Current thermoelectric (TE) materials often have low performance or containless abundant and/or toxic elements, thus limiting their large-scaleapplications. Therefore, new TE materials with high efficiency and low cost arestrongly desirable. Here we demonstrate that, SiS and SiSe monolayers made fromnon-toxic and earth-abundant elements intrinsically have low thermalconductivities arising from their low-frequency optical phonon branches withlarge overlaps with acoustic phonon modes, which is similar to thestate-of-the-art experimentally demonstrated material SnSe with a layeredstructure. Together with high thermal power factors due to theirtwo-dimensional nature, they show promising TE performances with large figureof merit (ZT) values exceeding 1 or 2 over a wide range of temperatures. Weestablish some basic understanding of identifying layered materials with lowthermal conductivities, which can guide and stimulate the search and study ofother layered materials for TE applications.
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