Luminescent diamond composites

Abstract Diamond is valuable material with extraordinary high thermal conductivity and transparency in a wide spectral range from UV to IR and longer wavelengths. Defects and impurities in the diamond lattice can absorb and emit light at wavelengths specific for each of such “color centers.” Particularly, the vacancy-related defects in diamond, such as nitrogen-vacancy (NV) or silicon-vacancy (SiV) centers, are actively investigated due to their potential for biomedicine, quantum optics, local thermometry and magnetometry. Although a great variety of different color centers in diamond are discovered, only a limited number of those point defects can be reliably reproduced in synthetic diamond, obtained either by chemical vapor deposition (CVD) or high-pressure high-temperature (HPHT). An alternative approach to producing luminescent diamond-based materials is to integrate stable non-diamond sources of luminescence in the form of nano- or microparticles of foreign materials into the pristine diamond. The produced diamond composites possess excellent properties of diamond combined with optical emission characteristics, which cannot be provided with intrinsic defects in diamond. The good candidates for the materials of such impurities are well-investigated fluorides and oxides doped by rare-earth elements (RE) or other luminescent chalcogenides such as sulfides, selenides and tellurides. Here we briefly review recent achievements in fabrication and properties of these new luminescent diamond-RE composites, compare them with luminescent properties of doped diamond, and outline prospects for applications of the luminescent diamond composites for photonics, markers, monitors of high-power synchrotron, X-ray beams and X-ray lasers.


Introduction
Diamond with its record-high thermal conductivity, high mobility for electrons and holes, unsurpassed hardness, biocompatibility and other physical properties is the material of choice for a variety of applications, including high power electronic devices [1][2][3], highly stable detectors of ionizing radiation [4][5][6], superhard tools [7,8], Raman lasers [9,10] and many others. While the diamond structure as perfect as possible is often required for specific applications, in some important cases defect engineering in diamond is the way to bring new valuable properties to the material. Diamond possesses a large number of optically active defects (color centers), particularly vacancy-related and impurity-related defects, which show luminescence on specific wavelengths in the visible and near-infrared range [11,12]. Some color centers, such as nitrogen-vacancy NV [13][14][15], silicon-vacancy SiV [16][17][18], germanium-vacancy GeV [19][20][21][22][23], Ni-based centers [24][25][26] have interesting optical and spin characteristics and can be well controlled in synthetic diamond nanoparticles, films and single crystals by doping process. This advanced the development of quantum applications based on diamond materials within the last decade. Such systems are very promising for quantum photonics and plasmonics [27,28], quantum computer [29], magnetometry [30][31][32], and temperature sensing [33,34]. Due to diamond biocompatibility, nanoscale diamonds with color centers attract a high interest for use as optical biomarkers [35,36]. Also, analysis of luminescence spectra and associated defects may shed light on origin of diamond crystals, differentiate natural and synthetic crystals, gives information on synthesis method and possible treatments [37][38][39], which is important in geology and jewelry. In addition, the use of X-ray luminescence (XRL) of bulk natural diamonds has great practical importance, since it forms the basis for the development of the X-ray luminescent separation of diamonds technology which is currently the main technological process for the extracting diamonds from ore [40]. Still, as a low-Z material, the pristine diamond is transparent to X-ray radiation, so the origin of XRL signal in bulk natural diamonds are various defects and impurities. Meanwhile, the synthetic membranes of pure polycrystalline diamond are transparent to X-ray radiation and are considered to be promising for X-ray lithography [41]. The high radiation resistance of diamond can be used for the formation of robust and reliable detectors and screens for high-energy radiation, such as ultraviolet light [42], X-ray [43], and ionizing radiation [44,45]. However, the typical color centers in diamond do not allow obtaining an intensive XRL signal in the case of thin films and membranes (only from bulk crystals [40,43,46]), so the search for new sources of XRL signal is a significant task.
The two well-established diamond synthesis methods are the high pressure -high temperature (HPHT) technique [15,[47][48][49][50] and chemical vapor deposition (CVD) [51][52][53][54]. The color centers are formed by mixing the dopant (in a solid or gas state) with a carbon precursor [20,[55][56][57]. However, the nomenclature of possible defects is limited because the extraordinary dense crystalline lattice of diamond (1.76 × 10 23 cm −3 ) can accommodate only a few elements, boron and nitrogen are among them, which atoms may be incorporated in the lattice to occupy substitutional sites. This list of dopants is somewhat extended due to the formation of more complex defects consisting of vacancies and foreign atoms, such as group-IV elements (Si, Ge, Sn, Pb) [54]. It's desirable to enrich the variety of lines in photoluminescence (PL) spectra by the introduction of new color centers in diamond, for example, by doping with rareearth (RE) elements, which include lanthanides, scandium, and yttrium. The electronic properties of REs provide sharp optical transitions with high quantum efficiency which are very useful for diverse photonic technologies and devices [58]. Therefore, the doping of diamond, which has a very wide optical transmission window, with REs looks very attractive, however, is a hard task due to the large size of RE atoms [59][60][61].
