surface state density [1].
Atomic Layer Deposition (ALD) Al2O3	Provides a uniform, dense, and inert capping layer that protects a freshly passivated surface from environmental degradation [1].
Oleylamine (OLA) / Oleic Acid (OA)	Common surface ligands that control nanocrystal growth, prevent aggregation, and can be manipulated for shell growth [35] [36].
6-(4-n-Butylphenyl)-6-oxohexanoic acid	6-(4-n-Butylphenyl)-6-oxohexanoic acid|CAS 951892-09-4
1-Methoxypropan-2-yl methanesulfonate	1-Methoxypropan-2-yl methanesulfonate

Workflow and Signaling Pathways

The following diagram synthesizes the key experimental pathways and their logical relationships for overcoming lattice mismatch, as detailed in the protocols above.

Optimizing Precursor Concentration and Reaction Conditions for Uniform Coating

Within the broader research on in-situ surface passivation techniques for quantum dots (QDs), achieving a uniform, defect-free coating is a critical determinant of final performance in applications ranging from light-emitting diodes to biomedical imaging [11] [1] [37]. The optimization of precursor concentration and reaction conditions directly governs the structural quality of the resulting nanocrystals, influencing their crystallinity, morphology, and ultimately, their optical and electronic properties [38]. This Application Note provides detailed, actionable protocols for systematically optimizing these parameters to achieve uniform coatings, a necessity for advancing high-performance QD-based technologies.

The Scientist's Toolkit: Essential Research Reagent Solutions

The following table catalogues key reagents and their functions in the synthesis and passivation of quantum dots.

Table 1: Key Research Reagent Solutions for QD Synthesis and Passivation

Reagent	Function/Application	Key Notes
Triphenylphosphine (TPP)	Multifunctional surface ligand, photoinitiator, and oxidation protector [11].	Enhances PLQY and enables direct photopatterning in air; acts as an L-type ligand [11].
9-Mercapto-BBN (BBN-SH)	Organoboron-based sulfur precursor for shell growth [38].	Its reactivity is tunable with Lewis bases; enables controlled, systematic shell deposition [38].
Lewis Bases (e.g., Picoline, DMAP)	Chemical additives to modulate precursor reactivity [38].	Fine-tunes the activation temperature (Tact) of BBN-SH independently of reaction temperature [38].
Ammonium Sulfide ((NH4)2S)	Sulfur-based surface passivation agent [1].	Eliminates surface dangling bonds on near-surface QDs; used in a two-step process with Al2O3 encapsulation [1].
Methanol	Electrolyte additive for QD-sensitized solar cells [39].	Improves wettability/permeability of polysulfide electrolyte and enhances stability of PbS QDs [39].
Core-Shell Perovskite QDs (e.g., MAPbBr3@tetra-OAPbBr3)	In-situ passivation agents for perovskite solar cells [4].	Epitaxially integrated at grain boundaries to suppress non-radiative recombination [4].
5-(Propylthio)-1,3,4-thiadiazol-2-amine	5-(Propylthio)-1,3,4-thiadiazol-2-amine, CAS:30062-49-8, MF:C5H9N3S2, MW:175.3 g/mol	Chemical Reagent
3-(Pentafluorosulfanyl)benzoic acid	3-(Pentafluorosulfanyl)benzoic acid, CAS:833-96-5, MF:C7H5F5O2S, MW:248.17 g/mol	Chemical Reagent

Quantitative Data on Precursor Concentration and Reaction Conditions

Optimization requires careful consideration of quantitative data from established methodologies. The tables below summarize key findings from relevant systems.

Table 2: Optimizing Precursor Concentration in the SILAR Method for PbS QDs [39]

Precursor Concentration	UV-Vis Absorbance	Inferred QD Quantity	Inferred QD Size	Recommended Application
< 0.06 M	Increases with concentration	Low to Moderate	Small	Limited use due to low light absorption.
0.06 M	Peak absorbance	Maximum	Moderate (Optimal)	Optimal for highest QD loading and device performance (PCE 4.01%) [39].
> 0.06 M	Decreases from peak	Slightly reduced	Large	Not recommended; large crystals may block pores [39].

Table 3: Modulating Precursor Reactivity with Lewis Bases for CdS Shell Growth [38]

Lewis Base (in order of BF3 affinity)	BF3 Affinity (ΔH kJ/mol)	Activation Temperature (Tact) for CdS Shell Growth	Resulting Shell Growth Rate
DMAP	~152	Lowest (~100°C)	Highest
Picoline	~136	~120°C	High
3-Chloropyridine	~125	~140°C	Moderate
4-CF3Pyridine	~115	~160°C	Low
4-Cyanopyridine	~113	Highest (~180-200°C)	Lowest

Experimental Protocols

This protocol details the use of Successive Ionic Layer Adsorption and Reaction (SILAR) to sensitize a mesoporous TiOâ‚‚ film with PbS QDs, with a focus on optimizing precursor concentration.

I. Materials and Reagent Preparation

• Substrate: Mesoporous TiOâ‚‚ film on FTO glass.
• Cationic Precursor Solution: Lead(II) nitrate (Pb(NO₃)â‚‚) or lead(II) acetate in a solvent such as ethanol. Prepare stock solutions of varying concentrations (e.g., 0.02 M, 0.04 M, 0.06 M, 0.08 M).
• Anionic Precursor Solution: Sodium sulfide (Naâ‚‚S) in a 1:1 methanol:water solvent.
• Rinsing Solvents: Ethanol and/or deionized water.

II. Step-by-Step SILAR Procedure

• Adsorption Cycle: a. Immerse the TiOâ‚‚ substrate in the cationic precursor solution (e.g., Pb²⁺ solution) for 1 minute to allow for the adsorption of Pb²⁺ ions onto the TiOâ‚‚ surface. b. Remove the substrate and rinse thoroughly with the appropriate solvent (e.g., ethanol) to remove loosely bound ions. c. Dry gently with a stream of inert gas (e.g., Nâ‚‚).
• Reaction Cycle: a. Immerse the substrate into the anionic precursor solution (e.g., S²⁻ solution) for 1 minute to react with the adsorbed Pb²⁺ ions and form PbS QDs. b. Remove and rinse again thoroughly to remove by-products. c. Dry gently with an inert gas. These two steps (1 & 2) constitute one SILAR cycle.
• Cycle Repetition: Repeat the SILAR cycle multiple times (e.g., 3-5 cycles) to achieve the desired QD loading and film thickness.
• Concentration Optimization: Execute the above procedure in parallel using the different concentrations of the Pb²⁺ precursor solution.

III. Analysis and Validation

• UV-Visible Spectroscopy: Measure the absorbance of each film. The optimal concentration (0.06 M) should yield the highest absorbance [39].
• Device Fabrication: Fabricate QD-sensitized solar cells and measure Photocurrent Density-Voltage (J-V) curves to confirm the highest power conversion efficiency is achieved with the 0.06 M precursor.

This protocol describes a method for growing high-quality CdS shells on CdSe cores using the tunable reactivity of the BBN-SH precursor.

I. Materials and Reagent Preparation

• QD Cores: Purified CdSe QDs in a non-coordinating solvent (e.g., octadecene).
• Sulfur Precursor: 9-Mercapto-BBN (BBN-SH).
• Metal Precursor: e.g., Cadmium oleate or cadmium carboxylate.
• Lewis Bases: A series of pyridine derivatives (e.g., Picoline, DMAP, 4-Cyanopyridine).
• Solvent: 1-Octadecene.

II. Step-by-Step Shell Growth Procedure

• Reaction Mixture Setup: a. In a three-neck flask, combine the CdSe QD core solution, the metal precursor, and BBN-SH. b. Add a selected Lewis base (e.g., Picoline). The Lewis base is consumed during the reaction, so its quantity must be stoichiometrically adequate for the desired shell thickness [38]. c. Evacuate and refill the flask with inert gas (Nâ‚‚ or Ar) three times.
• Temperature-Dependent Growth Monitoring: a. Slowly heat the reaction mixture under stirring (e.g., 5-10°C/minute). b. Monitor the photoluminescence (PL) peak position in real-time using a fluorometer. c. Record the Activation Temperature (Tact), defined as the temperature at which a persistent redshift in the PL peak is first observed, indicating the initiation of shell growth [38].
• Isothermal Shell Growth: a. Once the Tact for a given Lewis base is known, a new reaction can be set up and heated directly to a temperature 10-20°C above its specific Tact for controlled, isothermal shell growth. b. Maintain this temperature until the PL peak reaches the target wavelength, indicating the desired shell thickness has been achieved.
• Reactivity Optimization: Repeat the process using different Lewis bases to map the relationship between BF₃ affinity and Tact, allowing for the selection of the perfect precursor reactivity for a given core size and material [38].

III. Analysis and Validation

• Photoluminescence Quantum Yield (PLQY): Measure the PLQY before and after shell growth. Effective, uniform coating should significantly increase the PLQY.
• Transmission Electron Microscopy (TEM): Characterize the core/shell structure to confirm uniform shell thickness and spherical morphology.

Workflow and Mechanism Diagrams

The following diagrams illustrate the logical workflow for optimization and the chemical mechanism of precursor modulation.

Experimental Workflow for Optimization

Mechanism of Precursor Reactivity Modulation

Preventing Surface Aggregation and Ensuring Colloidal Stability Post-Passivation

The application of in-situ surface passivation techniques is a critical step in tailoring the optoelectronic properties of quantum dots (QDs) for advanced devices. However, a significant challenge that emerges post-passivation is the propensity for QDs to aggregate and lose colloidal stability, ultimately degrading device performance. This application note details robust experimental strategies to overcome these challenges, enabling the synthesis of high-quality passivated QDs for applications in photovoltaics, photodetectors, and quantum light sources. The protocols are framed within a research thesis exploring in-situ passivation, providing a practical framework for achieving stable, monodisperse QD inks and films.

Core Stability Challenges and Mechanistic Insights

Surface passivation often involves replacing native long-chain insulating ligands (e.g., oleic acid, oleylamine) with shorter functional ligands or inorganic layers. This process, while beneficial for electronic coupling and defect reduction, exposes the QD cores and reduces the steric hindrance that prevents aggregation [40]. The primary challenges include:

• Ligand Removal and Surface Exposure: The displacement of bulky native ligands creates surfaces that are prone to fusion, especially on non-polar facets like the (100) facets of large-size PbSe QDs [40].
• Incompatible Solubility: New passivating ligands can alter the QDs' solubility profile, leading to rapid precipitation and aggregation in undesirable solvents [41].
• Incomplete Passivation: Poorly executed ligand exchange leaves behind unpassivated surface defects, which act as trapping sites that quench luminescence and promote irreversible clustering [42].

Stabilization Strategies and Protocols

The following sections outline validated strategies for maintaining colloidal integrity during and after surface passivation.

Cascade Surface Modification (CSM) for Homogeneous Bulk Homojunctions

The CSM strategy is a two-step solution-phase process designed to achieve complete surface passivation and control over doping type while maintaining excellent colloidal solubility for homogeneous film formation [41].

Experimental Protocol: CSM for PbS QDs [41]

• Initial Halogenation (n-type ink formation):

  • Start with PbS QDs capped with oleic acid and dispersed in octane.
  • Perform a surface halogenation treatment using lead halide anions (e.g., PbIâ‚‚).
  • Purify the QDs and transfer them into a polar solvent like dimethylformamide (DMF), where they form a stable n-type colloid.
  • Function: This step provides an initial, robust surface passivation, infiltrating sites that are later inaccessible to bulkier organic ligands.

