In detonation synthesis, the energy of an explosion is used to drive the diamond phase formation [28]. In a closed metallic chamber, explosives with a negative oxygen balance are detonated; usually a mixture of 60% TNT (C₆H₂(NO₂)CH₃) and 40% hexogen (C₃H₆N₆O₆) is employed (Figure 2) [1]. The carbon atoms that eventually form the NDs are provided either by the molecules of the explosives or by precursor graphite put inside the detonation chamber. The synthesis is called “dry” or “wet” when the chamber is filled with a gas (N₂, Ar, CO₂) or with water (ice), respectively, that act as coolants. The cooling media play an important role in the carbon yield, which is usually around 10% of the explosive weight [29]. The process leading to NDs formation is schematized in Figure 2. After detonation, the carbon atoms released during the explosive molecule’s dissociation condense and crystallize into nanoclusters [30]. The pressure/temperature increase (Figure 1) reached in the chamber during the detonation leads to crystallization of the carbon nanoclusters into diamond phase. Finally, the formed NDs grow and agglomerate, resulting in NDs of size 4−5 nm. The main drawback of this technique is that the resulting sample, called detonation soot, must be purified to remove contaminants. Indeed, detonation soot is not only made of diamond phase, but also of non-diamond carbon (25–85 wt %) and metals coming from the wall of the chamber (1−8 wt %) [31]. To remove the impurities, cleaning of the detonation soot in a strong acid environment (usually a mixture of HNO₃/H₂SO₄/HClO₄) is required [28], which is a dangerous and expensive process. Another problem is NDs agglomeration occurring during synthesis, where NDs clusters of several hundreds of nanometers are obtained. To isolate the single 4−5 nm sized NDs, milling with ceramic microbeads (that again introduces contaminants) or ultrasonic disintegration are the standard de-aggregation methods [32].

Chemical vapor deposition is one of the most popular methods for the deposition of thin film, and it has been used for the synthesis of nanocrystalline diamond film [33]. In detail, the deposition of carbon atoms occurs during decomposition of a gas mixture with a carbon containing species, usually methane CH₄, in an excess of hydrogen. The gas phase is decomposed by using a hot filament or a microwave plasma to form radicals such as H^• and CH₃^•, which are essential for diamond growth. The ND film forms on a substrate, typically a silicon wafer coated with a micrometer sized diamond powder acting as a seed for ND nucleation [33], and eventually forms a continuous film. The typical set-up [34] for CVD synthesis is reported in Figure 3a. The size of the grains composing the film ranges to tens of micron down to a few nanometers, depending on CH₄/H₂ relative concentration [35].

A weak concentration of CH₄ produces microcrystalline diamonds, whereas a high value of the CH₄/H₂ ratio decreases grain size to the order of tens of nanometers for 1−5% CH₄/H₂. The standard diamond growth mechanism [35] is schematized in Figure 3b. Carbon atoms at the surface of diamond seeds are left with dangling bonds after hydrogen abstraction by H^• radicals. These bonds are then filled by CH₃^• molecules. When this process takes place in two adjacent sites, the new carbons can bond together, and finally be locked into the diamond lattice.

The HPHT synthesis technique resembles the natural process of diamond formation, where a carbon precursor, usually graphite, is brought to a state of high pressure and high temperature. Inside a chamber, the temperature is brought to ~2000 °C and a set of anvils increases the pressure up to several GPa [36]. This technique allows for the formation of bulk or microdiamonds, which must be milled to obtain NDs [21]. The milling process does not allow for good control of nanoparticle size and shape [37], so additional work-up is required. For instance, an acid treatment to remove the contaminants coming from the milling process or centrifugation and filtration to isolate NDs with a narrower size distribution.

Starting from the pioneering work of Yang 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 cyclohehexane [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.

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 sp² carbon atoms of graphite to sp³ 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:

• 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_C, a process known as phase explosion occurs [25], leading to the ejection of nanodroplets [42] that eventually turn into nanoparticles after solidification.
• 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.
• 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 10¹⁰–10¹¹ 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 sp² shell forms around the NDs. An efficient and simple way to remove the shell was found by Osawa Osswald [31]; it consists of air oxidation at ~400 °C. Furthermore, the main advantage of PLA for NDs production is the possibility of achieving diamond phase formation with a simple and cheap experimental apparatus, and performing the synthesis process at ambient pressure and temperature conditions.

Work-up and post-processing procedures aimed at purifying reaction products are often a critical step in the synthesis of NDs. Several methodologies have been developed to cope with the two most common and abundant contaminants: other forms of carbon (mostly graphitic) and metals and their oxides.

