  Based on first-principles calculations, we have investigated the structural
stability, electronic structures, and thermal properties of the monolayer
XSi2N4 (X= Ti, Mo, W) and their lateral (LH) and vertical heterostructures
(VH). We find that these heterostructures are energetically and dynamically
stable due to high cohesive and binding energies, and no negative frequencies
in the phonon spectra. The XSi2N4 (X= Ti, Mo, W) monolayers, the
TiSi2N4/MoSi2N4-LH, MoSi2N4/WSi2N4-LH, and MoSi2N4/WSi2N4-VH possess a
semiconducting nature with an indirect band gap ranging from 0.30 to 2.60 eV.
At room temperature, the Cv values are found to be between 100 and 416 J/K.mol
for the monolayers and their heterostructures, suggesting the better ability to
retain heat with respect to transition metal dichalcogenides. Our study unveils
the excellent attributes of XSi2N4 2D monolayers and their heterostructures,
proposing them as potential candidates in nanoelectronics and thermoelectric
applications.
