Single-Crystal and Twinned Zn2SnO4 Nanowires with Axial Periodical Structures Jianxiong Wang,† Xiao Wei Sun,*,† Shishen Xie,‡ Weiya Zhou,‡ and Yi Yang† School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore, 639798, and Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing, China, 100080
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 707–710
ReceiVed NoVember 3, 2006; ReVised Manuscript ReceiVed October 7, 2007
ABSTRACT: Single-crystal Zn2SnO4 (ZTO) nanowires were synthesized by thermal evaporation of a mixture of Zn and SnO powders at 1000 °C. The ZTO nanowires showed unique periodical structures in contrast to the traditional nanowires. It was found that the yield of the single-crystal ZTO nanowires was strongly dependent on the weight ratio of Zn and SnO in the source mixture. With the decrease of Zn content in the source, twinned ZTO nanowires with alternating (11j1) twins along the axial direction can be obtained. The formation mechanism of these ZTO nanostructures is discussed. The photoluminescence of the ZTO nanowires was measured. One-dimensional (1D) oxide nanostructures have stimulated great interest owing to their unique optical and electronic properties as well as their potential applications in fabrication of nanodevices..1,2 Recently, many efforts have been paid to fully control the structures of nanoscale materials because it is a crucial step toward the realization of functional nanodevices.3–6 In fact, simple 1D oxide nanostructures with various morphologies have been synthesized successfully by various methods.7,8 However, growths of binary oxide nanostructures with precise shape control are rare. Up to now, limited 1D binary oxide nanostructures have been synthesized using hydrothermal methods.9,10 However, the nanostructures obtained from this method are usually polycrystalline or amorphous. There is still a lack of an effective approach to synthesize single-crystalline binary oxide nanostructures on a large scale. As an important binary oxide, Zn2SnO4 (ZTO) is known to have high chemical sensitivity, low visible absorption, and excellent optical electronic properties, and it has been considered as a promising candidate for transparent electrode, sensor, and photovoltaic devices.11–13 In this paper, single-crystalline spinel ZTO binary oxide nanowires were synthesized on a large scale by a vapor phase transport method. The twinned ZTO nanowires showed unique periodical morphologies with twins alternating along the axial direction of the nanowires, different from traditional nanowires. These nanostructures are potentially useful in fabrication of nanodevices due to the periodical axial arrangement of nanocrystals in one-dimension. The ZTO nanowires were synthesized by a vapor phase transport method carried out in a horizontal furnace. A mixture of Zn and SnO powders with a weight ratio of 1:3 was loaded into a small quartz boat which was then placed in the center of a quartz tube mounted on a furnace. A {100} silicon wafer coated with a 10 nm Au thin film was used as the substrate, which was positioned about 5 cm downstream from the quartz boat. The quartz tube was first pumped down to 10 Torr and then heated to the working temperature of 1000 °C. The fabrication took about 4 h with a constant Ar flow of 50 scccm. After the reaction, the furnace was cooled down to room temperature and the resulting products were collected for * Corresponding author. E-mail:
[email protected]. † Nanyang Technological University. ‡ Chinese Academy of Sciences.
Figure 1. X-ray diffraction (XRD) pattern of the as-prepared products.
