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Register a new userAn atomically thin oxide layer on the elemental superconductor Ta(001) surface
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Recently the oxygen-reconstructed tantalum surface Ta(001)-p(3$times$3)-O has experienced considerable attention due its use as a potential platform for Majorana physics in adatom chains. Experimental studies using scanning tunneling microscopy and spectroscopy found rich atomic and electronic structures already for the clean Ta(001)-O surface, which we combine here with $ab~initio$ methods. We discover two metastable superstructures at the root of the different topographic patterns, discuss its emergence during annealing, and identify the electronic properties. The latter is determined as the sole origin for the contrast reversal seen at positive bias. The observed effects are essentially connected to the two distinct oxygen states appearing on the surface in different geometries. The second superstructure was found in simulations by introducing oxygen vacancies, what was also observed in tantalum pentoxide systems. Additionally we study the charge distribution on the oxidized surface and underline its importance for the adsorption process of polarizable atoms and molecules.
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The epitaxial deposition of the first oxide buffer layer (seed layer) on biaxially textured Ni tape for coated conductors is a critical step that is dependent on the atomistic surface condition of the metal. We present a study of the {100}<100> biaxially textured Ni (001) surface and seed-layer growth using in situ reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Our observations are consistent with formation of a c(2 x 2) 2-D superstructure due to surface segregation of sulfur contained in the metal. We show that this superstructure can have a dramatic effect on the heteroepitaxial growth of oxide seed layers. In particular, the surface superstructure promotes the (200) epitaxial oxide growth of Y2O3-stabilized ZrO2 (YSZ), which is necessary for the development of high-Jc superconducting films for coated conductors.
Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blue shift of the emission with increasing temperature is well described using a two oscillator model for the temperature dependence of the band gap.
Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p-n junction, one of the oldest electrical components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p-n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p-n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p-n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p-n junctions in electronic and optoelectronic devices. We discuss experiments on 2D p-n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a critical discussion about the future outlook and challenges of this incipient research field.
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