Symmetry Lowering Through Surface Engineering and Improved Thermoelectric Properties in MXenes

TL;DR

This study demonstrates that surface engineering of MXenes by symmetry lowering significantly enhances their thermoelectric properties, offering a pathway for designing high-performance thermoelectric materials.

Contribution

The paper introduces a novel surface engineering approach to lower symmetry in MXenes, leading to improved thermoelectric performance through electronic and anharmonicity modifications.

Findings

Symmetry lowering via surface modification enhances thermoelectric figure-of-merit.

Electronic band structure changes contribute to improved thermoelectric properties.

Bond strength dispersions and anharmonicity are key factors in the observed enhancements.

Abstract

Despite ample evidence of their influences on the transport properties of two-dimensional solids, the interrelations of reduced symmetry, electronic and thermal transport, have rarely being 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 M$_{2}$CO$_{2}$ MXenes are done by replacing transition metal $M$ on one surface, resulting in Janus compounds MM$^{\prime}$CO$_{2}$. 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 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 complex interplay of symmetry, electronic structure, bond strengths and anharmonicity which can be exploited to engineer thermoelectric materials with improved properties.