Rapid Synthesis of Sub-5 nm Sized Cubic Boron Nitride Nanocrystals

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Rapid Synthesis of Sub-5-nm-sized Cubic Boron Nitride Nanocrystals with High-piezoelectric Behavior via Electrochemical Shock Zhi-Gang Chen, Lian-Hui Li, Shan Cong, Jin-Nan Xuan, Dengsong Zhang, Fengxia Geng, Ting Zhang, and Zhigang Zhao Nano Lett., Just Accepted Manuscript • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Rapid Synthesis of Sub-5-nm-sized Cubic Boron Nitride Nanocrystals with High-piezoelectric Behavior via Electrochemical Shock

Zhigang Chen,†,§ Lianhui Li,†, § Shan Cong,† Jinnan Xuan,‡ Dengsong Zhang,§ Fengxia Geng,*‡ Ting Zhang,*† Zhigang Zhao*†

†

Key Lab of Nanodevices and Applications,

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China ‡

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China. §

Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, China.

Abstract: A key challenge in current superhard materials research is the design of novel superhard nanocrystals (NCs) whereby new and unexpected properties may be predicted. Cubic boron nitride (c-BN) is a superhard material which ranks next to diamond, however, downsizing c-BN material below the 10 nm scale is rather challenging, and the interesting new properties of c-BN NCs remain unexplored and wide-open. Herein we report an electrochemical shock method to prepare uniform c-BN NCs with a lateral size of only 3.4± 0.6 nm and a thickness of only 0.74±0.3 nm at ambient temperature and pressure. The fabrication process is simple and fast, with c-BN NCs produced in just a few minutes. Most interestingly, the NCs exhibit excellent piezoelectric performance with a large recordable piezoelectric coefficient of 25.7 pC/N, which is almost 6 times larger than that from bulk c-BN and even competitive to conventional piezoelectric materials. The phenomenon of enhancement in the piezoelectric properties of BN NCs might arise from the nanoscale

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surface-effect and nanoscale shape-effect of BN NCs. This work paves an interesting route for exploring new properties of superhard NCs. KEYWORDS: Cubic boron nitride, electrochemical shock, nanocrystals, piezoelectricity


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Cubic boron nitride (c-BN), featured by its second-highest hardness of any material, has been used in a wide range of applications, such as high-pressure devices, high-temperature electronics, super-abrasives and high-precision cutting tools, etc.1-3 Downsizing c-BN material below the 10 nm scale is rather meaningful and worthwhile, since its numerous physicochemical properties such as hardness, abrasion resistance, and damage resistance have been proven to increase dramatically with the reduced particle size.4 For example, c-BN nanocrystals (NCs) with sizes of 100 nm have Vickers hardness Hv ~56.5 GPa, while for c-BN nanocrystals with sizes of 3.4 nm this value is up to ~149.3 GPa.4 However, the synthesis of c-BN NCs smaller than 10 nm remains the least-explored and extremely challenging. Usually, c-BN is obtained from the conversion of graphite-like or amorphous BN precursor to c-BN at high temperature (1300-2000 K) and high pressure (several tens GPa).4 Under such fierce synthetic conditions, it is extremely difficult to obtain c-BN NCs under 10 nm due to the rapid growth of nuclei. Recently, Tang et al. reported one pioneering work which demonstrated a pulsed laser ablation method employing dioxane solution of ammonia borane as liquid target to synthesize c-BN NCs with sizes less than 10 nm. During this fabrication process, the dioxane solution of ammonia borane was irradiated under ambient conditions with a nanosecond-long pulsed laser with a wavelength of 1064 nm for 10 min.5 Clearly, such progress provides a feasible route to obtain small c-BN particles, however, there are some unsolved problems, such as special precursor materials is needed, the fabrication process is not easily optimized and scaled up, and the products are not pure. Therefore, much more efforts are needed to reduce the size of c-BN NCs to below 10 nm. On the other hand, the piezoelectricity of c-BN has recently received high attention from both theoretical and experimental scientists.6,7 Piezoelectricity is the ability of certain materials to generate electric charges in response to applied mechanical stress. c-BN is not only piezoelectric, but also of the highest acoustic velocity, highest Young’s modulus and


