Janus Ge2PX (X: N, As, Sb, Bi) Monolayers: First-principles Investigation of Electronic and Thermomelectric Properties

2D materials with unique features lead to new possibilities in future nanomaterial-based applications. One of their intriguing derivatives is Janus monolayers, constructed by substituting all atoms at one side of their binary counterpart with a different atom. As a result of broken out-of-plane mirror symmetry created by the addition of a third element, 2D Janus crystals can exhibit fascinating physical properties. In this respect, we systematically investigate the structural, electronic, and thermoelectric (TE) properties of Ge2PX (X = N, As, Sb, Bi) monolayers using first-principles methods. Our results show that the considered systems are dynamically stable. The electronic structure calculations at the level of HSE+SOC indicate that Ge2PAs and Ge2PSb are indirect semiconductors with a bandgap of 1.59 and 0.37 eV, respectively. Using Landauer formalism, the ballistic transport and TE properties of ternary Ge2PX structures are studied in a wide range of temperatures. Ge2PN possesses the highest phonon thermal conductance, while Ge2PBi has the lowest phonon thermal conductance owing to their phonon transmission spectra. Not only low phonon thermal conductance values but also high n-type power factors maximize n-type ZT values. Also, strain engineering can substantially improve the p-type ZT values at low temperatures. Our study suggests that ternary Ge2PAs and Ge2PSb can be promising n-type thermoelectric candidates at high temperatures.