Strain-Induced Electronic Gap Opening on PtSe₂ Thin Films: A Promising 2D Material with Excellent Thermoelectric Performance

Te-Hsien Wang^1*, Teng-Yu Su², Angus Huang³, Horng-Tay Jeng^3,4,5

¹Applied Science, National Taitung University, Taitung, Taiwan
²Materials Science and Engineering and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu, Taiwan
³Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴Physics, National Center for Theoretical Sciences, Hsinchu, Taiwan
⁵Physics, Academia Sinica, Taipei, Taiwan
⁶Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan

* Presenter:Te-Hsien Wang, email:pilgrim0613@gmail.com

Thermoelectric (TE) devices, with the ability to convert heat to electricity, have attracted increasing attention. In this work, we experimentally and theoretically investigate the thermoelectric properties of high-quality p-type PtSe₂ ultra-thin films [1]. Experiments show an excellent power factor of ≳200 μW/mK² with a Seebeck coefficient of >100 μV/K in the PtSe₂ layered film of 10 layers (∼5 nm in thickness) over a wide temperature range. This is much better than those of most of the two-dimensional materials reported in the literature. Such a large Seebeck coefficient indicates that the 10-layer specimen should be a semiconductor while previous theoretical studies show that PtSe₂ films of thickness larger than two layers are metals. Our density functional theory (DFT) calculation suggests that the semiconductor phase of the 10-layer specimen would originate from the internal compressive strain, which, ∼7% biaxial compressive strain, is then experimentally confirmed by transmission electron microscopy examination. With the help of optical absorption spectra and DFT calculations, we further find that the semiconductor-metal transition occurs at a critical thickness in between 7.7 (15 layers) and 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). Boltzmann transport calculation results suggested that both the strain and the hole concentration in the p-type specimens are well optimized. According to our calculations, an even better power factor can be achieved in n-type-doped PtSe₂.

Keywords: thermoelectric , PtSe₂, thin films, energy harvesting