Layer-Dependent Band Gaps of Platinum Dichalcogenides

Owing to the relatively strong interlayer interaction, the platinum dichalcogenides exhibit tunability of their electronic properties by controlling the number of layers. Both PtSe2 and PtTe2 display a semimetal to semiconductor transition as they are reduced to bi- or single layers. The value of the fundamental band gap, however, has been inferred only from density functional theory (DFT) calculations, which are notoriously challenging, as different methods give different results, and currently, there is no experimental data. Here, we determine the band gap as a function of the number of layers by local scanning tunneling spectroscopy on molecular beam epitaxy (MBE)- grown PtSe2 and PtTe2 islands. We find band gaps of 1.8 and 0.6 eV for mono- and bilayer PtSe2, respectively, and 0.5 eV for monolayer PtTe2. Trilayer PtSe2 and bilayer PtTe2 are semimetallic. The experimental data are compared to DFT calculations carried out at different levels of theory. The calculated band gaps may differ significantly from the experimental values, emphasizing the importance of the experimental work. We further show that the variations in the calculated fundamental band gap in bilayer PtSe2 are related to the computed separation of the layers, which depends on the choice of the van der Waals functional. This sensitivity of the band gap to interlayer separation also suggests that the gap can be tuned by uniaxial stress, and our simulations indicate that only modest pressures are required for a significant reduction of the gap, making Pt dichalcogenides suitable materials for pressure sensing.