Versatile band structure and electron-phonon coupling in layered PtSe2 with strong interlayer interaction

The large tunability in the band structure is ubiquitous in two-dimensional (2D) materials, and PtSe2 is not an exception, which has attracted considerable attention in electronic and optoelectronic applications due to its high carrier mobility and long-term air-stability. Such dimensional dependent properties are closely related to the evolution of electronic band structures. Critical points (CPs), the extrema or saddle points of electronic bands, are the cornerstone of condensed-matter physics and fundamentally determine the optical and transport phenomena of the layered PtSe2. Here, we have experimentally revealed the detailed electronic structures in layered PtSe2, including the CPs in the Brillouin zones (BZs), by means of reflection contrast spectroscopy and spectroscopic ellipsometry (SE). There are three critical points in the BZs attributed to the excitonic transition, quasi-particle band gap, and the band nesting effect related transition, respectively. Three CPs show red-shifting trends with increasing layer number under the mechanism of strong interlayer coupling. We have further revealed the electron-phonon (e-ph) interaction in such layered material, utilizing temperature-dependent absorbance spectroscopy. The strength of e-ph interaction and the average phonon energy also decline with the increasement of layer number. Our findings give a deep understanding to the physics of the layer-dependent evolution of the electronic structure of PtSe2, potentially leading to applications in optoelectronics and electronic devices.