The incorporation of Er and Yb in diamond by ion implantation was reported by Cajzl et al. [62]. Other researchers attempted to grow RE-doped diamonds by HPHT technique with the RE catalysts [60,63,64], but did not observe definite spectral features of the REs in the luminescence of the produced diamond crystals. Magyar et al. [65] reported the synthesis of Eu-doped diamond in a microwave plasma CVD (MPCVD) system by preliminary depositing an Eu-containing organic complex Eu(III) tris(dipicolinic acid) (Eu(DPA) 3 ) on a single crystal diamond substrate followed by diamond growth on the top. Similarly, Vanpoucke et al. [59] produced Eu-doped nanocrystalline diamond film by an MPCVD using Eu(DPA) 3 as the Eu-containing precursor and observed PL line at 614 nm assigned to Eu emission. As a new way to introduce elements on-demand into diamond, Sedov et al. [66] realized a synthesis of novel CVD diamond-based composites with compounds containing REs. They manufactured a diamond-RE composite material with EuF 3 nanoparticles embedded in the synthesized microcrystalline diamond films that show strong photoluminescence in the orange part of the visible spectrum. Synthesis of the aforementioned composite includes placement of EuF 3 nanoparticles on the diamond substrate and subsequent coating of them with an additional polycrystalline diamond layer grown by an MPCVD. The produced composite films exhibit high-intensity localized photoluminescence (PL) at 612 nm generated by the EuF 3 particles buried within a very stable transparent diamond matrix. The proposed synthetic approach is quite versatile, as it allows the preparation of the luminescent diamond-RE particles nanocomposites of different sizes and natures which perform well over a broad range of the visible spectrum.

Principles of diamond-RE composite fabrication
The scheme for diamond composites preparation is shown in Figure 1 using EuF 3 particles as an example. At first, the Si substrates are seeded with nanodiamond particles. Then, a layer of pure microcrystalline diamond is grown using a microwave plasma CVD. The formed diamond film then is covered with a thin non-coalesced layer of chosen RE-based nanoparticles (EuF 3 ) by spin-coating technique. Finally, an additional CVD growth was used in order to laterally overgrow the applied nanoparticles, encapsulating them in the bulk of the resulting PCD film and thus forming a composite material.

Preparation of RE nanoparticles
The typical conditions for CVD growth of diamond include substrate temperatures of 700-1100 °C in an "aggressive" methane-hydrogen plasma [53,67], which imposes strict requirements on the overall physical and chemical stability of RE-based particles used for incorporation into diamond. Substances should have chemical resistance up to 900-1100°С in a vacuum without decomposition, melting or sublimation. The substances must be resistant to the effects of plasma to avoid side plasma-chemical reactions. Thus, the particle size should be not less than 10 nm, as at smaller sizes even the slightest etching by plasma will lead to their decomposition. On the other side, a particle size larger than a few μm will lead to inhomogeneity of luminescence in composite material. In addition, for larger particles, the heat-sink effect of the diamond matrix is limited due to the low thermal conductivity of RE-based compounds [68,69]. Finally, large particles require thicker diamond layers for integration, while being affected by an aggressive plasma during the CVD process, which means they have reduced chances to retain their full functionality. Thus, particle sizes of 20-500 nm are seeming to be optimal for the task.
The desired particles can be prepared by a variety of both physical and chemical techniques [82,83]. The physical techniques for the formation of RE-based particles include grinding, ablation, and electron beam vapor deposition. Chemical techniques include a wide variety of synthesis methods such as synthesis in aqueous and non-aqueous solvents with the possibility to use normal or elevated temperatures and pressures [73][74][75][76][77]84]. The self-igniting technique is popular for the synthesis of oxide materials [85,86].
