SiO2 Nanochain Heterojunctions by

Synthesis and Properties of SiC/SiO2 Nanochain Heterojunctions by Microwave Method Guodong Wei, Weiping Qin,* Kezhi Zheng, Daisheng Zhang, Jingbo Sun, Jingjing Lin, Ryongjin Kim, Guofeng Wang, Peifen Zhu, and Lili Wang

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1431–1435

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, Changchun 130012, China ReceiVed August 2, 2008; ReVised Manuscript ReceiVed December 17, 2008

ABSTRACT: SiC/SiO2 one-dimensional nanochains and SiC/SiO2 two-dimensional X-junction and Y-junction nanochains were synthesized by using a simple and low-cost microwave method. Structural, morphological, and elemental analysis revealed that the SiC/SiO2 nanochains consisted of 3C-SiC strings with diameters of 20-80 nm and periodic SiO2 beads with diameters of 100-400 nm. Spectral analysis indicated that both SiC strings and SiO2 beads produced significant photoluminescence, and the presence of SiO2 beads enhanced the emissions from SiC strings. On the basis of experimental characterizations, a two-step growth mechanism of the nanochains was proposed to elucidate the growth process. Introduction During the fast development of nanotechnology in the past decade, most research efforts have been focused on the preparation of one-dimensional (1D) heterojunctions because of their important electronic and optical properties.1,2 Compared to single-component 1D nanostructures, 1D heterojunctions offer the same size and property benefit as single-component materials, but with the added benefit of multifunctionality or new properties arising from combining different materials.2 Until now, a great many of 1D heterostructures have been prepared, such as GaP/GaAsP/GaP core-multishell nanowire heterostructures,3 Si/Ge nanowire heterostructures,4 ZnS/SiO2 nanocables,5 Si/SiO2 nanochains,6 and SiC/SiO2, SiC/SiO2/C nanocables.7,8 Among them, SiC 1D heterojunctions have attracted extensive attention because of their excellent material properties, such as high strength, high thermal stability, and their broad applications in high frequency, high temperature, and high power nanoscale devices.9 Up to now, various types of SiC 1D heterostructures have been reported, such as SiC/ZnO nanobrush heterojunctions,10 SiC/SiO2, SiC/C, SiC/BN, and SiC/SiO2/C nanocable heterojunctions,7 and SiC/CNT (carbon nanotube) heterojunctions.11 These studies have had an enormous impact on nanotechnology and demonstrated the great potential of SiC 1D heterojunctions in a variety of applications. Nanochain is a new important member of 1D heterojunctions with unique properties6,12 (especially for optical and electronic properties). Rafiq et al. first reported the single-electron charging effects in a single Si/SiO2 nanochain.6 Inspired by Si/SiO2 nanochains, some work on 1D nanochain heterojunctions has been successfully achieved. Ni et al. reported B/SiO2 nanochains by simply sublimating the desired powders at high temperature.13 Tian et al. reported a two-step route to the synthesis of BC/SiO2 nanochains.14 Despite these efforts, very little work has been reported on 1D SiC/SiO2 nanochain heterojunctions. Furthermore, as the critical component of X/SiO2 (X ) Si, BC, B, SiC) 1D nanochain heterojunctions, SiO2 beads have interesting violet-blue light emission and waveguide properties. Therefore, the marriage of SiC nanowires and SiO2 beads in the form of 1D SiC/SiO2 nanochains is expected to exhibit unique electrical and optical properties and this structure is more * Corresponding author. Tel: 86-431-85168240-8325. Fax: 86-431-851682408325. E-mail: [email protected].

meaningful than SiC/SiO2 nanocables. In this paper, we report on the large-scale synthesis of 1D SiC/SiO2 nanochain heterojunctions by using a simple and low-cost microwave method. The SiC/SiO2 nanochains provide periodic semiconductor-oxide units for incorporation of different functionalities into a nanoscale system. The present results may inspire the exploring of other Si-related 1D heterojunctions materials, such as silicon nitride nanochains and zinc silicate nanochains, and their potential applications as building blocks for nanodevices in the future.

