Size-dependent Raman and SiV-center luminescence in polycrystalline nanodiamonds produced by shock wave synthesis

Abstract

Size-dependent structural and luminescent properties of the diamond polycrystals produced by shock wave synthesis followed by grinding and separation into fractions of different polycrystal median sizes (25–1000 nm) are studied by comparative Raman and luminescence spectroscopy. The intense 738 nm narrow band luminescence of the SiV-centers are observed for all fractions. The SiV luminescence intensity has a maximum at the median size of about 180 nm that is controlled by competition between deactivation of the SiV-centers by defects in the diamond nanocrystal lattice and that controlled by nonradiative recombination centers in the volume of the intergranular layers.

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Introduction

It is well-known that synthetic diamonds of different shapes and sizes can be produced by different methods, including CVD techniques and static synthesis at high pressure (up to 7 GPa) and high temperature (up to 2200 °C) which have found broad applications in the industry including nanophotonics.¹ One of most effective methods of large-scale synthesis of diamond powder is the shock wave synthesis of diamond developed several decades ago by DuPont in the US.^2–5 In this method, the direct conversion of graphite into a diamond-containing phase takes place for a very short period of time (≈200 μs) during the detonation of explosive material outside a metal tube filled with powder graphite.^3,6 The pressure and temperature during the process may be as high as ∼50 GPa and 1100 °C, respectively. Such high dynamic loads are responsible, inter alia, for the sintering of diamond nanocrystals into polycrystalline particles. An important feature of this synthesis is the use of metal powder with high thermal conductivity (copper as a rule), which does not form carbides, and is mechanically mixed with a precursor – graphite powder. Copper transmits well hydrostatic pressure in the fluid state at high pressures and temperatures and provides effective heat dissipation after synthesis, thereby preventing reverse graphitization of the nanodiamonds formed.⁵

Diamond powders manufactured by this technique using a thick-walled steel pipe driver are produced currently, in particular by Microdiamant USA, and commercially distributed by L. M. Van Moppes & Sons SA (Geneva, Switzerland) under the trade name Super Syndia™ SSX,⁷ and earlier (1976) were known under the trade mark Mypolex™ by E. I. Du Pont De Nemours.⁵ According to the manufacturer, after initial chemical treatment and deep removal of non-diamond phase, the product is a powder of polycrystalline diamond particles of micron size (10–60 μm) consisting of individual diamond nanocrystals with sizes not exceeding 20–25 nm each.⁷ Differently oriented diamond nanocrystals are tightly bound together by means of the spliced edges of the crystal lattices and by covalent bonds of shorter atomic groups existing on the grain boundaries. It is also assumed that the particles of diamond polycrystals may contain, in addition to cubic diamonds, up to 50 wt% of hexagonal diamonds including lonsdaleite,⁸ which may be formed inside the intergranular layers of thickness of up to several lattice constants that exist at the interface of cubic diamond nanocrystals.⁹ The presence of a dense “sintered” polycrystalline structure makes this material similar to so-called “Carbonado” and other granular diamond-containing materials of meteoric origin.^2,10,11 By grinding and fractionation of the as-synthesized polycrystalline diamonds, powders of polycrystals with smaller sizes (25–1000 nm) suitable for finishing the polishing of hard materials can be obtained. Fig. 1 illustrates schematically the fragmentation of a large as-synthesized polycrystalline diamond particle into smaller particles during milling.

Fig. 1 Schematic representation of the fragmentation of a large as-synthesized polycrystalline diamond particle into smaller particles during milling. The intergranular layers between the diamond nanocrystals are shown by solid black lines.

