Epitaxial Directional Growth of Tin Oxide (101) Nanowires on Titania

Oct 5, 2010 - Received 29 April 2010. Published online 5 October 2010. Published in print 3 November 2010. +. Altmetric Logo Icon More Article Metrics...
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DOI: 10.1021/cg100573a

Epitaxial Directional Growth of Tin Oxide (101) Nanowires on Titania (101) Substrate

2010, Vol. 10 4746–4751

Won-Sik Kim,† Daihong Kim,† Kyoung Jin Choi,‡ Jae-Gwan Park,‡ and Seong-Hyeon Hong*,† †

Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, Korea, and ‡Nano-Materials Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea Received April 29, 2010; Revised Manuscript Received July 16, 2010

ABSTRACT: Highly aligned single crystal SnO2 (101) nanowires were epitaxially grown on TiO2 (101) substrates by thermal evaporation via Au-catalyzed vapor-liquid-solid (VLS) growth. The orientation relationship and interface structure between the nanowires and the substrate were determined by X-ray pole figure and high resolution transmission electron microscopy (HR-TEM) combined with the focused ion beam (FIB) lift-out technique. Epitaxially grown SnO2 (101) nanowires exhibited three angular growth directions ([101], [011], and [011]) with different inclination angles to the substrate due to a tetragonal crystal structure. An atomic stacking model was proposed to describe the angular growth of SnO2 (101) nanowires with Æ101æ growth directions. The obtained results are expected to provide an understanding of the growth direction of nanowires and heteroepitaxial relationships between nanowires and substrate to synthesize the well-aligned SnO2 nanowires, which can be integrated into the electronic devices and lead to enhanced properties in the fields such as Li-ion batteries, dye-sensitized solar cells, and gas sensors.

Introduction Vertically grown, highly aligned, single crystalline nanowires have attracted much attention in recent years due to their potential applications as vertical surround-gate field effect transistors (FETs),1 light-emitting diodes,2 highly efficient dye-sensitized solar cells,3,4 nanogenerators,5 etc. Vertically well-aligned nanowires have been successfully synthesized using heteroepitaxial growth by various techniques in a variety of systems including Si,6 Ge,7 InAs,8 GaAs,9 In2O3,10 Ga2O3,11 and ITO.12 Tin dioxide (SnO2) is a wide bandgap (Eg =3.6 eV) n-type semiconductor13 that has been used in a wide range of applications such as gas sensors,14,15 UV-detectors,16,17 field-emitters,18 FETs,19 and solar cells.20 Various SnO2 nanostructures including nanotubes,21 nanoribbons,22 nanodiskettes,23 and nanocubes24 have been synthesized to improve the desired properties for many applications.25 In the epitaxial SnO2 thin films, the surface electronic structure and physicochemical characteristics such as gas sensing properties are strongly dependent on the surface orientation.25,26 The differently aligned or oriented nanostructures, particularly nanowires, are expected to exhibit different optical and electrical properties similar to the epitaxial films. However, there are only a few reports on highly aligned growth of SnO2 nanowires on single crystal substrates,27-32 and the understanding of aligned SnO2 nanowire growth behavior is very limited. In previous studies, SnO2 nanostructures showed various growth directions including [001],33 [301],34 [211],35 or [101],32,36 but the typical growth direction in vertically or angularly grown SnO2 nanowires was [100] irrespective of substrates (Al2O3, TiO2) and substrate orientations.27,31,32 On the other hand, the [101] growth direction is frequently observed in the synthesis of nonoriented SnO2 nanowires,22,32,36 but the *To whom correspondence should be addressed. Phone: 82-2-880-6273. Fax: 82-2-884-1413. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 10/05/2010

growth of highly aligned SnO2 nanowires with [101] growth direction has been reported once on the sapphire (100) substrate, to our knowledge.32 Furthermore, the interface between epitaxially grown SnO2 nanowires and substrate has not been investigated in detail even though epitaxial relationships between SnO2 films and substrates (TiO2, Al2O3) are well established.37-39 In this letter, we have demonstrated that highly uniform, well-aligned, single crystal SnO2 nanowires can be grown on TiO2 (101) substrates by combining carbothermal reduction with Au-catalyzed heteroepitaxial growth. The orientation relationship and interface structure between the nanowires and the substrate were determined by X-ray pole figure and high resolution transmission electron microscopy (HR-TEM). Experimental Section For SnO2 nanowire synthesis, the feedstock source was prepared by thoroughly mixing SnO2 powder (purity 99.99%) and graphite (purity 99.99%) in a 2:1 ratio. The feedstock was loaded into an alumina boat and placed 7 cm upstream from the Au-coated TiO2 (101) substrate in a quartz tube. The quartz tube was then inserted into a horizontal tube furnace and evacuated by a rotary pump to a pressure of 5  10-2 Torr. The furnace was heated to 800 C at a heating rate of 10 C/min and maintained at that temperature for 30 min under a constant Ar gas flow (300 sccm) at a total pressure of 10 Torr. After deposition, the furnace was cooled to room temperature and the samples were retrieved and characterized. The morphology of as-synthesized product was observed by field-emission scanning electron microscopy (FE-SEM, JSM-7401F, JEOL). The phase and in- and out of-plane orientation relationships between nanowires and substrate were examined by X-ray diffraction (XRD) and X-ray pole figure (X’Pert Pro, PANalytical). High-resolution transmission electron microscopy (HR-TEM, JEM-3000F, JEOL) analysis was further performed to investigate the crystal structure of nanowires and the nanowire/substrate interface. The conventional notations are adopted to describe the crystallographic directions and planes such that [uvw] is for an individual direction, for a r 2010 American Chemical Society

