Research Progress on Silicon Vacancy Color Centers in Diamond

Abstract

The silicon vacancy (SiV) color centers in diamond have significant potential in the fields of quantum information, nanophotonics, bioimaging and sensing, due to its good structural stability at room temperature, strong photostability, short luminescence lifetime, narrow zero-phonon line bandwidth, and avoidance of biological autofluorescence. They have attracted extensive attention and in-depth research from researchers. The applications of SiV color centers in the above areas require addressing key issues such as control over the number and charge state of SiV color centers in diamond, as well as improving the luminous efficiency. Therefore, this review summarizes some recent relevant research progress, including common methods for preparing SiV color centers, control over their luminescence intensity, preparation of SiV single-photon sources, and research on the control of SiV color center charge states. Based on these, an outlook is provided for the preparation and application of SiV color centers in diamond.

The figure of Table of Contents (TOC)

Keywords:

• Diamond
• SiV color centers
• photoluminescence
• charge state
• single-photon source

1. Introduction

In general, ideal crystalline materials do not exhibit fluorescence properties. However, when defects such as vacancies or impurity atoms stably exist within the crystal lattice, they can alter the original band structure, allowing electrons to undergo radiative transitions, resulting in the emission of a zero-phonon line (ZPL) [1]. If the bandgap of the crystal material is larger than the energy level width of the radiative transition, the fluorescence cannot be absorbed by the crystal material and will be emitted from the crystal material [2,3]. These optically active crystalline defects, whose fluorescence spectra typically fall within the visible light range, are called as color centers. Diamond, as a highly important indirect bandgap semiconductor material with a bandgap width of about 5.47 eV, has a wide bandgap compared to common semiconductor materials. This allows it to accommodate numerous potential color centers ranging from deep ultraviolet to far-infrared wavelengths [4].

In diamond lattice, due to its stiffness and relatively small atomic spacing, it tends to prefer forming defects in simple structures(such as vacancies, substitutional impurities, and interstitial impurities). So far, over 500 color centers have been discovered in diamond [5–7], among them, silicon vacancy (SiV) color centers and nitrogen vacancy (NV) color centers have received wide attention due to their room-temperature operability, high brightness, and high optical stability [5–12]. The NV color centers in nanodiamonds can be used for long-term biological imaging studies, but their luminescence peaks overlap with some optical labels and cell autofluorescence, making dual/multi-color imaging still challenging [13]. On the other hand, SiV color centers possess remarkable near-infrared (NIR) emission characteristics with a sharp ZPL near 738 nm. This leads to effective differentiation between the excitation fluorescence (signal) of SiV color centers and the autofluorescence (noise) caused by biological autofluorescence and ambient visible light, making them easier to be detected [13]. Nanodiamonds contained SiV color centers also exhibit good biocompatibility. Therefore, diamond containing SiV color centers has attracted extensive attention as a high-performance probe for biological imaging [14]. However, as diamond used for bioimaging is mostly in the form of nanocrystals, the number of color centers in the crystal is limited, and the luminescence intensity is weak. Therefore, the key to the widespread application of SiV color centers in the fields of biological labeling and sensing is improving the luminescence intensity and efficiency in the diamond nanocrystal.

Solid-state single-photon sources have a wide range of potential applications in quantum optical devices and are easily integrated with other nanophotonic devices in a scalable manner [15]. Over the past decade, color centers in diamond have emerged as highly promising candidate materials for solid-state single-photon sources due to their high brightness and optical stability at room temperature [16]. High-quality single-photon sources require structural stability, high quantum efficiency at room temperature, narrow emission bandwidth, and short luminescence lifetimes [16]. Currently, there is significant research focused on NV and SiV single-­photon sources. However, NV color centers exhibit broad fluorescence spectra with a linewidth of approximately 100 nm at room temperature, and only about 4% of photons are concentrated near its ZPL, making them challenging to use as single-photon sources [17–19]. On the other hand, the SiV centers have a strong ZPL at room temperature (containing about 70% of the emission) and a narrow width of about 5 nm, and short photoluminescence (PL) lifetimes, making them excellent solid-state single-photon sources [20,21]. Although SiV color centers can meet the requirements for high-quality single-photon sources, they also exhibit low single-photon emission rates and low radiative quantum yield [22]. Additionally, even at low temperature (4 K), SiV color centers have low quantum efficiency and nanosecond scale spin coherence times [23,24]. Therefore, to fully harness the exceptional performance of SiV color centers, challenges such as fabrication of single SiV color center, improvement of quantum efficiency, and charge state control need to be addressed.

Starting from the application challenges of diamond SiV color centers, this article provides an overview of the related research on the preparation, control of luminescence intensity, preparation of single-photon sources, and charge state control of SiV centers. Furthermore, the article offers prospects for the future development trends in this field.

1.1. Electronic structure and spectral properties of SiV color centers

The SiV color center in diamond consists of a silicon atom and two adjacent carbon vacancies, as shown in Figure 1(a), the silicon atom lies between two carbon lattice vacancies with D3d symmetry and its electronic orbitals have inversion symmetry [25,26]. The SiV color center exists in two charge states: the neutral state SiV⁰ and the negative state SiV^-. The SiV⁰ color center contains ten unpaired electrons, six of which arise from dangling bonds of six carbon atoms surrounding the two vacancies, while the remaining four come from the silicon atom, resulting in a spin state of S = 1. The SiV^- color center, on the other hand, has an additional electron, capturing from the lattice to form a negatively charged state, resulting in a spin state of S = 1/2 [25, 27]. This property paves the way for all-optical control of the electron spin in SiV color centers. The energy difference between the ground state and excited state of SiV⁰ and SiV^- color centers is 1.31 eV (ZPL 946 nm) and 1.68 eV (ZPL 738 nm), respectively [28]. Currently, research on SiV^- color centers has dominated the field, and relatively less research has been conducted on SiV⁰ color centers. Figure 1(b) depicts the energy level diagram of SiV^- color centers, consisting of the ground state and excited state separated by 1.68 eV at room temperature. At low temperatures, the spin-orbital coupling and Jahn-Teller effect cause the orbital degenerate states of the ground state and excited state to split into four energy levels with double spin degeneracy [20, 29]. Therefore, the PL spectrum of SiV^- color centers exhibit four peaks at low temperatures, as shown in Figure 1(c). At room temperature, the ZPL of SiV^- color centers are located around 738 nm [30], with a line width of approximately 7 nm due to inhomogeneous broadening effects, as shown in Figure 1(d) [31]. The ZPL of individual SiV color center can be as narrow as 0.7 nm [32], with approximately 70–80% of photons concentrated near the ZPL [33]. Furthermore, the unique physical structure and inversion symmetry of SiV color centers ensure its emission without spectral jitter or divergence, making different SiV color centers capable of producing indistinguishable photons, thereby rendering them ideal single-photon sources [29]. The excited-state lifetime of SiV color centers is short, only on the order of ∼1 ns [34], resulting in a high fluorescence emission rate, making them the brightest color centers among all reported color centers in diamond.

