  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
can not 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 recent
newly researched bulk thermoelectric materials 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 seen in electron doping.