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highest thermal conductivity among all piezoelectric materials, which has triggered its important use in high-frequency piezoelectric and surface acoustic wave devices. However, currently its piezoelectric property is not satisfactory, and the reported piezoresponse for bulk c-BN is only 3.3 pC/N,6,8 which is much lower than those of conventional piezoelectric materials such as ZnO, poly (vinylidene fluoride)(PVDF) and BaTiO3 (Table S1). Thus the piezoelectric behavior of c-BN definitely needs to be dramatically improved. Recent theories and experiments have discovered that the size effect promises to improve the piezoelectric properties of conventional piezoelectric materials such as PbTiO3, BaTiO3, KNbO3.9-13 For example, Hoshina et al. have reported that in the size effect of BaTiO3 fine particles, the maximum of dielectric permittivity or piezoelectric coefficient is observed at an optimal particle size of about 140 nm, which can be explained by a composite structure model including a gradient lattice strain layer (GLSL) having a very high permittivity.12,14 Wang et al. have also reported that the piezoelectric coefficient of BaTiO3 significantly increases with decreasing domain size.11 Herein, we develop a simple and fast electrochemical shock method to prepare uniform c-BN NCs with an average size of only 3.4 nm at ambient temperature and pressure, which is one of the smallest sizes ever reported. As a new type of piezoelectric material, the as-prepared ultrafine c-BN NCs exhibit a remarkably strong piezoresponse with piezoelectric coefﬁcient (d33) up to 25.7 pC/N, which is almost 6 times larger than the reported value for bulk c-BN and even comparative to that for conventional piezoelectric materials (Table S1, Figure S10). In sharp contrast, hexagonal BN (h-BN) NCs synthesized by the same method possess a much lower piezoelectric coefficient of 1.47 pC/N. Our approach for the preparation of c-BN NCs is based on the electrochemical shock of electrode materials. Electrochemical shock is usually described as a phenomenon in batteries in which the electrochemical cycling can cause mechanical fracture and structural disintergration of the electrode material originating from several distinct physical mechanisms


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including

concentration-gradient

stresses,

two-phase-coherency

stresses,

and

intergranular-compatibility stresses.15 The phenomenon is definitely detrimental to the performance of batteries, and should be averted in batteries. However, we believe this phenomenon can be used to disintegrate bulk crystals into ultrafine c-BN NCs, since it has been reported that severe electrochemical shock can also occur in cubic phase compounds due to coherency stresses in first-order cubic-to-cubic phase transformations.16 Inspired by this thought, the following steps are taken in our approach (Figure 1). Firstly, the pristine bulk c-BN material is coated onto a copper foil by mixing acetylene black and poly(vinylidene fluoride) (PVDF), and subjected to undergo Al3+ ion bombardment using galvanostatic discharge technique. Compared with Li+ ions, Al3+ has thrice the charge of Li+, but has a smaller radius, so the charge density of Al3+ is greater than that of Li+. As such, once trivalent Al3+ are employed as bombardment ions, much stronger electrostatic forces generated in the host lattice of bulk materials can be expected compared with the commonly used monovalent ions, which may cause the distortion and charge redistribution of the intra-plane covalent bonds and ready to break down the crystalline lattice of c-BN bulk materials. In a second step, the discharged samples are immersed in oleic acid/ethanol solution and subsequently sonicated for 5-10 minutes to be removed from copper foil. Oleic acid here used may play dual roles. First, oleic acid has good affinity for Al3+ and helps to extract Al3+ from c-BN material, which may be favorable to facilitate the efficient formation of small NCs.17 Second, it can also play a role as a stabilizing agent to modify the surface of these NCs and avoid agglomeration to bigger particles, endowing them with good dispersity in organic solvents. Finally, the suspension is centrifuged and washed with ethanol, and well-dispersed c-BN NCs are thus obtained. As can be seen, the fabrication process is simple and fast, with c-BN NCs produced in just a few minutes.


