Fig. 3. Tip-induced charge-state transition of a single NV.

a The process of charging the NV by local electric field from the AFM tip. Scale bar: 1 μm. (i) Confocal image including the target NV denoted by a dashed white ellipse before the charge state control. (ii) The tip with a high positive bias was guided close to the target NV, and the fluorescence of NV preserved. (iii) When the tip was grounded, the NV became completely dark. The small background denoted by red arrow arises from the tip-scattered light. (iv) Applying a high negative voltage to the tip switched the NV to bright state as before. (v) The bright state of NV remains when the tip was grounded and retracted. b Time-elapsed fluorescence shows the fast switch of NV charge state when the bias was set at +40 or +10 V alternatively. c Fluorescence-bias curves in the positive voltage range. Reproducible curves along with two cycles of bias ramps indicate the charge state control with high fidelity. d PL spectra of the same NV in (c) under different biases, confirming the different charge states of NV. The blue and red arrows indicate the ZPL of NV⁰ and NV⁻, respectively. All the fluorescence data were recorded under the 488-nm excitation laser with the 550-nm LP filter.