⇦Chinese Physics Letters, 2016, Vol. 33, No. 2, Article code 028101⇨ B–C Bond in Diamond Single Crystal Synthesized with h-BN Additive at High Pressure and High Temperature * Yong Li(李勇)1,3,4, Zhen-Xiang Zhou(周振翔)2**, Xue-Mao Guan(管学茂)3, Shang-Sheng Li(李尚升)3, Ying Wang(王应)1, Xiao-Peng Jia(贾晓鹏)5, Hong-An Ma(马红安)5 Affiliations 1Physical and Applied Engineering Department, Tongren University, Tongren 554300 2Beijing Sinoma Synthetic Crystals Co., Ltd, Beijing 100018 3School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000 4Institute of Cultural and Technological Industry Innovation of Tongren, Tongren 554300 5State Key Lab of Superhard Materials, Jilin University, Changchun 130012 Received 15 November 2015 ^*Supported by the National Natural Science Foundation of China under Grant No 51172089, the Natural Science Foundation of Guizhou Province Education Department under Grant No KY[2013]183, and the Natural Science Foundation of Guizhou Province Science and Technology Agency under Grant Nos LH[2015]7232 and LH[2015]7228.
^**Corresponding author. Email: zzx0826zzx@163.com Citation Text: Li Y, Zhou Z X, Guan X M, Li S S and Wang Y et al 2016 Chin. Phys. Lett. 33 028101 Abstract The synthesis of diamond single crystal in the Fe$_{64}$Ni$_{36}$-C system with h-BN additive is investigated at pressure 6.5 GPa and temperature range of 1300–1400$^{\circ}\!$C. The color of the obtained diamond crystals translates from yellow to dark green with increasing the h-BN addition. Fourier-transform infrared (FTIR) results indicate that $sp^{2}$ hybridization B-N-B and B-N structures generate when the additive content reaches a certain value in the system. The two peaks are located at 745 and 1425 cm$^{-1}$, respectively. Furthermore, the FTIR characteristic peak resulting from nitrogen pairs is noticed and it tends to vanish when the h-BN addition reaches 1.1 wt%. Furthermore, Raman peak of the synthesized diamond shifts down to a lower wavenumber with increasing the h-BN addition content in the synthesis system. DOI:10.1088/0256-307X/33/2/028101 PACS:81.05.uj, 81.15.Gh, 52.80.Pi, 47.11.Fg, 47.15.Cb © 2016 Chinese Physics Society Article Text The synthesis of novel materials with exceptional properties was the main challenge of material science. Diamond was well known for its excellent properties, such as its hardness, high thermal conductivity, high electron and hole mobility, high breakdown field strength and large band gap (5.5 eV).[1-10] However, it was not suitable for processing iron materials, due to the fact that C reacted with iron materials to generate iron carbide compounds. Recently, much research was devoted to the preparation of boron carbon nitrogen (BCN) composite material. Komatsu et al. successfully synthesized BC$_{2.5}$N material and the result indicated that the heat resistance of cubic phase BC$_{2.5}$N was superior to that of diamond.[11] Watanabe et al. reported that the band gap of the synthetic BC$_{2}$N by chemical vapor deposition (CVD) was about 0.03 eV.[12] Furthermore, it was theoretically investigated that it was possible to generate -N-B-N- structures (donor-acceptor-donor mode) when B and N were simultaneously introduced into the diamond lattices, which could endow n-type semiconductor properties to diamond crystal.[13] As we all know, cubic phase BN (c-BN) could be synthesized under high pressure and high temperature (HPHT) conditions, which could be used for processing iron material due to its characteristics. Its hardness (46–66 GPa) was only inferior to that of diamond ($\sim$96 GPa). Tian et al. prepared c-BN compounds with nano polycrystalline structure, of which hardness was about twice that of diamond.[14,15] Theoretically, BC$_x$N solids with higher carbon contents were more stable and had higher elastic moduli, making them more attractive as potential superhard materials.[16] Thus it was desirable to synthesize the new materials for combining h-BN and diamond at HPHT. In this Letter, we investigate the synthesis of diamond single crystal with h-BN additive under HPHT conditions and the B–C bond structure in the obtained crystals. It is expected that new diamond material could be prepared. Diamond crystallization experiments were carried out on a China-type large volume cubic high-pressure apparatus (CHPA) (SPD-6 $\times$ 1200) by temperature gradient growth (TGG). The temperature was calibrated by using a Pt6%Rh-Pt30%Rh thermocouple and the pressure was measured at room temperature by the change in resistance of standard substances and at high temperature by the graphite-diamond equilibrium. The (111) faces of high-quality seed crystals were selected as growth facets. High-purity graphite powder (99.9% purity) was employed as the carbon source. The h-BN powders with 99.9% in purity and size of 1–2 μm were selected as the additive in the experiments, which were clamped between the catalyst sheets. After the synthesized experiments, crystallization samples were dissolved in hot acids mixed with H$_{2}$SO$_{4}$ and HNO$_{3}$ to remove the graphite and metal catalyst, which remained on the crystal surfaces. Then the infrared spectra of the obtained diamond single crystals were measured by a Perkin–Elmer 2000 Fourier-transform infrared (FTIR) spectrometer in a spectral range of 400–4000 cm$^{-1}$ with a spectral resolution of 2 cm$^{-1}$ in transmittance mode at room temperature. Furthermore, the typical diamond crystals were characterized by an x-ray photoelectron spectroscopy (XPS) and Raman measurements. Experiments on diamond crystallization in Fe$_{64}$Ni$_{36}$-C system were carried out at 6.5 GPa in the temperature range of 1300–1400$^{\circ}\!$C with h-BN powders addition. The experimental parameters and results are listed in Table 1. According to TGG, the carbon source (high-purity graphite) was placed at the high temperature region and the seed crystal size about 0.6 mm was fixed at low temperature, respectively. The employed Fe$_{64}$Ni$_{36}$ catalyst with 2 mm thickness was established between the carbon source and the seed crystal to generate the temperature gradient. The dissolved C element in the molten catalyst would crystallize on the seed surface to realize diamond epitaxial growth.

