Three-Dimensional Structure of Twinned and Zigzagged One

Apr 13, 2010 - (1) Recently, the interest in twinned superlattice 1D nanostructures that have ... (15) Calculations predicted that a constant spacing ...
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Three-Dimensional Structure of Twinned and Zigzagged One-Dimensional Nanostructures Using Electron Tomography Han Sung Kim, Yoon Myung, Yong Jae Cho, Dong Myung Jang, Chan Soo Jung, and Jeunghee Park* Department of Chemistry, Korea University, Jochiwon 339-700, Korea

Jae-Pyoung Ahn* Nano Materials Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea ABSTRACT Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique threedimensional (3D) structures of twinned Zn3P2 (tetragonal) and InAs (zinc blende) nanowires synthesized by the vapor transport method. The Zn3P2 nanowires adopt a unique superlattice structure that consists of twinned octahedral slice segments having alternating orientations along the axial [111] direction of a pseudo cubic unit cell. The apexes of the octahedral slice segment are indexed as six equivalent 〈112〉 directions at the [111] zone axis. At each 30° turn, the straight and zigzagged morphologies appear repeatedly at the 〈112〉 and 〈011〉 zone axes, respectively. The 3D structure of the twinned Zn3P2 nanowires is virtually the same as that of the twinned InAs nanowires. In addition, we analyzed the 3D structure of zigzagged CdO (rock salt) nanowires and found that they include hexahedral segments, whose six apexes are matched to the 〈011〉 directions, linked along the [111] axial direction. We also analyzed the unique 3D structure of rutile TiO2 (tetragonal) nanobelts; at each 90° turn, the straight morphology appears repeatedly, while the in-between twisted form appears at the [011] zone axis. We suggest that the TiO2 nanobelts consist of twinned octahedral slices whose six apexes are indexed by the 〈011〉/〈001〉 directions with the axial [010] direction. KEYWORDS Electron tomography, twinned nanowires, superlattice, Zn3P2 nanowires, InAs nanowires

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semiconductors could have a significant impact on optically active band-structure engineering. In many semiconductor and noble metal NWs with a cubic structure (e.g., GaP, InP, InAs, GaAs, ZnS, ZnS, ZnTe, Si, SiC, B4C, Cr2O3, Zn2SnO4, Zn2TiO4, Ag, and Au), (111) twin planes are commonly found.2-22 For instance, Samuelson and co-workers reported the presence of twinned superlattices within GaP, InAs/InP, and InAs NWs, where their twinning periodicity, along the ZB [111] growth direction, increases with increasing diameter.2 However, only a few twinned structures have been reported for other crystal structures. The formation of (103) twin planes was found in hexagonal WZ ZnO and ZnSexS1-x NWs.9b,18b Shen et al. reported the twinned structure of tetragonal Zn3P2 NWs, where the twin planes exist along the [101] growth direction.12c Tao and Li investigated the defects of monoclinic Mg2B2O5 NWs, having the (010) twin planes at the cross section.14 For a better understanding of the twin structure, it becomes imperative to investigate their threedimensional (3D) geometry and the defect configurations down to the atomic level. Electron tomography, which is a method of reconstructing the 3D morphologies from a series of two-dimensional (2D) transmission electron microscopy (TEM) images or projections, has been successfully applied to analyze the

ne-dimensional (1D) nanostructures have attracted considerable attention due to their potential use as building blocks for assembling active and integrated nanosystems.1 Recently, the interest in twinned superlattice 1D nanostructures that have twin planes at a constant spacing has been steadily increasing, owing to their attractive morphology and electrical/optical properties.2-23 Since a twin boundary can act as a natural potential well for electrons, the discontinuous electron wave function leads to a reduction in the mobility of the charge carriers. For twinned zinc blend (ZB)/wurtzite (WZ) InP heterostructure nanowires (NWs), Bao et al. observed the excitation powerdependent blue-shift of the photoluminescence and explained it in terms of the staggered band alignment and concomitant diagonal transition between the localized electron/hole states.15 Calculations predicted that a constant spacing between the rotational twins would induce a direct bandgap in normally indirect bandgap semiconductors, such as group IV (Si, Ge) and III-V (GaAs) materials.23 Therefore, the controlled formation of a twinning structure in relevant