• Surface Reprogramming (p-type ink formation):

  • To the halogenated n-type QDs in DMF, introduce short thiol-based ligands (e.g., 1-thioglycerol (TG), 2-mercaptoethanol (ME), cysteamine (CTA)).
  • The thiols bind strongly to the QD surface, displacing the halides and reprogramming the QDs to p-type character.
  • Critical Solubility Control: The functional group (-L) on the thiol ligand (SH-R-L) determines colloidal stability. For miscible n-type and p-type inks in butylamine (BTA), ligands with terminal -NHâ‚‚ groups (e.g., CTA) provide optimal hydrogen bonding with BTA, preventing aggregation.

• Film Fabrication:

  • Blend the n-type and p-type inks to create a homogeneous bulk homojunction solution.
  • Deposit the film via a single-step spin-coating process. The resulting film exhibits a homogenous morphology, which is crucial for efficient charge transport.

The workflow for this protocol is illustrated in the diagram below.

Perovskite Intermediate Bridging for Large-Size PbSe QDs

This protocol addresses the specific instability of large-size (>5 nm) PbSe QDs, whose (100) facets are vulnerable to aggregation during ligand exchange in polar solvents [40].

Experimental Protocol: MAPbI₃₋ₓAcₓ Bridging for PbSe QDs [40]

• Ink Preparation:

  • Prepare a PbIâ‚‚ ligand solution in DMF.
  • Add methylammonium acetate (MAAc) directly into the PbIâ‚‚ ink to form a perovskite intermediate (MAPbI₃₋ₓAcâ‚“) solution.

• Ligand Exchange and Bridging:

  • Synthesize large-size PbSe QDs capped with oleic acid/OAm via the hot-injection method.
  • Perform a liquid-phase ligand exchange by mixing the PbSe QDs with the pre-formed MAPbI₃₋ₓAcâ‚“ solution.
  • The perovskite intermediate selectively bridges the vulnerable (100) facets of adjacent PbSe QDs, while the polar (111) facets are passivated by halide ligands. This bridging action temporarily prevents direct QD-QD fusion.

• Film Formation and Annealing:

  • Deposit the perovskite intermediate-bridged PbSe QDs into a film using spin-coating.
  • Anneal the film. Upon heating, the MAPbI₃₋ₓAcâ‚“ decomposes and evaporates, leaving behind only a short PbIâ‚‚ ligand shell on the QD surface.
  • The final film exhibits reduced defects and suppressed aggregation, enabling high-performance photodetectors.

The following diagram visualizes this stabilization mechanism.

Optimized Sulfur-Based Passivation for Near-Surface QDs

For QDs located near a semiconductor surface (e.g., in quantum light sources), a robust solid-state passivation process is essential to eliminate surface states and prevent degradation.

Experimental Protocol: (NH₄)₂S + ALD Al₂O₃ Passivation [1]

• Sample Preparation and System Setup:

  • Etch the sample to position the QDs within 40 nm of the surface.
  • Use a customized passivation system consisting of a glove box (Hâ‚‚O and Oâ‚‚ < 1 ppm) connected to an atomic layer deposition (ALD) system.

• Two-Step Passivation Process:

  • Step 1 - Chemical Passivation: Inside the glove box, filter a 20% (NHâ‚„)â‚‚S aqueous solution with a 0.02-μm syringe filter to remove polysulfide particles. Immerse the sample in the filtered solution for 10 minutes. This step eliminates surface dangling bonds.
  • Step 2 - Encapsulation: Under the inert atmosphere, immediately transfer the sample to the ALD load-lock chamber. Deposit a 10 nm thick Alâ‚‚O₃ layer at 150°C. This capping layer protects the passivated surface from re-oxidation and environmental degradation.

This optimized technique significantly reduces the optical linewidth and noise level of individual QDs, reviving resonance fluorescence signals [1].

Quantitative Data and Performance Metrics

The effectiveness of the described protocols is quantified by key performance indicators, as summarized in the table below.

Table 1: Performance Metrics of Stabilization Strategies

Stabilization Strategy	QD Material	Key Performance Indicator	Control / Pre-Passivation	Post-Stabilization	Reference
Cascade Surface Modification	PbS	Power Conversion Efficiency (PCE)	~12% (previous best)	13.3% (record)	[41]
		Photoluminescence Quantum Yield (PLQY)	6% (prior method)	18% (with TG ligands)	[41]
Perovskite Intermediate Bridging	Large-size PbSe	Dark Current Density (at 0V)	Not Specified	2.5 × 10⁻¹¹ A/cm²	[40]
		Specific Detectivity (@1550 nm)	Not Specified	1.16 × 10¹¹ Jones	[40]
Optimized S-Passivation + ALD	InAs/GaAs	Average RF Linewidth	43.23 ± 22.53 GHz	19.68 ± 6.48 GHz	[1]
		Average PL Linewidth	21.32 ± 5.48 GHz	16.49 ± 2.03 GHz	[1]
Z-type Ligand Passivation	CdTe	Photoluminescence Quantum Yield (PLQY)	8% (core only)	90% (with InCl₃ ligands)	[42]

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagent Solutions for Post-Passivation Stability

Reagent / Material	Function / Application	Key Consideration / Rationale
Lead Halide Salts (e.g., PbIâ‚‚)	Initial halogenation for n-type QD inks; provides foundational passivation [40] [41].	Infiltrates surface sites inaccessible to larger organic ligands, forming a stable base for subsequent exchanges.
Short-Chain Thiols (e.g., TG, ME, CTA)	Surface reprogramming for p-type QD inks; controls doping and solubility [41].	The -SH group binds strongly to the QD surface; the terminal -L group (e.g., -OH, -NHâ‚‚) dictates colloidal stability in blend inks.
Methylammonium Acetate (MAAc)	Forms a transient perovskite intermediate to bridge QD facets during exchange [40].	Prevents fusion of large-size PbSe QDs on vulnerable (100) facets; decomposes upon annealing to leave a clean surface.
Ammonium Sulfide ((NH₄)₂S)	Chemical passivation of near-surface QDs; eliminates surface dangling bonds [1].	Critical: Must be filtered (0.02-μm) to remove polysulfides and used in an inert atmosphere to prevent re-oxidation.
Atomic Layer Deposition (ALD) Al₂O₃	Forms a permanent, conformal encapsulation layer post-chemical passivation [1].	Protects the chemically passivated surface from long-term environmental degradation, ensuring sustained performance.
Functionalized Cinnamate Ligands	Solution-phase ligand exchange for precise band-edge tuning [43].	Conserves carboxylate coordination geometry, maintains solubility, and allows systematic tuning of QD electronic properties.

Preventing surface aggregation and ensuring colloidal stability are not ancillary concerns but are integral to the success of in-situ surface passivation in quantum dot research. The protocols detailed herein—Cascade Surface Modification, Perovskite Intermediate Bridging, and Optimized Sulfur-Based Passivation—provide actionable and validated pathways to overcome these challenges. By carefully selecting ligands and engineering the passivation process to address specific material vulnerabilities, researchers can reliably produce stable, monodisperse QD inks and high-quality solid films, thereby unlocking the full potential of quantum dots in next-generation optoelectronic devices.

Controlling the Passivation Layer Thickness for Optimal Performance

In-situ surface passivation is a pivotal technique in quantum dot (QD) research for mitigating surface defects that degrade optical and electronic performance. The thickness of the passivation layer is a critical determinant of its efficacy, influencing charge carrier dynamics, quantum confinement, and environmental stability. This Application Note synthesizes recent advances in passivation layer control, providing a structured quantitative analysis and detailed experimental protocols for achieving optimal performance in QD-based devices. The principles discussed are foundational for applications ranging from photovoltaics and photodetectors to quantum light sources and high-efficiency displays.

The relationship between passivation layer thickness, composition, and device performance is quantified across recent studies. The following table consolidates key performance metrics achieved through optimized passivation strategies.

Table 1: Performance Metrics of Quantum Dot Passivation Strategies

Passivation Method	Material System	Optimal Thickness / Condition	Key Performance Improvement	Reference
Sulfur-based + ALD Capping	InAs/GaAs QDs	~10 nm Al₂O₃ capping layer	RF linewidth reduced from 43.23 GHz to 19.68 GHz; Noise level reduction up to 42.27%	[1]
In-situ Epitaxial Core-Shell PQDs	MAPbBr₃@TOAPbBr₃ PQDs in PSCs	15 mg/mL PQD concentration	PCE increased from 19.2% to 22.85%; Voc enhanced from 1.120 V to 1.137 V; >92% PCE retention after 900 h	[44] [5]
Multifunctional Molecular Ligand	CdSe/ZnS RGB QDs with TPP	5% TPP mass fraction in ink	PLQY increased to 96.1% (R), 94.9% (G), 90.0% (B); EQE of 21.6% (Blue QLEDs)	[11]
Hybrid Conductive Polymer	PP@GQDs on Lu₀.₃₉In₀.₆₁O/GaN	40% GQDs to PEDOT:PSS volume ratio	Detectivity (D*) of 8.8 × 10¹¹ Jones; SBUV-Vis rejection ratio increased fourfold	[45]
Structural DBR Integration	Micro-LED QD Films	Optimized film thickness + DBR	Directional LCE within ±20° increased from 5.38% to 19.45%	[46]

Experimental Protocols for Passivation Layer Control

Protocol: Optimized Sulfur-Based Passivation with ALD Capping for Near-Surface QDs

This protocol details a two-step process for achieving a stable, ~10 nm thick passivation layer on near-surface semiconductor QDs, designed to minimize surface state density and suppress noise in resonant fluorescence applications [1].

• Primary Materials:
  • (NHâ‚„)â‚‚S Aqueous Solution (20%): Sulfur source for eliminating surface dangling bonds.
  • 0.02-µm Syringe Filters: For removing polysulfide particles from the solution.
  • Alâ‚‚O₃ ALD Precursors (e.g., TMA and Hâ‚‚O): For depositing a conformal, protective capping layer.

• Equipment:

  • Inert Atmosphere Glove Box (Hâ‚‚O and Oâ‚‚ < 1 ppm)
  • Atomic Layer Deposition (ALD) System
  • Load-lock Chamber connecting the glove box and ALD system

• Step-by-Step Procedure:

  • Sample Preparation: Etch the QD sample to achieve a dot-to-surface distance of less than 40 nm to enhance surface state effects for study [1].
  • Solution Filtration: Inside the inert atmosphere glove box, filter the 20% (NHâ‚„)â‚‚S aqueous solution through a 0.02-µm syringe filter directly into a clean container.
  • Sulfur Immersion: Immerse the sample in the filtered (NHâ‚„)â‚‚S solution for 10 minutes at room temperature.
  • Inert Transfer: Without exposing the sample to ambient air, transfer it directly from the glove box to the load-lock chamber of the ALD system.
  • ALD Capping: Deposit a 10 nm thick Alâ‚‚O₃ layer using precursors like trimethylaluminum (TMA) and Hâ‚‚O at a substrate temperature of 150 °C. This layer physically encapsulates the sulfur layer and prevents its re-degradation.
  • Post-processing: Proceed with standard device fabrication steps or optical characterization.

• Critical Thickness Control Parameters:

  • The 10 nm Alâ‚‚O₃ thickness is a critical outcome of the ALD process. It is controlled by the number of ALD cycles, with the growth per cycle (GPC) calibrated beforehand. This thickness provides optimal encapsulation without inducing significant strain.

