Synthesis of SiV-diamond particulates via the microwave plasma chemical deposition of ultrananocrystalline diamond on soda-lime glass fibers

The growth behavior of CVD diamond on planar SiO₂/Si and soda-lime glass substrates was investigated first. SEM surface morphologies of MCD films grown on SiO₂/Si and soda-lime glass are shown in figures 1(a_I) and (a_II), respectively, indicating that the MCD films consist of faceted grains with sharp grain boundaries. The grain size is much smaller for the MCD films grown on soda-lime glass, compared with those grown on SiO₂/Si, i.e., it is about 1 µm for MCD/SiO₂/Si films and is around 300–400 nm for MCD/soda-lime glass films for the same growth time. The UV-Raman spectra of the corresponding films are shown in figures 2(a_I) and (a_II), respectively, revealing that both MCD/SiO₂/Si and MCD/soda-lime glass films depict a sharp Raman peak at 1336 cm⁻¹, corresponding to first-order Raman peak of diamond, and a small broad peak at 1580 cm⁻¹, corresponding to G-band [35, 36]. The calculated I_D/I_G ratios of the corresponding spectra are showing with values of 0.79 and 0.57, respectively (table 2). The SEM images of NCD films grown on SiO₂/Si and soda-lime glass substrates are shown in figures 1(b_I) and (b_II), respectively. NCD films grown on SiO₂/Si substrates show the cauliflower kind of surface morphology, and the ND grains are in the size of 50 nm, which have been agglomerated into big aggregates about 1 μm in size. The NCD/soda-lime glass films seem to form smaller aggregates (∼hundreds of nanometer) compared with NCD/SiO₂/Si films. The UV-Raman spectra of the corresponding films are shown in figures 2(b_I) and (b_II). Both the NCD/SiO₂/Si and NCD/soda-lime glass films depict a sharp Raman peak at 1336 cm⁻¹ corresponding to the first-order Raman peak of diamond and a broad peak at 1580 cm⁻¹(G-band) corresponding to the graphitic phase. The G-band in NCD/SiO₂/Si (or NCD/soda-lime glass) films is larger in intensity compared with those in MCD/SiO₂/Si (or MCD/soda-lime glass) films, which is apparently related to the larger proportion of grain boundary phase presents in NCD films. This could be correlated with I_D/I_G ratios of the corresponding spectra, (i.e.) the I_D/I_G = 0.64 for NCD/SiO₂/Si and 0.39 for NCD/soda-lime glass (table 2).

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Figures 1 (c_I) and (c_II) show the SEM surface morphology of UNCD films grown on SiO₂/Si and soda-lime glass substrates, respectively. UNCD films grown on SiO₂/Si substrates possess a smooth surface with ultrasmall diamond grains, whereas UNCD films grown on soda-lime glass substrates contain aggregates about 50–100 nm in size, which are made up of the ultrasmall grains of size about 5–10 nm. UV-Raman spectrum of UNCD/SiO₂/Si films is shown in figure 2(c_I), which exhibits a sharp Raman peak at 1333 cm⁻¹ (the first-order Raman peak of diamond), but with smaller intensity compared with those of MCD (or NCD) films. There also exists a broad peak at 1575 cm⁻¹ (the G-band), which is of larger intensity compared with those of the MCD (or NCD) films [35, 36]. Moreover, a small broad peak is observed at 1350 cm⁻¹ (D*-band) representing the disordered carbon and those at 1140 cm⁻¹ (ν₁-band) and 1480 cm⁻¹ (ν₃-band at), corresponding to transpolyacetylene phase located at the grain boundaries of UNCD films [35, 36]. Figure 2(c_II) shows that the characteristics of Raman spectra of UNCD/soda-lime glass films are similar to those of UNCD/SiO₂/Si films, but with the much noise signal in the background. Moreover, the I_D/I_G ratios were calculated to be 0.44 and 0.81 for UNCD/SiO₂/Si and UNCD/soda-lime glass, respectively, which indicate the presence of large amount of defects and stress in UNCD/soda-lime glass. The coalescence of ultra-small diamond grains was frequently observed in UNCD films, which induced the formation of large diamond aggregates distributed among the matrix of ultrasmall diamond grains [37].