characterization by scanning electron microscopy (SEM, Hitachi S-5200), powder X-ray diffractometry (XRD; D/Max-2400) with Cu KR radiation, and high resolution transmission electron microscopy (HRTEM; JEM-2010F). The photoluminescence (PL) spectra were recorded in the spectral range 300-650 nm at room temperature using a He-Cd laser operating at 325 nm as the excitation source. Figure 1 shows the XRD pattern of the as-prepared product. All the diffraction peaks in the XRD pattern can be indexed to face-centered spinel ZTO with a ) 0.865 nm, according to the standard XRD data file (JCPDS file No. 24-1470). No other impurity phases such as ZnO, SnO2, etc. were found in Figure 1, indicating the good phase purity of the products. Figure 2 shows typical SEM images of the as-prepared products with various magnifications. The general morphology of the as-synthesized nanowires is shown in Figure 2a. Many nanowires were observed in the image. The nanowires have diameters ranging from 50 to 100 nm and lengths up to tens of micrometers. Enlarged SEM images of the nanowires dispersed on a copper grid are shown in Figure 2b-d. Different from the conventional nanowires, the nanowires with periodical morphologies were observed in Figure 2. The nanowires are formed by a row of inlaid uniform rhombohedral nanocrystals along the axial direction of the nanowire. These well-organized nanocrystals form ordered steps on the surface of the nanowires. The steps, though uniform for individual nanowires, show some variations for different nanowires, with the period ranging from
10.1021/cg060779+ CCC: $40.75 2008 American Chemical Society Published on Web 12/06/2007
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Figure 2. Typical SEM images of the as-prepared nanowires. (a) Morphology of the as-synthesized nanowires in low magnification. (b) Enlarged SEM images of the nanowires dispersed on the copper grid. (c) ZTO nanowires with a step of about 50 nm in length. (d) ZTO nanowires with a step of about 20 nm in length. (e) Branched nanowires with a definite branching angle.
Figure 3. (a) Bright-field TEM image of straight ZTO nanowires formed by several rhombohedral nanocrystals along the axis of the nanowire. The inset is the corresponding SAED pattern taken along the [110] zone axis. (b) HRTEM image of the ZTO nanowires. The black lines indicate small protrusion steps on the surface.
10 to 60 nm. Figure 2c and 2d show two nanowires with different step periods of 50 and 20 nm, respectively. In addition, branched nanowires are frequently observed in the product, as shown in Figure 2e, where the branch angle (the angle between the branching wire and stem wire) is measured to be about 71 °C. This branch angle is definite for various branched nanowires, indicating the branched nanowires and the stem nanowires have a definite orientation relationship. Figure 3a shows a bright-field TEM image of a straight ZTO nanowire formed by several rhombohedral nanocrystals along the axis of the nanowire. The corresponding selected area electron diffraction (SAED) pattern taken along the [110] zone axis is shown in the inset of Figure 3a. The sharp and bright spots in the SAED pattern indicate the single-crystal feature of the nanowires. According to the SAED pattern, the growth orientation of the ZTO nanowires is determined to be [11j1]. An HRTEM image recorded from the side of the nanowires is shown in Figure 3b. Three groups of parallel fringes with d-spacings of 0.43, 0.50, and 0.50 nm were clearly observed, which match those of the (002), (1j11), and (11j1) planes of the spinel ZTO, respectively. Though the surface of the rhombo-
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Figure 4. (a) Typical SEM image of the nanowires synthesized when the ratio of Zn in the source mixture is reduced. (b) A high resolution SEM image of a ZTO nanowire constructed by many coplanar octahedron nanocrystals. (c) A high resolution SEM image of ZTO nanowires with many ordered triangle steps distributed on the surface.
