17169
2007, 111, 17169-17172 Published on Web 10/26/2007
Bicrystal AlN Zigzag Nanowires H. Wang,† G. Liu,† W. Yang,‡ L. Lin,§ Z. Xie,*,† J. Y. Fang,| and L. An*,| State Key Laboratory of New Ceramics and Fine Processing, Tsinghua UniVersity, Beijing 100084, P.R. China, Institute of Materials, Ningbo UniVersity of Technology, Ningbo 315016, P.R. China, Department of Material Science and Engineering, Tianjin Institute of Urban Construction, Tianjin 300384, P.R. China, and AdVanced Materials Processing and Analysis Center, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed: September 15, 2007; In Final Form: October 14, 2007
Bicrystal AlN zigzag nanowires have been synthesized by directly nitriding aluminum powders using Fe(NO3)3 as a catalyst. The structural characterizations reveal that there are three types of grain boundaries between the two single crystals. The lattice mismatches along these boundaries cause asymmetrical residual stresses, which tilt these boundaries to different directions, leading to the formation of a zigzag morphology. The transitions between these boundaries offset these lattice mismatch stresses and stabilize the structure.
1. Introduction
2. Experimental Methods
Over the past decade, one-dimensional (1D) nanostructures (e.g., nanowires, nanobelts and nanorods) have attracted extensive attention for their potential applications in the fabrication of nanodevices and nanocomposites.1-3 The properties and applications of these 1D nanomaterials are strongly dependent on their morphologies and geometries.4,5 Self-assembly (bottomup approach) is an effective method to synthesize 1D nanomaterials with desired morphologies and crystallinity.6,7 The driving forces for nanoscale self-assembly include van der Waals forces and hydrogen bonding between passivating organic molecules,8,9 electrostatic forces of the nanostructures with charge-polarized surfaces,10,11 and surface energies.12,13 Recently, it has been reported that the lattice mismatch can also be a driving force for the formation of nanostructures with unique morphologies.14
The bicrystal AlN zigzag nanowires were synthesized on a SiC substrate by simply nitriding pure aluminum powders using Fe(NO3)3 as a catalyst. First, the SiC substrate of 10 × 10 mm and 3 mm thick was immersed in ethanol solution of Fe(NO3)3 of 0.05 mol/L concentration for 3 min. The substrate was then dried in air, leaving a thin layer of Fe(NO3)3 on the surface. The substrate was placed on the top of aluminum powder with 99.5% purity and ∼20 µm in size. An Al2O3 crucible containing the aluminum powders and the substrate was placed in the center of an induction-heated graphite tube furnace. Before heating, the furnace was vacuumed to a pressure of ∼20 Pa and then filled with ultrahigh purity nitrogen to a pressure of 0.1 MPa. Finally the furnace was heated to 1250 °C and held at that temperature for 1 h followed by furnace cooling. The synthesized products were collected from the substrate for further characterization by scanning electron microscopy (SEM, Hitachi S-5500, 30kV) and transmission electron microscopy (TEM, JEOL 2011, 200kV).
AlN is an important wide-band gap semiconductor with a band gap of 6.2 eV at room temperature. The material is promising for field-emission applications since it exhibits very small or even negative electron affinity.15-17 There have been great interests in synthesizing 1D AlN nanostructures for device applictions.18-24 In this paper, we report the synthesis of bicrystal AlN zigzag nanowires by directly nitriding aluminum powders using Fe(NO3)3 as a catalyst. Characterization of the crystal and boundary structures reveals that the zigzag morphology is formed due to the lattice mismatch between the two single-crystal components, although zigzag structures have been reported in various materials previously.25-31 To the best of our knowledge, this is the first report on bicrystal induced AlN nanozigzags. * Corresponding authors. Phone: +86-10-62794603. E-mail: xzp@mail. tsinghua.edu.cn (Z.X.); Phone: +407-823-1009. E-mail:
[email protected] (L.A.). † Tsinghua University. ‡ Ningbo University of Technology. § Tianjin Institute of Urban Construction. | University of Central Florida.
10.1021/jp077435u CCC: $37.00
3. Results and Discussion The morphology of the synthesized product was first observed by SEM. The product contains ∼10% zigzag nanowires and ∼90% straight nanowires. Figure 1a shows a typical lowmagnification SEM image of a zigzag-shaped nanowire. Zigzags are up to several micrometers in length and 30-70 nm in diameter. The length of repeating sections and the wave amplitude of zigzags are varied from wires to wires in the ranges of 130-500 and 80-160 nm, respectively. However, within an individual wire, the length of the repeating sections and the wave amplitude are the same. Close observation under high magnification SEM (Figure 1b) reveals a dividing line on the surface of the zigzag along its length direction, suggesting that the zigzag nanowires likely possess a bicrystal structure. The structure of the zigzag nanowires has been further analyzed by TEM. A low magnification TEM image of the zigzag nanowire is given in Figure 2a, which clearly displays a © 2007 American Chemical Society
17170 J. Phys. Chem. C, Vol. 111, No. 46, 2007
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Figure 1. (a) Typical SEM image of an AIN zigzag nanowire. (b) A high magnification SEM image showing a dividing line on the surface of an AIN zigzag nanowire.
Figure 3. (a) HRTEM image of a G2 boundary. (b) Schematic showing the G2 boundary and the relevant crystal parameters. (c and d) TEM images of typical G2 boundaries.
