Structural properties of amorphous GeSe2

Germanium Diselenide Ribbons with Orthorhombic Crystal Structure

Many materials are known to exist in several stable polymorphs, but synthesis only provides access to a subset. This situation is exemplified by the dichalcogenide semiconductor GeSe₂. Besides the amorphous form, which attracted intense interest, crystalline GeSe₂ in the bulk and in nanostructures such as flakes and nanobelts invariably adopts the 2D/layered monoclinic β-phase. Hence, the properties of other polymorphs such as the orthorhombic 3D GeSe₂ phase remain unknown. Here, we report the high-yield synthesis of orthorhombic GeSe₂ nanoribbons by GeSe/Se vapor transport over Au catalysts. Access to air-stable monocrystalline, single-phase ribbons enabled investigating the properties of orthorhombic GeSe₂ including its characteristic Raman spectrum. Optical absorption on ensembles and cathodoluminescence spectroscopy on individual ribbons show a wide bandgap and intense band-to-band emission in the visible, with a broad sub-bandgap emission tail. Our results establish orthorhombic GeSe₂ ribbons as a promising wide-bandgap semiconductor nanostructure for applications in optoelectronics and energy conversion.

Toward ultimate nonvolatile resistive memories: The mechanism behind ovonic threshold switching revealed

Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real technological breakthrough in the field of data storage memories. This breakthrough was achieved because of the successful 3D integration of so-called OTS selector devices with innovative phase-change memories, both based on chalcogenide materials. This paves the way for storage class memories as well as neuromorphic circuits. We elucidate the mechanism behind OTS switching by new state-of-the-art materials using electrical, optical, and x-ray absorption experiments, as well as ab initio molecular dynamics simulations. The model explaining the switching mechanism occurring in amorphous OTS materials under electric field involves the metastable formation of newly introduced metavalent bonds. This model opens the way for design of improved OTS materials and for future types of applications such as brain-inspired computing.

New Approaches to the Computer Simulation of Amorphous Alloys: A Review

In this work we review our new methods to computer generate amorphous atomic topologies of several binary alloys: SiH, SiN, CN; binary systems based on group IV elements like SiC; the GeSe₂ chalcogenide; aluminum-based systems: AlN and AlSi, and the CuZr amorphous alloy. We use an ab initio approach based on density functionals and computationally thermally-randomized periodically-continued cells with at least 108 atoms. The computational thermal process to generate the amorphous alloys is the undermelt-quench approach, or one of its variants, that consists in linearly heating the samples to just below their melting (or liquidus) temperatures, and then linearly cooling them afterwards. These processes are carried out from initial crystalline conditions using short and long time steps. We find that a step four-times the default time step is adequate for most of the simulations. Radial distribution functions (partial and total) are calculated and compared whenever possible with experimental results, and the agreement is very good. For some materials we report studies of the effect of the topological disorder on their electronic and vibrational densities of states and on their optical properties.