Determining Bandgaps in the Layered Group-10 2D Transition Metal Dichalcogenide PtSe₂

Unlike traditional group-6 transition metal dichalcogenides (TMDs), group-10 TMDs such as PtSe₂ and PdTe₂ possess highly tuneable indirect bandgaps, transitioning from semiconducting in the near-infrared to semimetal behavior with a number of monolayers (MLs). This opens up the possibility of TMD-based mid-infrared and terahertz optoelectronics. Despite this large potential, the optical properties of such materials have shown an extremely large disparity between that predicted and measured. For example, simulations show that a few MLs is required for the semiconductor–semimetal transition, whilst tens of MLs is found experimentally. This is a result of widely used optical extrapolation methods to determine bandgaps, such as the Tauc plot approach, that are not adapted here owing to i) nearby direct transitions, ii) the material dimensionality and iii) large changes in the non-parabolic bandstructure with MLs. Here, uniquely combining optical ellipsometry to determine the complex permittivity, terahertz time resolved spectroscopy for the complex conductivity and in-depth density functional theory (DFT) simulations, it is shown that the optical properties and bandstructure can be determined reliably and demonstrate clearly that the semiconductor-semimetal transition occurs for PtSe₂ layers ≈5 MLs. The microscopic origins of the observed transitions and the crucial role of the Coulomb interaction for thin semiconducting layers, and that of interlayer van der Waals forces for multilayer semimetallic samples are also demonstrated. This work of combining complimentary experimental techniques and extensive simulations avoids the application of constrained extrapolation methods to determine the optical properties of group-10 TMDs, and will be of importance for future mid-infrared and terahertz applications.