Wurtzite P-Doped GaN Triangular Microtubes as Field Emitters - The

May 7, 2010 - Ghulam Nabi , Chuanbao Cao , Sajad Hussain , Waheed S. Khan , R. R. Sagar , Zulfiqar Ali , Faheem K. Butt , Zahid Usman , Dapeng Yu...
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J. Phys. Chem. C 2010, 114, 9627–9633

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Wurtzite P-Doped GaN Triangular Microtubes as Field Emitters Lu-Tang Fu,†,⊥ Zhi-Gang Chen,‡,⊥ Da-Wei Wang,§ Lina Cheng,‡ Hong-Yi Xu,‡ Ji-Zi Liu,‡ Hong-Tao Cong,*,† Gao Qing (Max) Lu,§ and Jin Zou*,‡,| Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China, Materials Engineering, The UniVersity of Queensland, Brisbane, QLD 4072, Australia, ARC Centre of Excellence for Functional Nanomaterials, The UniVersity of Queensland, Brisbane, QLD 4072, Australia, and Centre for Microscopy and Microanalysis, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed: January 24, 2010; ReVised Manuscript ReceiVed: April 27, 2010

Novel P-doped GaN triangular microtubes were synthesized by a facile chemical vapor deposition method. This novel structure consists of a single hexagonal wurtzite phase with a triangular cross section. The tube lengths range from tens of to several hundred micrometers, and each side has a width between 0.5 and 1 µm, with a tube wall thickness of several tens of nanometers. The formation mechanism of this triangular tubular structure is a vapor-solid methanism, as determined by electron microscopy. Extraordinary and stable infrared emission (centered at ∼724 nm) from the P-doped GaN triangular microtubes was observed from their photoluminescence spectroscopy. The low turn-on field (2.9 V µm-1), high field-enhancement factor, large current density (3 mA cm-2 at a field of ∼9.5 V µm-1), and high stability indicate the suitability of P-doped GaN microtubes as potential field emitters. This field emission property is attributed to the specific crystallographic featuresthe rigid triangular structures with effective P doping and rough surface hillocks. 1. Introduction Tailoring one-dimensional (1D) micro/nanostructures as building blocks for functional materials and devices is the key for specific applications in micro/nanoelectromechanical systems, optoelectronics, field emitters, and light emission diodes.1-3 Among them, the tubular structure is of special interest due to the confinement effect and potentials for fluid path, fuel cells, hydrogen storage, field emitters, and other nanodevices.4-6 Different cross-sectional shapes of the tubular structures, such as round, hexagon, and rectangle, possess different optoelectronic and physiochemical properties. Compared with these shapes, the triangular cross section should be the most stable and rigid shape of all these geometries. Moreover, different crystal structures usually form different cross-sectional shapes.7 For layered materials, such as carbon nanotubes,8 BN nanotubes,9 and MoS2,10 the cross-sectional geometry is generally circular. On the other hand, as for the nonlayered, but anisotropic crystalline materials, such as GaN, AlN, ZnS, and ZnO,11-14 the preference is to form a faceted morphology (hexagonal, rectangular, and triangular cross sections). Therefore, it is of great importance to fabricate a desirable structure with a controllable morphology for specific optoelectric applications. GaN, an important semiconductor with a direct band gap of 3.4 eV, has received considerable attention because of its great potential for blue UV light emitters, powerful high-temperature electronic devices, and field emitters.15-24 Lee et al.25 predicted the stability and electronic structure of GaN nanotubes using density-function theory. Goldberger et al.26 fabricated faceted * To whom correspondence should be addressed. E-mail: [email protected] (H.-T.C.), [email protected] (J.Z.). † Chinese Academy of Sciences. ‡ Materials Engineering, The University of Queensland. § ARC Centre of Excellence for Functional Nanomaterials, The University of Queensland. | Centre for Microscopy and Microanalysis, The University of Queensland. ⊥ These authors contributed equally to this work.