**Table 1.** Experimental parameters and results of diamond growth synthesized in the Fe$_{64}$Ni$_{36}$-C system with h-BN powders added at 6.5 GPa.
Run	h-BN (wt%)	Temperature ($^{\circ}\!$C)	Time (h)	Color
a	0	1300	10	Yellow
b	0.4	1350	10	Light green
c	0.8	1380	10	Green
d	1.1	1400	10	Dark green

Fig. 1. Optical morphology of the synthesized diamond.

As shown in Fig. 1, the synthesized four samples exhibit dominating (111) surfaces and minor or disappeared (100) surfaces, consistently. Diamond crystal (Fig. 1(a)) obtained from the Fe$_{64}$Ni$_{36}$-C system without h-BN additive displayed typical yellow due to the involvement of nitrogen impurities, which is inevitable in the synthesis cell. The synthesized diamond crystal with 0.4 wt% (a weight ratio of h-BN additive to Fe$_{64}$Ni$_{36}$) h-BN additive exhibits light green and the crystal is transparent. The crystal (Fig. 1(c)) obtained from the synthesis system with 0.8 wt% h-BN additive shows a green color and the surfaces are rough. The prepared crystal appears dark green and nearly opaque when the addition of h-BN powders reaches 1.1 wt%. Obviously, the color of the obtained crystals is gradually deepened when the addition content of h-BN is increased in the synthesis system.

Fig. 2. FTIR spectra recorded for the synthesized diamond crystals.

To investigate the chemical bonding structure in more detail, Fourier transform infrared spectroscopy is employed. The method is well known to be a powerful nondestructive technique for analyzing impurities in diamond and for investigating chemical bonding structures.[17-19] It is well known that the FTIR absorption spectra of defects caused by nitrogen impurity are located in the phonon region of 900–1400 cm$^{-1}$. Recorded in Fig. 2(a), the incorporated nitrogen impurity in the form of single substitutional atoms (C-centers) is corresponding to the peaks at 1130 and 1344 cm$^{-1}$, which is attributed to typical Ib type synthetic diamond. Furthermore, it is seen from Fig. 2(b) that the characteristic absorption of nitrogen atoms in pairs (A-centers) appears and is located at 1282 cm$^{-1}$ in addition to 1130 and 1344 cm$^{-1}$. The formation of A-center nitrogen probably results in the supersaturation of nitrogen atoms in the synthesis cell due to the decomposition of h-BN additive.[4] To reduce the error, five points on the dominating surface of every crystal are selected and the nitrogen concentration is averaged for the corresponding crystal. The nitrogen concentration of the two crystals is valued according to the absorption coefficient of FTIR spectra, determined by[20,21] $$\begin{align} &{\rm C-centers~nitrogen~concentration~(ppm)}\\ =\,&\mu(1130\,{\rm cm}^{-1})/\mu(2120\,{\rm cm}^{-1})\times5.5\times25,\\ &{\rm A-centers~nitrogen~concentration~(ppm)}\\ =\,&\mu(1282\,{\rm cm}^{-1})/\mu(2120\,{\rm cm}^{-1})\times5.5\times16.5, \end{align} $$ where $\mu$(1130 cm$^{-1}$), $\mu$(1282 cm$^{-1}$) and $\mu$(2120 cm$^{-1}$) are absorption intensities and the dip at 2120 cm$^{-1}$, respectively. The calculation results of diamonds (a) and (b) are 320 and 850 ppm, respectively. Shown in Fig. 3, the peaks 1130 and 1344 cm$^{-1}$ are noticed in curves (c) and (d), indicating that C-center nitrogen exists in the samples. The presence of 745 and 1425 cm$^{-1[22,23]}$ when the h-BN addition content reaches 0.8 wt% is interesting, which results from $sp^{2}$ hybridization B-N-B and B-N vibration, respectively. However, the structures are not noticed in diamond crystals obtained in the synthesis system with boron addition.[24,25] Furthermore, the two peaks are not found in Fig. 2, proving that B-N-B and B-N structures are nonexistent in the corresponding crystals. The absorption intensities of the two peaks are enhanced with increasing the h-BN addition. However, the absorption at 1282 cm$^{-1}$ appears in curve (c) and it is absent in curve (d). Also, the nitrogen concentration of curves (c) and (d) are calculated and the values are 1250 and 230 ppm, respectively. Regrettably, up to now, we have no effective approach to calculate the concentrations of B-N-B and B-N in the doping diamond. It is possible that B-N-B and B-N structures in diamond begin to generate when h-BN addition reaches a certain amount in the synthesis system. Furthermore, the supersaturated nitrogen element tends to bond with B element and entered into diamond in the form of elementide. Hence, 1282 cm$^{-1}$ is absent in curve (d). The $sp^{2}$-bonded B-N-B and B-N are thermodynamically favored and hindered the formation of nitrogen pairs.

