No abstract

Transition metal dichalcogenides (TMDs) have recently gained tremendous interest for use in electronic and optoelectronic applications. Unfortunately, the electronic structure or band gap of most TMDs shows noncontinuously tunable characteristics, which limits their application to energy-variable optoelectronics. Thus, layered materials with better tunability in their electronic structures and band gaps are desired. Herein, we experimentally demonstrated that layered WSe2 possessed highly tunable transport properties under various pressures, with a linearly decreasing band gap that culminates in metallization. Pressure tuned the band gap of WSe2 linearly, at a rate of 25 meV/GPa. The high tunability of WSe2 was attributed to the larger electron orbitals of W2+ and Se2– in WSe2 compared to the Mo2+ and S2– in MoS2. WSe2 underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like bonding was observed in the valence electron localization function (ELF). Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering.

The correspondence between the temperature dependence of phase structures and the experimental physical properties was made clear for the first time in Bi1/2Na1/2TiO3 system according to in situ XRD, in situ Raman and impedance spectroscopy. XRD profiles show pseudo-cubic symmetry independent of temperature, while one of the Raman models disappears near 620K, indicating that the local symmetry increases. The temperature dependence of the main relaxation time of the electric modulus spectra can be divided into four regions, characterizing the thermal evolution of polar nanoregions (PNRs) with different symmetries and a phase transition between rhombohedral and tetragonal symmetry. Therefore, the relaxation times of the electric modulus provide a way to estimate the local phase transition and the thermal evolution of R3c and P4bm PNRs.

Aimed at improving vermiculite's thermal expansibility, a novel method of Na+ modification has been proposed. The influencing mechanism of Na+ modification on the thermal expansibility of vermiculite was explored.