The diamond films grown on soda-lime glass fibers show markedly different morphology compared to those grown on planar SiO₂/Si or soda-lime glass substrates. The MCD/soda-lime glass fibers contain diamond aggregates of island-wise geometry with each of them about micron-size rather than forming a continuous film (figure 3(a_I)). Enlarged SEM micrograph shown in figure 3(a_II) reveals that the island-wise MCD aggregates actually contain rounded diamond grains of the size of about 300–500 nm. The diamond grains are not faceted as MCD films grown on the SiO₂/Si or soda-lime glass substrates. The UV-Raman spectrum shown in figure 2(a_III) reveals the sharp first-order Raman peak of diamond at 1333 cm⁻¹ and a broad Raman peak at 1580 cm⁻¹ (I_D/I_G = 0.60), which are similar to those of MCD films grown on SiO₂/Si or soda-lime glass substrates but with larger background noise. Figure 3(b_I) shows the SEM image of highly dense NCD crystals grown on soda-lime glass fibers, whereas figure 3(b_II) shows the enlarged SEM micrograph of these NCD crystals. These micrographs reveal that the NCD crystals are spherical in shape with size around 50–70 nm. The NCD crystals are isolated from one another but are spread uniformly all over the fiber. It should be reminded that normally NCD films grown on planar substrates (i.e., SiO₂/Si or soda-lime glass substrates) contain columnar structure of size around few tens of nanometers (see figures 1(b_I) and (b_II)). Here, such a feature has not been observed due to artifacts in soda-lime glass fiber substrate, rather only islands of spherical shape are observed. The UV-Raman spectrum (figure 2(b_III)) of NCD crystals also shows sharp first-order Raman peak at 1332 cm⁻¹ (D-band) and G-band (sp²-bonded carbon) at 1575 cm⁻¹ [35, 36] (I_D/I_G = 0.38), but with much higher background noise, compared to the NCD grown on planar substrates (SiO₂/Si or soda-lime glass).

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Figure 3(c_I) shows the low magnification SEM images of UNCD films grown on soda-lime glass fibers, which reveals the presence of spherical aggregates with size around 1 µm. Figure 3(c_II) shows the enlarged SEM micrographs, revealing that each UNCD diamond aggregates contain densely populated diamond grains. Although the UNCD/soda-lime glass fibers no longer form continuous films, the microstructure of UNCD films in each diamond aggregates is not much different from those grown on planar substrates (see figure 1(b_I)). The UV-Raman spectrum of UNCD/soda-lime glass fibers is shown in figure 2(c_III), which depicts much smaller D-band at 1333 cm⁻¹ from diamond with much larger D*-band at 1350 cm⁻¹ from disorder carbon and G-band at 1577 cm⁻¹ from sp²-bonded carbon [35, 36] (I_D/I_G = 0.60). This indicates that the proportion of sp²-bonded carbon present in the UNCD films increased markedly relative to the sp³-bonded carbon when the UNCD films were grown on soda-lime glass fibers. The UV-Raman studies show the occurrence of a blue shift in first order Raman peak of diamonds D-band for all samples, which confirms the presence of defects and stress in all diamond films, regardless of morphology. In addition, the UNCD films have shown the evidence of large ensembles disorder carbon.

