Preferred Orientation of SiC Nanowires Induced by Substrates

at angles of 90 - x, 150 - x, and x - 30° to this plane, as illustrated in Figure 3f. Usually, two maximum angles take place. For further demonstrati...
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J. Phys. Chem. C 2010, 114, 2591–2594

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Preferred Orientation of SiC Nanowires Induced by Substrates Huatao Wang,*,†,‡ Lun Lin,§ Weiyou Yang,| Zhipeng Xie,*,† and Linan An⊥ State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 637371, Singapore, Department of Material Science and Engineering, Tianjin Institute of Urban Construction, Tianjin, 300384, People’s Republic of China, Institute of Materials, Ningbo UniVersity of Technology, Ningbo 315016, People’s Republic of China, and AdVanced Materials Processing and Analysis Center, UniVersity of Central Florida, Orlando, Florida 32816, USA ReceiVed: December 16, 2009

A technique to directly synthesize highly oriented SiC nanowire arrays on single-crystalline SiC substrates was present in this paper. The great impacts of substrate orientation on the growth habits of nanowires were systematically investigated. It has been found that nanowires can grow along the [1j102] direction or its’ equivalent ones on SiC (0001) substrates, while nanowires grow along the [101j0] direction or its’ equivalent ones on SiC (101j0) and (112j0) substrates. This technique for the preferred growth of SiC nanostructures can largely improve the quality of SiC nanowire arrays, which have wide applications in the fields of electronic nanodevices, optoelectronic nanodevices, and photocatalytic nanomaterials. 1. Introduction One-dimensional (1D) semiconducting nanostructures (e.g., nanowires and nanotubes) have gained extensive attention because of their potential applications in nanometer-scaled optoelectronics, electronics, and sensors.1-5 One of critical steps toward the realization of these applications is to grow them with controlled orientations and morphologies. It has been demonstrated that the orientation and morphology play a key role in determining the properties and applications of 1D nanostructures.6-9 For example, aligned silicon carbide (SiC) nanowire arrays exhibited an improved field emission behavior with a turn-on field of ∼10.5 MV/m as compared to 29.5 MV/m for randomly grown arrays.9 SiC possesses excellent thermomechanical properties, including high strength and stiffness, high-temperature stability, corrosion resistance, and high thermal conductivity.10-12 SiC is also an important wide band gap semiconductor with widespread applications in short wavelength and high-temperature optoelectronic devices. Up to now, many efforts have been put into the synthesis of 1D SiC nanostructures such as nanowires, nanobelts, nanotubes, and nanosprings.13-26 Importantly, synthesis of SiC nanostructures with preferred orientations has attracted extensive interest owing to their unique electronic, optical, and mechanical properties and their potential applications in the areas such as electronic nanodevices, optoelectronic nanodevices, nanocomposites, photocatalysts, and hydrophobic devices.27-39 Yang and co-workers synthesized oriented SiC porous nanowire arrays by in situ carbonizing aligned Si nanowire arrays.40 Pan et al. synthesized oriented SiC nanowires by reacting SiO with aligned carbon nanotubes as a template.41 Li et al. synthesized large-area oriented SiC nanowire arrays

by chemical vapor reaction using an ordered nanoporous anodic aluminum oxide (AAO) as a template.42 Kim et al. reported the direct synthesis of aligned SiC nanowires on a Si substrate using a novel catalytic reaction.43 Niu et al. prepared aligned SiC nanowire arrays by a simple thermal evaporation of ZnS and carbon on silicon wafer.44 However, controlled synthesis of highly oriented SiC arrays has still been a challenge. In this paper, we report a technique to directly synthesize aligned SiC nanowire arrays with high quality induced by single-crystalline SiC substrates. It was found that the preferred orientation of nanowires was completely determined by the substrate. 2. Experimental Methods The SiC nanowires were synthesized using a process similar to that reported previously.45 Commercially available polysilazane was used as the source material, which was first solidified by heating at 260 °C for 30 min and then ground into powders. 6H-SiC single-crystalline wafers with different orientations were used as the substrates. Before they were immersed in a ethanol solution of Fe(NO3)3 with a concentration of 0.1-0.2 mol/L, the substrates were cleaned by ultrasonic in acetone and ethanol, respectively. After being dried in air at room temperature, the substrates with a thin layer of Fe(NO3)3 were placed on the top of powders in an alumina crucible. The crucible was then placed in a graphite-heater furnace and heat-treated at 1350 °C for 0.5-2 h under the flow of ultrahigh purity Ar (99.99%, 1 atm). The obtained nanowire arrays were characterized using scanning electron microscopy (SEM, LEO-1530 German) and transmission electron microscopy (TEM, JEOL, 2011, Japan), equipped with energy-dispersive X-ray spectroscopy (EDS). 3. Results and Discussion

