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SnTe, as one type of topological crystal insulator (TCI), has crystal-symmetry-protected surface states, which is sensitive to its structure and lattice symmetry. Herein, the monolayer SnTe was well synthesized, presenting with tetragonal and hexagonal lattice symmetries, respectively. The lattice symmetry was found to be connected with precursors, as thermal evaporation of SnTe compounds led to a tetragonal type symmetry while co-deposition of Sn and Te elements can synthetize the hexagonal type. As a consequence of lattice mismatch with the underlying Au(111) substrate, SnTe with tetragonal lattice exhibits reconstructive stripe pattern, which would transform into hexagonal lattice symmetry with moiré pattern upon annealing at high temperature. Due to coupling with the substrate, SnTe exhibits metallicity in both symmetries, verified by corresponding differential conductance spectra (dI/dV). Comparative study on Cu(111) substrate was carried out and only copper telluride structures were formed, which may be related to the stronger surface decomposition of SnTe precursors on Cu(111) and direct copper tellurizing. Our work paves a practical way to control the surface structure and lattice symmetry of SnTe, providing a promising candidate for potential TCI applications. [ABSTRACT FROM AUTHOR]

Ge–Sb–Te superlattice (GST-SL) is a newly emerging electronic material for nonvolatile phase-change memory with ultralow energy cost. However, its switching mechanism is still unclear with intensive debates. In this work, by first-principles calculations and an electron counting model study, we study the possible mechanism of phase change and the accompanying property transition of GST-SL. GST-SL are separated into individual layers by van der Waals gaps. We demonstrate that both the global chemical stoichiometry of the material and the local chemical stoichiometry of individual layer block are required to have an insulating band gap according to an electron counting model analysis. The electrical property can be adjusted by changing the local stoichiometry, such as producing defects around van der Waals gaps. Inspired by a previous experiment, we propose that a stacking-fault motion can spontaneously alter the band gap and results in a metal–insulator transition. This transition may provide a significant change of carrier concentration and indicate an ultralow energy-consumption process with a low energy barrier. The present investigations reveal a picture of electrical transition in GST-SL and may guide us to improve its device performances. [ABSTRACT FROM PUBLISHER]

Abstract: Ge–Sb–Te alloys have played a critical role in present nonvolatile optical and electrical storages. It is generally accepted that their “data encoding” (i.e. amorphization) needs the crystal melting and subsequent quenching. Therefore, liquid should be an important intermediate state during the storage. In this study, based on first-principles molecular dynamics we compared the structural properties of liquid Ge–Sb–Te alloys with three compositions: Ge1Sb2Te4, Ge2Sb2Te5, and Ge4Sb1Te5. In long timescale mean square displacements (MSD), we observe that the element coupled state for Ge1Sb2Te4 and Ge2Sb2Te5 is significantly better than that of Ge4Sb1Te5. The careful analyses by pair correlation functions (PCF) and compositional disorder numbers (CDN) show that Ge2Sb2Te5 has the best stability among the three liquids. Bond angle distributions (BAD) further reflect that all the three liquids essentially retain the crystalline character of local structure with 90˚ bond angle. The present results are helpful to understand the rapid storage technique based on Ge–Sb–Te alloys. [Copyright &y& Elsevier]

Very recently, a new superconductor with T c = 80 K has been reported in nickelate (La 3 Ni 2 O 7 ) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with T c above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high T c state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I 4/ mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La 3 Ni 2 O 7 . The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the e g orbitals (3d z 2 and 3d x 2 - y 2 ) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between T c and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

The transition metal dichalcogenide (TMD)-metal interfaces constitute an active part of TMD-based electronic devices with optimized performances. Despite their decisive role, current strategies for nanoscale electronic tuning remain limited. Here, we demonstrate electronic tuning in the WSe 2 /Au interface by twist engineering, capable of modulating the WSe 2 carrier doping from an intrinsic p-type to n-type. Scanning tunneling microscope/spectroscopy gives direct evidence of enhanced interfacial interaction induced doping in WSe 2 as the twist angle with respect to the topmost (100) lattice of gold changing from 15 to 0°. Taking advantage of the strong coupling interface achieved this way, we have moved a step further to realize an n-p-n-type WSe 2 homojunction. The intrinsic doping of WSe 2 can be recovered by germanium intercalation. Density functional theory calculations confirm that twist angle and intercalation-dependent charge transfer related doping are involved in our observations. Our work offers ways for electronically tuning the metal-2D semiconductor interface.

By crystal structure prediction and first-principles calculations, we researched the structure transformation and electronic states for a typical transition metal dichalcogenides (TMDs): 1T-TaS 2 under hydrostatic pressure. The layered 1T-TaS 2 will first transform to an 8-fold monoclinic C2/m phase and then to a 10-fold coordinated tetragonal I4/mmm phase which has 3D covalent bond network linked in space. Our calculations suggest that the lone pair electrons of S in 1T-TaS 2 , which keep the stablity of the layered structure, will be activated by pressure and participate the chemical bonding with Ta, to form the high-pressure C2/m and I4/mmm phases. Additionally, collective electronic states of superconductivity also retains in the I4/mmm phase and the critical transition temperature of superconductivity is 9 K at 100 GPa.

Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO 2 ) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO 2 . X-ray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO 2 to give away the ionicity in exchange for covalency under pressure.

We report on the fabrication of a NO 2 gas sensor from room-temperature reduction of graphene oxide(GO) via two-beam-laser interference (TBLI). The method of TBLI gives the distribution of periodic dissociation energies for oxygen functional groups, which are capable to reduce the graphene oxide to hierarchical graphene nanostructures, which holds great promise for gaseous molecular adsorption. The fabricated reduced graphene oxide(RGO) sensor enhanced sensing response in NO 2 and accelerated response/recovery rates. It is seen that, for 20 ppm NO 2 , the response (R a /R g ) of the sensor based on RGO hierarchical nanostructures is 1.27, which is higher than that of GO (1.06) and thermal reduced RGO (1.04). The response time and recovery time of the sensor based on laser reduced RGO are 10 s and 7 s, which are much shorter than those of GO (34 s and 45 s), indicating that the sensing performances for NO 2 sensor at room temperature have been enhanced by introduction of nanostructures. This mask-free and large-area approach to the production of hierarchical graphene micro-nanostructures, could lead to the implementation of future graphene-based sensors.