For the carbon contaminants, the common principle underlying purification is selective oxidation, exploiting the higher reactivity of sp² carbon with respect to the sp³ carbon of the diamond phase. Examples are the above mentioned oxidation in air reported by Osawa Osswald [31], which was further investigated in a subsequent paper [46] where selectivity towards sp² carbon was achieved by keeping the temperature in the range of 400–430 °C. This procedure provides a simple and effective solution for the case of PLA NDs where, even though metallic impurities are avoided, the sp² carbon fraction can be so high as to actually be the main reaction product. A similar method, but employing boric anhydride to enhance selectivity, was also reported [47]. Another route that has been explored is oxidation with ozone-enriched air [48,49,50], though these procedures require significantly more complex equipment.

Liquid-phase oxidation reactions are by far the most commonly employed procedures. These generally require strong acids, which provide the additional advantage of also removing some metal-based impurities. Examples include HClO₄, concentrated HNO₃, H₂SO₄ alone and in combination with HNO₃, HCl in combination with HNO₃, and HF [29,51,52,53]. These treatments require temperatures from 80 °C to more than 200 °C, thus, specialized equipment is needed that adds to the complexity and cost. Yet, this is necessary, especially in the case of detonation NDs, where metal-based impurities are present in significant quantities, coming from the equipment involved in these processes, such as steel detonation chambers. For a comprehensive review, though it is focused on detonation NDs, the reader should consider Aleksenskii [54].

Recently, Reineck et al. [55] reported the observation of NV⁻ emissions from unprocessed detonation NDs, with a brightness comparable to what is obtained with highly processed fluorescent 100 nm HPHT NDs. Moreover, diamond CVD synthesis of highly-oriented NV centers was reported [56], with the possibility of crushing fluorescent bulk diamond into NDs [57].

Despite that, the standard way to produce NV-centers-enriched NDs is a complex, multi-step, and expensive process. Synthetic HPHT diamonds typically contain 100 ppm of nitrogen atoms as an impurity in the diamond structure [58] and to form NV centers, vacancies are created in the lattice by high-energy particle irradiation (electrons, protons, He+) followed by a vacuum annealing at 700–1000 °C [59]. The role of the annealing is to increase the mobility of the nitrogen atoms inside the lattice, which are then trapped by the carbon vacancies to form NVs. One could directly irradiate NDs to increase the NV-fluorescence, but it has been proven by density functional theory [60] and Monte Carlo simulations [61] that NV concentration increases non-linearly with crystal size. In particular, the probability of forming NV centers in 5 nm sized NDs is 4.5 and 25 times lower compared to 20 and 55 nm NDs, respectively [61]. Thus, it is more efficient to increase NV concentration in micro-sized diamonds and then reduce their size to obtain fluorescent NDs. This was done by Boudou et al. [23], in what has become the standard fluorescent NDs synthesis technique. The procedure is shown in Figure 6. The starting material is HPHT-synthesized microdiamonds that are then irradiated with a 10 MeV electron beam to form vacancies. The following step is annealing at 750 °C to form fluorescent microdiamonds. The next step is the reduction of diamond size, achieved by nitrogen jet milling and ball milling to obtain NDs with a crystal size smaller than 10 nm. For bioimaging, which is the main field of application for fluorescent NDs, the number of NV centers should be as high as possible. For this reason, only HPHT diamonds that contain a relevant amount of nitrogen impurities are used. In contrast, CVD diamond films contain a very small number of nitrogen atoms [62]. Consequently, highly fluorescent NDs are not produced by this mechanism; rather CVD is used to produce nanocrystalline diamond films with single NV centers, suitable for quantum information technologies.

PLA has been demonstrated to be a promising process for the direct synthesis of NV-enriched NDs. Narayan 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 sp² 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.

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 C⁴⁺), 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 N₂ molecules, having a bond dissociation energy at room temperature of 9.79 eV. Thus, the ablated energetic species can easily break N₂ 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).

The results, shown in Figure 9d, prove that the LN₂-NDs present a PL emission intensity (7000 ± 2000 a.u.) that is more than one order of magnitude higher with respect to DLC-NDs (400 ± 100 a.u.). The explanation of this larger NV-enriched ND production efficiency can be related to the different conditions attained inside the ablation plume. In particular, a strong sp² contamination of the NV-enriched NDs surface is observed for laser ablation in a gaseous environment [65] due to both lower pressure and smaller cooling rates achieved in the ablation plume.