First-principles study of the effect of Dirac phonons on the thermoelectric properties in monolayer Ge2H2

Abstract

The Dirac phonons have been widely attention due to their unique edge or surface states. However, research on their influence in the realm of thermoelectric transport in semiconductor materials remains limited. Here, we have systematically investigated the geometric structure, stability, electronic, thermal conductivity, and thermoelectric properties of the monolayer Ge2H2 and Ge2 based on density functional theory and the Boltzmann transport equation (BTE). The phonon spectrum of monolayer Ge2H2 exhibits both Hourglass-type and Dirac-type phonon dispersion curves. The band structure of monolayer Ge2H2 reveals a relatively large band gap of 1.02 eV (1.75 eV for HSE), indicating that it is a direct band gap semiconductor. These two features make monolayer Ge2H2 an ideal material for exploring the effect of Dirac phonons on thermoelectric transport properties. In this paper, we determined that the lattice thermal conductivity (κl) in Ge2H2 is 0.59 Wm−1K−1 at 300 K, but undergoes a sharp increase upon removal of H atoms. This is mainly attributed to the disappearance of the Dirac phonons and the change of lattice vibration modes by removing the H atoms. While also exhibiting high carrier mobilities (134.86 cm2/Vs for holes) and extended scattering time (4.22 × 10−14 s for holes), indicative of excellent thermoelectric performance. With increasing temperature, the ZTmax of p-type doping reaches 0.63 at 700 K in Ge2H2. Remarkably, at 300K, the Hourglass-type phonon contributes only 5.94 % to κl, while the Dirac phonon contributes 3.44 %. Further analysis reveals that the Dirac phonon within Hourglass-type in monolayer Ge2H2 contributes 2.5 % to the ZTmax at 300K. Our investigation elucidates the impact of Dirac phonons on thermoelectric properties, providing valuable insights for future design and exploration of high-ZT thermoelectric materials.