  Lead and tin chalcogenides have been studied widely due to their promising
thermoelectric (TE) properties. Further enhancement in their TE efficiency has
been reported upon the reduction of the dimension, which is an important
feature in modern device fabrications. Using density functional theory combined
with the Semi-classical Boltzmann transport theory, we studied the structural,
electronic and TE properties of two-dimensional (2D) MX (M = Sn, Pb; X = S, Te)
monolayers. Spin-orbit coupling was found to have significant effects on their
electronic structure, particularly for the heavy compounds. Structural
optimization followed by phonon transport studies prevailed that the
rectangular ({\gamma}-) phase is energetically the most favorable for SnS and
SnTe monolayers, whereas the square structure is found the most stable for PbS
and PbTe monolayers. Our results are in good agreement with previous studies.
These 2D materials exhibit high Seebeck coefficients and power factors along
with low lattice thermal conductivities, which are essential features of good
TE materials. The maximum figure of merits (ZT) of 1.04, 1.46, 1.51 and 1.94
are predicted for n-type SnS, SnTe, PBS and p-type PbTe monolayers respectively
at 700 K, which are higher than their bulk ZT values. Hence, these monolayers
are promising candidates for TE applications.
