Surface charges and optical characteristic of colloidal cubic SiC nanocrystals
© Li et al; licensee Springer. 2011
Received: 3 March 2011
Accepted: 15 July 2011
Published: 15 July 2011
Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.
Keywordscubic SiC nanocrystals surface charges photoluminescence quantum confinement
Silicon carbide (SiC) is an important wide band gap semiconductor with superior properties, such as excellent thermal conductivity, high breakdown field strength, and excellent physical and chemical stability [1–4]. It has been found to have applications in many harsh conditions, including high temperature, high power, and high frequency  For instance, SiC has been commercially applied for optoelectronic devices, such as photodiodes and light-emitting diodes which emit throughout the visible spectrum into ultraviolet . Its peculiar electronic properties make it a promising material for next-generation electronic devices. It is known that the miniaturization of devices is an irresistible trend for both industrial manufacture and academic research. Furthermore, design and fabrication at the nanoscale would lead to development of the novel materials and devices with target properties . Recent advances in the preparation of SiC nanocrystals have paved the way for wider uses in microelectronic devices and biological labels . Over the past few years, there have been increasing reports on the synthesis of SiC nanocrystals due to their potential applications as nanoscale light emitters . The large band gaps (2.3, 3.0, and 3.2 eV for the main polytypes, 3C-, 6H-, and 4H-SiC) of SiC render the nanocrystals a good candidate as blue and ultraviolet light emitters  when quantum confinement takes place. Hence, they could be good candidates for labels of biological molecules and will be superior to Si nanocrystals for its high chemical stability and good biocompatibility. Bulk SiC shows weak emissions at room temperature because of its indirect band gap. The emission intensity can be significantly enhanced when the crystallite size diminishes to several or tens of nanometers. The reason for the above phenomenon is thought to be depressed nonradiative recombination in the confined clusters . Despite the excellent luminescent and electronic properties, the fabrication of SiC nanocrystals has been hindered by its chemical inertness. It is still a great challenge to prepare bulk quantities of SiC nanocrystals with sizes smaller than 10 nm. Up to now, many methods have been developed to fabricate luminescent SiC nanocrystals, such as electrochemical etching of bulk SiC (mostly cubic or hexagonal SiC) [12, 13], C ion implantation into bulk Si followed by etching , C60 introduction into porous silicon followed by annealing , chemical etching of microscale cubic SiC powder, and subsequent ultrasonic vibration , and so on. Among these methods, catalyzed electrochemical etching of polycrystalline SiC wafers in a hydrofluoric acid (HF)/ethanol solution is most intensively investigated [17, 18]. Although hexagonal or cubic SiC nanocrystals with diameters ranging from 1 to 6 nm were achieved, the yield of SiC nanocrystals is limited by the surface area of expensive polycrystalline SiC wafers. Wet electroless chemical etching is a desirable substitute for electrochemical etching because it is relatively inexpensive, can be done with simple equipment, and has large-scale production. Recently, Zhu and coworkers have reported the synthesis of cubic SiC nanocrystals by wet electroless etching of microscale cubic SiC powder in a HNO3/HF solution and experimentally observed intense violet-blue photoluminescence (PL) from colloidal cubic SiC nanocrystals . However, the surface chemistry of etched cubic SiC powder from which cubic SiC nanocrystals derived and the influence of surface terminations of cubic SiC nanocrystals in different surrounding environments on its optical properties still need to be investigated in detail.
In this paper, we report successful fabrication of colloidal cubic SiC nanocrystals by anisotropic wet chemical etching of microsized cubic SiC powder. The obtained colloidal cubic SiC nanocrystals are about 4.4 nm in diameter and exhibit significant quantum confinement with strong blue and blue-green emissions. The surface charges and optical characteristic of the as-prepared colloidal cubic SiC nanocrystals are investigated, and the possible reasons are discussed.
Materials and methods
The cubic SiC powders which were fabricated by a combustion synthesis method using powders of silicon (99.9% pure, 325 mesh, General Research Institute for Nonferrous Metals, Beijing, China) and carbon black (Tian Hao Carbon Black Corporation, Tianjin, China) as starting materials in our lab, and the fabrication process has been described previously . The particle size of the cubic SiC powders determined by laser scattering method is 0.1 μm to approximately 0.3 μm. The cubic SiC powders were etched in a mixture (3:1) of 40% HF and 65% nitric acid (HNO3) solutions at 80°C for 3 h. After etching, cubic SiC powders were subsequently washed with deionized water several times and dried at 75°C for several hours. The dried etched cubic SiC powders were then mechanically ground in an agate mortar for 1 h to obtain numerous separated nanocrystals from the etched porous powder. The ground powder was then dispersed in deionized water and ethanol by ultrasonic vibration for 30 min, respectively. The formed suspension was centrifuged at 2,700 × g for 1 h. The top part of the suspension was collected, which contains uniformly dispersed cubic SiC nanocrystals. Several measuring and analytical techniques were employed to characterize the as-prepared samples. Transmission electron microscopy (TEM) studies were performed with a JEM-2100 (2,100-kv) microscope (JEOL Ltd., Tokyo, Japan). TEM samples were prepared by dropping a droplet of the cubic SiC nanocrystals in ethanol on a carbon-coated Cu grid. The infrared transmission spectra (FTIR) were acquired using an Excalibur 3100 Fourier transform infrared spectrometer (Varian, Walnut Creek, CA, USA). The UV/Visible absorption spectra of the colloidal cubic SiC nanocrystals were obtained on a Cary 5000 spectrometer (Agilent Technologies Co. Ltd., Beijing, China) over the wavelength range of 200-800 nm by putting the nanoparticle-contained suspension solution in a quartz cell. The PL spectral measurements of the samples were performed in a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp. All measurements were performed at room temperature.