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Figure 2. (a) Low magnification and (b) high resolution (HR) TEM images of Au/SnO2 interface. The interface was parallel to the (101) plane of SnO2. Incident beam is in the [010] direction of SnO2.

Figure 1. FESEM images of well-aligned SnO2 nanowires grown on TiO2 (101) substrate; (a) top-view, (b) tilted view, and crosssectional images viewed (c) along the [010] direction, and (d) along the [101] direction of the TiO2 Substrate. family of directions, (hkl) for an individual plane, and {hkl} for a family of planes.

Results and Discussion

Figure 3. (a) θ-2θ X-ray diffraction of SnO2 nanowires on TiO2 (101) substrate and (b) {101} pole figure of SnO2 nanowires at 33.89 Bragg angle (left) and TiO2 substrate at 36.09 Bragg angle (right).

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The phase and out-of-plane orientation of SnO2 nanowires on TiO2 (101) substrate were investigated by X-ray diffraction (Figure 3a). The as-grown SnO2 nanowires were crystalline, and only SnO2 (h0h) diffraction peaks were observed along with TiO2 (k0k) substrate peaks. This demonstrates a strongly preferred (101) orientation of nanowires normal to the substrate. Both SnO2 and TiO2 have a rutile structure with space group of P42/mnm, but the lattice constants of SnO2 are slightly larger than TiO2, resulting in SnO2 diffraction peaks at lower angles (2θ). In-plane orientation of nanowires was investigated by X-ray pole figure analysis, and {101} pole figures of SnO2 nanowires and TiO2 substrate are shown in Figure 3b. Only reflections from the {101} family of planes ((101), (101), (011), (011)) appeared in both pole figures and they corresponded to each other. This indicates that SnO2 (101) nanowires were epitaxially grown on TiO2 (101) substrate with in-plane relationships of [010]SnO2 [010]TiO2 and [101]SnO2 [101]TiO2. This was an expected result because the calculated lattice mismatch between SnO2 and TiO2 was small (2.98 and 4.39%, respectively). Thus, the obtained SnO2 nanowires were heteroepitaxially grown on TiO2 substrate similar to heteroepitaxial SnO2 films.37,38 )

The nanowire growth conditions such as source vapor generation, growth temperature, and source to substrate distance were precisely controlled to synthesize the wellaligned SnO2 nanowires on the substrate. Under high source vapor generation (or fast growth condition), the nanowires were too long to investigate the epitaxial relationships. On the other hand, the nanowires were randomly grown under the low growth temperature because thermal energy of adatoms for diffusion was too low (see Figure S1, Supporting Information). Figure 1 shows FESEM images of as-grown SnO2 nanowires on TiO2 (101) substrate. The average diameter of nanowires was ∼20 nm and the length was typically ∼1.5 μm. The presence of catalytic Au nanoparticles on the tips of the SnO2 nanowires, which are slightly larger than the nanowire diameter, implies the growth of nanowires via a vaporliquid-solid (VLS) mechanism (Figure 2). Top and tilted view images (Figure 1a,b) revealed that majority of the nanowires were well-aligned with an inclination to the substrate surface. A small fraction of nanowires were aligned in the other two different directions, which were also inclined to the substrate surface. The three observed aligned (or growth) directions were not related by 3-fold symmetry. Cross-sectional images (Figure 1c,d) clearly showed the characteristic angular orientations of the nanowires with respect to the TiO2 (101) substrate. When viewed along the [010] direction of TiO2 substrate (Figure 1c), most of the nanowires were inclined at an angle of 68 to the substrate plane and a few of them had an inclination angle of 56. Three aligned directions were evident when viewed along the [101] direction of TiO2 substrate (Figure 1d). The dominant aligned direction was vertical to the surface, while the other two minor aligned directions were projected at an angle of 29 to the surface forming the mirror images, which seem to have the same inclination angles when viewed along the [010] direction. In addition, there was no obvious evidence of a buffer layer between the TiO2 substrate and the base of the SnO2 nanowires implying that nanowires were epitaxially grown directly on the TiO2 substrate.