Still, many attractive RE-based substance classes cannot be used for the formation of diamond-based composites. For example, selenides, sulfides, and tellurides have exceptional quantum-well luminescent characteristics but their high tendency to sublimate in a vacuum hinders their use. Still, by using MoS 2 with a high melting temperature, layered electroluminescent diamond/ boron/MoS 2 /diamond composite films were recently obtained by Yang et al. [87]. Chlorides, bromides, and iodides typically have high vapor pressure and melting/ decomposition temperature below 900 °C, which makes them unsuitable for typical CVD process conditions. The majority of the remaining classes of substances in their totality do not meet the above-mentioned requirements and the chance of achieving an effective luminescence on their basis is rather low. On the other hand, matching these requirements allows us to highlight such substance classes as fluorides and chalcogenides (oxides), since they are generally stable in a vacuum and plasma at temperatures of 900-1100 °C, while also being able to form solid solutions with REs [79,80,[88][89][90], which makes them good candidates for the formation of diamond-based composites [91,92]. Such fluoride and oxide nanoparticles may be prepared by solvothermal technique in high boiling liquids [93], co-precipitation from aqueous solutions [83,94,95] and electron beam vapor deposition [91,92].

Diamond-based composites with integrated RE-based nanoparticles
The first results on the formation of the luminescent diamond-based composites were obtained by integration of europium(III) fluoride (EuF 3 ) into CVD-grown PCD film [66]. The described procedure was used for the formation of diamond composite with nanoparticles (average size of 50 nm) of pure EuF 3 . The final thickness of such "diamond-EuF 3 " composite film was 2.5 µm. In the cross-section SEM images of the films (Figure 2), the nanoparticles of EuF 3 were clearly observed both in "secondary electrons" (SE) and "energy-dispersive X-ray spectroscopy" (EDX) operating modes of the microscope. The position of the particles was exactly in the middle of the PCD film, precisely as expected. The closer investigation of the structure of PCD films and their Raman spectra did not reveal any defects or other unwanted traces of the performed incorporation of EuF 3 nanoparticles. In the case of small separate RE-based nanoparticles, they are quickly overgrown with diamond film, so the effect of their presence on the CVD growth itself is negligible.
In the PL spectra of the composite films, the new feature (in comparison of pure PCD films) was observed -the sharp peak at 612 nm, which is the distinct feature of Eu 3+ ions and is attributed to 5 D 0 → 7 F 2 transition in Eu (Figure 3a). The luminescence decay of 612 nm line in composite was 0.34 ms, which was similar to the PL decay time of EuF 3 powder (Figure 3b) and is somewhat typical for Eu 3+ -based PL in general [96,97].
However, the signal-to-noise ratio for "diamond-EuF 3 "-was quite low due to using of pure europium compounds. It is known that the use of individual RE substances results in concentration quenching of the luminescence. The introduction of these same active REs into the substances with solid solution formation leads to much more intensive luminescence (see, eg [93,98,99]). Keeping the Eu as the referent luminescence source was explained by the vast accumulated knowledge about characteristic bands of its 2+ and 3+ valence states and symmetry groups for various electronic transitions, observed in its absorption and luminescence spectra [100,101]. However, using of the Eu-doped substances in diamond rather than a pure compound was an important step toward increasing luminescence intensity. Thus, the low-temperature phase of NaGdF 4 with a hexagonal structure [93,94] was chosen as an effective non-luminescent matrix for Eu-based luminescent nanoparticles, that were used for the formation of diamond composites [102]. The obtained "diamond-NaGdF 4 :Eu" films showed an intensive peak at 612 nm both under the laser and, for the first time, X-ray excitation (Figure 4). The intensity of XRL linearly increased with the number of NaGdF 4 : Eu nanoparticles integrated into PCD film.
The major downside of using Eu 3+ -based sources of luminescence for scintillator applications is long PL decay times of ~1 ms (see Figure 3), and faster RE-based sources like Ce 3+ with decay times of ~30 ns are preferred [103][104][105]. However, cerium luminescence in fluorides is located predominantly in the UV spectral range [106], which is a serious drawback for scintillating applications. Thus, the yttrium-aluminum garnet (YAG) matrix with higher phonon energy was used to shift the luminescence of Ce 3+ ions to the visible range, as such powders of YAG doped with cerium (YAG:Ce) are already widely used as bright phosphors [107,108].
