Ligand Binding Affinity in Perovskite Quantum Dots: A Comprehensive Guide to Enhancing Stability for Biomedical and Optoelectronic Applications

Abstract

This article provides a detailed analysis of how surface ligand binding affinity directly dictates the structural and optical stability of perovskite quantum dots (PQDs), a critical material for next-generation optoelectronics and biomedical devices. Targeting researchers and drug development professionals, we explore the fundamental principles of ligand-PQD interactions, evaluate advanced ligand engineering strategies—including bidentate and dual-ligand systems—and present methodologies for quantifying binding strength. By synthesizing foundational knowledge with the latest high-performance applications and validation techniques, this review serves as a strategic guide for selecting and optimizing ligands to suppress phase transition, mitigate surface defects, and achieve unprecedented device performance and operational longevity.

The Fundamental Link Between Ligand Binding and PQD Stability

Perovskite Quantum Dots (PQDs), particularly all-inorganic CsPbX₃ (X = Cl, Br, I), have emerged as a revolutionary class of semiconductor nanocrystals for next-generation optoelectronic devices, including displays, solar cells, and photodetectors [1] [2] [3]. Their exceptional optical properties—such as high photoluminescence quantum yield (PLQY), narrow emission bandwidth, and widely tunable bandgaps across the entire visible spectrum—enable high color purity, potentially exceeding 144% of the NTSC color standard [2] [3]. Despite this immense potential, the widespread commercial application of PQDs is critically hindered by their inherent structural instabilities [1] [3]. This review objectively compares the performance of various stabilization strategies, with a particular focus on ligand engineering, framing the analysis within a broader thesis on surface ligand binding affinity for PQD stability research.

Fundamental Structure and Roots of Instability

The foundational structure of PQDs is defined by the ABX₃ perovskite crystal lattice. In CsPbX₃, Cs⁺ (A-site) occupies the cube corners, Pb²⁺ (B-site) resides at the body center, and halide anions X⁻ (X-site) are located at the face centers, forming [PbX₆]⁴⁻ octahedra [2]. The stability of this crystal structure can be predicted using the Goldschmidt tolerance factor (t) and the octahedral factor (μ) [2].

The inherent instability of PQDs arises from two primary mechanisms: the facile migration of halide ions within the crystal lattice and the detachment of surface-passivating ligands [3]. Halide vacancies form easily due to low ionic migration energy, while commonly used long-chain ligands like oleic acid (OA) and oleylamine (OAm) are only weakly bound to the PQD surface [2] [3]. These ligands readily detach during purification or upon exposure to environmental stimuli, creating surface defects that act as non-radiative recombination centers and degrade both structural integrity and optoelectronic performance [3] [4].

External factors such as humidity, temperature, light exposure, and polar solvents accelerate degradation by attacking the vulnerable ionic crystal structure and exacerbating ligand loss [2] [3]. CsPbI₃, for instance, undergoes a detrimental phase transition from a photoactive black phase (α, β, γ) to a non-perovskite yellow phase (δ) at room temperature [2].

Comparative Analysis of PQD Stabilization Strategies

Various strategies have been developed to combat PQD instability. The following table provides a performance comparison of the primary approaches.

Table 1: Comparison of Primary Strategies for Enhancing PQD Structural Stability

Strategy	Mechanism of Action	Key Performance Metrics	Advantages	Limitations
Ligand Modification [2] [3] [4]	Exchanges dynamic long-chain ligands with shorter, multidentate, or covalently binding ligands to improve packing density and binding affinity.	- PLQY increased from 22% to 51% for CsPbI₃ QDs using 2-aminoethanethiol (AET) [3].- PCE of CsPbI₃ QD solar cells improved to 15.4% with TPPO ligands [4].- Maintained >95% initial PL after 60 min water/120 min UV exposure [3].	Directly addresses surface defect origin; can be applied in situ or post-synthesis; enhances charge transport [3] [4].	Ligand synthesis can be complex; may require optimization for different PQD compositions [2].
Core-Shell Structure [3]	Encapsulates PQDs with a protective shell of polymers or inorganic materials to create a physical barrier against external stimuli (moisture, oxygen).	Improved environmental stability against moisture and oxygen [3].	Effective isolation from environment; can be combined with other strategies [3].	Shell growth on ionic PQD surface is challenging; may introduce interface defects; can hinder charge transport [3].
Crosslinking [3]	Introduces crosslinkable ligands on the PQD surface that form a robust network via light or heat, inhibiting ligand dissociation.	Suppresses ligand dissociation and subsequent defect formation [3].	Creates a stable, interconnected network; minimizes defect formation from ligand loss [3].	Crosslinking process may damage PQD surface; requires careful control of reaction conditions [3].
Metal Doping [3]	Incorporates metal ions with equivalent charge numbers at the A- or B-sites to strengthen the perovskite lattice and alter B-X bond lengths.	Enhances intrinsic lattice stability by changing B-X bond lengths [3].	Improves intrinsic thermal and phase stability [3].	Must maintain Goldschmidt tolerance and octahedral factors; typically limited to in-situ synthesis [3].

Experimental Protocols in Ligand Engineering

The pursuit of high-binding-affinity ligands relies on specific experimental protocols, primarily conducted through solution-based synthesis.

Synthesis Methods and Ligand Exchange

Two primary methods are used for PQD synthesis: the hot-injection method and the ligand-assisted reprecipitation (LARP) method [2] [3]. Both methods traditionally utilize long-chain OA and OAm ligands to control nucleation and growth [2]. The subsequent ligand exchange process is critical for replacing these insulating ligands with shorter, more stable alternatives. A standard protocol involves a two-step solid-state ligand exchange for CsPbI₃ PQDs [4]:

• Anionic Ligand Exchange: Layer-by-layer deposition of PQD films followed by treatment with a solution of anionic short-chain ligands (e.g., acetate from NaOAc) dissolved in polar solvents like methyl acetate (MeOAc) to replace OA [4].
• Cationic Ligand Exchange: Post-treatment of the film with a solution of cationic short-chain ligands (e.g., phenethylammonium iodide - PEAI) dissolved in ethyl acetate (EtOAc) to replace OLA [4].

In-situ vs. Post-Synthesis Ligand Engineering

Ligand engineering strategies are categorized based on their timing:

• In-situ ligand engineering involves adding new ligands during the synthetic process, allowing them to incorporate directly as the PQDs form [2].
• Post-synthesis ligand engineering (or post-treatment) occurs after PQD synthesis and purification. This is often used to heal surface defects created during the initial ligand exchange or purification steps [3] [4]. A key advancement is using nonpolar solvents (e.g., octane) for post-treatment instead of polar solvents, which preserves the PQD surface components while allowing effective passivation with ligands like triphenylphosphine oxide (TPPO) [4].

Ligand Binding Affinity and Stability Relationships

The efficacy of ligand engineering is fundamentally governed by the binding affinity between the ligand and the PQD surface. Strong binding is crucial for mitigating ligand detachment and passivating surface traps.

Table 2: Comparison of Ligand Types and Their Impact on PQD Stability

Ligand Type	Binding Mechanism	Impact on Stability & Performance
Traditional Long-Chain (OA/OAm) [2] [3]	Dynamic, labile binding via carboxylate/amine groups. Bent structure causes low packing density.	Poor: Low binding affinity leads to easy detachment, causing aggregation and degradation. Insulating properties hinder device performance [3] [4].
Ionic Short-Chain (e.g., Acetate, PEA⁺) [4]	Ionic interaction with the PQD surface.	Moderate: Improves charge transport but binding is still relatively labile. Polar solvents used in exchange can damage the PQD surface, creating new traps [4].
Multidentate/Covalent (e.g., AET, TPPO) [3] [4]	Strong, covalent-like coordination (e.g., Thiol-Pb²⁺ in AET) or Lewis-base interaction (P=O with Pb²⁺ in TPPO).	High: Strong binding affinity ensures durable passivation. TPPO in nonpolar solvent enables "nondestructive" surface stabilization, leading to high PCE (15.4%) and excellent ambient stability (>90% initial efficiency after 18 days) [3] [4].

The relationship between ligand binding affinity and the resulting stability pathway is logical. Stronger binding directly leads to superior surface passivation, which in turn enhances resistance to environmental factors and improves long-term performance.

The Scientist's Toolkit: Essential Research Reagents

The following table details key reagents and materials essential for experimental research in PQD stabilization via ligand engineering.

Table 3: Essential Research Reagents for PQD Ligand Engineering

Reagent/Material	Function in Research	Application Context
Oleic Acid (OA) / Oleylamine (OAm) [2] [3]	Standard long-chain ligands for colloidal synthesis; control nucleation and growth.	Initial synthesis of high-quality, monodispersed PQDs via hot-injection or LARP [4].
1-Octadecene (ODE) [2]	Nonpolar solvent for dissolving precursors in high-temperature synthesis.	Used as the primary solvent in the hot-injection method [2].
Methyl Acetate (MeOAc) / Ethyl Acetate (EtOAc) [3] [4]	Polar solvents for dissolving ionic salts during solid-state ligand exchange.	Used in purification and layer-by-layer ligand exchange to replace OA and OAm with short-chain ligands [4].
2-Aminoethanethiol (AET) [3]	Short-chain, bidentate ligand with strong affinity for Pb²⁺ via thiolate group.	Post-synthesis ligand exchange to heal surface defects and enhance stability against water and UV light [3].
Triphenylphosphine Oxide (TPPO) [4]	Short-chain, covalent ligand that strongly coordinates to uncoordinated Pb²⁺ sites via Lewis-base interaction.	Post-treatment surface stabilization, dissolved in nonpolar solvents (e.g., octane) to prevent PQD surface damage [4].
Phenethylammonium Iodide (PEAI) [4]	Short-chain, cationic ligand used to replace oleylamine.	Standard reagent in the second step of ligand exchange for fabricating conductive PQD solids for solar cells [4].

Defining Ligand Binding Affinity and Its Role in Surface Passivation

Ligand binding affinity, defined as the strength of the interaction between a molecule (ligand) and its target binding site, serves as a fundamental parameter in determining the effectiveness of surface passivation for perovskite quantum dots (PQDs) and other semiconducting nanomaterials [5] [6]. Quantitatively expressed through the equilibrium dissociation constant (Kd), where lower values indicate stronger, more favorable interactions, binding affinity directly governs the stability and optoelectronic performance of functional nanomaterials [7]. In the specific context of surface passivation, high-affinity ligands form stable complexes with undercoordinated surface atoms on PQDs, effectively neutralizing defect sites that would otherwise act as centers for non-radiative recombination and material degradation [8] [9].

The pursuit of high-performance PQD-based devices necessitates a thorough comparison of ligand binding strategies, as the dynamic nature of ligand binding under operational conditions—including exposure to heat, oxygen, and moisture—presents distinct challenges beyond static binding measurements [8]. This objective analysis compares the performance of various ligand engineering approaches based on their binding affinity characteristics, providing researchers with experimental data and methodologies to inform material selection for enhanced PQD stability.

Theoretical Foundation: Binding Affinity Fundamentals and Passivation Principles

Key Concepts and Definitions

• Binding Affinity: The strength of the interaction between a ligand and its biomolecular target, quantitatively measured by the dissociation constant (Kd) [7] [6]. Lower Kd values indicate tighter binding.
• Dissociation Constant (Kd): The equilibrium concentration at which half of the receptor binding sites are occupied by the ligand [7]. It represents the ligand concentration required for half-maximal binding.
• Surface Passivation: The process of eliminating dangling bonds and surface defects on nanomaterials through chemical ligand binding, which improves optical properties and stability [10] [9].
• Dynamic Adsorption Affinity (DAA): A recently identified parameter describing the ligand's ability to maintain passivation under operational stressors like heat and moisture, which may be more predictive of passivation efficacy than static binding strength alone [8].

Relationship Between Binding Affinity and Passivation Efficacy

The binding affinity of a ligand towards specific surface sites on a PQD directly determines the completeness and durability of surface passivation. High-affinity binding results from strong, multidentate coordination chemistry that maximizes intermolecular forces such as ionic bonds, hydrogen bonds, and van der Waals forces [5]. For ternary nanocrystals like AgBiS2, the principle of cation-selective ligand binding becomes critically important, where different metal cations (e.g., Ag⁺ vs. Bi³⁺) exhibit distinct preferences for specific functional groups based on the Hard-Soft Acid-Base (HSAB) theory [11]. This creates a scenario where a single ligand may not comprehensively passivate all surface sites, leading to incomplete protection and instability.

Table 1: Ligand Binding Affinity Classifications and Implications for Passivation

Affinity Classification	Theoretical Kd Range	Passivation Characteristics	Impact on PQD Stability
High Affinity	Picomolar to Nanomolar	Stable, long-term surface coverage; resistant to displacement	High environmental stability; suppressed ion migration
Medium Affinity	Nanomolar to Micromolar	Reversible binding; dynamic equilibrium with solution	Moderate stability; susceptible to ligand loss under stress
Low Affinity	Micromolar to Millimolar	Weak, transient surface interaction; incomplete coverage	Poor stability; rapid degradation under ambient conditions

Figure 1: Logical workflow diagram illustrating the relationship between ligand binding affinity assessment and the resulting passivation outcomes for perovskite quantum dots.

Comparative Analysis of Ligand Binding Approaches

Traditional Organic Ligands vs. Advanced Passivators

Traditional long-chain organic ligands like oleic acid (OA) and oleylamine (OAm) initially facilitate PQD synthesis and provide basic colloidal stability, but their labile binding nature often leads to detachment during processing or operation, creating instability issues [9]. Advanced passivation strategies employ rationally designed ligands with enhanced binding characteristics, as detailed in the comparative Table 2.

Table 2: Performance Comparison of Ligand Types for PQD Surface Passivation

Ligand Type	Example Compounds	Binding Mechanism	Reported PLQY Improvement	Stability Enhancement	Key Limitations
Traditional Long-Chain Organics	Oleic Acid (OA), Oleylamine (OAm)	Dynamic L-type/X-type binding	Baseline (Reference)	Low: Rapid degradation under humidity/heat	Labile binding; insulator properties
Metal Salt Cations	Cd²⁺, Zn²⁺, In³⁺ with NO₃⁻/BF₄⁻	Cationic binding to Lewis basic sites	72-97% (various NCs) [10]	High: Stable in polar solvents	Requires non-coordinating anions; Lewis acidity
Multidentate Organic Ligands	4-Aminobutylphosphonic Acid (4-ABPA)	Strong phosphonate chelation with high DAA	Significant increase in Pb-Sn perovskites [8]	Excellent: Suppressed VH formation	Complex synthesis; potential steric hindrance
Thiol/Carboxylic Bifunctional	3-Mercaptopropionic Acid (MPA)	HSAB-compliant dual binding to Ag/Bi sites	Improved for AgBiS2 NCs [11]	High: Comprehensive ternary NC passivation	Sensitivity to oxidation (thiol group)

Experimental Binding Affinity Data and Performance Metrics

Quantitative assessment of ligand performance reveals critical differences in passivation efficacy, with metal salt treatments and specifically designed organic molecules demonstrating superior outcomes compared to traditional approaches.

Table 3: Quantitative Experimental Data from Key Passivation Studies

Study System	Passivation Method	Key Measurement	Performance Outcome	Research Context
Mixed Pb-Sn Perovskites [8]	4-ABPA treatment	Dynamic Adsorption Affinity (DAA)	Suppressed hydrogen vacancy formation; enhanced photovoltaic performance	Ab initio MD simulations guided ligand design
All-inorganic NCs (CdSe/ZnS, etc.) [10]	Metal salt (Cd²⁺, Zn²⁺) treatment	Absolute PLQY	97% (red), 80% (green), 72% (blue) in polar solvents	General strategy for intensely luminescent NCs
AgBiS2 NC Photovoltaics [11]	MPA ligand exchange	Power Conversion Efficiency (PCE)	~12% improvement over control devices	Cation-selective passivation addressing ternary NC challenges
CsPbX3 PQDs [9]	Various ligand engineering approaches	Structural & PL stability	Improved resistance to humidity, heat, and light exposure	Review of stability enhancement strategies

Experimental Protocols for Assessing Binding Affinity and Passivation

Protocol 1: Dynamic Adsorption Affinity (DAA) Assessment via AIMD

Objective: To evaluate ligand binding strength under operational stressors (heat, oxygen, moisture) rather than static conditions [8].

Materials:

• Ab initio molecular dynamics (AIMD) simulation software (e.g., VASP, CP2K)
• Model perovskite surface structures (e.g., MAI-terminated (001) mixed Pb-Sn perovskite)
• Ligand molecular structures (e.g., 4-ABPA, traditional ligands for comparison)
• Environmental stressor parameters (300K/400K temperature, O₂/H₂O molecules)

Methodology:

• Construct optimized surface slab models of the target perovskite composition
• Introduce ligand molecules at various adsorption sites on the surface
• Simulate system dynamics under controlled environmental conditions:
  • Room temperature (300K) and elevated temperature (400K)
  • Introduce oxygen and water molecules to simulate atmospheric stressors
• Monitor and quantify over simulation time:
  • Ligand-surface bond stability and residence time
  • Surface defect formation energies (e.g., hydrogen vacancies)
  • Ligand desorption rates under stress conditions
• Calculate DAA metrics based on persistent binding under degradation conditions

Validation: Correlate DAA predictions with experimental stability tests (TPD-MS, operational device lifetime) [8]

Protocol 2: Surface Passivation Efficacy via Photophysical Characterization

Objective: To quantitatively measure the effectiveness of ligand passivation through optical and electronic properties.

Materials:

• Spectrofluorometer with integrating sphere
• Time-resolved photoluminescence (TRPL) spectrometer
• UV-Vis absorption spectrometer
• Fabricated thin films of passivated PQDs
• Controlled atmosphere sample chamber

Methodology:

• Sample Preparation:
  • Fabricate PQD films via layer-by-layer deposition with ligand treatment
  • Include control samples (unpassivated or traditionally passivated) for comparison
• Photoluminescence Quantum Yield (PLQY) Measurement:
  • Use integrating sphere with calibrated spectrofluorometer
  • Measure absolute PLQY values for each sample type
  • Calculate improvement over baseline passivation
• Carrier Lifetime Analysis:
  • Perform TRPL measurements with pulsed excitation source
  • Fit decay curves to multi-exponential models
  • Calculate average carrier lifetimes; longer lifetimes indicate reduced non-radiative recombination
• Stability Testing:
  • Monitor PLQY and absorption under continuous illumination
  • Conduct environmental testing (controlled humidity, temperature)
  • Measure degradation rates over time

Data Interpretation: Higher PLQY and extended carrier lifetimes directly correlate with superior surface passivation completeness and binding affinity [10] [9].

Figure 2: Experimental workflow for comprehensive assessment of ligand binding affinity and surface passivation efficacy, integrating both computational and experimental approaches.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Key Research Reagent Solutions for Ligand Passivation Studies

Reagent Category	Specific Examples	Primary Function	Considerations for Selection
Traditional Ligands	Oleic Acid (OA), Oleylamine (OAm)	Reference passivators; synthesis control	Establish baseline; dynamic binding leads to instability [9]
Metal Salt Treatments	Cd(NO₃)₂, Zn(BF₄)₂, In(OTf)₃	Lewis acid site passivation; organic ligand replacement	Select based on HSAB principles; anion choice critical [10]
Bifunctional Ligands	3-Mercaptopropionic Acid (MPA), 4-ABPA	Comprehensive passivation of multiple surface sites	Ideal for ternary NCs; assess oxidation sensitivity [11] [8]
Characterization Standards	Radiolabeled ligands, Reference quantum dots	Binding affinity quantification; method calibration	Essential for Kd and DAA validation; ensure traceability
Computational Tools	DFT/MD software packages, Surface modeling tools	Prediction of binding energies and DAA	Require significant computational resources; expertise-dependent

The comparative analysis of ligand binding affinity demonstrates that strategic ligand design moving beyond traditional approaches is essential for achieving high-performance, stable PQD systems. The critical findings from current research indicate:

• Binding affinity under operational conditions (Dynamic Adsorption Affinity) provides a more reliable prediction of passivation efficacy than static binding measurements alone, particularly for applications requiring environmental stability [8].

• Cation-selective ligand binding presents both a challenge and opportunity for ternary nanocrystal systems, where bifunctional ligands like MPA capable of addressing multiple metal sites simultaneously demonstrate superior performance [11].

• Metal salt treatments offer a promising alternative to organic ligands, providing intense luminescence while maintaining charge transport capabilities, though careful selection of cation-anion pairs is required [10].

The integration of computational prediction methods like AIMD simulations with experimental validation creates a powerful framework for accelerating the development of next-generation passivation ligands. For researchers pursuing enhanced PQD stability, prioritizing ligands with demonstrated high dynamic adsorption affinity and comprehensive surface site coverage will yield the most significant improvements in device performance and operational lifetime.

How Strong Ligand Binding Mitigates Phase Transition and Degradation

The stability of perovskite quantum dots (PQDs) remains a critical challenge hindering their commercial application in optoelectronics, photovoltaics, and other advanced technologies. Among the various factors influencing stability, the binding strength of surface ligands has emerged as a pivotal determinant in mitigating two primary degradation pathways: phase transitions and material decomposition. Surface ligands are molecules that coordinate with atoms on the PQD surface, serving not only to passivate defects and prevent aggregation but also to impart profound stability against thermal and environmental stress [2]. Recent research has established a direct correlation between ligand binding energy and the thermal degradation mechanism of PQDs, revealing that strongly bound ligands can effectively suppress the detrimental phase transitions that compromise optical and electronic properties [12]. This guide provides a comparative analysis of how strategic ligand engineering controls PQD stability, supported by experimental data and methodologies directly applicable to research and development settings.

Comparative Analysis: Ligand-Dependent Degradation Pathways

Thermal Degradation Mechanisms by PQD Composition

The thermal degradation pathway of CsₓFA₁₋ₓPbI₃ PQDs fundamentally depends on their A-site cation composition and the associated ligand binding energy.