It has been noted that the rapid synthesis and direct conversion of graphite to diamond in air results in the formation of fairly large numbers of defects (dangling C–C bonds and vacancies) and in the presence of impurity atoms in the crystalline lattice of polycrystalline diamond produced by the synthesis method mentioned above. This leads to the appearance of NV- and SiV-centers, for example, which can be identified by electron paramagnetic resonance¹² and luminescence spectroscopy. The presence of luminescent NV-centers in diamond polycrystals of meteoric origin (carbonado) has been reported.¹³ Other impurities in the charge used for shock-wave synthesis (Si, Cr, or Cu) may also incorporate into the diamond lattice as substitutional defects, leading to the appearance of luminescen t color centers such as SiV. It was proposed that the intergranular layers between the diamond nanocrystals are the major locations of paramagnetic defects and various color centers. Some parameters of the nanodiamond synthesis by shock wave and of the subsequent chemical treatments may influence the content in luminescent NV- or SiV-centers. To investigate such dependencies, structural characterization of the nanodiamond powders and diagnostics of luminescence centers content are instrumental. To this aim, Raman and luminescence spectroscopy are particularly adequate as they permit the quick and contactless investigation of the samples without disturbing the structure of the constituent materials. Moreover, these techniques are already widely used to determine the structural features of different nanocarbon materials, including diamond^14–22 and graphite^23–27 nanoparticles obtained by different methods.

In the present work, optical spectroscopy methods (Raman and luminescence analysis) were employed for investigating polycrystalline diamond powders obtained by shock wave synthesis and subjected to grinding followed by separation into fractions of different polycrystal median size (Z_M) in the 25–1000 nm range.

Experimental

Materials

We have investigated powders of polycrystalline diamonds produced by shock wave synthesis. Polycrystals composed of tightly connected differently oriented diamond nanocrystals with mean sizes of 10–15 nm were formed by merging the boundary areas of the crystal lattices of adjacent nanocrystals and short covalent bonds of atomic groups of different carbon fragments present on the boundaries between the nanocrystals.

Polycrystalline nanodiamonds of the Super Syndia™ SSX series were provided by Van Moppes & Sons (Geneva, Switzerland).⁷ This nanodiamonds were treated by mild low-energy stainless steel bead milling of large starting as-synthesized polycrystalline microdiamonds (ash content < 0.1 wt%) obtained after shock wave synthesis and extensive acid cleaning (including aqua regia and HF) of the synthesized product. The raw graphite material used as a precursor for such synthesis contained 97.5 wt% of carbon, 0.5 wt% of moisture, and approximately 2 wt% of ash, where the content of SiO₂ is around 0.8–1 wt%. The SiO₂ present in graphite is the origin of the silicon present in the synthesized diamond product. According to ICP elemental analysis the graphite content of other elements is as follows: Fe – 9000 ppm, Si – 4870 ppm, Ca – 790 ppm, Al – 260 ppm, Cu – 170 ppm, Mg – 150 ppm, Ni – 50 ppm, Ti – 30 ppm, Mn – 30 ppm. The average content of Si over batches in raw polycrystalline diamond powder is ∼100 ppm as measured by XRF, although Si cannot be considered as a characteristic contaminant of products of the SSX series. The goal of the standard bead milling was to produce polycrystalline microdiamonds with selected size ranges (varying in the wide range from <1–2 to 10–20 μm) by avoiding loss as fine diamond dust (both submicro- and nano-diamonds). The composition of the steel milling beads includes iron (∼70%), Cr (17.0–19.5%), Ni (8.0–10.5%), Mn (≤2%), Si (≤1%), and other remaining elements.

The studied set of polycrystalline diamond powders with a median size of diamond polycrystals from 1000 to 25 nanometers was obtained by step-by-step fractionation of once milled starting powder. Size fractionation of the submicron dust was accomplished by centrifugation or sedimentation in water the micron fraction with median particle size 1–2 microns. This process gave nine Super Syndia SSX samples (S1–S9), each containing submicron polycrystalline diamond nanoparticles with a bell-shaped size distribution. The nanoparticle size ranges of the samples and corresponding median particle size (Z_M) is presented in Table 1.