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Figure 4. (a) Low magnification TEM image of a SnO2 nanowire, (b) SAED pattern of the nanowire in panel a, and (c) HR-TEM image of the SnO2 nanowire in panel a.

The SnO2 nanowires were further analyzed using transmission electron microscopy (TEM) to confirm their crystallographic growth direction. Figure 4a shows a low magnification, bright-field image of an individual nanowire. It reveals a constant diameter and a straight, smooth surface. The average diameter of nanowires was ∼22 nm with a relatively narrow distribution that increased slightly with increasing the growth time (see Figure 2S of the Supporting Information). The selected area electron diffraction (SAED) pattern in Figure 4b showed that the nanowire is single crystal, rutile SnO2 with a zone axis of [010]. High resolution TEM image of SnO2 nanowire showed clear lattice fringes without obvious defects or dislocations (Figure 4c), further confirming its high single crystallinity. The fast Fourier transformation (FFT) pattern, taken from the image marked by the box, was completely indexed to rutile viewed along the [010] zone axis (inset of Figure 4c). It also represents that the sidewall of the nanowire, which was straight and atomistically sharp, was parallel to the (101) plane. Only the {101} and {200} families of planes appeared in the FFT image, and the corresponding interplanar spacings were determined to be 0.259 and 0.347 nm, respectively, which are in good agreement with (101) and (200) interplanar spacings of rutile SnO2. In a tetragonal structure with lattice constants (a=b = 4.738 A˚, c = 3.187 A˚), the (101) plane and [101] direction are not perpendicular and they are inclined at an angle of 68 (see Figure 3S, Supporting Information). The growth direction of nanowire, angled 68 to the (101) plane, was determined to be [101]. The SnO2 nanowires grew along the [101] direction with

Kim et al.

Figure 5. (a) Cross-sectional TEM image of [101] grown SnO2 nanowires on TiO2 (101). Incident beam is in the [010] direction. (b, c) FFT images of selected areas on SnO2 nanowire and TiO2 substrate marked by the boxes in panel a, respectively.

an inclination angle of 68 to (101) plane. Thus, the structural characteristics of the SnO2 nanowires were very close to those of SnO2 nanobelts synthesized by the vapor-solid method where the growth direction, top surfaces, and side surfaces were [101], ( (101), and ( (010), respectively.40 To further investigate the interface between nanowires and substrate, cross sectional TEM specimens were prepared using the focused ion beam (FIB) lift-out technique.41 The crosssectional TEM image viewed along the [010] direction of TiO2 substrate is shown in Figure 5. Consistent with SEM observations, most of nanowires were inclined at an angle of 68 to the substrate. The nanowires had a larger diameter (broadened) base, marked by a circle in Figure 5a. Also, spherical or semispherical particles were frequently observed between nanowires. Although it was difficult to observe clean interfaces between nanowire and substrate due to the overlapping neighbor particles, nonoverlapping interfaces revealed epitaxial, clean interfaces between nanowires and substrate. FFT patterns (Figure 5b,c), taken from the nanowire and substrate marked by the boxes, completely matched each other, indicating that SnO2 (101) nanowires were grown with the same crystal orientation as the TiO2 (101) substrate, consistent with the XRD results. A high resolution cross-sectional TEM image of the interfacial region between nanowire and substrate is shown in Figure 6. It indicates that the (101) plane of the SnO2 nanowire is parallel to the (101) plane of the TiO2 substrate, consistent

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Figure 6. (a) HR-TEM image of a SnO2 nanowire on TiO2 (101) substrate. Incident beam is in the [010] direction, (b) high magnification image of the interfacial area marked by the box in panel a, and (c) schematic representation of the atomic configuration of the SnO2/TiO2 interface viewed along the [010] direction of TiO2.

with the XRD results. The high magnification image in Figure 6b revealed that the interface between nanowire and substrate is clean and atomistically flat, and both (101) lattice fringes are connected at the interface with a ∼2 tilt resulting from the lattice mismatch between SnO2 and TiO2. The tilting of the (101) lattice fringes at the interface is schematically shown in Figure 6c using the crystal structure model. From these observations, it can be concluded that the SnO2 (101) nanowires were heteroepitaxially grown on the TiO2 (101) substrate without a distinct buffer layer. In addition, (101) lattice fringes were clearly observed in the broadened base of nanowire marked by circle in Figure 6a. The FFT pattern of this area exactly matched with that of nanowire indicating that this region had the same crystallographic orientation as the nanowire. At some interfaces, planar defects were observed between broadened base and nanowire, but the growth direction and epitaxial relationships were still conserved. In other cases, the interface could not be distinguished due to overlapping with neighboring particles (see Figure 4S of the Supporting Information). The larger diameter of the nanowire base could be formed by either surface tension (or line tension) acting on the droplet during VLS growth42 or a collision between two nanowires resulting in a growth termination of one of the nanowires. At present, it is not obvious which mechanism is responsible for the broadened base of

