Semiconductor nanocrystals, known as quantum dots (QDs), have unique optical
properties that makes them useful for application in various light-emitting devices, such
as LEDs, displays and lasers. For actual application of QDs, energy efficiency of their light
emission and material stability are crucial. To achieve high energy efficiency, it must be
understood which types of atomic defects on the nanocrystals lead to energy losses and
how to prevent them. To achieve stable QDs, it is important to understand which kinds of
chemical reactions the QDs can undergo, if these reactions lead to the formation of new
defects and how to prevent this...@en

Electrochemical charging of films of semiconductor nanocrystals (NCs) allows precise control over their Fermi level and opens up new possibilities for use of semiconductor NCs in optoelectronic devices. Unfortunately, charges added to the semiconductor NCs are often lost due to electrochemical side reactions. In this work, we examine which loss processes can occur in electrochemically charged semiconductor NC films by comparing numerical drift-diffusion simulations with experimental data. Both reactions with impurities in the electrolyte solution, as well as reactions occurring on the surface of the nanomaterials themselves, are considered. We show that the Gerischer kinetic model can be used to accurately model the one-electron transfer between charges in the semiconductor NC and oxidant or reductant species in solution. Simulations employing the Gerischer model are in agreement with experimental results of charging of semiconductor NC films with ideal one-electron acceptors ferrocene and cobaltocene. We show that reactions of charges in the semiconductor NC film with redox species in solution are reversible when the reduction potential is in the conduction band of the semiconductor NC material but are irreversible when the reduction potential is in the band gap. Experimental charging of semiconductor NC films in the presence of oxygen is always irreversible in our system, even when the reduction potential of oxygen is in the conduction band of the semiconductor NC material. We show that the Gerischer model in combination with a coupled reversible-irreversible reaction mechanism can be used to model oxygen reduction. Finally, we model irreversible reduction reactions with the semiconductor NC material itself, such as reduction of ligands or surface ions. Simulations of semiconductor NC cyclic voltammograms in the presence of material reduction reactions strongly resemble experimental cyclic voltammograms of InP and CdSe NC films. This marks material reduction reactions at the semiconductor NC surface as a likely candidate for the irreversible behavior of these materials in electrochemical experiments. These results show that all reduction reactions with redox potentials in the band gap of semiconductor NCs must be suppressed in order to achieve stable charging of these materials.

@en

InP/ZnSe/ZnS core/shell/shell quantum dots are the most investigated quantum dot material for commercial applications involving visible light emission. The inner InP/ZnSe interface is complex since it is not charge balanced, and the InP surface is prone to oxidation. The role of oxidative defects at this interface has remained a topic of debate, with conflicting reports of both detrimental and beneficial effects on the quantum dot properties. In this study we probe the structure of the InP/ZnSe interface at the atomic level using 31P, 77Se and 17O ssNMR and HAADF-STEM. We observe clear differences in Se NMR spectra and crystal orientation of core and shell when the InP/ZnSe is oxidized on purpose. High levels of interface oxidation result in an amorphous phosphate layer at the interface, which inhibits epitaxial growth of the ZnSe shell.@en

Indium phosphide (InP) quantum dots (QDs) are considered the most promising alternative for Cd and Pb-based QDs for lighting and display applications. However, while core-only QDs of CdSe and CdTe have been prepared with near-unity photoluminescence quantum yield (PLQY), this is not yet achieved for InP QDs. Treatments with HF have been used to boost the PLQY of InP core-only QDs up to 85%. However, HF etches the QDs, causing loss of material and broadening of the optical features. Here, we present a simple postsynthesis HF-free treatment that is based on passivating the surface of the InP QDs with InF_3. For optimized conditions, this results in a PLQY as high as 93% and nearly monoexponential photoluminescence decay. Etching of the particle surface is entirely avoided if the treatment is performed under stringent acid-free conditions. We show that this treatment is applicable to InP QDs with various sizes and InP QDs obtained via different synthesis routes. The optical properties of the resulting core-only InP QDs are on par with InP/ZnSe/ZnS core-shell QDs, with significantly higher absorption coefficients in the blue, and with potential for faster charge transport. These are important advantages when considering InP QDs for use in micro-LEDs or photodetectors.

