Defect-Assisted Photoemission in the hBN and TMDs/hBN Heterostructures

Ultrafast electron pulses have broad applications in the investigation of the ultrafast dynamics of materials and near-field nanophotonics. A hexagonal boron nitride (hBN) photoemission source has been proposed recently, characterized by a nanoscale working area and high brightness. However, the photoemission mechanisms of hBN are still not clear because the wide bandgap and low electron density of states of hBN are believed to result in low photoemission brightness. Here, we experimentally demonstrated the defect-assisted two-photon photoemission process in hBN by electron microscopy. The laser-induced defect states work as intermediate states to enhance the photoemission process by transferring a two-photon process to a cascaded one-photon process. In addition, we proposed another strategy to improve the photoemission brightness by increasing the density of states with a narrow-bandgap, two-dimensional material stacked on hBN. The photoemission intensity of the monolayer transition metal dichalcogenides (TMDs)/hBN heterostructure was largely enhanced, whereas the emission angle and energy spread remained similar to hBN. However, the photoemission intensity of TMDs/hBN can be influenced negatively by laser-induced defect trapping from the actual intermediate energy levels of TMDs. The defect-assisted photoemission process and heterostructure stacking strategy proposed here are instructive for the design of next-generation photoemission sources.