hexagonal GaN nanotubes by an “epitaxial casting” approach using hexagonal ZnO nanowires as templates. Hu et al.27 synthesized single-crystalline cubic GaN nanotubes with rectangular cross sections via a simple high-temperature catalystfree template route. However, to our best knowledge, GaN nanoscaled or microscaled tubes with triangular cross sections have not been produced. In this study, we demonstrate a facile chemical vapor deposition (CVD) method to fabricate P-doped GaN triangular microtubes with lengths ranging from tens of micrometers to several hundred micrometers and each side having a tube width of between 0.5 and 1 µm and a tube wall thickness of several tens of nanometers. The formation mechanism of the triangular tubular structure is presented based on the structural characteristics determined by electron microscopy. It was found that the triangular tubular structure is a combination of several growth features in the wurtzite structured GaN. These novel P-doped triangular microtubes possess the low turn-on field (2.9 V µm-1), high field enhancement factor, large current density (3 mA cm-2 at a field of ∼9.5 V µm-1), and small fluctuation, suggesting that our synthesized GaN microtubes have the potential to be used as field emitters and optoelectronic devices. 2. Experimental Section Synthesis of GaN Structures. The uniform high-energy ballmilled mixture of Ga2O3 powders and red phosphorus was used as the raw material and placed on a long quartz piece with dimensions of 180 mm ×13 mm ×2 mm. The substrates used were p-type Si(001) wafers with a resistivity of 0.1-5 Ω · cm. The Si substrate was chemically cleaned by using the Shiraki method28 and placed downstream at a distance of 100 mm from the center of the quartz piece to collect the product. The quartz piece was then inserted into a small quartz test tube (Φ15 mm × 200 mm) with a small hole at the downstream end. The small quartz test tube was transferred into a quartz reaction tube (Φ35

10.1021/jp100689s  2010 American Chemical Society Published on Web 05/07/2010

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Figure 1. SEM images (a-d) and XRD pattern (e) of the GaN microtubes.

mm ×1000 mm) and mounted in the CVD furnace. Before heating, a small amount of water vapor was injected into the small quartz test tube in order to activate the atmosphere. Ar gas was piped into the quartz tube to flush out any residual air. The mixture gases of NH3 and H2 with flow rates of 200-250 and 30-60 mL min-1, respectively, were then introduced instead of Ar as the temperature was increased to ∼950 °C. After a reaction time of 20 min, a yellowish wool-like product was found on the Si substrate. Structural and Property Characterizations. The synthesized products were comprehensively characterized using X-ray diffraction (XRD, RINT2200, Cu KR), scanning electron microscopy (SEM, JEOL 890, 6400, and LEO SUPRA 35), transmission electron microscopy (TEM, FEI Tecnai F30), and Raman spectroscopy (JY Labram HR 800, 632.8 nm laser, and 325 nm laser). Field Emission. The field emission properties of GaN triangular microtubes were measured in a ball-type chamber that was evacuated to 6 × 10-7 Pa by an ultra-high-vacuum system.29 The anode was a cylinder-shaped platinum probe with a diameter of 1 mm, and the P-doped GaN triangular microtubes were fixed

onto a copper stage with conductive paste as the cathode. The distance between the two electrodes was 200 µm. High voltage was supplied by a power source (Keithley 248) and the current signal, which was increased by 20 V steps, under an increasing applied field and recorded on an electrometer (Keithley 6514) with picoampere sensitivity. 3. Results and Discussion Figure 1a-d shows the general morphology of the synthesized product, revealing that over 75% of the product consists of straight, tubular structures with triangular cross sections with the remainder being nanowires or nanoparticles. The length of the triangular GaN microtubes ranges from tens of micrometers to several hundred micrometers (Figure 1a). Figure 1b-d shows that the surface of the GaN microtubes is considerably rough with regularly aligned hillocks and each side of the triangular tubes is between 0.5 and 1 µm in width and several tens of nanometers in tube wall thickness (normally 40-60 nm). Figure 1e is an XRD pattern of the synthesized product, in which all reflection peaks can only be indexed as hexagonal wurtzite GaN, with lattice constants of a ) 0.31891 nm and c ) 0.51855 nm

Wurtzite P-Doped GaN Triangular Microtubes

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Figure 2. (a-c) Typical low-magnification TEM image, its corresponding SAED, and high-magnification image of a P-doped GaN triangular microtube aligned along the zone axis of [0001]. (d-f) TEM image of the microtube tilted along one side with an angle of 30°, its corresponding SAED image, indexed as the zone axis of [21j1j0], and high-magnification image. (g) Structural model.