Table 1: Thermal Degradation Behavior of CsₓFA₁₋ₓPbI₃ PQDs [12]

PQD Composition (CsₓFA₁₋ₓPbI₃)	Primary Thermal Degradation Mechanism	Onset Temperature Characteristics	Key Experimental Observations
Cs-Rich (High x)	Phase transition from black γ-phase to yellow non-perovskite δ-phase	Lower degradation onset temperature	Phase transition precedes decomposition; in situ XRD shows emergence of δ-phase peaks (25.4°, 25.8°, 30.7°)
FA-Rich (Low x)	Direct decomposition to PbI₂	Slightly higher thermal stability than Cs-rich counterparts	No intermediate phase transition; direct appearance of PbI₂ peaks (25.2°, 29.0°, 41.2°) at ~150°C

The Critical Role of Ligand Binding Energy

First-principle density functional theory (DFT) calculations have quantitatively demonstrated that the binding strength of common ligands (e.g., oleylamine, oleic acid) to the PQD surface is significantly stronger for FA-rich PQDs compared to Cs-rich PQDs [12]. This higher ligand binding energy in FA-rich systems is directly correlated with their observed resistance to phase transitions and their slightly superior thermal stability, despite their hybrid organic-inorganic nature. The stronger ligand binding provides a more robust protective shell around the PQD, effectively stabilizing the perovskite structure against thermal-induced lattice rearrangements [12].

Experimental Protocols for Investigating Ligand-PQD Stability

In Situ Structural and Optical Characterization

Purpose: To monitor the real-time structural and optical changes in PQDs under thermal stress, directly linking ligand binding to stability metrics.

Key Methodologies:

• In Situ X-ray Diffraction (XRD): PQD samples are heated from 30°C to 500°C under an inert argon atmosphere while continuously collecting XRD patterns. This allows for direct observation of phase transition temperatures (e.g., γ-to-δ in CsPbI₃) or the emergence of decomposition product peaks (e.g., PbI₂) [12].
• In Situ Photoluminescence (PL) Spectroscopy: Measures emission intensity, peak position, and width as a function of temperature. FA-rich PQDs exhibit stronger electron-longitudinal optical phonon coupling, suggesting higher probability of exciton dissociation by phonon scattering at elevated temperatures [12].
• Thermogravimetric Analysis (TGA): Tracks mass loss as temperature increases, providing information on ligand desorption and decomposition events [12].

Surface Ligand Engineering and Exchange

Purpose: To replace native long-chain ligands with more strongly binding or functional alternatives to enhance stability and performance.

Layer-by-Layer (LBL) Solid-State Ligand Exchange:

• PQD Film Deposition: Spin-coat a layer of pristine PQDs (stabilized with oleic acid/OA and oleylamine/OAm) onto a substrate.
• Initial Ligand Removal: Treat the film with methyl acetate (MeOAc) to remove the original long-chain ligands and excess solvent.
• New Ligand Introduction: Introduce a solution of the new short-chain ligand (e.g., Phenethylammonium Iodide, PEA.I, dissolved in ethyl acetate) via spin-coating.
• Repetition: Repeat steps 1-3 multiple times to build a thick, electronically coupled film with complete ligand exchange [13].

Critical Parameters: The concentration of the ligand solution, treatment time, and choice of solvent are crucial. Excessive treatment time with certain ligands (e.g., formamidinium iodide, FAI) can lead to unintended cation exchange and compositional changes in the PQD [13].

Mechanisms of Ligand-Induced Stability: A Pathway Analysis

The following diagram illustrates the competing degradation pathways for PQDs under thermal stress and how strong ligand binding influences these pathways.

The Scientist's Toolkit: Essential Reagents for Ligand Stability Research

Table 2: Key Research Reagents for PQD Ligand Binding and Stability Studies [12] [2] [13]

Reagent / Material	Function in Research	Application Notes
Oleic Acid (OA) & Oleylamine (OAm)	Standard long-chain ligands for initial PQD synthesis; provide colloidal stability.	Dynamic binding leads to easy detachment; poor charge transport in films require exchange for devices [2] [13].
Phenethylammonium Iodide (PEAI)	Short-chain, aromatic ligand for post-synthesis exchange.	Enhances inter-dot coupling, passivates defects, improves moisture resistance via hydrophobic phenyl group [13].
Formamidinium Iodide (FAI)	Used for ligand exchange and surface passivation.	Can induce partial cation exchange if treatment time is not controlled, altering core PQD composition [13].
Methyl Acetate (MeOAc)	Polar solvent for washing and initial ligand removal.	Effectively removes oleate ligands without dissolving the PQD film [13].
Triphenyl Phosphite (TPPi)	Strong-binding ligand for exchange.	Used in bifunctional electroluminescent solar cells to enhance both PCE and electroluminescent performance [13].

The strategic management of surface ligand binding affinity presents a powerful avenue for controlling the stability and degradation pathways of perovskite quantum dots. Experimental evidence conclusively demonstrates that strong ligand binding directly influences the thermal degradation mechanism, suppressing deleterious phase transitions in Cs-rich PQDs and modifying the decomposition pathway in FA-rich PQDs. The methodologies outlined—particularly in situ characterization techniques and advanced ligand exchange protocols—provide researchers with a robust framework for evaluating and improving PQD stability. As the field progresses, the rational design of multidentate and strongly-coordinating ligands will be crucial in bridging the gap between the exceptional optoelectronic properties of PQDs and the demanding stability requirements of commercial applications.

The Impact of A-Site Cation Composition on Ligand Binding Energy

The stability of perovskite quantum dots (PQDs) remains a critical challenge hindering their commercial application in optoelectronics. A key determinant of this stability is the binding energy of surface ligands, which passivate the nanocrystal surface and prevent degradation. This guide objectively compares the impact of A-site cation composition (Cs⁺ vs. FA⁺) on ligand binding affinity, a fundamental relationship that directly dictates the thermal degradation pathway and ultimate device longevity. Experimental data and theoretical calculations demonstrate that A-site cation engineering is a powerful strategy for modulating surface chemistry and enhancing PQD stability.

Fundamental Concepts and Key Experimental Findings

The Role of A-Site Cations and Surface Ligands

In perovskite quantum dots with the general formula ABX₃ (e.g., CsₓFA₁₋ₓPbI₃), the A-site is occupied by cations such as Cesium (Cs⁺) or Formamidinium (FA⁺), while the surface is capped by organic ligands like oleic acid (OA) and oleylamine (OLA). These ligands are crucial for stabilizing the nanocrystal colloid and passivating surface defects; however, their effectiveness is intrinsically linked to the chemical identity of the A-site cation. The strength of this interaction, quantified as the ligand binding energy, has been proven to be composition-dependent, thereby influencing critical properties such as phase stability and electron-phonon coupling [12].

Comparative Analysis of Binding Energy and Stability

The following table summarizes the core experimental findings on how A-site composition affects ligand binding and subsequent material properties.

Table 1: Impact of A-Site Cation Composition on PQD Properties

Property	Cs-Rich PQDs	FA-Rich PQDs
Ligand Binding Energy	Lower	Higher [12]
Primary Thermal Degradation Mechanism	Phase transition from black γ-phase to yellow δ-phase [12]	Direct decomposition into PbI₂ [12]
Electron-LO Phonon Coupling	Weaker	Stronger [12]
Implication for Exciton Dissociation	Lower probability of exciton dissociation by phonon scattering	Higher probability of exciton dissociation by phonon scattering [12]
Typical Experimental Phase Assignment	γ-phase (black)	α-phase (black) [12]

A seminal in-situ study constructed a detailed picture of the temperature-dependent behavior of CsₓFA₁₋ₓPbI₃ PQDs across the entire compositional range. The research established that the thermal degradation mechanism is not universal but depends critically on the A-site chemistry and the associated ligand binding energy. First-principles Density Functional Theory (DFT) calculations directly correlated these observed stability trends with ligand binding strength, showing that the bond strength of ligands to FA-rich PQD surfaces is larger than that to Cs-rich surfaces [12].

Detailed Experimental Protocols

To ensure reproducibility and provide a clear framework for comparative research, this section outlines the key methodologies used in the cited investigations.

Synthesis of CsₓFA₁₋ₓPbI₃ PQDs

Method: Hot-injection colloidal synthesis [12] [14]. Procedure:

• Precursor Preparation: PbI₂ is complexed with Oleic Acid (OA) and Oleylamine (OLA) in a non-coordinating solvent (e.g., 1-Octadecene).
• Cation Injection: A pre-heated solution of Cs-oleate (for Cs⁺) or FAI (for FA⁺) in a suitable solvent is swiftly injected into the lead precursor solution.
• Reaction Quenching: The reaction is terminated by cooling the mixture in an ice-water bath after a short reaction period (5-10 seconds).
• Purification: The crude solution is centrifuged with an anti-solvent (e.g., methyl acetate) to precipitate the PQDs. The supernatant containing excess ligands and reaction byproducts is discarded. The pellet is re-dispersed in a non-polar solvent (e.g., toluene or hexane) for further use. Key Control Parameter: The molar ratio of Cs⁺ to FA⁺ in the precursor solutions is varied to achieve the desired composition across the entire range (x = 0 to 1) [12].

In-Situ Temperature-Dependent X-ray Diffraction (XRD)

Objective: To monitor structural and phase evolution in real-time under thermal stress [12]. Protocol:

• Sample Preparation: A thin film of PQDs is deposited on a suitable substrate (e.g., a silicon wafer or a Pt heater strip).
• Measurement Setup: The sample is placed in a temperature-controlled stage or chamber under an inert atmosphere (e.g., argon flow).
• Data Acquisition: XRD patterns (e.g., 2θ range of 10°-50°) are continuously collected as the temperature is ramped from room temperature (30 °C) to high temperature (up to 500 °C).
• Data Analysis: Diffraction peaks are assigned to specific crystal phases (e.g., perovskite γ-phase, δ-phase, or PbI₂). The appearance, disappearance, and shifting of peaks are tracked to identify degradation onset temperatures and mechanisms.

Theoretical Calculation of Ligand Binding Energy

Objective: To quantitatively compute the strength of the interaction between surface ligands and the PQD surface [12] [15]. Method: First-principles Density Functional Theory (DFT) calculations. Workflow:

• Model Construction: A slab or cluster model representing the relevant surface termination (e.g., PbI₂-terminated) of the perovskite is built.
• Geometry Optimization: The atomic positions of the model and the ligand molecule are relaxed to find the most stable configuration.
• Energy Calculation: The total energy of the optimized PQD-ligand complex (Ecomplex), the isolated PQD model (EPQD), and the isolated ligand (E_ligand) are computed.
• Binding Energy Determination: The binding energy (ΔEbind) is calculated using the formula: ΔEbind = Ecomplex - (EPQD + Eligand) A more negative value of ΔEbind indicates a stronger, more favorable binding interaction.

The diagram below illustrates the logical relationship between A-site composition, ligand binding, and material stability established through these experiments.

Diagram 1: The causal relationship between A-site cation composition, ligand binding energy, and the resulting thermal degradation pathway in perovskite quantum dots.

Advanced Ligand Engineering Strategies

The pursuit of enhanced stability has moved beyond simple ion exchange to sophisticated ligand design. A prominent strategy involves developing multi-site binding ligands that form stronger, more robust connections with the PQD surface.

Multi-Site Binding for Enhanced Stability

Conventional ligands like OA and OLA typically bind through a single active site, which can lead to labile passivation. Recent research has identified complexes like Sb(SU)₂Cl₃ (antimony chloride-N,N-dimethyl selenourea) that can bind to four adjacent sites on the perovskite surface via two Se and two Cl atoms [15]. DFT calculations confirm that as the number of binding sites increases, the adsorption energy becomes more negative, indicating a stronger and more stable bond. This multi-dentate binding significantly enhances moisture resistance and overall device stability, leading to record operational lifetimes for perovskite solar cells [15].

Covalent Ligands in Non-Polar Solvents

Another advanced approach addresses the destructive side-effects of conventional ligand exchange processes, which often use polar solvents that strip away surface ions and create defects. A successful mitigation strategy employs covalent short-chain ligands like triphenylphosphine oxide (TPPO) dissolved in non-polar solvents (e.g., octane) [4]. The TPPO ligand coordinates strongly to uncoordinated Pb²⁺ sites via Lewis-base interactions, while the non-polar solvent prevents the loss of surface components. This synergetic effect simultaneously improves the optoelectronic properties and ambient stability of the resulting PQD solids [4].

Table 2: Advanced Ligand Strategies for PQD Surface Passivation

Ligand Strategy	Key Feature	Mechanism of Action	Demonstrated Outcome
Multi-Site Binding [15]	Single molecule with multiple anchoring points (e.g., 2Se + 2Cl).	Forms multiple simultaneous chemical bonds with the perovskite surface, increasing adsorption energy.	Enhanced crystallinity, suppressed defect formation, dramatically improved thermal and operational stability.
Covalent Ligands in Non-Polar Solvents [4]	Covalent ligands (e.g., TPPO) processed in non-polar solvents (e.g., octane).	Strong Lewis-acid/base interaction with undercoordinated Pb²⁺; non-polar solvent preserves surface ions.	Higher PL intensity, reduced surface trap density, improved PCE and device longevity.

The Scientist's Toolkit: Essential Research Reagents and Materials

This section catalogs key materials and reagents essential for experimental research in this field.

Table 3: Essential Reagents for PQD Synthesis and Ligand Binding Studies

Reagent/Material	Function/Application	Key Considerations
Cesium Carbonate (Cs₂CO₃)	Precursor for Cs⁺ A-site cation.	Reacted with OA to form Cs-oleate injection solution.
Formamidinium Iodide (FAI)	Precursor for FA⁺ A-site cation.	High purity required to avoid unwanted impurities affecting crystallization.
Lead Iodide (PbI₂)	Source of Pb²⁺ and I⁻ in the perovskite framework.	Must be thoroughly dried and stored in a controlled environment.
Oleic Acid (OA) & Oleylamine (OLA)	Standard long-chain surface ligands for colloidal synthesis.	Ratio and concentration control nucleation, growth, and final size of PQDs [14].
1-Octadecene (ODE)	Non-coordinating solvent for high-temperature synthesis.	Often purified to remove peroxides and other reactive species.
Methyl Acetate (MeOAc) / Ethyl Acetate (EtOAc)	Polar anti-solvents for purification and ligand exchange.	Used to precipitate PQDs and for solid-state ligand exchange [4].
Sodium Acetate (NaOAc) / Phenethylammonium Iodide (PEAI)	Ionic short-chain ligands for ligand exchange.	Replace long-chain OA/OLA to improve inter-dot charge transport [4].
Triphenylphosphine Oxide (TPPO)	Covalent short-chain ligand for surface stabilization.	Dissolved in non-polar solvents (e.g., octane) for post-treatment passivation [4].

The following diagram maps the experimental workflow from synthesis to stability assessment, integrating the reagents and strategies detailed above.

Diagram 2: A comprehensive experimental workflow for synthesizing, passivating, and characterizing the stability of perovskite quantum dots.

The experimental data unequivocally demonstrates that the A-site cation composition is a powerful lever for controlling ligand binding energy in perovskite quantum dots. FA-rich PQDs exhibit higher ligand binding energy than their Cs-rich counterparts, which directly influences their thermal degradation pathway. While FA-rich compositions benefit from stronger ligand binding, they also exhibit stronger electron-phonon coupling, which may influence charge carrier dynamics. The choice of A-site cation is therefore a multifaceted decision. Future research directions highlighted in this guide, including the use of multi-anchoring and covalent ligand systems, provide a clear roadmap for overcoming current stability limitations and advancing PQD technologies toward commercialization.

Perovskite quantum dots (PQDs) have emerged as a revolutionary class of semiconducting nanomaterials with exceptional optoelectronic properties, including narrow-band emission, high photoluminescence quantum yield (PLQY), and widely tunable bandgaps. These characteristics make them highly promising for applications in next-generation displays, photovoltaics, and bioimaging. However, their commercial deployment faces significant challenges due to inherent instability under environmental stressors. The ionic crystal structure of PQDs renders them particularly vulnerable to degradation from humidity, temperature fluctuations, and light exposure, leading to rapid deterioration of their optical and electronic properties.

This guide objectively compares the stability performance of various PQD stabilization strategies, with a specific focus on how surface ligand engineering modulates resilience against these key instability factors. The binding affinity and molecular structure of surface ligands directly influence defect formation, ion migration, and interfacial interactions—fundamental processes governing PQD degradation pathways. By examining experimental data across multiple studies, we provide researchers with a quantitative framework for evaluating stabilization approaches and selecting optimal ligand systems for specific application environments.

Comparative Analysis of PQD Stability Performance

Quantitative Stability Metrics Across Ligand Strategies

Table 1: Comparative performance of PQD stabilization strategies against environmental stressors

Stabilization Strategy	Humidity Stability	Temperature Stability	Photostability	Key Performance Metrics
Silica Coating [16]	Greatly improved	Improved	Improved	• Maximum CRI fluctuation of 2.3 over 20 days• Maximum CCT fluctuation of 7 K• LER fluctuation of 5.2 lm/Wopt
Mn-Doping + Silica Shell [16]	Enhanced	Enhanced	Enhanced	• 15-day PL spectrum stability• Dual emission stability (host and Mn-related)
Dual-Ligand (Eu(acac)₃ & Benzamide) [17]	Excellent solvent compatibility	-	High stability	• 98.56% PLQY• 69.89 ns fluorescence lifetime• PGMEA solvent compatibility
Multifaceted Anchoring Ligand (ThMAI) [18]	Greatly enhanced	Enhanced black phase stability	-	• 15.3% PCE in solar cells• 83% PCE retention after 15 days
Traditional Long-Chain Ligands (OA/OLA) [18]	Poor	Poor black phase stability	Poor	• Rapid phase transition to δ-phase• Poor charge transport

Impact of Relative Humidity on Material Systems

Table 2: Relative humidity effects on diverse material systems

Material System	RH Impact	Stability Threshold	Performance Degradation
PQDs (Unprotected)	Severe degradation	Low RH environments	• PLQY reduction• Structural decomposition
Amorphous Solid Dispersions [19]	Variable by polymer	• HPMCAS: Most stable at high RH• PVP: Most affected at high RH	• RH significantly reduces kinetic stabilization• API solubility decreases with RH for NAP
Dry Eye Disease [20]	Clinical correlation	Lower RH increases risk	• Reduced RH increases relative risk of DED outpatient visits

Experimental Protocols and Methodologies

Dual-Ligand Synergistic Passivation Engineering

The DLSPE strategy represents a sophisticated approach for simultaneously addressing bulk and interfacial defects in CsPbBr₃ PQDs [17]. The protocol involves:

• PQD Synthesis: CsPbBr₃ QDs are synthesized via hot-injection method. Cs₂CO₃ (0.3258 g, 1 mmol) and 10 mL of octanoic acid (OTAc) are loaded into a 20 mL vial and stirred at room temperature for 10 minutes to prepare the Cs precursor.
• Europium Doping: A PbBr₂ precursor solution is prepared by dissolving PbBr₂ (1 mmol), tetraoctylammonium bromide (TOAB, 2 mmol), and varying amounts of europium acetylacetonate (Eu(acac)₃ (0, 0.1, 0.2, 0.3, 0.4 mmol) in 10 mL of toluene.
• Ligand Exchange: Benzamide solution in toluene is introduced for surface ligand exchange, introducing short-chain ligands with electron-rich amide groups that coordinate with Br⁻ surface sites.
• Characterization: HR-XRD, FT-IR, XPS, and TEM analyses confirm successful incorporation of Eu(acac)₃ and benzamide, demonstrating defect suppression and structural modulation.

Multifaceted Anchoring Ligands for Phase Stability

The ThMAI ligand exchange process addresses both conductivity and stability challenges in CsPbI₃ PQD solar cells [18]:

• PQD Synthesis: CsPbI₃ PQDs stabilized with oleic acid (OA) and oleylamine (OLA) are synthesized by hot injection method, confirmed by TEM analysis to exhibit black phase with average size of 11 nm.
• Ligand Exchange: A solution of 2-thiophenemethylammonium iodide (ThMAI) in acetonitrile is used for ligand exchange, replacing initial long-chain ligands.
• Film Fabrication: PQD thin films are fabricated via layer-by-layer spin-coating with ThMAI treatment between layers.
• Device Characterization: J-V measurements, EQE spectra, electrochemical impedance spectroscopy, and GIWAXS are employed to analyze photovoltaic performance, charge transport, and crystal structure.

Stability Testing Protocols

Standardized stability assessment methodologies enable direct comparison between different stabilization approaches:

• Environmental Testing: PQD films and devices are subjected to controlled environmental conditions: 25°C/0% RH, 25°C/60% RH, and 40°C/75% RH [19].
• Temporal Monitoring: Long-term stability is evaluated through 15-20 day observations with regular measurements of PL spectrum, CRI, CCT, and efficiency parameters [16].
• Accelerated Aging: Devices are operated under continuous illumination or elevated temperatures to simulate extended operational lifetimes.

Mechanisms of Action and Signaling Pathways

Ligand-Mediated Stabilization Pathways

Diagram 1: Ligand-mediated stabilization pathways against environmental stressors

Dual-Ligand Defect Passivation Mechanism

Diagram 2: Dual-ligand defect passivation mechanism

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key research reagents for PQD stability studies

Reagent/Chemical	Function in Research	Application Context
Tetramethoxysilane (TMOS)	Silica shell precursor for encapsulation	Creates protective barrier against humidity [16]
Europium Acetylacetonate (Eu(acac)₃)	Trivalent dopant for bulk defect passivation	Compensates Pb²⁺ vacancies in DLSPE strategy [17]
Benzamide	Short-chain surface ligand	Passivates surface defects via amide coordination [17]
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchoring ligand	Enhances phase stability and charge transport [18]
Manganese Chloride (MnCl₂)	Transition metal dopant	Creates dual emission and improves structural stability [16]
Oleic Acid/Oleylamine	Traditional long-chain ligands	Initial stabilization but limit charge transport [18]
Propylene Glycol Monomethyl Ether Acetate (PGMEA)	Polar solvent for photolithography	Tests solvent compatibility in patterning processes [17]

The strategic engineering of surface ligand binding affinity presents a powerful approach for mitigating the key instability factors affecting perovskite quantum dots—humidity, temperature, and light exposure. Experimental data demonstrate that advanced ligand strategies, particularly dual-ligand systems and multifaceted anchoring ligands, significantly outperform traditional stabilization methods across quantitative metrics including PLQY retention, phase stability, and environmental resilience.

The integration of computational design with experimental validation has enabled rational ligand development targeting specific degradation pathways. As research progresses, the continued refinement of ligand architectures promises to bridge the gap between laboratory demonstration and commercial deployment of PQD-based technologies across display, energy, and semiconductor applications.

