Frank C. M. Spoor

Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals

Luminescent copper indium sulfide (CIS) nanocrystals are a potential solution to the toxicity issues associated with Cd- and Pb-based nanocrystals. However, the development of high-quality CIS nanocrystals has been complicated by insufficient knowledge of the electronic structure and of the factors that lead to luminescence quenching. Here we investigate the exciton decay pathways in CIS nanocrystals using time-resolved photoluminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. Our measurements support the model of photoluminescence by radiative recombination of a conduction band electron with a localized hole. Moreover, we find that photoluminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway determines whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.

A phonon scattering bottleneck for carrier cooling in lead chalcogenide nanocrystals.

The cooling dynamics of hot charge carriers in colloidal lead chalcogenide nanocrystals is studied by hyperspectral transient absorption spectroscopy. We demonstrate a transient accumulation of charge carriers at a high energy critical point in the Brillouin zone. Using a theoretical study of the cooling rate in lead chalcogenides, we attribute this slowing down of charge carrier cooling to a phonon scattering bottleneck around this critical point. The relevance of this observation for the possible harvesting of the excess energy of hot carriers by schemes such as multiexciton generation is discussed.

Hole Cooling Is Much Faster than Electron Cooling in PbSe Quantum Dots

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with processes such as carrier multiplication or hot charge extraction that may improve the light conversion efficiency of photovoltaic devices. Understanding charge carrier cooling is therefore of great interest. We investigate high-energy optical transitions in PbSe QDs using hyperspectral transient absorption spectroscopy. We observe bleaching of optical transitions involving higher valence and conduction bands upon band edge excitation. The kinetics of rise of the bleach of these transitions after a pump laser pulse allow us to monitor, for the first time, cooling of hot electrons and hot holes separately. Our results show that holes cool significantly faster than electrons in PbSe QDs. This is in contrast to the common assumption that electrons and holes behave similarly in Pb chalcogenide QDs and has important implications for the utilization of hot charge carriers in photovoltaic devices.

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials.

Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Cooling and Auger Recombination of Charges in PbSe Nanorods: Crossover from Cubic to Bimolecular Decay

The cooling and Auger recombination of electron-hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump-probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30-50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.

Photogeneration and Mobility of Charge Carriers in Atomically Thin Colloidal InSe Nanosheets Probed by Ultrafast Terahertz Spectroscopy

The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8-1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.

Broadband Cooling Spectra of Hot Electrons and Holes in PbSe Quantum Dots

Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1–5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.

Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals

Carrier multiplication is a process in which one absorbed photon excites two or more electrons. This is of great promise to increase the efficiency of photovoltaic devices. Until now, the factors that determine the onset energy of carrier multiplication have not been convincingly explained. We show experimentally that the onset of carrier multiplication in lead chalcogenide quantum confined and bulk crystals is due to asymmetric optical transitions. In such transitions most of the photon energy in excess of the band gap is given to either the hole or the electron. The results are confirmed and explained by theoretical tight-binding calculations of the competition between impact ionization and carrier cooling. These results are a large step forward in understanding carrier multiplication and allow for a screening of materials with an onset of carrier multiplication close to twice the band gap energy. Such materials are of great interest for development of highly efficient photovoltaic devices.

Efficient carrier multiplication in CsPbI 3 perovskite nanocrystals

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.In semiconductor nanocrystals, efficient carrier multiplication counteracts hot carrier thermalization, increasing the overall carrier generation yield. Here, de Weerd et al. observe a quantum yield of up to 98% in CsPbI3 nanocrystals as a result of efficient carrier multiplication.