(JCPDS No. 50-0792). No characteristic diffraction peaks of GaP or Ga2O3 phases were detected in the XRD pattern, suggesting that the synthesized product has high single-phase crystallinity. Figure 2a is a bright-field TEM image of a typical triangular GaN microtube and shows clearly that the surface of the triangular GaN microtube is rough with hillock structures that are arranged in regular rows perpendicular to the tube length. A selected area electron diffraction (SAED) pattern (Figure 2b), taken from the microtube in Figure 2a, was indexed as the [0001] zone axis of hexagonal GaN. The diffraction spots indicate that each side of the triangular GaN microtubes is a single crystal. The corresponding high-resolution (HR) TEM image (Figure 2c) of the bottom edge reveals a d spacing of 0.28 nm, which corresponds to GaN{011j0} atomic planes. All these indicate that each side has high crystallinity. Figure 2d,e shows the morphology and the diffraction pattern of the same

microtube (Figure 2a), viewed along the direction that is rotated 30° around the axial direction of the microtube. By tilting such an angle, one facet is parallel to the electron beam, as the dark vertical line shown in Figure 2d. The corresponding SAED pattern confirms the [2j110] zone axis of GaN, suggesting that the side surfaces are parallel to the (0001) planes, and the axial direction of the microtube is along the direction. Figure 2f shows its corresponding HRTEM image, clearly revealing a two-dimensional lattice image perpendicular to each other with d spacings of 0.28 and 0.52 nm, which are consistent with those of the (011j0) and (0001) planes of hexagonal GaN, respectively. The triangular structural model is shown in Figure 2g, in which each side of the triangular microtube corresponds to a singlecrystalline GaN nanobelt. An EDS trace of the triangular GaN microtubes is shown in Figure 3a, indicating that the microtubes contain Ga, N, and P elements (note that the Cu peaks have originated from the TEM grid). Composition distributions were

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Figure 3. (a) EDS pattern of P-doped GaN microtubes. (b) TEM BF image of a GaN microtube and the corresponding Ga, N, and P elemental maps.

Figure 4. Schematic atomic structure and growth models. (a) A projection of the GaN wurtzite lattice along the (011j0) plane. The {0001} and {1j013} plane families are the growth directions of the side of the microtubes and sharing planes of the triangular microtube. (b) A schematic representation of the twin structures and growth directions of the triangular microtube. (c) Schematic growth model.

determined by electron energy loss spectroscopy (EELS) elemental mapping (Figure 3b), indicating that the element P is uniformly distributed within the triangular GaN microtube. EDS analysis showed that the P signals recorded on various GaN microtubes vary slightly with an elemental composition of 4.0 ( 0.5 atom %. From these observations, the observed triangular cross sections of the GaN microtubes have several unique features: (1) The shape and crystallographic facets are well-defined. (2) Each side of a GaN microtube is a single-crystalline GaN nanobelt. (3) The triangular GaN microtubes possess the wurtzite structure, uniform shape, high crystallinity, and P-doping. (4) The triangular cross sections of the GaN microtubes have a rough surface with regularly aligned hillock structures perpendicular to the tube length, uniform P doping, and a micrometersized base. The atomic nature of the triangular GaN microtubes can be determined from the above structural characterizations. Their atomic schematic models are shown in Figure 4a,b. By projecting the structural unit cell along the [011j0] direction, the (011j0) atomic plane can be clearly seen, which is the most typical (0001) polar surface that is terminated with Ga and N centers, respectively. The (1j013) atomic plane also can be clearly observed. From Figure 4b, two sides of the triangular GaN microtubes share the {1j013} plane and grow along the [0001]

