## 2 Nanodiamonds Synthesis Techniques
### Pulsed Laser Ablation  
Starting from the pioneering work of [PERSON] et al. [38], PLA has become a viable method for the synthesis of NDs. A standard PLA experimental setup for NDs is given in Figure 4. The laser beam is focused on the top surface of a graphite target that is immersed in a liquid, which is typically water, but NDs production has been observed in other liquid environments, such as cyclohexane [39]. A pulsed laser is used in order to release an extremely high power on the target; usually, laser pulses in the ns region with energies in the order of 100 mJ, for a final power density of up to GW cm\({}^{-2}\), are employed. Figure 3: (**a**) Experimental setup for chemical vapor deposition (CVD) synthesis. The hot filament is required to dissociate the gas phase (CH\({}_{4}\)/H\({}_{2}\) mixture) into radicals, whereas the electrode plate is used to attract those radicals (mainly H\({}^{\bullet}\) and CH\({}^{\bullet}\)) toward the substrate. Reproduced with permission from [34], Elsevier, 2016. (**b**) Schematization of the standard growth mechanism of nanocrystalline diamond film during CVD synthesis. In pulsed laser ablation, the water plays a double role; it is a medium, where nanoparticles are suspended and collected during ablation, and it confines the ablation plume, which favors the diamond-phase formation. Indeed, the presence of a confining layer in front of the target is a fundamental aspect of the transition from sp\({}^{2}\) carbon atoms of graphite to sp\({}^{3}\) hybridization of the diamond phase. The details of the formation mechanism are provided in [40, 41]. The synthesis of NDs upon the PLA of graphite in water occurs in three steps:  
1. The absorption of the high power laser pulse induces melting of the graphite surface. As the laser energy is deposited in a short timescale, the target surface rapidly becomes a superheated liquid. When the temperature of this liquid reaches a value of about 90% of the critical temperature, T\({}_{\text{C}}\), a process known as phase explosion occurs [25], leading to the ejection of nanodroplets [42] that eventually turn into nanoparticles after solidification. 2. Strong vaporization of the graphite target and phase explosion lead to the emission of a plasma plume containing the ablated material; inside the liquid, a state of extreme thermodynamic conditions of temperature and pressure is created, of 5000-6000 K [43] and 2-4 GPa [41, 44], respectively. 3. The last step concerns the cooling of the ablation plume. Due to the confinement effect, the ablation plume dissipates excess heat very efficiently through collisional cooling with the liquid molecules, resulting in a short quenching time. The fast cooling rate is the most peculiar characteristic of laser ablation in liquid, which can be in the order of 10\({}^{10}\)-10\({}^{11}\) K s\({}^{-1}\)[45]. This strong and fast reduction in the temperature is enough to produce the carbon nanodroplets in an undercooling regime in a few nanoseconds. In this condition of undercooling, NDs form as a metastable phase, starting from the nanodroplets expelled by phase explosion. The transition to the thermodynamic stable allotrope of carbon, namely graphite, is prevented by the rapid quenching; the metastable phase is literally frozen under the undercooling action. Figure 5 shows the characterization of NDs produced through the PLA of graphite in water as reported in [40]. PLA for NDs synthesis presents some advantages compared to the standard production processes, such as HPHT or detonation. For instance, the sample is not contaminated by metallic impurities that require harsh chemical purification. NDs produced by laser ablation only require removal of the graphitic shell covering the nanoparticles. Indeed, when the temperature and pressure conditions inside the ablation plume are no longer the ones required for diamond phase formation, a sp\({}^{2}\) shell forms around the NDs.