Molecular-beam epitaxy of monolayer and bilayer WSe2: A scanning tunneling microscopy/spectroscopy study and deduction of exciton binding energy

TL;DR

This study demonstrates the epitaxial growth of atomically flat monolayer and bilayer WSe2 using molecular-beam epitaxy, and investigates their electronic properties and exciton binding energies through scanning tunneling microscopy and spectroscopy.

Contribution

It presents the first detailed STM/STS analysis of MBE-grown WSe2 monolayer and bilayer, revealing atomically flat films with no domain boundaries and insights into their electronic structure.

Findings

Monolayer and bilayer WSe2 are atomically flat with no domain boundary defects.

Bandgap narrows from monolayer to bilayer WSe2.

Exciton binding energies are estimated from spectroscopy and photoluminescence data.

Abstract

Interests in two-dimensional transition-metal dichalcogenides have prompted some recent efforts to grow ultrathin layers of these materials epitaxially using molecular-beam epitaxy. However, growths of monolayer and bilayer WSe2, an important member of the transition-metal dichalcogenides family, by the molecular-beam epitaxy method remain uncharted probably because of the difficulty in generating tungsten fluxes from the elemental source. In this work, we present a scanning tunneling microscopy and spectroscopy study of molecular-beam epitaxy-grown WSe2 monolayer and bilayer, showing atomically flat epifilm with no domain boundary defect. This contrasts epitaxial MoSe2 films grown by the same method, where a dense network of the domain boudaries defects is present. The scanning tunneling spectroscopy measurements of monolayer and bilayer WSe2 domains of the same sample reveal not only the bandgap narrowing upon increasing the film thickness from monolayer to bilayer, but also a band-bending effect across the boundary between monolayer and bilayer domains. This band-bending appears to be dictated by the edge states at steps of the bilayer islands. Finally, comparison is made between the scanning tunneling spectroscopy-measured electronic bandgaps with the exciton emission energies measured by photoluminescence, and the exciton binding energies in monolayer and bilayer WSe2/MoSe2 are thus estimated.