Table 1 Size range of the Super Syndia SSX samples obtained by size fractionation (see text) and corresponding median particle size (Z_M) according to the Van Moppes & Sons Product Catalog⁷

Sample	Size range (μm)	ZM (nm)
S1	0–0.05	25
S2	0–0.10	50
S3	0–0.15	75
S4	0–0.20	90
S5	0–0.35	180
S6	0–0.50	210
S7	0.25–0.5	350
S8	0.5–1.00	750
S9	0.75–1.25	1000

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Each fraction of Super Syndia SSX nanodiamonds was subsequently extra-purified in acid mixture in autoclave at ∼160 °C for removal traces of metals and graphitic phase and then washed in rinsing water several times. According the XRF data, the main pollutants in the extra-purified SSX nanodiamonds among transition metals are Fe (50–170 ppm) and Cu (6–17 ppm). The content of silicon in the six finest fractions of the samples (S1–S6) is on the 5–6 ppm level, while the content of other metal pollutants (except alkaline metals) is below 1–3 ppm.²⁸

Characteristic HRTEM images of the as-fabricated SSX0.05 polycrystalline diamond fraction (smallest grade powder) are shown in Fig. 2. JEOL JEM-2100F equipped with CEOS Cs corrector operated at 80 keV was used for TEM observation, and Gatan Digital Micrograph 1.71 was used for FFT and image analysis. Diamond nanocrystal domain was analyzed by using the method similar to ref. 29, which uses the FFT spots to determine the domains. The crystalline lattice of cubic diamond crystallites together with a few twinning boundaries are clearly seen on the images. No evident signs of lonsdaleite or other hexagonal-type diamond phases were seen in the HRTEM images, but there may exist such a phase in very small amount, in intergranular layers.

Fig. 2 (a)–(c) Characteristic HRTEM images of the S1 polycrystalline diamond fraction. Areas shaded in different colors correspond to the individual crystallites with different orientation. Scale bars of 4 nm are shown.

The X-ray diffraction (XRD) pattern of the S1 sample obtained with a Smart Lab X-ray Diffractometer (Rigaku Co., Japan) using CuK_α radiation (λ = 1.541 Å) is shown in Fig. 3. The (111), (220) and (311) reflections from cubic diamond phase together with weak (002) reflection from graphite phase are clearly seen. Weak shoulders on the left and the right sides of the (111) peak come from the stacking faults and/or twinning boundaries inside the diamond lattice as shown in ref. 28. The X-ray coherent scattering size for cubic diamond crystallites obtained on the base of analysis (220) and (311) reflections was estimated as ∼7–8 nm. This value matches well with the size of individual crystallites of cubic diamonds observed in the high resolution TEM images.

Fig. 3 Wide angle XRD pattern of the S1 sample. The (111), (220), (311), and (400) reflections from cubic diamond (d) phase and (002) from graphite phase (g) are shown. The CuKα radiation at 1.541 Å was used.

Methods

A micro-Raman spectrometer “inVia” (Renishaw, UK) equipped with a Leica microscope and a CCD detector cooled to −70 °C was used for the acquisition of Raman spectra of the samples in backscattering geometry and with a spectral resolution of 2 cm⁻¹. The Raman modes were excited by radiation of 488 nm and 514 nm from an argon-ion laser with power lower than 20 W cm⁻² on the sample surface. A standard scheme of focusing of the laser beam with a 50× and NA = 0.75 microscope objective allows to collect scattered light from a spot of ∼2 μm diameter. However, in this study the recording of the Raman spectra was performed with use of a new technique for focusing the laser radiation and collection of the scattered light, the Stream-Line™ Plus (Renishaw, UK). This technique significantly reduces the incident light power density on the sample due to its focus on a ∼2 × 60 μm² stripe. At the same time, there is no loss of measurement sensitivity since the whole matrix of the CCD camera is used to record the spectrum. The Stream-Line™ Plus technique allows not only to avoid the influence of sample heating on the parameters of the Raman and luminescence spectra but also obtaining averaged spectra from a sufficiently large volume of the sample, which is important for samples with micron-scale structure heterogeneities. Luminescence spectra of the samples were obtained with the “inVia” spectrometer together with Raman spectra, with one scan recording Raman and luminescence spectra from exactly the same portion of the sample. For a correct comparison of the intensities of Raman and luminescence bands the spectra were normalized to the spectral sensitivity of the spectrometer measured with a black-body radiation unit. All measurements were done at room temperature.