Figure 1. (a) Representative atomistic diagram of a SiV center with interstitial silicon (red), vacancies (gray), and carbon (black) atoms shown within a diamond unit cell. (b) The energy level scheme of the SiV center at cryogenic temperatures, showing the four optical transitions between the doublet ground and excited states [26]. (c) Fluorescence spectrum at 4K for an SiV ensemble in [001]-oriented bulk diamond obtained by non-resonant excitation at 700 nm [29]. (d) Photoluminescence spectra of SiV centers in NCD films [31].

1.2. Preparation methods of SiV color centers in diamond

In the process of preparing SiV centers, both silicon atoms and vacancies are required, where the silicon atoms can be doped by in-situ doping or injected into the diamond lattice. The representative in-situ doping methods include high-temperature and high-pressure method, chemical vapor deposition method and detonation method. Ion implantation is a representative method among non-in-situ doping methods.

1.2.1. High-pressure high-temperature (HPHT) method

The formation of diamond color centers can be achieved through in-situ doping using the HPHT method during the diamond synthesis process [21]. By introducing specific dopant elements into the reactants, doped single crystals [35,36], micro/nano crystalline diamonds can be obtained [37]. As shown in Figure 2, precursors contained silicon are used to generate SiV color centers in diamond by applying HPHT method in device [22]. Compared to color centers prepared by chemical vapor deposition (CVD), the color centers produced through HPHT method exhibit higher stability [35].

Figure 2. (a) Schematic diagram of the HPHT apparatus and precursor used to obtain SiV color center. (b) TEM image of diamond nanoparticles prepared by HPHT method. (c) Typical spectra of SiV centers fluorescence at room temperature [22].

1.2.2. Chemical vapor deposition (CVD) method

The HPHT method cannot grow thin films or control the fine structure of diamond, which can be achieved by CVD method. There are two common sources of impurities in CVD method: one is the substrate which will be etched by plasma or vapored at high temperature, for example, in the preparation of SiV color centers in diamond, a silicon-­containing material such as a silicon wafer or quartz substrate can be chosen [20]; the other is the gas precursor materials contained doping elements such as silane [38]. Generally, using solid doping sources is advantageous for preparing shallow SiV color centers, ie, those close to the substrate surface. To better control the formation of color centers, gas doping source is needed. CVD method has many advantages in diamond color center preparation, such as the size of diamond crystals and the content of diamond color centers can be well controlled by the growth time and atmosphere, and there is a diverse selection of substrates for diamond growth [1]. Among different CVD methods, microwave plasma CVD (MPCVD) is considered the most suitable for the deposition of epitaxial diamond film [39], polycrystalline film, and isolated micro/nanocrystals, as it can deposit high-purity diamond materials with nitrogen impurity content less than 1 ppb and control over the doping dose and film thickness. However, the CVD method is not suitable for synthesizing a large amount of SiV color center-containing nanodiamond powders [40,41], which are essential for medical applications. Therefore, understanding the mechanism of the formation of SiV color centers and improving the yield and quality of SiV color centers in CVD method, are crucial for their practical applications [42].

1.2.3. Detonation

The SiV color center-containing nanodiamonds produced by CVD method exhibit improved fluorescence characteristics. However, their production is currently limited to small-scale laboratory synthesis. This poses significant limitations on the practical applications of SiV color center-containing nanodiamonds [43]. The detonation method can be used to prepare nanodiamonds with an average particle size of 3–5 nm and containing SiV color centers by detonating a mixture of explosives (such as TNT) and Si-containing precursors. This method has a simple process and can produce detonation nanodiamonds in large quantities at a low cost [41]. As shown in Figure 3, using triphenylsilanol (TPS) as the silicon source, detonation nanodiamonds containing SiV color centers with a particle size of 4–10 nm can be directly synthesized by the detonation method [44]. Compared to nanodiamonds produced by CVD method and derived from the fragmentation of large diamonds produced by the HPHT method, detonation nanodiamonds have smaller particle sizes (approximately 5 nm) and more uniform size and shape. The detonation SiV nanodiamonds have great potential to drive advancements in the field of biomedical applications [45]. However, the surface effects resulting from the extremely small particle size of nanodiamonds enhance the ­electron-phonon coupling and increase the probability of nonradiative transitions of SiV color centers in detonation nanodiamonds [44]. Meanwhile, during the detonation process, the uneven distribution of electrons within the nanodiamonds can lead to a broadening of the ZPL of SiV color centers, resulting in a decrease in their luminescent performance. The surface effect stemming from the extremely small particle size of DNDs (detonation of nanodiamonds) enhances electron–­phonon coupling and increases the nonradiative transition probability of SiV-DNDs. Moreover, because of the detonation process, the electronic states of the SiV centers inside the DNDs are not uniform. Aggregates of SiV-DNDs with slightly nonuniform electronic states exhibit the ZPL with inhomogeneous broadening [43].

Figure 3. (a) Chemical structure of the silicon dopant: triphenylsilanol (TPS). (b–d) TEM, XRD, and particle size distribution images of detonation nanodiamonds. (e) PL spectrum of detonation nanodiamonds [44].

1.2.4. Ion implantation method

Ion implantation is used to create diamond SiV color centers by bombarding diamond with high-energy Si ions, which are then implanted into the diamond lattice. The quantity and location of the color centers in diamond can be controlled by adjusting the acceleration voltage and ion dose. As shown in Figure 4, Si ions are injected into the sample to create SiV color centers in the diamond lattice, with the location, acceleration voltage and ion dose controlled [46,47]. Ion implantation allows us to introduce desired impurities into diamond with high controllability [15]. This method can be used to create various optical defects, including NV [48,49], SiV [50], stannum vacancy (SnV) [45], germanium vacancy (GeV) [51] and others. To minimize lattice damage and residual stress induced by ion implantation, annealing is typically performed after ion implantation [52], vacancies created by ion implantation will undergo recombination during annealing, forming impurity atom-vacancy complexes that are color centers.

Figure 4. (a) Ion implantation schematic and corresponding fluorescence spectrum obtained through confocal scanning [46]. (b) Confocal scanning image and photoluminescence spectrum under ion implantation dose of 2 × 10¹⁵cm⁻² [47].