or [011j0] to form three-dimensional triangular tubular structures. Because no catalyst was used to synthesize the microtubes, it is anticipated that the growth of the triangular GaN microtubes is governed by the vapor-solid (VS) mechanism so that the formation mechanism may be proposed as follows (illustrated in Figure 4c). (1) The wurtzite structured GaN nuclei with a triangular morphology form first. Although energetic calculations30 suggest that the triangular GaN nanotubes may not be stable, our experiment shows that wurtzite structured triangular GaN microtubes are stable, which may be from our tubes being of micrometer size and doped with P. If so, P may stabilize triangular nuclei formation. In fact, our experiment demonstrated that no GaN triangular microtubes can be found without P accession in the precursors. However, the intrinsic P-doping stability mechanism is still not clear and will need further clarifications. Therefore, the formation of the wurtzite structured GaN nuclei could be governed by thermodynamics.31 (2) The wurtzite structured GaN nuclei preferentially grow along directions into two-dimensional (2D) triangular bases (Figure 1c). Under stable conditions, the previous positions of formed triangular bases may be new nucleation sites for further growth. In this way, the structure tends to grow into a triangular shape with stacking along the directions, and each side grows along the [0001] direction to ultimately form

Wurtzite P-Doped GaN Triangular Microtubes

Figure 5. PL spectrum of a P-doped GaN triangular microtube: T ) 300 K and excitation at 325 nm.

three-dimensional triangular structures. Rough surface hillocks grown on the surface of triangular GaN microtubes could be due to the GaN vapor depletion and faster facet growth velocity.32 As mentioned above, the synthesized P-doped triangular GaN microtubes may have unique optoelectronic properties. For this reason, the photoluminescence of these GaN microtubes is measured, and their typical spectrum is shown in Figure 5. The photoluminescence spectrum of P-doped triangular GaN microtubes is composed of one extraordinary and stable visible emission, centered at ∼724 nm with a shoulder at 880 nm. Normally, the high-purity, highly crystalline GaN nanowires or single crystals can produce an ultraviolet emission at ∼364 nm (3.4 eV),16,33 which can be attributed to band-edge-related emission. Other emissions at 436,33 451,34 468,34 470,18 524,34

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9631 and 550 nm33 can also be observed in GaN nanostructures. Those emission peaks are probably associated with the deep level or defect levels. In this study, the emission is shifted greatly to the infrared range (the emission centered at ∼724 nm). Such strong and stable infrared emission is a new emission, which could originate from uniform P doping when compared with those pure GaN nanostructures. It is of interest to note that this infrared emission is important for the optoelectronic applications in scatheless biological diagnosing and medical analysis and that the emission is able to be tuned by band-gap engineering, for example, by varying the doping level for GaN nanostructures. Figure 6 shows the typical electric field characteristics. The turn-on field (Eto), which is defined as an electric field to produce a current density of 10 µA cm-2, is low at ∼2.9 V µm-1. The maximum emission current density can reach 3 mA cm-2 at an applied field of ∼9.5 V µm-1. The turn-on field is far lower than that of GaN nanowires (12,19 5.1,35 7.5,36 8.517) and nanobelts37 and comparable with other GaN nanostructures,38,39 as summarized in Table 1. These data suggest that our GaN triangular microtubes may be promising candidates for field emitters.40,41 To avoid systematic errors caused by different field emission test equipment, the field emission properties of the undoped GaN nanowires and P-doped GaN nanowires were measured using the same test equipment for comparison, and their turn-on fields were ∼8.6 and 5.9 V µm-1 (Figure 6a), respectively; which are similar to previous results.17,35 Moreover, blank experiments (no samples) show that no detectable field emission current was observed. From these comparisons, the reason for the low Eto and high field emission current density can be attributed to their superior structural characteristics, that

Figure 6. FE properties of the P-doped GaN triangular microtube: (a) J-E curves with a turn-on field of GaN microtubes, P-doped nanowires, and undoped nanowries. (b) A Fowler-Nordheim plot. The straight line is a linear fit of the ln(J/E2) - (1/E) plot. (c) Field emission current stability of the GaN microtubes.

