Atomic and Electronic\nStructure of Defects in hBN:\nEnhancing Single-Defect Functionalities

Defect centers in insulators play a critical role in\ncreating important\nfunctionalities in materials: prototype qubits, single-photon sources,\nmagnetic field probes, and pressure sensors. These functionalities\nare highly dependent on their midgap electronic structure and orbital/spin\nwave function contributions. However, in most cases, these fundamental\nproperties remain unknown or speculative due to the defects being\ndeeply embedded beneath the surface of highly resistive host crystals,\nthus impeding access through surface probes. Here, we directly inspected\nthe atomic and electronic structures of defects in thin carbon-doped\nhexagonal boron nitride (hBN:C) by using scanning tunneling microscopy\n(STM) and scanning tunneling spectroscopy (STS). Such investigation\nadds direct information about the electronic midgap states to the\nwell-established photoluminescence response (including single-photon\nemission) of intentionally created carbon defects in the most commonly\ninvestigated van der Waals insulator. Our joint atomic-scale experimental\nand theoretical investigations reveal two main categories of defects:\n(1) single-site defects manifesting as donor-like states with atomically\nresolved structures observable via STM and (2) multisite defect complexes\nexhibiting a ladder of empty and occupied midgap states characterized\nby distinct spatial geometries. Combining direct probing of midgap\nstates through tunneling spectroscopy with the inspection of the optical\nresponse of insulators hosting specific defect structures holds promise\nfor creating and enhancing functionalities realized with individual\ndefects in the quantum limit. These findings underscore not only the\nversatility of hBN:C as a platform for quantum defect engineering\nbut also its potential to drive advancements in atomic-scale optoelectronics.