Preparation and luminescent properties of SiC nanoparticles by strong pulse discharge in organic liquid phase environment

SiC nanoparticles with relatively uniform particle distribution were successfully prepared using a continuous strong pulse discharge method using hexamethyldisilane as the organic liquid phase environment. The samples were subjected to x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Raman spectroscopy (Raman) tests. XRD and Raman data indicate that the particle composition is mainly composed of β-SiC. The electron microscope image illustrates particle sizes ranging from 5–20 nm and existing in an agglomerated state. Further photoluminescence (PL) results indicate that the annealing temperature under vacuum conditions has a significant positive effect on the luminescence intensity.


Introduction
SiC is a high-hardness material, second only to diamond and boron carbide.SiC nano powder can be used directly as an abrasive, or as a main component or auxiliary additive to improve the hardness and strength of alloy or ceramic matrix, forming particle-reinforced composites [1][2][3][4][5].Especially when combined with aluminum alloy to form composite materials [2,6,7], there are broad application prospects in the fields of aerospace, automotive, shipbuilding, and construction.SiC is also the third-generation wide-bandgap semiconductor material developed after the first-generation elemental semiconductor material (Si) and the second-generation compound semiconductor material (GaAs, GaP, InP, etc).Due to its high critical breakdown field, high thermal conductivity, high carrier saturation concentration, and mobility, etc, it has enormous potential applications in high-temperature, high-frequency, high-power, optoelectronic, and radiationresistant devices [8][9][10][11][12].
In recent years, research on SiC in the field of photoluminescence has been increasing [13][14][15][16][17][18][19][20][21][22].Researchers have successively discovered photoluminescence peaks of SiC nanostructures in the 300-450 nm wavelength range and conducted related mechanism studies.In order to achieve large-scale production and application in photoluminescence materials, selecting simple, fast, and continuous production methods is attractive and necessary.Various preparation methods, including the sol-gel method, electrospinning, and carbothermic reduction, have been utilized to synthesize SiC nanostructures [18,[23][24][25][26][27].However, simplifying the process and achieving continuous production remains a challenge.The electrospinning process is relatively simple and allows for continuous production.The resulting SiC structures are uniform, but the particle sizes are relatively large, typically in the micrometer or submicrometer range [23,24].The synthesis steps of sol-gel method and carbon thermal reduction method are relatively complex, although the particle sizes are relatively small, but still above 50 nm [18,[25][26][27].The arc plasma method is a newly developed preparation method [17,20].This method can achieve continuous production and produce particles with smaller size, but it often requires the assistance of inert gas, which limits the amount of precursor entering and increases the production cost.
We use the method of continuous strong pulse discharge to efficiently produce SiC nanoparticles for a long time, with small particle size, narrow size distribution, and easy to popularize.Furthermore, we further studied the photoluminescence characteristics of the samples after vacuum annealing.The results show that the SiC sample has a photoluminescence peak centered at 400 nm, and vacuum annealing studies on this peak indicate its association with defects.Vacuum annealing of SiC samples within a certain temperature range will contribute to the enhancement of the 400 nm emission peak, providing a reference for their application in the field of ultraviolet detectors [28][29][30].

Method
The continuous strong pulse discharge device in the experiment is designed and manufactured based on the breakdown discharge principle of the dielectric under high voltage.It mainly consists of high-voltage DC(direct current) power supply, discharge capacitor, plasma switch, electrical control system, and discharge chamber as shown in figure 1(a).The voltage of the high-voltage DC power supply can be adjusted from 0 to 40kV.The discharge capacitor is composed of multiple capacitors in parallel and can be increased or decreased according to the situation.The plasma switch provides pulse electrical energy by generating plasma discharge in the air to form a conducting circuit.The electrical control system can conveniently adjust the discharge voltage and determine the pulse discharge period.The discharge chamber is a core component of the liquid phase pulse discharge device, consisting of a thick-walled cylindrical electrical insulation, a polytetrafluoroethylene inner barrel, and upper and lower metal electrodes in a sealed manner, as illustrated in figure 1(b).The outer diameter of the discharge chamber is 170 mm, the inner diameter is 60 mm, and the height is 100 mm.The polytetrafluoroethylene inner barrel is set to prevent sample contamination, with an outer diameter of 60 mm, inner diameter of 40 mm, and height of 100 mm.The spacing between the upper and lower electrodes can be adjusted as needed (generally 3-5 mm).During discharge, the upper electrode is buried below or close to the liquid surface of the liquid raw material.During discharge, strong pulse currents are generated between the upper and lower electrodes in the reaction liquid.
Experimental process: Approximately 40-50 ml of organic liquid-hexamethyldisilane is added to the discharge chamber, which is then sealed by the top cover equipped with the upper electrode.Connect the upper and lower electrodes to the circuit and start the circuit to charge the capacitor.During operation, the voltage of the high-voltage DC power supply is adjusted to a certain value (3-10kV) according to the experimental requirements, the charging switch is turned on, and the capacitor is charged until the voltage reaches the predetermined value, forming a pulse discharge in the liquid precursor.The organic liquid decomposes and rapidly forms the solid precipitates.The entire discharge process is carried out at room temperature and pressure, without the need for any additional protective or reducing gases.Other organic liquids containing Si and C can also be used as reactants, such as other silane liquids or dimethyl silicone oil.However, considering environmental pollution issues, it is advisable to avoid using reactants that produce toxic and harmful gases or liquids after the decomposition of organic liquids.Depending on the circumstances, continuous periodic discharge can be performed, or manual control can be carried out.The interval between two consecutive discharges in continuous discharge is determined by the charging and discharging cycle of the capacitor, generally ranging from 1-2 s.After the synthesis is completed, the charging switch is turned off, and the capacitor is short-circuited using the discharge rod to prevent electric shock risk.Then, the control power supply is turned off, the discharge chamber is removed, and the sample is taken out.The sample is subjected to alcohol ultrasonic cleaning 2-3 times to remove organic solvents, distilled water washing 2-3 times, centrifugation, and drying to obtain black powder for collection and further use.
The crystal structure was characterized using a standard x-ray powder diffractometer (XRD, Rigaku D/max-rA) with CuKα radiation, while the morphology and microstructure were examined using field emission scanning electron microscopy (FESEM, JSM-6700F) and transmission electron microscopy (TEM, Hitachi-H8100).Raman spectra were acquired using a Renishaw-Invia FT-RAMAN spectrometer with Ar + laser excitation at a 325 nm wavelength.

