Direct d−d hybridization mechanism for strong anisotropic carrier transport in layered Mo2SBr2

Abstract

Transition metal sulfide halides such as ${\mathrm{Mo}}_{2}{\mathrm{SBr}}_{2}$ have exhibited excellent anisotropy in their electronic and optical properties. To explore the mechanism behind this strong anisotropic behavior, we performed first-principles calculations on the anisotropic mechanical, optical, and electronic properties of monolayer ${\mathrm{Mo}}_{2}{\mathrm{SBr}}_{2}$. We find that monolayer ${\mathrm{Mo}}_{2}{\mathrm{SBr}}_{2}$ demonstrates obvious anisotropy in its electron mobility, ${\ensuremath{\mu}}_{e}$, with an extremely high ${\ensuremath{\mu}}_{e}$ of $10,356.08\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{--1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{--1}$ along the $b$ direction. We attributed this strong anisotropy in monolayer ${\mathrm{Mo}}_{2}{\mathrm{SBr}}_{2}$ to the unique characters in the orbital coupling. Our further studies show that direct $d\text{\ensuremath{-}}d$ coupling between the nearest-neighboring Mo atoms plays a critical role in the unique carrier transport properties and strong anisotropy. Direct $d\text{\ensuremath{-}}d$ coupling provides a fast hole transport channel along the $c$ direction. Furthermore, wavefunction delocalization at the valence band maximum is significantly enhanced by this $d\text{\ensuremath{-}}d$ hybridization, which further reduces the effective mass of holes. Our work provides physical insights into the origin of strong anisotropy in the photoelectric properties of transition metal sulfide halides. Transition metal sulfide halides are demonstrated to have great potential applications in novel nanoelectronic devices.