Protocol: In-situ Integration of Core-Shell Perovskite QDs for Photovoltaics

This protocol describes the integration of core-shell MAPbBr₃@TOAPbBr₃ PQDs during the antisolvent step of perovskite solar cell (PSC) fabrication, which enables epitaxial passivation at grain boundaries with an optimal concentration of 15 mg/mL [5].

• Primary Materials:
  • Core-Shell PQD Ink (MAPbBr₃@TOAPbBr₃ in chlorobenzene): Pre-synthesized as described in the reagent toolkit.
  • Perovskite Precursor Solution: (e.g., 1.6 M PbIâ‚‚, 1.51 M FAI, 0.04 M PbBrâ‚‚, 0.33 M MACl, 0.04 M MABr in DMF:DMSO (8:1 v/v)).
  • Antisolvent (Chlorobenzene, CB).

• Equipment:

  • Spin Coater
  • Hotplate for Annealing
  • Centrifuge

• Step-by-Step Procedure:

  • Substrate Preparation: Prepare the FTO/TiOâ‚‚ (compact layer)/TiOâ‚‚ (mesoporous) substrate stack and pre-heat it on a hotplate.
  • Perovskite Deposition: Spin-coat the perovskite precursor solution using a two-step program (e.g., 2000 rpm for 10 s, then 6000 rpm for 30 s).
  • In-situ Antisolvent Passivation: During the final 18 seconds of the second spin-coating step, dynamically dispense 200 µL of the core-shell PQD ink (in chlorobenzene) at the optimized concentration of 15 mg/mL onto the spinning substrate. This acts as the antisolvent.
  • Film Crystallization: Immediately transfer the film to a hotplate and anneal at 100 °C for 10 min, followed by 150 °C for 10 min in a dry air atmosphere. This step facilitates perovskite crystallization and the epitaxial integration of the PQDs at grain boundaries.
  • Device Completion: Continue with the deposition of subsequent charge transport layers (e.g., Spiro-OMeTAD) and metal electrodes.

• Critical Thickness/Concentration Control Parameters:

  • The PQD concentration of 15 mg/mL in the antisolvent is crucial for achieving a sub-monolayer to monolayer coverage of PQDs at the grain boundaries without causing aggregation, which could impede charge transport. This optimal concentration balances defect passivation with charge extraction efficiency [5].

Visualization of Passivation Strategies and Workflows

Core-Shell Quantum Dot Passivation Process

The following diagram illustrates the structure of a core-shell quantum dot and its mechanism for defect passivation within a host material, a key strategy for enhancing performance and stability.

Optimized Sulfur-Based Passivation Workflow

This workflow outlines the critical steps for the optimized sulfur-based passivation process, highlighting the importance of an inert environment for achieving a high-quality, stable passivation layer.

The Scientist's Toolkit: Research Reagent Solutions

The following table catalogues essential materials and their functions for implementing the advanced in-situ passivation techniques described in this note.

Table 2: Essential Reagents for Advanced QD Passivation

Reagent / Material	Function / Role in Passivation	Key Consideration / Property
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Eliminates surface dangling bonds on III-V semiconductors, reducing surface state density.	Requires filtration to remove polysulfides and inert atmosphere processing to prevent re-oxidation. [1]
Atomic Layer Deposition (ALD) Al₂O₃	Provides a conformal, dense, and pinhole-free capping layer that stabilizes the underlying sulfur passivation.	Thickness (e.g., ~10 nm) is precisely controlled by the number of deposition cycles. [1]
Core-Shell PQDs (MAPbBr₃@TOAPbBr₃)	Acts as an epitaxially compatible passivant at perovskite grain boundaries, healing halide vacancies and Pb²⁺ defects.	Optimal concentration in antisolvent (15 mg/mL) is critical for efficacy without aggregation. [5]
Triphenylphosphine (TPP)	Multifunctional ligand providing surface passivation, oxidation protection, and photo-patterning capability.	A mass fraction of 5% in QD ink optimizes PLQY without compromising optical properties. [11]
GQDs-Decorated PEDOT:PSS (PP@GQDs)	Serves as a conductive defect-passivating window layer in photodetectors, improving charge collection.	A 40% GQDs to PEDOT:PSS volume ratio optimizes electrical properties and passivation effect. [45]

Strategies to Suppress Detrimental Hydroxyl Ligands and Prevent Reoxidation

The presence of detrimental hydroxyl (OH) ligands on quantum dot (QD) surfaces represents a significant challenge in the development of high-performance optoelectronic devices. These surface hydroxyl groups, which often originate from synthesis procedures or ambient exposure, introduce sub-bandgap trap states that promote non-radiative recombination, reduce charge carrier mobility, and ultimately diminish device performance and operational stability [47]. Within the broader context of in-situ surface passivation techniques for quantum dot research, developing effective strategies to suppress these hydroxyl ligands while simultaneously preventing surface reoxidation is paramount for advancing QD-based technologies.

This Application Note details two proven surface engineering approaches that directly address the hydroxyl ligand problem: a halide-based additive strategy using hydroiodic acid (HI) and a pseudohalide pretreatment utilizing nitrosonium tetrafluoroborate (NOBF4). The protocols and data presented herein provide researchers with methodologies to significantly improve the optoelectronic properties and environmental stability of quantum dots.

Key Strategic Approaches

The following table summarizes the core strategies for suppressing hydroxyl ligands and preventing reoxidation, highlighting their distinct mechanisms and applications.

Table 1: Core Strategies for Hydroxyl Ligand Suppression and Reoxidation Prevention

Strategy	Core Mechanism	Target Material	Key Outcome	Compatibility Notes
Halide Additive (HI) [47]	Acid-promoted ligand exchange; replaces OH⁻ with I⁻	PbS QDs in QD-ink solar cells	Reduced trap states, enhanced iodine passivation, improved carrier mobility	Compatible with solution-phase ligand exchange and QD-ink processes
Pseudohalide Pretreatment (NOBF4) [48]	Oxide peeling and defect passivation; removes TeOâ‚‚ and oleate ligands	CdSeTe QD-sensitized solar cells	Simultaneous oxide removal and surface passivation	Effective for QDs prone to oxidation (e.g., those containing Te)

Quantitative Performance Data

Implementation of these surface passivation strategies yields significant, quantifiable improvements in both material properties and final device performance.

Table 2: Quantitative Performance Enhancement from Surface Passivation Strategies

Metric	Control Device Performance	Performance with Surface Passivation	Enhancement	Citation
Solar Cell PCE	8.39%	10.02% (Standard); 12.05% (Champion)	~20% increase; >12% champion	[48]
Solar Cell PCE (QD-ink)	9.56%	10.78%	~13% relative increase	[47]
Open-Circuit Voltage (Voc)	1.120 V	1.137 V	17 mV increase	[4]
Short-Circuit Current Density (Jsc)	24.5 mA/cm²	26.1 mA/cm²	1.6 mA/cm² increase	[4]
Fill Factor (FF)	70.1%	77.0%	~7% absolute increase	[4]
Stability (PCE Retention)	~80% after 900 h	>92% after 900 h	>12% improved retention	[4]

Experimental Protocols

Protocol: Hydroiodic Acid Additive Treatment for PbS QD-Ink

This protocol describes a solution-phase ligand exchange process that introduces hydroiodic acid (HI) to a PbIâ‚‚-PbS QD system to suppress hydroxyl ligands and improve surface passivation [47].

Research Reagent Solutions

• Lead Iodide (PbIâ‚‚) Solution: 30 mg/mL in DMF. Serves as the primary source of iodide passivating ligands.
• Hydroiodic Acid (HI) Additive: Mild concentration (e.g., 45% solution used in source). Critical for hydroxyl removal and enhanced iodination.
• Oleate-Capped PbS QDs (OA-PbS): Synthesized per standard methods, dispersed in octane (30 mg/mL). The starting material for ligand exchange.
• Mixed Solvent for QD-Ink: Butylamine (BTA) and N,N-Dimethylformamide (DMF). Redisperses the exchanged QDs for film deposition.

Step-by-Step Procedure

• Solution Preparation: Combine the PbIâ‚‚-DMF solution with a controlled, suitable amount of HI additive to create the modified ligand exchange solution.
• Ligand Exchange: Mix the OA-PbS QDs in octane with the as-prepared PbIâ‚‚/HI-DMF solution. Vigorously stir the biphasic mixture to facilitate the ligand exchange across the solvent interface.
• Purification: Isolate the ligand-exchanged PbS QDs from the mixture via centrifugation. Remove the supernatant containing displaced oleic acid, hydroxyl byproducts, and excess reactants.
• QD-Ink Formulation: Dry the resulting iodide-passivated PbS QD powder and re-disperse it in the mixed solvent (BTA:DMF) at the desired concentration for the absorber layer deposition.
• Film Fabrication: Deposit the QD-ink onto the substrate (e.g., ZnO-coated ITO) via spin-coating or other suitable methods to form the photoactive layer.

Protocol: Pseudohalide Pretreatment with NOBFâ‚„ for CdSeTe QDs

This protocol outlines a surface pretreatment strategy using nitrosonium tetrafluoroborate (NOBFâ‚„) to remove surface oxides and passivate defects on CdSeTe QDs prior to solar cell assembly [48].

Research Reagent Solutions

• Nitrosonium Tetrafluoroborate (NOBFâ‚„): Serves as the pseudohalide precursor for oxide etching and surface passivation.
• CdSeTe QDs: Synthesized near-infrared QDs, typically capped with native oleate ligands and containing surface oxides like TeOâ‚‚.
• Dichloromethane (CHâ‚‚Clâ‚‚) or Hexane: Used as solvents for the pretreatment reaction and purification.

Step-by-Step Procedure

• Pretreatment Solution: Prepare a solution of NOBFâ‚„ in a suitable anhydrous solvent (e.g., dichloromethane).
• Surface Reaction: Disperse the as-synthesized CdSeTe QDs in the NOBFâ‚„ solution. Allow the reaction to proceed for a predetermined time with stirring. The NOBFâ‚„ effectively cleaves metal atom dangling bonds, strips insulating long-chain oleate ligands, and peels off unwanted oxides (e.g., TeOâ‚‚).
• Purification: Precipitate the pretreated QDs by adding a non-polar antisolvent (e.g., hexane). Isolate the QDs via centrifugation and discard the supernatant containing reaction byproducts and removed ligands.
• Sensitization: Redisperse the purified, pretreated QDs in an appropriate solvent for subsequent deposition onto the photoanode (e.g., mesoporous TiOâ‚‚) to create the sensitized electrode for solar cell assembly.

Signaling Pathways and Workflow Diagrams

The following diagrams illustrate the chemical mechanism of hydroxyl suppression and the experimental workflow for QD surface passivation.

Figure 1: Mechanism of HI Additive Suppressing Hydroxyl Ligands. The diagram illustrates the chemical pathway by which hydroiodic acid (HI) removes detrimental hydroxyl (OH⁻) ligands from the PbS QD surface and introduces passivating iodide (I⁻) ions, leading to improved optoelectronic properties.

Figure 2: Experimental Workflow for QD Surface Passivation. The workflow outlines the two primary strategies detailed in this note—Halide Additive and Pseudohalide Pretreatment—from initial QD synthesis to final device fabrication.

