  Surface and strain engineering are among the cheaper ways to modulate
structure property relations in materials. Due to their compositional
flexibilities, MXenes, the family of two-dimensional materials, provide enough
opportunity for surface engineering. In this work, we have explored the
possibility of improving thermoelectric efficiency of MXenes through these
routes. The Janus MXenes obtained by modifications of the transition metal
constituents and the functional groups passivating their surfaces are
considered as surface engineered materials on which bi-axial strain is applied
in a systematic way. We find that in the three Janus compounds Zr$_{2}$COS,
ZrHfO$_{2}$ and ZrHfCOS, tensile strain modifies the electronic and lattice
thermoelectric parameters such that the thermoelectric efficiency can be
maximised. A remarkable reduction in the lattice thermal conductivity due to
increased anharmonicity and elevation in Seebeck coefficient are obtained by
application of moderate tensile strain. With the help of first-principles
electronic structure method and semi-classical Boltzmann transport theory we
analyse the interplay of structural parameters, electronic and dynamical
properties to understand the effects of strain and surface modifications on
thermoelectric properties of these systems. Our detailed calculations and in
depth analysis lead not only to the microscopic understanding of the influences
of surface and strain engineering in these three systems, but also provide
enough insights for adopting this approach and improve thermoelectric
efficiencies in similar systems.