hedral nanocrystals seems smooth in the SEM picture, some small protrusions on the surface were found, as indicated in the HRTEM image. No dislocations and planar defects were found in the HRTEM image, which further confirmed the singlecrystalline nature of the nanowires. Different nanostructures were obtained when the ratio of Zn in the reactant was varied. Figure 4a shows a typical SEM image with low magnification of the nanowires synthesized when the ratio of Zn in the reactant was reduced from 1:3 to 1:4 (ZnO to SnO weight ratio). Two different types of ZTO nanostructures were further found by high-resolution SEM. Figure 4b shows one type of ZTO nanowire constructed by many coplanar octahedral nanocrystals. Each of the two adjacent octahedral nanocrystals shares one common plane and forms a periodic pattern along the nanowire axis. Figure 4c shows another type of ZTO nanowire formed by many ordered triangle steps distributed on the surface of the nanowire, forming a periodic pattern along the axis of the nanowire. Figure 5a displays the bright-field TEM image of the nanowire shown in Figure 4b. Uniform octahedron crystals can be found along the nanowire axis. The corresponding SAED pattern taken at the boundary of two octahedron nanocrystals (indicated in Figure 5a) is shown in Figure 5b. The SAED pattern reveals typical twin reflections. Without titling of the nanowires, the diffraction pattern recorded from different boundaries along the wire axis gives exactly the same pattern, indicating the regular twin structure along the length direction. In Figure 5b, the “twin” reflections are indexed with a subscript “T” and the remaining matrix reflection indices are marked without a subscript. The zone axis of the diffraction pattern is indexed to be [110] for the spinel structured ZTO. The twinning plane is determined to be (11j1), and the twinning direction is [1j12], perpendicular to the axis of the nanowires. This twin can also be regarded as being formed by rotating one part of the crystal (twin) by 180 °C along the normal direction of the (11j1) crystal plane, while the remaining sections of the crystal (matrix) maintain the original orientation. Shown in Figure 5c is an HRTEM image around the twin boundary. The image clearly
Single-Crystal and Twinned Zn2SnO4 Nanowires
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Figure 6. Schematic of the proposed growth mechanism of singlecrystal ZTO nanowires (a and b) and twinned ZTO nanowires (c and d).
Figure 5. (a) Bright-field TEM image of the ZTO nanowire shown in Figure 4b. (b) The corresponding SAED pattern taken along the [110] axis reveals typical twin diffraction reflections. The “twin” reflections are indexed with a subscript “T”. (c) HRTEM image around the twin boundary. The twinned plane is indexed to be (11j1). (d) TEM image of the ZTO nanowire shown in Figure 4c. (e) The corresponding SAED pattern of the nanowire in Figure 5d. (f) HRTEM image of the nanowires in Figure 5d, which clearly reveals the typical twin boundary.
reveals the twin plane and the twin direction, which is consistent with the electron diffraction pattern. The TEM image of the nanowire is shown in Figure 4c, and the corresponding SAED patterns are shown in parts d and e, respectively, of Figure 5. The SAED patterns also show twin diffraction reflections, which are similar to those of the nanowire shown in Figure 5a. The same pattern can be obtained from different parts of the wire axis. From the SAED pattern, the spinel structures of ZTO can also be indexed and the twin plane and twin orientations were determined to be (11j1) and [1j12], respectively. The HRTEM images of the nanowries are also given in Figure 5f. The typical twin boundary is clearly seen in the image. The twin plane and twin direction determined from the image agree with the SAED results. The single-crystal ZTO nanowires and the twinned nanowires show unique periodical morphologies, which may have potential applications in fabrication of a range of precisely defined nanodevice arrays on an individual nanowire. Recently, Wang et al. suggested that the different surface chemical activities are the main reason for the formation of different ZnO nanostructures such as nanosaws and dendrites.14,15 In the present case, we believe that the formation of the ZTO nanostructures is most likely due to the asymmetric growth of the different facets during the growth process (Supporting Information). In fact, singlecrystal ZTO nanowires can be considered as many inlaid rhombohedral nanocrystals along the [11j1] orientations. For each rhombohedral nanocrystal, the surfaces can be determined to be (002), (1j11), and (111j) from the experimental results