The samples for the optical measurements were prepared as follows. Powders of polycrystal diamond particles of different fractions were pressed into cylinders 5 mm in length and 2 mm in diameter using a special Renishaw sealing press. Raman and luminescence spectra were recorded from the end of cylinder.

The studied fractions of polycrystalline diamond particles of various sizes are powders of color varying from light gray to black due to the presence of various amounts of amorphous carbon formed during grinding. The presence of amorphous carbon leads to different absorption of incident light as well as Raman and luminescence spectra, which impedes the quantitative comparison of their intensities and, therefore, of the concentration of different allotropic forms of carbon and luminescence centers. For the quantitative comparison of the emission intensities of the various factions, potassium nitrate (KNO₃) powder was added to diamond powders in order to use the intensity of the KNO₃ characteristic Raman band at ∼1049 cm⁻¹ as a reference. Firstly, KNO₃ was mixed homogeneously to diamond in a weight ratio of 5 : 1 (KNO₃ : diamond), and then the mixture was pressed into cylinders as described above, yielding a weakly absorbing solid solution of diamond polycrystal particles in KNO₃ with volume ratio of ∼1 : 8.5.

Results

A typical example of the secondary emission spectrum of the sample S5 (Z_M ∼ 180 nm) in the 500–800 nm spectral range is shown in Fig. 4. The spectrum was excited by 488 nm radiation. The spectrum shows photoluminescence (PL) and Raman bands observed in the spectra of all fractions of polycrystalline diamonds. The inset in the figure shows the comparison of this spectrum with the one obtained by excitation at 514.5 nm wavelength, that allows to clearly distinguish (i) a broad band luminescent background, (ii) a narrow PL line at 738 nm and (iii) Raman bands. An analogous luminescence broadband background is usually observed in secondary emission spectra of diamond particles produced both by detonation techniques¹⁷ and CVD methods^18,19,26 and is attributed to a broad orange-to-red PL caused by NV centers and H₃ centers at wavelengths around 520–530 nm in distorted diamond lattices. The 738 nm narrow line observed in the spectra of all fractions is attributed to PL from SiV colour centers formed by the intercalation of silicon atoms into the crystal lattice of diamond particles.^19,21,22

Fig. 4 Spectrum of secondary emission of the S5 polycrystalline diamond fraction (Z_M = 180 nm) for excitation wavelength of 488 nm. The deconvolution of the broad PL background by Gaussians is shown. The pink circle shows the region of the first order Raman spectrum of the sample. Inset: comparison of the spectra excited at 488 nm (red line) and 514.5 nm (blue line). The constancy of the PL bands and shift of the Raman bands with excitation wavelength is marked by vertical dotted lines.

Fig. 4 shows also an example of the deconvolution of the broad luminescence background. This deconvolution was performed for the spectra of all samples studied and used for subtracting the luminescence background, a procedure necessary for the quantitative comparison of the Raman and SiV luminescence band intensities.

An example of the spectrum after subtraction for the S4 fraction is shown in Fig. 5a. By construction, this subtracted spectrum only contains (i) the Raman lines from the nanodiamond sample, and (ii) the 738 nm narrow line attributed to PL from SiV colour centers.

Fig. 5 (a) An example of the spectrum after PL background subtraction for the S4 fraction; (b) representative set of Gaussian fits of SiV-center PL bands at 738 nm for different fractions of polycrystalline diamonds after normalization to KNO₃ Raman line intensity. (c) Size dependence of the integral intensity of the SiV-center luminescence band. Excitation laser wavelength: 488 nm.

Fig. 5b depicts the 738 nm PL band for different fractions of the polycrystalline diamond particles after the normalization of its maximum to KNO₃ 1049 cm⁻¹ Raman line. Fig. 5c shows the dependence of 738 nm PL band intensity on Z_M, exhibiting a maximum for fraction S5 (Z_M = 180 nm).

