Indium phosphide (InP) quantum dots (QDs) are promising heavy-metal-free materials for optoelectronics, but their redox stability, trap-state landscape, and charge carrier dynamics are not well understood. Here we investigate InP and InP/ZnSe/ZnS QD films with different ligands by using spectroelectrochemistry. For both core-only and core/shell/shell QD films, the absorption spectra remain unchanged during charging, indicating that injected charges do not populate the conduction or valence bands. InP/ZnSe/ZnS QD films with original ligands exhibit reversible photoluminescence (PL) modulation: an increase at modest cathodic potentials, followed by quenching at more negative potentials. Solid-state ligand exchange using ethylenediamine (2DA) and sodium sulfide (Na₂S) enhances conductivity and induces stronger PL changes at both cathodic and anodic potentials. These results are in line with the population of electron traps at modest cathodic potentials (i.e., near the midbandgap), suppressing nonradiative recombination and increasing the PL. At more negative potentials, electrochemical reactions of surface species result in new trap states quenching the PL. Our findings provide insights into the stability and trap-state-mediated carrier dynamics during electrochemical charging of InP-based QDs.

Metal nanoparticles and their environment can be locally heated on an ultrafast time scale using femtosecond pulsed illumination of their plasmon resonance, making them of interest for spatiotemporal temperature control. Here, we propose experimental approaches to obtain time-resolved particle and medium temperatures using gold nanoparticles. 23.5 and 39 nm nanoparticles dispersed in water and DMF:water mixture were heated and probed using transient absorption spectroscopy. Simulations indicate that the change in absorbance >10 ps after excitation arises from temperature-induced alterations in the dielectric functions of the particle and the medium. Thus, we measured the temperature-dependent absorbance spectra of nanoparticles, where the signal reflects the combined response of the particle and the medium to heating for a known temperature. We then disentangled the spectra obtaining the particle (Method 1) and the medium contributions (Method 2) to heating independently, followed by a consistency check between the two approaches (Method 3). Accordingly, the transient absorbance spectrum was resolved to extract particle and medium temperatures at each time delay. The resulting profiles are in line with each other, revealing temperature increases of ∼80 K for the particle and 5–15 K for the medium when excited at 400 nm with ∼4 J/m² fluence. A faster particle temperature decay was observed with decreasing particle size and a faster medium temperature decay with increasing medium thermal diffusivity, in agreement with expectations. Overall, we demonstrate an experimental methodology for simultaneous determination of particle and medium temperatures under a spatiotemporal gradient which is relevant for studies with transient heating and nanoparticles as sensors.

Control over the electron density and conductivity is a cornerstone of semiconductor technology. Here, we report electrochemical control over electron density and conductivity in films of InAs colloidal quantum dots (QDs) capped with ethanedithiol ligands. The quantum-confined twofold degenerate 1S1/2(e) electron state can be reversibly and completely filled. Increasing the electron population yields four bleach features in the optical-absorption spectrum associated transitions to the 1S1/2(e) state, and state-resolved electronic conductivity which follows the 1S1/2(e) density of states, reaching a maximum of 0.45 S/m at 0.5 electrons per QD. The absence of 1P(e) bleach features and state-resolved conductivity imply a wide separation between the 1S1/2(e) and 1P(e) states resulting in electronic transport between 1S(e) states exclusively. The reversible electrochemistry of InAs QDs films allows determination of the absolute energy levels. InAs QDs of 4.2 nm in edge length and capped with ethanedithiol ligands are natively n-doped with the Fermi level at -4.6 eV, the 1S1/2(e) state at -4.28 eV and the 1S3/2(h) state at -5.54 eV vs vacuum. This work establishes a way to precisely control the charge carrier density and conductivity and gives insights into the charge transport properties and electronic structure of InAs QD films, opening the possibility of making devices with InAs QDs in which the charge carrier density is precisely controlled electrochemically.