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TABLE 1: Field Emission Properities of Different 1D GaN Nanostructures features GaN nanowries P-doped GaN nanowires needle-like GaN nanowires thin GaN nanowires herringbone GaN nanobelts triangular GaN nanowires GaN nanowires GaN nanowires P-doped GaN nanowires P-doped triangular GaN microtubes

turn-on field at 10 µA cm-2 (V µm-1) 12 5.1 7.5 8.5 6.1 at 0.1 µA cm-2 3.96 8.5 at 0.1 µA cm-2 8.5 5.9 2.9

β value

1170 β1 ) 1600; β2 ) 750 1050 1170 βL ) 3884 and βH ) 1435 using Φ ) 4.1 eV; βL ) 2774 and βH ) 1115 using Φ ) 3.5 eV

is, the single-crystalline triangular-shaped structure with rough surface hillocks and P-doping. The field emission current-voltage characteristics were analyzed by using the Fowler-Nordheim (F-N) equation43

J ) (Aβ2E2 /Φ)exp(-BΦ3/2 /βE) where J is the current density, E is the applied field, Φ is the work function, β is the field enhancement factor, and A and B are constants with values of 1.56 × 10-10 A eV V-2 and 6.83 × 103 eV-3/2 µm-1,44 respectively. The work function (Φ), defined as the difference between the vacuum level and the Fermi energy, was predicted using first-principles calculations within the macroscopic average approach.45,46 In particular, the calculations were performed on the P-doping GaN structures, and its corresponding work function was approximately 3.5 eV. Figure 6b shows the F-N plot for the triangular GaN microtubes. The approximate linear relationship of the F-N plot confirms that the origin of the observed high current is from tunneling electron emission. It should be noted that there exist two different slopes in the F-N plot. Accordingly, the β value can be divided into βL and βH under low and high electric fields, respectively, and the two β values can be calculated as ∼2774 and ∼1115 from their gradients, respectively. As a comparison, using the work function of bulk GaN (4.1 eV),42 the two β values can be calculated as 3884 and 1435, respectively. In fact, these β values are comparable with those β values shown in Table 1, suggesting that, as far as the β values are concerned, our synthesized microtubes are sufficient for practical applications.47 On the basis of the comprehensive SEM and TEM observations, a large number of small hillocks grew on the surface of the triangular GaN microtubes. For each individual triangular GaN microtube, there could exist two kinds of electron emission origins: body emission and surface hillock emission due to their different geometrical enhancement factors. The surface hillocks, which have higher geometry enhancement factors, would be the first to emit electrons under the low electric field, whereas the body emission would take effect under the high electric field. Therefore, this structural variety in the microtubes results in two different β values.48 Another plausible mechanism for these phenomena (FE current saturates at high voltages and the F-N relationship shows an inflection point) is current saturation due to an intrinsic resistance or depletion of electron in GaN semiconductor emitters. Figure 6c shows the stability of the emission current density of the triangular GaN mircotubes within 160 min under an applied electric field of 3.40 V µm-1. The initial current density and the average current density are 12.0 and 12.4 µA cm-2,

stability

6% at 250 µA cm-2 8.7% at 7.91 µA cm-2 3.3% at 12 µA cm-2 or 2.8% at 10 µA cm-2

references 19 35 36 17 37 38 42 this work this work this work

respectively. No notable current density degradation was observed, and the emission current fluctuation was as low as ∼3.3%, proving the high stability of the triangular GaN microtubes as a field emitter. Nevertheless, to understand the FE emission uniformity, we have recorded results from more than 10 test points (the probe size is 1 mm in diameter for each test point), from which we confirmed that these results have similar FE properties. To further valuate the emission current stability at a low applied electric field of 2.9 V µm-1 where the initial current density and the average current density are 10.0 and 10.28 µA cm-2 within ∼160 min of testing, the emission current fluctuation can be calculated as 2.8%. Generally, a given field emission device would exhibit poor stability if the emitter was not thermally stable because significant heat can be generated when a high electric field is applied. The stable field emission performance shown in this study is related to the rigid triangular geometrical structure and tubular morphology, which has a large contact area (with each side of the triangular microtubes) that can quickly transmit heat from the field emission area to the substrate and a surface area (enhanced by the surface hillocks) that can efficiently disperse heat so that the microtubes can be well protected from damage due to superheating. Therefore, the excellent field emission properties of triangular GaN microtubes are mainly attributed to the following three aspects. First, effective uniform P doping in triangular GaN microtubes brings an impurity band into the band gap of GaN. As indicated in the energy band diagram in Figure 7, the P doping can raise the Fermi level toward a higher energy, that is, from EF0 to EF. This position shift of the Fermi level can favorably improve the conductivity of the GaN; more electrons are easier to tunnel to the vacuum through the impurity band under the electric field. Previous studies49,50 have confirmed that the doping could notably improve the field emission performance of semiconductor nanomaterials. Second, the numerous

