An unconventional phase-change memory (PCM) made of In Se , which utilizes reversible phase changes between a low-resistance crystalline β phase and a high-resistance crystalline γ phase is reported for the first time. Using a PCM with a layered crystalline film exfoliated from In Se crystals on a graphene bottom electrode, it is shown that SET/RESET programmed states form via the formation/annihilation of periodic van der Waals' (vdW) gaps (i.e., virtual vacancy layers) in the stack of atomic layers and the concurrent reconfiguration of In and Se atoms across the layers. From density functional theory calculations, β and γ phases, characterized by octahedral bonding with vdW gaps and tetrahedral bonding without vdW gaps, respectively, are shown to have energy bandgap value of 0.78 and 1.86 eV, consistent with a metal-to-insulator transition accompanying the β-to-γ phase change. The monolithic In Se layered film reported here provides a novel means to achieving a PCM based on melting-free, low-entropy phase changes in contrast with the GeTe-Sb Te superlattice film adopted in interfacial phase-change memory.

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The hydrogen economy is seen as a potential alternative to overcome the depletion of traditional fossil fuels and environmental pollution; therefore, the demand for high-purity hydrogen has rapidly increased. To produce hydrogen in a sustainable and environmentally friendly manner, numerousThe metallic 1T phase of WS 2 (1T-WS 2 ), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS 2 (2H-WS 2 ) is inherently stable, methods for synthesizing 1T-WS 2 are limited and complicated. Herein, a uniform wafer-scale 1T-WS 2 film is prepared using a plasma-enhanced chemical vapor deposition (PE-CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T-WS 2 films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as-grown 1T-WS 2 are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a 0 × a 0 superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T-WS 2 film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T-WS 2 films synthesized with CVD and hydrothermal methods. The 1T-WS 2 does not transform to stable 2H-WS 2 , even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T-phase 2D materials for electrocatalysis applications.

Structural properties of single-phase films of κ-In 2 Se 3 and γ-In 2 Se 3 were investigated.Both films were polycrystalline but their microstructure differed considerably. The alattice parameter of κ-In 2 Se 3 has been measured. Comparison between these two materials indicates that κ-In 2 Se 3 has a significantly larger unit cell (∆c = 2.5 ± 0.2 % and ∆a = 13.5 ± 0.5 %) and a structure more similar to the α-phase of In 2 Se 3 . a)

Articles you may be interested inPhotoluminescence properties and crystallization of silicon quantum dots in hydrogenated amorphous Si-rich silicon carbide films Structural and electronic properties of amorphous and single-phase polycrystalline films of ␥and -In 2 Se 3 have been measured. The effect of deposition conditions on the film phase was studied extensively. The stable ␥ phase nucleates homogeneously in the film bulk and has a high resistivity, while the metastable phase nucleates at the film surface and has a moderate resistivity. The microstructures of polycrystalline hot-deposited and postannealed, cold-deposited ␥ films are quite different but their electronic properties are similar. The increase in the resistivity of amorphous In 2 Se 3 films upon annealing is interpreted in terms of the replacement of In-In bonds with In-Se bonds during crystallization. Great care must be taken in the preparation of In 2 Se 3 films for electrical measurements as the presence of excess chalcogen or surface oxidation may greatly affect the film properties.