Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

We investigate the energy relaxation of excitons during the real-space transport in ZnSe quantum wells by using microphotoluminescence with spatial resolution enhanced by a solid immersion lens. The spatial evolution of the LO-phonon sideband, originating from the LO-phonon assisted recombination of hot excitons, is measured directly. By calculating the LO-phonon assisted recombination probability, we obtain the nonthermal energy distribution of excitons and observe directly the energy relaxation of hot excitons during their transport. We find the excitons remain hot during their transport on a length scale of several micrometers. Thus, the excitonic transport on this scale cannot be described by classical diffusion.Comment: 4 pages, 4 figure

Hot exciton transport in ZnSe quantum wells

The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Froehlich coupling, the initial kinetic energy of the excitons is well controlled by choosing the excess energy of the excitation laser. When increasing the laser excess energy, we find a general trend of increasing transport length and more importantly a pronounced periodic quenching of the transport length when the excess energy corresponds to multiples of the LO-phonon energy. Such features show the dominant role of the kinetic energy of excitons in the transport process. Together with the excitation intensity dependence of the transport length, we distinguish the phonon wind driven transport of cold excitons and defect-limited hot exciton transport.Comment: 4 pages, 4 figure

Coherence Length of Excitons in a Semiconductor Quantum Well

We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300 ~ 400 nm, about 25 ~ 30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by only considering the acoustic phonon scattering suggests an important influence of static disorder.Comment: 4 Pages, 4 figure

Etude des propriétés optiques de boîtes quantiques semiconductrices II-VI pour leur application à l'émission à un photon à haute température

Président: Roger GROUSSON; Rapporteurs: Pierre GILLIOT, Pierre LEFEBVRE; Examinateur: Manfred BAYERWe studied the optical properties of single CdSe and CdTe quantum dots with the objective of using them as sources that can emit single photons at high temperatures (beyond 100 K). So we characterised the origins of dephasing (random charge fluctuations, exciton-phonon interaction) that limit the emission of indistinguishable photons. The study of our charged CdSe dots allows to show the importance of the mixing between heavy holes and light holes that limits the usage of the dots as sources of pairs of entangled photons. Moreover, we studied two inter-dot transfer mechanisms that affect the emission of single photons on demand: the transfer by tunnelling, occurring at high dot densities, and the thermoactivated transfer that appears at low carrier confinements. Finally, the proof of single photon emission is furnished by means of photon correlation experiments with a continuous wave excitation.Nous avons étudié les propriétés optiques de boîtes quantiques uniques de CdSe et de CdTe dans le but de les utiliser comme sources pouvant émettre des photons uniques à haute température (au dessus de 100 K). Nous avons ainsi caractérisé les sources de déphasage (les fluctuations aléatoires de charges, l'interaction exciton – phonon), qui limitent l'émission de photons indiscernables. L'étude de nos boîtes chargées de CdSe permet de montrer l'importance du mélange entre trous lourds et trous légers, qui limite l'utilisation des boîtes comme sources de paires de photons intriqués. De plus, nous avons étudié deux mécanismes de transfert inter-boîte affectant l'émission de photons uniques sur demande: le transfert par effet tunnel, qui intervient à des fortes densités de boîtes, et le transfert thermoactivé, apparaissant à des faibles confinements de porteurs. Finalement, la preuve de l'émission de photons uniques est apportée par des expériences de corrélation de photons avec une excitation continue

Etude des propriétés optiques de boîtes quantiques semiconductrices II-VI pour leur application à l'émission à un photon à haute température

Nous avons étudié les propriétés optiques de boîtes quantiques uniques de CdSe et de CdTe dans le but de les utiliser comme sources pouvant émettre des photons uniques à haute température (au dessus de 100 K). Nous avons ainsi caractérisé les sources de déphasage (les fluctuations aléatoires de charges, l'interaction exciton phonon), qui limitent l'émission de photons indiscernables. L'étude de nos boîtes chargées de CdSe permet de montrer l'importance du mélange entre trous lourds et trous légers, qui limite l'utilisation des boîtes comme sources de paires de photons intriqués. De plus, nous avons étudié deux mécanismes de transfert inter-boîte affectant l'émission de photons uniques sur demande: le transfert par effet tunnel, qui intervient à des fortes densités de boîtes, et le transfert thermoactivé, apparaissant à des faibles confinements de porteurs. Finalement, la preuve de l'émission de photons uniques est apportée par des expériences de corrélation de photons avec une excitation continue.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Operando electrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2

Semiconductor-based solar energy conversion devices are often multilayer structures with each layer serving a distinct purpose towards generating an efficient and stable device. In water splitting, the use of atomic layer deposited TiO2 (ALD-TiO2) layers enables the stable operation of materials that would normally photocorrode in the aqueous electrolyte. Interestingly, thick ALD-TiO2 (>50 nm) has been successfully used to protect high performance photoanodes, despite an apparent band mismatch that should preclude charge transfer. The understanding of the charge transfer through the relatively thick TiO2 layer remains controversial and warrants further study. Here, we introduce an operando methodology to study charge carrier processes in the ALD-TiO2 protected photoanode by utilizing photoelectrochemical impedance spectroscopy (PEIS) combined with the dual-working-electrode (DWE) technique to resolve if the charge transport through the TiO2 is a conduction band process or involves a hopping through defect states. Two silicon-based systems were evaluated, one featuring a buried homojunction (np+Si/TiO2/Ni) and the other a purely n-type Si directly interfaced with TiO2 (nSi/TiO2/Ni). The additional series resistance imparted by the TiO2 layer (RTiO2) was extracted from the PEIS measurements. Both the potential and thickness dependence of RTiO2 were analyzed, and the DWE technique enabled the sensing of the potential of the TiO2 layer under operation, indicating a strong band bending with the conduction band even more positive than the oxygen evolution potential. Together, these data suggest a conduction band-based transport mechanism, in spite of the presence of defect states in the bandgap of ALD-TiO2, and a detailed picture of the charge transfer through the multilayer structured photoanodes was obtained

Sb2S3/TiO2 Heterojunction Photocathodes: Band Alignment and Water Splitting Properties

Antimony sulfide (Sb2S3) is a promising light-absorbing semiconductor for photovoltaic applications, though it remains vastly unexplored for photoelectrochemical water splitting. Sb2S3 was synthesized by a simple sulfurization of electrodeposited antimony metal at relatively low temperatures (240–300 °C) with elemental sulfur. Using a TiO2 buffer layer and a platinum co-catalyst, photocurrent densities up to ∼9 mA cm–2 were achieved at −0.4 V vs RHE in 1 M H2SO4 under one sun illumination. Using X-ray photoelectron spectroscopy band alignment studies and potential-dependent incident photon-to-current efficiency measurements, a conduction band offset of 0.7 eV was obtained for the Sb2S3/TiO2 junction as well as an unfavorable band bending at the heterointerface, which explains the low photovoltage that was observed (∼0.1 V). Upon inserting an In2S3 buffer layer, which offers a better band alignment, a 0.15 V increase in photovoltage was obtained. The excellent photoelectrochemical water splitting performance and the identification of the origin of the low photovoltage of the Sb2S3 photocathodes in this work pave the way for the further development of this promising earth-abundant light-absorbing semiconductor for solar fuel generation

Enhanced carrier confinement in quantum dots by raising wetting layer state energy

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Influence of Mg on the temperature‐dependent optical properties of CdTe quantum dots embedded in Zn 0.7 Mg 0.3 Te

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