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Figure 1. a) Schematic representation of electrochemical shock process for the fabrication of ultrafine c-BN NCs from the bulk materials. b) The formation mechanism of c-BN NCs synthesized via electrochemical shock. Ⅰ,Ⅱ) Al3+ ions with high charge density are first bombarded into the lattice of bulk c-BN, causing serious cracking in the stacking plane of c-BN. The unit cell of c-BN after electrochemical shock is inserted in (Ⅱ). Ⅲ) Oleic acid helps to extract Al3+ ions from c-BN lattice, which disintegrate bulk c-BN crystals into ultrafine c-BN NCs. Ⅳ) Oleic acid plays a role as a stabilizing agent to modify the surface of these NCs, endowing them good dispersity in organic solvents. The quickly-prepared c-BN NCs can be well dispersed in ethanol or N-methylpyrrolidone (NMP), showing a faint white color (insert in Figure 2a), which is stable without any noticeable aggregation at room temperature for more than 1 month, confirmed by observing a Tyndall effect in the form of a discernible red line that results from light scattering when a laser beam is passed through the solution. The phase purity and crystal structure of the


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as-prepared NCs is verified by X-ray diffraction (XRD) studies in comparison with bulk sample, as shown in Figure 2a. Three clear diffraction peaks at values of 43.54º, 50.92º and 75.10º can be observed in the XRD pattern of the pristine bulk powder, which correspond to the (111), (200), and (220) crystalline planes of c-BN phase (JCPDS No. 35-1365), respectively.18 However, the intensities of the observed diffraction peaks of the c-BN NCs are conspicuously decreased, indicating that the c-BN has been exfoliated and disintegrated during the reaction. Moreover, a clear shift of the distinct peak of (111) toward lower angle is observed for c-BN NCs, suggesting that the lattice of NCs is expanded with respect to that of bulk sample. The morphology of as-prepared c-BN NCs are characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As can be seen, large amounts of c-BN NCs are obtained (Figure 2b). These c-BN NCs have good crystallinity with an average size of 3.4±0.6 nm (Figure 2c). High-resolution TEM (HRTEM) images of c-BN NCs give two-dimensional lattice fringes of ~0.21 and 0.18 nm (Figure 2d), which can be ascribed to the (111) and (200) plane of c-BN, in good consistency with XRD results. AFM line analysis suggests that the height of c-BN NCs are 0.74±0.3 nm, which confirms these NCs are rather thin (Figure 2e, f), corresponding to ~3 B-N double layers in thickness.7,19 Figuratively speaking, our c-BN NCs are roughly the shape of a disk with extremely small three-dimensional sizes. The absorption spectrum provides information about the optical properties of c-BN NCs (Figure 2g). These NCs are highly transparent in the range of 300-900 nm but only have two strong absorptions in the ultraviolet window. The peak at 228 nm is probably due to the stacking faults in c-BN NCs (Figure S2b), while the peak around 270 nm is attributed to the impurity or defect-related bands, which is similar to that reported from Tang et al.5 Fourier-transform infrared (FTIR) spectroscopy gives information on the surface of c-BN NCs (Figure 2h). The c-BN NCs show a strong peak near 1089 cm-1, which is characteristic of sp3-bonded c-BN. Besides the strong peak, other significant peaks at 937,


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1285, 2854 and 2925 cm-1 can be observed, which can be assigned to of O-H stretch, C-O stretch, symmetric CH2 stretch and asymmetric CH2 stretch of oleic acid, respectively.20 However, it is worth noting that the most intense peak at 1710 cm-1 of oleic acid deriving from C=O stretch is absent in the FTIR spectrum of c-BN NCs. On the contrary, there appear two new peaks at 1379 and 1647 cm-1 instead, which are characteristic of the asymmetric νas(COO-) and symmetric νs(COO-) stretch. This result can be explained by the fact that oleic acid are chemisorbed onto the c-BN NCs through a covalent bond between the carboxylate head of oleic acid and boron atom. Considering the c-BN NCs are produced with the assistance of oleic acid, this result confirms the as-prepared NCs are assumed to be surface-wrapped by a layer of oleic acid molecule via covalent interaction.