Figure 7. Energy band diagram showing that the P doping states raise the Fermi level.

Wurtzite P-Doped GaN Triangular Microtubes surface hillocks possess high field enhancement factors because the rough surface can remarkably enhance materials’ field emission performance.51 Third, the unique configuration of GaN microtubes possesses a large surface area and a triangular cross section. The large surface can effectively disperse some of the heat, and the triangular configuration can enhance the structural stability. Therefore, the triangular GaN microtubes are not destroyed when they are subjected to superheating or high electric field. 4. Conclusions In summary, P-doped GaN microtubes with a triangular cross section were synthesized via a CVD method. The GaN microtubes are straight, and the length is from tens of to several hundred micrometers. Each side of the triangular GaN microtubes has a width between 0.5 and 1 µm and a tube wall thickness of tens of nanometers. The extraordinary and stable infrared emission (centered at ∼724 nm) from the P-doped GaN triangular microtubes is attributed to the P-doping, indicating that this material has great potential as nano-optical and/or nanooptoelectronic devices in nanoscale surgery and spectroscopy. Field emission measurements show that the turn-on field of the triangular GaN microtubes is ∼2.9 V µm-1 with high stability, which are attributed to the P doping, rough surface hillocks, and the unique configuration. Acknowledgment. This work was supported by the External Cooperation Program of the Chinese Academy of Sciences (Grant No. GJHZ200815) and the Australian Research Council, UQ start-up grant and UQ ECR grant. The authors thank Professor H. M. Cheng and Mr. G. J. Auchterlonie for their help. References and Notes (1) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (2) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966. (3) Chen, Z. G.; Cheng, L.; Zou, J.; Yao, X. D.; Lu, G. Q.; Cheng, H. M. Nanotechnology 2010, 21, 065701. (4) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami, T.; Iijima, S. AdV. Mater. 2004, 16, 397. (5) Li, W. Z.; Liang, C. H.; Qiu, J. S.; Zhou, W. J.; Han, H. M.; Wei, Z. B.; Sun, G. Q.; Xin, Q. Carbon 2002, 40, 791. (6) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (7) Xu, B.; Lu, A. J.; Pan, B. C.; Yu, Q. X. Phys. ReV. B 2005, 71, 125434. (8) Iijima, S. Nature 1991, 354, 56. (9) Golberg, D.; Bando, Y.; Bourgeois, L.; Kurashima, K.; Sato, T. Appl. Phys. Lett. 2000, 77, 1979. (10) Chhowalla, M.; Amaratunga, G. A. J. Nature 2000, 407, 164. (11) Liu, B. D.; Bando, Y.; Tang, C. C.; Shen, G. Z.; Golberg, D.; Xu, F. F. Appl. Phys. Lett. 2006, 88, 93120. (12) Wu, Q.; Hu, Z.; Wang, X. Z.; Lu, Y. N.; Chen, X.; Xu, H.; Chen, Y. J. Am. Chem. Soc. 2003, 125, 10176. (13) Yin, L. W.; Bando, Y.; Zhan, J. H.; Li, M. S.; Golberg, D. AdV. Mater. 2005, 17, 1972. (14) Tong, Y. H.; Liu, Y. C.; Shao, C. L.; Liu, Y. X.; Xu, C. S.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Phys. Chem. B 2006, 110, 14714. (15) Zhong, Z. H.; Qian, F.; Wang, D. L.; Lieber, C. M. Nano Lett. 2003, 3, 343. (16) Kim, H. M.; Kang, T. W.; Chung, K. S. AdV. Mater. 2003, 15, 567.

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