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Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe ₂ and MoS ₂. We observe a large Stokes-like shift of ∼100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

The conditions whereby epitaxy is achieved are commonly believed to be mostly governed by misfit strain. We report on a systematic investigation of growth and interface structure of single crystalline tungsten thin films on two different metal oxide substrates, Al 2 O 3  ( 11 2 ¯ 0 ) and MgO (001). We demonstrate that despite a significant mismatch, enhanced crystal quality is observed for tungsten grown on the sapphire substrates. This is promoted by stronger adhesion and chemical bonding with sapphire compared to magnesium oxide, along with the restructuring of the tungsten layers close to the interface. The latter is supported by ab initio calculations using density functional theory. Finally, we demonstrate the growth of magnetic heterostructures consisting of high-quality tungsten layers in combination with ferromagnetic CoFe layers, which are relevant for spintronic applications.

Nanoscaling interstitial metal hydrides offers opportunities for hydrogenation applications by enhancing kinetics, increasing surface area, and allowing for tunable properties. The introduction of interfaces impacts hydrogen absorption properties and distribution heterogeneously, making it, however, challenging to examine the multiple concurrent mechanisms, especially at the atomic level. Here, the effect of proximity on interstitial hydrogen in ultrathin single‐crystalline vanadium films is demonstrated by comparing hydride formation in identically strained Fe/V‐ and Cr/V‐superlattices. Pressure concentration and excess resistivity isotherms show higher absolute solubility of hydrogen, higher critical temperature, and concentration in a Cr/V‐superlattice. Direct measurements of hydrogen site location and thermal vibrations show identical site occupation of octahedral z at room temperature with a vibrational amplitude of 0.20–0.25 Å over a wide range of hydrogen concentrations. These findings are consistent with a more extended region of hydrogen depletion in the vicinity of Fe compared to Cr, which showcases an inverse of the hydrogen spillover effect. Advancing the understanding of interface effects resolves previously puzzling differences in the hydrogen loading of Fe/V‐ and Cr/V‐superlattices and is relevant for advancing both catalysis and storage. Interstitial hydrogen behaves distinctly at interfaces compared to the bulk. Vanadium films show proximity‐dependent hydrogen depletion near interfaces–an inverse of hydrogen spillover. Ion beam and resistivity measurements reveal Fe/V‐superlattices have lower hydrogen solubility and higher critical temperature than Cr/V due to proximity‐induced finite size effects, with similar site occupation.

We investigate the effect of finite size on the chemical diffusion of deuterium in extremely thin V(001) layers. A five fold increase in the diffusion coefficient is observed at concentrations around 0.2 [D/V], when the thickness of the V is decreased from 28 to 14 atomic layers ( 2.1-4.2 nm). The size dependent deuterium-deuterium interaction energy is argued to be the root of the observed changes as the diffusion rates are similar at low concentrations. The results demonstrate the feasibility of using finite-size effects to enhance the chemical diffusion of light interstitials in solids. We discuss the general applicability of these effects to other systems.

Because of its light weight and small size, hydrogen exhibits one of the fastest diffusion rates in solid materials, comparable to the diffusion rate of liquid water molecules at room temperature. The diffusion rate is determined by an intricate combination of quantum effects and dynamic interplay with the displacement of host atoms that is still only partially understood. Here we present direct observations of the spatial and temporal changes in the diffusion-induced concentration profiles in a vanadium single crystal and we show that the results represent the experimental counterpart of the full time and spatial solution of Fick's diffusion equation. We validate the approach by determining the diffusion rate of hydrogen in a single crystal vanadium (001) film, with net diffusion in the [110] direction. Understanding hydrogen diffusion in metals is a challenge because of limited access to spatial evolution of the concentration profiles. Using time- and spatially resolved optical measurements, Palsson et al . determine the diffusion rate of hydrogen by directly monitoring its transit through a vanadium thin film.

We demonstrate a lossless switching between vortex and collinear magnetic states in circular FePd disks arranged in a square lattice. Above a bifurcation temperature we show that thermal fluctuations are enough to facilitate flipping between the two distinctly different magnetic states. We find that the temperature dependence of the vortex annihilation and nucleation fields can be described by a simple power law relating them to the saturation magnetization.

GenL is a flexible program that can be used to simulate and/or fit X-ray reflectivity and X-ray diffraction data from epitaxial thin films exhibiting, for example, Laue oscillations. It utilizes a differential evolution within a genetic algorithm for fitting data and uses a modular approach based on either the kinematic theory of diffraction or the dynamic theory. Effects of polarization, absorption, the Lorentz factor, as well as instrumental resolution and lattice vibrations are taken into account. Useful parameters that can be extracted after fitting include atomic interplanar spacings, number of coherently scattering atomic planes, strain profiles along the film thickness, and crystal roughness. The program has been developed in MATLAB and employs a graphical user interface. The deployment strategy is twofold, whereby the software can either be obtained in source code form and executed within the MATLAB environment, or as a pre-compiled binary for those who prefer not to run it within MATLAB. Finally, GenL can be easily extended to simulate multilayered film systems, superlattices, and films with atomic steps. The program is released under the GNU General Public License.

We demonstrate the feasibility of obtaining accurate pair distribution functions of thin amorphous films down to 80 nm, using modern laboratory-based x-ray sources. The pair distribution functions are obtained using a single diffraction scan (one-shot) without the requirement of additional scans of the substrate or of the air. By using a crystalline substrate combined with an oblique scattering geometry, most of the Bragg scattering of the substrate is avoided, rendering the substrate Compton scattering the primary contribution. By utilizing a discriminating energy filter, available in the latest generation of modern detectors, we demonstrate that the Compton intensity can further be reduced to negligible levels at higher wavevector values. We minimize scattering from the sample holder and the air by the systematic selection of pixels in the detector image based on the projected detection footprint of the sample and the use of a 3D printed sample holder. Finally, x-ray optical effects in the absorption factors and the ratios between the Compton intensity of the substrate and film are taken into account by using a theoretical tool that simulates the electric field inside the film and the substrate, which aids in planning both the sample design and measurement protocol.

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically. An electronic asymetry has long been observed in the LaAlO3/SrTiO3 system. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first principles calcula- tions to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

We investigate the growth of thin Fe layers on MgAl\\(_2\\)O\\(_4\\)~(001) and MgO~(001) substrates using dc magnetron sputtering. The crystal quality of Fe layers deposited on MgAl\\(_2\\)O\\(_4\\) is found to be substantially higher as compared to Fe grown on MgO substrates. The effects of the crystal quality on the magnetic and electronic transport properties are discussed.