Results and discussions
HNO3 acts as the oxidizing agent, and the superfluous HF removes the produced silicon oxide. The etching process was not uniform and very selective to some parts of the particles, resulting in the formation of interconnected nanostructure network. It has been reported that there was a large difference in etching rates between different polytypes of SiC . For example, cubic SiC could easily be etched by HNO3 and HF, while hexagonal SiC is very stable in such a mixture. The SiC powders prepared by a combustion synthesis method contain a fair concentration of stacking faults due to the nonequilibrium reaction process. The large amount of stacking faults can be verified by the X-ray diffraction profiles  and high-resolution electron microscopy observations . Zhang et al. have revealed that the stacking faults in the cubic SiC nanorods are threefold in nature, and the region near the threefold stacking faults resemble the structure of hexagonal SiC. Therefore, the structure of the powders may be viewed as a cubic SiC matrix containing nanoscaled hexagonal SiC regions. Different etching rates of hexagonal SiC as compared with the bulk cubic SiC may explain the preferential etching of cubic SiC powders.
where μ is the reduced mass of the exciton, μ = m e m h/(m e + m h), m e = 0.394m 0, m h = 0.387m 0, E g = 2.25 eV. ε is the high frequency dielectric constant of cubic SiC (ε ≅10), ε 0 = 8.854 × 10-12 F/m, and r is the radius of the nanocrystals. For the cubic SiC nanocrystals with 4.4 nm, we can estimate that the band gap will be widened to 2.51 eV, which is in agreement with the emission peak located at approximately 490 nm.
The surface charges of cubic SiC nanocrystals will form new electronic states in the band gap, and this will influence the PL energy levels. It is reported that the radiative recombination takes place in the surface/defect states. On the surface of cubic SiC nanocrystals, a large number of C and Si terminations will be formed after anisotropic chemical etching and ultrasonic treatment. In the water suspension, the water molecules will be dissociated into -OH and -H groups, which will bond to C and Si terminations. In room temperature, -OH groups only react with Si terminations . The interaction between water molecules and bulk cubic SiC (001) Si-terminated surface leads to dissociation of water molecules accompanied by changes in both its structural and electronic properties. The -OH and -H groups bond mainly on the Si-terminated cubic SiC nanocrystals. Cicero and coworkers  have investigated the interaction of water molecules with polar Si-terminated surfaces of cubic SiC by means of ab initio molecular dynamics simulations.
After storage in air for more than 3 months, the colloidal cubic SiC nanocrystals were found to be uniformly dispersed and not show any appreciable PL spectral shifts or intensity loss. Given their high chemical stability, small size, good biocompatibility, and simple fabrication method, colloidal cubic SiC nanocrystals could find applications in biological labeling as well as in optoelectronic applications.
In summary, nearly monodispersed colloidal cubic SiC nanocrystals with an average diameter of 4.4 nm have been fabricated via anisotropic wet chemical etching microsized cubic SiC powder. Carboxylic acid, SiH, CH, and CHx groups are observed on the etched cubic SiC powder by FTIR spectra. The as-fabricated cubic SiC nanocrystals in water and ethanol exhibit strong, above band gap blue and blue-green PL emissions. Our results reveal that quantum confinement and surface charges in water and ethanol suspensions play a great role in determining the optical characteristic of colloidal cubic SiC nanocrystals. Because of the good biocompatibility, high chemical stability and simple fabrication method, we believe that our results will contribute to the current endeavors aimed at building blocks of nanostructured devices as violet light sources and developing new biological probes in life science.
fourier transform infrared
full width at half maximum
transmission electron microscopy
The finical support from the following projects are greatly appreciated: Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS, National Natural Science Foundation Key Project, No.50932006, National Natural Science Foundation Project, No.50772116 and No.60807008, Shanghai Rising-Star Program under Grant 10QA1403500, Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) under Grant 200802481028, and SMC Excellent Young Faculty Project of "ChenXing Scholar" Prize Program under Grant SMC2009GB16.
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