*4.2.3 Electron beam irradiation method*

The principle of electron beam irradiation technique is to use electron beam to irradiate diamond to generate vacancies, which can bind with nitrogen atoms to form NV defects after high-temperature annealing, as shown in **Figure 10b**. The concentration and depth of the generated NV defects can be controlled by adjusting the energy

#### **Figure 10.**

*(a) Schematic of the preparation of NV defect in diamond using femtosecond laser irradiation. Reproduced with permission from ref. [51]. Copyright 2022 AIP Publishing, and (b) Schematic of the generation of NV defect in diamond by electron beam irradiation in a transmission electron microscope (TEM). Reproduced with permission from ref. [52]. Copyright 2016 American Chemical Society.*

of electron beam, the irradiation time, and the distance between the diamond and the electron beam gun. In 2017, a TEM-based method was used to create localized NV defect with coherence time exceeding 1 ms [52].

Due to the high sensitivity and high spatial resolution of TEM-based technique, it is well suited for the creation of NV defects. By using electron beam irradiation with the energy of 200 keV followed by high-temperature annealing at 1200°C, a surface density of ~2500 μm−2 of NV defects in diamond can be achieved. This technique can enable the given arbitrary in-depth control of NV centers.