Results and discussion
Figure 2 shows the XRD pattern of the sample, indicating the XRD peaks of the sample before and after purification treatment.The treatment process involved annealing the obtained black powder sample at 600 °C in air for 30 min to oxidize the Si impurities into SiO 2 , followed by HF acid treatment to remove the SiO 2 from the sample.Subsequently, the sample was washed with water and ethanol three times each to obtain a pure sample.From the figure, it can be seen that the diffraction peaks in pattern a match the data of JCPDS Card No.75-0254 (β-SiC), JCPDS Card No.80-0018 (Si), and JCPDS Card No.75-1541 (6H-SiC), indicating that the main component of the sample is β-SiC, with a small amount of Si and trace amounts of 6H-SiC.Spectrum line b shows that the diffraction peak intensity of the β-SiC particles significantly increased after treatment, the curve is smoother, and the purity is significantly improved compared to before treatment.Furthermore, the diffraction peak of silicon impurities in the sample has completely disappeared after treatment, indicating complete removal, and no significant SiO 2 diffraction peak is observed.
Afterwards, we characterized the sample using FESEM and TEM.The SEM image of the sample (figure 3(a)) indicates that the particle size is in the range of 5-20 nm, with a relatively uniform size distribution and a spherical morphology, but with significant particle aggregation.The TEM image in figure 3(b) shows that the particles in the sample are aggregated together, which is related to the presence of numerous dangling bonds on the surface of the nanomaterial, making it easy to form weak binding bonds.The particle outlines in the grey area are more distinct, while the particles in the dark area are covered by a layer of black material.The three diffraction rings obtained from electronic diffraction correspond to the (111), (220), and (311) crystal planes of β-SiC.It can be observed from the shape of the diffraction rings that β-SiC is in a polycrystalline state.In order to further determine the composition of the substance, the sample was subjected to Raman spectroscopy testing.The Raman spectrum of the sample (figure 4) shows the presence of transverse optical phonon of β-SiC (783.3cm−1 ), longitudinal optical phonon mode (955.2cm−1 ) and signal of elemental silicon (514.8cm−1 ), with all three Raman peaks having weak intensities.Comparatively more prominent is the broad band related to graphite, from which two graphite Raman peaks can be distinguished, with their central positions located around 1390cm −1 and 1580cm −1 respectively.This indicates that in the β-SiC particles, there are also small amounts of silicon and amorphous carbon embedded (which were not detected in the XRD spectrum of the sample in previous figure 2(b), suggesting low content).Since Raman detection probes a relatively shallow area on the particle surface, and the prepared nano SiC appears black, it is believed that the detected silicon and amorphous carbon only exist near the surface of the SiC particles, with a higher content of amorphous carbon on the surface.This may be due to the high heat generated by the strong pulse discharge during the sample preparation process, causing the precursor to thermally decompose, resulting in the formation of SiC accompanied by silicon and amorphous carbon particles.As the SiC particles form, these impurity micro-powders are adsorbed near their surface due to the adsorption effect of dangling bonds on the surface of the SiC nano particles.
We investigated the influence of pulse discharge times on the size of generated β-SiC particles.XRD tests were conducted on nano SiC particle samples prepared with different discharge times (20, 40, 60 times), and the results are shown in figure 5.The average particle sizes calculated using the Scherrer formula were 7.5 nm (20 times), 7.7 nm (40 times), and 7.6 nm (60 times), indicating that the discharge times did not have a significant impact on the particle size.According to the XRD spectrum, the particle size calculated by the Scheler formula was smaller than that obtained from SEM and TEM results.This is mainly due to the influence of other factors such as lattice distortion or sub-grains caused by defects on the broadening of the XRD peak [32][33][34].Conclusion on the influence of pulse discharge times suggested that multiple pulse discharges did not have a clear effect on the initial formation of SiC particle size.Therefore, it is believed that when using this method for discharge experiments, the cyclic discharge mode will not affect the particle size, enabling continuous production and increasing product yield.
