Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films

TL;DR

This study investigates the electronic, surface doping, and optical properties of high-quality epitaxial WSe2 thin films grown on bilayer graphene, revealing key insights into their band structure, spin-splitting, exciton binding energy, and doping effects.

Contribution

It provides new detailed experimental data on the electronic structure and optical response of epitaxial WSe2 films, including the largest observed spin-splitting among MX2 materials and effects of surface doping.

Findings

Bilayer WSe2 is a direct bandgap semiconductor in heterostructures.

Monolayer WSe2 exhibits a 475 meV spin-splitting at the K point.

Exciton binding energy in monolayer WSe2/BLG is 0.21 eV.

Abstract

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy (MBE). The combination of angle-resolved photoemission (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional 3D semiconductors, yet small as compared to other 2D transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.