The defect structure formed during ion implantation process is crucial for the formation of color centers [46]. However, the application of ion implantation in quantum device manufacturing is still limited because it currently lacks reliable sub-50 nm positioning accuracy, is expensive, and cannot be produced on a large scale. This method is mainly used for the preparation of SiV color center single photon sources, which can meet the requirements of narrow phonon sidebands and short excited state lifetimes [47]. However, high-energy ion beams can damage the diamond lattice, resulting in non-radiative transitions and significantly reducing the SiV color center’s luminescence efficiency. Implantation of ions always causes damage to the crystal lattice of any substrate (radiation defects) along the ion tracks. In the case of diamond, the radiation defects are mainly single vacancies, interstitial carbon atoms and their simple complexes. In order to restore the damaged crystal lattice, the ion-implanted diamonds are usually annealed at high temperatures [53]. Annealing is usually required after ion implantation, and typical annealing temperatures for group IV elements are in the range of 800–1200 °C [52]. Lattice vacancies caused by ion implantation are reorganized after annealing to form atomic vacancy complexes, ie, optically active color centers.

From current research, it appears that the preparation of SiV color centers in diamond is relatively easy. However, the application of SiV color centers in diamond still faces several challenges [31, 54], such as controlling the luminescence intensity of SiV color centers, preparing single photon sources, and controlling the charge state of SiV color centers. The following is a brief introduction to the current research status and progress on these issues.

2. Controlling the luminescence intensity of SiV color centers

SiV color centers generated in situ during the CVD growth process exhibit better fluorescence properties [55]. However, heteroepitaxial diamond grown by CVD often contains non-diamond phases and lattice defects, which can lead to a decrease in the luminescence intensity and broadening of the ZPL of SiV color centers [56–58]. In fields such as biosensing and nanoscale temperature sensing, high PL intensity of SiV color centers is desired. However, the high refractive index of diamond (around 2.4) and the formation of sp² amorphous carbon or graphite phases or hydrogen termination result in low collection efficiency and photoluminescence quenching, respectively. To improve the luminescence of SiV color centers, various methods can be employed. Before fluorescence is generated, the suppressed SiV color centers can be controlled to fluoresce through techniques such as optical irradiation [54], electrical biasing [27, 59], and surface modifications. After fluorescence is generated, photon collection efficiency is able to be enhanced through constructing nanostructure, resonant cavities [60] and other means. However, challenges still exist for the application of SiV color centers in nanodiamonds [49]. The following are some commonly used approaches to control the luminescence intensity of diamond SiV color centers.

2.1. Adjusting the amount of doped Si atom

Under equivalent irradiation and annealing procedures, the doping dose of Si atoms in diamond directly determines the formation of SiV color centers in diamond, thereby affecting their PL intensity. Zhang et al. used HPHT methods to prepare SiV color centers in diamond, and adjusted the doping dose by controlling the amount of Si powder under conditions of 5.0–5.2 GPa and 1420–1500 °C [61]. They fabricated microcrystalline diamonds with different concentrations of SiV color centers. As shown in Figure 5(a), the PL intensity of SiV color centers in diamond increases with the increase of silicon doping level, and it was also demonstrated that the presence of nitrogen impurities inhibits the formation of SiV color centers in diamond [61]. Yang et al. studied the impact of Si/C ratio on the microstructure and PL intensity of diamond films grown by CVD method using trimethylsilane (TMS) as dopant source [62]. As shown in Figure 5(b), for diamond films grown at 870 °C, the diamond grains transformed from microcrystalline to nanocrystalline when the Si/C ratio was increased from 0 to 1/100, indicating that the addition of Si atoms results in grain refinement. The SiV PL intensity exhibits a non-monotonic variation trend with the increase of Si/C ratio. When the Si/C ratio is 1/3100, the SiV color center shows the highest PL intensity, with a strong emission peak at about 738 nm and a full width at half maximum (FWHM) of approximately 5.1 nm. Increasing the Si/C ratio promotes the formation of nanocrystalline structure and reduces the PL intensity of SiV color centers. The results indicate that the quenching of SiV centers with the increase of Si/C ratio is attributed to the formation of amorphous carbon. In addition, Sedov et al. grew nanocrystalline diamond (NCD) and microcrystalline diamond (MCD) films using MPCVD with bright SiV color centers [63]. As shown in Figure 5(c), by adding SiH₄ into the CH₄-H₂ reaction gas mixture to dope the film with Si, the PL intensity of SiV color centers in NCD and MCD films reaches its maximum value at SiH₄/CH₄ ratios of 0.2% and 0.6%, respectively. The maximum intensity of MCD is an order of magnitude higher than that of NCD. Under higher SiH₄ addition, the films are prone to fluorescence quenching, but no significant degradation of film structure such as the formation of Si-induced amorphous carbon was observed within the studied SiH₄ concentration range (0%–0.9%). According to Raman spectroscopy, the higher PL intensity of MCD films is related to their lower crystal defects.

Figure 5. (a) Raman and PL spectra of samples with different silicon doping levels measured using a 532 nm laser at room temperature [61]. (b) SEM images and PL spectra of Si-doped diamond films with different Si/C ratios deposited at a growth temperature of 870 °C [62]. (c) SEM images of undoped 1 µm thick NCD and MCD films, as well as Raman and PL spectra of NCD films, grown in MW plasma at SiH₄/CH₄ ratios of 0% to 0.8% [63].

In summary, the presence of nitrogen impurities inhibits the formation of SiV color centers in diamond, while increasing the Si doping level helps to enhance the luminescence intensity. However, it is not favorable to obtain SiV color centers with excessively high Si doping levels, as it can affect the crystal morphology and size of diamond and lead to the formation of amorphous carbon on the diamond surface, resulting in the PL quenching of SiV color center. While regulating the amount of silicon atoms doped, annealing treatment is a necessary step to allow the vacancies to find silicon atoms, while electrons are required for the formation of SiV color centers in diamond. Mindarava et al. report here the characterization of surface cleaned fluorescent micro- and nanodiamonds, obtained by irradiation of commercial diamond powder with high-energy (10 MeV) electrons and simultaneous annealing at 800 °C [64].