Advanced Ligand Engineering Strategies for Enhanced Performance

The stability and optoelectronic performance of perovskite quantum dots (PQDs) are critically dependent on their surface chemistry. Ligands—molecules bound to the PQD surface—play a dual role: they passivate surface defects to enhance photoluminescence and influence charge transport between neighboring dots [9]. However, conventional ligands often bind through a single functional group, leading to dynamic and labile attachment that compromises stability [15] [9]. Multifaceted anchoring ligands represent a transformative strategy, employing multiple functional groups that bind coordinatively to different surface sites simultaneously. This approach creates a more robust and stable surface passivation, significantly improving the performance and longevity of PQD-based devices such as solar cells and light-emitting diodes [18] [15].

Comparative Analysis of Multifaceted Anchoring Ligands

The following section objectively compares the performance of several advanced multifaceted ligands against conventional alternatives, with data summarized in Table 1 and binding mechanisms detailed in Table 2.

Table 1: Performance Comparison of Multifaceted Anchoring Ligands in Perovskite Optoelectronics

Ligand Name	Material/ System	Key Functional Groups	Primary Binding Mechanism	Reported Power Conversion Efficiency (PCE)	Key Stability Outcomes
2-Thiophenemethylammonium Iodide (ThMAI)	CsPbI₃ PQD Solar Cells [18]	Thiophene, Ammonium [18]	Multifaceted Anchoring [18]	15.3% [18]	Retained 83% of initial PCE after 15 days in ambient conditions [18]
Sb(SU)₂Cl₃ Complex	Fully Air-Processed Perovskite Solar Cells [15]	Selenourea (Se), Chloride (Cl) [15]	Multi-site (Quadruple) Binding [15]	25.03% [15]	Projected T₈₀ shelf lifetime of 23,325 hours; T₈₀ of 5,004 h at 85°C [15]
Triphenylphosphine Oxide (TPPO)	CsPbI₃ PQD Solar Cells [4]	Phosphine Oxide (P=O) [4]	Covalent L-type Binding [4]	15.4% [4]	Maintained >90% of initial efficiency after 18 days in ambient conditions [4]
Conventional Ionic Short-Chain (e.g., Acetate, PEA⁺)	CsPbI₃ PQD Solar Cells [18] [4]	Carboxylate, Ammonium [18] [4]	Ionic / Labile Binding [18] [4]	13.6% (Control) [18]	Retained only ~8.7% of initial PCE after 15 days [18]

Table 2: Binding Mechanism and Function of Multifaceted Ligands

Ligand	Binding Target on PQD Surface	Theoretical/Measured Binding Affinity	Secondary Function
ThMAI [18]	Thiophene to uncoordinated Pb²⁺; Ammonium to Cs⁺ vacancies [18]	Strong binding energy due to reinforced dipole moment [18]	Larger ionic size of ThMA⁺ restores beneficial surface tensile strain, stabilizing the black phase [18]
Sb(SU)₂Cl₃ [15]	Two Se and two Cl atoms to four adjacent undercoordinated Pb²⁺ sites [15]	Stronger charge transfer and more stable adsorption energy with more binding sites [15]	Forms an extended hydrogen-bonding network (NH...Cl) and fills Iodine vacancies with its Cl atoms [15]
TPPO [4]	Phosphine oxide group to uncoordinated Pb²⁺ sites [4]	Strong Lewis-base interaction (covalent) [4]	Nonpolar solvent (octane) enables nondestructive application, preserving PQD surface components [4]

The data reveals a consistent trend: ligands capable of multi-site, coordinative binding universally outperform conventional ionic ligands. ThMAI's dual-group approach provides effective defect passivation and enhances phase stability [18]. The Sb(SU)₂Cl₃ complex represents the pinnacle of this strategy, with its quadruple-site binding leading to record-breaking stability metrics for air-processed devices [15]. Similarly, TPPO's strong, covalent Lewis-acid-base interaction effectively suppresses surface traps despite having a single primary binding group, especially when applied with a non-destructive solvent [4].

Experimental Protocols for Ligand Application and Analysis

Reproducible experimental protocols are fundamental for comparing ligand efficacy. Below are detailed methodologies for applying and characterizing multifaceted ligands, with the workflow for ThMAI-treated PQD solar cells visualized in Diagram 1.

Synthesis and Ligand Exchange Procedure for CsPbI₃ PQDs

• PQD Synthesis: CsPbI₃ PQDs are typically synthesized via the hot-injection method [18] [4]. A lead iodide (PbI₂) precursor is dissolved in a high-boiling-point solvent (e.g., 1-octadecene) with coordinating long-chain ligands, typically oleic acid (OA) and oleylamine (OLA), at elevated temperatures (e.g., 150-180 °C) under inert atmosphere. A cesium precursor (e.g., Cs₂CO₃-oleate) is then swiftly injected to initiate nucleation and growth of monodisperse PQDs [18] [4].
• Conventional Ligand Exchange (Two-Step): The insulating OA and OLA ligands must be replaced with shorter, conductive ligands to fabricate solid films.
  • Anionic Ligand Exchange: The pristine PQD solution is spin-coated onto a substrate, followed by treatment with a solution containing short anionic ligands (e.g., acetate ions from sodium acetate) dissolved in a polar solvent like methyl acetate (MeOAc). This step replaces OA ligands [4].
  • Cationic Ligand Exchange: The film is subsequently treated with a solution of short cationic ligands (e.g., phenethylammonium iodide, PEA⁺) dissolved in another polar solvent like ethyl acetate (EtOAc). This step replaces OLA ligands [4]. This two-step process is repeated in a layer-by-layer fashion to build the desired film thickness.
• Multifaceted Ligand Treatment: The innovative ligands are often applied as a post-synthesis treatment on the ligand-exchanged PQD solid.
  • For ThMAI, the ligand is dissolved in a solvent like chlorobenzene or a hexane/octane mixture and dynamically spin-coated onto the fabricated PQD film [18] [21].
  • For TPPO, the ligand is dissolved in a nonpolar solvent (n-octane) and applied to the ligand-exchanged PQD solids. The use of a nonpolar solvent is crucial to prevent the dissolution or degradation of the PQD surface components that can occur with polar solvents [4].

Characterization Techniques for Binding Affinity and Surface Analysis

• Fourier-Transform Infrared (FT-IR) Spectroscopy: This technique verifies the successful replacement of ligands by tracking the disappearance of characteristic vibrational peaks of oleyl chains (e.g., C-H stretches) and the emergence of peaks corresponding to the new ligands [4].
• Photoluminescence (PL) Spectroscopy: This is a key metric for assessing defect passivation. A significant increase in PL intensity and carrier lifetime after treatment with a multifaceted ligand indicates successful reduction of non-radiative recombination centers (surface traps) [18] [4].
• X-ray Diffraction (XRD): Used to analyze the crystal structure and phase stability of the PQDs. Ligands like ThMAI that impart tensile strain can help stabilize the metastable black perovskite phase (α-, β-, γ-phase) against transformation into a non-perovskite yellow phase (δ-phase) [18] [9].
• Density Functional Theory (DFT) Calculations: Theoretical modeling is indispensable for understanding the binding mechanism at the atomic level. DFT can calculate adsorption energies of different ligand configurations, map charge transfer, and determine the most stable binding modes, such as the quadruple-site binding of Sb(SU)₂Cl₃ [15].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Multifaceted Ligand Research

Reagent/Material	Function/Application	Example in Context
2-Thiophenemethylammonium Iodide (ThMAI) [18]	Multifaceted anchoring ligand for defect passivation and strain engineering in PQD films.	Primary ligand in Seo et al. study for enhancing CsPbI₃ PQD solar cell performance and stability [18] [21].
Antimony Chloride-N,N-dimethyl Selenourea Complex (Sb(SU)₂Cl₃) [15]	Multi-site (quadruple) binding passivator for bulk perovskite films, suppressing defects.	Key additive in fully air-processed perovskite solar cells, achieving high efficiency and record stability [15].
Triphenylphosphine Oxide (TPPO) [4]	Covalent short-chain ligand for passivating uncoordinated Pb²⁺ sites via Lewis-base interaction.	Used as a post-treatment on ligand-exchanged CsPbI₃ PQD solids, dissolved in nonpolar octane [4].
Oleic Acid (OA) & Oleylamine (OLA) [18] [4] [9]	Standard long-chain ligands for the initial synthesis and stabilization of colloidal PQDs.	Universally used in the hot-injection synthesis of CsPbI₃ PQDs; later replaced via ligand exchange [18] [4].
Phenethylammonium Iodide (PEAI) [4]	Conventional short-chain cationic ligand used in standard ligand exchange procedures.	Used in the control group and as part of the initial ligand exchange before advanced ligand treatment [4].
Methyl Acetate (MeOAc) / Ethyl Acetate (EtOAc) [4]	Polar solvents used in the conventional two-step ligand exchange process.	Employed to dissolve ionic short-chain ligands (e.g., acetate, PEAI) and wash away long-chain ligands [4].
n-Octane [4]	Nonpolar solvent for dissolving covalent ligands in post-synthesis treatments.	Used to dissolve TPPO for a nondestructive surface treatment that preserves PQD surface components [4].

The strategic development of multifaceted anchoring ligands marks a significant leap forward in perovskite material science. By moving beyond single-site, labile binding to multi-functional, coordinative anchoring, researchers can simultaneously address the critical challenges of surface defect passivation, inter-dot charge transport, and phase stability. As demonstrated by ThMAI, TPPO, and the sophisticated Sb(SU)₂Cl₃ complex, this approach enables devices that combine high performance with exceptional durability, a crucial combination for commercial applications. Future research will likely focus on designing even more complex ligands with tailored binding groups, exploring lead-free perovskite systems, and scaling up synthesis and application protocols for industrial manufacturing.

In the pursuit of high-performance perovskite quantum dot (PQD) optoelectronics, surface ligand engineering has emerged as a critical frontier. The central challenge lies in overcoming the inherent trade-off between surface passivation for stability and efficient charge transport for device performance. Long-chain insulating ligands used in synthesis provide excellent colloidal stability but severely hinder inter-dot charge transport, while short-chain ligands often suffer from incomplete coverage and weak binding. Within this landscape, bidentate anchoring ligands represent a transformative strategy, simultaneously addressing the dual requirements of robust surface passivation and enhanced electrical conductivity through their unique coordination geometry and strong binding affinity.

This review objectively compares the performance of emerging bidentate and multifaceted ligand systems, focusing on quantitative metrics critical for PQD device advancement. We examine experimental data on binding energies, conductivity enhancement, and device performance, providing researchers with a structured comparison of ligand strategies that are pushing the boundaries of PQD applications in photovoltaics and light-emitting devices.

Experimental Approaches in Ligand Performance Evaluation

Evaluating ligand performance requires a multifaceted experimental approach to quantify both binding strength and its impact on material properties. Standard protocols include:

Binding Energy Calculation via Density Functional Theory (DFT)

• Protocol: First-principles DFT calculations are performed on cluster or slab models of the PQD surface. The binding energy (E_b) is calculated as E_b = E_total - E_surface - E_ligand, where E_total is the energy of the surface-ligand complex, E_surface is the energy of the pristine surface, and E_ligand is the energy of the isolated ligand. More negative values indicate stronger, more favorable binding [22] [23].

Photoluminescence Quantum Yield (PLQY) Measurement

• Protocol: PQD films are excited at their absorption band edge using a monochromatic light source. The integrated intensity of the emitted photons is compared against a calibrated reference standard using an integrating sphere. PLQY = (number of photons emitted / number of photons absorbed) × 100%. Higher PLQY indicates superior passivation of non-radiative recombination centers [22] [23].

Two-Terminal Device Conductivity Measurement

• Protocol: Ligand-exchanged PQD films are deposited onto substrates with pre-patterned electrodes. Current-voltage (I-V) characteristics are measured under dark conditions. Electrical conductivity (σ) is calculated from the slope of the I-V curve, factoring in film geometry. This directly quantifies improved inter-dot charge transport after ligand exchange [22] [23].

The experimental workflow below illustrates how these characterization techniques are integrated to evaluate ligand performance from molecular binding to final device functionality.

Figure 1: Experimental workflow for evaluating ligand performance in PQD applications

Comparative Performance of Ligand Systems

Bidentate Ligands for Near-Infrared Perovskite LEDs

The application of formamidine thiocyanate (FASCN) as a bidentate liquid ligand demonstrates remarkable improvements in near-infrared PQD-LEDs. The comparative data reveals its superior performance against conventional ligands.

Table 1: Performance comparison of FASCN versus conventional ligands in FAPbI₃ PQDs

Parameter	Oleate (OA)	Oleylammonium (OAm)	FAI	MAI	FASCN
Binding Energy (eV)	-0.22	-0.18	-0.31	-0.30	-0.91
Relative Binding	1×	1×	1.4×	1.4×	4.1×
Film Conductivity (S/m)	Baseline	Baseline	-	-	8× improvement
Exciton Binding Energy (meV)	39.1	39.1	-	-	76.3
LED External Quantum Efficiency	~11.5%	~11.5%	-	-	~23%
Turn-on Voltage at 776 nm	-	-	-	-	1.6 V

FASCN's bidentate coordination through soft sulfur and nitrogen atoms enables fourfold higher binding energy than oleate ligands and threefold higher than formamidine iodide (FAI) and methylammonium iodide (MAI) [22] [23]. This tight binding prevents ligand desorption during film preparation, eliminating interfacial quenching centers. The short carbon chain (<3 atoms) enables eightfold higher film conductivity compared to control samples, while the liquid characteristics of FASCN avoid the need for high-polarity solvents that could damage PQD surfaces [22] [23].

Multifaceted Anchoring Ligands for Perovskite Quantum Dot Photovoltaics

In photovoltaic applications, the ligand design strategy expands to include multifaceted anchoring groups that simultaneously address multiple surface defects.

Table 2: Performance comparison of anchoring ligands in CsPbI₃ PQD solar cells

Ligand	Anchor Groups	Binding Affinity	PCE (%)	Stability (Initial PCE Retained)	Key Advantages
ThMAI	Thiophene + Ammonium	High (Dipole-enhanced)	15.3%	83% after 15 days	Multifaceted anchoring, tensile strain restoration
TPPO	Phosphine Oxide	Strong (Covalent/Lewis base)	15.4%	>90% after 18 days	Nonpolar solvent compatibility, strong Pb²⁺ coordination
Conventional Ionic Short-chain	Single ammonium/carboxylate	Labile/Weak	13.6%	<9% after 15 days	Baseline for comparison

The electron-rich thiophene ring in ThMAI acts as a Lewis base that strongly binds to uncoordinated Pb²⁺ sites, while the ammonium segment efficiently occupies cationic Cs⁺ vacancies [18]. This multifaceted anchoring, reinforced by charge separation and strong dipole moment, enables effective defect passivation and uniform PQD ordering. The larger ionic size of ThMA⁺ compared to Cs⁺ helps restore surface tensile strain, enhancing black-phase stability [18].

Triphenylphosphine oxide (TPPO) employs a different strategy, using covalent short-chain ligands dissolved in nonpolar solvents to avoid damaging the ionic PQD surface [4]. The TPPO ligand strongly coordinates with uncoordinated Pb²⁺ sites via Lewis acid-base interactions, while the nonpolar solvent octane completely preserves PQD surface components. This approach yields a PCE of 15.4% with excellent ambient stability [4].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key reagents for bidentate ligand research in perovskite quantum dots

Reagent	Function/Application	Key Characteristics
Formamidine Thiocyanate (FASCN)	Bidentate liquid ligand for NIR PQD-LEDs	Short chain (<3C), liquid state, S/N coordination
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchor for PQD photovoltaics	Thiophene + ammonium groups, large ionic size
Triphenylphosphine Oxide (TPPO)	Covalent ligand for surface stabilization	Lewis base, nonpolar solvent compatibility
Oleic Acid (OA) / Oleylamine (OLA)	Standard long-chain ligands for PQD synthesis	Provides initial stability, requires replacement
Methyl Acetate (MeOAc) / Ethyl Acetate (EtOAc)	Polar solvents for conventional ligand exchange	Removes long-chain ligands, can damage PQD surface
Octane	Nonpolar solvent for nondestructive ligand treatment	Preserves PQD surface components during treatment
Phenethylammonium Iodide (PEAI)	Short cationic ligand for comparison studies	Conventional short-chain ligand, ionic nature

Molecular Coordination Mechanisms

The superior performance of bidentate ligands stems from their fundamental coordination chemistry with the PQD surface. The diagram below illustrates how these ligands achieve enhanced surface coverage and stability compared to conventional monodentate ligands.

Figure 2: Molecular coordination mechanisms of ligand classes on PQD surfaces

Bidentate ligands like FASCN form chelate complexes with surface metal atoms, achieving significantly higher binding energies (-0.91 eV) compared to monodentate ligands (-0.18 to -0.31 eV) [22] [23]. This chelate effect dramatically reduces ligand desorption during processing. Multifaceted anchors like ThMAI further enhance this approach through dipole-enhanced binding, where charge separation between electron-rich thiophene and electron-deficient ammonium groups creates stronger surface adhesion [18].

The strategic implementation of bidentate and multifaceted anchoring ligands represents a paradigm shift in perovskite quantum dot surface engineering. The experimental data clearly demonstrates that these advanced ligand architectures simultaneously address the historical challenges of surface passivation and charge transport that have limited PQD device performance.

Through strong chelation, short conductive chains, and multifunctional anchoring, these ligands enable record device efficiencies—achieving ~23% EQE in NIR-LEDs and over 15% PCE in photovoltaics—while significantly enhancing operational stability. The fundamental coordination chemistry of these systems provides a versatile foundation for further innovation, offering researchers a expanding toolkit to tailor PQD surfaces for specific applications. As the field progresses, the continued refinement of bidentate ligand design promises to unlock the full commercial potential of perovskite quantum dot technologies.

In the evolving landscape of nanoscience and drug development, complementary dual-ligand systems represent an advanced paradigm where two distinct ligand molecules work synergistically to enhance material performance and functionality. Unlike single-ligand approaches that often address stability or binding affinity in isolation, dual-ligand systems integrate complementary functionalities that collectively overcome individual limitations. This strategy is particularly valuable in perovskite quantum dot (PQD) stabilization, where environmental vulnerability has historically constrained practical application. By engineering ligand pairs with cooperative interactions, researchers can create robust nanostructures that maintain optical excellence while withstanding harsh operational conditions.

The fundamental thesis governing this approach posits that strategic ligand pairing can produce emergent properties—benefits that transcend the simple summation of individual ligand contributions. These systems typically combine one ligand providing strong surface attachment with another offering steric protection or additional functional capabilities. This guide examines the comparative performance of leading dual-ligand and encapsulation strategies, providing experimental data and methodologies to inform research directions in PQD stabilization and surface ligand binding affinity optimization.

Comparative Analysis of Dual-Ligand and Encapsulation Strategies

The pursuit of PQD stability has yielded various strategic approaches, from surface ligand engineering to complete nanostructure encapsulation. The table below systematically compares the performance characteristics of two prominent strategies documented in recent literature.

Table 1: Performance Comparison of PQD Stabilization Strategies

Strategy & Materials	Stability Performance	Optical Properties	Key Advantages	Experimental Affinity Metrics
Polydimethylsiloxane (PDMS) Encapsulation [24]	Maintains 99.8% PL intensity after 2 hours water immersion	Amplified spontaneous emission (ASE) with ultralow threshold of 1.72 μJ cm−2; Emission intensity 10× stronger than conventional PL	Waterproof protection; Ultrahigh-speed monitoring capability (108 fps)	Superior linearity (R² = 0.999) for concentration quantification (0–3.5 μM tartrazine)
Metal-Organic Framework (UiO-66) Encapsulation [25]	Luminescence maintenance over 30 months ambient; Several hours underwater	Strong exciton-polariton coupling; Anti-crossing behavior in dispersion curves; Identifiable lattice fringes (0.58 nm) = (100) plane of CsPbBr₃	Long-term environmental stability; Enhanced exciton-phonon interaction	BET surface area decreases from 1,510 m²/g (UiO-66) to 320 m²/g (PQD@UiO-66) confirms pore filling

The experimental data reveal that both encapsulation strategies successfully address PQD instability through distinct mechanisms. The PDMS encapsulation creates a protective barrier that enables exceptional retention of photoluminescence (99.8%) in aqueous environments while enhancing optical gain properties [24]. Conversely, the UiO-66 framework provides nanoscale confinement within its porous structure, offering remarkable long-term stability over 30 months while maintaining strong light-matter interactions [25]. These encapsulation approaches differ fundamentally from traditional surface ligand binding by creating physical barriers that shield PQDs from environmental degradants while preserving—and sometimes enhancing—their innate optical properties.

Experimental Protocols for Dual-Ligand System Validation

PDMS Encapsulation Methodology for PQDs

The PDMS encapsulation process follows a sequential fabrication approach that prioritizes interfacial compatibility between the quantum dots and polymer matrix [24]:

• PQD Synthesis Preparation: Synthesize CsPbBr₃ PQDs using standard hot-injection methods, with precise control over precursor ratios and reaction temperatures to achieve uniform size distribution.

• Surface Ligand Treatment: Implement a dual-ligand surface engineering step using oleic acid and oleylamine to ensure optimal dispersion compatibility with the PDMS matrix.

• PDMS Matrix Formation: Prepare a transparent PDMS precursor by thoroughly mixing silicone elastomer base and curing agent in a 10:1 ratio, followed by degassing under vacuum to remove entrapped air bubbles.

• PQD-PDMS Integration: Combine PQD solution with PDMS precursor using gradual titration (approximately 1:4 volume ratio) with continuous mechanical stirring to ensure homogeneous distribution without aggregation.

• Curing Protocol: Cure the composite material thermally at 65°C for 4 hours, followed by post-curing at 85°C for 2 hours to achieve optimal cross-linking density without damaging the PQDs.

• Film Fabrication: For sensor applications, spin-cast the uncured composite onto glass substrates at 2000 rpm for 30 seconds before implementing the curing protocol to create uniform thin films.

The critical validation step involves water immersion testing, where encapsulated films are submerged in deionized water while monitoring photoluminescence intensity at regular intervals using a fluorescence spectrometer. The exceptional stability (99.8% PL retention after 2 hours) confirms effective encapsulation [24].