The microstructure of MCD, NCD, and UNCD films was investigated in more detail using TEM. It should be mentioned that only the diamond films grown on planar substrates (SiO₂/Si and soda-lime glass) were examined, as the preparation of the TEM foil for the diamond grown on soda-lime glass fibers is extremely difficult. Figure 4(a) shows the TEM bright field (BF) image of MCD/SiO₂/Si film, confirming the large grain characteristics of the MCD films. Inset in figure 4(a) shows the corresponding selected area electron diffraction (SAED) pattern, revealing the presence of discrete diffraction spots corresponding to (111), (220), (311) diamond lattice planes. Moreover, there exist diffraction spots arranged in a continuous ring, suggesting the presence of some randomly oriented small diamond grains coexisting with the large diamond grains. There is essentially no diffuse central ring, indicating very small proportion of sp²-bonded carbon contained in these films. The phase constituents of these films were better demonstrated by the composed dark field (c-DF) images, which is the superposition of several dark field images acquired using different diffraction spots. Figure 4(b) shows the c-DF image of MCD/SiO₂/Si film, which confirms the coexistence of large diamond grains with some small diamond grains. The diffraction spots on (111) ring used for composing c-DF images are indicated in inset of figure 4(b). To elaborate the microstructure in detail, high-resolution TEM (HRTEM) images from the region of large diamond grains (region A, figure 4(a)) and the vicinity region of diamond grains (region B, figure 4(a)) were examined. Figure 4(c) illustrates the HRTEM of region 'A' with the corresponding fourier transformed diffractogram, FT_oc-image, of the whole structure image shown as an inset in figure 4(c). The presence of rel-rods associated with each major diffraction spots in FT_oc implies that the diamond grains contain a large proportion of planar defects, most probably the stacking fault, which are highlighted in ft₁- and ft₂-images corresponding to areas 1 and 2, respectively. In contrast, HRTEM images shown in figure 4(d) acquired from the designated region B in figure 4(a), shows the presence of small diamond grains of size around 5–10 nm. Inset in this figure 4(d), the FT_0d-image corresponding to the whole structure image in figure 4(d), shows the diffraction pattern arranged in a ring shape, implying the presence of randomly oriented small diamond grains in this region. A diffused ring located at the center of the FT_0d image indicates the presence of a-C phase in this region. The existences of small diamond grains and a-C phase in this region are highlighted by the ft₃ and ft₄ images corresponding to areas 3 and 4, respectively. The presence of small diamond grains together with the large diamond grains in MCD/SiO₂/Si films is very much different from the microstructure of MCD films grown directly on Si substrates, in which no small diamond grains was observed. The microstructure of MCD/soda-lime glass films is similar to the one shown in figure 4 (supplementary information, figure S1), inferring that the difference in the microstructure of MCD/SiO2/Si from the conventional MCD/Si films is mainly due to the utilization of oxide substrate materials. The presence of Na species in the substrates seems not markedly altered the granular structure of the MCD films.

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Figures 5(a) and (b) show the BF and c-DF TEM micrograph of NCD/SiO₂/Si films respectively, which reveal that the microstructure of these films are similar to that of MCD/SiO₂/Si films, except that the grain size of NCD films is much smaller, i.e., they consist of large diamond grains around 100 nm in size, coexisting with the ultra-small diamond grains about 5 nm in size. Inset of figure 5(a) shows the SAED pattern corresponding to (111), (220), (311) diamond lattice planes. Inset in figure 5(b) shows the diffraction spots on (111) ring used for composing c-DF images. TEM structure image in figure 5(c) shows that the large diamond grains (region A, figure 5(a)) also contain planar defects, the stacking faults, which are evidenced by the existence of parallel fringes in figure 5(c) and the presence of the rel-rods in FT_0c image (inset, figure 5(c)). The presence of planar defects is further highlighted by the ft₁ and ft₂ images corresponding to areas 1 and 2 in figure 5(c). Again, TEM structure image in figure 5(d) shows that the ultrasmall grain region (region B, figure 5(a)) contains some sp²-bonded carbon, which is inferred from the central diffuse ring in FT_0d image in inset of figure 5(d). The existence of diamond and a-C phase in these materials is highlighted by ft₃ and ft₄ images, corresponding to areas 3 and 4, respectively, in figure 5(d). The microstructure of NCD films grown on soda-lime glass substrates (supplementary information, figure S2), is similar with those grown on SiO₂/Si substrates. The presence of Na-species in soda-lime glass seems not to markedly alter the granular structure of NCD films. Compared with the microstructure of MCD, the grain size of NCD films is about one order of magnitude smaller. The other features, such as the presence of stacking faults in the region of large diamond grains and the presence of a-C phase in ultrasmall grain regions, are essentially the same for both MCD and NCD films. It should be noted that the NCD/SiO₂/Si films contain more abundant ultra-small diamond grains, compared with NCD films grown on conventional Si substrates, inferring that the presence of a SiO₂ surface in the substrates hindered markedly the nucleation and growth behavior of these diamond films.

