Monolayer enhanced thermoelectric properties compared with bulk for BiTeBr

It is believed that nanostructuring is an effective way to achieve excellent thermoelectric performance. In the work, by combining the first-principles calculations and semiclassical Boltzmann transport theory, we investigate the thermoelectric properties of bulk and monolayer BiTeBr including both the electron and phonon transports. The generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is employed for the electron part, and GGA for the phonon part. It is found that SOC has important effects on electronic transport coefficients because of SOC-induced obvious influences on energy band structures. In p-type doping, monolayer has larger Seebeck coefficient than bulk in wide doping range, which is beneficial to excellent thermoelectric performance. The calculated average lattice thermal conductivity of bulk is 1.71 $\mathrm{W m^{-1} K^{-1}}$ at room temperature, which is close to experimental value 1.3 $\mathrm{W m^{-1} K^{-1}}$. Calculated results show that monolayer has better $ZT_e$ and lower lattice thermal conductivity than bulk, which suggests that monolayer has better thermoelectric performance than bulk. The lower lattice thermal conductivity in monolayer than bulk is due to shorter phonon lifetimes. By comparing the experimental electrical conductivity of bulk with calculated value, the scattering time is determined for 3.3 $\times$ $10^{-14}$ s. Based on electron and phonon transport coefficients, the thermoelectric figure of merit $ZT$ of bulk and monolayer are calculated. It is found that monolayer has higher peak $ZT$ than bulk, and the peak $ZT$ of monolayer can be as high as 0.55 in n-type doping and 0.75 in p-type doping at room temperature. These results imply that monolayer BiTeBr may be a potential two-dimensional (2D) thermoelectric material, which can stimulate further experimental works to synthesize monolayer BiTeBr.