@en

The optoelectronic properties of colloidal quantum dots (cQDs) depend critically on the absolute energy of the conduction and valence band edges. It is well known these band-edge energies are sensitive to the ligands on the cQD surface, but it is much less clear how they depend on other experimental conditions, like solvation. Here, we experimentally determine the band-edge positions of thin films of PbS and ZnO cQDs via spectroelectrochemical measurements. To achieve this, we first carefully evaluate and optimize the electrochemical injection of electrons and holes into PbS cQDs. This results in electrochemically fully reversible electron injection with >8 electrons per PbS cQDs, allowing the quantitative determination of the conduction band energy for PbS cQDs with various diameters and surface compositions. Surprisingly, we find that the band-edge energies shift by nearly 1 eV in the presence of different solvents, a result that also holds true for ZnO cQDs. We argue that complexation and partial charge transfer between solvent and surface ions are responsible for this large effect of the solvent on the band-edge energy. The trend in the energy shift matches the results of density functional theory (DFT) calculations in explicit solvents and scales with the energy of complexation between surface cations and solvents. As a first approximation, the solvent Lewis basicity can be used as a good descriptor to predict the shift of the conduction and valence band edges of solvated cQDs.

@en

Electrochemical charging of nanocrystal films opens up new possibilities for designing quantum dot-based device structures, but a solid theoretical framework of this process and its limitations is lacking. In this work, drift-diffusion simulations are employed to model the charging of nanocrystal films and gain insight into the electrochemical doping process. Through steady state simulations it is shown that the Fermi level and doping density in the nanocrystal film depend on the concentration of the electrolyte in addition to the value of the applied potential. Time-resolved simulations reveal that charging is often limited by transport of electrolyte ions. However, ion transport in the film is dominated by drift, rather than diffusion, and the concentration profile of ions differs substantially from concentration profiles of diffusing redox species at flat electrodes. Classical electrochemical theory cannot be used to model this type of mass transport limited behavior in films of nanocrystals, so a new model is developed. We show that the Randles-Ševčík equation, which is derived for electrochemical species diffusing in solution, but is often applied to films as well, results in a significant underestimation of the diffusion coefficients of the charge compensating electrolyte ions.

@en

Owing to their tunable band gap, high absorption coefficient, narrow emission linewidths and unrestricted composition, InP-based colloidal quantum dots (QDs) have become industrially relevant for visible and near-infrared photonic technologies. Although their development has so far been strongly driven by their suitability for green and red light-emitting diodes, the spectrum of applications for this class of materials is much broader. This Review covers the multidisciplinary field of InP-based QDs from its genesis in the mid-1990s to date, drawing on relevant knowledge from other classes of QDs and from III–V semiconductors as a whole. We discuss the optoelectronic properties of InP QDs, their fabrication, their defects and passivation strategies and the design of InP-based QD heterostructures. Finally, we outline the technological status of these QDs for various photonic applications.

@en

Indium phosphide quantum dots are the main alternative for toxic and restricted Cd-based quantum dots for lighting and display applications, but in the absence of protecting ZnSe and/or ZnS shells, InP quantum dots suffer from low photoluminescence quantum yields. Traditionally, HF treatments have been used to improve the quantum yield of InP to ~50%, but these treatments are dangerous and not well understood. Here, we develop a postsynthetic treatment that forms HF in situ from benzoyl fluoride, which can be used to strongly increase the quantum yield of InP core-only quantum dots. This treatment is water-free and can be performed safely. Simultaneous addition of the z-type ligand ZnCl2 increases the photoluminescence quantum yield up to 85%. Structural analysis via XPS as well as solid state and solution NMR measurements shows that the in situ generated HF leads to a surface passivation by indium fluoride z-type ligands and removes polyphosphates, but not PO3 and PO4 species from the InP surface. With DFT calculations it is shown that InP QDs can be trap-free even when PO3 and PO4 species are present on the surface. These results show that both polyphosphate removal and z-type passivation are necessary to obtain high quantum yields in InP core-only quantum dots. They further show that core-only InP QDs can achieve photoluminescence quantum yields rivalling those of InP/ZnSe/ZnS core/shell/shell QDs and the best core-only II-VI QDs.

@en