The Scientist's Toolkit

Table 3: Essential Research Reagent Solutions for Surface Passivation

Reagent / Material	Function / Role in Passivation	Example Application / Note
Hydroiodic Acid (HI)	Removes hydroxyl (OH⁻) ligands via acid reaction; promotes iodide binding.	Used as an additive in PbI₂-based ligand exchange for PbS QDs [47].
Nitrosonium Tetrafluoroborate (NOBFâ‚„)	Pseudohalide precursor for oxide peeling and simultaneous surface defect passivation.	Effective for treating CdSeTe QDs with surface oxides like TeOâ‚‚ [48].
Lead Iodide (PbI₂)	Standard source of iodide (I⁻) ligands for effective surface passivation of PbS QDs.	Foundation of the halide exchange process; often used in DMF solution [47].
Metal Salts (e.g., Cd(NO₃)₂, In(OTf)₃)	Cations strip organic ligands; bind to Lewis basic sites on NC surface to enhance PLQY.	Enables creation of intensely luminescent all-inorganic nanocrystals (ILANs) [49].
Ammonium Sulfide ((NH₄)₂S)	Sulfur-based passivator for near-surface QDs; eliminates surface dangling bonds.	Used in a two-step process with ALD Al₂O³ capping for robust passivation [1].

Quantitative Performance Validation and Technique Comparison

Surface states are inherent limiting factors that degrade the performance of semiconductor quantum dots (QDs), particularly as device architectures become more compact and QDs are positioned closer to surfaces for enhanced light extraction and integration into photonic circuits [1]. These surface states introduce significant linewidth broadening and intensity quenching, which critically impair the performance of QDs as quantum light sources [1] [50]. Surface passivation techniques have therefore emerged as an indispensable strategy to mitigate these detrimental effects.

This Application Note provides a comprehensive framework for benchmarking the efficacy of surface passivation protocols, with specific focus on two key metrics: emission linewidth narrowing and fluorescence intensity recovery. We situate this discussion within the context of a broader thesis on in-situ surface passivation techniques, which integrate the passivation process directly during QD synthesis or device fabrication to achieve more uniform and stable defect termination [5]. The protocols and data analysis methods presented herein are designed for researchers and scientists developing optimal QD-based quantum light sources, single-photon emitters, and other quantum nanophotonic devices.

Quantitative Benchmarking of Passivation Efficacy

Key Performance Metrics Before and After Passivation

Robust benchmarking requires quantitative comparison of critical optical parameters before and after application of a passivation protocol. The following table summarizes typical performance gains achievable through optimized surface passivation, as demonstrated in recent literature.

Table 1: Quantitative Benchmarking of Passivation Efficacy on Quantum Dot Optical Properties

Performance Metric	Pre-Passivation Value	Post-Passivation Value	Relative Improvement	Measurement Conditions
Resonance Fluorescence (RF) Linewidth [1]	43.23 ± 22.53 GHz	19.68 ± 6.48 GHz	~54% narrowing (avg.)	Pulsed resonance excitation, individual InAs/GaAs QDs
Non-Resonant PL Linewidth [1]	21.32 ± 5.48 GHz	16.49 ± 2.03 GHz	~23% narrowing (avg.)	Non-resonant excitation, ensemble of InAs/GaAs QDs
RF Signal-to-Background Ratio (SBR) [1]	Low SBR	High SBR	Substantial increase	Pulsed resonance excitation, Fabry–Pérot cavity detection
Photon Number Fluctuation Variance [1]	0.2749	0.1587	~42% reduction (noise suppression)	Time-resolved photon statistics on single QD
Photoluminescence Intensity [51]	Weak emission	Strong, stable lasing emission	Significant intensity recovery & lasing achieved	532 nm pulsed laser excitation, QDs in cholesteric liquid crystal

Efficacy Classification of Observed Outcomes

Passivation outcomes can be categorized into distinct efficacy levels, which are valuable for qualifying the success of a given protocol.

Table 2: Passivation Efficacy Classification Based on Observed Outcomes

Efficacy Level	Description	Observed Experimental Manifestations
Performance Enhancement	Improvement of existing optical signals from functional QDs.	Reduction of emission linewidth, lowering of noise levels, increase in fluorescence intensity and signal-to-background ratio [1].
Signal Revival	Recovery of measurable optical signals from previously non-functional QDs.	Emergence of bright, pulsed resonance fluorescence from QDs that showed no RF prior to passivation [1].
Stabilization	Increased resistance to environmental degradation and improved long-term performance.	Retention of >92% of initial solar cell efficiency after 900 hours under ambient conditions for perovskite QD-passivated devices [5].

Experimental Protocols for Passivation and Benchmarking

Optimized Sulfur-Based Passivation Protocol for Near-Surface QDs

This protocol describes a two-step, optimized sulfur passivation technique combined with atomic layer deposition (ALD) encapsulation, specifically developed for near-surface III-V QDs (e.g., InAs/GaAs or GaAs QDs) [1].

Principle: The method eliminates surface dangling bonds using a filtered ammonium sulfide solution and subsequently protects the passivated surface with a conformal oxide layer to prevent re-oxidation, thereby stabilizing the interface and reducing surface state density [1] [52].

Materials & Reagents:

• (NHâ‚„)â‚‚S aqueous solution (20%)
• 0.02-μm syringe filters
• Inert atmosphere glove box (Hâ‚‚O and Oâ‚‚ < 1 ppm)
• Atomic Layer Deposition (ALD) system
• Alâ‚‚O₃ precursor (e.g., trimethylaluminum) and Hâ‚‚O
• Solvents: Acetone, Isopropanol

Procedure:

• Sample Preparation: Etch the sample to achieve the desired dot-to-surface distance (e.g., < 40 nm). Clean the sample surface sequentially in acetone and isopropanol using an ultrasonic bath.
• Solution Filtration: Inside an inert atmosphere glove box, filter the 20% (NHâ‚„)â‚‚S aqueous solution using a 0.02-μm syringe filter to remove polysulfide particles.
• Sulfur Passivation: Immerse the sample in the filtered (NHâ‚„)â‚‚S solution for 10 minutes at room temperature.
• Transfer: Without exposure to ambient air, transfer the sample directly to the load-lock chamber of the ALD system.
• Oxide Encapsulation: Deposit a 10 nm thick Alâ‚‚O₃ film at 150°C via ALD to encapsulate the passivated surface.

Workflow Diagram:

In-Situ Epitaxial Passivation with Core-Shell Perovskite QDs

This protocol describes an in-situ method for integrating core-shell perovskite quantum dots (PQDs) during the antisolvent step of perovskite film crystallization, enabling epitaxial passivation of grain boundaries and interfaces [5].

Principle: Pre-synthesized core-shell PQDs, which share compositional similarity with the host perovskite matrix, are incorporated during film formation. They epitaxially bind to the growing grains, passivating surface and grain boundary defects and suppressing non-radiative recombination [5].

Materials & Reagents:

• Core-Shell PQDs (e.g., MAPbBr₃@tetra-OAPbBr₃) dispersed in chlorobenzene
• Perovskite precursor solution (e.g., PbIâ‚‚, FAI, MABr, MACl in DMF/DMSO)
• Standard solvents (Chlorobenzene, Isopropanol)

Procedure:

• PQD Synthesis: Synthesize core-shell PQDs (e.g., MAPbBr₃@tetra-OAPbBr₃) via colloidal synthesis and purify by centrifugation. Redisperse in chlorobenzene at a defined concentration (e.g., 15 mg/mL) [5].
• Film Fabrication: Deposit the perovskite precursor solution onto the substrate using a two-step spin-coating process (e.g., 2000 rpm for 10 s, then 6000 rpm for 30 s).
• In-Situ Integration: During the final 15-18 seconds of the second spin-coating step, dynamically introduce the PQD-chlorobenzene solution as an antisolvent.
• Annealing: Anneal the film sequentially (e.g., 100°C for 10 min, then 150°C for 10 min) to complete crystallization with the integrated PQDs.

Workflow Diagram:

Protocol for Optical Benchmarking Measurements

A. Resonance Fluorescence (RF) Linewidth and Intensity

• Setup: Use a confocal micro-photoluminescence (μ-PL) setup with pulsed resonant laser excitation.
• Spectral Analysis: Direct the RF emission to a high-resolution spectrometer (or a Fabry–Pérot interferometer for ultra-fine resolution) to acquire the spectrum.
• Data Analysis: Fit the spectrum to a Lorentzian function (for homogeneous linewidth) or a Gaussian/Voigt profile to extract the full width at half maximum (FWHM). The integrated intensity under the peak provides the intensity metric [1].
• Noightness Level: Perform time-correlated photon counting and calculate the variance of the photon number fluctuations to quantify noise suppression [1].

B. Non-Resonant Photoluminescence (PL)

• Setup: Employ a standard PL setup with above-bandgap (non-resonant) continuous-wave or pulsed laser excitation.
• Measurement: Record the PL spectrum and determine the FWHM. The PL quantum yield (QY) can be measured using an integrating sphere for absolute intensity recovery metrics [51].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagent Solutions for Quantum Dot Passivation Research

Reagent/Material	Function/Application	Exemplary Use Case
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Sulfur precursor for passivating III-V semiconductor surfaces.	Eliminates dangling bonds on GaAs/InAs surfaces, reducing surface state density [1] [52].
Al₂O₃ (from ALD)	Dielectric encapsulation layer.	Protects the sulfur-passivated surface from re-oxidation, ensuring long-term stability [1] [50].
Thiol Surfactants (e.g., C₆H₁₄S)	Organic ligand for colloidal QD passivation.	Passivates surface traps on colloidal QDs, improving PL quantum yield and stability in soft matter composites [51].
Core-Shell Perovskite QDs	In-situ epitaxial passivation agents.	MAPbBr₃@tetra-OAPbBr₃ PQDs incorporated during film processing to passivate grain boundaries in perovskite solar cells [5].
Tetraoctylammonium Bromide (t-OABr)	Shell precursor for core-shell PQDs.	Forms a wider bandgap shell around PQD cores, enhancing stability and reducing surface recombination [5].

Rigorous benchmarking of passivation efficacy, centered on the quantitative metrics of linewidth narrowing and intensity recovery, is fundamental to advancing quantum dot technologies. The protocols and benchmarking frameworks detailed in this note provide a standardized approach for evaluating and qualifying passivation techniques. The demonstrated success of both ex-situ methods like sulfur passivation and, more importantly, in-situ strategies such as epitaxial integration of core-shell PQDs, underscores a critical pathway in materials engineering. These approaches directly address the critical challenge of surface and interface defects, paving the way for the development of optimal, high-performance QD-based quantum light sources and optoelectronic devices.

The integration of quantum dots (QDs) into optoelectronic devices, such as perovskite solar cells (PSCs) and quantum light sources, is often hindered by performance degradation induced by surface defects. Surface states act as non-radiative recombination centers, reducing photoluminescence (PL) efficiency and compromising device-level performance, including stability and efficiency [1]. In-situ surface passivation techniques have emerged as a critical strategy to mitigate these defects directly during device processing. This application note provides a detailed statistical framework and experimental protocols for quantitatively analyzing performance enhancements, from fundamental PL characteristics to functional device-level metrics, following the application of in-situ passivation techniques. The content is structured to enable researchers to reliably characterize and validate the efficacy of novel passivation methods.

The following tables consolidate key quantitative improvements observed after the application of advanced surface passivation techniques, providing a benchmark for researchers.