MOF Encapsulation via Self-Limiting Solvothermal Deposition

The UiO-66 encapsulation employs a confinement-based stabilization strategy through a multi-step process [25]:

• UiO-66 Matrix Synthesis: Prepare UiO-66 powder with missing-linker defects by combining zirconium chloride and terephthalic acid in N,N-dimethylformamide with acetic acid as modulators, followed by solvothermal reaction at 120°C for 24 hours.

• Activation and Purification: Activate the synthesized UiO-66 by solvent exchange with methanol and subsequent thermal activation under vacuum at 150°C for 12 hours.

• Lead Ion Functionalization: Create Pb-UiO-66 through self-limiting solvothermal deposition (SIM method) where Pb²⁺ ions coordinate on hexa-zirconium nodes of the MOF, forming metal-oxygen bonds between guest metal ions and the cluster.

• Perovskite Crystallization: Introduce CsBr precursor solution to the Pb-UiO-66 powder mixture, initiating reaction that breaks Pb-O bonds and generates CsPbBr₃ QDs within the MOF pores.

• Purification and Characterization: Remove excess precursors through repeated centrifugation and washing cycles, followed by characterization through XRD, TEM, and BET surface area analysis.

Validation of successful encapsulation includes BET surface area analysis showing reduction from 1,510 m²/g (pristine UiO-66) to 320 m²/g (PQD@UiO-66), confirming pore filling with perovskite QDs [25]. Additional confirmation comes from TEM imaging showing lattice fringes with 0.58 nm spacing corresponding to the (100) plane of CsPbBr₃.

Analytical Techniques for Binding Affinity Assessment

Binding Affinity Measurement Methods

Quantifying ligand interactions remains fundamental to understanding dual-ligand system efficacy. The table below compares established techniques for evaluating binding affinity.

Table 2: Analytical Methods for Binding Affinity Assessment

Method	Working Principle	Sample Requirements	Affinity Range	Key Applications in Ligand Systems
Microscale Thermophoresis (MST) [26]	Fluorescence variation measurement in response to temperature gradients	Minimal (10 μL volume); nM target concentration; One fluorescent partner	pM-mM	Ligand/receptor binding in native membranes; D2R/spiperone-Cy5 affinity (5.3 ± 1.7 nM)
Native Mass Spectrometry [27]	Gentle ionization to transfer folded proteins and intact complexes to gas phase	Low consumption; Can analyze complex mixtures without purification	Varies by system	Protein-ligand binding affinity from tissue samples; Kd determination without protein concentration knowledge
Equilibrium Dialysis [28]	Physical separation of bound and free ligands through semi-permeable membrane	Requires precise concentration knowledge; Controlled buffer conditions	nM-mM	Fundamental binding constant determination; RNA-protein interactions (Puf4)
Electrochemical Analysis (SWV) [29]	Current changes from target molecule adsorption on electrode surface	Peptide-modified gold electrodes; Buffer solution with target	nM range	Peptide-viral protein interactions; Kd of 70.02 nM for HA BP2 peptide

Experimental Considerations for Reliable Binding Measurements

Recent methodological evaluations highlight critical factors often overlooked in binding studies. A survey of 100 binding studies revealed that 70% failed to report varying incubation time to establish equilibration, while approximately 25% risked titration artifacts [28]. To ensure measurement reliability:

• Establish Equilibration Time: Determine that fraction of bound complex remains constant over time, with most exponential binding processes reaching ~97% completion after five half-lives [28].

• Avoid Titration Regime: Ensure the limiting component concentration remains sufficiently low relative to the dissociation constant (KD), with systematic concentration variation confirming absence of titration artifacts [28].

• Account for Active Concentration: Determine the fraction of functional protein or ligand, as impurities or inactive material can significantly distort apparent affinity measurements [28].

For membrane proteins like GPCRs, recent MST advancements enable binding affinity determination directly from cell membrane fragments without purification, overcoming historical challenges with solubilization effects on protein functionality [26].

Mechanism Visualization: Dual-Ligand Stabilization Approaches

Diagram 1: Dual-Ligand Stabilization Mechanisms and Functional Enhancement Pathways

This schematic illustrates how complementary stabilization strategies address specific PQD instability challenges while enabling enhanced functionality. The encapsulation approaches create physical barriers that prevent environmental degradants from reaching the quantum dots while potentially enhancing optical properties through improved charge confinement and interface engineering.

Research Toolkit: Essential Materials and Reagents

Table 3: Essential Research Reagents for Dual-Ligand System Development

Reagent/Material	Specifications	Research Function	Exemplary Applications
Polydimethylsiloxane (PDMS)	Silicone elastomer kit (base & curing agent); Optical grade transparency	Protective encapsulation matrix; Enhanced light extraction	Waterproof PQD films for aqueous environment sensing [24]
UiO-66 MOF	Zr₆(μ₃-O)₄(μ₃-OH)₄(BDC)₆ with missing-linker defects; BET ~1,510 m²/g	Nanoscale confinement template; Pore size ~1-2 nm	Spatial confinement of CsPbBr₃ QDs; Long-term stabilization [25]
Oleic Acid/Oleylamine	Technical grade, 90%; Purified by degassing	Surface ligand pair for PQD synthesis; Coordination bonding	Primary surface stabilization during PQD synthesis [24]
CsPbBr₃ Precursors	Cesium carbonate (99.9%), Lead bromide (99.99%), Oleic acid (90%)	Perovskite quantum dot synthesis	High-purity PQD preparation with controlled stoichiometry [24] [25]
Spiperone-Cy5	Cy5-labeled antagonist; ≥95% purity	Fluorescent tracer for binding studies	MST-based binding affinity determination [26]
Phage Display Library	M13 peptide library (Ph.D.-12 or C7C)	High-affinity peptide screening	Identification of target-specific binding peptides [29]

Complementary dual-ligand and encapsulation strategies represent a transformative approach to overcoming the historical stability limitations of perovskite quantum dots. The experimental data demonstrate that both PDMS and UiO-66 encapsulation enable not only remarkable stability improvements but also functional enhancement of optical properties. The PDMS approach offers exceptional aqueous protection with 99.8% photoluminescence retention after 2 hours water immersion, while the UiO-66 framework provides unprecedented long-term stability over 30 months [24] [25].

The future development of dual-ligand systems will likely incorporate computational approaches like chemical language models for de novo design of multi-target ligands and hierarchical interactive learning to predict binding affinities with greater accuracy [30] [31]. As these computational and experimental methodologies converge, researchers will increasingly design ligand systems with precisely tailored cooperativity, unlocking new applications for PQDs in sensing, optoelectronics, and quantum technologies that demand both exceptional performance and environmental resilience.

The pursuit of high-performance perovskite quantum dot (PQD) optoelectronics necessitates a delicate balance between two competing demands: achieving exceptional electronic coupling between quantum dots for efficient charge transport and maintaining robust surface passivation for structural and photoluminescent stability. Traditional long-chain insulating ligands (e.g., oleic acid and oleylamine), essential for synthesizing high-quality colloids, create significant charge transport barriers in solid films, severely limiting device performance. Short-chain conductive ligands have emerged as a transformative solution, directly addressing this intrinsic conflict. This guide provides a comparative analysis of advanced short-chain ligand strategies, evaluating their effectiveness in modulating surface chemistry, enhancing optoelectronic properties, and ultimately improving the performance and stability of PQD-based devices. By systematically examining experimental data and methodologies, we aim to equip researchers with the knowledge to select and implement optimal ligand engineering approaches for their specific applications.

Comparative Performance of Ligand Strategies

The development of short-chain ligands has progressed along several innovative pathways, including conjugated molecular designs, multifaceted anchoring groups, and covalent binding schemes. The table below summarizes the performance of different ligand classes in key applications.

Table 1: Performance Comparison of Different Short-Chain Ligand Strategies

Ligand Class & Example	Device Type	Key Performance Metrics	Stability Findings	Citation
Conjugated Ligands (e.g., 4-CH3 PPABr)	Green QLED	- Peak EQE: 23.88% (with light extraction) - 1.67x EQE improvement vs. control	- Enhanced carrier mobility via π-π stacking - Reduced surface defect densities	[32]
Multifaceted Anchoring Ligands (e.g., ThMAI)	CsPbI3 PQD Solar Cell	- PCE: 15.3% - Control PCE: 13.6%	- Maintained 83% of initial PCE after 15 days (Control: 8.7%) - Improved cubic-phase stability	[18]
Conjugated Polymer Ligands (e.g., Th-BDT)	CsPbI3 PQD Solar Cell	- PCE: >15% - Control PCE: 12.7% - Enhanced Jsc and FF	- Retained >85% initial efficiency after 850 hours	[33]
Alkali-Augmented Hydrolysis (MeBz with KOH)	FA0.47Cs0.53PbI3 PQD Solar Cell	- Certified PCE: 18.3% - Steady-state PCE: 17.85%	- Improved storage and operational stability	[34]
Covalent Ligands in Nonpolar Solvents (TPPO in Octane)	CsPbI3 PQD Solar Cell	- PCE: 15.4%	- Maintained >90% initial efficiency after 18 days in ambient conditions	[4]

Experimental Protocols for Ligand Implementation

A critical understanding of ligand performance requires a detailed examination of the experimental methods used to integrate them into PQD solids. The following section outlines standardized protocols for the most effective ligand exchange and post-treatment strategies.

Solid-State Ligand Exchange for Multifaceted Anchoring

This protocol, adapted for ligands like ThMAI, focuses on replacing pristine long-chain ligands after PQD film deposition [18].

• PQD Film Deposition: Spin-coat a layer of synthesized CsPbI3 PQDs (capped with OA/OAm) onto the target substrate.
• Ligand Solution Preparation: Prepare a solution of the short-chain ligand (e.g., 2-thiophenemethylammonium iodide, ThMAI) in a suitable solvent like ethyl acetate at a specified concentration (e.g., 1.0 mg mL⁻¹).
• Interlayer Rinsing: While the PQD film is still wet, dynamically rinse it with the ligand solution. This step displaces the weakly bound long-chain ligands.
• Spin and Repeat: Spin-dry the film to remove the solvent and by-products. Repeat steps 1-4 to build up the desired film thickness in a layer-by-layer fashion.
• Post-Treatment Annealing: Anneal the final film on a hotplate at a mild temperature (e.g., 70°C for 5 minutes) to remove residual solvent and improve inter-dot coupling.

Conjugated Ligand Post-Treatment

This method, used for ligands like 4-CH3 PPABr, involves treating already ligand-exchanged PQD films to enhance transport and passivation [32].

• Base PQD Film Preparation: Fabricate a control PQD solid film using a standard ligand exchange process (e.g., with ThPABr).
• Post-Treatment Solution Preparation: Dissolve the conjugated ligand (e.g., 4-CH3 PPABr) in isopropyl alcohol (IPA) at an optimized concentration.
• Spin-Coating Treatment: Spin-coat the conjugated ligand solution directly onto the pre-formed, dry PQD solid film.
• Final Annealing: Anneal the film at a specific temperature (e.g., 70°C for 1 minute) to facilitate strong binding to the PQD surface without degrading the material.

Alkali-Augmented Antisolvent Hydrolysis (AAAH)

This advanced protocol enhances the conventional ester rinsing process by creating an alkaline environment to boost the hydrolysis of ester-based antisolvents, producing a higher density of conductive capping ligands [34].

• PQD Film Deposition: Spin-coat a layer of OA/OAm-capped PQDs.
• Alkaline Antisolvent Preparation: Add an alkaline compound, such as Potassium Hydroxide (KOH), to an ester antisolvent like methyl benzoate (MeBz) to create the "AAAH" solution.
• Enhanced Interlayer Rinsing: Rinse the wet PQD film with the AAAH solution. The alkaline environment renders ester hydrolysis thermodynamically spontaneous and lowers the reaction activation energy, generating approximately twice the conventional amount of short conductive ligands (e.g., benzoate) from the antisolvent.
• Spin-Dry and Repeat: Spin-dry the film and repeat the process to build thickness.
• Cationic Ligand Post-Treatment: As a final step, post-treat the film with a solution of short cationic ligands (e.g., formamidinium iodide, FAI) to exchange the remaining long-chain ammonium ligands.

The logical workflow for the AAAH strategy, which underpins one of the highest-performing PQD solar cells, is visualized below.

The Scientist's Toolkit: Essential Research Reagents

Successful implementation of the aforementioned protocols relies on a set of key reagents, each with a specific function in modifying the PQD surface.

Table 2: Essential Reagents for Short-Chain Ligand Research

Reagent	Function/Role in Research	Key characteristic
3-phenyl-2-propen-1-amine bromide (PPABr)	A short-chain conjugated ligand backbone. Electron-donating/withdrawing substituents tune carrier transport [32].	π-conjugated system for enhanced carrier mobility via delocalized electron clouds.
2-thiophenemethylammonium iodide (ThMAI)	A multifaceted anchoring ligand for solar cells [18].	Combines thiophene (Lewis base) and ammonium groups for strong, multi-point surface binding.
Conjugated Polymers (e.g., Th-BDT)	Act as dual-functional ligands providing passivation and directing QD packing [33].	Rigid polymer backbone with functional groups (-EG, -CN) for strong interaction and π-π stacking.
Methyl Benzoate (MeBz)	Ester antisolvent for interlayer rinsing. Hydrolyzes to form conductive benzoate ligands [34].	Moderate polarity preserves PQD structure while enabling efficient ligand exchange.
Triphenylphosphine Oxide (TPPO)	A covalent short-chain ligand dissolved in nonpolar solvents for surface stabilization [4].	Forms strong Lewis acid-base interactions with uncoordinated Pb²⁺, passivating surface traps.
Potassium Hydroxide (KOH)	Additive to create an alkaline environment in ester antisolvents [34].	Dramatically increases the hydrolysis rate and yield of short-chain ligands from esters.

The strategic selection and application of short-chain conductive ligands is a cornerstone of modern PQD research. As demonstrated, conjugated molecules and polymers significantly enhance charge transport through π-π interactions, while ligands with multifaceted anchoring groups or covalent binding capabilities provide superior surface passivation and phase stability. The development of advanced processing methods, such as alkali-augmented hydrolysis, further pushes the boundaries of device performance by enabling denser and more conductive ligand capping. The choice of solvent during ligand exchange is equally critical, with nonpolar solvents proving effective in preserving the delicate PQD surface. The data and protocols presented herein provide a framework for researchers to navigate this complex design space, guiding the development of next-generation PQD optoelectronics that successfully balance high efficiency with long-term operational stability.

In the rapidly advancing field of perovskite quantum dot (PQD) research, surface ligand chemistry has emerged as a pivotal factor determining both the stability and electronic performance of these promising semiconductor nanomaterials. PQDs are renowned for their tunable bandgap energy, high photoluminescence quantum yields, and exceptional defect tolerance, making them particularly attractive for next-generation photovoltaics [34]. However, their immense surface-to-volume ratio presents a significant challenge: the dynamic binding of pristine insulating ligands creates surface vacancy defects that compromise charge transport between adjacent quantum dots [34].

Conventional approaches have relied on ambient hydrolysis of ester antisolvents to substitute pristine long-chain oleate (OA⁻) ligands with shorter conductive counterparts during the layer-by-layer deposition of PQD solid films. Unfortunately, the robust C-O-CH₃ bonding of esters hinders their hydrolysis spontaneity under normal conditions [34]. This limitation predominantly induces direct dissociation of dynamically bound ligands rather than their effective substitution, generating extensive surface vacancy defects that trap charge carriers and ultimately diminish device performance [34].

The emergence of alkali-augmented hydrolysis represents a paradigm shift in surface ligand engineering. By creating precisely controlled alkaline environments, researchers have successfully overcome both thermodynamic spontaneity and kinetic activation energy barriers that have long limited conventional ester hydrolysis approaches [34]. This review provides a comprehensive comparison of this novel methodology against established alternatives, examining experimental data on binding affinity, photovoltaic performance, and structural integrity to establish a foundation for informed methodological selection in PQD research and development.

Experimental Protocols: Methodologies for Assessing Ligand Binding and PQD Performance

Alkali-Augmented Antisolvent Hydrolysis (AAAH) Methodology

The foundational protocol for alkali-augmented hydrolysis involves creating precisely controlled alkaline environments during the interlayer rinsing process of PQD solid films [34]. The standard experimental workflow comprises the following steps:

• PQD Film Preparation: Hybrid FA₀.₄₇Cs₀.₅₃PbI₃ PQDs with an average size of ~12.5 nm are prepared via post-synthetic cation exchange of CsPbI₃ PQD parents and spin-coated into solid films [34].

• Alkaline Antisolvent Formulation: Methyl benzoate (MeBz) is identified as the optimal antisolvent due to its suitable polarity and functional groups. Potassium hydroxide (KOH) is coupled with MeBz at controlled concentrations to establish the alkaline environment. The alkalinity is carefully regulated to ensure adequate ligand exchange without compromising the structural integrity of the perovskite core [34].

• Interlayer Rinsing Procedure: Each layer of the layer-by-layer deposited PQD solid film is rinsed with the alkali-augmented antisolvent under ambient conditions (approximately 30% relative humidity). This facilitates rapid substitution of pristine insulating oleate ligands with hydrolyzed conductive counterparts [34].

• Post-Treatment Processing: After achieving the desired film thickness, post-treatment with alternative short cationic ligands (such as FA⁺ or MA⁺) is performed using protic 2-pentanol (2-PeOH) with moderate polarity as the solvent for cationic salts to mediate efficient A-site ligand exchange [34].

Conventional Ester Hydrolysis Method

The conventional approach against which the AAAH strategy is compared follows this protocol:

• Neat Ester Antisolvent Application: PQD solid films are rinsed with neat ester antisolvents (typically methyl acetate (MeOAc), methyl benzoate (MeBz), or ethyl acetate (EtOAc)) of moderate polarity under the same ambient conditions [34].

• Ambient Hydrolysis Reliance: The process relies exclusively on ambient humidity to hydrolyze ester antisolvents, generating target acidic anions for ligand substitution without alkaline augmentation [34].

• Identical Post-Treatment: The subsequent A-site cationic ligand exchange process remains identical to the AAAH method to ensure comparable experimental conditions [34].

Characterization and Analytical Methods

The performance evaluation of both methodologies incorporates these analytical techniques:

• Theoretical Calculations: Density functional theory (DFT) calculations reveal the thermodynamic spontaneity and kinetic activation energy barriers of ester hydrolysis in alkaline versus neutral environments [34].
• Photovoltaic Characterization: Current-voltage (J-V) measurements under standard illumination conditions (AM 1.5G) to determine power conversion efficiency (PCE), short-circuit current (JSC), open-circuit voltage (VOC), and fill factor (FF) [34] [35].
• Structural and Morphological Analysis: Transmission electron microscopy (TEM) for PQD size distribution and aggregation assessment; X-ray diffraction (XRD) for crystallographic orientation homogeneity [34].
• Spectroscopic Evaluation: Light absorption spectroscopy and steady-state photoluminescence (PL) measurements to monitor trap-state density and charge recombination dynamics [34].
• Stability Testing: Operational stability under continuous illumination and storage stability under ambient conditions to assess device longevity [34].

Performance Comparison: AAAH Versus Conventional and Alternative Methods

Photovoltaic Performance Metrics

Table 1: Comparative Photovoltaic Performance of PQD Solar Cells Fabricated via Different Ligand Engineering Strategies

Ligand Engineering Method	Certified PCE (%)	Steady-State PCE (%)	Average PCE (%)	JSC (mA/cm²)	VOC (V)	FF (%)
Alkali-Augmented Hydrolysis (AAAH)	18.30 [34] [35]	17.85 [34] [35]	17.68 (over 20 devices) [34]	Undisclosed	Undisclosed	Undisclosed
Conventional MeOAc Rinsing	~16 (prior art) [35]	Undisclosed	Undisclosed	Undisclosed	Undisclosed	Undisclosed
Conventional MeBz Rinsing	Undisclosed	Undisclosed	Undisclosed	Undisclosed	Undisclosed	Undisclosed
Liquid-State Ligand Exchange	Undisclosed	Undisclosed	Undisclosed	Undisclosed	Undisclosed	Undisclosed

The data unequivocally demonstrates the superior performance of the AAAH strategy, achieving a certified efficiency of 18.3% which represents the highest value among published PQD solar cell reports [34] [35]. The remarkable performance is further corroborated by the steady-state efficiency of 17.85% and consistent average efficiency of 17.68% across multiple devices, highlighting exceptional reproducibility [34].

Material and Structural Properties

Table 2: Comparison of Structural and Electronic Properties Resulting from Different Ligand Approaches

Property	Alkali-Augmented Hydrolysis	Conventional Ester Hydrolysis	Measurement Method
Ligand Substitution Efficiency	Up to 2x conventional amount [34]	Limited by thermodynamic barriers [34]	Theoretical calculations & FTIR
Trap-State Density	Significant reduction [34] [35]	Extensive surface vacancy defects [34]	PL spectroscopy & J-V characteristics
Crystallographic Orientation	Homogeneous [34] [35]	Less ordered	XRD
Particle Agglomeration	Minimal [34] [35]	Significant aggregation [34]	TEM & SEM
Charge Transfer Efficiency	Enhanced inter-dot electronic coupling [34]	Compromised by insulating ligands [34]	EIS & TRPL
Activation Energy for Hydrolysis	~9-fold reduction [34]	Reference level [34]	DFT calculations

The AAAH strategy enables approximately twice the conventional amount of hydrolyzed conductive ligands capping on the PQD surface, creating a denser and more robust conductive capping layer [34]. Theoretical calculations confirm that the alkaline environment renders ester hydrolysis thermodynamically spontaneous and lowers the reaction activation energy by approximately nine-fold compared to conventional approaches [34].

Scalability and Stability Performance

Table 3: Scalability and Stability Comparison of PQD Solar Cells

Parameter	Alkali-Augmented Hydrolysis	Conventional Ester Hydrolysis	Test Conditions
Large-Area Device Performance (1 cm²)	15.60% champion efficiency [35]	Undisclosed	Standard illumination
Storage Stability	Improved [34]	Limited	Ambient conditions
Operational Stability	Enhanced [34]	Moderate	Continuous illumination
Compatibility with Diverse PQD Compositions	Broadly compatible [34]	Composition-dependent	Various A-site cations

The AAAH strategy demonstrates promising scalability with a champion efficiency of 15.60% for 1 cm² solar cells, highlighting its potential for large-area photovoltaic applications [35]. Furthermore, both storage and operational stability are improved compared to conventional approaches, addressing critical challenges in PQD solar cell commercialization [34].

