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

We study V_B^- centres generated by helium focused ion beam (FIB) irradiation in thin $sim$70 nm hBN nanoflakes, in order to investigate the effect of implantation conditions on the key parameters that influence the magnetic field sensitivity of V_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_B^- concentration against the degradation in spin coherence and lattice quality observed at high ion fluences. Our results indicate that both the V_B^- spin properties and hBN lattice parameters are largely preserved up to an ion fluence of 10¹⁴ ions/cm², and beyond this significant degradation occurs in both. At the optimal implantation dose, an AC magnetic sensitivity of sim 1 μT/sqrt{Hz} is achieved. Using the patterned implantation enabled by the FIB, we find that V_B^- centres and the associated lattice damage are well localised to the implanted regions. This work demonstrates how careful selection of fabrication parameters can be used to optimise the properties of V_B^- centres in hBN, supporting their application as quantum sensors based on 2D materials.