Thermally Strain-Induced Band Gap Opening on Platinum Diselenide-Layered Films: A Promising Two-Dimensional Material with Excellent Thermoelectric Performance

In this work, we, for the first time, observed the remarkable thermoelectric properties of a few high-quality PtSe2 layered films fabricated by a post selenization of Pt thin films. An excellent power factor of greater than or similar to 200 mu W/mK(2) with a Seebeck coefficient of >100 mu V/K in the PtSe2 layered film of 10 layers can be experimentally demonstrated over a wide temperature range, which is much better than those of most of the two-dimensional materials reported in the literature. Optical absorption spectra and DFT (density functional theory) calculations indicate a semiconductor-metal transition at a critical thickness once the thickness increases from 7.7 (15 layers) to 14.3 nm (30 Layers). The results are consistent with the experimental results of the dramatic reduction in the power factor, the magnitude of the Seebeck coefficient, and the resistivity when the thickness increases from 7.7 (15 layers) to 14.3 nm (30 Layers). Nevertheless, the semiconductor-metal transition would occur when the thickness increases from 1.5 nm (3 layers) to 2 nm (4 layers). To figure out this unusual performance, a detailed material examination has been conducted. After the transmission electron microscopy examination, 7% biaxial compressive strain built in the polycrystalline PtSe2 thin film can be observed. The strain, as revealed by our DFT calculations, plays an important role in opening the electronic energy gap and hence significantly improves the thermoelectric performance. Boltzmann transport calculation results suggested that both the strain and the hole concentration in the p-type specimens are well optimized. We further propose that an even better power factor can be achieved with n-type-doped PtSe2.