Optimization of sensing sensitivity and coherence properties of spin defects in hexagonal boron nitride†

Hexagonal boron nitride (hBN) nanoflakes embedded with spin defects can be easily integrated into two-dimensional materials and devices to serve as both substrate materials and quantum sensors. In particular, the negatively charged boron vacancy (V_B⁻) spin defects are garnering increasing interests in sensing applications. However, optimal irradiation parameters for generating V_B⁻ ensemble in hBN flakes with a thickness of several hundred nanometers are still lacking. In this work, we investigated the influence of the irradiation dose on the spin properties of the V_B⁻ ensemble with determined density using continuous and pulsed optically detected magnetic resonance (ODMR) techniques. A trend of saturation dependence was observed among the ODMR contrast, linewidth, magnetic sensitivity, and bias magnetic field for the V_B⁻ ensemble in hBN flakes that were irradiated with varying doses. For 50 keV helium ion irradiation, the optimal dose was 2 × 10¹⁴ ions per cm², which produced a V_B⁻ ensemble with superior magnetic sensitivity and spin relaxation and coherence times. Furthermore, the impact of an external magnetic field on the spin relaxation dynamics of the V_B⁻ ensemble and the role of lattice damage in reducing the coherence time were discussed. These results provide a framework for optimizing the sensing sensitivity and coherence properties of the V_B⁻ ensemble in hBN as layered quantum sensors and offer insights into the mechanisms that limit the spin properties.