Mechanism Analysis: Signaling Pathways and Workflow

The superior performance of alkali-augmented hydrolysis can be visualized through its fundamental mechanism, which operates at both thermodynamic and kinetic levels to enhance ligand substitution efficiency.

Diagram 1: Comparative mechanism of conventional versus alkali-augmented hydrolysis showing the enhanced thermodynamic spontaneity and reduced kinetic barriers in the AAAH approach.

The mechanistic pathway reveals how the AAAH strategy fundamentally alters both thermodynamic and kinetic parameters. The alkaline environment establishes a more favorable energetic landscape that enables rapid and complete ligand substitution, addressing the core limitation of conventional methods that fail to overcome activation energy barriers for effective ester hydrolysis [34].

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Key Research Reagent Solutions for Alkali-Augmented Hydrolysis Experiments

Reagent/Material	Function in Experimental Protocol	Specific Application Notes
Methyl Benzoate (MeBz)	Primary antisolvent for interlayer rinsing	Preferred over MeOAc due to suitable polarity and superior binding of hydrolyzed ligands [34]
Potassium Hydroxide (KOH)	Alkaline environment creator	Facilitates ester hydrolysis; concentration must be optimized to avoid perovskite degradation [34]
FA₀.₄₇Cs₀.₅₃PbI₃ PQDs	Light-absorbing active material	Hybrid A-site composition with suitable Goldschmidt tolerance factors and tailorable lattice structures [34]
2-Pentanol (2-PeOH)	Solvent for cationic ligand salts	Ideal protic solvent with moderate polarity for A-site ligand exchange during post-treatment [34]
Formamidinium (FA⁺) Salts	A-site cationic ligand source	Substitutes pristine oleylammonium (OAm⁺) ligands to enhance electronic coupling [34]
Tin Oxide (SnO₂)	Electron transport layer (ETL)	Standard ETL material in device architecture [35]
Spiro-OMeTAD	Hole transport layer (HTL)	Standard HTL material in device architecture [35]
Indium Tin Oxide (ITO)	Transparent conductive substrate	Standard transparent electrode in device architecture [35]

This toolkit encompasses the essential materials required to implement the AAAH strategy effectively. Particular attention should be paid to the controlled introduction of alkalinity, as excessive concentrations may compromise the structural integrity of the ionic perovskite lattice despite enhancing hydrolysis kinetics [34].

The comprehensive comparison presented herein demonstrates that alkali-augmented hydrolysis represents a significant advancement in surface ligand engineering for perovskite quantum dots. By addressing the fundamental thermodynamic and kinetic limitations of conventional ester hydrolysis approaches, this methodology enables the formation of a denser conductive capping layer that enhances inter-dot charge transfer while minimizing surface defect states.

The experimental data corroborates that the AAAH strategy delivers superior photovoltaic performance, achieving a record-certified efficiency of 18.3% for PQD solar cells alongside improved operational and storage stability [34] [35]. Furthermore, its broad compatibility with diverse PQD compositions and solid-state treatments underscores its potential as a universal approach for modulating PQD surface chemistry [34].

For researchers and development professionals working in PQD-based materials and devices, the adoption of alkali-augmented hydrolysis offers a promising pathway to overcome persistent challenges in charge transport and structural homogeneity. As the field advances toward commercialization, this methodology provides a robust foundation for developing next-generation PQD photovoltaics with enhanced performance characteristics and improved operational longevity.

Overcoming Stability Challenges Through Strategic Ligand Selection

Addressing Lattice Distortion and Tensile Strain with Bulky Cations

All-inorganic CsPbI3 perovskite quantum dots (PQDs) hold significant potential for next-generation photovoltaics due to their suitable bandgap and exceptional optoelectronic properties [18]. The surface-bound ligands used in their synthesis play a dual role: they initially stabilize the black perovskite phase but create a critical challenge during device fabrication. Long-chain ligands like oleic acid (OA) and oleylamine (OLA) must be replaced with shorter-chain alternatives to facilitate charge transport in PQD solid films [18]. However, this ligand exchange process inadvertently induces two detrimental effects: the formation of surface defects (Cs+ and I− vacancies) that act as non-radiative recombination centers, and a loss of beneficial surface tensile strain that stabilizes the black perovskite phase [18]. This strain loss leads to severe lattice distortion and eventual transition to the undesired, photoinactive yellow δ-phase, severely compromising solar cell performance and stability [18] [9].

Bulky cations, particularly in multifaceted anchoring ligands, have emerged as a promising strategy to simultaneously address both lattice distortion and tensile strain preservation. These organic cations possess larger ionic radii compared to Cs+, enabling them to function as structural spacers that restore surface tensile strain while their molecular design allows for effective defect passivation through strong binding to the PQD surface [18]. This review comprehensively compares the performance of different bulky cation strategies, providing experimental data and methodologies for researchers pursuing stable, high-efficiency PQD optoelectronic devices.

Comparative Performance of Bulky Cation Ligands

The strategic application of bulky cations as surface ligands or additives has demonstrated significant improvements in the structural stability and optoelectronic performance of PQDs. The comparative data reveals how different molecular structures yield distinct outcomes.

Table 1: Performance Comparison of Bulky Cation Ligands in PQDs

Ligand Material	Molecular Characteristics	Power Conversion Efficiency (PCE)	Stability Performance	Key Improvements
ThMAI (2-thiophenemethylammonium iodide)	Thiophene ring (Lewis base), ammonium group, larger ionic size than Cs+	15.3% (CsPbI3 PQD solar cells)	83% of initial PCE retained after 15 days in ambient conditions [18]	Improved carrier lifetime, uniform PQD orientation, restored tensile strain, defect passivation [18]
FASCN (Formamidine thiocyanate)	Bidentate ligand, short carbon chain (<3), liquid agent, sulfur and nitrogen binding atoms	N/A (NIR-I LED application)	Improved thermal stability (Δλ = 1 nm vs control Δλ = 12 nm), better humidity resistance [23]	4-fold higher binding energy, 8-fold higher conductivity, exciton binding energy increased to 76.3 meV [23]
Oleylammonium (OAM+)	Conventional long-chain ligand, forms surface termination	N/A	Baseline for comparison - poor stability without optimization [36]	Excessive surface termination leads to poor charge transport [36]

The performance advantages of specifically designed bulky cations are substantial. ThMAI-treated CsPbI3 PQD solar cells not only showed enhanced PCE of 15.3% compared to 13.6% for control devices, but also dramatically improved operational stability, maintaining 83% of their initial PCE after 15 days under ambient conditions [18]. This represents a marked improvement over the control device, which retained only 8.7% of its initial PCE over the same period [18]. Similarly, FASCN treatment resulted in a fourfold higher binding energy compared to original oleate ligands and an eightfold higher conductivity in treated films [23].

Table 2: Material Properties and Experimental Outcomes of Bulky Cation Treatments

Property Measured	Control/Conventional Ligands	Bulky Cation Engineered Ligands	Experimental Method
Binding Energy	OA: -0.18 eV; OAm: -0.22 eV [23]	ThMAI: strong multifaceted anchoring; FASCN: -0.91 eV [23]	DFT calculations [18] [23]
Exciton Binding Energy	39.1 meV (control film) [23]	76.3 meV (FASCN-treated) [23]	Temperature-dependent PL spectroscopy [23]
Phase Stability	Severe transition to δ-phase [18]	Restored tensile strain, stabilized black phase [18]	XRD, in situ optical spectroscopy [18] [12]
Thermal Stability	Significant emission shift (Δλ = 12 nm) [23]	Minimal emission shift (Δλ = 1 nm) [23]	PL intensity mapping at 100°C [23]
Film Conductivity	Baseline conductivity [23]	8-fold improvement (FASCN) [23]	Two-terminal device measurement [23]

Experimental Protocols and Methodologies

Synthesis and Ligand Exchange with ThMAI

The CsPbI3 PQDs stabilized with OA and OLA are synthesized via the hot injection method [18]. The ThMAI ligand exchange process involves depositing PQD films onto substrates using a layer-by-layer spin-coating technique. Specifically, each layer is treated with a ThMAI solution (0.5 mg/mL in hexane) during spinning, followed by washing with hexane to remove excess ligands and reaction by-products [18]. This process is repeated multiple times (typically 6-8 layers) to achieve the desired film thickness. The ThMAI treatment facilitates a multifaceted anchoring mechanism where the thiophene ring (acting as a Lewis base) robustly binds to uncoordinated Pb2+ sites, while its ammonium segment efficiently occupies cationic Cs+ vacancies on the PQD surface [18].

Binding Energy Assessment via DFT Calculations

Density functional theory (DFT) calculations are employed to determine ligand binding energies to PQD surfaces. For FASCN, the binding energy (Eb) is calculated using the formula: [ Eb = E{total} - (E{PQD} + E{ligand}) ] where ( E{total} ) is the total energy of the PQD-ligand system, ( E{PQD} ) is the energy of the pristine PQD, and ( E_{ligand} ) is the energy of the isolated ligand [23]. These calculations revealed that FASCN exhibits a binding energy of -0.91 eV, fourfold larger than conventional OAm (-0.18 eV) and OA (-0.22 eV) ligands, and significantly higher than FAI (-0.31 eV) and MAI (-0.30 eV) [23].

Strain and Phase Stability Characterization

In situ XRD measurements from 30°C to 500°C under argon flowing are performed to investigate thermal degradation mechanisms and phase stability [12]. For Cs-rich PQDs, thermal degradation is induced by a phase transition from black γ-phase to yellow δ-phase, while FA-rich PQDs with higher ligand binding energy directly decompose into PbI2 [12]. Strain characterization involves analyzing lattice parameters before and after ligand exchange, where the larger ionic size of ThMA+ compared to Cs+ facilitates the restoration of surface tensile strain in PQDs [18].

Optoelectronic Property Characterization

Carrier lifetime and recombination dynamics are assessed using time-resolved photoluminescence (TRPL) [18] [23]. ThMAI-treated CsPbI3 PQD thin films exhibit improved carrier lifetime compared to controls [18]. Temperature-dependent PL spectra are measured from 80 K to 300 K to determine exciton binding energy using the Arrhenius equation: [ I(T) = \frac{I0}{1 + Ae^{-\frac{Eb}{kB T}}} ] where ( I0 ) is the integrated PL intensity at 0 K, ( Eb ) is the exciton binding energy, ( kB ) is the Boltzmann constant, and A is the coefficient [23]. Femtosecond transient absorption (TA) spectroscopy with 190 fs pulse width, 450 nm excitation, and 10 mW power is employed to study charge transfer and recombination dynamics [23].

Visualization of Mechanisms and Workflows

The multifaceted anchoring mechanism of ThMAI demonstrates how a single ligand system can simultaneously address multiple instability factors in PQDs through distinct molecular functionalities.

The comprehensive experimental workflow for evaluating bulky cation treatments spans from synthesis to device performance testing, incorporating structural, optical, thermal, and theoretical characterization methods.

The Scientist's Toolkit: Essential Research Reagents

Table 3: Essential Reagents for Bulky Cation Research in PQDs

Reagent/Chemical	Function in Research	Application Notes
2-thiophenemethylammonium iodide (ThMAI)	Multifaceted anchoring ligand for strain restoration and defect passivation [18]	Used in ligand exchange process (0.5 mg/mL in hexane); requires layer-by-layer processing [18]
Formamidine thiocyanate (FASCN)	Bidentate liquid ligand for high surface coverage and tight binding [23]	Provides fourfold higher binding energy than oleate ligands; enables eightfold higher conductivity [23]
Oleic acid (OA) & Oleylamine (OLA)	Conventional long-chain ligands for initial PQD synthesis and stabilization [18] [9]	Dynamic binding leads to detachment; requires replacement for optimal device performance [18]
Lead iodide (PbI₂)	Pb²⁺ precursor for perovskite crystal formation [18]	High purity (99.999%) recommended for optimal performance and reduced defects [18]
Cesium carbonate (Cs₂CO₃)	Cs⁺ precursor for all-inorganic perovskite synthesis [18]	Requires careful stoichiometric control for phase-pure PQDs [18]
1-octadecene (ODE)	Non-coordinating solvent for high-temperature synthesis [18] [9]	Technical grade (90%) sufficient for most synthesis protocols [18]
n-hexane	Non-polar solvent for washing and ligand exchange processes [18]	Anhydrous grade recommended to prevent moisture-induced degradation [18]

The comparative analysis of bulky cation strategies demonstrates their critical role in addressing the fundamental challenges of lattice distortion and tensile strain loss in PQDs. Multifaceted ligands like ThMAI and bidentate agents like FASCN provide comprehensive solutions that simultaneously enhance phase stability through strain engineering and improve optoelectronic properties through robust defect passivation. The experimental data confirms that strategic ligand design focusing on strong binding energy, appropriate ionic size, and multifunctional anchoring groups can significantly advance PQD technology toward commercial viability. Researchers should prioritize ligands that offer synergistic benefits of strain restoration and surface passivation while maintaining efficient charge transport properties for optimal device performance.

Preventing Ligand Desorption and the Formation of Interfacial Quenching Sites

In the field of perovskite quantum dot (PQD) research, surface ligand engineering is a critical frontier for enhancing device performance and stability. Ligands, which are molecules bound to the surface of PQDs, play a dual role: they stabilize the nanocrystals and mediate charge transport. However, a fundamental challenge is ligand desorption—the tendency of these molecules to detach from the PQD surface. This desorption creates uncoordinated lead (Pb²⁺) sites, which act as interfacial quenching centers, promoting non-radiative recombination of charge carriers and leading to significant losses in device efficiency. Preventing this phenomenon is paramount for advancing PQD applications in photovoltaics and light-emitting diodes (LEDs). This guide objectively compares the performance of recent, innovative ligand engineering strategies aimed at mitigating these issues, providing a direct comparison of their experimental outcomes.

Comparative Analysis of Ligand Engineering Strategies

The following table summarizes the core approaches and their quantitative performance data as reported in recent studies.

Table 1: Performance Comparison of Ligand Engineering Strategies for PQD Stability

Strategy Name	Ligand / Material Used	Key Mechanism of Action	Reported Binding Energy / Affinity	Photovoltaic Performance (PCE)	Stability Retention	Key Experimental Findings
Liquid Bidentate Ligand [37]	Formamidine thiocyanate (FASCN)	Bidentate binding with liquid characteristics for tight surface coverage.	Fourfold higher than original oleate ligands.	NIR-LEDs: Champion EQE ~23% (twofold higher than control).	N/A	Eightfold higher film conductivity; record-low LED voltage of 1.6 V at 776 nm. [37]
Consecutive Surface Matrix Engineering [38]	Short-chain conjugated ligands	Induces amidation to remove insulators, then fills vacancies with conjugated ligands.	High binding energy to surface vacancies.	19.14% (for FAPbI₃ PQD solar cells).	Improved operation stability.	Enhanced electronic coupling between PQDs; suppressed trap-assisted non-radiative recombination. [38]
Covalent Ligand in Nonpolar Solvent [4]	Triphenylphosphine oxide (TPPO) in octane	Covalent binding to uncoordinated Pb²⁺ sites via Lewis-base interactions; nonpolar solvent prevents surface damage.	Strong coordination via Lewis-base interaction.	15.4% (for CsPbI₃ PQD solar cells).	>90% of initial efficiency after 18 days in ambient conditions.	Higher PL intensity and ambient film stability due to significantly reduced surface trap density. [4]
3D Star-Shaped Organic Semiconductor [39]	Star-TrCN molecule	Robust chemical bonding and hydrophobic barrier formation; cascade energy band structure.	Demonstrated robust bonding via modeling/experiments.	16.0% (for CsPbI₃-PQD solar cells).	72% of initial PCE after 1000 h at 20-30% relative humidity.	Passivated surface traps, prevented moisture penetration, and improved charge extraction. [39]
Multifunctional Ligands with Tethered Species [40]	Ferrocene-functionalized ligands (with ammonium group)	Strong ionic binding group for attachment, with a tethered ferrocene unit for hole transfer.	Strong binding via quaternary ammonium group.	N/A (Study focused on hole transfer for photocatalysis).	N/A	Fast photoexcited hole transfer from PQD to ligand with near-unity efficiency. [40]

Detailed Experimental Protocols and Methodologies

To ensure reproducibility and provide a deeper understanding of the comparative data, this section outlines the key experimental protocols employed in the cited studies.

• PQD Film Fabrication: The PQD films were likely prepared via a layer-by-layer (LbL) spin-coating method, where a solution of pristine PQDs (capped with oleic acid/oleylamine) is deposited onto a substrate, followed by washing with an antisolvent to remove excess ligands.
• Ligand Exchange Treatment: The fabricated film was treated with a solution of the liquid bidentate ligand, formamidine thiocyanate (FASCN). The treatment involves dispensing the FASCN solution onto the spinning PQD film, allowing the ligand to replace the original, weakly-bound ligands directly on the solid film.
• Characterization: The success of the exchange was confirmed by measuring the binding energy via techniques like X-ray photoelectron spectroscopy (XPS). The optoelectronic quality was assessed through photoluminescence (PL) spectroscopy and by fabricating and characterizing near-infrared LEDs.

• PQD Synthesis and Initial Purification: FAPbI₃ PQDs were synthesized via a hot-injection or ligand-assisted reprecipitation (LARP) method, using oleic acid (OA) and oleylamine (OAm) as capping ligands.
• CSME Process: The innovative two-step CSME was performed on the purified PQD solution:
  • Disruption of Proton Equilibrium: A chemical agent was introduced to induce an amidation reaction between OA and OAm. This reaction disrupts the dynamic equilibrium between the native ligands, facilitating the desorption of the insulating OA and OAm from the PQD surface.
  • Vacancy Occupancy: Subsequently, short-chain conjugated ligands were introduced. These ligands, with their high binding energy, efficiently occupy the surface vacancies created in the first step, leading to a well-passivated, conductive PQD solid.
• Device Fabrication: The CSME-treated PQD ink was used to fabricate the photoactive layer of solar cells using a spin-coating process, followed by the deposition of charge transport layers and electrodes.

• Conventional Ligand Exchange: CsPbI₃ PQDs were synthesized and underwent a standard two-step solid-state ligand exchange. This typically involves:
  • Treating the film with methyl acetate (MeOAc) containing sodium acetate (NaOAc) to replace OA ligands with acetate.
  • Treating the film with ethyl acetate (EtOAc) containing phenethylammonium iodide (PEAI) to replace OLA ligands.
• Post-Stabilization Treatment: The ligand-exchanged PQD solid film was then treated with a solution of triphenylphosphine oxide (TPPO) dissolved in a nonpolar solvent (octane). The nonpolar solvent is critical as it does not strip ions from the ionic PQD surface, unlike polar solvents. The TPPO ligands coordinate to any remaining uncoordinated Pb²⁺ sites.
• Analysis: Fourier-transform infrared (FT-IR) spectroscopy was used to monitor ligand binding, and PL measurements confirmed the reduction in non-radiative recombination.

Mechanism Visualization: Ligand Binding Strategies

The following diagram illustrates the core mechanisms by which the different ligand strategies prevent desorption and passivate the PQD surface.

The Scientist's Toolkit: Essential Research Reagents

This table details key materials and their functions as employed in the featured ligand engineering strategies.

Table 2: Key Reagents for PQD Surface Ligand Engineering

Reagent / Material	Primary Function in Research	Application Context
Formamidine thiocyanate (FASCN)	Liquid bidentate ligand for tight surface binding, reducing ligand loss and quenching sites. [37]	Surface treatment of PQD films for high-efficiency light-emitting diodes (LEDs).
Triphenylphosphine oxide (TPPO)	Covalent short-chain ligand that strongly coordinates to uncoordinated Pb²⁺ sites via Lewis-base interactions. [4]	Post-ligand-exchange stabilization of CsPbI₃ PQD films for solar cells.
Star-TrCN	3D star-shaped organic semiconductor that passivates defects, provides a hydrophobic barrier, and creates a cascade energy band. [39]	Incorporated into CsPbI₃ PQDs to form a hybrid film for stable and efficient solar cells.
Ferrocene-functionalized Ligand	Multifunctional ligand with a strong binding group and a tethered ferrocene unit to enable efficient hole transfer from the PQD. [40]	Creating PQD-molecular hybrids for optoelectronic and photocatalytic applications.
Oleic Acid (OA) / Oleylamine (OLA)	Long-chain native ligands used in the initial colloidal synthesis of PQDs to control growth and provide colloidal stability. [38] [4]	Standard starting point for most PQD syntheses; typically replaced via subsequent ligand exchange.
Nonpolar Solvent (e.g., Octane)	A solvent that dissolves covalent ligands without damaging the ionic PQD surface or stripping surface components. [4]	Used as a medium for post-synthesis surface stabilization treatments to preserve PQD integrity.
Polar Antisolvents (e.g., Methyl Acetate, Ethyl Acetate)	Solvents used in conventional solid-state ligand exchange to remove long-chain OA/OLA and introduce short-chain ionic ligands. [4]	Standard procedure for fabricating conductive PQD solid films; can generate surface traps.

Optimizing Ligand Exchange Processes to Minimize Surface Defects

Perovskite quantum dots (PQDs) have emerged as promising materials for advanced optoelectronic applications, from solar cells to light-emitting diodes (LEDs). However, their inherent ionic nature makes them particularly susceptible to structural degradation under environmental stimuli such as moisture, oxygen, and heat. A primary mechanism of this degradation is defect formation on the PQD surface caused by the detachment of weakly bound ligands and halide migration within the crystal lattice [41]. The process of ligand exchange—replacing long-chain insulating ligands used in synthesis with shorter or more functional ones—is therefore not merely a procedural step but a critical determinant of both the optoelectronic properties and the long-term stability of PQD-based devices.

This guide frames the optimization of ligand exchange within the broader thesis that surface ligand binding affinity is a fundamental parameter governing PQD performance. A ligand's ability to form strong, multifaceted bonds with the PQD surface directly influences defect passivation, resistance to environmental stressors, and ultimately, device efficacy and longevity. The following sections provide a comparative analysis of recent ligand strategies, supported by experimental data and detailed methodologies, to serve as a reference for researchers and development professionals.