Experimental Section In our synthesis, the microwave heating system was similar to that described in the literature.15 The microwave power can be adjusted from 0 to 800 W. Therefore, the reaction temperature can be controlled through adjusting the microwave power. With use of this microwave heating system, the temperature of raw materials can be heated over 1300 °C in just 10 s (power, 800 W), and can be reduced to lower than 1000 °C in just 3 s when stopping microwave heating. In a typical synthesis, (1) SiO2 was synthesized by hydrolysis of tetraethoxysilane (TEOS) in the alcohol-water mixed solvent using ammonia as a catalyst.16 (2) An alumina crucible containing Si, SiO2, and charcoal powder was placed at the center in the microwave oven. The molar ratio of Si, SiO2, and charcoal was 1:1:2. (3) The furnace was purged with pure argon gas at a constant flow rate of 50 mL min-1 for 2 h and then the gas flow rate was adjusted down to 10 mL min-1. This flow rate was maintained throughout the entire fabrication process. After being heated for 10 min, the raw materials were intermittently heated for another 10 min by turning the microwave on 30 s (heating stage) and off 10 s (cooling stage). The product was cooled down to room temperature naturally and heated in air at 600 °C for 8 h to remove the residual carbon. Finally, the as-grown sample with light-green color was obtained. The obtained sample was then characterized using X-ray powder diffraction (XRD, RU-200b) with Cu KR radiation (λ ) 1.54178 Å), field emission scanning electron micrographs (SEM, Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM, Philips H-8100), and high-resolution transmission electron microscopy (HRTEM, JEOL-2011). The Raman scattering spectrum was recorded with a micro-Raman spectrometer (Renishaw 2000) at room temperature. The 514-nm line of an Ar+ laser was used as the excitation source. Photoluminescence (PL) spectra were

10.1021/cg800845h CCC: $40.75  2009 American Chemical Society Published on Web 01/20/2009

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Figure 1. (a, b) Low magnification SEM images of the nanochains; (c) high magnification SEM images of a single nanochain; (d) EDS spectrum of the beads; (e) Raman spectrum of the nanochains; (f) XRD pattern of the nanochains. recorded using a UV-lamp micro-Raman spectrometer under the excitation of a 325 nm He-Cd laser at room temperature.

Results and Discussion SEM was employed to characterize the morphology of the as-synthesized sample. The most evident characteristic of the sample is that it mainly consists of beaded nanochains, as shown in Figure 1a,b. The content of the nanochains in the sample is estimated to be more than 70%. As can be seen from SEM images, the beads are connected with nanowires (strings) forming nanochain structures. Most of the nanochains have one string, while some have two or more, as indicated by the arrow in Figure 1b. Figure 1c displays an extremely straight nanochain with a length of several microns, which has equirotal beads of 300 nm and a uniform string with a diameter of 70 nm. The distance between two neighboring beads is 600 nm, which is almost identical, and thus can be regarded as the period of the nanochain (T ) 600 nm). The chemical compositions of the nanochain were analyzed using EDS under SEM. As shown in Figure 1d, the beads consist of Si, C, and O elements, revealing that the beads contain SiO2. Since EDS is less accurate quantitatively for light atoms such as C and O, we use Raman spectroscopy to identify compositions of the nanochains, particularly compositions beyond SiO2. As shown in Figure 1e,

the nanochains only have the compositions of SiC and SiO2.17 The peak at 779 cm-1 is assigned to the transverse optical (TO) phonon of 3C-SiC and the amorphous bulge centered at 930 cm-1 corresponds to the Raman peak of amorphous SiO2. The broadening and asymmetric Raman peak, compared to that of bulk SiC and SiO2, could be attributed to size confinement effects, structure defects, and planar faults.17,18 XRD was conducted to investigate the structure of the nanochains. As shown in Figure 1f, all peaks in this pattern can be readily indexed as the 3C-SiC structure with a lattice a ) 4.361 Å (JCPDS, No. 29-1129). The low intensity peak marked with S.F. is attributed to stacking faults in the SiC structure.19 Further characterization was carried out using TEM. Typical TEM images of the nanochains are displayed in Figure 2a-c. It is worth noticing that, besides further confirming the nanochain structure, the nanochain string clearly goes through the beads. In order to verify whether the beads are made of SiO2 or not, the nanochains were etched in dilute HF solution for 6 h and then characterized using TEM. As shown in Figure 2d, the beads disappeared and only nanowires were left behind. It is well-known that silicon carbide cannot be etched in usual acids, including hydrogen fluoride, but SiO2 can be easily etched away in HF solution. From EDS, Raman spectroscopy, and the result of etching, we can conclude that the beads are amorphous

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Figure 2. (a, b) TEM images of X-junction and Y-junction nanochain; (c) TEM image of a single nanochain; (d) TEM image of the nanochains after being washed with diluted HF solution for 6 h, showing only the nanowires.