The obtained "diamond-YAG:Ce" composites [109] with embedded 80 nm nanoparticles showed the intensive broad band near 550 nm both in PL and XRL spectra (Figure 5a), which corresponds well to 5d → 4f interconfigurational transitions in Ce 3+ ion [110]. However, surprisingly, the intensive SiV peak at 738 nm [17,18,111] was also observed for the first time in XRL spectra of diamond-based composites. An interesting detail is that the SiV luminescence yield followed the same trend as Ce 3+ emission (Figure 5b), not only at the K-edge, but also at higher energies, reproducing extended X-ray absorption fine structure (EXAFS) oscillations (see inset in Figure 5b). This trend for SiV emission was quite unexpected since diamond absorption has no features in this region. The SiV luminescence, which might be directly excited by X-rays [43], should not be affected by changing excitation energy near 17 keV. Such correlated behavior indicates that the energy is transferred from YAG:Ce nanoparticles to the diamond matrix, which results in the stepwise increase of the SiV yield and its oscillating behavior. The jump of the yield value at the yttrium edge was ~50% of the yield value below the edge. The mechanism of the energy transfer can be either radiative or by photoelectrons emitted from the nanoparticles. A comparison of luminescence kinetics of Ce 3+ and SiV ( Figure  5c) suggests that the energy is delivered to the matrix by photoelectrons rather than radiatively since there was no delayed rise-on part in SiV kinetics.
The further optimization of both the exact compound material and the size of integrated particles is still in progress. In work [112] the optimization of YAG:Ce composition was attempted by adding stabilizing Sc 3+ atoms as partial replacements in Ce 3+ positions. However, the highest XRL signal was registered from [Y 2.98 Ce 0.02 ] {Al 2 }Al 3 O 12 , which contained none of Sc atoms at all. In work [113] the new compound with ~1 µm luminescent particles was tested -gadolinium-scandium-aluminum garnet, doped with cerium (GSAG:Ce) with Gd 2.73 Ce 0.02 Sc 0.5 Al 4.75 O 12 composition. For that, a total of 25 powders with a slight variation in their composition were synthesized and tested for the intensity of X-ray luminescence. The powders with optimized content were used for the formation of "diamond-GSAG:Ce" composites. Then, for the first time, composite membrane structures (Figure 6a) were formed on the basis of the obtained films by the selective local removal of the Si substrate [114]. Such membranes allowed registering the bright yellow X-ray luminescence, which was visible to the eye (Figure 6b). In the X-ray spectra, the Ce 3+related peak at ~570 nm was registered, while the intensity of the peak changed with the number of particles integrated into the PCD film (Figure 6c).
From all reviewed composites, the highest XRL intensity so far was registered for the "diamond-YAG:Ce" films, in which the size of integrated nanoparticles was ~80 nm. In general, the investigated class of composites showed great promise for their application in reliable robust detectors and visualizers of high-intensity X-ray radiation in synchrotrons and free-electron lasers. Still, the search for the ideal phosphor and the overall design of the composite material is still ongoing and remains to be a very important task.

Diamond composites with IV group elements
The preparation of diamond-RE composites assumes separated synthesis of RE nanoparticles and, then, diamond film overgrowth. There is another class of  and SiV (738 nm) centers in the obtained "Diamond-YaG:ce" composite material, excitation 19 keV, Rt. inset in (b) -normalized spectra of ce 3+ and SiV emission. the results in (c) for the best-suited fitting components are shown near experimental lines: the decay times, and the relative contribution of each component [109].
composites, such as diamond-silicon carbide [115][116][117] and diamond-germanium [118], which are produced by co-deposition of the two phases in a single process.
The approach to fabricating SiC/diamond composites by a CVD process was developed by Jiang's group [115,119]. The composite films with the nanocrystalline or microcrystalline structures were deposited on Si substrates by an MPCVD in H 2 -CH 4 -tetramethylsilane Si(CH 3 ) 4 (TMS) mixtures. The addition of rather low TMS concentrations of about 0.01 at.% to CH 4 -H 2 gas mixtures is enough to make cubic polytype β-SiC formation competitive and to obtain mixed diamond/β-SiC films. Alternatively, Sedov et al. [117] added silane (SiH 4 ) gas as Si precursor into standard CH 4 -H 2 mixtures to obtain similar composite films. Independent on the specific Si precursor used the presence of silicon atoms and SiH x radicals in the plasma automatically leads to diamond doping with Si and the formation of SiV centers with luminescence at 738 nm. In the diamond-SiC films deposited with silane precursor, a bright PL emission with the SiV peak in the spectrum ~50 times stronger than that for the diamond Raman peak was observed [117].