* To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. Received for review: 01/04/2010 Published on Web: 04/13/2010 © 2010 American Chemical Society

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morphology of various 1D nanostructures.24-29 All of the twinned structures in the previous studies were analyzed by examining their TEM 2D projections. There are, however, potentially some cases where structural information is missed or erroneous information is obtained when using this technique alone. Herein, we employed both electron tomography and high-resolution TEM images to study the 3D structure of various twinned and zigzagged 1D nanostructures. Their 3D reconstruction images, acquired from a series of 2D projections, were obtained by high-angle annular dark field (HAADF) scanning TEM (STEM).30 As one of the important II-V group semiconductors, Zn3P2 has a direct bandgap of 1.5-1.6 eV, which is the optimum range for photovoltaic solar energy conversion. The highest energy conversion efficiency of 5.96% was reported so far from polycrystalline transparent magnesium Zn3P2 diodes.31 Zn3P2 belongs to a unique tetragonal system, whose lattice constant, c/a, ratio (1.414) leads to its having a pseudo face-centered cubic (fcc) symmetry.32,33 Zn3P2 NWs (including twinned NWs), nanobelts (NBs), and nanotubes were usually synthesized using vapor phase transport, and their electronic and optoelectronic properties were also indentified.12c,34-36 The present work demonstrates that tetragonal Zn3P2 NWs have virtually the same twinned superlattice structure as that of ZB InAs NWs. In addition, the 3D structure of rock-salt cubic CdO NWs was analyzed to show their unique zigzagged structures, which are worth comparing with the twinned structures. Furthermore, we studied the novel 3D structure of zigzagged rutile tetragonal TiO2 NBs, which appear to have another unique twinned structure model. The results should open up a new field of investigation of nanostructures with high spatial resolution that could have an impact on our understanding of the growth mechanism or application of nanodevices in many fields, in which the crystal structure plays an important role in the final properties. The Zn3P2, InAs, and CdO NWs were all synthesized by the vapor transport method utilizing the vapor-liquid-solid growth mechanism. To synthesize the Zn3P2 NWs, a mixture of Zn (Aldrich, 99.999%) and InP (Aldrich, 99.99%) powders was placed in a ceramic boat, in order to generate Zn and P vapors, and 3 nm-thick Au film-deposited Si substrates were placed 20-cm apart from the source. An argon flow at 50-200 sccm was introduced into the reactor tube under ambient pressure, followed by heating to 800-850 °C. The growth reaction temperature at the substrates was maintained at 600 °C for 60 min. The InAs NWs were synthesized using the thermal evaporation of InAs (Aldrich, 99.99%) powders at 900 °C and 3 Torr. The substrates were Au film (3 nm thickness)-deposited Si substrates. An argon flow at 100 sccm was maintained during the whole growth process and the reaction time was 30-60 min. To synthesize the CdO NWs, a mixture of Cd (Aldrich, 99.999%) and CdO (Aldrich, 99.99%) powders was placed in a ceramic boat, in order to generate Cd vapor, and 3 nm-thick Au film© 2010 American Chemical Society