The efficiency of quantum dot (QD) light-emitting diodes is limited by inefficient hole injection into the valence levels of the QDs. Electrochemical doping, where mobile ions form electrical double layers (EDLs) at electrodes, offers a route to removing injection barriers. While QD light-emitting electrochemical cells (QLECs) have shown promise, prior studies relied on additional charge injection layers, complicating the study of charge injection into QDs. In this work, devices with a simple ITO/QD active layer/Al structure were fabricated using highly photoluminescent ligand-exchanged CdSe/CdS/ZnS QDs, poly(ethylene oxide), and lithium trifluoromethanesulfonate as electrolyte. We show that the dense QD films in these QLECs can be electrochemically doped, transport charges, and exhibit electroluminescence. Symmetrical cyclic voltammograms and operando photoluminescence measurements prove that these devices function as electrochemically doped LECs. Spectroelectrochemical experiments on separately n- and p-doped QD films indicate that hole injection remains the primary limitation in QLEC performance. These findings demonstrate that using EDLs to facilitate charge injection in QD light-emitting devices is promising, but significant challenges remain to be solved before electron and hole injections are balanced.

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.

Indium phosphide is the leading material for commercial applications of colloidal quantum dots. To date, however, the community has failed to achieve successful operation under strong excitation conditions, contrasting sharply with other materials. Here, we report unusual photophysics of state-of-the-art InP-based quantum dots, which makes them unattractive as a laser gain material despite a near-unity quantum yield. A combination of ensemble-based time-resolved spectroscopy over timescales from femtoseconds to microseconds and single-quantum-dot spectroscopy reveals ultrafast trapping of hot charge carriers. This process reduces the achievable population inversion and limits light amplification for lasing applications. However, it does not quench fluorescence. Instead, trapped carriers can recombine radiatively, leading to delayed—but bright—fluorescence. Single-quantum-dot experiments confirm the direct link between hot-carrier trapping and delayed fluorescence. Hot-carrier trapping thus explains why the latest generation of InP-based quantum dots struggle to support optical gain, although the quantum yield is near unity for low-intensity applications. Comparison with other popular quantum-dot materials—CdSe, Pb–halide perovskites, and CuInS₂—indicate that the hot-carrier dynamics observed are unique to InP.

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.

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.

Lead halide perovskites have attracted significant attention for their wide-ranging applications in optoelectronic devices. A ubiquitous element in these applications is that charging of the perovskite is involved, which can trigger electrochemical degradation reactions. Understanding the underlying factors governing these degradation processes is crucial for improving the stability of perovskite-based devices. For bulk semiconductors, the electrochemical decomposition potentials depend on the stabilization of atoms in the lattice-a parameter linked to the material’s solubility. For perovskite nanocrystals (NCs), electrochemical surface reactions are strongly influenced by the binding equilibrium of passivating ligands. Here, we report a spectro-electrochemical study on CsPbBr₃ NCs and bulk thin films in contact with various electrolytes, aimed at understanding the factors that control cathodic degradation. These measurements reveal that the cathodic decomposition of NCs is primarily determined by the solubility of surface ligands, with diminished cathodic degradation for NCs in high-polarity electrolyte solvents where ligand solubilities are lower. However, the solubility of the surface ligands and bulk lattice of NCs are orthogonal, such that no electrolyte could be identified where both the surface and bulk are stabilized against cathodic decomposition. This poses inherent challenges for electrochemical applications: (i) The electrochemical stability window of CsPbBr₃ NCs is constrained by the reduction potential of dissolved Pb²⁺ complexes, and (ii) cathodic decomposition occurs well before the conduction band can be populated with electrons. Our findings provide insights to enhance the electrochemical stability of perovskite thin films and NCs, emphasizing the importance of a combined selection of surface passivation and electrolyte.

Mixed Sn-Pb halide perovskites are promising absorber materials for solar cells due to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are susceptible to oxidation affecting the optoelectronic properties. In this work, we investigated qualitatively and quantitatively metastable oxygen-induced doping in isolated ASn_xPb_1-xI₃ (where A is methylammonium or a mixture of formamidinium and cesium) perovskite thin films by means of microwave conductivity, structural and optical characterization techniques. We observe that longer oxygen exposure times lead to progressively higher dark conductivities, which slowly decay back to their original levels over days. Here oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and creation of free holes. The metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. Next, we show that doping not only leads to the reduction in the photoconductivity signal but also induces long-term effects even after loss of doping, which is thought to derive from consecutive oxidation reactions leading to the formation of defect states. On prolonged exposure to oxygen and light, optical and structural changes can be observed and related to the formation of SnO_x and loss of iodide near the surface. Our work highlights that even a short-term exposure to oxygen immediately impairs the charge carrier dynamics of the perovskite, while structural perovskite degradation is only noticeable upon long-term exposure and accumulation of oxidation products. Hence, for efficient solar cells, exposure of mixed Sn-Pb perovskites to oxygen during production and operation should be rigorously blocked.