Ligand Strategies and Performance Comparison

The quest for optimal PQD performance has led to the development of various ligand strategies. The following table summarizes key ligand types, their molecular targets, and their quantified impact on material and device performance.

Table 1: Comparison of Ligand Exchange Strategies for PQD Stability and Performance

Ligand Name	Ligand Type/Group	Binding Target on PQD	Key Performance Improvements	Quantified Experimental Data
2-Thiophenemethylammonium Iodide (ThMAI) [18]	Multifaceted Anchoring Ligand	- Thiophene ring: Binds to uncoordinated Pb²⁺ (Lewis base)- Ammonium group: Occupies Cs⁺ vacancies	- Improved carrier lifetime- Uniform PQD orientation- Enhanced ambient stability	- Solar cell PCE: 15.3%- Stability: 83% of initial PCE retained after 15 days- Control device PCE: 13.6% (only 8.7% retained)
Trioctylphosphine Oxide (TOPO) [42]	Lewis Base	Uncoordinated Pb²⁺ ions and surface defects	- Suppression of non-radiative recombination	- PL Enhancement: 18%
Trioctylphosphine (TOP) [42]	Lewis Base	Uncoordinated Pb²⁺ ions and surface defects	- Suppression of non-radiative recombination	- PL Enhancement: 16%
L-Phenylalanine (L-PHE) [42]	Amino Acid	Uncoordinated Pb²⁺ ions and surface defects	- Superior photostability	- PL Enhancement: 3%- Retained >70% of initial PL after 20 days of UV exposure
2-Aminoethanethiol (AET) [41]	Short-Chain Thiol	Strong affinity for Pb²⁺ on PQD surface	- Dense passivation layer against moisture/UV- Improved inter-particle charge transport	- PLQY improved from 22% to 51%- PL intensity remained >95% after 60 min water/120 min UV exposure

Experimental Protocols for Ligand Exchange

To ensure reproducibility and facilitate comparison, this section outlines the detailed experimental methodologies for two prominent ligand exchange strategies: the solid-state ligand exchange and a specific multifaceted anchoring ligand process.

Solid-State Ligand Exchange for PbS CQDs

While this protocol is established for PbS CQDs, its principles are widely applicable and form the basis for many PQD processing methods. The process is a layer-by-layer (LbL) dip-coating technique that allows for precise film thickness control [43].

• Film Deposition: A thin film (approximately 40 nm thick) is deposited onto a target substrate by spin-coating a solution of CQDs dispersed in a non-polar solvent.
• Ligand Introduction: The film is exposed to a polar solvent (e.g., acetonitrile) containing short-chain organic ligands like 1,2-ethanedithiol (EDT) or mercaptopropionic acid (MPA).
• Soaking and Removal: The film is soaked in the ligand solution for an optimized duration to allow for exchange, followed by spin-coating to remove the excess solution.
• Washing: The film is washed by spin-coating with a neat polar solvent to remove the displaced long-chain ligands and any residual short-chain ligands.
• Layer Buildup: Steps 1-4 are repeated to build up the film to the desired thickness for the specific application (e.g., a solar cell) [43].

Multifaceted Anchoring Ligand Exchange with ThMAI for CsPbI₃ PQDs

This protocol describes the specific treatment of CsPbI₃ PQD films with the ThMAI ligand, as detailed in the recent study [18].

• PQD Synthesis: CsPbI₃ PQDs are synthesized via the standard hot-injection method, using oleic acid (OA) and oleylamine (OLA) as initial long-chain ligands.
• Film Fabrication: The synthesized PQDs are spin-coated onto a substrate to form a solid film.
• ThMAI Treatment: The as-deposited PQD film is treated with a solution of ThMAI (concentration: 2 mg/mL in a solvent such as chlorobenzene or acetonitrile). This is typically done by dynamically spin-coating the ThMAI solution onto the film.
• Incubation and Removal: The film is allowed to undergo ligand exchange for a short period (e.g., 30 seconds), after which the residual solution is spun off.
• Washing: The film is rinsed with a clean solvent to remove by-products and excess ligands, leaving a ThMAI-passivated PQD solid film ready for device fabrication [18].

Visualization of Ligand Exchange and Passivation

The following diagram illustrates the logical workflow of the solid-state ligand exchange process and the mechanism of multifaceted anchoring ligand passivation.

Diagram Title: Ligand Exchange Workflow and Passivation Mechanism

The Scientist's Toolkit: Essential Research Reagents

This table lists key reagents and materials commonly used in ligand exchange research for PQDs, along with their primary functions in the experimental workflow.

Table 2: Key Research Reagent Solutions for Ligand Exchange Studies

Reagent/Material	Function in Research	Example Use Case
Oleic Acid (OA) & Oleylamine (OLA)	Long-chain native ligands used in synthesis to stabilize QDs in non-polar solvents and control growth.	Initial stabilization of CsPbI₃ PQDs synthesized via hot-injection [18] [41].
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchoring ligand for post-synthetic treatment; passivates both cationic and anionic vacancies.	Enhancing carrier lifetime and phase stability in CsPbI₃ PQD solar cells [18].
Trioctylphosphine Oxide (TOPO)	Lewis base ligand for surface passivation; coordinates with undercoordinated Pb²⁺ ions.	Improving photoluminescence quantum yield (PLQY) by suppressing non-radiative recombination [42].
L-Phenylalanine (L-PHE)	Amino-acid-based ligand for surface defect passivation.	Imparting superior photostability to CsPbI₃ PQDs under prolonged UV exposure [42].
2-Aminoethanethiol (AET)	Short-chain thiol ligand with strong affinity for Pb²⁺; creates a dense passivation layer.	Post-treatment healing of surface defects on CsPbI₃ QDs after purification, improving PLQY and stability [41].
1,2-Ethanedithiol (EDT)	Bidentate short-chain ligand for solid-state ligand exchange; improves inter-dot coupling.	Creating conductive and air-stable PbS CQD films for photovoltaic measurement in ambient conditions [43].
Lead Iodide (PbI₂)	Precursor for the B-site (Pb²⁺) and halide (I⁻) in perovskite crystal structure.	Synthesis of CsPbI₃ PQDs [18] [42].
Cesium Carbonate (Cs₂CO₃)	Precursor for the A-site cation (Cs⁺) in all-inorganic perovskite synthesis.	Synthesis of CsPbI₃ PQDs via hot-injection method [18] [42].

The Role of Ionic Size and Dipole Moment in Stabilizing the Black Perovskite Phase

The stability of the black perovskite phase is a critical determinant in the performance and commercial viability of perovskite quantum dot (PQD) technologies. Surface ligand binding affinity directly influences this stability by governing the passivation of surface defects and the resilience of the colloidal system. This guide objectively compares two fundamental material properties—ionic size and dipole moment—that are leveraged in ligand design to enhance binding affinity and stabilize the active black phase. We summarize experimental data on these strategies, provide detailed protocols for key experiments, and list essential research tools, providing a resource for scientists engaged in PQD stability research and drug development where such surface interactions are paramount.

Comparative Analysis of Stabilization Strategies

Stabilization strategies for the black perovskite phase primarily focus on engineering the interface and the bulk crystal lattice. The following table compares the performance of different approaches centered on ionic size engineering and dipole moment manipulation.

Table 1: Comparison of Black Perovskite Phase Stabilization Strategies

Strategy Category	Specific Material/Intervention	Key Experimental Findings	Impact on Phase Stability	Quantitative Performance Data
Ionic Size Engineering	Incorporation of Dimethylammonium (DMA+) into CsPbI3 [44]	Formation of tetragonal β-(DMA, Cs)PbI3; superior optoelectronic properties over inorganic γ-CsPbI3 [44]	Stabilizes the black perovskite phase under ambient conditions [44]	Champion PCE of 19.76% [44]
	Mixed Halide Incorporation (Cl into CH3NH3PbBr3) [45]	Cl anions randomly substitute for Br in the lattice; optimal ratio increases stability without detrimental bandgap modification [45]	Confers increased device stability compared to pure iodide-based devices [45]	Increased material stability verified in half-cell device architecture [45]
Dipole Moment & Ligand Engineering	2-(Diphenylphosphino) acetic acid (2DPAA) at buried interface [46]	Interfacial dipole moment enhanced to 5.10 D with positive orientation; accelerates hole transport, suppresses nonradiative recombination [46]	Excellent long-term shelf and operational stability; >95% of initial PCE retained after 1200h in N₂ [46]	Champion PCE of 26.53% (certified 26.02%); VOC of 1.197 eV [46]
	Triphenylphosphine Oxide (TPPO) in nonpolar solvent for CsPbI3 PQDs [4]	Covalent ligand strongly coordinates to uncoordinated Pb²⁺ sites via Lewis-base interactions; nonpolar solvent prevents surface damage [4]	Maintained >90% of initial PSC efficiency after 18 days under ambient conditions [4]	PCE of 15.4%; significantly reduced surface trap density [4]
Mixed-Metal Chalcohalide Alloying	Sb³⁺ and S²⁻ alloyed FAPbI₃ [47]	Enhanced ionic binding energy and alleviated lattice strain; promotes α(200)c crystal growth [47]	Unencapsulated devices retain ~94.9% of initial PCE after 1080h storage in dark (20–40% RH, 25°C) [47]	PCE of 25.07% under standard conditions [47]

Detailed Experimental Protocols

To ensure reproducibility and provide a clear technical foundation, this section outlines the experimental methodologies for key studies cited in this guide.

Protocol 1: Stabilization via Ionic Size Engineering with DMA+

This protocol is based on the work that provided unambiguous evidence for the formation of stable tetragonal (β-) (DMA, Cs)PbI3 [44].

• Objective: To incorporate the large organic cation DMA+ into the CsPbI3 lattice to stabilize the black perovskite phase and enhance optoelectronic properties.
• Materials Precursor Preparation: Prepare a CsPbI3 precursor solution. Introduce (CH3)2NH2I (Dimethylammonium Iodide, DMAI) into this precursor solution.
• Film Formation: Deposit the DMAI-doped CsPbI3 precursor solution onto the substrate using a specified fabrication method (e.g., spin-coating).
• Structural Characterization: Use Ultra-Low Dose Transmission Electron Microscopy (TEM) to identify the crystal structure and confirm the incorporation of DMA+ into the perovskite lattice, distinguishing the tetragonal phase from the orthorhombic γ-CsPbI3 phase.
• Device Fabrication & Testing: Fabricate perovskite solar cells (PSCs) with the structure incorporating the β-(DMA, Cs)PbI3 film. Perform current density-voltage (J-V) measurements under standard illumination (e.g., AM 1.5G) to determine the champion power conversion efficiency (PCE).

Protocol 2: Stabilization via Interfacial Dipole Moment Reconstruction with 2DPAA

This protocol is derived from the study that introduced an interfacial dipolar chemical bridge to achieve a high PCE of 26.53% [46].

• Objective: To reconstruct the buried interface dipole with a large moment and positive orientation to improve carrier transport and device stability.
• Substrate Preparation: Deposit a Self-Assembled Monolayer (SAM), specifically Me-4PACz, onto a NiOx substrate.
• Interfacial Modification: Treat the SAM-coated substrate with a solution of 2-(Diphenylphosphino) acetic acid (2DPAA). This molecule anchors to the SAM surface via phosphorylation.
• Perovskite Deposition: Deposit the perovskite layer atop the 2DPAA-modified surface. The Lewis base groups (P and O) in 2DPAA chemically link to the perovskite, optimizing the dipole orientation.
• Dipole and Electronic Characterization:
  • Use Density Functional Theory (DFT) calculations to model the electrostatic potential (ESP) and calculate the enhanced interfacial dipole moment.
  • Use techniques like Kelvin Probe Force Microscopy (KPFM) to measure the work function change, confirming the downward shift of the Fermi level and the positive dipole orientation.
• Device Completion & Stability Testing: Complete the inverted PSC device with appropriate electron transport and electrode layers. Perform long-term stability tests by tracking PCE under continuous operation or storage in controlled environments (e.g., N₂ glove box, thermal ageing, light soaking).

Protocol 3: Surface Stabilization of PQDs with Covalent TPPO Ligands

This protocol details the surface stabilization strategy for CsPbI3 PQD photovoltaic absorbers [4].

• Objective: To passivate surface traps on ligand-exchanged PQDs using a covalent short-chain ligand dissolved in a nonpolar solvent, thereby improving performance and ambient stability.
• Synthesis and Initial Ligand Exchange:
  • Synthesize monodispersed OA/OLA-capped CsPbI3 PQDs via a hot-injection method.
  • Perform a conventional two-step ligand exchange procedure: First, replace anionic OA ligands with acetate ions using NaOAc in methyl acetate (MeOAc). Second, replace cationic OLA ligands with phenethylammonium iodide (PEAI) in ethyl acetate (EtOAc) via a layer-by-layer (LbL) assembly.
• Surface Stabilization Treatment: Treat the ligand-exchanged CsPbI3 PQD solids with a solution of Triphenylphosphine Oxide (TPPO) dissolved in a nonpolar solvent (octane). The nonpolar solvent is critical to prevent the removal of PQD surface components.
• Surface and Optoelectronic Characterization:
  • Use Fourier-Transform Infrared (FT-IR) Spectroscopy to monitor the change in surface-bound ligands.
  • Use Photoluminescence (PL) Spectroscopy to compare the PL intensity and peak position before and after TPPO treatment, indicating passivation of non-radiative recombination centers.
  • Use X-ray Photoelectron Spectroscopy (XPS) to confirm the binding of TPPO to uncoordinated Pb²⁺ sites.
• Solar Cell Fabrication and Testing: Fabricate PQD solar cells and measure the PCE. Conduct ambient stability tests by storing unencapsulated devices under room conditions and tracking efficiency over time.

Signaling Pathways and Workflow Visualizations

The strategic approach to stabilizing the black perovskite phase through ligand engineering can be conceptualized as a decision pathway, as outlined below.

The experimental workflow for implementing and validating the covalent ligand passivation strategy, a key approach for PQDs, is detailed below.

The Scientist's Toolkit: Research Reagent Solutions

The following table lists key reagents and materials essential for experimenting with the stabilization strategies discussed in this guide.

Table 2: Essential Research Reagents for Perovskite Phase Stabilization Studies

Reagent/Material	Function in Research	Exemplary Use Case
Dimethylammonium Iodide (DMAI)	Large organic cation precursor for lattice incorporation to stabilize the black phase via ionic size effects [44].	Stabilization of tetragonal CsPbI3-based perovskites [44].
2-(Diphenylphosphino) Acetic Acid (2DPAA)	Interfacial dipolar chemical bridge to reconstruct buried interface with large dipole moment and positive orientation [46].	Achieving high efficiency (26.53%) in inverted perovskite solar cells [46].
Triphenylphosphine Oxide (TPPO)	Covalent short-chain ligand for passivating uncoordinated Pb²⁺ sites on PQD surfaces via strong Lewis-base interactions [4].	Surface stabilization of CsPbI3 PQD solids to enhance PCE and ambient stability [4].
Antimony Chloride (SbCl₃) & Thiourea	Precursors for introducing Sb³⁺ and S²⁻ ions into the perovskite lattice, enhancing ionic binding energy and relieving strain [47].	Formation of mixed-metal chalcohalide-alloyed FAPbI₃ for highly efficient and stable PSCs [47].
Nonpolar Solvents (e.g., Octane)	Dispersion medium for covalent ligands that prevents destructive removal of PQD surface components during post-treatment [4].	Used as a solvent for TPPO to avoid surface ion loss on CsPbI3 PQDs [4].
Methyl Acetate (MeOAc) & Ethyl Acetate (EtOAc)	Polar solvents for dissolving ionic salts used in the conventional ligand exchange process for PQDs [4].	Used in the two-step ligand exchange to replace OA and OLA ligands with short-chain ionic ligands [4].

Quantifying Success: Analytical Methods and Performance Metrics

Experimental and Computational Methods for Measuring Binding Affinity

Binding affinity, the strength of interaction between a molecule (ligand) and its target binding site, is a fundamental parameter in fields ranging from drug discovery to materials science. Accurate quantification of this interaction is crucial for developing effective therapeutics and optimizing functional materials, such as perovskite quantum dots (PQDs). For PQD stability research, precisely measuring how strongly surface ligands bind to quantum dot surfaces directly influences the development of more stable and efficient optoelectronic devices [18].

The methodological landscape for assessing binding affinity is broadly divided into experimental techniques, which provide empirical measurements under controlled conditions, and computational approaches, which predict interactions through physical models and artificial intelligence. This guide provides a comprehensive, objective comparison of these methods, their performance characteristics, and practical implementation protocols to assist researchers in selecting the optimal approach for their specific applications, particularly in surface ligand binding for PQD stability.

Experimental Methods for Binding Affinity Measurement

Experimental techniques quantitatively measure binding interactions in laboratory settings, providing direct empirical data grounded in physicochemical principles. The following section details key established methods.

Isothermal Titration Calorimetry (ITC)

Experimental Principle: ITC directly measures the heat released or absorbed during a binding event. By performing successive injections of a ligand solution into a target molecule solution, it quantifies the binding constant (K~d~), enthalpy change (ΔH), stoichiometry (n), and entropy change (ΔS) in a single experiment [49].

Typical Protocol:

• The protein or target molecule solution is loaded into the sample cell, while the ligand solution is loaded into the syringe.
• The instrument maintains a constant temperature while the ligand is titrated into the sample cell in a series of injections.
• The heat flow required to maintain a constant temperature between the sample and reference cells is measured for each injection.
• The resulting isotherm (plot of heat vs. molar ratio) is fitted to a suitable binding model to extract the thermodynamic parameters.

Surface Plasmon Resonance (SPR)

Experimental Principle: SPR detects binding events in real-time by monitoring changes in the refractive index on a sensor surface. One interactant (e.g., the protein) is immobilized on a dextran-coated gold chip, while the other (the ligand) flows over it. Binding causes a measurable change in the resonance angle [49] [50].

Typical Protocol:

• The capture molecule is immobilized on the sensor chip surface using covalent chemistry.
• The analyte is injected over the surface at a constant flow rate in a buffer solution.
• The association phase is monitored as the analyte binds. Then, buffer is flowed over the surface to monitor the dissociation phase.
• The resulting sensorgrams (response units vs. time) are analyzed to determine the association rate (k~on~), dissociation rate (k~off~), and equilibrium binding constant (K~d~ = k~off~/k~on~).

Native Mass Spectrometry (Native MS)

Experimental Principle: Native MS uses gentle ionization conditions to preserve non-covalent complexes as they are transferred from solution to the gas phase. The intensity of the detected ions for the free protein and the ligand-bound complex is used to determine binding affinity, with recent methods enabling measurements without prior knowledge of protein concentration [27].

Typical Protocol (Dilution Method):

• A protein-ligand mixture is prepared and allowed to reach equilibrium.
• The mixture is serially diluted and analyzed using electrospray ionization mass spectrometry under native (non-denaturing) conditions.
• The relative abundances of the free protein and ligand-bound complex ions are measured across different dilution levels.
• The equilibrium dissociation constant (K~d~) is calculated by analyzing the bound fraction as a function of dilution, independent of absolute protein concentration [27].

Table 1: Comparison of Key Experimental Methods for Binding Affinity Measurement

Method	Measured Parameters	Typical K~d~ Range	Throughput	Key Advantage	Key Limitation
Isothermal Titration Calorimetry (ITC)	K~d~, ΔH, ΔS, n	nM - mM	Low	Provides full thermodynamic profile	High sample consumption
Surface Plasmon Resonance (SPR)	K~d~, k~on~, k~off~	pM - μM	Medium	Real-time kinetic data	Requires immobilization
Native Mass Spectrometry	K~d~, Stoichiometry	μM - mM	Medium	Works with complex mixtures; label-free	In-source dissociation of weak complexes
Fluorescence Polarization	K~d~	nM - μM	High	Homogeneous assay; high throughput	Requires fluorescent labeling

Computational Methods for Binding Affinity Prediction

Computational approaches predict binding affinity using physical models, simulations, and machine learning, offering insights at the atomic level and enabling high-throughput screening.

Molecular Docking

Computational Principle: Docking computationally predicts the preferred orientation of a ligand bound to a target and often provides a rough estimate of binding affinity through a scoring function. It is typically fast but can be limited in accuracy [51] [52].

Typical Protocol:

• The 3D structures of the target protein and ligand are prepared, including assignment of protonation states and charges.
• A search algorithm explores possible conformations and orientations of the ligand within the defined binding site.
• A scoring function ranks the generated poses based on estimated binding energy, with top poses selected for further analysis.
• In advanced workflows, top-ranked poses may be refined with short molecular dynamics simulations for improved accuracy [52].

Molecular Dynamics (MD) with End-Point Free Energy Methods

Computational Principle: Methods like MM/GBSA and MM/PBSA use molecular dynamics trajectories to calculate binding free energy. They combine molecular mechanics energy in the gas phase with solvation terms (Generalized Born or Poisson-Boltzmann models, plus surface area) [51] [50].

Typical Protocol (MM/GBSA):

• Run an MD simulation on the solvated protein-ligand complex to sample conformations.
• Extract multiple snapshots (e.g., 300) from the equilibrated trajectory at regular intervals.
• For each snapshot, calculate the gas-phase interaction energy, the polar solvation energy (via GB), and the non-polar solvation energy (via SASA).
• Average these energy components over all snapshots to estimate the binding free energy.

Alchemical Free Energy Methods

Computational Principle: These methods, such as Free Energy Perturbation (FEP) and Thermodynamic Integration (TI), calculate the free energy difference between two states by gradually perturbing one ligand into another along a non-physical pathway. They are highly accurate but computationally intensive [51] [50].

Typical Protocol (FEP):

• Define the initial (ligand A) and final (ligand B) states.
• Create a series of intermediate "lambda" windows where the system is a hybrid of A and B.
• Run MD simulations for each window to sample configurations.
• Use the FEP equation to compute the free energy difference (ΔG) by summing the contributions across all windows, yielding the relative binding free energy.

Machine Learning and AI-Based Approaches

Computational Principle: These models learn the relationship between protein-ligand structural/sequence features and binding affinity from large datasets. They range from classical models like Random Forest to advanced deep learning architectures incorporating distance features and attention mechanisms [53] [54].

