Direct Reconstruction of the Band Diagram of Rhombohedral-Stacked Bilayer WSe2–Graphene Heterostructure via Photoemission Electron Microscopy

Abstract

The determination of energy levels at heterointerfaces is important for understanding charge transport mechanisms, enabling judicious assembly of various electronic and optoelectronic devices. Herein, we investigated the interface properties of a heterostructure consisting of two-dimensional (2D) transition-metal dichalcogenides rhombohedral 3R (AB stacking) bilayer WSe₂ (3R-2 ML WSe₂) and epitaxial graphene using photoemission electron microscopy (PEEM) with a femtosecond laser excitation source. The 2D energy band diagram was imaged in an energy-resolved mode (ER-PEEM). For the 3R-2 ML WSe₂, the conduction band minimum and the exciton were located at 2.0 and 2.6 eV, respectively, while the valence band maximum was at 4.18 eV. The Fermi level of graphene was located at 4.08 eV. These observations were supported by photoluminescence and Kelvin probe atomic force microscopy results. Furthermore, we investigated carrier dynamics using the system in the time-resolved mode (TR-PEEM). We evidenced that irradiation with 2.4 eV pulses induced a surface photovoltage that relaxed within ∼25 ps. This methodology, coupling spectral and dynamic properties with space, time, and energy resolutions, allows the reconstruction of energy band diagrams and observation of the recombination mechanisms in nanoscale heterostructures. These parameters are instrumental for modeling and fabricating a wide range of heterojunction devices for photovoltaic and optoelectronic applications.