Table 1: Statistical Analysis of Quantum Dot Optical Improvements Post-Passivation (Data from [1])

Performance Metric	Pre-Passivation Value (Mean ± SD)	Post-Passivation Value (Mean ± SD)	Relative Improvement	Measurement Technique
Non-Resonant PL Linewidth	21.32 ± 5.48 GHz	16.49 ± 2.03 GHz	-22.7%	Photoluminescence Spectroscopy
Resonance Fluorescence (RF) Linewidth	43.23 ± 22.53 GHz	19.68 ± 6.48 GHz	-54.5%	Resonance Fluorescence Spectroscopy
Pulsed-RF Linewidth (QD2)	14.23 ± 2.34 GHz	7.84 ± 0.48 GHz	-44.9%	High-Resolution Fabry-Pérot Interferometer
Photon Noise Level (Variance, QD2)	0.2749	0.1587	-42.3%	Photon Statistics Analysis

Table 2: Device-Level Performance Metrics for Passivated Perovskite Solar Cells (Data from [2] [44])

Device Parameter	Control Device (No QD Passivation)	Device with QD Passivation	Improvement	Measurement Standard
Power Conversion Efficiency (PCE)	~21.5% [2]	22.85% [44]	~1.35% (absolute)	Current-Voltage (J-V) Characterization
Open-Circuit Voltage (VOC)	Reported ~1.137 V [44]	1.137 V [44]	Notable stability improvement	Current-Voltage (J-V) Characterization
Short-Circuit Current Density (JSC)	Reported ~26.1 mA/cm² [44]	26.1 mA/cm² [44]	Maintained with reduced loss	Current-Voltage (J-V) Characterization
Fill Factor (FF)	Reported ~77% [44]	77% [44]	Maintained with reduced loss	Current-Voltage (J-V) Characterization
Stability (Light/Thermal)	Baseline	Significantly Enhanced [2]	Superior operational lifetime	Accelerated Aging Tests

Experimental Protocols

This section outlines detailed methodologies for key experiments in the passivation and characterization workflow.

Optimized Sulfur-Based Passivation of Quantum Dot Surfaces

Objective: To create a stable, uniform passivation layer on near-surface QDs that eliminates dangling bonds and protects against environmental degradation [1].

Materials:

• (NH(4))(2)S aqueous solution (20%)
• 0.02-µm syringe filters
• Atomic Layer Deposition (ALD) system capable of depositing Al(2)O(3)
• Inert atmosphere glove box (H(2)O and O(2) < 1 ppm)

Procedure:

• Sample Transfer: Place the QD sample (e.g., on a DBR-CBG structure) inside the inert atmosphere glove box.
• Solution Filtration: Filter the (NH(4))(2)S aqueous solution using a 0.02-µm syringe filter to remove polysulfide particles.
• Sulfur Treatment: Immerse the sample in the filtered 20% (NH(4))(2)S solution for 10 minutes to passivate surface dangling bonds.
• Inert Transfer: Without exposing the sample to ambient air, transfer it directly to the load-lock chamber of the ALD system.
• Al(2)O(3) Capping: Deposit a 10 nm thick Al(2)O(3) capping layer at 150°C via ALD. This layer seals the passivated surface, preventing reoxidation and ensuring long-term stability [1].

In-Situ Epitaxial Quantum Dot Passivation in Perovskite Films

Objective: To integrate core-shell perovskite QDs (e.g., MAPbBr(3)@TOAPbBr(3)) into a perovskite precursor solution to simultaneously passivate bulk and surface defects during film formation [44].

Materials:

• Core-shell Perovskite QDs (PQDs)
• Perovskite precursor solution (e.g., lead halide-based in DMF/DMSO)
• Antisolvent (e.g., chlorobenzene or toluene)

Procedure:

• QD Dispersion: Disperse a precise concentration of core-shell PQDs into the antisolvent.
• Film Fabrication: Spin-coat the perovskite precursor solution onto the substrate.
• In-Situ Passivation: During the spin-coating process, at the appropriate time for crystal nucleation, drop-cast the PQD-dispersed antisolvent onto the spinning film.
• Crystallization: Allow the film to crystallize. The PQDs, delivered via the antisolvent, integrate epitaxially into the growing perovskite matrix, passivating defects at grain boundaries and surfaces [44].
• Annealing: Proceed with standard annealing steps to form the final perovskite film.

Statistical Optical Characterization of Single Quantum Dots

Objective: To quantitatively compare the optical properties of individual QDs before and after passivation.

Materials:

• Confocal micro-photoluminescence (µ-PL) setup
• Pulsed resonant laser source
• High-resolution spectrometer
• Fabry-Pérot interferometer

Procedure:

• Pre-Passivation Baseline:
  • Location Marking: Identify and record the spatial coordinates of specific, single QDs on the sample.
  • Non-Resonant PL: For a randomly selected set of QDs (e.g., n=25), acquire PL spectra and measure the full-width at half-maximum (FWHM) [1].
  • Resonant Fluorescence (RF): For a subset of QDs (e.g., n=9) with existing RF signals, use a pulsed resonant laser to excite and record the RF linewidth with a spectrometer. For higher resolution, use a Fabry-Pérot cavity [1].
  • RF Revival Check: Identify and record the locations of QDs that show no RF signal under pulsed resonant excitation.

• Post-Passivation Analysis:
  • Re-location: Return to the pre-marked coordinates of the same QDs.
  • Re-measurement: Repeat the non-resonant PL, RF linewidth, and RF signal checks on the exact same QDs.
  • Dot-to-Dot Comparison: Perform a paired statistical analysis (e.g., paired t-test) on the pre- and post-passivation metrics for each QD to confirm significance [1].

Electrochemical Characterization of Passivation Stability

Objective: To quantitatively assess the stability and quality of a passive film using electrochemical methods. Note: While initially for steel, this methodology is adaptable for assessing conductive layers in electronic devices. [53]

Materials:

• Electrochemical workstation (potentiostat)
• Standard three-electrode cell (Working, Counter, and Reference electrodes)
• Simulated concrete pore solution (or other relevant electrolyte)

Procedure:

• Open-Circuit Potential (OCP): Immerse the passivated sample (working electrode) in the electrolyte and monitor the OCP over time until it stabilizes. An increasing and stabilizing OCP indicates successful passivation [53].
• Electrochemical Impedance Spectroscopy (EIS): At set intervals, apply a small AC voltage perturbation (e.g., 10 mV) over a frequency range (e.g., 10(^5) to 10(^{-2}) Hz). A significant increase in the impedance arc radius indicates the formation and development of a protective passive film [53].
• Potentiodynamic Polarization: After stable passivation is achieved (e.g., after 5 days), perform a polarization scan (e.g., ±250 mV vs. OCP) to determine the corrosion current density and corrosion potential. A decrease in the anodic Tafel slope and an increase in corrosion potential confirm the formation of a stable oxide film [53].

Workflow and Signaling Visualizations

Passivation Efficacy Analysis Workflow

Surface State Passivation Signaling Pathway

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Reagents and Materials for In-Situ Passivation Research

Item Name	Function / Role in Experiment	Key Characteristics / Notes
Ammonium Sulfide ((NHâ‚„)â‚‚S)	Sulfur-based passivation agent that eliminates surface dangling bonds on semiconductor surfaces [1].	Aqueous solution (e.g., 20%); requires filtration to remove polysulfides. Use in an inert atmosphere.
Core-Shell Perovskite QDs (e.g., MAPbBr₃@TOAPbBr₃)	Dual-function agents for bulk and surface defect passivation in perovskite films; act as both dopant and passivant [44].	Engineered for epitaxial compatibility with the host perovskite material to minimize lattice strain.
Pluronic F127	Non-ionic surfactant used for surface passivation in biochemical studies of condensates; can be adapted for nanoparticle dispersion and stabilization [54].	Forms a dense brush-like layer that prevents non-specific binding; provides robust passivation across diverse conditions.
Al₂O₃ ALD Precursors	Source for depositing a conformal, protective capping layer to seal the passivated surface from environmental degradation [1].	Enables low-temperature (e.g., 150°C), pin-hole-free growth of thin films (e.g., 10 nm).
Atomic Layer Deposition (ALD) System	Instrument for depositing ultra-thin, highly uniform inorganic capping layers [1].	Essential for ensuring long-term stability of the passivated interface. Integrated glove box is ideal.
Inert Atmosphere Glove Box	Provides a controlled environment (Hâ‚‚O and Oâ‚‚ < 1 ppm) for handling air-sensitive materials and processes [1].	Prevents reoxidation of the freshly passivated surface before ALD capping.

Surface passivation is a critical technology in quantum dot (QD) research, directly influencing their optical properties, electronic performance, and stability. Effective passivation eliminates surface dangling bonds and suppresses defect formation, which are detrimental to quantum yield and emission linewidth. This application note provides a comparative analysis of three prominent passivation strategies—sulfur, oxide, and halide ligands—framed within the context of in-situ surface passivation techniques. We summarize quantitative performance data and provide detailed experimental protocols to guide researchers in selecting and implementing optimal passivation methods for specific applications, from photoelectrochemical cells to quantum light sources.

Comparative Performance Analysis of Passivation Ligands

The choice of passivation ligand significantly impacts key performance metrics of quantum dots. The table below summarizes the comparative effects of sulfur, oxide, and halide-based treatments on optical and electronic properties.

Table 1: Comparative Analysis of Passivation Ligand Effects on Quantum Dot Properties

Ligand Type	Effect on PL Quantum Yield (QY)	Effect on Emission Linewidth	Charge Transport Characteristics	Primary Applications
Sulfur Ligands	Marked improvement (up to ~95% in Ag-In-Ga-S QDs) [55]	Reduces RF linewidth by ~47% (InAs/GaAs QDs) [1]	Improved charge transfer, stability in PEC cells [56]	Quantum light sources [1], Photoelectrodes [56]
Halide Ligands	High QYs achieved; precursor-dependent [55]	Not explicitly quantified	Electron mobility increases with ligand size (F⁻ to I⁻) [57]	Solar cells, FETs (n-type behavior with I⁻) [57]
Oxide Ligands	Not explicitly quantified	Not explicitly quantified	Acts as protective capping layer [1]	Surface stabilization, quantum light sources [1]

Table 2: Electronic Properties of PbS Quantum Dots with Different Halide Ligands

Halide Ligand	Electron Mobility (cm²/(V s))	Hole Mobility (cm²/(V s))	Transport Behavior
Fluoride (F⁻)	3.9 × 10⁻⁴ [57]	~1 × 10⁻⁵ to 10⁻⁴ [57]	p-type [57]
Chloride (Cl⁻)	Data not explicitly given [57]	~1 × 10⁻⁵ to 10⁻⁴ [57]	p-type [57]
Iodide (I⁻)	2.1 × 10⁻² [57]	~1 × 10⁻⁵ to 10⁻⁴ [57]	n-type [57]

Experimental Protocols & Methodologies

Atomic Sulfur Passivation for ZnSe Nanorods (Ligand Exchange)

Application: This protocol is designed for passivating heavy metal-free ZnSe nanorods (NRs) to improve performance in photoelectrochemical cells (PECs) by enhancing charge transfer and stability [56].

Reagents:

• ZnSe NRs (synthesized with OAm and 1-DDT ligands)
• 3-mercaptopropionic acid (3-MPA)
• Sodium sulfide (Naâ‚‚S) solution
• Toluene, ethanol, hexane

Procedure:

• Initial Ligand Exchange:
  • Transfer 40 mL of ZnSe NRs in toluene to a reaction vessel.
  • Add 90 µL of 3-MPA to the solution to initiate partial ligand exchange from original oleylamine (OAm) and 1-dodecanethiol (1-DDT) to 3-MPA [56].
  • Purify the NRs via precipitation and centrifugation.

• Photoanode Fabrication:

  • Deposit the ligand-exchanged ZnSe NRs onto a TiOâ‚‚ substrate to form the photoanode [56].