deposited Si substrates were placed 20-cm apart from the source. An argon flow at 200 sccm was introduced into the reactor tube under ambient pressure, followed by heating to 1100 °C. The growth reaction at the substrates was maintained at 900 °C for 60-90 min. The TiO2 NBs were synthesized by the thermal oxidation method, following the solid-liquid-solid (SLS) growth mechanism. Ultrasonically cleaned Ti foils (10 × 10 × 0.25 mm) with a purity of 99.7% (Aldrich) were coated with a 3 nmthick Au film and used as both the reagent and substrates for the growth of the TiO2 NBs. The Ti foil was placed on the top of a quartz boat, located inside a quartz tube reactor. The temperature of the Ti foil was set to 800 °C. A flow of oxygen (>99.999%) with a rate of 20-50 sccm was introduced only for the reaction time of 1 h. Then the Ti foil was covered homogeneously with the white-colored NW array. The products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, FEI Co. TECNAI F20 G2 200 kV and Jeol JEM 2100F), high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), electron diffraction (ED), and energydispersive X-ray fluorescence spectroscopy (EDX). Highresolution X-ray diffraction (XRD) patterns were obtained using the 8C2 and 3C2 beamlines of the Pohang Light Source (PLS) with monochromatic radiation (λ ) 1.54520 Å). 3D electron tomography was performed using a STEM (FEI Co., TECNAI F20 G2 200 kV), with a tilt holder (Dual Orientation Tomography Holder 927, Gatan Co.) and a Fischione model 3000 HAADF detector operated at 200 kV. A series of 130 HAADF-TEM images was collected from +75° to -75° in 1.5° steps under a nominal magnification of 40 000-110 000×, resulting in a pixel size of 1-3 nm on the computer-controlled sample stage. The images were spatially aligned by a cross-correlation algorithm using Inspect3D software (FEI Co.), and the 3D reconstructions were achieved using a simultaneous iterative reconstruction algorithm (SIRT) from consecutive 2D slices. Visualization was performed using AMIRA 4.0.26 Figure 1a shows the SEM image of the high-density zigzagged Zn3P2 NWs synthesized using the vapor transport method. These zigzagged Zn3P2 NWs were obtained when the evaporation rate was kept low using a low argon flow rate (50-100 sccm). The smooth-surface single-crystalline NWs were obtained at a higher argon flow rate. The XRD pattern confirms the tetragonal structure of Zn3P2 (JCPDS No. 65-2854, a ) 8.097 Å, c ) 11.450 Å), as shown in the Supporting Information, Figure S1. The TEM image reveals that the structure is entirely zigzagged over the whole NW (Figure 1b). The zigzag period is 140 nm on average and the zigzag angle is uniformly 140°. We turned the TEM grid holder in order to tilt the NW around the axial direction. Figures 1c corresponds to the TEM image at a tilt of 30°, showing the straight morphology. After further tilting the NW by 30°, the straight form turns into a zigzagged form (Figure 1d). For many other NWs, the TEM images at various tilt also 1683

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FIGURE 1. (a) SEM micrograph of high-density Zn3P2 NWs homogeneously grown on the substrate. (b) TEM image reveals the zigzagged morphology of the Zn3P2 NW having a zigzagged period of 140 nm. Consecutive 30° tilt around the wire axis changes the morphology to (c) straight and (d) zigzagged. Lattice-resolved TEM images of the (e) 0, (f) 30, and (g) 60°-turn morphologies. The insets show the corresponding FFT ED patterns at the [011¯]c zone axis for the 0° turn, [112¯]c for the 30° turn, and [101¯]c for the 60° turn. The [111]c axial direction remains the same for each tilt. The distance between the adjacent (111)c planes of the pseudo cubic unit cell is 6.6 Å. At the [011¯]c zone axis, two segments share the [111]c/[1¯1¯1¯]c spots. The (i) and (iii) ED patterns correspond to those of the twin segments and (ii) the ED pattern corresponds to their twin plane region. (h) Images of the Zn3P2 NW obtained through tomographic 3D reconstruction; (i-iv) correspond to a series of 0°, 30°, 60°, and 90° tilt around the axial direction, respectively; (v-viii) images show the sliced views along the NWs (as marked in (iv)), having a triangular cross section.

showed the same change in their morphology. The average diameter of the NWs is 100 nm. This symmetric and periodic structure probably arises from the twinned symmetric segments (or units). Therefore, we analyzed the crystal structure using their pseudo cubic (fcc) unit cell (a ) 11.45 Å), rather than the tetragonal unit cell. The cell parameters a (and b) of the tetragonal unit are directed along the diagonals of the faces of the elementary cube, and the parameter c is equal to the doubled edge of the cube. The volume of the tetragonal cell corresponds to four volumes of the elementary cube (Figure 2a). We referred to the indices in the pseudo cubic and tetragonal unit cell systems as the subscripts “c” and “t”, respectively. © 2010 American Chemical Society

