Out-of-Plane Dielectric Susceptibility of Graphene in Twistronic and Bernal Bilayers.

We describe how the out-of-plane dielectric polarizability of monolayer graphene influences the electrostatics of bilayer graphene-both Bernal (BLG) and twisted (tBLG). We compare the polarizability value computed using density functional theory with the output from previously published experimental data on the electrostatically controlled interlayer asymmetry potential in BLG and data on the on-layer density distribution in tBLG. We show that monolayers in tBLG are described well by polarizability αexp = 10.8 Å3 and effective out-of-plane dielectric susceptibility ϵz = 2.5, including their on-layer electron density distribution at zero magnetic field and the interlayer Landau level pinning at quantizing magnetic fields.

Structure and Metallicity of Phase V of Hydrogen.

A new phase V of hydrogen was recently claimed in experiments above 325 GPa and 300 K. Because of the extremely small sample size at such record pressures the measurements were limited to Raman spectroscopy. The experimental data on increase of pressure show decreasing Raman activity and darkening of the sample, which suggests band gap closure and impending molecular dissociation, but no definite conclusions could be reached. Furthermore, the available data are insufficient to determine the structure of phase V, which remains unknown. Introducing saddle-point ab initio random structure searching, we find several new structural candidates of hydrogen which could describe the observed properties of phase V. We investigate hydrogen metallization in the proposed candidate structures, and demonstrate that smaller band gaps are associated with longer bond lengths. We conclude that phase V is a stepping stone towards metallization.

Optoelectronic properties of atomically thin ReSSe with weak interlayer coupling.

Rhenium dichalcogenides, such as ReS2 and ReSe2, have attracted a lot of interests due to the weak interlayered coupling in these materials. Studies of rhenium based dichalcogenide alloys will help us understand the differences between binary rhenium dichalcogenides. They will also extend the applications of two-dimensional (2D) materials through alloying. In this work, we studied the optoelectronic properties of ReSSe with a S and Se ratio of 1 : 1. The band gap of the ReSSe alloy is investigated by optical absorption spectra as well as theoretical calculations. The alloy shows weak interlayered coupling, as evidenced by the Raman spectrum. A field-effect transistor based on ReSSe shows typical n-type behavior with a mobility of about 3 cm(2) V(-1) s(-1) and an on/off ratio of 10(5), together with the in-plane anisotropic conductivity. The device also shows good photoresponse properties, with a photoresponsivity of 8 A W(-1). The results demonstrated here will provide new avenues for the study of 2D materials with weak interlayer interactions and in-plane anisotropy.