2.2. Controlling the surface termination state of diamond

Although the concentration of doped Si atoms is high, the SiV color centers in nanodiamond particles or films deposited by CVD consistently exhibit weak PL intensity [62,63]. Similar phenomena have also been observed for NV color centers in nanodiamonds. Hu et al. conducted a series of thermal oxidations on nanodiamond films at different temperatures to create various surface termination states of nanodiamond grains and studied their effects on the photoluminescence of SiV color centers [5]. As shown in Figure 6(a,b), experimental and first-principles calculations indicated that negative ­electron-affinitive surfaces induced by C-H termination leads to the quenching of photoluminescence of SiV color centers, while positive electron-affinitive surfaces originating from C = O termination eliminate this quenching effect [5]. This study discovered the quenching mechanism of SiV photoluminescence and developed a way to improve the PL intensity. Yang et al. referenced this result and investigated the influence of different surface oxidation methods on the microstructure evolution of nanodiamond films and the photoluminescence of SiV color centers [12]. All oxidation methods resulted in the incorporation of oxygen functional groups on the diamond surface. Compared to the as-grown films, the photoluminescence intensity of SiV color centers in the thermally oxidized samples was enhanced by approximately 100 times. The schematic diagram of the oxidation process in the nanodiamond film, as shown in Figure 6©, demonstrated that the enhancement of photoluminescence originated from the formation of different thicknesses of oxide layers [12]. A candidate ground states of the technically important oxidized diamond (100) surface was identified by theoretical calculations (Figure 6(d)) by Hu et al. [65] It indicates that the proposed methoxyacetone surface structure, containing both carbonyl and epoxide groups, is the most stable reconstruction at ambient and elevated temperatures. This finding solved the long-standing structural uncertainty regarding the oxidized diamond (100) surface. Hu et al. utilized intrinsic silicon substrate as the source of silicon and prepared separated domain formed nanodiamond films using hot-filament chemical vapor deposition (HFCVD) method, which provided more oxidation sites compared to continuous films. After thermal oxidation, the photoluminescence intensity of SiV color centers increased by 22.7 times compared to before oxidation [66]. The reduced amount of amorphous carbon and increased diamond content in the oxidized film, along with the sufficient exposure of nanodiamond grains, significantly enhanced the luminescence intensity of the film. Yang et al. also employed air annealing of Si-doped nanodiamond films to enhance the photoluminescence intensity of SiV color centers by approximately 1473 times compared to that of the untreated films [31]. The significant increase in the photoluminescence intensity of SiV color centers is primarily also attributed to the transformation of the diamond surface from hydrogen-terminated to ­oxygen-terminated, as well as the optimization of the crystal quality of the annealed films.

Figure 6. (a) Schematic illustration of the possible modes for thermal oxidation processes of NCD films [5]. (b) The electronic affinity and calculation model of carbonyl (C = O) terminated surfaces [5]. (c) The schematic illustration of oxidation behavior and corresponding PL emission in a nanocrystalline diamond particle and film [12]. (d) Electronic band structure of the proposed methoxyacetone surface [65].

These studies collectively demonstrate that the conversion of diamond surface termination from hydrogen to oxygen (particularly carbon-oxygen double bonds) can significantly enhance the photoluminescence intensity of SiV color centers in diamond. Additionally, after deposited films were treated by thermal oxidation at certain temperatures, the amorphous carbon and graphite phases on the diamond surface can be etched, thereby greatly reducing their inhibitory effect on the photoluminescence of SiV color centers.

2.3. Constructing nanoscale structures and resonant cavities on the diamond surface

The optical properties of color centers in diamond can be significantly improved through their interaction with the environment, particularly in the presence of resonant cavities where the spontaneous emission rate of atoms is enhanced (Purcell effect). This effect leads to a higher spontaneous emission rate compared to when the atoms are in free space [67], resulting in an increase in the spontaneous emission rate of quantum emitters and impacting their directionality and quantum efficiency, thus enhancing the photoluminescence intensity of SiV color centers in diamond. For example, nanoscale structures [7, 66], photonic crystals [68–70], plasmonic nanostructures, and other optical resonators [71,72], have been fabricated to enhance the photoluminescence characteristics of SiV color centers in diamond.

Hu et al. successfully constructed nanostructures on the surface of diamond particles through oxidation, greatly enhancing the PL intensity of SiV color centers [7]. As shown in Figure 7(a), since the crystal orientation of these nano-structures matches the <111> aligned split-vacancy structure of the SiV center, the collection efficiency of SiV luminescence increased, such that the SiV emission intensity increased by 27-fold and 4-fold for the nano-pyramid and the irregular nano-porous structure, respectively [7]. Thermal oxidation significantly improved the crystal quality of diamond, reducing the lattice strain around SiV color centers and narrowing their ZPL widths to approximately 3 nm. Hu et al. [66] also performed thermal oxidation method on nanocrystalline diamond films which are comprised of separated domains and constructed flower-shaped diamond aggregates which is comprised of radially arranged diamond grains. SiV PL intensity of the film is largely enhanced by about 22.7 times after oxidation, see Figure 7(b).

Figure 7. (a) SEM images of diamond particles grown at 2.5 kPa and variation of intensity of the SiV PL peaks of diamond particles grown at 2.5 and 3.5 kPa after 0, 30, 60, 90, and 120 min thermal oxidation [7]. (b) SEM images, Raman spectra and PL spectra of flower-shaped diamond aggregates comprised of radially arranged diamond grains after different thermal oxidation times (0, 30, 40, 130, and140 min) [66].

Additionally, Hamers et al. developed Ag-diamond core-shell structures using MPCVD [73], as shown in Figure 8(a). Ag is encapsulated by a diamond film containing SiV color centers. The Ag-diamond core-shell structures exhibit enhanced photoluminescence of SiV color centers, suggesting their potential utility in bioimaging, nanophotonics, bright single-photon sources, nanoscale thermometry, and other quantum-based applications. In another study, Yang et al. successfully fabricated diamond nanoneedle arrays, as depicted in Figure 8(b). By removing the nanodiamond and sp² carbon from the sidewalls of the diamond nanoneedles through thermal oxidation, the collection efficiency of PL significantly increased [74]. This study identified the constraints of sp² carbon on the optical collection of sidewall color centers in nanoneedles. A schematic model based on surface energy band bending theory was proposed to explain the enhancement mechanism of photoluminescence in SiV color centers: the near-surface light-trapping layer formed in hydrogen-terminated nanodiamond undergoes a transformation during oxidation, resulting in the additional collection of fluorescence from inner SiV color centers, thereby enhancing the PL of SiV color centers in the diamond nanoneedle array. The similar schematic also has been mentioned before in the work of Lu and Hu et al. [65].

Figure 8. (a) Fabrication process of Ag–diamond patterned arrays [73]. (b) SEM and HRTEM images of diamond nano pins formed after annealing of diamond films [74].

