  Current thermoelectric (TE) materials often have low performance or contain
less abundant and/or toxic elements, thus limiting their large-scale
applications. Therefore, new TE materials with high efficiency and low cost are
strongly desirable. Here we demonstrate that, SiS and SiSe monolayers made from
non-toxic and earth-abundant elements intrinsically have low thermal
conductivities arising from their low-frequency optical phonon branches with
large overlaps with acoustic phonon modes, which is similar to the
state-of-the-art experimentally demonstrated material SnSe with a layered
structure. Together with high thermal power factors due to their
two-dimensional nature, they show promising TE performances with large figure
of merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. We
establish some basic understanding of identifying layered materials with low
thermal conductivities, which can guide and stimulate the search and study of
other layered materials for TE applications.
