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Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states. [ABSTRACT FROM AUTHOR]

2D materials like transition metal dichalcogenides (TMDCs) have been widely studied and are a gateway to modern technologies. While research today is mostly carried out on a laboratory scale, there is an intensive need for reliable processes on a wafer‐scale, starting with monolayer‐precise deposition of high‐quality films. In this work, a plasma‐enhanced atomic layer deposition (PEALD) process is developed on a 200 mm SiO2/Si substrate. The layers are investigated regarding crystallinity, composition, homogeneity, microstructure, topography, and electrical properties. The process is then applied on 200 mm alkali‐free glass wafers aiming toward flexible electronics and compatibility with Si processes. A complete coverage of the wafer with a satisfying uniformity is achieved on both substrates and direct polycrystalline growth of MoS2 films is verified on the entire wafer at a substrate temperature of T = 230 °C. On glass, the deposited MoS2 films exhibit a higher crystallinity and are more planar compared to the SiO2/Si substrate. Furthermore, application relevant few‐nanometer thick layers are investigated in detail. This low‐temperature process inspires optimism for future direct integration of 2D‐materials in an economical bottom‐up approach on a wide variety of substrates, thus paving the way for industrial mass production. [ABSTRACT FROM AUTHOR]

An Al2O3/ZnO heterojunction was grown on a Si single crystal substrate by subsequent thermal and plasma-assisted atomic layer deposition (ALD) in situ. The band offsets of the heterointerface were then studied by consecutive removal of the layers by argon sputtering, followed by in situ X-ray photoelectron spectroscopy. The valence band maximum and conduction band minimum of Al2O3 are found to be 1.1 eV below and 2.3 eV above those of ZnO, resulting in a type-I staggered heterojunction. An apparent reduction of ZnO to elemental Zn in the interface region was detected in the Zn 2p core level and Zn L3MM Auger spectra. This suggests an interface formation different from previous models. The reduction of ZnO to Zn in the interface region accompanied by the creation of oxygen vacancies in ZnO results in an upward band bending at the interface. Therefore, this study suggests that interfacial properties such as the band bending as well as the valence and conduction band offsets should be in situ controllable to a certain extent by careful selection of the process parameters. [ABSTRACT FROM AUTHOR]

Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 Å per cycle and a thickness inhomogeneity of ≤1.2% across 4-in. substrates in the temperature range of 100–150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off-stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates. [ABSTRACT FROM AUTHOR]

Metal oxide semiconductors are promising as photoanodes for solar water splitting, but they typically suffer from poor charge transfer properties due to the slow surface reaction kinetics for oxygen evolution. To overcome this, their surfaces are usually modified by depositing earth-abundant, efficient, and inexpensive water oxidation co-catalysts. While this effort has been successful in enhancing the photoelectrochemical performance, a true understanding of the nature of the improvement is still under discussion. This is due to the fact that the co-catalyst can have multiple functionalities, e.g., accelerating charge transfer, passivating surface states, or modifying band bending. Disentangling these factors is challenging, but necessary to obtain a full understanding of the enhancement mechanism and better design the semiconductor/co-catalyst interface. In this study, we investigate the role of atomic layer deposited (ALD) MnOx co-catalysts and their thickness in the photoelectrochemical performance of BiVO4 photoanodes. Modified MnOx/BiVO4 samples with an optimum thickness of ∼4 nm show higher photocurrent (a factor of >3) as well as lower onset potential (by ∼100 mV) compared to the bare BiVO4. We combine spectroscopic and photoelectrochemical measurements to unravel the different roles of MnOx and explain the photocurrent trend as a function of the thickness of MnOx. X-ray photoelectron spectroscopy (XPS) studies reveal that the surface band bending of BiVO4 is modified after the addition of MnOx, therefore reducing surface recombination. At the same time, increasing the thickness of MnOx beyond the optimal 4 nm provides shunting pathways, as shown by energy dispersive X-ray scanning transmission electron microscopy (EDX-STEM) and redox electrochemistry. This cancels out the band bending effect, which explains the observed photocurrent trend. Therewith, this study provides additional insights into the understanding of the charge transfer processes occurring at the semiconductor–catalyst interface. [ABSTRACT FROM AUTHOR]

Highly (00.1)-textured polycrystalline WSe2 films are developed as hydrogen evolving photoelectrodes and improved through deposition of a thin ammonium thiomolybdate (NH4)2Mo3S13 catalyst film. This semiconducting thiomolybdate forms a heterojunction with the p-type WSe2 film passivating recombination centers of excited electron–hole pairs at the edges of the (00.1) textured WSe2 nanoflakes. In addition, thiomolybdate acts as a photoelectrocatalyst at the electrode – aqueous electrolyte interface during light-driven hydrogen evolution. Whereas the photoelectrochemical activity of pure WSe2 is dominated by charge carrier recombination processes at edge states of the hexagonal nanoflakes, we obtain homogeneous charge transfer across the van der Waals planes concomitant with the passivation of these edge states. A photocurrent density of up to 5.6 mA cm−2 at 0 V vs. RHE is obtained with the proposed homogeneously distributed (NH4)2Mo3S13/WSe2 heterojunction system under AM 1.5 illumination in the 0.5 M H2SO4 electrolyte. We conclude that homogeneously distributed semiconducting catalysts on the van der Waals planes of WSe2 nano-crystallites are a feasible strategy towards solar hydrogen evolution with large-area photoelectrocathodes. [ABSTRACT FROM AUTHOR]

P-type CuBi2O4 has recently been reported as a promising photocathode material for photoelectrochemical water reduction due to its optimal optical band gap and positive photocurrent onset potential. However, despite these favourable attributes, CuBi2O4 photocathodes have shown limitations in charge carrier transport within CuBi2O4 and across the interface with n-type fluorine doped tin oxide (FTO). To overcome the later limitation, a very thin and transparent p-type Cu doped NiO (Cu:NiO) back contact layer is inserted between the FTO substrate and CuBi2O4. The Cu:NiO layer is prepared by electron beam evaporation of Ni and Cu followed by post annealing in air. CuBi2O4 photocathodes with a 7 nm thick Cu:NiO back contact layer produce photocurrent densities up to 2.83 mA cm−2 at 0.6 V versus RHE under back illumination with H2O2 as an electron scavenger, which is 25% higher than photocathodes without the back contact layer. This is also the highest reported photocurrent density for CuBi2O4 to date. The observed improvement in photocurrent density with the Cu:NiO back contact layer is attributed to hole selective transport across the CuBi2O4–Cu:NiO interface with a decrease in barrier height compared to the CuBi2O4–FTO interface. [ABSTRACT FROM AUTHOR]

Two studies were conducted in conjunction with an inner-city mental health clinic's development and implementation of an integrated HIV prevention and group psychotherapy program for 262 individuals living with HIV. Results indicated that HIV seropositive individuals, particularly those with high levels of sexual compulsivity and who were engaged in the HIV care system, were representative of those most appropriate for an HIV prevention program that integrated the best practice principles of mental health and public health. However, results also indicated that the development and implementation of such integrated programs may be challenged by unique clinical issues related to sexual compulsivity and by administrative and policy barriers that are inherent in the existing community-based HIV services infrastructure. [ABSTRACT FROM AUTHOR]

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated: modification of the precursors for the spray pyrolysis recipe and post‐deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N‐modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X‐ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen (i.e., N2). Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of ≈0.1 eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower‐bandgap absorber materials for photoelectrochemical energy conversion applications. [ABSTRACT FROM AUTHOR]

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development. While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses. [ABSTRACT FROM AUTHOR]