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Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

ABSTRACT: Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl₃ or CdCl₂ afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

SUBMITTER: Kirkwood N

PROVIDER: S-EPMC6257620 | biostudies-literature | 2018 Nov

REPOSITORIES: biostudies-literature

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Publications

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

Kirkwood Nicholas N Monchen Julius O V JOV Crisp Ryan W RW Grimaldi Gianluca G Bergstein Huub A C HAC du Fossé Indy I van der Stam Ward W Infante Ivan I Houtepen Arjan J AJ

Journal of the American Chemical Society 20181112 46

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstra ...[more]

PMID: 30375226

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Project description:Pairs of coupled quantum dots with controlled coupling between the two potential wells serve as an extremely rich system, exhibiting a plethora of optical phenomena that do not exist in each of the isolated constituent dots. Over the past decade, coupled quantum systems have been under extensive study in the context of epitaxially grown quantum dots (QDs), but only a handful of examples have been reported with colloidal QDs. This is mostly due to the difficulties in controllably growing nanoparticles that encapsulate within them two dots separated by an energetic barrier via colloidal synthesis methods. Recent advances in colloidal synthesis methods have enabled the first clear demonstrations of colloidal double quantum dots and allowed for the first exploratory studies into their optical properties. Nevertheless, colloidal double QDs can offer an extended level of structural manipulation that allows not only for a broader range of materials to be used as compared with epitaxially grown counterparts but also for more complex control over the coupling mechanisms and coupling strength between two spatially separated quantum dots. The photophysics of these nanostructures is governed by the balance between two coupling mechanisms. The first is via dipole-dipole interactions between the two constituent components, leading to energy transfer between them. The second is associated with overlap of excited carrier wave functions, leading to charge transfer and multicarrier interactions between the two components. The magnitude of the coupling between the two subcomponents is determined by the detailed potential landscape within the nanocrystals (NCs). One of the hallmarks of double QDs is the observation of dual-color emission from a single nanoparticle, which allows for detailed spectroscopy of their properties down to the single particle level. Furthermore, rational design of the two coupled subsystems enables one to tune the emission statistics from single photon emission to classical emission. Dual emission also provides these NCs with more advanced functionalities than the isolated components. The ability to better tailor the emission spectrum can be advantageous for color designed LEDs in lighting and display applications. The different response of the two emission colors to external stimuli enables ratiometric sensing. Control over hot carrier dynamics within such structures allows for photoluminescence upconversion. This Account first provides a description of the main hurdles toward the synthesis of colloidal double QDs and an overview of the growing library of synthetic pathways toward constructing them. The main discoveries regarding their photophysical properties are then described in detail, followed by an overview of potential applications taking advantage of the double-dot structure. Finally, a perspective and outlook for their future development is provided.

Dataset Information

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

Publications

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

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