A comparative study of thermoelectric properties between bulk and monolayer SnSe

Electron-crystal and phonon-glass are regarded as two essential factors for ideal thermoelectric materials, which require both an enhanced electronic transport and a depressed phononic transport. These two characteristics usually cannot coexist in complete bulk semiconductors because of the strong interaction between electrons and phonons. Introducing nanostructures into bulk thermoelectric materials has long been applying to allow for diverse phonon scattering mechanisms to reduce lattice thermal conductivity while keeping the electrical conductivity intact. However, some two-dimensional materials do the opposite. Here, we propose an example of monolayer SnSe introduced from newly researched bulk thermoelectric material SnSe. Through first-principles electronic structure calculations and Boltzmann transport theory, we show that the lattice thermal conductivity of monolayer SnSe is instead higher than that of bulk SnSe, leading to a depressed thermoelectric figure of merit compared with bulk one. Besides, we also find the anisotropic level in monolayer SnSe is reduced, and better thermoelectric performance can be realized in electron doping. The present work may provide a fundamental understanding of thermoelectric transport in SnSe-based materials.