Csete et al. demonstrated superradiance in a fragmented symmetric array of SiV diamond color centers embedded in a concave plasmonic nano resonator [75]. Figure 9(a,b) demonstrate the advantages of the lower number of color centers composed of bare spheres and coated ellipses and the elliptical geometry in achieving greater total fluorescence enhancement. Additionally, Gritsienko et al. proposed coupling nanodiamonds containing SiV color centers with gold nanocavities [76], as shown in Figure 9(c,d). In this configuration, the emission intensity of SiV color centers was enhanced by 62 times compared to nanodiamonds on the gold surface. This coupling structure improved the optical excitation efficiency by four times compared to nanodiamonds on the gold surface, and the main mechanism for fluorescence enhancement may be related to the plasma resonance switching of the charge state of SiV color centers. Therefore, by constructing a coupling between the nano-resonator and SiV color centers in nanodiamonds, the fluorescence characteristics of SiV color centers can be greatly enhanced.

Figure 9. Optical response of bare spherical and ellipsoidal core-shell nanoresonators. Wavelength dependent (a) Purcell factor and quantum efficiency. (b) Radiative rate enhancement in the case of the reference (upper) and completely seeded (lower) system [75]. (c) Schematic of picking up and placing individual nanodiamonds, nano-pit arrays, and scans of SiV fluorescence of NDs on the gold surface. (d) Optical properties of SiV for ND1 before and after the pick-and-place manipulation. Background-corrected fluorescence spectra of ND1-G (orange, right scale) and ND1-GNP (blue, left scale). The excitation power was 2.5 mW. Saturation measurements for ND1. Excitation-beam waist reflection microscopy profiles over ND1-G (orange dashed line), bare nanopit (red dashed line), and ND1-GNP (blue solid line). The horizontal axis corresponds to the position of the waist relative to the scanned object. The vertical axis contains reflected intensities normalized to the reflected intensities of the gold surface. The minima of the reflected intensity near 2.5 µm indicate the degree of field localization in the plasmonic nanostructure. Fluorescence decay curves for ND1-G (orange) and ND1-GNP (blue) under pulsed laser excitation at 650 nm. The gray curve represents the instrument response function (IRF) [76].

In Figure 10(a), Fait et al. demonstrate a new and simple two-step method for fabricating diamond photonic crystals slabs with leaky modes overlapping the emission line of the silicon vacancy (SiV) centers [77]. A thin layer of SiV-rich diamond is then deposited over the photonic crystals slab so that the spectral position of the photonic crystals leaky modes is adjusted to the emission line of the SiV centers. An intensity enhancement of the ZPL of the SiV centers by a factor of nine is achieved. In Figure 10(b), Ondič et al. designed suspended photonic crystal cavity made of polycrystalline diamond to increase the intensity of the zero-phonon line at the center of the SiV by more than a factor of 2.5 by coupling to the cavity photonic modes [78]. This enhancement is comparable to what has been achieved on single-crystal diamond [79]. By improving the optical structures and structural quality, further enhancement in fluorescence intensity is expected. Kubanek et al. deterministically place SiV-containing nanodiamonds inside one hole of one-dimensional, freestanding, Si₃N₄-based photonic crystal cavities and coherently couple individual optical transitions to cavity modes. By optimizing the coupling using cavity resonance, Purcell enhancement, and waveguide tuning, the emission intensity of SiV color centers was enhanced by more than 14 times compared to free-space emission in Figure 10(c) [80].

Figure 10. (a) SEM images and measured transmission spectra of the original sample and the sample after deposition of the tuning layer, PL spectra measured on the original structure (black on photonic crystal (PhC), red on reference layer; the green rectangle shows the spectral position of the SiV center ZPL). PL spectra measured on the structure with the tuning layer [77]. (b) Photoluminescence of the selected PhC cavity possessing the highest enhancement of the SiV⁻ center PL emission with respect to the PL spectra of the adjacent periodically patterned PhC slab and with respect to the adjacent homogeneous diamond slab suspended in air. Confocal microscope PL map of the photonic structure measured through a filter transmitting 740 ± 13 nm placed in front of the detector. The dashed line depicts the border of the PhC structure, ie, the interface of the PhC and the homogeneous diamond slab; micro-PL spectra of the PhC cavity (black curve), the PhC structure (red), and the homogeneous diamond air-suspended slab. The most prominent peak at 738 nm is the emission from SiV⁻ centers enhanced by the coupling to the higher order photonic crystal cavity modes. The peak at around 780 nm superimposed on the broad PL is a fundamental cavity mode. The inset shows PL decay at 738 nm of the PhC cavity, of the PhC structure, and of the homogeneous diamond slab; In-plane electric field components (Ex, Ey) and vertical magnetic field component (Hz) of the L3 PhC cavity TE modes obtained by the FDTD simulation; Experimental PL enhancement factor given by the ratio of the PL intensity measured at the cavity and at the PhC structure. The enhancement factor is fitted by Lorentzian peaks. Below the fits of the experimental data, cavity modes acquired from FDTD simulation of the PhC cavity are plotted as well [78]. (c) Simulation of emitter and waveguide/PCC. Three different cases are taken into account: First, the emitter in vacuum; Second, the emitter placed inside the waveguide without holes; Third, the emitter placed inside the second hole of the PCC at the same location as in the second case. For the simulation, the position of the emitter along the z-axis of the waveguide is varied. βλ-factor at 737 nm of an emitter coupled to a waveguide vs distance of the emitter to the center of the beam. As the emitter is moved closer to the center of the beam, the β_λ factor increases from 0.2 to above 0.8. Simulation of coupled power, normalized to the maximum achievable power into the waveguide vs the distance from the emitter to the cavity axis. The cavity is simulated to possess a mode at 740 nm and the emitter is shifted from zero displacement to over 100 nm away from the cavity axis. A drop in coupled power of almost 50% can be observed [80].

In addition to the methods mentioned above for enhancing the emission intensity of SiV color centers in diamond, Hu et al. [10] utilized HFCVD with adjusted growth pressure to prepare diamond with specific crystal facets and strong SiV photoluminescence. Diamond with (111) as the growth face exhibited higher emission intensity, as shown in Figure 11(a). Furthermore, deep Raman analysis indicated that SiV color centers were more likely to form in the near-surface region of the diamond particles, where the crystal quality was lower and lattice strain was higher, as illustrated in Figure 11(b). The complex lattice strain environment in the near-surface region broadened the photoluminescence peak of SiV color centers. The crystalline quality of diamond also influenced the emission of SiV color centers to a certain extent, as an improvement in crystalline quality could enhance the emission intensity and collection efficiency of SiV color centers [26]. For instance, Chu et al. employed salt-assisted air oxidation (SAAO) nanodiamonds as CVD seeds to grow high-quality diamond containing SiV color centers [26]. As shown in Figure 11(c,d), the photoluminescence intensity of SiV color centers in these high-quality diamond particles was significantly enhanced.

