Impact of irradiation conditions on the magnetic field sensitivity of spin defects in hBN nano flakes

We study $V_{\mathrm B}^-$ centres generated by 25 keV helium focused ion beam (FIB) irradiation in thin (∼70 nm) hexagonal boron nitride (hBN) nanoflakes, to investigate the effect of implantation conditions on the key parameters that influence the magnetic field sensitivity of $V_{\mathrm B}^-$ quantum sensors. Using a combination of photoluminescence, optically detected magnetic resonance, and Raman spectroscopy we examine the competing factors of maximising signal intensity through larger $V_{\mathrm B}^-$ concentration against the degradation in spin coherence and lattice quality at high ion fluences. Both the $V_{\mathrm B}^-$ spin properties and hBN lattice parameters are largely preserved up to an ion fluence of 10 ^14 ions cm ^−2 , and beyond this significant degradation occurs in both. Our measurements give an inferred AC magnetic field sensitivity of ${\sim}1\,\unicode{x03BC}\text{T}/\sqrt{\text{Hz}}$ at the optimal implantation dose. Using the patterned implantation enabled by the FIB, the $V_{\mathrm B}^-$ centres and the associated lattice damage are well localised to the implanted regions. This work demonstrates how careful selection of fabrication parameters optimises the properties of $V_{\mathrm B}^-$ centres in hBN, supporting their application as quantum sensors based in 2D materials.