YNbC, novel Janus MXene: a first-principles study

The unique out-of-plane structural asymmetry of Janus MXenes makes them an attractive class of two-dimensional materials, enabling flexibility in tailoring their physical properties. Employing first-principles calculations, we systematically explored the structural stability, mechanical properties, electronic structure, and thermoelectric transport properties of the novel Janus YNbC monolayer in comparison with its parent Nb₂C and Y₂C MXenes. The mechanical and dynamic stability of the proposed structure was established through elastic-constant analysis and phonon-dispersion spectra, respectively. Cohesive-, formation-, and decomposition-energy analyses indicate favorable bonding with respect to isolated atoms and a metastable character relative to the considered parent-MXene phases. Its finite-temperature thermodynamic behaviour was investigated within the harmonic phonon approximation. The calculated mechanical properties indicate that YNbC possesses favorable mechanical robustness with characteristics intermediate between those of Nb₂C and Y₂C. Electronic-structure calculations reveal a substantial modification induced by Janus symmetry breaking. While the parent MXenes exhibit metallic characteristics, at the PBE level, Janus YNbC shows semimetallic behaviour, characterized by a small indirect band overlap of approximately 14.6 meV, whereas the HSE functional produces a finite indirect band gap of approximately 0.1 eV, indicating narrow-gap semiconducting behaviour at this level of theory. Furthermore, the calculated thermoelectric transport coefficients demonstrate a favorable thermoelectric response for YNbC compared with the parent MXenes. Overall, Janus engineering offers a promising route to transforming metallic MXenes into narrow-gap materials with favorable electronic thermoelectric transport properties. These results suggest that narrow-bandgap YNbC holds promise for electronic and thermoelectric applications.