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Figure 7. (a) Cross-sectional TEM image of a [011] or [011] grown SnO2 nanowire on TiO2 (101) surface, (b, c) FFT patterns of SnO2 nanowire and TiO2 substrate from panel a, respectively, and (d) HR-TEM image of the SnO2 nanowire.

nanowire, and further studies are required to clarify the dominant mechanism. As mentioned earlier in SEM micrographs (Figure 1), the nanowires inclined at an angle of 56 to the substrate when viewed along the [010] direction were less frequently observed in cross-sectional TEM images. So the interface between the nanowires and substrate could not be seen due to the overlapping with the neighboring particles (Figure 7). FFT patterns (Figure 7b,c), taken from the nanowire and substrate, exactly matched each other revealing that these SnO2 nanowires also had the same (101) crystallographic orientation with TiO2 substrate. As shown in the SEM micrographs (Figures 1c,d and 5S of the Supporting Information), this nanowire was not located in the cutting plane, which was parallel to the TiO2 (010) plane and it was inclined either forward or backward to the cutting the plane. There was no Au catalyst present at the tip of nanowire (Figure 7d), and it appears that the middle of the nanowire was cut during the FIB process. On the basis of these observations and crystallographic considerations (Figure 8), the growth direction was determined to be either [011] or [011]. In this [011] or [011] grown nanowire, the observed side surface was the (200) plane (Figure 7d). The [101], [011], and [011] growth in tetragonal SnO2 are crystallographically equivalent, but in this study, the [101] growth was predominant. It is speculated that the [011] or [011] grown nanowires had a low inclination angle compared to the [101] grown nanowires (56 vs 68, 29 vs 90)

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TiO2) and nanowires grown in the [011] or [011] direction have an inclination angle of 56 (Figure 8b). When viewed along the [101] direction of TiO2 substrate, the [101] grown nanowires are vertical to the TiO2 (101) surface and the [011] or [011] grown nanowires are inclined at an angle of 29.15 to the surface ([010] direction of TiO2) forming a mirror image (Figure 8c). The observed angular directional growth of SnO2 (101) nanowires was successfully explained by atomic stacking models. The optical and electrical (gas sensing) properties of heteroepitaxially grown SnO2 (101) nanowires will be reported in future work. Acknowledgment. This research was supported by Midcareer Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 20100000396). The work at KIST was supported by the program 2N32280. Supporting Information Available: Figures S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 8. (a) 3D-illustration of directional growth of SnO2 nanowires grown on a TiO2 (101) substrate and atomic stacking models of SnO2 nanowires on the TiO2 (101) substrate viewed along the (b) [010] and (c) [101] directions of TiO2.

resulting in a high probability of collision with other nanowires, which could terminate growth early in the process.43 Consequently, [011] or [011] grown nanowires were filtered out as growth proceeded and the [101] grown nanowires became dominant. It has been reported that the crystalline orientation of the catalyst might determine the growth direction and side surfaces of nanowires and nanobelts.44,45 In this study, SnO2 (101) nanowires were heteroepitaxially grown on TiO2 (101) substrate and three growth directions were crystallographically equivalent, and thus, the influence of Au catalyst on the dominance of [101] grown nanowires is less likely. In addition, 180 rotated, twin-like [101] grown nanowires were occasionally observed (see Figure 6S of the Supporting Information). The FFT patterns, taken from the SnO2 nanowire and TiO2 substrate, were mirror images (Figure 6Sc,d of Supporting Information) and HR-TEM images showed line defects at the interface between nanowire and substrate (Figure 6Sb). It is believed that these defects resulted in the formation of twin-like [101] grown nanowires, but further studies are required to clarify the formation mechanism. Conclusion In summary, atomic stacking models for the heteroepitaxially grown SnO2 (101) nanowires on TiO2 (101) substrate are schematically shown in Figure 8. Both SnO2 and TiO2 have the tetragonal rutile structure where the {101} planes and Æ101æ directions are not perpendicular. This resulted in three crystallographically equivalent growth directions, that is, [101], [011], and [011] (Figure 8a). The angles between nanowire and substrate were calculated using the lattice constants of a = b = 4.738 A˚ and c = 3.187 A˚ (JCPDS # 41-1445), and they are the same as those observed in FESEM and TEM images. When viewed along the [010] direction of TiO2 substrate, nanowires grown in the [101] direction are inclined at an angle of 68 to the substrate surface ([101] direction of

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