Figure 11. (a) SEM images of diamond particles grown at a pressure of 1.5–4.5 kPa for 4 h [10]. (b) Schematic illustration of Raman and PL depth profiles tests. The depth profiles of Raman spectra of diamond particles in sample 2.5 kPa-4 h. The inset shows the ratios of the intensity of Si and diamond peak (I_Si/I_diamond) in Raman spectra. The depth profiles of PL spectra of diamond particles in sample 2.5 kPa-4 h. The inset shows the enlarged view of the red dotted box area in the PL spectra, which is responsible for the diamond Raman peak [10]. (c) The PL spectra of the DND- and SAAO ND-grown diamond microparticles containing SiV centers (532 nm as the excitation wavelength) at 25 1 C. (d) XRD of the CVD-grown diamond microparticles [26].

It is currently unclear whether defects in polycrystalline diamond may act as traps for the excitation of SiV color centers, thus reducing the photoluminescence efficiency of SiV color centers within diamond [4]. This effect is relatively low in the thicker surface of polycrystalline diamond (several micrometers). However, in surface thin layers of 100–300 nm, grain boundaries and sp² carbon may have a significant impact on the photoluminescence efficiency of SiV color centers. Additionally, the fabrication of photonic crystals and the construction of diamond nano-resonators can enhance the emission intensity of SiV color centers. Changes in the local density of optical states within a photonic crystal cavity can modify the photoluminescence emission rate of SiV color centers through the Purcell effect. However, these methods rely on relatively complex and expensive manufacturing processes. Due to the low coupling efficiency between SiV color centers and defects, diamond films fabricated on low-refractive-index substrates are considered promising diamond photonic structural materials [15]. The formation of SiV color centers in diamond is influenced by various factors [81], making it still challenging to enhance the emission intensity of SiV color centers by precisely controlling the doping of silicon atoms through growth conditions or through the construction of resonators.

3. Diamond SiV color centers as single photon sources

Silicon and nitrogen are the most common impurity elements in natural and synthetic diamonds. However, due to their high solubility in diamond, it is difficult to control the introduction of SiV and NV color centers in the crystal growth process [53, 82]. The NV center is widely studied due to its exceptional photostability and unparalleled spin properties, which are highly relevant to numerous applications. For the NV center, its long fluorescent lifetime (∼11 ns) and weak emission into the ZPL (only ∼4% at room temperature) put an upper bound to the maximum photon rates achievable when employing NV centers in basic quantum photonic devices [21]. The SiV⁻ center in diamond has been demonstrated to be a more suitable candidate as a single photon source when compared. This is attributed to its strong ZPL. It serves as a highly efficient emitter of indistinguishable single photons, with the notable characteristic of exhibiting 70% of its emission in the ZPL, which possesses a narrow width of about 5 nm [20]. Nevertheless, it remains a challenging to directly obtain SiV color center single photon sources during the fabrication of diamond using CVD and HPHT methods. Only a small number of successful cases. For example, Akimov et al. obtained 10 nm diamond particles with single color centers through HPHT methods [83]. Ion implantation can control the quantity and position of SiV color centers to some extent [46], This method has been widely used to fabricate various types of single photon color centers in diamond [84–86]. It has been reported that the yield of SiV color centers is less than 25% when Si ions are implanted at energies of 15–50 keV [19]. Additionally, high-energy Si ion implantation causes lattice damage in diamond, leading to strong non-radiative recombination of carriers due to crystal defects [87]. The extent of structural damage caused by ion implantation is determined by the type and energy of the implanted ions, as well as the dose. High-temperature annealing can partially restore the damaged lattice structure [85], but it cannot completely eliminate the possibility of local formation of non-diamond carbon after annealing [88].

For SiV color centers, nanodiamonds with sizes smaller than the emission wavelength are crucial for reducing losses caused by reflection and refraction at grain boundaries and improving the collection efficiency of single photon sources [89]. As shown in Figure 12(a), Walsh et al. directly and masklessly created individual SiV color centers in diamond nanostructures by focused ion beam implantation. Second-order correlation function measurements yielded g²(0) = 0.38 ± 0.09, revealing antibunching behavior of SiV color centers and confirming single photon emission [19]. Boltasseva et al. injected Si into nanodiamonds with an average size of ∼20 nm, as shown in Figure 12(b). The emission spectra at room temperature indicated a ZPL wavelength between 730 and 800 nm with a linewidth below 10 nm. The measured second-order correlation function were g²(0) = 0.22 and g²(0) = 0.01, indicating the generation of single SiV color centers with stable, high-purity single photon emission [90]. Beyond that, without ion implantation, Akimov et al. used high pressure and high temperature to synthesize nanodiamonds with dimensions of about 10 nm, which contain optically active, single-silica vacancy color centers. They obtained the achieved g²(0) value, 0.41, which was below the threshold of 0.5 for two equally bright color centers [83]. In addition to ion implantation in nanodiamonds, Agio et al. demonstrated the scalable fabrication of SiV color centers through ion implantation in single crystal diamonds. As shown in Figure 12(c), the position of SiV color centers was controlled by a 1 µm pinhole placed in front of the sample, which was finely positioned using a piezoelectric stage. The initial implantation position was controlled by monitoring the ion beam position with a camera [46]. The obtained second-order correlation function was g²(0) = 0.41 ± 0.12, and the Lorentz fit displayed a clear ZPL near 738 nm, demonstrating the generation of SiV color centers as single photon sources. Wu et al. generated individual negatively charged SiV^- color centers by focusing a femtosecond (fs) laser on high-purity diamond coated with a layer of silicon nanospheres. Under the interaction of the high-intensity fs laser, silicon atoms are ionized and injected into the diamond, simultaneously generating vacancies and they obtained g²(0) values of 0.14 and 0.07, respectively [54].