• Atomic Sulfur Treatment:

  • Immerse the fabricated TiOâ‚‚/ZnSe NR photoanode in an aqueous sodium sulfide (Naâ‚‚S) solution [56].
  • The S²⁻ ions from the solution substitute a portion of the 3-MPA ligands and bond with under-coordinated Zn atoms on the NR surface, forming an atomic ZnS monolayer [56].

Mechanism Insight: This treatment specifically targets and passulates surface traps originating from under-coordinated Se atoms and defects introduced during the initial ligand exchange, without impeding hole transfer efficiency [56].

Optimized Sulfur-Based Passivation for Near-Surface QDs

Application: This optimized two-step process is designed for near-surface semiconductor QDs (e.g., InAs/GaAs) to reduce surface state density and recover high-quality resonance fluorescence (RF) for quantum light sources [1].

Reagents:

• Ammonium sulfide ((NHâ‚„)â‚‚S) solution (20%)
• Solvents for ALD (as required by system)
• Precursors for Alâ‚‚O³ deposition (e.g., trimethylaluminum and Hâ‚‚O)

Equipment:

• Inert atmosphere glovebox (Hâ‚‚O and Oâ‚‚ < 1 ppm)
• Atomic Layer Deposition system
• 0.02-µm syringe filters

Procedure:

• Solution Preparation and Filtration:
  • Inside an inert atmosphere glovebox, filter the (NHâ‚„)â‚‚S aqueous solution using a 0.02-µm syringe filter to remove polysulfide particles [1].

• Sulfur Treatment:

  • Immerse the sample (e.g., etched DBR-CBG structure with near-surface QDs) in the filtered 20% (NHâ‚„)â‚‚S solution for 10 minutes [1].
  • This step eliminates surface dangling bonds.

• Oxide Capping:

  • Under the inert atmosphere, transfer the sample directly to the load-lock chamber of the ALD system.
  • Deposit a 10-nm-thick Alâ‚‚O₃ film at 150°C [1].
  • This oxide layer acts as a protective barrier, preventing re-degradation of the freshly passivated surface.

Critical Note: The integrated system (glovebox connected to ALD) and the two-step process are crucial for achieving a stable, uniform, and high-quality passivation layer, enabling the observation of reduced RF linewidth and revived single-photon emission [1].

Halide Ligand Exchange for PbS CQDs

Application: This protocol details solid-state ligand exchange on PbS colloidal quantum dot films to tune electronic transport properties for optoelectronic devices like field-effect transistors and solar cells [57].

Reagents:

• PbS CQDs (2.7 - 3.5 nm diameter) capped with oleic acid
• Anhydrous tetraalkylammonium halide salts: Tetramethylammonium fluoride (TMAF), Tetrabutylammonium chloride (TBACl), Tetrabutylammonium bromide (TBABr), Tetrabutylammonium iodide (TBAI)
• Solvents for layer-by-layer processing (e.g., octane, acetonitrile)

Procedure:

• Film Preparation:
  • Deposit a thin film of PbS CQDs on the desired substrate via spin-coating [57].

• Ligand Exchange:

  • Treat the film with a solution of the selected halide salt (e.g., TMAF for F⁻, TBAI for I⁻).
  • This displaces the native long-chain oleate ligands with the smaller atomic halide ions [57].

• Washing and Layer Buildup:

  • Wash the film with a solvent to remove excess ligands and by-products.
  • Repeat the spin-coating and ligand exchange steps in a layer-by-layer fashion to build a thick, electronically coupled CQD solid [57].

Transport Property Tuning: The choice of halide allows for tuning electron mobility, which increases with ligand size (F⁻ < Cl⁻ < Br⁻ < I⁻), while hole mobility remains relatively consistent, thus controlling the n-type vs. p-type character of the film [57].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Quantum Dot Passivation Protocols

Reagent	Function / Role in Passivation	Example Application
Ammonium Sulfide ((NH₄)₂S)	Source of S²⁻ ions; eliminates surface dangling bonds [1]	Optimized passivation of InAs/GaAs QDs [1]
Sodium Sulfide (Na₂S)	Source of S²⁻ ions for ligand substitution & surface monolayer formation [56]	Atomic sulfur passivation of ZnSe NRs [56]
Tetramethylammonium Fluoride (TMAF)	Source of small F⁻ ligands for solid-state exchange [57]	Halide passivation of PbS CQDs (p-type layer) [57]
Tetrabutylammonium Iodide (TBAI)	Source of I⁻ ligands; induces n-type behavior [57]	Halide passivation of PbS CQDs (n-type layer) [57]
Al₂O₃ (ALD Precursors)	Forms protective oxide capping layer to stabilize passivated surface [1]	Protective capping in optimized S-based passivation [1]
3-Mercaptopropionic Acid (3-MPA)	Short-chain bifunctional ligand for initial ligand exchange [56]	Initial surface functionalization of ZnSe NRs [56]

Ligand Selection and Application Guidelines

The optimal passivation strategy is highly dependent on the target application, as illustrated in the following decision workflow.

Sulfur, oxide, and halide passivation techniques each offer distinct mechanisms and advantages for quantum dot engineering. The choice of ligand directly controls critical parameters such as PL quantum yield, emission linewidth, and charge carrier mobility. As research progresses, the development of hybrid passivation strategies that combine the strengths of different ligands—such as sulfur treatment for defect suppression followed by oxide capping for stability—presents a promising path toward the realization of optimal QD-based devices for a wide range of scientific and industrial applications.

The integration of quantum dots (QDs) into commercial devices, from solar cells to bio-imaging agents, is critically dependent on their long-term stability. A significant challenge in the field is the degradation of QD performance under operational stresses, particularly continuous illumination and exposure to ambient conditions such as oxygen and moisture. These factors can lead to the formation of surface defects—dangling bonds and trap states—that quench photoluminescence (PL), increase emission linewidth, and ultimately cause device failure [58] [16]. In-situ surface passivation has emerged as a pivotal strategy to mitigate these issues by directly treating these surface defects during or immediately after QD synthesis, leading to more robust and reliable nanomaterials [59] [16]. These application notes provide a detailed protocol for validating the long-term stability of in-situ passivated QDs, supplying researchers with the methodologies and benchmarks necessary to assess performance for industrial and biomedical applications.

Stability Testing Protocols

A comprehensive stability assessment requires controlled testing across multiple environmental stressors. The following protocols detail the procedures for photostability, ambient air stability, and chemical stability testing.

Photostability Testing Under Continuous Illumination

Principle: This test evaluates the resistance of QD photoluminescence (PL) to photodegradation (photobleaching) under prolonged light exposure, simulating operational conditions in LEDs or bio-imaging.

Materials:

• Quantum dot sample (passivated and non-passivated control)
• Continuous-wave laser or high-power LED light source (e.g., 365 nm UV, 5 W)
• Integrating sphere or fiber-optic spectrometer
• Neutral density filters
• Temperature-controlled sample holder
• Power meter

Methodology:

• Initial Characterization: Measure the initial absolute PL quantum yield (QY) and PL intensity spectrum of the QD sample.
• Setup: Place the QD sample in a temperature-controlled holder (e.g., 25°C). Use neutral density filters to adjust the illumination intensity to a standard level (e.g., 30 mW/cm²) at a set distance (e.g., 20 cm) from the light source [58].
• Illumination: Expose the sample to continuous illumination for the test duration (e.g., 264 hours).
• Monitoring: At predetermined time intervals (e.g., 0, 1, 2, 4, 8, 24, 48... 264 hours), temporarily shutter the light source and measure the PL intensity at the peak emission wavelength.
• Data Analysis: Plot the normalized PL intensity (I/Iâ‚€, where Iâ‚€ is the initial intensity) versus time. Calculate the time taken for the PL intensity to drop to 50% of its initial value (Tâ‚…â‚€) and the percentage of initial intensity retained at the end of the test.

Stability Testing Under Ambient Conditions

Principle: This test evaluates the degradation of QDs when stored in ambient air, assessing the effectiveness of passivation layers against oxygen and moisture.

Materials:

• Quantum dot sample (passivated and non-passivated control)
• Environmental chamber (capable of controlling temperature and relative humidity)
• Spectrometer

Methodology:

• Baseline Measurement: Record the initial PL QY and emission linewidth of the QDs.
• Storage: Store the QD samples in the environmental chamber set to standard ambient conditions (e.g., 25°C, 60% relative humidity) or accelerated aging conditions (e.g., 40°C, 75% RH).
• Periodic Sampling: At regular intervals (e.g., daily for the first week, weekly thereafter), remove samples for characterization and immediately measure PL QY and emission linewidth.
• Data Analysis: Monitor the change in PL QY and emission linewidth over time. Effective passivation will manifest as minimal change in both parameters over extended periods.

Quantitative Data from Stability Studies

The efficacy of passivation is quantitatively demonstrated through key performance metrics. The tables below summarize stability data for different types of quantum dots from recent studies.

Table 1: Photostability of Passivated Carbon Quantum Dots (CQDs) under UV Illumination [58]

Sample Type	Passivation/Encapsulation Method	Initial PL Intensity (%)	Final PL Intensity (%) (after 264 hrs)	Retention (%)	Observed Tâ‚…â‚€
CDs in Silica Matrix	Organosilane functionalization	100	~10	~10%	< 4 hours
S-CDs in NaCl	Salt crystal embedding	100	~70	~70%	~60 hours
S-CDs in KBr	Salt crystal embedding	100	~60	~60%	Not specified

Table 2: Optical Performance of Passivated InAs/GaAs Quantum Dots [16]

Measurement Type	Sample Condition	Average Linewidth (GHz)	Improvement	Key Finding
Non-resonant PL	Before Passivation	21.32 ± 5.48	-	Passivation reduces ensemble linewidth
Non-resonant PL	After (NH₄)₂S + Al₂O₃ Passivation	16.49 ± 2.03	22.7% reduction
Pulsed Resonance Fluorescence (RF)	Before Passivation	43.23 ± 22.53	-	Passivation revives RF signal in some QDs
Pulsed Resonance Fluorescence (RF)	After (NH₄)₂S + Al₂O₃ Passivation	19.68 ± 6.48	54.5% reduction
Pulsed RF (Single QD - QD2)	Before Passivation	14.23 ± 2.34	-	Noise level reduced by 42.27%
Pulsed RF (Single QD - QD2)	After (NH₄)₂S + Al₂O₃ Passivation	7.84 ± 0.48	44.9% reduction

Experimental Workflows & Stability Mechanisms

The following diagrams illustrate the core experimental workflow for surface passivation and the mechanism by which it enhances stability.

In-situ Passivation and Stability Validation Workflow

Mechanism of Surface Passivation for Enhanced Stability

The Scientist's Toolkit: Key Research Reagents & Materials

A successful passivation and stability study requires carefully selected reagents. The following table details essential materials and their functions.

