The origin of shifted fermi level and improved thermoelectric performance of monolayer BiCuSeO under pressures

Abstract

The layered BiCuSeO has been widely investigated because of high thermoelectric (TE) performance. However, there is few reports about the TE property of two-dimensional (2D) BiCuSeO so far. In this work, it is found that the Fermi level passes through the conduction band of [Bi2O2]2+ layer and the valence band of [Cu2Se2]2- layer simultaneously. Therefore, there is no band gap appeared for monolayer BiCuSeO, which would lead to very poor TE performance. First-principles calculation shows that negative hydrostatic pressure could weaken the intra-layer bonding and enhance the inter-layer coupling. Thus, the Fermi level would gradually get out of conduction and valence bands along with the increased pressure. To improve the TE performance of monolayer BiCuSeO, negative hydrostatic pressures are applied and successfully increased its band gap values from 0 eV to 0.85 eV. At the same time, an obvious band convergence near conduction band minimum (CBM) is found under the −2.22 GPa pressure. Besides, the band convergence near the Fermi level under negative pressure will greatly improve the TE performance of n- and p-type monolayer BiCuSeO.