## 2 Nanodiamonds Synthesis Techniques
### Fluorescent NDs Synthesis by Pulsed Laser Ablation  
PLA has been demonstrated to be a promising process for the direct synthesis of NV-enriched NDs. [PERSON] et al. [63] achieved synthesis of NV-doped diamond in the form of single-crystal nanodiamonds, nanoneedles, microneedles, and thin films by pulsed laser irradiation of N-doped carbon film. Diamond phase formation is obtained through conversion of the sp\({}^{2}\) carbon atoms through rapid melting in a super undercooled state and consequent quenching at ambient temperatures and pressures in air. Recently, we demonstrated direct synthesis of NV-centers-enriched NDs through the PLA of graphite in two different nitrogen-containing media: controlled nitrogen atmosphere [64] and liquid nitrogen [65]. In the first case, graphite is laser ablated inside a vacuum chamber filled with 1 Pa of nitrogen gas, and the ablated material deposits onto a silicon substrate. The resulting sample is a diamond-like carbon (DLC) film embedded with the fluorescent NDs (Figure 7). In the case of ablation in liquid nitrogen, the NDs are directly dispersed into the liquid medium. In both cases, NDs show intense native photoluminescence (PL) without the need of post-synthesis thermal activation or additional procedures. Proof of NV centers formation is obtained through optically detected magnetic resonance (ODMR), reported in Figure 8, for synthesis in liquid nitrogen. Figure 7: (**a**) Experimental setup of pulsed laser ablation (PLA). (**b**) SEM image of the synthesized sample: on the Si substrate a thin diamond-like carbon (DLC) film is deposited over which NDs are dispersed. (**c**) NV\({}^{-}\) center energy levels. When the NV is excited by using a green laser (green arrow), it can relax either radiatively (red arrow) or non-radiatively (yellow dotted arrow). The effect of the lattice strain, both in its parallel and perpendicular components with respect to the NV axis, on the degenerated \(m_{s}\) = 1 spin states is represented (see text for details). In the red box: scheme of diamond crystal structure with an NV center, consisting of a nitrogen atom (blue sphere) and a nearest-neighbor carbon-vacancy (yellow circle) along the (111) direction. (**d**) Left-hand side: SEM image of one microparticle deposited on the DLC film, consisting of clustered C-nanoparticles with sizes below 50 nm. Right-hand side: corresponding EDXS spectrum. The carbon and nitrogen peaks are detected at, respectively, 0.27 and 0.38 KeV, indicating that the nitrogen is contained inside the nanoparticles’ cluster. In addition, the silicon peak from the Si substrate was detected at 1.74 KeV (not shown). Reproduced with permission from [64], Royal Society of Chemistry, 2018. The mechanism responsible for ND formation is the same reported in Section 2.4. The whole graphite-liquid-diamond transition is represented by the purple arrow in the carbon phase diagram [41] in Figure 9a. The tilted red lines indicate the region where undercooled liquid carbon and metastable diamond coexist, referred to as a \"diamond-like\" liquid [66]. The transition to diamond phase occurs because in this thermodynamic condition, the energy barrier for diamond formation is lower than that for graphite formation [67]. It is important to note the presence of atomic nitrogen, which is required for NV-centers formation. The energy of the atoms, ions (from C\({}^{+}\) to C\({}^{4+}\)), and electrons leaving the laser ablated target can be higher than 100 eV for the typical laser fluence used [68]. This energy is enough to break the N\({}_{2}\) molecules, having a bond dissociation energy at room temperature of 9.79 eV. Thus, the ablated energetic species can easily break N\({}_{2}\) molecules, and single N atoms may be trapped inside the nanodroplets expelled from the graphite target. Interestingly, we performed a comparison between these two methods (ablation of graphite in gas and liquid nitrogen) in term of the total PL emission of the produced sample (Figure 9b,c).