Fig. 3. FTIR spectra recorded for the synthesized diamond crystals.

Fig. 4. Raman shift of diamond with h-BN additive.

It is well known that the Raman peak could be affected by the concentration, existing forms and distribution of impurities in materials. Shown in Fig. 4, the Raman peak of the obtained diamond with 0.4 wt% h-BN additive located at 1332.42 cm$^{-1}$. Furthermore, the Raman peaks shift down to 1331.74 and 1331.16 cm$^{-1}$ when the addition content of h-BN is increased in the synthesis system.

Fig. 5. XPS spectrum of the synthesized diamond for B 1$s$ core level.

To analyze the B–C bonding state of the diamond synthesized from the Fe$_{64}$Ni$_{36}$-C system with h-BN additive, XPS spectrum is recorded in Fig. 5. The deconvolution of B 1$s$ spectra shows three peaks centered at 188.5, 190.1 and 192.1 eV, respectively. According to the previous report, B 1$s$ spectra in B$_{4}$C and BC$_{3.4}$ have peaks at 188.4 and 189.4 eV, respectively.[26] Hence, the resolved peak of 188.5 eV could be contributed to by B–C bonding. Furthermore, the peak at 190.1 eV is attributed to the B 1$s$ in h-BN. Thus the peak at 190.1 eV should be the B–N bond. Based on the previous finding, the peak energy of 192.1 eV is due to the B–O bond.[27] In summary, diamond single crystals are successfully synthesized in the Fe$_{64}$Ni$_{36}$-C system with h-BN additive, and are investigated at 6.5 GPa in the temperature range of 1300–1400$^{\circ}\!$C. The color of the obtained diamond crystals translates from yellow to dark green with increasing the h-BN addition. The structures of $sp^{2}$ hybridization B-N-B and B-N structures generate in diamond crystal when h-BN additive content reaches a certain value in the system. Furthermore, h-BN addition would result in the diamond Raman peak moving to lower wavenumber. References Nano-Crystalline Diamond Films Grown by Radio-Frequency Inductively Coupled Plasma Jet Enhanced Chemical Vapor DepositionReaction Mechanism of Al and N in Diamond Growth from a FeNiCo-C SystemElectrical properties of diamond single crystals co-doped with hydrogen and boronThe characteristics of diamond crystallized from carbonyl nickel powders at high pressure and high temperature conditionsEvolution of nitrogen structure in N-doped diamond crystal after high pressure and high temperature annealing treatmentEffects of FeNi-phosphorus-carbon system on crystal growth of diamond under high pressure and high temperature conditionsSynthesis and characterization of a shock-synthesized cubic B?C?N solid solution of composition BC2.5NBonding characterization of BC2N thin filmsCodoping method for the fabrication of low-resistivity wide band-gap semiconductors in p-type GaN, p-type AlN and n-type diamond: prediction versus experimentUltrahard nanotwinned cubic boron nitrideNanotwinned diamond with unprecedented hardness and stability Structural stability, mechanical and electronic properties of cubic BC _x N crystals within a random solid solution model Quantitative analysis of hydrogen in chemical vapor deposited diamond filmsThe relationship between infrared absorption and the A defect concentration in diamondSynthesis of diamond with the highest nitrogen concentrationThe control of BN and BC bonds in BCN films synthesized using pulsed laser depositionSynthesis and Characterization of New “BCN” Diamond under High Pressure and High Temperature ConditionsEffects of the additive boron on diamond crystals synthesized in the system of Fe-based alloy and carbon at HPHTEffect of additive boron on type-Ib gem diamond single crystals synthesized under HPHTStructural and electrical characterization of BC2N thin filmsStructural and chemical characterization of BN thin films deposited onto Si(100) and graphite substrates by pulsed laser deposition

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