The lattice-resolved images of the morphologies observed after tilting of 0, 30, and 60° are shown in Figure 1e-g, respectively, and their corresponding fast Fourier-transform (FFT) electron diffraction (ED) pattern generated from the inversion of the TEM images using Digital Micrograph GMS1.4 software (Gatan Inc.) are shown in the corresponding insets. The zone axis is indexed as [011¯]c of the pseudo cubic unit cell for the 0° turn, [112¯]c for the 30° turn, and [101¯]c for the 60° turn. The [111]c axial direction remains the same for each tilt. The distance between the adjacent (111)c lattice planes is 6.6 Å, which is consistent with that of a pseudo cubic unit cell. The alternating orientation of the twin segments is evident from the [011¯]c and [101¯]c zone axes, 1684

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FIGURE 2. (a) Zn3P2 NWs; (i) pseudo cubic unit (fcc) cell in which the parameters a of the tetragonal cell are directed along the diagonals of the faces of the elementary cube, and the parameter c is equal to the doubled edge of the cube. The table shows the index correlation between tetragonal and cubic unit cells. (ii) Schematic model constructed for the twinned Zn3P2 NWs at the [111]c zone axis; the twin octahedral slice blocks have six equivalent 〈112〉c apexes. The six side facets of the blocks would be enclosed by the ((112¯), ((12¯1), and ((21¯1¯) surfaces. Schemes (iii) and (iv) correspond to the views for the [112]c and [011]c zone axes, respectively, showing the straight (symbolized as “s” in (ii)) and zigzagged (symbolized as “z” in (ii)) morphologies. (b) Schematic model constructed for the twinned InAs NWs. (i) At the [111] zone axis, the twin octahedral slice blocks have six equivalent 〈112〉 apexes. The side facet of the blocks is enclosed by the {112} planes. Schemes (ii) and (iii) correspond to the views at the 〈112〉 and 〈011〉 zone axes, respectively, showing the straight and zigzagged morphologies. (c) Schematic model constructed for the CdO NWs. (i) The apexes of the hexahedral segments have six equivalent 〈011〉 directions at the [111] zone axis: [011¯], [01¯1], [101¯], [1¯01], [1¯10], [11¯0], and the axial direction is [111]. (ii) At the 〈011〉 zone axes, the segments appear as hexagons, (iii) while at the 〈112〉 zone axes, their shape becomes rhombohedral.

where the two segments share the [111]c and [1¯1¯1¯]c spots, that is, have common (111)c twin planes. (i) and (iii) correspond to the ED patterns of the individual segment, and (ii) corresponds to that of the twin plane region of the segments. The 30° tilt allowed us to view the structure from the [112¯]c zone axis, where the twinned spots cannot be seen. The indices of the pseudo cubic cell match those of the tetragonal unit cell, i.e., [011]c ) [111]t ) [001]t, [112]c ) [011]t, [110]c ) [100]t (see Figure 2a). The ED patterns indexed by the tetragonal unit cell system are shown in the Supporting Information, Figure S2. The growth direction of the Zn3P2 NWs is [011]t in the tetragonal system. As seen in Figure 1g, there are extra ED spots that are not assigned by the [101¯]c zone axis, but matched to the [100]t zone axis. Since [100]t corresponds to [110]c () [101]c), the ED spots of the [100]t zone axis can appear together with those of the [101¯]c zone axis,. Figure 1h displays the tomographic 3D reconstruction images of the Zn3P2 NW, (i)-(iv) images for the sequential 30° turns. The corresponding movie is supplied in the Supporting Information, movie S1. The sliced views along the NW (as marked in (iv)), (v-viii), reveal the triangular cross-section of the twinned segments. Movie S2 in the Supporting Information provides the vertical-direction tilting view. The movie of the STEM images that used for this © 2010 American Chemical Society

