Observation of Boron Vacancy Concentration in Hexagonal Boron Nitride at Nanometer Scale

Negatively charged boron vacancy (V_B^-) ensembles in hexagonal boron nitride (h-BN) have attracted considerable attention as a promising platform for quantum sensing. Current challenges include the experimental validation of the spatial distribution and electronic states of optically active V_B^- and optically inactive neutral boron vacancy (V_B⁰) defects. To address these issues, we employ electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) using monochromated 30-keV electrons, effectively reducing background interference. This approach unveils distinct spectral peaks at 2.5 and 1.9 eV, corresponding to V_B^- and V_B⁰ defects, respectively. Furthermore, we achieve nanometer-scale concentration mapping for V_B^- and V_B⁰ defects, advancing insights into spin defect configurations crucial for optimizing quantum sensor performance.