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Figure 2. a) XRD patterns of bulk c-BN and c-BN NCs. Insert: the optical photo of the c-BN NCs suspension in ethanol showing the as-prepared c-BN NCs were uniform without noticeable aggregation. b) TEM image of c-BN NCs. c) Particle size distribution of c-BN NCs. d) HRTEM image of a c-BN NC and its corresponding fast Fourier transform diffraction. e) AFM image of c-BN NCs. f) Thickness distribution of c-BN NCs. g) UV-Vis absorbance spectrum of c-BN NCs. h) FTIR spectra of pure oleic acid (red line), bulk c-BN (black line) and c-BN NCs (blue line). Further spectroscopy works give clue to the subtle structural changes of c-BN NCs in the formation process of NCs. Figure 3a shows the Raman spectra of the three samples of c-BN including pristine bulk c-BN, Al3+-shock c-BN without oleic acid treatment and c-BN NCs with oleic acid treatment. In order to more clearly identify the differences between the spectra, the wavenumbers and the full-widths at half-maximum (FWHM) of the bands are given in Supporting Information (Table S2). The pristine bulk c-BN show two strong peaks at about 1045.8 and 1297.8 cm-1, which can be assigned to scattering by the transverse optical (TO) and longitudinal optical (LO) phonon modes of c-BN, respectively, in agreement with the characteristics of c-BN previously reported.4 However, some difference at wavenumbers and FWHM of the bands has been observed for Al3+-bombarded c-BN and c-BN NCs. For Al3+-bombarded c-BN, the two characteristic peaks are both transferred to low-frequency region by around 3.4 cm-1, which results from the crystallite size reduction by electrochemical shock.21 By contrast, the final NCs with oleic acid treatment are transferred to high-frequency region by around 7.8 and 5.2 cm-1, respectively. This phenomenon is possible due to the presence of oleic acid. As mentioned before, the electron-donating carboxylate group of oleic acid chemically bonds to c-BN NCs, and increase the electron-cloud density of B atom in B-O bond, which causes Raman peaks to shift to higher frequency.22,23 Moreover, the Raman spectra of Al3+-bombarded c-BN and c-BN NCs show a strong broadening and lowering of


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two characteristics peaks (Table S2), which is also be attributed to the effects of decreasing particle size on the force constants and vibrational amplitudes of the nearest neighbor bonds.24 The X-ray photoelectron spectroscopy (XPS) results give evidence of the change in the bonding characteristics of c-BN NCs (Figure 3b, S5). For pristine bulk c-BN, the Al element is not detected. In the case of bulk c-BN after soaking in Al(ClO4)3 electrolyte solution, Al element is observed in XPS analysis arising from the adsorption of electrolytes. In contrast, the Al2p core level spectrum of Al3+-shock c-BN without oleic acid treatment shows three peaks after deconvolution, which can be attributed to the bonding configurations of Al-ClO4, Al-N and Al-O, respectively,25 derived from the bombardment of c-BN with Al3+. For the final NCs with oleic acid treatment, it is found that the Al element is not detected again, confirming that Al3+ has been removed completely from c-BN NCs.

Figure 3. a) Raman spectra of bulk c-BN (black line), Al3+-bombarded c-BN without oleic acid treatment (red line) and c-BN NCs with oleic acid treatment (blue line). b) XPS spectra of Al2p core level peak for b1) Pristine bulk c-BN, b2) Bulk c-BN after soaking in Al(ClO4)3 electrolyte solution, b3) Al3+-shock c-BN without oleic acid treatment, b4) c-BN NCs with oleic acid treatment. Recently, dimensionality has proved to be one of the most momentous material parameters for piezoelectricity.26 The same material can exhibit dramatically different piezoelectric


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properties depending on whether it is arranged in one-dimensional (1D) and two-dimensional (2D) crystal structures. 1D nanostructure with larger length-to-diameter ratio can produce larger strain under the action of mechanical force, and atomically-thin 2D nanostructure can become non-centrosymmetric because of the absence of inversion symmetry, both of which lead to their enhanced piezoelectric properties compared to their bulk counterparts. However, to the best of our knowledge, zero-dimensional (0D) NCs have seldom been experimentally exploited to demonstrate piezoelectricity although the theory has predicted that piezoelectric effects are proportional to the size of nanoparticles (or the size of the strained region).27 Here we report the first experimental study of the piezoelectric properties of c-BN NCs and its application in piezoelectronic sensing. Figure 4a shows the schematic diagram for the fabrication of a c-BN NCs-based piezoelectric sensing device, which is composed of a PDMS/ITO substrate as the bottom electrode, a PDMS/ITO substrate as top electrode, and a c-BN NC film in between. More detailed information regarding the fabrication process can be found in Supporting Information. Once a vertical external force is applied to the top of the device, the stress results in the deformation of the unit cell of BN, and subsequently the positive- and negative-charge centers are displaced with respect to each other, which generates positive and negative piezopotentials at the top and bottom electrodes and induces the flow of electrons through external circuit. When releasing the force, the unit cell and dipole component will return to the initial state. The piezopotentials fade away and the electrons flow back, thus producing the reverse output signals, which is the basic working principle of our piezoelectric device.28 We use four samples including bulk c-BN, c-BN NCs, bulk h-BN and h-BN NCs synthesized by our method for piezoelectric measurements to find if there is significant difference between the piezoelectric properties of the bulk BN and their corresponding NCs. The ambient environment during the piezoelectric property test is as follows: temperature 23℃, RH 60%, respectively. Figure 4b shows the piezoelectric output