Fig. 6a displays a representative set of Raman spectra (in the 1100–2000 cm⁻¹ range) of samples of diamond polycrystals of different median size containing the main features of Raman scattering. Like for Fig. 5b, the spectra were obtained by first subtracting the continuous luminescence background from the raw spectra, and then normalize the intensity by the one of KNO₃.

Fig. 6 (a) Representative set of Raman spectra of the polycrystalline diamond fractions S2–S9 for excitation wavelength 488 nm. (b) Size dependence of the diamond Raman peak intensity (I_DIA). (c) Ratio of the intensities of amorphous carbon (I_A) and diamond Raman bands (I_A/I_DIA).

Raman band at 1049 cm⁻¹. Since the Raman intensity drops with decreasing size of the diamond polycrystals, we were not able to obtain Raman spectrum with for the sample S1 (Z_M = 25 nm) due to a too low signal-to-noise ratio.

Raman spectra of different samples show the same set of bands. The diamond Raman band at about 1330 cm⁻¹ as well as well-known Raman bands of disordered nanocarbon structures^23–27 dominate all spectra. The Raman bands of diamond phase, marked as DIA, are comparable in intensity to the other lines in the spectrum. However, keeping in mind that the Raman cross-section for diamond at 488 nm excitation wavelength is smaller by more than one order of magnitude than that for sp² carbon nanostructures we can conclude that the structure of the studied samples is dominated by the diamond phase. Other bands in the spectra indicate the presence of disordered forms of carbon compounds. The spectra shown in Fig. 6a exhibit bands at ∼1350 cm⁻¹ (D), ∼1500 cm⁻¹ (A), ∼1587 cm⁻¹ (G), and ∼1625 cm⁻¹ (D′) which are characteristic for disordered nanocarbon.^23–25,27 The presence of the G-band in the spectra points out to the presence of sp²-bonded C atoms. The D and D′ bands correspond to the breathing vibration of aromatic rings in the carbon network and their intensities are proportional to the degree of structural disorder in graphite-like structures.^26,27 The broad A band at about 1500 cm⁻¹ is usually assigned to different kinds of amorphous carbon structures, including short atomic groups on different carbon fragments,^30,31 and its intensity can serve as an indication of the relative content of amorphous carbon in the samples.^27,32

A simple comparison of the Raman spectra of the fractions shows a general tendency of increased intensity of graphite-related bands with respect to diamond bands with decreasing size of the polycrystalline particles. A more detailed analysis allows finding other important differences. In particular, more pronounced size-dependence is observed for the DIA-band intensity (Fig. 6b), which remains virtually unchanged when Z_M decreases from 1000 to 180 nm but drops about 3 times with further reduction of the particle size down to 50 nm. Analogous size-dependence is also observed for the amorphous carbon band intensity (I_A) that results in an about 5-fold reduction of the ratio of intensities of amorphous carbon and diamond Raman bands (I_A/I_DIA) in the same range of the particle size.

Discussion

According to the Microdiamant AG data^5,7,28 the as-synthesized polycrystalline diamond particles consist of cubic diamond nanocrystals of size about 10–15 nm with 1–2 nm thick intergranular layers of less ordered carbon material between the diamond nanocrystals. The relative volume of the intergranular layers may be estimated to about 10% of that of the polycrystalline diamond particle. Mechanical fragmentation of the as-synthesized diamond polycrystals into particles of smaller sizes destructs these intergranular layers and, probably, damages the diamond nanocrystals. It is expected that this does not increase the number of the luminescent centers like SiV- or NV-centers formed at shock wave induced conversion of graphite to diamond. However, we observed unexpected Z_M dependence of the SiV PL accompanied by changes of the Raman spectra of the fractions. These changes in Raman and PL spectra can be explained in the framework of the following scenario.