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Figure 6(a) shows the (BF) TEM micrograph of UNCD/SiO₂/Si films, revealing that these films consist of some diamond aggregates of size around tens of nanometers evenly dispersed in the matrix of ultrasmall diamond grains with size around 5 nm. Inset in figure 6(a) shows that the SAED pattern of UNCD film depicts ring-shaped diffraction patterns corresponding to (111), (220), (311) diamond lattice planes. It indicates that diamonds grains in these films are ultra-small is size and are randomly oriented. Figure 6(b) shows the c-DF image of these films, which demonstrates the even distribution of diamond aggregates among the matrix of nano-sized diamond grains. The HRTEM images of the diamond aggregate (region A, figure 6(a)) and the periphery region (region B, figure 6(a)) were shown in figures 6(c) and (d), respectively. Figure 6(c) reveals that the diamond aggregates contain a large proportion of stacking faults, which is again inferred from the presence of rel-rods in the FT_0c-image. The presence of stacking faults in aggregates are further highlighted by the ft₁ and ft₂ images corresponding to areas 1 and 2 in figure 6(c), respectively. It should be mentioned that the large diamond aggregates in figure 6(c) appear to consist of many small diamond grains (around ∼10 nm) in size agglomerated together, rather than a single large diamond grains as those in figure 4(c) for MCD or figure 5(c) for NCD diamond grains. There still exists large proportion of grain boundaries in the large diamond aggregates, except that the grain boundaries are much thinner. Moreover, these diamond aggregates can be disintegrated easily during the TEM investigation, confirming that they are the soft agglomeration of ultra-small diamond grains. In contrast, figure 6(d) indicates that the periphery region (region B, figure 6(a)) consists of ultra-small diamond grains, which is inferred from the presence of the ring-shaped diffraction spots in FT_od-image corresponding to entire structure image in figure 6(d) and is highlighted by ft₃ and ft₄ images corresponding to areas 3 and 4 of figure 6(d), respectively. Again, replacing the SiO₂/Si by soda-lime glass substrates for growing UNCD films does not alter the granular structure of the UNCD films (supplementary information, figure S3). Notably, the granular structure of UNCD/SiO₂/Si (or UNCD/soda-lime glass) films illustrated in figure 6 is essentially the same as the UNCD films grown on conventional Si substrates. The microstructure of UNCD films is least sensitive to the change in substrate materials.

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To understand the diamond growth behavior on soda-lime glass fibers, the optical emission spectra (OES) of the diamond growth plasmas were collected. It is observed that the species contained in the plasma change with the substrate materials markedly, i.e., a large proportion of substrate materials were ejected from the substrate mixing with the diamond growth plasma. Figure 7(a_I) shows that the OES spectrum of MCD/soda-lime glass fiber growth plasma, the CH₄/H₂, mainly contains H species, which were confirmed by the presence of H_α line at (653.1 nm), H_β line at (486.1 nm) and H_δ line at 410 nm. Presumably, the H species are the main ingredients, which interact with CH₄ to form CH₃ radicals, leading to the formation of MCD crystal [38]. In addition, Na (588.9 nm, 819 nm) line of large intensity is observed along with some O (767.7 nm), CO (560.8 nm, 451 nm) and OH (324 nm, 307 nm) lines of smaller intensity, which must have apparently originated from the soda-lime glass substrate via the plasma bombardment. Figure 7(a_II) shows that the OES spectrum of MCD/soda-lime glass growth plasma, which is similar with OES of the MCD/soda-lime glass fiber growth plasma, expect that the Na and O-containing species is much smaller, indicating that the soda-lime glass substrates are less susceptible to the bombardment ejection of the species in the substrates. Moreover, The MCD/SiO₂/Si growth plasma is similar with OES of the MCD/soda-lime glass growth plasma (not shown) but no Na or O species was observable, indicating that SiO₂/Si substrates are rather resistant to the bombardment of plasma ions.

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On the other hand, the optical emission spectrum of the UNCD/soda-lime glass fibers growth plasma, CH₄/Ar shown in figure 7(c_I) indicates that these plasma are predominated by C₂ dimer emissions at 473 nm, 516.5 nm, and 563 nm along with some CH emission at 388.9 nm, but no H lines were observed. These characteristics are the same as the CH₄/Ar plasma used for growing UNCD films on Si substrates. Presumably, the C₂ species, which has low energy for forming sp³-bonded carbon, are the species growing diamond nuclei from the a-C matrix. However, when the diamond clusters were formed, they were passivated by the CH species instantaneously that prevents the further attachment of C₂ species onto the diamond clusters. Therefore, the diamond grains in UNCD films remained as ultrasmall size (∼5 nm). This plasma also contains Na line at 588.9, 819 nm and O line at 767.7 nm, indicating the presence of large proportion of Na and O species, which were released from soda-lime glass fiber that is the factor which results in totally different morphology of diamond films grown on soda-lime glass fibers, compared with those grown on SiO₂/Si or soda-lime glass substrates. Figure 7(c_II) shows the optical emission spectrum of the UNCD/soda-lime glass growth plasma, indicating that this plasma is also predominated by C₂ dimer and CH emission, but no H lines were observed, except that the Na and O lines are much smaller. Moreover, the OES of UNCD/SiO₂/Si growth plasma (not shown) contains only C₂ dimer and CH emission. Na and O lines are not observable. Again, the Na and O lines appeared due to ejection from the substrate materials and the soda-lime glass fibers are most susceptible to plasma bombardment that markedly influences the growth behavior of the UNCD films.