demonstrated above. Parts a and b of Figure 6 give a model to schematically illustrate the formation of the ZTO nanowires. During the growth process, the ZTO nanocrystals will first nucleate in the substrate. Then the octahedral nanocrystals grow rapidly along the [11j1] direction and stretch to a nanowire. At the same time, the nanocrystal will grow along the [1j11], [1j11], and [002] directions. The ZTO nanocrystals grow along the three competitive directions and then form the periodical single-crystal ZTO nanowires. Various growth rates in different directions thus form different steps on the surface of the nanowires. The formation of branched nanowires is due to the epitaxial growth of nanowires from equivalent crystal directions.16 In this work, we believe the branched ZTO nanowires originate from the equivalent [11j1] and [1j11] or [111j] directions. The angle between any two directions of a face centered cubic crystal is calculated to be 70.5 °C, which agrees with the angle we measured in Figure 2e. According to the TEM data (Figure 5), we found that the twinned ZTO nanowires shown in Figure 4b and c had almost the same structure. All these nanowires are formed by many octahedral ZTO nanocrystals (Figure 4b) or truncated octahedral nanocrystals (Figure 4c) connected with a series of (11j1) twinned planes. Compared to the twinned nanowires in Figure 4b, the twinned nanowires in Figure 4c have many regular triangle steps on the surface sharing a common base with a neighboring triangle at the twin boundary. Thus, we believe the formation of these triangle steps may be due to the secondary growth of the ZTO layer on the surface of the nanowires. To further understand the structure of twinned nanowires, the structure relations of the twinned nanowires are schematically illustrated in Figure 6c and d. However, the reason why the nanowires are formed by many ordered twinned crystals is not yet clear at this stage. From the view of the total formation energy, the twinned crystal with higher energy is not favored. During the growth process, the energy of the nanowires may accumulate because of the strain, defects, and other disturbances. The formation of the ordered twinned nanowires can be considered as a relaxation of the energy accumulated. Obviously, these twinned nanowires are in a metastable state and their energy is higher than that of the single-crystal nanowires. The detailed growth mechanism of the ZTO nanowires and twinned nanowires are yet to be confirmed. Room temperature photoluminescence (PL) spectra of the single-crystal ZTO nanowires and the twinned nanowires are shown in Figure 7. Both the single-crystal ZTO nanowires and twinned ZTO nanowires show a stable and strong green emission band centered at 494 nm in the PL spectrum. Obviously, this green emission peak is not the band-to-band emission of ZTO (band gap ∼ 3.6 eV).17 Therefore, the green emission of ZTO nanowires should be due to other luminescent centers, such as oxygen vacancies, defects, etc. The ZTO nanowires are faceted and have many periodical structures on
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Acknowledgment. Sponsorships for National Natural Science Foundation and “973” National Key Basic Research of China, Research Grant Manpower Fund (RGM 21/04) of Nanyang Technological University, and Science and Engineering Research Council Grant (#0421010010) from Agency for Science, Technology and Research (A*STAR), Singapore, are gratefully acknowledged. Supporting Information Available: Figure showing SEM images of a sawlike ZTO nanostructure and a kinked ZTO nanostructure. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 7. Room temperature PL spectra of single-crystal ZTO nanowires (solid line), annealed ZTO nanowires (dashed-dotted line), and twinned ZTO nanowires (dashed line).
the surface, resulting in a high specific surface area. In addition, the ZTO nanowires are prepared in an oxygen-deficient ambient. Therefore, the existence of large quantities of oxygen vacancies is possible. The oxygen vacancies would induce new energy levels in the band gap and contribute to the green emission of ZTO nanowires.18,19 The spectrum of the single-crystal ZTO nanowires annealed in oxygen ambient (1 atmospheric pressure) at 600 °C for 3 h is also shown in Figure 7. After annealing, the green emission of the ZTO nanowires reduced due to the decrease of the oxygen vacancies. The slight difference in the emission spectra between the single-crystal nanowires and the twinned nanowires, especially on the right shoulder, is probably due to the difference in residual strains in these two types of nanowires. In summary, single-crystal ZTO nanowires and twinned ZTO nanowires with periodical structures along the axial direction were synthesized via the thermal evaporation of a mixture of ZnO and SnO powders. It is possible that these nanowires can be extended as a nanoscale for fabrication of optical, chemical, and biochemical electronic nanodevices, making use of their one-dimensional periodical feature.
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