There are two diamond crystal phases in the samples: cubic diamond in the nanocrystals and hexagonal diamond, lonsdaleite, located in the intergranular layers. The DIA Raman band integral intensity reflects the content of diamond crystal phases in the form of cubic diamond in the nanocrystals and hexagonal diamond, lonsdaleite, in the intergranular layers.⁹ Since the relative volume of the intergranular layers represents only ∼10%, this band can be assigned mainly to the Raman signal from cubic diamond nanocrystals. However, some evidence for the presence of lonsdaleite in the intergranular layers comes from the up-shift of the DIA band from 1328 cm⁻¹ to 1331.2 cm⁻¹ observed with decreasing Z_M from 1000 to 50 nm. Indeed, the Raman shift of the lonsdaleite (∼1325 cm⁻¹)³³ is smaller than that of cubic diamond (1332 cm⁻¹) and the DIA band may consist of overlapping diamond and lonsdaleite bands unresolved spectrally in our experiment. Then, a reduction of volume of the intergranular layers containing lonsdaleite with decreasing Z_M will result in a decrease of the lonsdaleite Raman signal and an up-shift of the DIA band. At the same time, the removal of lonsdaleite cannot explain the about three-fold reduction of the DIA band intensity observed for fractions from S5 to S2 (Fig. 6b) since its content in the fractions does not exceed ∼10%. This threshold reduction of the DIA band intensity indicates the increasing presence of damaged cubic diamond nanocrystals in the fractions of Z_M smaller than 180 nm. Correspondently, the content of disordered amorphous carbon structures in the fractions increases proportionally, resulting in an about five-fold reduction of the ratio of intensities of amorphous carbon and diamond Raman bands (I_A/I_DIA), as it shown in Fig. 6c.

As a results of the analysis of the Raman spectra of the fractions of diamond polycrystals of different Z_M we can conclude that S9–S5 fractions (Z_M decreasing from 1000 nm to 180 nm, respectively) consist of polycrystals of as-synthesized 10–15 nm cubic diamond nanocrystals with a relatively small number of defects. These fractions have intergranular layers probably containing lonsdaleite and some amount of disordered carbon structures due, in particular, to the destruction of the intergranular layers. In this range of polycrystal size we have not found evidence of the damage of diamond nanocrystals, but the damage of the intergranular layers with the increase of amorphous carbon content has been observed. For the fractions from S5–S2, where Z_M is smaller than 180 nm, a strong decrease of the intensity of the DIA band indicates the onset of damage of the cubic diamond nanocrystals with the appearance of disordered carbon structures, which increases with decreasing Z_M.

The size dependences of the SiV-center luminescence band at 738 nm shown in Fig. 5c differ from that of the DIA band intensity (Fig. 6b) by the presence of a clear maximum at Z_M = 180 nm. Fig. 5c shows that decreasing the nanocrystal size from 1000 nm to 180 nm (S9–S5 samples) leads to an increase of SiV luminescence intensity. As mentioned earlier, the size decrease is accompanied by a decrease of volume of intergranular layers. Therefore, SiV PL intensity increase suggests that the SiV-centers are localized mainly in the diamond nanocrystals, and not in the intergranular layers. This conclusion is supported by the parallel decrease of the SiV-center PL intensity and the DIA band Raman intensity for the S5–S2 series, where increasing damage of the nanocrystals takes place. It has been shown that for CVD diamonds with luminescent SiV-centers an increasing number of defects inside the crystals leads to a parallel decrease in the diamond Raman and SiV-centers PL intensities.¹⁹ The latter was associated to the formation of recombination centers, opening non-radiative decay channels in disordered regions. At the same time, the decrease of the PL intensity upon increase of size from 180 nm to 1000 nm (S5–S9 series) requires additional assumptions. Since the most evident change in the structure in samples when moving from fraction S5 to fraction S9 is an increase in the volume fraction of the intergranular layers, it is natural to assume that nonradiative recombination centers simply exist there. Then, increasing the intergranular layer volume in the S5–S9 series leads to a decrease of the SiV-center PL intensity. Therefore, the maximum observed in the size dependence of the PL intensity should be the result of competition between processes of deactivation of the luminescent SiV-centers due to the formation of defects in the cubic diamond nanocrystals and due to the reduction of nonradiative recombination centers in the volume of the intergranular layers. Typically, efficient non-radiative recombination requires a nanometer range proximity of luminescent and recombination centers. The quenching of SiV-center PL by recombination centers located in intergranular layers observed in our experiments indicates that a large number of the SiV centers is located near the surface of the diamond nanocrystals of 10–15 nm primary size. The nature of the recombination centers is not well established yet and the size dependence of the PL decay time should be analyzed for a better understanding of the PL quenching mechanism. Nevertheless, we have shown that there exists an optimum median size of the diamond polycrystals produced by shock wave synthesis for which a maximum intensity of the SiV-center luminescence takes place.