Figure 2. (a) TEM image of a zigzag nanowire. (b) A convoluted SAED pattern taken from the area cross the grain boundary; the red and white arrowheads indicate the diffraction patterns from left and right crystals, respectively. (c and d) HRTEM images taken from areas 1 and 2 marked in panel a, respectively.
grain boundary in the middle of the nanowire, confirming that the nanowire is bicrystal in nature. Electron diffraction (Figure 2b) from the area cross the grain boundary reveals that the two single-crystal components are tilted from each other by 61.6°, as labeled in Figure 2a. Figures 2c and d are typical HRTEM images taken from the areas 1 and 2 marked in Figure 2a. The HRTEM images are identical over the entire left and right components, indicating both of them are a single crystal. The lattice fringe spacing is measured to be 0.50 and 0.27 nm, which is in good agreement with (0001) and (101h0) planes of wurtzite AlN (w-AlN), respectively. The HRTEM results confirm the crystal orientations of the two single-crystal components. The orientation relationship between the two single-crystal components and the orientations of the grain boundary lead to three different types of grain boundaries, labeled as G1, G2, and G3 in the inset of Figure 2a. The shorter G2 boundary separates
relatively the longer G1 and G3 boundaries in a sequence of G1-G2-G3-G2-G1. Closer observation of the G2 boundary under HRTEM suggests that it approximately bisects the angle between the (0001) planes of the two single crystals to form two angles of ∼30° between the boundary and the two (0001) planes (Figure 3a). This suggests that the G2 boundary is formed by the (101h1) plane of left single crystal and the (101h1h) plane of right single crystal (Figure 2b), which are equivalent planes and have the same packing pattern. The orientation relationship of the two single crystals determines that the two planes are not parallel but rather tilted from each other by 4.8°; thus, the G2 is a smallangle tilt boundary. The G2 boundary cannot grow for a long distance due to the residual stress caused by the lattice mismatch. The length of the G2 boundaries is measured to be ∼10-25 nm (Figure 3, panels c and d). Note that, although there are residual stresses at the G2 boundary due to the lattice mismatch, the stresses are symmetric in the two single crystals: having the same level and sign because of the symmetrical character of this boundary. HRTEM examination of the G3 boundary reveals that it consists of a set of sub-boundaries which are ∼6 nm long and parallel to the (0001) plane of the right single crystal (Figure 4a). Along the G3 boundary the lattice constant of the left single crystal is ∼0.283 nm and that of the right single crystal is ∼0.269 nm (Figure 4b). The distance of 20 layers on the left single crystal (20 × 0.283 ) 5.66 nm) is approximately equal to the distance of 21 layers on the right single crystal (21 × 0.269 ) 5.65 nm), that is to say, the atomic bonding pattern at the boundary repeats itself after 5.66 nm. This value is equal to the measured length of a sub-boundary, implying that the subboundary is one section of the repeating lattice pattern. The lattice mismatch between the two single crystals is ∼5% at the G3 boundary, which causes residual stresses in the both single
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J. Phys. Chem. C, Vol. 111, No. 46, 2007 17171 G1 boundary and tilt it to right. Although the residual stresses are independent of the boundary length, this force is expected to increase with the length of the G1 boundary and eventually tilts the boundary to the G2 and G3 boundaries. This same argument also accounts for the G3 f G2 f G1 transition. Since the stress levels in the G1 area are the same as in the G3 area, the length of the G1 and G3 boundaries should be similar, which agrees with our observation. Because the asymmetric residual stresses, and thereby the elastic energies, are at the same level for the G1 and G3 boundaries, the alternative growth of the G1 and G3 boundaries cannot result in the release of stresses and energies. However, the alternative growth of the G1 and G3 can offset the asymmetric residual stresses; thus, the zigzag structure can be formed and retained stable. 4. Conclusion
Figure 4. (a) HRTEM image of a G3 boundary, displaying a set of sub-boundaries. (b) Schematic showing the atomic patterns around the G3 boundary.
We report the synthesis of bicrystal AlN zigzag nanowires via directly nitriding pure aluminum powder. The structural characterization reveals that there are three different types of boundaries between the two single-crystal components: G1, G2, and G3. The residual stresses along the G1 and G3 boundaries are driving force for the formation of the zigzag morphology. Acknowledgment. The work is financially supported by the National Science Foundation of China (No. 50372031), the twobased projects of NSFC (No. 50540420104), and the Specialized Research Foundation for the Doctoral program of Higher Education (No. 20050003004). References and Notes
Figure 5. HRTEM image of a G1 boundary, displaying a set of subboundaries. The inset is a schematic showing the atomic patterns around the G1 boundary.
crystals. These stresses are asymmetric: compression in the left crystal and tension in the right crystal. It is worthwhile to point out that these stresses remain the same with increase in the length of the boundary. The G1 boundary also contains a set of sub-boundaries which are ∼6 nm long and parallel to the (0001) plane of the left single crystal (Figure 5). This is exactly opposite to the G3 boundary. Consequently, the asymmetric residual stresses caused by the G1 boundary are also opposite to that of the G3 boundary: tension in the left crystal and compression in the right crystal. It has been shown that the residual stresses which are generated by the lattice mismatch can bend nanowires into circles or spirals.32-34 In our case, the asymmetric residual stresses lead to the formation of zigzag nanowires. For the G1 boundary, the asymmetric residual stresses, which are tension in the left crystal and compression in the right crystal, are expected to bend the crystal toward the left. Instead, we find that the G1 boundary tilts to right to form the next G2 and G3 boundaries (Figure 2a). The formation of zigzag nanowires has not been fully understood. One possibility is that the stiffness of the nanowire is high and residual stresses are not large enough to bend it. As the result, a force is generated that acts on the
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