SiO2 and the strings are SiC. Figure 2a shows an X-junction nanochain formed by two 1D nanochains joined to each other at one bead. The two SiC stings are contacting each other at the junction. Similarly, a Y-junction nanochain has been found, as shown in Figure 2b, which is formed by three 1D nanochains joined to each other at one bead. The nanochains with X-junction or Y-junction are gaining increasing interest as building blocks for a two-dimensional (2D) or three-dimensional (3D) network of nanoelectronic devices. As shown in Figure 2c, it is interesting that the SiC string has a high stacking fault inside the SiO2 beads. Thus, we can infer that the SiO2 beads prefer to grow on the high stacking-fault surface of the SiC string because of the much lower growth energy and much better wettability. (The high stacking faults could result in the formation of a rough surface. Because of the surface tension of liquid SiO2, liquid SiO2 is favored to deposit onto high stackingfault surface to form SiO2 beads. In other words, the high stacking-fault surface has better wettability.) HRTEM provides further insight into the structure of the nanochains. Figure 3a displays a TEM image of a nanochain. Figure 3b is a HRTEM image of the string marked by a rectangle in Figure 3a, which shows that the string is structurally uniform and no obvious defects, such as stacking faults or twins, are observed. The string shows a periodic lattice structure and the d-spacing between two adjacent lattice fringes is 0.25 nm for the string, in good agreement with the (111) plane of 3CSiC. This result indicates that the string grows along the [111] direction. In addition, a thin shell (∼3 nm) of SiO2 out of the SiC string could be observed in the inset of Figure 3a. The inset of Figure 3b is the corresponding selected area electron diffraction (SAED) pattern recorded along the zone axis [001j], indicating that the string is a single crystal and has a cubic structure. A digital diffractogram, based on the Fourier transform of the experimental image, was used to produce the simulated image (Figure 3c) that clearly reveals the lattice fringes are perfectly arrayed. The white dots in the HRTEM images represent the holes among atoms and the dark-gray regions are those of Si-C atomic pair projections.20 Figure 3d is atom structure model of stacking sequences in perfect crystals of 3CSiC. It is revealed that the 3C-SiC structure repeats the stacking

Figure 3. (a) A higher magnification TEM image of a nanochain; the inset is a HRTEM image of SiO2 shell; (b) HRTEM image of the SiC strings; the inset is a corresponding SAED pattern recorded from the string; (c) a digital diffractogram based on the Fourier transform of the experimental image; (d) stacking sequences in perfect crystals of 3C-SiC.

sequence by ABCABCABC... The distance, a, between neighboring silicon or carbon atoms is approximately 0.31 nm and the distance, b, between the C atoms to the Si atoms is approximately equal to 0.19 nm. The distance between two silicon planes is approximately 0.25 nm. It is worthwhile to notice that the ABCABC stacking sequence of 3C structure creates straight atomic plane structures along all of the (111), (111j), (11j1j), (1j11) planes; that is, there are translational symmetry and periodicity in these planes, which can be seen from Figure 3c. Here A, B, C represents the three basic positions of atom centers for a close spherical packing in SiC.21 Previous studies demonstrated that the SiC nanowires were grown via a VS process without metal catalysts.22 However, sole VS mechanism is not appropriate for explaining the formation of SiC/SiO2 nanochains, because of the periodic SiO2 beads form. On the basis of the above experimental results and analysis, we propose a two-step process for the growth of SiC/ SiO2 nanochains: VS and modulation process. VS step: under microwave radiation, four different reactions are involved in the synthesis process, as illustrated below:

C(s) + SiO2(s) f SiO(g) + CO(g)

(1)

SiO2(s) + Si(s) f 2SiO(g)

(2)

C(s) + CO2(g) f 2CO(g)

(3)

SiO(g) + 3CO(g) f SiC(g) + 2CO2(g)

(4)

where (s) and (g) stands for solid and gaseous, respectively. Si is known to react with SiO2 to yield SiO(g) at above 1100 °C.

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Figure 4. (a) PL spectrum of the nanochains; (b) the comparison spectra of the SiC nanowires and the nanochains.

