Thickness-Dependent Electronic Properties and Work Function in HfSe₂ Thin Films

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are known to exhibit changes in their electronic properties when transitioning from the bulk to a single layer. In this study, we have investigated the thickness-dependent electronic properties of 1T-HfSe₂ films. The HfSe₂ films were grown in a molecular beam epitaxy (MBE) chamber and the growth process was monitored by in situ high-energy reflection electron diffraction (RHEED). Scanning tunneling microscopy (STM) revealed a well-ordered atomic structure with a hexagonal 1T lattice. Scanning tunneling spectroscopy (STS) showed a band gap of ~1.1 eV for monolayer thickness. Angle-resolved photoemission spectroscopy (ARPES) was used to investigate the electronic structure, which showed the valence bands show splitting with increasing film thickness. The valence band maximum does not change depending on the thickness. On the other hand, we found that the work function of the film gradually increases when increasing thickness. Density functional theory (DFT) calculations well explain the valence band structure and show that the thickness dependence of work function is due to the screening effect from the underlying graphene substrate. Our results provide fundamental understanding on both 2D semiconductor-based device design and electrical contact engineering.