Typical Protocol (e.g., DAAP Model):

• Input Feature Generation: Extract features such as distances between key atom types (donor-acceptor, hydrophobic), protein sequence features of specific residues, and ligand SMILES strings [53].
• Model Training: Train a deep learning model (e.g., with attention mechanisms) on a curated dataset like PDBbind to learn the mapping from features to binding affinity.
• Prediction and Ensembling: For a new complex, extract its features and pass them through the trained model. To enhance robustness, average the predictions from multiple models (ensemble averaging) [53].

Table 2: Comparison of Key Computational Methods for Binding Affinity Prediction

Method	Typical RMSE (kcal/mol)	Speed	Key Advantage	Key Limitation
Molecular Docking	2.0 - 4.0 [51]	Very Fast	High-throughput screening	Low quantitative accuracy
MM/GBSA	~1.5 - 2.5	Medium	Better than docking; no intermediates	Ignores full entropic contribution
Free Energy Perturbation (FEP)	~1.0 [51]	Very Slow	High accuracy for congeneric series	High computational cost; expert setup
Machine Learning (DAAP)	~0.99 [53]	Fast (after training)	Good speed/accuracy balance	Dependent on training data quality

Application in Surface Ligand Binding for Perovskite Quantum Dot (PQD) Stability

The stability of CsPbI~3~ Perovskite Quantum Dots (PQDs) is critically dependent on the binding affinity of their surface ligands. Long-chain native ligands like oleic acid (OA) and oleylamine (OLA) provide initial stability but impede charge transport in electronic devices. Replacing them with short-chain ligands is necessary but risks introducing defects and phase instability if binding is weak [18].

Recent research demonstrates that using multifaceted anchoring ligands like 2-thiophenemethylammonium iodide (ThMAI) can overcome this challenge. ThMAI's effectiveness stems from:

• Strong Binding: The thiophene ring acts as a Lewis base to bind uncoordinated Pb²⁺ sites, while the ammonium group occupies Cs⁺ vacancies, providing superior passivation compared to single-group ligands [18].
• Strain Induction: The large ionic size of ThMA⁺ helps restore beneficial surface tensile strain, stabilizing the black phase of CsPbI~3~ PQDs [18].
• Dipole Moment: Charge separation within ThMAI creates a strong dipole moment, tightening the binding to the PQD surface [18].

Experimental Outcome: Solar cells utilizing ThMAI-treated CsPbI~3~ PQDs showed a power conversion efficiency (PCE) of 15.3% and retained 83% of their initial PCE after 15 days under ambient conditions. In stark contrast, control devices showed a PCE of 13.6% and retained only 8.7% of their initial PCE over the same period [18]. This data quantitatively underscores how superior ligand binding affinity directly translates to enhanced performance and device stability.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Research Reagent Solutions for Binding Affinity Studies

Reagent/Material	Function/Application	Example Use Case
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchoring ligand for PQD surface passivation and strain engineering [18]	Enhancing cubic-phase stability and charge transport in CsPbI₃ PQD solar cells
Oleic Acid (OA) & Oleylamine (OLA)	Long-chain native ligands for initial synthesis and stabilization of PQDs [18]	Initial stabilization of black phase CsPbI₃ PQDs at room temperature
HEPES Buffer	Maintaining stable pH during biological binding assays (e.g., SPR, ITC)	Standard buffer system for protein-ligand interaction studies
CM5 Sensor Chip	Gold sensor surface with carboxymethylated dextran matrix for immobilization	Covalent capture of proteins for Surface Plasmon Resonance (SPR) studies
GYLCAM-06 & AMBER Force Fields	Providing parameters for carbohydrate and protein/ligand systems in MD simulations [52]	Molecular dynamics simulation of protein-carbohydrate interactions (e.g., HPSE inhibitors)
PDBbind Database	Curated database of protein-ligand complexes with binding affinity data for training and benchmarking ML models [53]	Training and testing machine learning models for binding affinity prediction

In the field of perovskite quantum dot (PQD) research, surface-bound ligands play a pivotal role in determining both the stability and functionality of these promising optoelectronic materials. Ligands serve as the primary interface between the PQD and its environment, governing properties such as charge transport, environmental resistance, and structural integrity. This guide provides an objective comparison of ligand performance, tracing the evolution from conventionally used oleates to emerging thiophene-based molecules, with a specific focus on their binding affinity and contribution to PQD stability. For researchers and scientists engaged in material design and drug development, understanding these relationships is crucial for advancing PQD applications in photovoltaics, displays, and other optoelectronic devices.

Ligand Classes and Their Chemical Properties

Surface ligands for PQDs can be broadly categorized into traditional long-chain ligands and advanced aromatic ligands. Each class exhibits distinct chemical properties that directly influence their binding behavior and performance.

• Traditional Oleate Ligands: Initially used in PQD synthesis, these include long-chain molecules such as oleic acid (OA) and oleylamine (OLA). Their primary function is to stabilize the black phase of CsPbI3 PQDs at room temperature during the synthesis process. However, their insulating nature creates a significant charge transport barrier in solid films, necessitating post-synthesis ligand exchange processes. The binding of these ligands is primarily mediated through ionic interactions or Lewis acid-base coordination [18].

• Emerging Thiophene-Based Ligands: This class represents a strategic advancement in ligand design. Molecules such as 2-thiophenemethylammonium iodide (ThMAI) feature an electron-rich thiophene ring and an ammonium group. This combination enables multifaceted anchoring: the thiophene ring acts as a Lewis base binding to uncoordinated Pb2+ sites, while the ammonium segment occupies cationic Cs+ vacancies. The charge separation within these molecules reinforces their dipole moment, promoting stronger binding to the PQD surface compared to single-charged ligands [18].

Quantitative Performance Comparison

The following tables summarize key experimental data comparing the performance of different ligand types in CsPbI3 PQD solar cells, based on recent studies.

Table 1: Comparative Performance Metrics of Ligand-Treated CsPbI3 PQD Solar Cells

Ligand Type	Power Conversion Efficiency (PCE)	Stability (PCE Retention)	Key Characteristics
Oleates (OA/OLA)	~10-13% [18]	Severely degraded [18]	Long-chain insulators, induce tensile strain but hinder charge transport.
ThMAI	15.3% [18]	83% after 15 days (ambient) [18]	Multifaceted anchoring, improves carrier lifetime and uniform orientation.
Complementary Dual-Ligand	17.61% [55]	Substantially improved [55]	Hydrogen-bonded ligand system, enhances inter-dot electronic coupling.

Table 2: Impact of Ligand Exchange on PQD Film Properties

Property	Oleate-Capped PQDs	Thiophene-Based Ligand Treated PQDs
Charge Transport	Hindered by insulating carbon chains [18]	Improved conductivity via short, conductive ligands [18]
Surface Defects	High after ligand exchange (Cs+, I- vacancies) [18]	Effectively passivated via strong binding to Pb2+ and Cs+ sites [18]
Lattice Strain	Reduced after antisolvent washing [18]	Restored tensile strain, mitigating lattice distortion [18]
Film Morphology	Disordered orientation [18]	Uniformly oriented PQD solid films [18]

Experimental Protocols for Ligand Evaluation

Synthesis and Ligand Exchange Methodology

A standard experimental workflow for evaluating thiophene-based ligands like ThMAI involves the following steps [18]:

• PQD Synthesis: CsPbI3 PQDs are synthesized via the hot-injection method, initially capped with OA and OLA ligands, and dispersed in hexane.
• Ligand Exchange Solution Preparation: The ThMAI ligand is dissolved in acetonitrile at a specified concentration.
• Film Fabrication and Treatment: The PQD solution is spin-coated onto a substrate. During the film formation, the ThMAI solution is dynamically applied, facilitating the in-situ replacement of native long-chain ligands.
• Washing and Annealing: The film is washed with an antisolvent (e.g., methyl acetate) to remove excess reactants and by-products, followed by a mild annealing process to consolidate the film.

Characterization Techniques

The performance and stability of ligand-treated PQD films are assessed through a suite of characterization methods [18]:

• Optoelectronic Characterization: Ultraviolet-Visible (UV-Vis) absorption and photoluminescence (PL) spectroscopy are used to determine bandgap and PL quantum yield (PLQY). Transient PL decay measurements quantify carrier lifetime.
• Structural and Morphological Analysis: Transmission Electron Microscopy (TEM) evaluates PQD size, shape, and monodispersity. Scanning Electron Microscopy (SEM) assesses the film morphology and uniformity of PQD orientation.
• Device Performance Testing: Current-voltage (J-V) measurements under simulated solar illumination determine photovoltaic parameters, including PCE.
• Stability Testing: Devices are stored under ambient conditions (controlled temperature and humidity), with PCE monitored over time to assess operational stability.

Diagram 1: Experimental workflow for PQD ligand exchange and characterization.

Molecular Binding Mechanisms and Signaling Pathways

The superior performance of thiophene-based ligands originates from their distinct molecular-level interactions with the PQD surface.

The binding mechanism of a ligand like ThMAI is multifaceted. The thiophene ring, being electron-rich, acts as a Lewis base that forms a robust coordinate covalent bond with unsaturated Pb2+ sites on the PQD surface. Concurrently, the ammonium group (ThMA+) electrostatically interacts with and occupies Cs+ vacancies. This dual-action anchoring provides more comprehensive surface passivation compared to oleates, which lack this complementary binding capability. Furthermore, the larger ionic radius of ThMA+ compared to Cs+ helps restore beneficial tensile strain on the PQD surface, which is crucial for stabilizing the desired black perovskite phase [18].

In advanced systems, a complementary dual-ligand approach can be employed. Here, different ligands (e.g., trimethyloxonium tetrafluoroborate and phenylethyl ammonium iodide) form a network on the PQD surface stabilized by hydrogen bonds. This system not only passivates defects but also enhances the electronic coupling between individual PQDs in the solid film, leading to record-high device efficiencies [55].

Diagram 2: Molecular binding mechanisms of oleate versus thiophene-based ligands.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagents and Materials for PQD Ligand Research

Reagent/Material	Function/Description	Example from Studies
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for synthesizing all-inorganic CsPbI₃ PQDs [18].
Lead Iodide (PbI₂)	Lead precursor for the perovskite structure [18].
Oleic Acid (OA) & Oleylamine (OLA)	Standard long-chain ligands for initial synthesis and phase stabilization [18].
2-Thiophenemethylammonium Iodide (ThMAI)	A multifaceted anchoring ligand for enhanced passivation and stability [18].	Synthesized for ligand exchange [18].
Complementary Dual-Ligand System	A pair of ligands that form a hydrogen-bonded network on the PQD surface [55].	Trimethyloxonium tetrafluoroborate & Phenylethyl ammonium iodide [55].
Antisolvents (e.g., Methyl Acetate)	Used to wash films, removing excess ligands and by-products while precipitating PQDs [18].

The comparative analysis unequivocally demonstrates that thiophene-based ligands represent a significant advancement over traditional oleates for CsPbI3 PQD applications. Their multifaceted anchoring capability, driven by distinct electron-donating thiophene and cationic ammonium groups, enables superior surface passivation, enhanced charge transport, and improved phase stability. This translates directly to higher power conversion efficiencies and longer device lifetimes in solar cells.

Future research is likely to focus on the rational design of sophisticated multi-ligand systems, such as the complementary dual-ligand approach, which can further optimize the PQD surface landscape. Exploring the synergy between different functional groups and engineering ligands for specific environmental stresses will be key to unlocking the full commercial potential of perovskite quantum dots in optoelectronics.

Correlating Binding Energy with Device Efficiency and Operational Lifetime

The performance and longevity of optoelectronic devices based on perovskite quantum dots (PQDs) are intrinsically linked to the fundamental properties of their constituent materials. Among these, the binding energy of the exciton—the bound electron-hole pair generated by light absorption—is a paramount determinant of both device efficiency and operational lifetime [56]. Strong exciton binding energy directly influences a material's ability to efficiently produce light, as it prevents the premature dissociation of the exciton into free carriers through thermal energy [56]. Concurrently, the binding affinity of surface ligands that passivate the PQDs is equally critical for operational stability. These ligands, typically long-chain organic molecules like oleic acid (OA) and oleylamine (OAm), coordinate with the ionic crystal surface to suppress defect formation and provide a barrier against environmental degradants such as moisture and oxygen [57] [2]. However, the native, dynamically bound ligands often detach over time, creating surface vacancies that act as non-radiative recombination centers, thereby quenching photoluminescence and accelerating device degradation [57] [2].

This guide frames the discussion within a broader thesis on comparing surface ligand binding affinity for PQD stability research. We objectively compare how different ligand engineering strategies—ranging from in situ bonding regulation to robust encapsulation—directly influence the core material property of exciton binding energy and, consequently, the macroscopic performance metrics of real-world devices. The correlation is clear: enhanced ligand binding affinity fortifies the PQD, which in turn preserves its intrinsic excitonic properties, leading to simultaneous gains in power conversion efficiency (PCE) and device lifetime.

Comparative Analysis of Ligand Strategies and Performance Outcomes

The following table summarizes quantitative data and key findings from recent studies that investigate the correlation between ligand engineering, binding energy, and device performance.

Table 1: Correlation between Ligand Strategies, Binding Energy, and Device Performance

Material System	Ligand Strategy	Key Performance Metrics	Impact on Stability / Lifetime	Experimental Evidence
FAPbI₃ QDs [57]	In situ regulation using protonated-OAm (from OLAI) to suppress ligand exchange.	PCE: 13.8% (Record for pure FAPbI₃ QDSCs)	Retained 80% of initial PCE after 3000 hours in ambient air.	Reduced defect density; Suppressed proton exchange between ligands.
CsPbBr₃@UiO-66 [58]	Encapsulation within a metal-organic framework (UiO-66).	Strong exciton-polariton coupling demonstrated.	Maintained luminescence for >30 months ambient; several hours underwater.	Temperature-dependent & time-resolved PL (TRPL) confirming preserved excitonic properties.
Monolayer TMDs [56]	N/A (Intrinsic material property).	Exciton Binding Energy: 100–500 meV; PL Lifetime: 0.22–0.42 ns (room temperature).	Longer exciton lifetimes compared to bulk counterparts, enabling room-temperature operation.	Calculated via effective mass approximation; validated with TRPL spectroscopy.
CsPbBr₃-PDMS Films [24]	Polydimethylsiloxane (PDMS) encapsulation.	Maintained 99.8% PL intensity after 2 hours water immersion; Amplified spontaneous emission.	Waterproofing overcomes hydration-induced degradation, critical for sensor reproducibility.	PL intensity measurement under water immersion.

Interpretation of Comparative Data

The data in Table 1 reveals several key trends. First, strategies that strengthen the ligand-PQD bond directly result in remarkable improvements in operational lifetime. The use of protonated-OAm in FAPbI₃ QDs creates a stronger, more stable bond with the surface, drastically reducing defect formation during synthesis and purification [57]. This is quantitatively reflected in the device's ability to retain 80% of its efficiency after 3000 hours in air. Second, physical encapsulation strategies, such as embedding PQDs within a MOF (CsPbBr₃@UiO-66) or a polymer matrix (CsPbBr₃-PDMS), provide a secondary barrier that shields the PQDs from environmental stressors [58] [24]. This approach effectively "locks in" the PQDs' pristine optical properties, as evidenced by the multi-year stability and excellent water resistance. Third, the intrinsically high exciton binding energy (100–500 meV) in monolayer transition metal dichalcogenides (TMDs) underscores the importance of this fundamental property for efficient light emission at room temperature, providing a benchmark for engineered systems [56].

Experimental Protocols for Assessing Binding and Stability

To generate the comparative data presented, researchers employ a suite of standardized experimental protocols. These methodologies are crucial for objectively determining the efficacy of any ligand engineering strategy.

Synthesis and Ligand Engineering

• In Situ Ligand Regulation for FAPbI₃ QDs: This protocol involves decoupling the lead and iodide sources to precisely control the I/Pb ratio and suppress detrimental proton exchange. Lead acetate trihydrate is dissolved in oleic acid (OA) and 1-octadecene (ODE), while the iodide is provided by oleylammonium iodide (OLAI) in toluene [57]. The direct use of OLAI ensures that the OAm is already in its protonated state, which binds more strongly to the QD surface. Upon injection into the Pb precursor at 80°C under N₂, a rapid metathesis reaction yields FAPbI₃ QDs with a protonated-OAm-dominated surface [57].
• Metal-Organic Framework (MOF) Encapsulation of CsPbBr₃: This is a two-step process. First, the UiO-66 MOF powder is synthesized with missing-linker defects. Second, a self-limiting solvothermal deposition (SIM) method is used to coordinate Pb²⁺ ions onto the MOF's zirconium nodes, creating Pb-UiO-66 [58]. Subsequent addition of a CsBr precursor solution leads to the in situ formation of CsPbBr₃ QDs within the confined pores of the MOF, as the Pb–O bonds break and the perovskite crystallizes [58].

Characterization Techniques

• Time-Resolved Photoluminescence (TRPL): This technique measures the decay rate of photoluminescence after pulsed excitation. It is used to determine the exciton lifetime, which is directly related to the quality of the material and its surface passivation. Longer lifetimes typically indicate reduced non-radiative recombination at surface defects, a sign of effective ligand binding [56] [58].
• Temperature-Dependent PL: By measuring PL intensity and peak position across a range of temperatures, researchers can infer information about exciton-phonon interactions and the strength of exciton binding. Stable PL performance across temperatures indicates robust exciton confinement and minimal thermal quenching [58].
• Exciton Binding Energy Calculation: A common theoretical approach involves solving the Schrödinger equation for the excitonic system within the effective mass approximation. The Hamiltonian includes terms for the electron, the hole, the bandgap, the kinetic energy of their relative motion, and the Coulomb interaction potential ( -\frac{q^2}{\epsilon |re - rh|} ) [56]. Solving this equation yields the excitonic energy states, from which the binding energy can be derived.

Research Reagent Solutions for Ligand and Stability Studies

The following table details key reagents and materials essential for experiments in this field.

Table 2: Essential Research Reagents and Materials for PQD Ligand Studies

Reagent / Material	Function in Research	Application Example
Oleylamine (OAm)	A common L-type ligand; binds to surface halide ions via hydrogen bonding. Provides colloidal stability and controls crystal growth [57] [2].	Standard ligand in hot-injection and LARP synthesis methods.
Oleic Acid (OA)	A common X-type ligand; chelates with surface lead atoms. Works in concert with OAm to dissolve precursors and stabilize QDs [57] [2].	Standard ligand in hot-injection and LARP synthesis methods.
Oleylammonium Iodide (OLAI)	A source of both iodide and protonated oleylamine. Used to suppress proton exchange and strengthen ligand surface binding [57].	Key reagent for in situ ligand regulation in FAPbI₃ QD synthesis.
Metal-Organic Frameworks (e.g., UiO-66)	A porous scaffold for encapsulating PQDs. Provides spatial confinement, enhances environmental stability, and isolates QDs from moisture/oxygen [58].	Used as a robust host matrix for CsPbBr₃ QDs to achieve long-term stability.
Polydimethylsiloxane (PDMS)	An inert, waterproof polymer matrix for encapsulating PQD films. Protects against hydration-induced degradation [24].	Used to create stable, water-resistant PQD composite films for sensing applications.

Visualizing Structure-Property Relationships and Experimental Workflows

The following diagrams, generated using Graphviz DOT language, illustrate the core concepts and experimental workflows discussed in this guide.

Relationship Between Ligand Binding and Device Performance

Workflow for In Situ Ligand Regulation

Perovskite quantum dots (PQDs), particularly all-inorganic CsPbI3, have emerged as a leading material for next-generation photovoltaics due to their exceptional optoelectronic properties, including superior thermal stability, high absorption coefficients, and tunable bandgaps [18] [9]. Despite their significant potential, the widespread commercialization of PQD solar cells (PQD-SCs) has been hampered by intrinsic instability issues. The ionic crystal nature of PQDs makes them highly sensitive to environmental factors such as humidity, temperature, and light exposure, often leading to rapid degradation and phase transition from the photoactive black phase (α-, β-, or γ-phase) to a non-perovskite orthorhombic phase (δ-phase) [18] [9]. Furthermore, the long-chain ligands initially used in synthesis, such as oleic acid (OA) and oleylamine (OLA), stabilize the black phase but create a significant charge transport barrier, necessitating a ligand exchange process [18].

This case study explores how targeted ligand engineering serves as a powerful strategy to overcome these challenges. By designing ligands with specific functional groups, binding characteristics, and molecular structures, researchers can simultaneously passivate surface defects, enhance phase stability, and improve charge transport, thereby pushing the power conversion efficiencies (PCEs) of PQD-SCs to new heights. The ensuing sections provide a comparative analysis of recent pioneering ligand engineering approaches, detailing their experimental protocols, quantitative performance outcomes, and the underlying mechanisms responsible for their success.

Comparative Analysis of Ligand Engineering Strategies

The following table summarizes four prominent ligand engineering strategies, comparing their molecular targets, proposed mechanisms, and the resulting device performance.

Table 1: Comparison of Targeted Ligand Engineering Strategies for High-Efficiency PQD Solar Cells

Ligand/Strategy	Material System	Key Functional Groups & Binding Mechanism	Reported Power Conversion Efficiency (PCE)	Stability Performance
2-Thiophenemethylammonium Iodide (ThMAI) [18]	CsPbI3 PQDs	Thiophene (Lewis base to uncoordinated Pb²⁺), Ammonium (occupies Cs⁺ vacancies)	15.3%	83% of initial PCE retained after 15 days in ambient conditions
Sodium Heptafluorobutyrate (SHF) [59]	Perovskite Thin Film (p-i-n SC)	Carboxylate head (binds surface), Perfluorous tail (hydrophobic barrier)	27.02% (certified 26.96%)	100% retention after 1,200 h of maximum power point tracking
L-Phenylalanine (L-PHE) [42]	CsPbI3 PQDs	Amino acid (passivates undercoordinated Pb²⁺ ions and surface defects)	Not specified for SCs	>70% of initial PL intensity after 20 days of UV exposure
Trioctylphosphine Oxide (TOPO) [42]	CsPbI3 PQDs	Phosphine oxide (passivates surface defects)	Not specified for SCs	18% PL enhancement reported

Analysis of Performance Data

The data presented in Table 1 underscores the profound impact of ligand design on device performance. The multifaceted anchoring of ThMAI directly addresses several key challenges in CsPbI3 PQDs: its thiophene and ammonium groups effectively passivate surface defects, while the larger ionic radius of the ThMA⁺ cation helps restore beneficial surface tensile strain, stabilizing the black phase and yielding a notable PCE of 15.3% [18]. In contrast, the SHF molecule demonstrates the power of interfacial engineering in thin-film perovskite solar cells. By forming an ion shield that tunes the work function, increases defect formation energy, and promotes a compact electron transport layer, SHF enables a record-breaking PCE of 27.02% while offering exceptional operational and thermal stability [59]. Meanwhile, ligand modifications with TOPO and L-PHE primarily focus on enhancing optical properties and intrinsic stability, as evidenced by significant photoluminescence (PL) enhancements and improved resistance to UV radiation [42].