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voltages for two samples under the same stress of 30 N. It can be seen that the piezoelectric output voltages show maxima of 1, 2.1, 4.5 and 27.3 mV for pristine bulk c-BN, bulk h-BN (Figure S9a), h-BN NCs (Figure S9b) and c-BN NCs, respectively, clearly demonstrating the large piezoelectric response achievable only in c-BN NCs. The output voltage for c-BN NCs is almost 6 times than those produced by pristine bulk c-BN, 13 times for h-BN NCs and 27 times for bulk h-BN. Correspondingly, bulk c-BN, bulk h-BN and h-BN NCs yield very small current output of 0.52, 0.13 and 0.22 nA, respectively; in contrast, c-BN NCs produce much higher piezoelectric current up to 3.2 nA (Figure S9c). The output voltage change as a function of the applied external force from four samples is also investigated. The output performance is increased with the increasing the external force (Figure 4c), revealing typical piezoelectric power output behavior and good mechanical durability of our piezoelectric devices. Remarkably, c-BN NCs exhibit much higher piezoelectric response at various applied stresses (2-30N) (Figure 4d), compared with other samples. Piezoelectric coefficient (d33), which quantifies the volume change when a piezoelectric material is subject to an electric field, is one of important gauge parameters for piezoelectric performance. The d33 are calculated from the slope of the plot of the voltage against the force (Figure 4e), yielding 0.23, 1.47, 4.3 and 25.7 pC/N for pristine bulk h-BN, h-BN NCs, bulk c-BN and c-BN NCs. Encouragingly, the value for c-BN NCs is much larger than that for bulk BN materials, even comparative to that for conventional piezoelectric materials (Table S1). The stability test results of c-BN NCs are shown in Figure 4f. The generated output voltage is still stable even after 3200 cycles and only a small decrease of output voltage (about 2.5% after 3200 pulses) is observed (inert in Figure 4f).


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Figure 4. a) Schematic diagram for the fabrication of a BN-based piezoelectric device.Ⅰ,Ⅱ) Spin coating of PDMS layer on ITO glass, Ⅲ) Spray of c-BN NCs onto PDMS layer, Ⅳ, Ⅴ) Affixed to the top electrode. b) Output voltage for bulk c-BN and c-BN NCs under the same stress of 30N. c) Output voltage for bulk c-BN and c-BN NCs under different applied stress (2~30N). d) Column distribution of output voltages at different stress levels for bulk h-BN (white), h-BN NCs (black), bulk c-BN (blue), c-BN NCs (red), respectively. e) Linear fitting curves of output voltage vs applied stress. f) Stability test of voltage output from a c-BN NCs device. As shown above, BN NCs synthesized by our method, in either cubic or hexagonal phase, are indeed piezoelectric with the corresponding coefficient significantly greater than that of their bulk counterparts. The piezoelectric effect arises from the deformation of non-centrosymmetric unit in both h-BN and c-BN when stressed by external force, that is, a deformation changes bonding angles (B-N-B or N-B-N bond angle) in the crystal and changes the charge distribution giving a polarization (Figure 5a,d).29 The observation of largely enhanced piezoelectricity in NCs might arise from the nanoscale surface-effect of BN NCs. In NCs, the surface effect becomes much more significant because of the high surface to volume