Table 3: Essential Reagents and Materials for In-situ Passivation and Stability Testing

Item Name	Function/Application	Example from Literature
Ammonium Sulfide ((NH₄)₂S)	A sulfur-based passivator used to eliminate surface dangling bonds on semiconductor QDs (e.g., InAs/GaAs), reducing surface state density [16].	Used in a two-step process with Al₂O₃ capping for III-V QDs [16].
BODIPY-OH (Borondipyrromethene)	A functional organic dye ligand. Its conjugated system and -F groups facilitate charge carrier separation and act as a passivating ligand for perovskite QDs, improving photocatalytic performance [59].	Ligand for MAPbBr₃ QDs in photocatalytic antibacterial applications [59].
Oleic Acid (OA) & Oleylamine (OAm)	Common long-chain ligands used in the synthesis of various QDs. They coordinate with surface metal atoms, providing initial passivation and colloidal stability [59].	Standard ligands in the ligand-assisted reprecipitation (LARP) method for MAPbBr₃ QDs [59].
Al₂O₃ (Aluminum Oxide)	A thin, inert capping layer deposited via Atomic Layer Deposition (ALD). It physically protects the passivated QD surface from re-degradation by oxygen and moisture [16].	Capping layer in the optimized two-step (NH₄)₂S + Al₂O₃ passivation process [16].
TOAPbBr₃	A perovskite shell material used for epitaxial growth on a perovskite core. It creates a core-shell structure (e.g., MAPbBr₃@TOAPbBr₃) that passivates surface defects and enhances intrinsic stability [44].	Used for in-situ epitaxial passivation in perovskite solar cells, improving efficiency and stability [44].
Silica (SiOâ‚‚)	An inorganic matrix used for encapsulation. It provides a robust physical barrier against environmental factors like water, enhancing stability for aqueous applications [59].	Used to coat BODIPY-passivated perovskite QDs (SiOâ‚‚@BDP/QDs) for antibacterial applications in water [59].
Salt Crystals (NaCl, KBr)	An encapsulation matrix. Embedding QDs in salt crystals provides a simple and effective method to shield them from UV and thermal degradation [58].	CQDs embedded in NaCl crystals showed 77% PL retention after one week in a UV-LED [58].

Surface states are inherent limiting factors that degrade the performance of solid-state semiconductor devices, including both classical and quantum systems. For quantum devices, the issue is particularly acute because the quantum light source regions are often located close to the surface, making them more vulnerable to surface effects. This case study demonstrates the critical importance of optimized surface passivation techniques for improving the resonance fluorescence (RF) characteristics of near-surface semiconductor quantum dots (QDs) through direct dot-to-dot comparisons. The ability to maintain high-quality pulsed-RF signals is essential for quantum information technologies, as pulsed-resonance excitation represents the practical on-demand excitation technique widely applied in this field [1].

The degradation caused by surface states manifests primarily through linewidth broadening and increased noise levels in RF signals, ultimately limiting the performance and reliability of QD-based quantum light sources. This study validates optimized passivation methodologies that not only enhance existing RF signals but can revive vanishing RF signals, enabling more robust quantum photonic technologies. Through systematic dot-to-dot comparisons, we establish a rigorous framework for evaluating passivation efficacy that moves beyond ensemble averaging to provide single-dot resolution of passivation effects [1].

Experimental Design and Methodology

Sample Structure and Optical Collection Enhancement

Our investigation utilized a sample grown by a molecular beam epitaxy (MBE) system with a specialized structure designed to facilitate both surface proximity and efficient photon collection [1]:

• DBR-CBG Hybrid Structure: The sample incorporated 30 pairs of λ/4-thick AlAs/GaAs distributed Bragg reflector (DBR) with a λ-thick GaAs layer at the top. A layer of self-assembled InAs/GaAs QDs was embedded in the middle of this GaAs layer [1].
• Surface Proximity: The surface was etched to achieve dot-to-surface distances of less than 40 nm to emphasize surface state effects [1].
• Collection Efficiency Enhancement: A hybrid DBR-CBG structure combining circular Bragg gratings with DBR was implemented to overcome the reduced collection efficiency from broken symmetry in the source region. This design achieved an 8.81-fold enhancement in collection efficiency (increasing to 16.28% at 890 nm) while maintaining a Purcell factor of 2.29 [1].

Table 1: DBR-CBG Structure Parameters

Parameter	Specification
DBR Pairs	30
CBG Periods	6
Width Parameters	12 (optimized via BOBYQA algorithm)
Purcell Factor	2.29
Quality Factor (Q)	53
Collection Efficiency Enhancement	8.81-fold
Final Collection Efficiency	16.28%

Optimized Surface Passivation Protocol

We implemented a customized two-step passivation process specifically designed to eliminate surface dangling bonds while protecting the passivated surface from re-degradation [1]:

Equipment Setup

• Custom Passivation System: A glove box connected directly to an atomic layer deposition (ALD) system maintained an inert atmosphere (Hâ‚‚O and Oâ‚‚ < 1 ppm) to prevent reoxidation of the sulfur layer before ALD deposition [1].

Two-Step Passivation Procedure

• Chemical Passivation:

  • Filter (NHâ‚„)â‚‚S aqueous solution using 0.02-μm syringe filters within the glove box environment to remove polysulfide particles [1].
  • Immerse the sample in 20% (NHâ‚„)â‚‚S solution for exactly 10 minutes at room temperature [1].
  • Transfer the sample directly to the ALD load-lock chamber under continuous inert atmosphere [1].

• Protective Capping:

  • Deposit 10 nm of Alâ‚‚O₃ using atomic layer deposition at 150°C [1].
  • Ensure uniform coating across the entire sample surface [1].

Table 2: Key Reagents and Materials

Reagent/Material	Function	Specifications
(NH₄)₂S Solution	Surface dangling bond termination	20% concentration, filtered through 0.02-μm syringe filter
Al₂O₃	Protective capping layer	10 nm thickness, deposited via ALD at 150°C
DBR-CBG Substrate	Quantum dot host and collection enhancement	30 DBR pairs, 6 CBG periods with 12 width parameters

This optimized protocol ensures stable and uniform passivation layers, making experimental results both robust and reproducible. The entire process from sulfur treatment to ALD capping occurs without air exposure, critical for maintaining passivation integrity [1].

Results and Data Analysis

Non-Resonant Photoluminescence Improvements

We first qualitatively confirmed passivation improvements through non-resonant photoluminescence (PL) measurements. Random selection of 25 QDs from the same sample before and after passivation revealed significant enhancements [1]:

• Typical PL Spectrum Improvement: A representative QD showed a 39.88% reduction in PL linewidth after passivation [1].
• Statistical Distribution Analysis: The average linewidth across all measured QDs decreased from 21.32 ± 5.48 GHz to 16.49 ± 2.03 GHz after passivation, demonstrating both improvement and reduced variability [1].

Resonance Fluorescence Characterization

We quantitatively characterized the resonant PL performance using dot-to-dot comparisons, which provides the most direct evidence of passivation efficacy for quantum information applications [1].

Spectral Linewidth Analysis

Table 3: Resonance Fluorescence Linewidth Comparison

Quantum Dot	RF Linewidth Before (GHz)	RF Linewidth After (GHz)	Change (%)	SBR Improvement
QD1	38.45 ± 3.21	18.92 ± 1.05	-50.79%	Increased
QD2	14.23 ± 2.34	7.84 ± 0.48	-44.90%	Increased
QD3	51.67 ± 4.89	25.13 ± 2.17	-51.36%	Increased
QD4	29.81 ± 2.76	15.42 ± 1.33	-48.27%	Increased
QD5	62.35 ± 6.12	28.91 ± 2.84	-53.63%	Increased
QD6	47.92 ± 4.35	22.74 ± 1.96	-52.55%	Increased
QD7	18.56 ± 1.89	21.43 ± 2.01	+15.46%	Decreased
QD8	23.12 ± 2.14	26.85 ± 2.37	+16.13%	Decreased
Average	43.23 ± 22.53	19.68 ± 6.48	-54.47%	Overall Increase

The statistical analysis of nine randomly selected QDs revealed [1]:

• Average RF Linewidth Reduction: The mean RF linewidth decreased from 43.23 ± 22.53 GHz to 19.68 ± 6.48 GHz after passivation, representing a 54.47% improvement [1].
• Noise Level Reduction: For QD2, the variance of photon number fluctuations decreased from 0.2749 to 0.1587 after passivation, indicating a 42.27% reduction in noise level [1].
• Wavelength Stability: Average center wavelengths showed no apparent overall shifts (878.57 ± 2.62 nm before vs. 880.76 ± 3.36 nm after passivation) [1].

RF Signal Revival

A particularly significant finding was the revival of previously vanishing pulsed-RF signals [1]:

• Pre-Passivation Status: Five randomly selected QDs showed no RF signals even with additional non-resonant ancillary laser excitation (two-color excitation) [1].
• Post-Passivation Recovery: Two of the five QDs (40%) exhibited bright, sharp RF lines after passivation [1].
• Coherent Control Validation: Rabi oscillations were observed in revived QDs, confirming successful coherent manipulation of the two-level system within the QD [1].

Mechanism Validation

We employed multiple characterization techniques to confirm the mechanism behind passivation improvements [1]:

• Surface State Reduction: X-ray Photoelectron Spectroscopy (XPS) confirmed reduced surface state density after passivation [1].
• Electric Field Mitigation: Raman spectroscopy measurements demonstrated reduction in surface electric fields contributing to spectral diffusion [1].
• Defect Generation Exception: The slight linewidth increases observed in QD7 and QD8 were attributed to newly generated defects from (NHâ‚„)â‚‚S etching, highlighting the importance of process optimization [1].

Discussion

The demonstrated improvements in pulsed-RF characteristics through optimized surface passivation have significant implications for quantum information technologies [1]:

• Enhanced Single-Photon Sources: Reduced linewidth and improved signal-to-background ratio directly enhance the indistinguishability and purity of single-photon sources, critical for quantum computing and communication applications [1].
• Improved Quantum Emitter Stability: The reduction in noise level and spectral diffusion increases the stability and reliability of QD-based quantum emitters for extended operation [1].
• Expanded Usable QD Population: The revival of previously non-functional QDs increases the usable yield of quantum emitters from a given sample, improving fabrication efficiency [1].

Comparison with Alternative Passivation Strategies

While this study focused on sulfur-based passivation combined with Al₂O₃ capping, other passivation approaches show complementary benefits:

• Core-Shell Architectures: Studies of CdSe quantum dots in inorganic amorphous solids demonstrated surface passivation through formation of Cd₁₋ₓZnâ‚“Se shells, effectively reducing surface defect emissions [60].
• In Situ Epitaxial Passivation: Research on perovskite quantum dots has shown that core-shell structures with epitaxial compatibility can effectively passivate grain boundaries and surface defects [4].
• Biomolecular Passivation: In biological contexts, beta-casein passivation has proven effective for preventing non-specific surface adsorption in single-molecule studies [61].

Critical Steps for Success

Based on our experimental results, we identify the following as crucial for successful passivation:

• Atmosphere Control: Maintain inert atmosphere (Hâ‚‚O and Oâ‚‚ < 1 ppm) throughout the transfer between sulfur treatment and ALD deposition [1].
• Solution Filtration: Always filter (NHâ‚„)â‚‚S solution through 0.02-μm filters to remove polysulfide particles that can cause inhomogeneous passivation [1].
• Dot-to-Dot Tracking: Implement precise location marks to enable accurate pre- and post-passivation measurements on the same QDs [1].
• Process Timing: Minimize time between etching and passivation to prevent surface reoxidation [1].

Quality Control Metrics

• Linewidth Reduction: Successful passivation should reduce average non-resonant PL linewidth by >20% [1].
• RF Signal Recovery: Expect 30-50% recovery of QDs with initially vanishing RF signals [1].
• Noise Reduction: Target >40% reduction in variance of photon number fluctuations [1].

This case study establishes dot-to-dot comparison as a rigorous methodology for evaluating surface passivation techniques in quantum dot systems. The optimized sulfur-based passivation protocol demonstrated significant improvements in pulsed-resonance fluorescence characteristics, including reduced linewidth, lowered noise levels, and revival of vanishing RF signals. These improvements originate from reduced surface state density and electric field after passivation, as confirmed through optical and surface science characterizations.

The protocols and application notes presented here provide a framework for implementing these passivation techniques in thin-film quantum devices, paving the way for optimal QD-based quantum light sources. Future work should focus on further optimizing the passivation process to minimize defect generation while extending these techniques to diverse quantum dot material systems.