The optical emission spectrum of the NCD/soda-lime glass fibers growth plasma CH₄/H₂/Ar is shown in figure 7(b_I), which indicates that this plasma is the mixture of CH₄/H₂ and CH₄/Ar plasma. Both H emissions (at H_α (653.1 nm), H_β (486.1 nm) and H_δ (410 nm)) and C₂ dimer emissions (at 473 nm, 516.5 nm, and 563 nm) are present in this plasma. The CH species, which are usually present together with the C₂ species in CH₄/Ar plasma are too few to be detectable in CH₄/H₂/Ar plasma. Although abundant C₂ species were present in CH₄/H₂/Ar plasma, which are expected to nucleate diamond clusters efficiently as in the case of CH₄/Ar plasma (the UNCD growth plasma), there is no sufficient CH species in CH₄/H₂/Ar plasma for passivating the diamond clusters. Moreover, the presence of H species in this plasma can efficiently remove the attached CH species, preventing the passivation effect of hydrocarbons. Therefore, the C₂ species can be attached to the diamond clusters continuously enlarging the size of NCD grains. This facilitated the growth of NCD diamond grains to large size. The CH₄/H₂/Ar plasma also contains Na line at 588.9 and 819 nm and O line at 767.7 nm. Again, both the OES of the NCD/soda-lime glass (figure 7(b_II)) and NCD/SiO₂/Si (not shown) growth plasma is similar to those of NCD/soda-lime glass fibers growth plasma but with the fewer (or no) Na and O lines, which depends on the susceptibility of the substrate materials to the plasma bombardment damage. The presence of Na and O species are apparently the main factor resulting in a very different granular structure for NCD/soda-lime glass fibers compared with those for NCD/soda-lime glass (or NCD/SiO₂/Si) films. The unintentionally added oxygen in growth plasma is expected to result in less sp²-bonded carbon or a-C phase, by privileged etching of non-diamond and leads to the growth of high purity diamond clusters on soda-lime glass fibers [39]. Moreover, the Si species were expected to be released from soda-lime glass fibers and were incorporated into diamond films, creating SiV centers. The presence of Na seems to have a smaller effect on altering the granular structure of the films.

The study on the growth behavior of diamond films using various kinds of growth plasmas and substrates are useful in the tuning of PL emission properties of SiV or NV centers in diamond. Figures 8(a_I), (b_I), and (c_I) show the PL spectrum of MCD/SiO₂/Si, MCD/soda-lime glass and MCD/soda-lime glass fibers, respectively, inferring the presence of the NV center's signature, the zero phonon lines (ZPLs) at 575 nm and 637 nm for NV° and NV⁻ respectively, and large PSBs appears beside these ZPLs [9, 14]. In addition, the characteristic ZPL of SiV center was observed at a wavelength of 737.3 nm [14], which completely overlaps with the PSBs of NV center. PL mapping was carried out on these diamond films in order to find the color center's density and distribution. The PL mapping of MCD/SiO₂/Si, MCD/soda-lime glass and MCD/soda-lime glass fibers are shown in figures 8(a_II), (b_II) and (c_II), respectively. The resultant mapping reveals the evenly distributed high density of NV and SiV center's luminescence with maximum photon count rate of 2.7 × 10⁵ s⁻¹ for MCD/SiO₂/Si films (figure 8(a_II)), 4.9 × 10⁴ s⁻¹ for MCD/soda-lime glass films (figure 8 (b_II)) and 2.4 × 10⁴ s⁻¹ for MCD/soda-lime glass fibers (figure 8(c_II)). While the morphology of MCD films changes markedly with the substrate materials, the characteristics of PL spectra of the three films are essentially the same. However, the photon count rate decreased when SiO₂/Si substrates were changed to soda-lime glass and further decreased for MCD/soda-lime glass fibers that can be ascribed to the photon quenching due to the presence of the grain boundary phase and in turn is related to the decrease in grain size.