Figure 12. (a) Confocal scan of SiV center array. Sites are separated by 2.14 mm. Overlaid are regular grid points from an aberration-corrected reference lattice. Analysis of implantation precision. We determine the 2D position uncertainty of the created SiV be 40 ± 20 nm. Blue curve: fit to Rayleigh distribution. Inset: Scatter plot of created single SiV sites relative to their grid points with one and two σ guides to the eye, where the radius σ = 26 nm corresponds to the expected implantation s.d. resulting from the combination of beam width and implant straggle. Normalized second-order autocorrelation function of a single SiV with g² (0) = 0.38 ± 0.09. Red points indicate data (without background subtraction), and the blue line is a fit to the function. The black dashed line indicates g² (τ) = 0.5, while the blue dashed lines indicate the 95% confidence interval on the fit. Ensemble (black) and single-emitter (red) SiV room temperature fluorescence spectra. The characteristic ZPL at 737 nm is prominent [19]. (b) Photophysics of typical single-photon emitters (SPEs) found on sample D. Two PL maps containing bright, stable emitters collected on sample D under a 690 nm CW laser excitation. The emission spectra of emitter SPE-1 (top) and SPE-2 (bottom) circled under the excitation of the 690 nm CW laser. Insets are the fluorescence stability curves of both emitters recorded for 60 s. Second-order autocorrelation measurement of SPE-1 (top) and SPE-2 (bottom). Gray lines are experimental data and red lines are fitted curves using the three-level model mentioned in the main text. The graph indicates the fitted values of g²(0), t1,and t2. Fluorescence saturation curves of typical SPEs found on sample D, showing saturation counts of around 70 kcps. Solid black dots are experimental data and red lines are fitted curves. Inset: a schematic illustrating the collection configurations used for SiV centers in NDs on a quartz coverslip [90]. (c) The expected positions of the matrix spots are indicated by the white circles. One of the identified emitters is marked with a white arrow. Emission spectrum of the marked spot. The Lorentzian fit shows a clear ZPL with the parameters given in the figure. Corresponding second order correlation measurement verifying the creation of a single SiV emitter by the presence of antibunching (deviation from zero due to background emission and electronic/photonic noise) [46]. (d) Detailed analysis of a diamond grain with a sub-4-nm size, containing less than three SiV emitters. Confocal fluorescence image obtained using a fluorescent bandpass filter around SiV fluorescence, in which the excitation laser wavelength was 532 nm. Pixel size was 200*200 nm². Autocorrelation function showing the background-corrected raw data (black line) and its fitted line (red). HRTEM image of a particle obtained from the O-20 sample showing a few isolated diamond grains on the edge of the particle. Left inset: TEM image of the particle showing the full view. The red circle denotes the area at which the HRTEM image was obtained. Right inset: enlarged image of the red square in the HRTEM image. Background-subtracted SiV PL spectra of the particle measured at room temperature with 532 nm laser excitation [9].

In addition, the interaction between SiV color centers and the surface of nanodiamonds suppresses the emission, leading to intermittent or blinking emission [20], which limits the minimum size of nanodiamonds [91]. Meanwhile, the nanoscale implantation cross-section and high sensitivity to ion beam damage make it challenging to optimize ion irradiation conditions during the ion implantation of nanodiamonds. Therefore, achieving faster, simpler, and more accurate fabrication of single SiV centers with better physical properties remains a challenge. Previously, fluorescent SiV nanodiamonds of molecular size (∼1.6 nm) containing ∼3 emitters have been found in meteorites [92], and the synthesis of ultrafine SiV nanodiamonds has so far remained a great challenge. Hu et al. reduced the size of nanodiamonds obtained by CVD growth to near the minimum value through oxygen plasma treatment. As shown in Figure 12(d), the size of nanodiamonds uniformly reduced from 11 nm to 1.7 nm at different treatment durations [9]. It was found that nanodiamonds with sizes between 2.1 and 4 nm contain fewer than three emitters. This is the first time that ultrafine photoluminescent nanodiamonds containing less than three SiV color centers have been artificially prepared, and at the same time provides another possible method for obtaining a single-photon source that avoids lattice damage caused by ion implantation.

Previously, we have mentioned coupling SiV color ­center-containing nanodiamonds into plasma cavities with volumes the same size as the nanodiamonds themselves. Integrating nanodiamonds with nanophotonic structures holds the promise of achieving on-demand, ultrafast single photon emission [93,94]. The key step to realizing this is to create single SiV color centers within isolated nanodiamonds of specified sizes, which would allow precise identification and manipulation of plasma/photon coupling. As mentioned before, focused ion beam technology [19] is an important tool for studying and controlling the generation of single SiV color centers in isolated nanodiamonds. However, there is still a long way to go.

4. Charge state control of SiV color centers

Among the various diamond color centers, NV and SiV are two typical color centers that have been widely studied and exhibit different charge states, ie, a neutral charge state and a negative charge state [21]. The SiV⁰ color center holds great potential in the field of quantum information due to its long electronic spin coherence and ideal optical properties. SiV⁰ is an S = 1 spin system, which is less subject than its negatively charged counterpart to decoherence caused by phonon interaction and electric field fluctuations [95]. Spin polarization of the system, needed to initialize the spin controllably, has been observed [96], similar to what happens with NV^- centers [97]. In addition, the coherence time is as long as ∼1 ms at temperatures lower than 20 K (using the Hahn echo scheme), and the spin-lattice relaxation time is exceptionally long ∼40 s [21]. SiV^- color centers exhibit narrow ZPL widths at room temperature, with over 70% of the signal concentrated in the ZPL [98]. Sukachev et al. demonstrated high-fidelity coherent manipulation and single-shot readout of individual SiV⁻ spin qubits in a dilution refrigerator. These results establish the SiV⁻ as a promising solid-state candidate for the realization of quantum networks [99]. The fluorescence intensity of SiV⁰ color centers cannot generally be observed at room temperature, and even at low temperatures their fluorescence intensity is relatively low, which is the main drawback of SiV⁰ color centers [100]. The different charge states of SiV color centers can undergo interconversion. Therefore, there is an urgent need for reliable methods to control the charge state of SiV color centers in diamonds [89, 97].

Currently, several methods have been proposed to manipulate the charge state of diamond color centers. One of them is optical modulation via laser illumination that involves the charge-state conversion upon the combination of different wavelength lasers. Dhomkar et al. studied the charge dynamics of NV and SiV color centers in diamond using dual-wavelength confocal microscopy [101]. By examining the nonlocal fluorescence patterns emerging from local laser excitation, they show that, in the simultaneous presence of photogenerated electrons and holes, SiV (NV) centers selectively transform into the negative (neutral) charge state [101]. As shown in Figure 13(a), for uniformly distributed SiV under the green beam scanning, the color centers exposed to the beam transitioned (or remained) in a neutral state, while the color centers near it transitioned into a negative charge state (represented by regions I and II in the inset) [101]. As the beam moved from one spot to another, the locally generated SiV⁰ color centers converted to SiV^- by capturing electrons. This indicates that the number and distribution of SiV^- color centers are controllable and increase with increasing laser intensity.

Figure 13. (a) (Left) As the green beam scans the diamond crystal, electrons and holes diffuse away from the illuminated spot. (Right) Effect of a strong green scan on SiV⁻; I and II in the inset indicate the areas exposed to the beam and affected by electron capture, respectively. The middle figure, (Right) SiV⁻-selective (top) and NV⁻selective (bottom) scanning confocal images (632 nm read-out) obtained after a 532 nm, 50 μW scan. (Left) Optical spectrum averaged over the same area using bandpass BP1. Same as in the middle figure but for a 1 mW green laser initialization scan. The integration time during the confocal imaging and spectroscopy scans are 2 and 5 ms, respectively; in both cases, the red laser power is 100 μW [101]. (b) (two columns) Comparison of SiV centers absorption spectra before and during X-ray at low temperature (T = 10 K) [102].

