Deformation-Driven Enhancement of Spin Defect Emission in Hexagonal Boron Nitride

The negatively charged boron vacancy $\left(V_B^{-}\right)$ in hexagonal boron nitride (hBN) has been extensively investigated, as it offers a playground for two-dimensional quantum sensing with the closest possible proximity to target samples. However, its practical sensitivity is limited by the intrinsically weak photoluminescence of the spin ensemble. Here, we report a photoluminescence enhancement of up to 30 times from $V_B^{-}$ centers in suspended regions of hBN compared to those in substrate-supported areas. The key spin properties, such as the optically detected magnetic resonance (ODMR) contrast, line width, and the spin lifetime, of the $V_B^{-}$ centers in this region are well preserved. Detailed investigations, including measurements of zero-field ODMR, Raman spectroscopy, and Kelvin probe force microscopy, reveal a correlation between the emission enhancement and local deformation in the sample. It is concluded that the suspended regions exhibit higher local deformation compared to the supported areas, breaking local symmetry and thereby activating otherwise forbidden or weak optical transitions of the $V_B^{-}$ centers.