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Germanium is a small-gap semiconductor that efficiently absorbs visible light, resulting in photoexcited electrons predicted to be sufficiently energetic to reduce H2O for H2 gas evolution. In order to protect the surface from corrosion and prevent surface charge recombination in contact with aqueous pH 7 electrolyte, we grew epitaxial SrTiO3 layers of different thicknesses on p-Ge (001) surfaces. Four-nanometer SrTiO3 allows photogenerated electrons to reach the surface and evolve H2 gas, while 13 nm SrTiO3 blocks these electrons. Ambient pressure x-ray photoelectron spectroscopy indicates that the surface readily dissociates H2O to form OH species, which may impact surface band bending.
With the development of affordable aberration correctors, analytical scanning transmission electron microscopy (STEM) studies of complex interfaces can now be conducted at high spatial resolution at laboratories worldwide. Energy-dispersive X-ray spectroscopy (EDS) in particular has grown in popularity, as it enables elemental mapping over a wide range of ionization energies. However, the interpretation of atomically resolved data is greatly complicated by beam–sample interactions that are often overlooked by novice users. Here we describe the practical factors—namely, sample thickness and the choice of ionization edge—that affect the quantification of a model perovskite oxide interface. Our measurements of the same sample, in regions of different thickness, indicate that interface profiles can vary by as much as 2–5 unit cells, depending on the spectral feature. This finding is supported by multislice simulations, which reveal that on-axis maps of even perfectly abrupt interfaces exhibit significant delocalization. Quantification of thicker samples is further complicated by channeling to heavier sites across the interface, as well as an increased signal background. We show that extreme care must be taken to prepare samples to minimize channeling effects and argue that it may not be possible to extract atomically resolved information from many chemical maps.
A method has been developed for specifying the growth location of Cu2O nanodotson SrTiO3 (100) substrates. Growth location has been specified by using a focused ion beam (FIB) to modify microscopic and nanoscopic regions of the SrTiO3substrate prior to Cu2O deposition. Deposition onto the modified regions under carefully selected process conditions has generated nanodot growth at the edge of microscopic FIB-induced features and on top of nanoscopic FIB-induced features. For this work, an array of evenly spaced FIB implants was first patterned into several regions of each substrate. Within each sub-division of the array, the FIB implants were identical in Ga+ energy and dosage and implant diameter and spacing. After FIB surface modification and subsequent in-situ substrate cleaning, Cu2O nanodots were synthesized on the patterned SrTiO3 substrates using oxygen plasma assisted molecular beam epitaxy. The substrates and nanodots were characterized using atomic force microscopy at various stages of the process; in-situ X-ray photoelectron spectroscopy and Auger electron spectroscopy analysis demonstrated that the final stoichiometry of the nanodots was Cu2O. The photocatalytic decomposition of water on Cu2O under visible light irradiation has been reported. If the Cu2O can be located in the form ofislands on a carefully selected substrate, then it could be possible to greatly enhance the efficiency of the photochemical process.
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