* To whom correspondence should be addressed. E-mail: xzp@ mail.tsinghua.edu.cn (Z.X.); [email protected] (H.W.). † Tsinghua University. ‡ Nanyang Technological University. § Tianjin Institute of Urban Construction. | Ningbo University of Technology. ⊥ University of Central Florida.

Figure 1a shows SEM images of typical SiC nanowires grown on a SiC (0001) substrate. It is interesting to see that the nanowires are not perpendicular to the growth surface. In stead, they grow obliquely along six directions. The angle between these directions is 60° (top view, the inset in Figure 1a), and

10.1021/jp911911e  2010 American Chemical Society Published on Web 01/22/2010

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Figure 1. (a) A SEM image of SiC nanowires grown on a singlecrystalline SiC (0001) substrate showing the preferred orientation with six growth directions (top view). (b) A typical enlarged SEM image suggesting the oblique growth of the nanowires on the substrate with an angle of 20° (side view) as illustrated by the inset schematic model.

the angle between the substrate and the nanowire axis is measured to be ∼20° (side view, Figure 1b). To understand the effect of the substrate orientation on the growth of the nanowires, we examined the nanowires using TEM. Figure 2a is a typical TEM image of the nanowire. The catalytic droplet suggests that the nanowires could be grown via a typical vapor-liquid-solid (VLS) process.46 And screw dislocations may also play another crucial role in the growth of these nanowires,47,48 especially since screw dislocation exists extensively in SiC.49-51 EDS analyses of the droplet and nanowire reveal that the droplet contains Si, C, and Fe only with a small amount of Cr (Figure 2b), while the nanowire contains only Si and C (Figure 2c) (Cu is from the sample holder), confirming that the nanowire is SiC. Figure 2e is a highresolution transition electron microscopy (HRTEM) taken from the rectangular area marked in Figure 2d. The d spaces of two sets of fringes are measured to be 0.25 and 0.22 nm, corresponding to (1j102) and (011j4) planar distances of 6H-SiC (JCPDS: 49-1428), respectively. Figure 2f is the corresponding selected area electron diffraction (SAED) pattern. Both SAED and HRTEM results reveal that the nanowire grew along the [1j102] direction. The growth orientation of the nanowires can be attributed to the lattice matching effect. The lattice parameters of 6H-SiC are a ) 0.3081 nm and c ) 1.5120 nm. The angle between the [1j102] and (0001) is 20° and equals to the angle between the substrate and the growth direction of the nanowire (Figure 1b), clearly suggesting that the nanowires grow epitaxially on SiC (0001) surface induced by the lattice match to minimize the strain energy. The effect of substrate orientation on the growth habit of SiC nanowires can also be seen from the nanowires grown on the substrates with other orientations (Figure 3). Figure 3a shows

Wang et al.

Figure 2. (a) A typical TEM image of SiC nanowires grown on a SiC (0001) substrate. (b) and (c) Respective EDS spectrum recorded from the droplet and the body of the nanowire. (d) A TEM image of the nanowire tip. (e) A HRTEM image taken from the marked area in (d). (f) Corresponding SAED pattern of the nanowire body.

