Symmetry lowering through surface engineering and improved thermoelectric properties in Janus MXenes

Despite ample evidence of their influence on the transport properties of two-dimensional solids, the interrelations of reduced symmetry, electronic and thermal transport have rarely been discussed in the context of thermoelectric materials. With the motivation to design new thermoelectric materials with improved properties, we have addressed these by performing first-principles density functional theory based calculations in conjunction with semi-classical Boltzmann transport theory on a number of compounds in the MXene family. The symmetry lowering in parent M₂CO₂ (M = Ti, Zr, Hf, Mo) MXenes is achieved by replacing the transition metal M on one surface, resulting in Janus compounds MM^'CO₂ (M = Ti, Zr, Hf and M' = Mo, Zr, Hf; M ≠ M'). Our calculations show that the thermoelectric figure-of-merit can be improved significantly by such surface engineering. We discuss in detail, both qualitatively and quantitatively, the origin behind high thermoelectric parameters for these compounds. Our in-depth analysis shows that the modifications in the electronic band structures and degree of anharmonicity driven by the dispersions in the bond strengths due to the lowering of symmetry, an artefact of surface engineering, are the factors behind the trends in the thermoelectric parameters of the MXenes considered. The results also substantiate that the compositional flexibility offered by the MXene family of compounds can generate a complex interplay of symmetry, electronic structure, bond strength and anharmonicity which can be exploited to engineer thermoelectric materials with improved properties.