Although density functional theory (DFT) calculations have been crucial in our understanding of colloidal quantum dots (QDs), simulations are commonly carried out on QD models that are significantly smaller than those generally found experimentally. While smaller models allow for efficient study of local surface configurations, increasing the size of the QD model will increase the size or number of facets, which can in turn influence the energetics and characteristics of trap formation. Moreover, core-shell structures can only be studied with QD models that are large enough to accommodate the different layers with the correct thickness. Here, we use DFT calculations to study the electronic properties of QDs as a function of size, up to a diameter of ∼4.5 nm. We show that increasing the size of QD models traditionally used in DFT studies leads to a disappearance of the band gap and localization of the HOMO and LUMO levels on facet-specific regions of the QD surface. We attribute this to the lateral coupling of surface orbitals and the formation of surface bands. The introduction of surface vacancies and their a posteriori refilling with Z-type ligands leads to surface reconstructions that widen the band gap and delocalize both the HOMO and LUMO. These results show that the surface geometry of the facets plays a pivotal role in defining the electronic properties of the QD.

Ytterbium-doped LiYF ₄ (Yb:YLF) is a commonly used material for laser applications, as a photon upconversion medium, and for optical refrigeration. As nanocrystals (NCs), the material is also of interest for biological and physical applications. Unfortunately, as with most phosphors, with the reduction in size comes a large reduction of the photoluminescence quantum yield (PLQY), which is typically associated with an increase in surface-related PL quenching. Here, we report the synthesis of bipyramidal Yb:YLF NCs with a short axis of ∼60 nm. We systematically study and remove all sources of PL quenching in these NCs. By chemically removing all traces of water from the reaction mixture, we obtain NCs that exhibit a near-unity PLQY for an Yb ³⁺ concentration below 20%. At higher Yb ³⁺ concentrations, efficient concentration quenching occurs. The surface PL quenching is mitigated by growing an undoped YLF shell around the NC core, resulting in near-unity PLQY values even for fully Yb ³⁺-based LiYbF ₄ cores. This unambiguously shows that the only remaining quenching sites in core-only Yb:YLF NCs reside on the surface and that concentration quenching is due to energy transfer to the surface. Monte Carlo simulations can reproduce the concentration dependence of the PLQY. Surprisingly, Förster resonance energy transfer does not give satisfactory agreement with the experimental data, whereas nearest-neighbor energy transfer does. This work demonstrates that Yb ³⁺-based nanophosphors can be synthesized with a quality close to that of bulk single crystals. The high Yb ³⁺ concentration in the LiYbF ₄/LiYF ₄ core/shell nanocrystals increases the weak Yb ³⁺ absorption, making these materials highly promising for fundamental studies and increasing their effectiveness in bioapplications and optical refrigeration.

Indium phosphide colloidal quantum dots (CQDs) are the main alternative for toxic and restricted Cd based CQDs for lighting and display applications. Here we systematically report on the size-dependent optical absorption, ensemble, and single particle photoluminescence (PL) and biexciton lifetimes of core-only InP CQDs. This systematic study is enabled by improvements in the synthesis of InP CQDs to yield a broad size series of monodisperse core-only InP CQDs with narrow absorption and PL line width and significant PL quantum yield.

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.

Quantum dots (QDs) are known for their size-dependent optical properties, narrow emission bands, and high photoluminescence quantum yield (PLQY), which make them interesting candidates for optoelectronic applications. In particular, InP QDs are receiving a lot of attention since they are less toxic than other QD materials and are hence suitable for consumer applications. Most of these applications, such as LEDs, photovoltaics, and lasing, involve charging QDs with electrons and/or holes. However, charging of QDs is not easy nor innocent, and the effect of charging on the composition and properties of InP QDs is not yet well understood. This work provides theoretical insight into electron charging of the InP core and InP/ZnSe QDs. Density functional theory calculations are used to show that charging of InP-based QDs with electrons leads to the formation of trap states if the QD contains In atoms that are undercoordinated and thus have less than four bonds to neighboring atoms. InP core-only QDs have such atoms at the surface, which are responsible for the formation of trap states upon charging with electrons. We show that InP/ZnSe core-shell models with all In atoms fully coordinated can be charged with electrons without the formation of trap states. These results show that undercoordinated In atoms should be avoided at all times for QDs to be stably charged with electrons.