Temperature Effects on the Electronic Structures of Epitaxial 1T′-WSe₂ Monolayers

Transition metal dichalcogenides (TMDCs) with a 1T' structural phase are predicted to be two-dimensional topological insulators at zero temperature. Although the quantized edge conductance of 1T'-WTe₂ has been confirmed to survive up to 100 K (Wu, S.; Fatemi, V.; Gibson, Q. D.; Watanabe, K.; Taniguchi, T.; Cava, R. J.; Jarillo-Herrero, P., <i>Science</i> 2018, 359, 76-79), this temperature is still relatively low for industrial applications. Addressing the limited studies on temperature effects of 1T'-TMDCs, our research focuses on the crystal and electronic properties of epitaxial 1T'-WSe₂ monolayers grown on bilayer graphene (BLG) and SrTiO₃(100) substrates at various temperatures. For the 1T'-WSe₂ grown on BLG (1T'-WSe₂/BLG), we observed a significant thermal expansion effect on its band structures with a thermal expansion coefficient of ∼60 × 10^-6 K^-1. In contrast, 1T'-WSe₂ grown on SrTiO₃(100) (1T'-WSe₂/SrTiO₃) exhibits minimal changes with varied temperatures due to the enhanced stress exerted by the substrate. In addition, a significant Coulomb gap (CG) was observed to be pinned at the Fermi level for both 1T'-WSe₂/BLG and 1T'-WSe₂/SrTiO₃ in the angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS). The CGs show different sizes depending on the different dielectric environments and interfacial doping from the substrates. The CG was also found to decrease with increasing temperatures and can persist up to 200 K for 1T'-WSe₂/BLG, consistent with our Monte Carlo simulations. The observation of CG at Fermi level endows the epitaxial 1T'-WSe₂ monolayers with a huge potential for realizing quantum spin Hall devices at high temperature and the topological computing designations in the future.