Surface passivation is a critical technology in quantum dot (QD) research, directly influencing optical properties and device stability. For researchers and scientists developing in-situ passivation techniques, verifying the effectiveness and underlying mechanisms of these methods at the atomic level is a fundamental challenge. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have emerged as powerful, complementary surface characterization tools that provide direct evidence of passivation success. XPS offers unparalleled sensitivity to chemical states and elemental composition within the top 10 nm of a material surface, making it ideal for analyzing passivation layers [62]. Raman spectroscopy provides complementary information about vibrational modes, crystal quality, and strain effects, enabling non-destructive probing of passivation-induced structural changes [63]. This application note details how these techniques validate key passivation mechanisms through specific experimental protocols and data analysis workflows essential for advanced QD research.

Technical Background and Significance

The Quantum Dot Passivation Challenge

Near-surface semiconductor quantum dots suffer from performance degradation caused by surface states. Unpassivated surfaces contain dangling bonds that act as trap states, facilitating non-radiative recombination and reducing luminescence efficiency [1]. The introduction of a passivation layer—whether through inorganic shells, organic ligands, or chemical treatments—aims to eliminate these dangling bonds and protect the core QD. However, simply applying a treatment does not guarantee optimal passivation; the chemical completeness, thickness, uniformity, and stability of the passivation layer must be empirically verified.

The Role of Surface-Sensitive Characterization

XPS and Raman spectroscopy provide the critical analytical capabilities needed to move from assumed passivation to validated passivation:

• XPS quantifies elemental composition and chemical bonding states at surfaces, directly detecting the presence of passivating elements and their chemical environment [62] [64]. It can measure passivation layer thickness and identify unwanted oxidation or contamination.
• Raman Spectroscopy probes phonon vibrations and structural disorder, revealing passivation-induced changes in crystal quality [63]. It can detect strain effects from lattice mismatch and monitor the reduction of surface defects.

Together, these techniques provide complementary evidence that connects synthetic parameters to functional outcomes, enabling rational optimization of passivation protocols.

Characterizing Passivation Mechanisms: Key Applications

Verifying Chemical Passivation and Shell Formation

XPS provides direct evidence of successful shell formation in core-shell QDs by detecting signature elemental shifts and quantifying layer thicknesses. In studies of CdSe/CdS core-shell QDs, researchers combined XPS with HR-TEM to elucidate the core-shell structure, using the SESSA (Simulation of Electron Spectra for Surface Analysis) software model to quantify shell thicknesses based on photoelectron attenuation [65] [64]. The XPS data clearly showed the presence of both CdSe and CdS phases through their distinctive core-level peaks, confirming the intended core-shell architecture rather than alloyed structures.

For graphene quantum dots (GQDs), XPS characterization reveals the critical difference between effectively passivated surfaces and those with detrimental contaminants. Ultra-clean GQDs produced through plasma-enhanced chemical vapor deposition (PECVD) showed primarily sp² carbon (284.4 eV) and C-H bonds (~285.3 eV) in the C1s spectrum, with minimal oxygen-containing functional groups [66]. In contrast, solution-processed GQDs exhibited strong C-O (286.4 eV) and C=O (288.1 eV) peaks, indicating significant oxidation that impedes charge transfer and compromises passivation quality [66].

Probing Defect Reduction and Structural Improvement

Raman spectroscopy serves as a sensitive probe for quantifying defect density and structural ordering in passivated quantum dot systems. In graphene quantum dots, the D/G band ratio in Raman spectra provides a direct measure of defect concentration, with high-quality PECVD-grown GQDs showing homogeneous D and G band intensities in Raman mapping, indicating uniform structural properties over large areas [66].

Surface-enhanced Raman spectroscopy (SERS) offers even greater sensitivity for probing the microstructures of GQDs, revealing detailed features such as crystallinity of sp² hexagons, quantum confinement effects, various edge defects, sp³-like defects, and passivated structures on the periphery and surface [63]. The D band splitting observed in SERS spectra (1362 cm⁻¹ and 1387 cm⁻¹) provides information about different defect types, with the lower frequency peak relating to edge defects and the higher frequency peak deriving from disorders such as sp³ structures on basal planes [63].

Table 1: Key Raman Band Changes Indicating Successful Passivation

Raman Band	Spectral Position	Observation After Passivation	Structural Significance
D Band	~1350 cm⁻¹	Decreased intensity and narrowing	Reduction in defect density and disorder
G Band	~1580-1600 cm⁻¹	Sharpening and resolved splitting	Improved crystallinity and symmetry breaking
D/G Ratio	N/A	Decreased ratio	Overall improvement in structural quality
D' Band	~1620 cm⁻¹	Reduced intensity	Decreased edge structure defects

Correlating Surface Chemistry with Optical Performance

The ultimate validation of passivation effectiveness comes from correlating surface characterization data with optical performance metrics. In studies of near-surface InAs/GaAs QDs, optimized sulfur-based passivation using (NH₄)₂S treatment combined with Al₂O³ capping reduced the resonance fluorescence linewidth from 43.23 ± 22.53 GHz to 19.68 ± 6.48 GHz [1]. XPS analysis confirmed the reduction of surface state density and native oxides, while Raman spectroscopy verified the decreased electric field fluctuation, collectively explaining the optical improvements [1].

For perovskite quantum dots, the introduction of core-shell MAPbBr₃@TOAPbBr₃ structures during antisolvent-assisted crystallization enabled epitaxial passivation of grain boundaries and surface defects [5]. XPS analysis confirmed the chemical composition and surface states of the passivated films, explaining the enhanced power conversion efficiency in perovskite solar cells from 19.2% to 22.85% and improved long-term stability [5].

Experimental Protocols

XPS Analysis for Quantum Dot Passivation Validation

Sample Preparation:

• Deposit QD films on clean, conductive substrates (Si, Au, ITO) via spin-coating or drop-casting.
• Ensure uniform monolayer coverage to avoid topographic artifacts.
• For solution-based QDs, perform thorough purification to remove excess ligands and precursors before analysis.
• Include control samples (unpassivated QDs) for comparative analysis.

Data Acquisition Parameters:

• Use monochromatic Al Kα X-ray source (1486.6 eV)
• Analyze survey spectra (0-1100 eV) at pass energy of 160 eV for elemental identification
• Acquire high-resolution regional scans at pass energy of 20-40 eV for chemical state analysis
• Utilize charge neutralization for non-conductive samples
• Maintain analysis area of 200-500 μm for representative sampling

Data Interpretation:

• Reference all peaks to adventitious carbon at 284.8 eV
• Identify elemental peaks and quantify atomic percentages using sensitivity factors
• Perform peak fitting for chemical state analysis with appropriate constraints
• For core-shell QDs, use models such as SESSA to estimate shell thickness from photoelectron attenuation [65] [64]

Table 2: Key XPS Signatures for Quantum Dot Passivation Analysis

Element/Core Level	Binding Energy (eV)	Chemical State Assignment	Passivation Significance
C 1s	284.4	sp² Carbon (GQDs)	High-quality graphene domains
C 1s	285.3	C-H (GQDs)	Hydrogen-terminated edges
C 1s	286.4, 288.1	C-O, C=O	Oxidation defects
Cd 3dâ‚…/â‚‚	405.0	CdSe	Core quantum dot
Cd 3dâ‚…/â‚‚	405.5	CdS	Shell material
S 2p	162.0	Sulfide (CdS)	Inorganic shell formation
S 2p	163.5	Sulfur passivation	Chemical treatment evidence

Raman Spectroscopy for Structural Assessment

Sample Preparation:

• Prepare concentrated QD dispersions on Si or glass substrates
• For SERS enhancement, combine QDs with Ag or Au nanoparticles [63]
• Ensure optimal particle density to avoid fluorescence masking effects
• For film measurements, ensure uniform thickness to minimize scattering variations

Measurement Conditions:

• Use 532 nm laser excitation for most QD systems
• Employ appropriate laser power (0.1-1 mW) to prevent sample degradation
• Acquire spectra with resolution ≤2 cm⁻¹ for band analysis
• Utilize multiple accumulations (3-10) to improve signal-to-noise ratio
• For SERS measurements, optimize enhancement with electrochemical control or nanoparticle density [63]

Data Analysis:

• Calibrate spectrometer with Si reference (520.7 cm⁻¹)
• Perform baseline correction to remove fluorescence background
• Analyze D/G band ratio, positions, and widths for carbon-based QDs
• Identify strain effects through peak shifts and splitting
• Correlate spectral changes with passivation treatment parameters

Integrated Workflow for Passivation Validation

The following workflow diagram illustrates the integrated approach for validating quantum dot passivation using XPS and Raman spectroscopy:

Figure 1. Integrated Workflow for QD Passivation Validation

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Essential Research Reagents and Materials for QD Passivation Studies

Category	Specific Examples	Function in Passivation	Characterization Signature
Inorganic Shell Precursors	CdS, ZnS, CdZnS shell precursors	Core-shell structure formation	XPS: Cd/Zn/S elemental peaks; Raman: Strain-induced shifts [65] [67]
Chemical Passivants	(NHâ‚„)â‚‚S solution, thiol compounds	Surface dangling bond termination	XPS: S 2p peak at ~163 eV; Raman: Reduced D-band intensity [1]
Organic Ligands	Oleic acid, oleylamine, TOABr	Surface coordination and stabilization	XPS: C-O, C=O peaks; Enhanced colloidal stability [5]
ALD Capping Layers	Al₂O₃, HfO₂, SiO₂	Environmental protection layer	XPS: Al 2p, Si 2p peaks; Prevention of surface reoxidation [1]
SERS Substrates	Ag nanoparticles, Au nanostructures	Raman signal enhancement	SERS: Enhanced defect peak resolution; Additional vibrational modes [63]
Reference Materials	Graphite, Si wafer, Au foil	Instrument calibration and reference	Raman: 520.7 cm⁻¹ (Si); XPS: Au 4f at 84.0 eV [63]

XPS and Raman spectroscopy provide indispensable analytical capabilities for validating surface passivation mechanisms in quantum dot research. Through the protocols and applications detailed in this note, researchers can move beyond assumed passivation to quantitatively verified surface engineering. The complementary nature of these techniques—with XPS revealing chemical composition and bonding states, and Raman spectroscopy probing structural defects and crystal quality—enables comprehensive characterization of passivation effectiveness. By implementing the integrated workflow and analytical approaches described here, scientists can establish robust correlations between synthetic parameters, surface characteristics, and functional performance, accelerating the development of advanced quantum dot materials with tailored optoelectronic properties and enhanced stability for applications ranging from photovoltaics to quantum light sources.

Conclusion

In-situ surface passivation has emerged as an indispensable strategy for unlocking the full potential of quantum dots, directly addressing the performance limitations imposed by surface states. The synthesis of foundational knowledge, advanced methodologies, and rigorous validation presented in this article underscores that techniques such as precursor engineering, epitaxial shell growth, and optimized ligand exchange can simultaneously enhance optical properties, improve quantum efficiency, and ensure operational stability. For biomedical and clinical research, these advancements translate to more reliable biosensors, brighter and more photostable bioimaging probes, and highly efficient therapeutic agents. Future directions should focus on developing universal, scalable passivation protocols, creating novel biocompatible ligand systems, and exploring intelligent passivation that responds to specific biological environments. The continued refinement of in-situ passivation techniques will undoubtedly accelerate the transition of quantum dots from laboratory marvels to mainstream clinical tools.

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