**4. Creation methods of NV defect in diamond**

The development of diamond NV centers has been relatively mature so far. Not only can NV centers be formed during diamond growth (inter-growth methods), but also ions, electrons, and laser beams can be used to obtain holes after diamond growth and then combine with nitrogen atoms to form NV centers (post-growth methods).

#### **4.1 Inter-growth methods**

The formation of NV centers in diamond can be realized during the growth of diamond by CVD (inter-growth). By adding a nitrogen gas source in the mixed gas source, the NV centers can possibly be created during the diamond growth. There are several advantages to grow NV defect by inter-growth method: (1) by adjusting the gas-phase environment and growth conditions, the NV defect concentration of the diamond film can be controlled within 200 ppb, ideally suitable for optical and electronic applications; [43] (2) The NV centers by this method can not only achieve a high concentration, but also the lifetime of electron spin coherence in ground state can reach the order of milliseconds. However, the inter-growth method by CVD to create NV defect has obvious drawbacks, like the uncontrollable formation site of NV defect, which needs further investigations. By optimizing the nitrogen concentration in the range of 0.2–20 ppm and E-beam irradiation conditions to study the formation of NV defect in CVD-grown diamond, the spin–spin coherence times of 45.5–549 μs for CVD diamond containing 168–1 ppb NV centers can be achieved [44]. In addition, with low growth temperature to enhance the nitrogen incorporation and with N2O as a dopant gas for diamond growth by CVD on [113]-oriented substrate, the obtained nitrogen-doped diamond combines a high NV defect creation yield and long coherence time of several tens of μs together with a partial preferential orientation, which are desirable requirements for diamond-based sensors (**Figure 8**) [45].
