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.

Inserting hexagonal boron nitride (hBN) as barrier layers into bilayer transition metal dichalcogenides heterointerface has been proved an efficient method to improve two dimensional tunneling optoelectronic device performance. Nevertheless, the physical picture of interlayer coupling effect during incorporation of monolayer (1L-) hBN is not explicit yet. In this article, spectroscopic ellipsometry was used to experimentally obtain the broadband excitonic and critical point properties of WS₂/MoS₂ and WS₂/hBN/MoS₂ van der Waals heterostructures. We find that 1L-hBN can only slightly block the interlayer electron transfer from WS₂ layer to MoS₂ layer. Moreover, insertion of 1L-hBN weakens the interlayer coupling effect by releasing quantum confinement and reducing efficient dielectric screening. Consequently, the exciton binding energies in WS₂/hBN/MoS₂ heterostructures blueshift comparing to those in WS₂/MoS₂ heterostructures. In this exciton binding energies tuning process, the reducing dielectric screening effect plays a leading role. In the meantime, the quasi-particle (QP) bandgap remains unchanged before and after 1L-hBN insertion, which is attributed to released quantum confinement and decreased dielectric screening effects canceling each other. Unchanged QP bandgap as along with blueshift exciton binding energies lead to the redshift exciton transition energies in WS₂/hBN/MoS₂ heterostructures.