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ratio, which has direct impact on the atomic polarizations for surface atoms resulting in an enhancement of the piezoelectric performance.30 Another important factor contributing to the enhanced piezoelectric performance of BN NCs may be due to the nanoscale shape effect of BN NCs, which is recently vividly advocated by Wang et al.31 Bulk h-BN exhibits negligible piezoelectric property, since it has centrosymmetric crystalline structure (Figure 5b). In contrast, h-BN NCs show piezoelectricity as a result of the transition from centrosymmetric to noncentrosymmetric in crystal structure when thinned down to few layers (Figure 5c). However, although both h-BN NCs and c-BN NCs are small-sized NCs, major differences in piezoelectric performance exist between two types of NCs, which is probably due to the following reason: For h-BN NCs, its crystal structure is a stacking of two-dimensional layers of hexagonally linked sp2 hybridized B-N bonds aligned in the z direction. In this case, piezoelectric polarization is only produced when strain is applied in the xy-plane direction normal to the z axis.31 In sharp contrast, c-BN structure starts from an eight-atom zinc-blende-structured cubic unit cell, and is intrinsically noncentrosymmetric. The strains in all three directions could generate piezoelectric polarization when subjected to mechanical deformation (Figure 5f), which causes larger piezoresponse in c-BN NCs. Finally, nanostructuring of c-BN usually improve its hardness by virtue of the Hall-Petch effect-the tendency for hardness to increase with decreasing grain size, giving rise to a large lattice stiffening in c-BN.32 However, as for piezoelectricity, enhancement in d33 is normally associated with lattice softening.33-35 The curious inconsistency likely arises from the subtle differences in various sample preparation processes, where different surface states of nanoparticles, size and shape could be engineered.


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Figure 5. a, d) Distortion of the unit cell of c-BN and h-BN under the compression of an external force, showing the bond angle change. For h-BN, the angle θ=0 corresponds to an orientation where a group of opposite atoms (N and B) lies along a mirror plane of the crystal. For c-BN, the angle θ corresponds to the N-B-N angle. Atomic structures of BN bulk materials and the corresponding exfoliated NCs. b) Bulk h-BN, c) h-BN NCs, e) Bulk c-BN, f) c-BN NCs. Black and blue spheres represent nitrogen and boron atoms, respectively. The green arrows indicate the direction of force applied to the sample, while the direction of piezoelectric polarization is labeled with orange arrows. In summary, we have demonstrated a novel, easy and fast electrochemical shock method to prepare ultrafine c-BN NCs with a lateral size of only 3.4±0.6 nm and a thickness of only 0.74±0.3 nm. The c-BN NCs show markedly improved piezoelectric coefficient of 25.7 pC/N, which is almost 6 times larger than that from bulk c-BN. Our electrochemical shock method might build a bridge between 0D nanocrystals and bulk materials, as well as their fascinating properties and important applications.


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■ASSOCIATED CONTENT Supporting Information: Experimental details; Galvanostatic discharge curves for h-BN and c-BN; HRTEM image of c-BN NCs, TEM images of c-BN sample after electrochemical shock without oleic acid treatment and the corresponding EDS analysis; XPS spectra of B1s and N1s core level peaks for Pristine bulk c-BN, Al3+-shock c-BN without oleic acid treatment, c-BN NCs with oleic acid treatment; Morphology characterization of pristine bulk c-BN and h-BN NCs; XRD patterns of bulk h-BN and its corresponding h-BN NCs; Schematic diagram of the external force; The output voltage for bulk h-BN and h-BN NCs under the same stress of 30 N; The output voltage for bulk h-BN and h-BN NCs under the same stress of 30 N; The piezoelectric data for commercial BaTiO3 nanoparticles; The effective piezoelectric coefficient (d33) from literature; The full-widths at half-maximum (FWHM) of the transverse optical (TO) and longitudinal optical (LO) photon Raman peaks for pristine bulk c-BN, Al3+-bombarded c-BN without oleic acid treatment and c-BN NCs with oleic acid treatment; Calculation about the piezoelectric coefficient.

■AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected], tzhang2009@sinano. ac.cn, [email protected] Notes The authors declare no competing financial interest.

Acknowledgements


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This work was supported by the National Natural Science Foundation of China (51372266, 51572286), the Outstanding Youth Fund of Jiangsu Province (BK20160011), the China Postdoctoral Science Funding (2015M571837).

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Rapid Synthesis of Sub-5 nm Sized Cubic Boron Nitride Nanocrystals

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