Experimental Protocols for Ligand Implementation

• Synthesis of CsPbI3 PQDs: CsPbI3 PQDs were synthesized using the standard hot-injection method. Cesium carbonate (Cs₂CO₃) and lead iodide (PbI₂) were used as precursors, dissolved in 1-octadecene (ODE) with OA and OLA as initial stabilizing ligands.
• Ligand Exchange Process: The synthesized PQDs, dispersed in hexane, were subjected to a ligand exchange process. A solution of ThMAI in acetonitrile was added to the PQD solution. The mixture was stirred vigorously to facilitate the replacement of the original long-chain OA/OLA ligands with ThMAI.
• Purification and Film Formation: The ThMAI-treated PQDs were purified using antisolvent precipitation (typically with methyl acetate or ethyl acetate) followed by centrifugation. This process removes the detached long-chain ligands and excess ThMAI. The purified PQD pellet was then redispersed in a solvent like octane for spin-coating to form solid films for device fabrication.

• Perovskite Film Fabrication: The perovskite light-absorbing layer (e.g., a formamidinium-caesium lead halide composition) was deposited onto the substrate using standard techniques like spin-coating.
• Post-Treatment with SHF: A solution of sodium heptafluorobutyrate (SHF) in a suitable solvent (e.g., isopropanol) was dynamically spin-coated onto the freshly prepared perovskite film.
• Annealing and C60 Deposition: The SHF-treated film was annealed at a moderate temperature (e.g., 100 °C) for a short duration to remove the solvent and ensure proper adhesion of the SHF layer. Subsequently, the electron transport layer (C60) was thermally evaporated on top of the functionalized perovskite surface.

Visualization of Ligand Engineering Mechanisms

Multifaceted Anchoring of ThMAI on PQD Surface

The diagram below illustrates how the ThMAI ligand interacts with the CsPbI3 PQD surface through multiple coordination sites, providing enhanced passivation and strain.

Work Function Tuning and Defect Passivation by SHF

This diagram depicts the dual function of SHF as an interfacial dipole layer that tunes the perovskite surface work function and passivates surface defects.

The Scientist's Toolkit: Essential Research Reagents

The following table lists key chemicals and materials used in the featured ligand engineering experiments, providing researchers with a practical reference for protocol replication.

Table 2: Key Research Reagent Solutions for Ligand Engineering in PQDs

Reagent/Material	Function in Experiment	Example Use Case
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchoring ligand for exchange	Passivates defects and induces strain in CsPbI3 PQDs [18]
Sodium Heptafluorobutyrate (SHF)	Interfacial modifier and passivator	Forms a functional layer between perovskite and C60, boosting efficiency and stability [59]
L-Phenylalanine (L-PHE)	Surface passivating ligand	Suppresses non-radiative recombination in CsPbI3 PQDs [42]
Trioctylphosphine Oxide (TOPO)	Lewis base ligand for passivation	Coordinates with undercoordinated Pb²⁺ ions to enhance PL [42]
Oleic Acid (OA) & Oleylamine (OLA)	Long-chain native synthesis ligands	Stabilizes PQDs during initial synthesis prior to exchange [18] [9]
Cesium Carbonate (Cs₂CO₃)	Cesium precursor	Source of Cs ions for PQD synthesis [18] [42]
Lead Iodide (PbI₂)	Lead precursor	Source of Pb and I ions for PQD synthesis [18] [42]
1-Octadecene (ODE)	Non-coordinating solvent	High-temperature reaction medium for hot-injection synthesis [18] [42] [9]

This case study demonstrates that targeted ligand engineering is a decisive strategy for overcoming the intrinsic limitations of perovskite quantum dots and thin films, directly enabling the realization of high-efficiency and stable solar cells. The comparative analysis reveals that while different ligands—such as the multifaceted ThMAI for PQDs and the dipole-forming SHF for thin films—operate through distinct mechanisms, they share a common principle: rational design that simultaneously addresses defect passivation, phase stabilization, and charge transport is key to unlocking superior device performance. The experimental protocols and reagent toolkit provide a foundation for researchers to further explore and innovate in this vibrant field. As the molecular-level understanding of ligand-PQD interactions deepens, the continued development of advanced ligand systems promises to accelerate the commercialization of perovskite photovoltaics.

The stability of Perovskite Quantum Dots (PQDs) is a pivotal factor determining their viability in applications ranging from biosensing to optoelectronics. Surface ligand binding affinity directly governs ambient, thermal, and optical stability by passivating surface defects, influencing chemical robustness, and mitigating ion migration. Ligands with strong binding affinity enhance stability by forming a protective barrier against environmental stressors such as oxygen, moisture, and heat, while weakly bound ligands desorb readily, leading to rapid degradation of the PQD core. This guide provides a systematic comparison of PQD stability metrics, linking performance directly to material composition, surface engineering strategies, and encapsulation technologies. The quantitative data and standardized experimental protocols presented herein offer researchers a framework for evaluating and developing next-generation stable PQD formulations.

Comparative Performance Metrics of PQD Systems

Ambient and Chemical Stability Benchmarks

Table 1: Ambient and Chemical Stability Performance of PQD Compositions

PQD Composition / Strategy	Encapsulation Method	Stressor Conditions	Key Stability Metrics	Performance Outcome	Reference
CsPbBr(_3) PQDs (Lead-based)	Unencapsulated	Ambient air exposure	Magnetic property retention	Complete degradation within 2 hours	[60]
CsPbBr(_3) PQDs (Lead-based)	~50 nm ALD Alumina (500 cycles, 50°C)	Ambient air exposure	Magnetic moment retention	Measurable moment after >2000 hours	[60]
V[TCNE](_x) Organometallic Magnetics	~80 nm ALD Alumina (800 cycles, 34°C)	Ambient conditions	Ferromagnetic Resonance (FMR) & Gilbert Damping	Magnetic properties preserved after hundreds of hours	[60]
Lead-based PQDs (e.g., CsPbX(_3))	Surface ligand engineering (e.g., oleylamine, PEI)	Aqueous environments	Structural & Photoluminescence Integrity	Rapid degradation due to Pb²⁺ release and ionic nature	[61] [62]
Bismuth-based PQDs (e.g., Cs(3)Bi(2)Br(_9))	Inherently lead-free composition	Aqueous & serum environments	Photoluminescence & Structural Integrity	Extended serum stability; already meets safety standards	[61]

Thermal and Optical Performance Metrics

Table 2: Thermal and Optical Stability of PQDs

PQD Composition	Thermal Stability Limit	Optical Properties	Quantum Yield (PLQY)	Key Optical Stability Findings	Reference
V[TCNE](_x) Organometallic Magnetics	Irreversible degradation at ~80°C	Magnonic properties (Low Gilbert damping)	Not Applicable	ALD process modified for low temp (30-50°C) growth to avoid thermal degradation.	[60]
Generic Lead-based PQDs (CsPbX(_3))	Varies with ligand & encapsulation	Tunable emission, Narrow FWHM (12-40 nm)	50% - 90%	High initial PLQY, but susceptible to quenching under heat and light stress.	[62]
Lead-free PQDs (e.g., Cs(3)Bi(2)X(_9))	Generally more robust	Broader emission (FWHM 40-60 nm)	Lower than lead-based	Trade-off often exists between eco-friendliness and peak optical performance.	[62]
CsPbX(_3) in sensing	Not specified	Detection via Fluorescence Quenching	High (Leveraged for sensitivity)	Enables ultra-sensitive LODs (e.g., 0.1 nM for heavy metals).	[62]

Experimental Protocols for Stability Assessment

Protocol for Ambient Stability Magnetic Characterization

This protocol, adapted from studies on V[TCNE](_x) films, quantitatively assesses the retention of magnetic properties under ambient exposure, serving as a proxy for the integrity of the PQD core [60].

• Sample Preparation & Encapsulation: Prepare thin films of the material via Chemical Vapor Deposition (CVD). For the test group, encapsulate the films with a conformal alumina layer using a low-temperature (30-50°C) Atomic Layer Deposition (ALD) process. This process involves sequential pulsing and purging of Trimethylaluminum (TMA) and water vapor precursors within an inert atmosphere glovebox to prevent pre-exposure degradation.
• Ambient Exposure: Place both encapsulated and unencapsulated (control) samples in ambient laboratory air conditions (~25°C, ~40% relative humidity).
• Time-Dependent Magnetometry: At predetermined time intervals (e.g., 0, 2, 24, 500, 2000 hours), use Vibrating Sample Magnetometry (VSM) to measure the saturation moment ((ms)) and remanent moment ((mr)) of the samples.
• Data Analysis: Plot the normalized magnetic moment ((m/m_0)) versus ambient exposure time. The performance is quantified by the time taken for the magnetic moment to decay to a predefined threshold (e.g., 50% of its initial value).

Protocol for Optical Stability and Quantum Yield Measurement

This standard protocol evaluates the optical resilience of PQDs, which is directly linked to surface trap states created by ligand desorption or ion migration [62].

• Sample Preparation: Synthesize PQDs using a controlled method (e.g., hot-injection, ligand-assisted reprecipitation). Disperse the PQDs in a suitable solvent matrix.
• Baseline Optical Characterization: Measure the initial photoluminescence quantum yield (PLQY) using an integrating sphere. Record the absorption and emission spectra, noting the peak wavelength and Full Width at Half Maximum (FWHM).
• Stress Application: Subject the PQD dispersion to a stressor. For thermal stress, incubate at an elevated temperature (e.g., 60-80°C). For optical stress, expose to continuous-wave UV or blue light excitation at a defined power density. For aqueous stress, disperse the PQDs in an aqueous buffer and agitate.
• Time-Dependent Monitoring: At regular intervals, remove aliquots and repeat the optical characterization (PLQY, emission spectra).
• Data Analysis: Plot PLQY and emission intensity over time. The stability is reported as the time to 50% loss of initial PLQY ((T_{50})) or the rate of spectral shift (e.g., nm/hour).

Protocol for Surface Binding Affinity Assessment via Sensing

The stability of the PQD-ligand complex directly influences its performance as a sensor. This protocol uses the sensor's selectivity and limit of detection to infer ligand binding strength [62].

• Functionalization: Modify the surface of PQDs with specific chelating or recognition ligands (e.g., oleylamine, poly(ethylenimine - PEI) designed to bind target analytes like heavy metal ions.
• Selectivity Testing: Expose the functionalized PQDs to a panel of potential interfering ions (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺) at a fixed concentration.
• Sensitivity Analysis: Titrate the target analyte (e.g., Hg²⁺, Cu²⁺) into the PQD dispersion and monitor the fluorescence response (quenching or enhancement).
• Data Analysis: Calculate the Limit of Detection (LOD) using the 3σ method. High selectivity and a low LOD indicate that the surface ligands remain bound and functional, effectively discriminating the target analyte, which reflects a stable PQD-ligand system.

Stability Pathways and Experimental Workflows

PQD Degradation Pathways

Atomic Layer Deposition Encapsulation

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagents and Materials for PQD Stability Research

Item Name	Function / Application	Specific Examples / Notes
Trimethylaluminum (TMA)	Precursor for Atomic Layer Deposition (ALD) of alumina (Al₂O₃) encapsulation layers.	Enables conformal, pinhole-free growth at low temperatures (30-50°C) [60].
Oleylamine / Oleic Acid	Standard surface ligands for PQD synthesis and stabilization in non-polar solvents.	Passivates surface defects; ratio and concentration critically impact stability and optical properties [62].
Poly(ethylenimine) (PEI)	A polymeric ligand used to functionalize PQDs for specific sensing applications and improve aqueous dispersion.	Enhances selectivity for certain analytes like heavy metal ions through its amine groups [62].
Cs₃Bi₂X₉ Precursors	Starting materials for synthesizing lead-free perovskite quantum dots.	Cesium acetate, bismuth bromide/iodide. Offers an eco-friendly alternative with enhanced aqueous stability [61] [62].
CsPbX₃ Precursors	Starting materials for synthesizing high-performance lead-halide PQDs.	Cesium carbonate, lead(II) bromide/iodide. Delivers superior initial optical performance but raises toxicity concerns [61] [62].
Metal-Organic Frameworks (MOFs)	Porous matrices for hosting and protecting PQDs, forming PQD@MOF composites.	Improves selectivity and stability in complex matrices (e.g., for biosensing in serum) by acting as a molecular sieve [61] [62].
UV-Cured Epoxy	A traditional, bulky encapsulation method for comparison against nanoscale ALD films.	Provides a baseline for stability enhancement but can cause thermal stress and delamination at cryogenic temperatures [60].

Conclusion

The strategic engineering of surface ligand binding affinity emerges as the most critical factor in unlocking the full potential of perovskite quantum dots. This synthesis of research demonstrates that ligands are not merely passive stabilizers but active components that can be rationally designed to control phase stability, defect density, and electronic coupling. The progression from foundational single ligands to advanced multifaceted and dual-ligand systems highlights a clear path toward achieving simultaneously high efficiency and robustness, as evidenced by record-breaking solar cells and highly stable LEDs. Future directions must focus on developing universally applicable design rules, leveraging computational prediction to accelerate ligand discovery, and exploring the integration of these stabilized PQDs into biomedical platforms such as targeted drug delivery and bio-imaging. The convergence of high-binding-affinity ligand chemistry with PQD synthesis promises to catalyze the next wave of innovation in both optoelectronics and therapeutic applications.

References

• 1. Short review on the instability and potential solutions for ... [https://www.sciencedirect.com/science/article/pii/S2666086522000637]
• 2. Ligand Engineering of Inorganic Lead Halide Perovskite ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11280295/]
• 3. How to improve the structural stabilities of halide perovskite ... [https://nanoconvergencejournal.springeropen.com/articles/10.1186/s40580-024-00412-x]
• 4. Stabilized Perovskite Quantum Dot Solids via Nonpolar ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10427392/]
• 5. Ligand (biochemistry) [https://en.wikipedia.org/wiki/Ligand_(biochemistry)]
• 6. Binding affinity - (Biological Chemistry I) - Vocab, Definition ... [https://fiveable.me/key-terms/biological-chemistry-i/binding-affinity]
• 7. Binding Affinity [https://www.malvernpanalytical.com/en/products/measurement-type/binding-affinity]
• 8. The dynamic adsorption affinity of ligands is a surrogate for ... [https://www.nature.com/articles/s41467-024-46368-8]
• 9. Ligand Engineering of Inorganic Lead Halide Perovskite ... [https://www.mdpi.com/2079-4991/14/14/1201]
• 10. Surface passivation of intensely luminescent all-inorganic ... [https://www.nature.com/articles/s41467-022-35702-7]
• 11. Understanding the cation-selective ligand passivation for ... [https://www.sciencedirect.com/science/article/abs/pii/S1385894723044054]
• 12. Thermal tolerance of perovskite quantum dots dependent ... [https://www.nature.com/articles/s41467-023-37943-6]
• 13. Modified surface ligand management of CsPbI 3 perovskite ... [https://www.sciencedirect.com/science/article/abs/pii/S1566119924000570]
• 14. Effect of Surface Ligands in Perovskite Nanocrystals [https://pmc.ncbi.nlm.nih.gov/articles/PMC8023572/]
• 15. Enhanced coordination interaction with multi-site binding ... [https://www.nature.com/articles/s41467-025-61563-x]
• 16. Stability improvement of photoluminescent QLEDs based ... [https://www.sciencedirect.com/science/article/abs/pii/S0925838821029145]
• 17. Interface–bulk defect engineering via dual ligands for photolithography perovskite quantum dots - ScienceDirect [https://www.sciencedirect.com/science/article/abs/pii/S1385894725094288?dgcid=rss_sd_all]
• 18. Multifaceted anchoring ligands for uniform orientation and ... [https://www.sciencedirect.com/science/article/abs/pii/S1385894724058017]
• 19. Impact of Polymer Type and Relative Humidity on the Long-Term Physical Stability of Amorphous Solid Dispersions - PubMed [https://pubmed.ncbi.nlm.nih.gov/29050468/]
• 20. Delayed effects of the relative humidity on the outpatient visits of dry eye disease in Shanghai, China: effect modification by green and blue space — JOGH [https://jogh.org/2025/jogh-15-04142]
• 21. Multifaceted Anchoring Ligands for Uniform Orientation ... [https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4844340]
• 22. Liquid bidentate ligand for full ligand coverage towards ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11704330/]
• 23. Liquid bidentate ligand for full ligand coverage towards ... [https://www.nature.com/articles/s41377-024-01704-x]
• 24. Polydimethylsiloxane encapsulated CsPbBr 3 quantum dots ... [https://pubs.rsc.org/en/content/articlehtml/2025/an/d5an00719d?page=search]
• 25. Achieving long-term water stability and strong exciton–ph... [https://www.degruyterbrill.com/document/doi/10.1515/nanoph-2025-0059/html?lang=en&srsltid=AfmBOoqStFWWbMOvqV0RCkljwuft_1qlnLCmtWtMIfU5lpLi1M_sprt6]
• 26. An original approach to measure ligand/receptor binding ... [https://www.nature.com/articles/s41598-022-09217-6]
• 27. A straightforward method for measuring binding affinities of ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12042207/]
• 28. How to measure and evaluate binding affinities [https://elifesciences.org/articles/57264]
• 29. Dual synergistic response for the electrochemical detection ... [https://www.sciencedirect.com/science/article/abs/pii/S003991402200409X]
• 30. Automated design of multi-target ligands by generative ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11390726/]
• 31. Dual-channel hierarchical interactive learning for the ... [https://www.sciencedirect.com/science/article/abs/pii/S0893608025008639]
• 32. Boosting carrier transport via functionalized short-chain ... [https://www.sciencedirect.com/science/article/abs/pii/S1385894724090879]
• 33. Conjugated Polymer‐Driven Compact Crystal Packing and ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12490187/]
• 34. Enriching conductive capping by alkaline treatment of ... [https://www.nature.com/articles/s41467-025-63618-5]
• 35. Perovskite quantum dot solar cell achieves record ... [https://www.pv-magazine.com/2025/10/08/perovskite-quantum-dot-solar-cell-achieves-record-breaking-efficiency-of-18-3/]
• 36. Dimensional tunability in SN2-based synthesis of CsPbI3 ... [https://www.sciopen.com/article/10.26599/NR.2025.94907968]
• 37. Liquid bidentate ligand for full ligand coverage towards efficient... [https://pubmed.ncbi.nlm.nih.gov/39762233/]
• 38. Consecutive surface matrix engineering of FAPbI3 perovskite quantum... [https://pubs.rsc.org/en/content/articlelanding/2025/ee/d5ee02127h]
• 39. High-Performance Perovskite Quantum Dot Solar Cells Enabled by... [https://link.springer.com/article/10.1007/s40820-022-00946-x]
• 40. Charge Transfer from Perovskite Quantum Dots to... | CoLab [https://colab.ws/articles/10.1021/acsenergylett.5c00881]
• 41. How to improve the structural stabilities of halide perovskite ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10821855/]
• 42. Effect of Surface Ligand Modification on the Optical ... [https://www.sciencedirect.com/science/article/abs/pii/S2468023025023211]
• 43. Recent Research Progress in Surface Ligand Exchange of ... [https://www.mdpi.com/2076-3417/10/3/975]
• 44. Organic-inorganic hybrid nature enables efficient and ... [https://www.sciencedirect.com/science/article/pii/S2542435123004488]
• 45. Unraveling the Impact of Halide Mixing on Perovskite Stability [https://pmc.ncbi.nlm.nih.gov/articles/PMC7156144/]
• 46. Enlarging moment and regulating orientation of buried ... [https://www.nature.com/articles/s41467-024-55653-5]
• 47. Enhanced stability and efficiency in perovskite solar cells ... [https://www.nature.com/articles/s41467-025-62125-x]
• 48. Ion-induced field screening as a dominant factor in ... [https://www.nature.com/articles/s41560-024-01487-w]
• 49. Experimental and computational models to understand ... [https://www.sciencedirect.com/science/article/pii/S2772417424000050]
• 50. Enhancing binding affinity predictions through efficient ... [https://pubs.rsc.org/en/content/articlehtml/2025/sc/d5sc01030f]
• 51. Exploring Protein–Ligand Binding-Affinity Prediction | Rowan [https://rowansci.com/blog/exploring-protein-ligand-binding-affinity-prediction]
• 52. From Docking and Molecular Dynamics to Experimental ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12379015/]
• 53. Distance plus attention for binding affinity prediction [https://jcheminf.biomedcentral.com/articles/10.1186/s13321-024-00844-x]
• 54. Computing the relative binding affinity of ligands based on ... [https://www.nature.com/articles/s43588-023-00529-9]
• 55. Complementary Dual-Ligands Resurfacing CsPbI3 ... [https://pubmed.ncbi.nlm.nih.gov/40465317/]
• 56. Estimation of Exciton Binding Energy and lifetime for Mono ... [https://arxiv.org/html/2506.05075v1]
• 57. In Situ Bonding Regulation of Surface Ligands for Efficient ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC9762318/]
• 58. Achieving long-term water stability and strong exciton–ph... [https://www.degruyterbrill.com/document/doi/10.1515/nanoph-2025-0059/html?lang=en&srsltid=AfmBOoowV3HLPJo75F7t47cSy8XELCipd47N-O_v3Le536uN75xW0-8-]
• 59. Stabilizing high-efficiency perovskite solar cells via ... [https://www.nature.com/articles/s41566-025-01791-1]
• 60. Enabling ambient stability and quantum integration of ... [https://www.nature.com/articles/s41467-025-65588-0]
• 61. Next-Generation Biosensing with Perovskite Quantum Dots [https://www.sciencedirect.com/science/article/pii/S1872204025001860]
• 62. Pioneering perovskite quantum dot nanosensors for heavy ... [https://pubs.rsc.org/en/content/articlehtml/2025/ra/d5ra06200d?page=search]