Let us note that silicon is an embedded substitutional impurity in polycrystalline diamond which appears unintentionally in the diamond lattice during the shock wave synthesis. This is because precursor graphite used for synthesis contains silica on the 0.8–1 wt% level. Thus, explosive-induced graphite-to-diamond phase transformation of such silica containing graphite seems very effective for making diamond polycrystals doped by silicon and having vacancies inside the crystallites at the same time. To, the best of our knowledge, apart the so-called meteoritic diamonds, it is the first report of the presence of SiV-color centers in diamonds produced by dynamic synthesis with 100 μs short synthesis time. It means that this method may be used for further intentional doping of diamond polycrystals by substitutional silicon for the purpose of enrichment the diamond polycrystals in SiV-centers. The use of special silicon-containing additives to precursor graphite in the solid, liquid or even gas form (instead of silica) should promote the doping and make the technology of obtaining polycrystalline diamonds with SiV more efficient and controllable. We envision a 10–30 fold increase of SiV concentration in polycrystalline diamonds by means of optimization the type of silicon-containing additive to graphite.

Conclusion

Raman and luminescence spectroscopy were used for investigating polycrystalline diamond powders produced by shock wave synthesis followed by grinding and separation into fractions of different polycrystal median size in the range 25–1000 nm. The TEM data showed that the diamond polycrystals consist of 10–15 nm nanocrystals with thin (2–3 nm) intergranular layers of partially ordered carbon material between the nanocrystals. A distinctive feature of the diamond powders studied is the presence of a narrow intense PL band of SiV centers at 738 nm. It is the first case of SiV-color centers found in diamonds produced by short duration dynamic synthesis. The analysis of the size dependencies of Raman bands showed that in the 1000–180 nm median size range the diamond polycrystals consist mainly of 10–15 nm cubic diamond nanocrystals with a fairly good crystalline structure and intergranular layers which, most probably, contain lonsdaleite and disordered carbon material. When the median size decreases from 180 nm to 25 nm, the damaged diamond nanocrystals fraction grows up, along with an increasing amount of disordered carbon structures.

As for the luminescent SiV-centers, related to silicon impurity incorporated as substitutional impurity in the diamond lattice during the shock wave synthesis, our data allows to conclude that the majority of these centers are located inside the diamond nanocrystals. SiV PL intensity depends on the polycrystalline powder mean size and has a maximum at size ≈180 nm. The latter is the result of a competition between two SiV non-radiative decay channels with opposite size dependencies. One channel was tentatively associated to the formation of crystalline defects inside the 10–15 nm primary size nanocrystals upon size reduction of the polycrystalline powder. The concentration of such defects increases when the polycrystal size decrease. The other non-radiative channel, was tentatively related to the presence of recombination centers in the volume of the intergranular layers. The concentration of these recombination centers is expected to decrease when the polycrystal size decreases, hence the observed SiV PL maximum at a compromised polycrystal size of 180 nm. The nature of the recombination centers is not established yet and requires further investigations.

Acknowledgements

Authors express their gratitude to Dr Jean-Paul Boudou (CNRS, France) for his help in chemical purification of nanodiamond samples and Christophe Bangerter from L. M. Van Moppes & Sons SA (Geneva, Switzerland) for help in elemental analysis and sample supply. K. T. was supported by JSPS KAKENHI Grant No. 26107532. V. Y. O. was supported by the Russian Scientific Foundation (project N 14-13-00795). K. V. B. and A. V. B. acknowledge financial support from the Ministry of Education and Science of the Russian Federation, Government Assignment No. 3.109.2014/K.

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