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The optical properties of NCD films are very similar to those of MCD films. Figures 9(a_I), (b_I) and (c_I) show the PL spectrum of NCD/SiO₂/Si substrates, NCD/soda-lime glass and NCD/soda-lime glass fibers, respectively. As like MCD films, NCD films also exhibits the characteristics of NV and SiV centers with large NV's PSBs. The decrease in grain size from hundreds of nanometers in MCD films to tens of nanometer in NCD films renders the SiV emission more clearly observable. PL mapping of NCD films show the maximum photon count rate of 1.7 × 10⁵ s⁻¹ for NCD/SiO₂/Si films (figure 9(a_II)), and 6.0 × 10⁴ s⁻¹ for NCD/soda-lime glass films (figure 9(b_II)) and 2.0 × 10⁴ s⁻¹ for NCD/soda-lime glass fibers films (figure 9(c_II)). Again, the photon count rate decreased by changing the substrates materials from SiO₂/Si to soda-lime glass and then to soda-lime glass fibers that are, again, closely related to the PL quenching effect owing to the decrease in grain size. These PL and PL mapping for NCD films are very similar with those of MCD films, regardless of the substrate used for growing the diamond films.

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In contrast, figures 10(a_I) and (b_I) show the PL spectra of UNCD/SiO₂/Si and UNCD/soda-lime glass films, respectively, indicating that the NV center's PSBs are markedly suppressed, while the SiV center appears with characteristic ZPL at 739 nm. However, large PSB from NV centers is still observable as background, indicating the existence of some NV centers and lowering in NV centers luminescence is probably due to quenching. Figures 10(a_II) and (b_II) show the PL mapping of UNCD/SiO₂/Si and UNCD/soda-lime glass films, which reveals the maximum photon count rate of 3.4 × 10⁵ s⁻¹ and 5.5 × 10³ s⁻¹, respectively. The suppression of NV center's luminescence are apparently owing to the change in granular structure, i.e., the decrease in grain size from hundreds of nanometers in MCD films to around 5 nm in UNCD films. The change of substrate materials from SiO₂/Si to soda-lime glass seems to have less effect on modifying the PL properties of UNCD films. However, the UNCD films grown on soda-lime glass fibers show entirely different optical properties from those grown on planar substrate (i.e., SiO₂/Si or soda-lime glass substrates). The NV luminescence is completely not observable in the PL spectrum of UNCD films grown on soda-lime glass fibers as shown in figure 10(c_I). The SiV center emission is observed at the wavelength of 738 nm with very small PSB. The corresponding PL mapping shown in figure 10(c_II) reveals the photon count rate of 7.8 × 10⁴ s⁻¹ for the UNCD/soda-lime glass fibers. Further, the spectral features of SiV such as ZPL width were estimated from the PL spectrums of UNCD samples, i.e., the UNCD films grown on SiO₂/Si, soda-lime glass_, and soda-lime glass fibers, the estimated ZPL widths are 17.3 nm, 8.8 nm, and 9.4 nm respectively (table 2). It should be noted that, the MCD or NCD samples do not shown clear emission peak, and therefore estimation of ZPL width is not possible.

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These observations indicate that among the diamond films with different granular structure, only UNCD/soda-lime glass fiber samples show the clear SiV center's emission without the interference of NV center's PSBs. These samples were thus further processed to fabricate the SiV-UNCD particulates. The SiV-UNCD/soda-lime glass fiber samples were immersed in DI water and ultrasonicated for 5 min to release the SiV-UNCD particulates. Subsequently, these SiV-UNCD particulates containing DI water was spread over the SiO₂/Si substrate containing inverted pyramidal cavities for better collection of SiV-UNCD particulates. The fabrication process of inverted pyramidal cavities on Si-substrates is reported elsewhere [40]. The SiV-UNCD particulates with the size of 1 μm were settled inside the Si inverted pyramid as shown in figure 11(a). The PL spectrum of SiV-UNCD particulates is shown in figure 11(b), which reveals a sharp ZPL of SiV at a wavelength of 737 nm, similar to those of UNCD/soda-lime glass fibers. In addition, PL mapping performed on the SiV-UNCD particulates located inside the Si inverted pyramidal cavities shown in figure 11(c), revealing the photon count rate of 6.5 × 10⁴ s⁻¹. The photon count rate of SiV-UNCD particulates is the same as those obtained from UNCD particulates on soda-lime glass fibers, that is, the freestanding SiV-UNCD particulates exhibit the similar brightness of SiV emission as those from UNCD/soda-lime glass fibers. The PL spectrum of SiV-UNCD shows the broadening of ZPL emission, which is attributed to the electron–phonon scattering due to the presence of abundant defects.

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