Direct Reconstruction of the Band Diagram of Rhombohedral-Stacked\nBilayer WSe<sub>2</sub>–Graphene Heterostructure via Photoemission\nElectron Microscopy

The\ndetermination of energy levels at heterointerfaces is important\nfor understanding charge transport mechanisms, enabling judicious\nassembly of various electronic and optoelectronic devices. Herein,\nwe investigated the interface properties of a heterostructure consisting\nof two-dimensional (2D) transition-metal dichalcogenides rhombohedral\n3R (AB stacking) bilayer WSe<sub>2</sub> (3R-2 ML WSe<sub>2</sub>)\nand epitaxial graphene using photoemission electron microscopy (PEEM)\nwith a femtosecond laser excitation source. The 2D energy band diagram\nwas imaged in an energy-resolved mode (ER-PEEM). For the 3R-2 ML WSe<sub>2</sub>, the conduction band minimum and the exciton were located\nat 2.0 and 2.6 eV, respectively, while the valence band maximum was\nat 4.18 eV. The Fermi level of graphene was located at 4.08 eV. These\nobservations were supported by photoluminescence and Kelvin probe\natomic force microscopy results. Furthermore, we investigated carrier\ndynamics using the system in the time-resolved mode (TR-PEEM). We\nevidenced that irradiation with 2.4 eV pulses induced a surface photovoltage\nthat relaxed within ∼25 ps. This methodology, coupling spectral\nand dynamic properties with space, time, and energy resolutions, allows\nthe reconstruction of energy band diagrams and observation of the\nrecombination mechanisms in nanoscale heterostructures. These parameters\nare instrumental for modeling and fabricating a wide range of heterojunction\ndevices for photovoltaic and optoelectronic applications.