the nanowires grown on a (101j0) substrate. It was found that most of the nanowires grow perpendicularly to the growth surface, suggesting well-aligned SiC nanowires have been prepared successfully on this substrate (Figure 3h). Figure 3b is the SEM image of the nanowires grown on a (112j0) substrate, which tilt at the angles of 60 and 120° to the growth surface. For nanowires on a (123j0) substrate, they grow at the angles of 49 and 109° to the growth surface (Figure 3c). Note that these growth surfaces are all parallel to the substrate [0001] direction, and all of the nanowires on them grew in [101j0] direction or its’ equivalent ones. Calculations based on lattice parameters of 6H-SiC show that the angles between 〈101j0〉 and (112j0) are 60 or 120°, and those between 〈101j0〉 and (123j0) are 49 or 109°, which agree well with the measured angles between the nanowires and the substrate surface, suggesting the growth of the SiC nanowires on the substrates are controlled by lattice match. Figure 3e-f disclose the details of the substrate orientation and nanowires growth on (0001), (101j0) and (112j0) planes. There are six growth directions (actually one orientation in 〈1j102〉) at an angle of 20° to substrate surface for nanowires on (0001), while there are also six growth directions (actually one orientation in 〈101j0〉) at angles of 90 or 60° to substrate surfaces for nanowire on (101j0) and (112j0), respectively. And all the directions show 3-fold symmetries as the crystal structure of hexagonal SiC and match together with [101j0] or its’ equivalent ones as illustrated in Figure 3g (top view). For nanowires on other planes that are parallel to the substrate [0001] direction and at an angle of x degrees to (101j0), they may grow at angles of 90 - x, 150 - x, and x - 30° to this plane, as illustrated in Figure 3f. Usually, two maximum angles take place. For further demonstrating this point, we grew nanowires on a plane with an angle of 35° to (101j0) (Figure 3d). The results

Preferred Orientation of SiC Nanowires

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2593 or its’ equivalent ones on SiC (0001) substrates, while nanowires grow along [101j0] direction or its’ equivalent ones on SiC (101j0) and (112j0) substrates. Well-aligned SiC nanowire arrays were successfully synthesized on (101j0) substrates. This technique for the preferred growth of SiC nanostructures can significantly improve the quality of SiC nanowire alignments, which have wide applications in the fields of electronic nanodevices, optoelectronic nanodevices and photocatalytic nanomaterials. Acknowledgment. The work is financially supported by the National Science Foundation of China (Nos. 50372031 and 50872058), the two-based projects of NSFC (No. 50540420104), the Specialized Research Foundation for the Doctoral program of Higher Education (No. 20050003004), and International Cooperation Project of Ningbo Municipal Government (Grant No. 2008B10044). The authors also thank the support of TanKeBlue Semiconductor Co. Ltd (China, Beijing) in SiC single crystals. References and Notes

Figure 3. (a-c) Respective SEM images of nanowires grown on SiC (101j0), (112j0), and (123j0) substrates. The dash dot lines indicate crystallographic planes of (101j0), (112j0), and (123j0), respectively. (d) Nanowires grown on x plane that is parallel to [0001] and with a tilted angle of x to (101j0), form angles of 55 and 115° between the nanowire and substrate when x equals to 35°. The two dotted lines indicate x plane and (101j0). (e) A 3D model of nanowires grown on SiC (101j0), (112j0), and (0001) planes. (f-g) Schematic diagrams showing the relationships between substrate orientation and nanowires axes. (h) One well-aligned SiC nanowire array fabricated on (101j0) substrate. The inset is an enlarged image with a scale bar of 1 µm.

clearly displayed that the angles between the nanowires axes and the substrate are ∼55° and ∼115°. Above all, the growth of SiC nanowires on single-crystalline SiC substrates behaved the obviously preferred orientations, quite different from the random growth of nanowires/whiskers on SiC ceramics.52-56 Actually, the dependence of preferred growth of SiC nanowires on the orientation of single-crystalline SiC substrates can be extended from the preparation of InP, Si, Ge, and ZnO nanowire alignments on the corresponding singlecrystalline substrates or homogeneous buffer-layers with the same composition and structure as the nanowires,57-61 suggesting it is an universal route for the preparation of aligned nanostructure arrays. For example, InP nanowires with vertical orientation and 〈111〉 growth direction were grown by the metal organic vapor-phase epitaxial technique on Fe-doped (111) B InP substrates.61 4. Conclusions In summary, the great impacts of substrate orientation on the preferred growth of nanowires were systematically investigated. It is found that nanowires can grow along the [1j102] direction

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