Under our experimental conditions (above 1300 °C), SiO and CO vapors are produced through the reactions 1-3 and SiC nanowires grow on SiC nuclei via the VS mechanism. The growth reaction is identical to reaction 4. The SiC nanowire generally grows along the [111] direction at a rapid rate. It is noted that there is excessive SiO(g) in the reaction atmosphere because of excessive Si and SiO2 used in raw materials. Therefore, after SiC nanowires formed, SiO vapor pressures keep increasing and reach a supersaturated state. Modulation step: when the temperature decreases in the cooling stage, the saturated vapor pressures of SiO decreases from 102.27 at 1300 °C to 10-0.38 Pa at 1000 °C,23 and then SiO molecules start to nucleate on the surface of the SiC nanowires. More and more SiO molecules deposit onto the SiC nanowire surface, which result in the formation of a thin layer of SiO2. This process is very quick due to excessive SiO in the atmosphere. When the temperature increases again in the heating stage, the as-grown SiO2 shell will form a viscous liquid (It is noted that SiC has a strong absorption to microwave. Once the SiO2 shell forms, the absorbed microwave energy in SiC cores transfers into thermal energy and heats the SiO2 shell via heat conduction. When the temperature reaches above the melting point of SiO2, the SiO2 shell will be melted and form a viscous liquid.). Liquid SiO2 flows along the SiC nanowires and covers their surfaces. Because of the Rayleigh instability and the poor wettability between SiC and SiO2, the liquid SiO2 (in the form of a cylindrical form) is completely separated into spheres and forms the first batch of SiO2 beads. Once the SiO2 beads form, their morphology is relatively unchanged, but the SiO2 beads experience continuous growth afterward.13 As time goes on, the vapor pressures of SiO reach a supersaturated state again in the heating stage and form a new SiO2 shell in the cooling stage. This cycle can occur repeatedly without interruption providing the excess SiO in the atmosphere. It is noted that the content of SiO molecules in the reaction atmosphere has a significant effect on the SiO2 layer thickness. The more SiO molecules are in the reaction atmosphere, the thicker SiO2 layer thickness is. The thicker SiO2 layer thickness is, the larger the SiO2 beads diameters are. Therefore, the excess SiO has an important role in determining the diameters of SiO2 beads. To investigate the optical properties of the SiO2 beads on the SiC nanowires, the photoluminescence (PL) spectra were recorded from the nanochains and SiC nanowires at room temperature (Figure 4). The most striking property of the nanochains is that they emit stable and intensive violet-blue light. As shown in Figure 4a, the spectrum shows a broad emission band with a maximum at 3.25 eV. This broadband can be further deconvolved into five peaks (the Gauss-fit peaks), centered at

2.37, 2.52, 2.78, 3.25, and 3.59 eV, respectively. The similar emission peaks (2.37, 2.52, and 2.78 eV) are also observed from the SiC nanowires, suggesting that they can be attributed to the emissions from the 3C-SiC nanowires (Figure 4b). Compared with the emissions of pure SiC nanowires, the emission peaks centered at 3.25 and 3.59 eV are assigned to the emissions from the SiO2 beads. The effect of the oxygen discrepancy24,25 in the SiO2 beads near the interface boundary and/or several defects related to the oxygen deficiency are believed to be the origin of emission (luminescence centers).24 The similar band emission peak but very low emission intensity centered at 3.15 eV in SiC nanowires can be attributed to the neutral oxygen vacancy formed at the interface boundary of SiC/SiO2 when etched the SiO2 in dilute HF solution. As shown in Figure 4a, the PL of the SiC/SiO2 nanochains is quite different from those of SiC/ SiOx nanocables reported in the literature.26 In these reports, the PL of SiC/SiOx nanocables was mainly from the defects in SiOx shell rather than from the SiC cores. Because SiOx reveals remarkable emission capability and SiC shows very low luminescence efficiency at room temperature on account of its indirect bandgap, the contribution from SiC cores of SiC/SiOx nanocables to PL emission bands could be ignored. However, as shown in Figure 4a,b, PL from SiC strings and SiO2 beads can be both observed in our SiC/SiO2 nanochains spectrum. In addition, compared to the previous reports with the origin of the PL for SiC/SiO2 nanocables mainly from their SiO2 shell rather than the SiC cores or the interface between the core and the shell,26 our SiC/SiO2 nanochains revealed more distinct tailoring to the optical property. Why can we observe the SiC emission in the PL of SiC/SiO2 nanochains? Further, the detailed effects of the diameter and the period of the SiO2 beads on the emission from the nanochains are not clear at this moment. They need further study. The current results suggest that the optical properties of SiC nanowires can be tailored by SiO2 beads. Conclusions In conclusion, 1D SiC/SiO2 nanochain heterojunctions have been achieved via a simple and low-cost microwave method. The synthesized nanochains are composed of quite uniform 3CSiC strings and periodic SiO2 beads. On the basis of the experimental results, it is suggested that the nanochains are formed by a two-step growth mechanism: VS process and modulation process. The optical properties of SiC nanowires can be tailored by the SiO2 beads because each SiO2 bead may give off violet-blue light. The special composite structure accompanied by its optical properties may have some potential applications in photoelectricity and nanodevices. Our approach

SiC/SiO2 Nanochain Heterojunctions

presented here would be helpful in designing and preparing other Si-related heterostructures and the obtained nanochain heterostructures could meet the growing demands of optical and electronic nanodevices. Acknowledgment. The authors gratefully acknowledge Prof. Hanchen Huang and Weiyou Yang for help in explaining the growth mechanism and acknowledge the financial support from the National Nature Science Foundation of China (Grant Nos. 50672023 and 10874058).

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