Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (X = S, Se) monolayers

Abstract

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m−1 K−1 (0.75 W m−1 K−1)] and high p-type Seebeck coefficient [1300 μV K−1 (850 μV K−1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s−1 (3 km s−1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4–5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along the z direction causes out-of-plane piezoelectricity. |d33| ∼ 21.6 pm V−1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young’s modulus ∼41 N m−1 (31 N m−1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m−1) and TcTe2 (34 N m−1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2 V−1 s−1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.