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

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.

References in corpus (9)

• Two Dimensional Atomic Crystals
• Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition
• Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor
• Tightly bound excitons in monolayer WSe2
• Probing Excitonic Dark States in Single-layer Tungsten Disulfide
• Exciton Binding Energy of Monolayer WS2
• Non-linear Optical Spectroscopy of Excited Exciton States for Efficient Valley Coherence Generation in WSe2 Monolayers
• Mono- and Bilayer WS2 Light-Emitting Transistors
• Dense network of one-dimensional mid-gap metallic modes in monolayer MoSe2 and their spatial undulations

Cited by in corpus (13)

• Generating extreme electric fields in 2D materials by dual ionic gating
• Line and Point Defects in MoSe2 Bilayer Studied by Scanning Tunneling Microscopy and Spectroscopy
• Luminescent emission of excited Rydberg excitons from monolayer WSe2
• Millimeter-scale layered MoSe2 grown on sapphire and evidence for negative magnetoresistance
• Electrical tuning of moiré excitons in MoSe${}_2$ bilayers
• Tunneling-current-induced local excitonic luminescence in p-doped WSe${}_2$ monolayers
• Niobium doping induced mirror twin boundaries in MBE grown WSe2 monolayers
• Phonon-assisted exciton and trion conversion efficiency in transition metal Dichalcogenides
• Exciton dissociation in two-dimensional transition metal dichalcogenides: Excited states and substrate effects
• In-situ scanning tunneling microscopy observation of thickness-dependent air-sensitive layered materials and heterodevices
• Photoluminescence efficiency of MBE-grown MoSe${}_2$ monolayers featuring sharp excitonic lines and diverse grain structures
• Effect of manganese incorporation on the excitonic recombination dynamics in monolayer MoS${}_2$
• Resonant interlayer coupling in NbSe${}_2$-graphite epitaxial moir{é} superlattices