Recently, diamond/SiC composite films were prepared in an MPCVD system with a 915 MHz reactor with a variable flow rate of TMS gas (5-30 sccm) added to the base CH 4 (4 sccm)/H 2 (400 sccm) mixture. The addition of more TMS gas resulted in grain refinement of diamond crystals, the average grain size is always less than 100 nm, and a larger volume fraction of SiC (an example of SEM image of the 5 sccm TMS sample is shown in Figure 7a,b). All the composites revealed a clear SiV emission in the PL spectrum, with a tendency to a gradual reduction in SiV intensity with TMS flow (Figure 7c), which could be assigned to a diamond structure degradation. The maximum ratio of SiV PL zero-phonon line (ZPL) intensity to diamond Raman peak was ~21 for a low TMS flow regime. The chemical removal of the SiC component produced a porous diamond structure with some increase in SiC PL/Raman  ratio. It was suggested to use these composite films including porous structures in the areas of nano-thermometry or bio-marking.
The co-deposition concept was by applied Ralchenko et al. [118] to produce polycrystalline diamond-Ge films on (100) oriented Si substrates by an MPCVD in CH 4 -H 2 -GeH 4 mixtures at high enough germane GeH 4 concentration in gas. The process regime was found that provided close growth rates for diamond and crystalline Ge grains, which demonstrated well-faceted shapes (Figure 8a). The diamond crystals were doped with Ge owing to GeH x radical in the plasma (the process quite similar to that upon diamond doping from SiH 4 ) and revealed a bright luminescence of GeV color center with ZPL at 602 nm (Figure 8b). The GeV intensity depends on the substrate temperature, the highest brightness being achieved at 800 °C. The PL GeV intensity was proved to be uniform over a large area by mapping Raman peak and PL emissions for diamond and Ge with confocal spectroscopy. The laser beam was focused on a spot of ≈1 µm diameter and scanned across the 30 × 30 µm 2 area with a small step of 500 nm measuring the spectrum in each location and revealing a reasonably homogeneous intensity distribution. Potentially the co-deposition techniques could be extended to grow thin-film diamond composites with components other than SiC and Ge.
Such diamond-based optically active composites can be used for application in nanothermometry and optical biolabels, in which very fine diamond particles (nanodiamonds with sizes of ~50-500 nm) are needed [34,36]. One way of producing such diamond nanoparticles is crashing (sonic disintegration) of polycrystalline diamond films with SiV centers, as demonstrated by Neu et al. [121]. One can suggest that the milling of the diamond films could be performed easier if the film would be preliminary converted to a porous skeleton. Zhuang et al. [122] obtained a porous diamond membrane by selective wet chemical etching of SiC grains from SiCdiamond. The further milling of similar Si-doped or Ge-doped porous diamond films is expected to give luminescent diamond particles suitable for thermal sensing and biomarkers.

Conclusions and outlook
Engineering of specific optically active defects (color centers) in synthetic diamonds currently attracts great interest due to applications of the luminescent polycrystalline and single-crystal diamonds in quantum optics, local thermometry, high sensitivity magnetometry, X-ray scintillators, and biomedicine. Because only a very limited number of elements could be built in the dense diamond lattice, the extension of a family of luminescence sources imbedded in diamond can be achieved by combining diamond with nanoparticles of other substances, for example, containing rare-earth elements, with diverse luminescence spectra. The nanoparticles are overgrown with CVD diamond to form a unified composite body, which has a robust transparent matrix with extremely high thermal conductivity. The diamond-based composites with REs in form of YAG:Ce, NaGdF 4 :Eu, EuF 3 and gadolinium-scandium-aluminum garnet, doped with cerium (GSAG:Ce) are already demonstrated with bright photo-and X-ray luminescence. Other REs are waiting to be explored in the future for this aim. As an option, there is a special sort of diamond composites containing SiC and Ge nano-and micrograins, which are grown by co-deposition in CH 4 -H 2 -SiH 4 and CH 4 -H 2 -GeH 4 mixtures, respectively, and demonstrate intensive luminescence of SiV and GeV centers incorporated in the diamond skeleton during the growth.
The diamond overgrowth method to fabricate such composites is quite flexible, it allows the synthesis of multilevel composites by placing the nanoparticles of the same sort on each level, thus increasing the total volume of the luminescent component. In addition, a combined PL signal from multiple RE sources can be achieved by using nanoparticles of different substances the Pl spectra are normalized to the diamond Raman peak area and shifted vertically for clarity [118].
in one diamond-based composite material. One can choose an appropriate type of RE nanoparticles to get a fast or slow luminescence response, in the desired spectral range. Another possibility is a selective-area arrangement of nanoparticles to have a patterned (grid, array of squares, or more complex structure) luminescent object. Further development of technology of luminescent CVD diamond composites with controllable structure and properties seems to be an interesting task for researchers.

Disclosure statement
No potential conflict of interest was reported by the authors.