Arjan J. Houtepen

Electronic doping underpins many semiconductor applications but has remained elusive for lead halide perovskite nanocrystals because their high density and low formation energy of defects easily compensate introduced dopants. Here, we use electrochemical doping as a strategy to avoid doping compensation. Electrochemical injection of electrons or holes into the nanocrystal is compensated by surface-adsorbed electrolyte counterions, therefore not compromising crystal structure. We accomplished ambipolar (n- and p-type) doped CsPbBr3 nanocrystals with charge carrier concentrations ranging from intrinsic up to 4.8 × 1017 cm–3 (corresponding to 0.25 charges per nanocrystal). We show that electrochemical doping is reversible and stable at these free carrier concentrations. The extent of doping and stability are enabled by the use of lecithin, a zwitterionic ligand that strongly binds to the surface of the CsPbBr3 nanocrystals while still allowing ion and electron transfer. Lecithin stabilizes the surface atoms by shifting the potentials for Pb2+ reduction and Br– oxidation outside of the bandgap. The inferred surface-chemistry design rules for stable electrochemical doping can also serve as a guideline for developing CsPbBr3 nanocrystals stable under high free-carrier concentrations in optoelectronic devices, such as light-emitting diodes or electronically triggered quantum light sources.

Shortwave infrared (SWIR) photodetectors are crucial for emerging technologies such as autonomous driving, smart agriculture, industrial inspection, and eye-safe LiDAR. Colloidal indium arsenide (InAs) quantum dots (QDs) offer a low-cost, RoHS-compliant alternative to epitaxial III–V semiconductors. But their performance is limited by a high density of surface trap states that induce nonradiative recombination and potentially give rise to a high dark current in photodiodes. Here, we demonstrate that InF3 can effectively passivate these surface traps, significantly improving both optical and electronic properties. In nonpolar solvents, a simple InF3 treatment on oleylamine/chloride-capped In(As,P) and oleate-capped InAs QDs increases the photoluminescence quantum yield to 13% and 4%, respectively─the highest reported for core-only InAs QDs. In polar media, the InF3 treatment remains effective but reduces colloidal stability. To overcome this limitation, a solid-state in situ InF3 treatment of QD films was used. The treatment doubles the carrier lifetime and lowers the dark carrier density, confirmed by ultrafast transient absorption spectroscopy, microwave conductivity, and electrochemically gated transistor measurements. In SWIR photodetectors, this leads to an order-of-magnitude reduction in dark current density and a 3-fold increase in the calculated specific detectivity, assuming it is shot-noise-limited. Temperature-dependent dark current density and thermal admittance spectroscopy measurements reveal that deep trap states dominate the dark current in InAs QDs photodetectors. The InF3 treatment strongly suppresses these traps. These findings establish InF3 passivation as a powerful, promising strategy for mitigating surface-trap-induced dark current and advancing high-performance, RoHS-compliant SWIR photodetectors.

Metal nanoparticles strongly absorb light at their localized surface plasmon resonance (LSPR). The absorbed energy generates plasmons under femtosecond excitation, which decay and lead to ultrafast heating of the particles. Thus, these materials can be utilized as nanoheaters to increase the temperature of their local environment on picosecond to nanosecond timescales. The temperature increase of the surrounding medium, as well as their cooling timescales are governed by several factors. These include excitation conditions, the optical and thermal properties of both the metal and the medium, as well as the nature of the interface. Despite extensive studies, particularly on interfacial effects in heat transfer dynamics, a framework linking these experimental parameters to key system descriptors such as the particle temperature decay time and the temperature increase in the medium is lacking. Here, we address this issue by identifying the governing parameters controlling heat transfer rates via a scaling analysis using a continuum heat transfer model. We use transient absorption (TA) spectroscopy to probe ultrafast heating in colloidal gold nanoparticles in a liquid solution. We experimentally obtain temperature profiles of both the particles and the medium and determine the interface thermal conductance (ITC), G. The scaling analysis reveals distinct regimes limited by interface thermal conductance or thermal diffusion in the medium. We demonstrate a shift from an interfacial effects dominated heat transfer regime to a thermal diffusion dominated regime by changing particle size, surface ligands, and the surrounding medium. This way, we show that a simple scaling approach captures the tunability of the particle cooling timescales and temperature increases in the medium based on experimental parameters. This provides a basis for rational parameter selection. The approach can be extended to different materials, particle sizes and shapes, and surrounding media under transient heating conditions.

Indium phosphide-based quantum dots (QDs) are leading Cd- and Pb-free alternatives for photonic applications spanning 400-920 nm. However, their long-term photostability under prolonged illumination remains poorly understood. In this study, we systematically investigate the optical response of colloidal InP/ZnSe/ZnS core/shell/shell QDs exposed to white light under a dynamic inert atmosphere (glovebox filled with circulating N₂ with O₂ < 0.1 ppm, H₂O < 1.0 ppm). Upon illumination, the QDs exhibit irreversible photodarkening, defined as a reduction in photoluminescence quantum yield with a nearly unchanged absorption spectrum. Photodarkening shows a linear dependence on absorbed photon dose, with photoexcitation deep into the conduction band inducing it more rapidly than resonant photoexcitation. Additionally, prolonged photoexposure leads to an emission redshift and an enhanced delayed-emission component, consistent with the selective photodarkening of individual QDs. Experiments in air and controlled humidity (10% and 90% relative humidity) indicate that photodarkening primarily arises from photooxidation by oxygen, whereas water promotes QD photoetching. This is further corroborated by control experiments in which the QDs are illuminated under dry oxygen (2O) and in a cuvette sealed by fusion under a N₂ atmosphere with 2 and 2O. These experiments show that photodarkening does not occur in QDs contained in the fusion-sealed cuvette, while under dry oxygen the photodarkening rates of the QDs are even faster than in dry air. These findings advance understanding of InP-based QD photodegradation and provide a framework for quantifying their stability under light stress.

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 1S_1/2(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 1S_1/2(e) state, and state-resolved electronic conductivity which follows the 1S_1/2(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 1S_1/2(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 1S_1/2(e) state at -4.28 eV and the 1S_3/2(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.

Metal nanoparticles and their environment can be locally heated on an ultrafast timescale 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, 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/m^2 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 spatiotemporal gradient which is relevant for studies with transient heating and nanoparticles as sensors.

Control over 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 2 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 to determine the absolute energy levels. InAs QDs 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 where the charge carrier density is precisely controlled electrochemically.

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.