We further studied the photoluminescence characteristics of SiC nanoparticles.The results indicate that the photoluminescence signal of the original sample is extremely weak.Only one photoluminescence peak at 400 nm was observed in the emission spectrum.Pang et al [21] reported the discovery of a luminescence peak at 410 nm in the PL spectrum, while Shen et al [27] reported the discovery of a luminescence peak at 401 nm.They attributed the blue emission to the presence of quantum confinement effects and microstructural defects in SiC nanostructures.But the quantum confinement effect only occurs when the particle size is below 10 nanometers or even smaller [13,15].The nanowires synthesized by Shen et al have a size range of 40-120 nm.The authors of previous reports [13,14] believed that the SiC nanowires/cables were separated into segments of several nanometers by the defects, thus satisfying quantum confinement effect.Generally, blue-shifted peak positions resulting from uantum confinement effect typically occur around 450 nm or at longer wavelengths [15,20,22].Therefore, we tend to believe that the PL peak of 400 nm is caused by microstructural defects.
To further validate, we conducted vacuum annealing treatment on the samples.The samples were then annealed at temperatures of 800 °C, 1000 °C, 1200 °C, and 1600 °C, with a heating rate of 10 °C min −1 , maintained at the set temperature for 1 h, and then naturally cooled to room temperature.The intensity of this peak increased with the annealing temperature, with the photoluminescence peak intensity of the sample annealed at 1000 °C being approximately twice that of the sample annealed at 800 °C, and the peak intensity of  the sample annealed at 1200 °C being three times that of the sample annealed at 1000 °C.The peak intensity variation in the photoluminescence spectrum in figure 6 shows that the growth rate of the peak intensity is much higher in the 1000 °C-1200 °C range than in the 800 °C-1000 °C range, and it significantly decreases when the temperature exceeds 1200 °C.Although the intensity of the 400 nm peak did not reach its maximum at 1600 °C (the highest safe temperature of the furnace), the trend of the peak intensity change suggests that the peak's maximum value should appear at a temperature slightly higher than 1600 °C.Additionally, it was observed that there was no significant peak shift in the 400 nm emission peak under vacuum conditions.
The intensity variation of the emission peak at 400 nm with annealing temperature can be explained as follows: under vacuum conditions, as the temperature rises, the rich carbon region near the surface of β−SiC nanoparticles transforms towards the direction of carbon crystallization, tending to form more small-sized nanocrystals, thereby creating a larger number of phase interfaces and leading to more defect centers, resulting in the enhancement of the emission peak.When the temperature exceeds 1200 °C, the growth rate of the emission peak intensity decreases.This is partly due to the reduction in the formation rate of small-sized nanocrystals inside the SiC nanoparticles with increasing temperature, and partly due to the crystallization of the SiC nanoparticles, resulting in the annihilation of some defects, which contributes to a slowdown in the intensity enhancement.Based on this, it is believed that a further increase in temperature will lead to a stabilization of the number of small-sized nanocrystals inside the SiC nanoparticles and the disappearance of a large number of defects, thereby causing a rapid decrease in intensity.

Conclusion
Nano-SiC particles with particle sizes ranging from 5-20 nm were successfully prepared using the method of continuous discharge in an organic liquid phase environment.XRD data show that the components of the particles are mainly β-SiC, while Raman data further indicate the presence of a very small amount of Si and amorphous carbon.The study of the effect of discharge frequency on the formation of β-SiC particle size shows minimal impact, making it suitable for continuous production.The photoluminescence results indicate that β-SiC exhibits an emission peak at 400 nm in the 350-450 nm spectral range.Annealing under vacuum conditions, the photoluminescence peak intensity of SiC nanoparticles significantly increases with annealing temperature, and this conclusion has reference value for the research of SiC in the field of luminescence of small-sized particles.

Figure 1 .
Figure 1.(a) schematic diagram of the continuous strong pulse discharge device, (b) discharge chamber.

Figure 3 .
Figure 3. Electron microscope images of nano SiC particles: a. Scanning electron microscope, b.Transmission electron microscope.

Figure 5 .
Figure 5. XRD spectra of nano SiC particles synthesized with different discharge times.