Boldyrev et al. utilized X-ray irradiation on diamond crystals to alter the charge state of SiV color centers [102]. As shown in Figure 13(b), absorption spectra of SiV color centers before and during low-temperature X-ray irradiation were obtained and analyzed, determining the initial concentrations of SiV color centers in negative and neutral charge states, as well as their changes during and after X-ray irradiation. It was observed that a fraction of SiV^- color centers converted to SiV⁰ under X-ray exposure. Furthermore, switching off the X-ray radiation allowed the total concentration and charge distribution of the color centers to recover to the initial state within approximately 1 h at a temperature of 10 K.

The aforementioned studies have shown that in the presence of photo-generated electrons and holes, SiV color centers selectively transition to the negative or neutral charge state. However, this method, due to its high excitation energy, always excites other surrounding impurities, which may lead to charge redistribution and instability of the color centers in the negative charge state.

Another approach to regulate the charge states of color centers in diamond is through Fermi level engineering, namely to alter the position of the charge transition levels of color centers relative to the Fermi level in diamond [4]. Multiple studies have confirmed that the transition on the surface of nanodiamonds from hydrogen termination to oxygen termination causes the charge transition energy levels of the color centers to bend downwards, thereby improving the optical performance of SiV^- color centers [97, 103]. Furthermore, the charge state of color centers can be adjusted by applying a bias voltage on vertical diamond homojunction or heterojunction to inject or release charge carriers. Yang et al. fabricated a diamond/n-Si heterojunction, wherein a bias voltage was applied on the heterojunction to facilitate the transfer of carriers from the diamond/silicon interface, resulting in enhanced forward current and higher photoluminescence intensity in the heterojunction [103]. As shown in Figure 14(a,b), it is evident that the heterojunction of diamond with heavily doped n-type materials can overall increase the number of negatively charged color centers in diamond through the electron tunneling effect. Another effective method for controlling the relative positions of charge transition energy levels in diamond is by forming junctions between diamond and other materials. As shown in Figure 14(c,d), this research group studied the electroluminescence of SiV color centers in parallel structures of p-i-n diodes and Schottky diodes [104]. In the luminescence spectrum, only a line at a wavelength of 738 nm is detected. The electroluminescence of color centers is observed only in the p–i–n region of the diode, that is, it is experimentally demonstrated that both types of charge carriers are necessary to excite electroluminescence.

Figure 14. (a) Schematic diagram of the fabricated diamond-Si diode, including an n-type Si substrate, an about 3.5 mm-thick diamond layer containing SiV centers, and two 30 nm Ti/30 nm Au electrodes (radius of 2 mm). (b) The schematic energy band diagrams of both the diamond/n⁺-Si and diamond/n^--Si heterojunction under different bias: zero bias; forward bias; and reverse bias [103]. (c) Electroluminescence spectrum at a current of 1 mA in a merged diode (the inset shows a map of the emission intensity of SiV centers in the groove region);. (d) dependence of the emission intensity of the SiV center at a wavelength of 738 nm on the current flowing through the diode (the inset shows the profiles of the zero-phonon lines of the SiV center at different currents flowing in the diode) [104].

However, so far, designing neutral charge M-V defects as desired has proven to be challenging because the corresponding negatively charged defects seem to form more favorably. Other neutral charged defects, such as PbV⁰, GeV⁰, and SnV⁰, have not been reported. This could be due to the unfavorable Fermi level in the diamond for their formation or their potential low quantum efficiency hindering their detection in standard photoluminescence measurements [103]. Currently, the strategy for obtaining SiV⁰ centers relies on boron doping (as an acceptor) in diamond while limiting the amount of nitrogen atoms (as donors) in diamond. There is also an ongoing debate about the feasibility of isolating individual SiV⁰ defects in a controlled manner.

5. Summary and outlook

The methods for preparing SiV color centers in diamond are relatively limited. Each method has its own limitations. For example, CVD and HPHT methods have difficulties in controlling the quantity of color centers. Although ion implantation offers high controllability, it is expensive and often leads to lattice damage. Therefore, based on the characteristics of these two types of preparation methods, they correspond to two main application directions. The first direction is the use of multiple high-brightness color centers for fluorescence labeling and other fields, where the main challenge lies in controlling the intensity of emission. The second direction is the use of single-photon color centers for quantum information and other fields, where the main challenges are the preparation of single-photon sources and the control of the charge state of color centers.

Meanwhile, SiV color centers in diamond have also shown potential for applications based on electroluminescence [21]. It has been suggested that other IV group defects can also be electrically excited, and research in this field is still in its early stages, but current predictions and preliminary results are promising. The use of diamond diodes is feasible [104,105], although the high turn-on voltage and current remain major challenges, mainly due to difficulties in achieving high-quality n-type doping. Another promising approach is the formation of heterojunctions [21], such as p-type diamond and other n-type materials, to realize optoelectronic diodes. These optoelectronic diodes can be used for manufacturing quantum circuits on chips, as well as for tuning spin transitions and generating coherent photons from color centers, which is advantageous for achieving integration on a single chip. With these recent advancements, diamond has demonstrated its potential to become the preferred material for commercialization in nanophotonics technology.

Author contributions

Chengke Chen: Conceptualization, writing – review & editing. Bo Jiang: literature survey, writing of original drafts. Xiaojun Hu: Conceptualization, supervision, writing – review & editing.

Abbreviations
CVD	=	chemical vapor deposition
DNDs	=	detonation of nanodiamonds
Fs	=	femtosecond
FWHM	=	full width at half maximum
GeV	=	germanium vacancy
HPHT	=	High-pressure high-temperature
HFCVD	=	Hot-Filament Chemical Vapor Deposition
MCD	=	microcrystalline diamond
MPCVD	=	microwave plasma chemical vapor deposition
NCD	=	nanocrystalline diamond
NIR	=	near-infrared
NV	=	nitrogen vacancy
PhC	=	photonic crystal
PL	=	photoluminescence
SAAO	=	salt-assisted air oxidation
SiV	=	silicon vacancy
SnV	=	stannum vacancy
SPEs	=	single-photon emitters
TMS	=	trimethylsilane
TPS	=	triphenylsilanol
ZPL	=	zero-phonon line

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Natural Science Foundation of China (Grant Nos. 52102052); Key Project of National Natural Science Foundation of China (Grant No. U1809210); and Key Research and Development Program of Zhejiang Province (2018C04021).