reconstruction has also shown in the Supporting Information, movie S3. Based on the assigned ED pattern, we built a schematic model for the twinned Zn3P2 NW, as shown in schemes (ii-iv) of Figure 2a. The octahedral slice segments have six apexes directed to the six equivalent 〈112〉c directions: [112¯], [1¯1¯2], [1¯21¯], [12¯1], [2¯11], and [21¯1¯] at the [111]c zone axis, as shown in scheme (ii). The top and bottom planes of the segment are triangular in shape and the side facets are enclosed by the ((112¯), ((1¯21¯), and ((2¯11) planes. The segments stack with alternating orientations along the [111]c direction, forming the twinned superlattices. When the incident electron beam is projected at the six equivalent 〈112〉c zone axes (symbolized as “s” in scheme (ii)), the apexes become collinear, so the straight morphology appears, as shown in scheme (iii). At the six equivalent 〈011〉c zone axes (symbolized as “z” in scheme (ii)), the two apexes stick out, resulting in the zigzagged morphology appears, as shown in scheme (iv). The twinned InAs NWs were previously synthesized by metal organic vapor phase epitaxy (MOVPE).2d,22 In contrast, we synthesized the twinned ZB InAs NWs using the vapor transport method. The growth of the twinned InAs NWs was achieved at a lower growth temperature (900 °C) than that of the straight single-crystalline ones (950 °C). Figure 3a 1685

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FIGURE 3. (a) SEM micrograph of high-density InAs NWs homogeneously grown on the substrate. (b) TEM image reveals the zigzagged morphology of the InAs NW (diameter ) ∼100 nm) with a period of 80 nm. (c) TEM image for its 30° tilt, showing the straight morphology. (d) Another 30° sequential tilt leads to the production of a zigzagged shape. Lattice-resolved TEM images of the (e) 0, (f) 30, and (g) 60°-turn morphologies and their corresponding FFT ED patterns are shown in the insets. The zone axis is indexed as [011¯] for the 0° turn, [112¯] for the 30° turn, and [101¯] for the 60° turn. The distance between the adjacent (111) planes is 3.5 Å. At the [011¯] and [101¯] zone axes, ED patterns (i) and (iii) correspond to those of the twin segments, and pattern (ii) corresponds to the twin plane region. (h) Tomographic 3D reconstruction images; (i-iv) images for the 30° sequential turns. The sliced views along the NW (as marked in (iv), (v-viii)), reveal the hexagonal crosssection.

zone axis is [011¯] for the 0° turn, [112¯] for the 30° turn, and [101¯] for the 60° turn. The [111] axial direction remains the same for each tilt. The distance between the adjacent (111) planes is 3.5 Å, which is consistent with that of bulk ZB InAs. The twin segments are evident in the ED pattern of the [011¯] and [101¯] zone axes. Figure 3h displays the tomographic 3D reconstruction images, viz. images (i)-(iv) for the sequential 30° turns. The corresponding movie is supplied in the Supporting Information, movie S4. The sliced views (as marked in (iv)), (v)-(viii), reveal the truncated-apex triangle shaped (hexagonal) crosssection. Movie S5 in the Supporting Information provides the vertical-direction tilting view. The movie of the STEM images that used for this reconstruction has also shown in the

shows the SEM image of the high-density twinned InAs NWs grown on the substrates. The XRD pattern confirms the formation of ZB InAs (JCPDS No. 79-1984, a ) 6.036 Å), as shown in the Supporting Information, Figure S1. The TEM image shows the zigzagged structure over the entire NW (Figure 3b). The averaged zigzag period is 80 nm and the zigzag angle is about 150°. We turned the TEM grid holder in order to tilt the NW around the axial direction. Figures 3c corresponds to the TEM image for its 30° tilt, showing a straight morphology. The diameter of the NW is 100 nm. Further tilting the NW by 30° turns it into a zigzagged NW (Figure 3d). Figure 3e-g show the lattice-resolved TEM images for the 0, 30, and 60°-turn morphologies, respectively, and their corresponding FFT ED patterns (insets). The © 2010 American Chemical Society

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FIGURE 4. (a) SEM image showing high-density chain-like CdO NWs grown on the substrates. (b) TEM image reveals that the NW uniformly consists of linked rhombohedral segments. (c) As it is tilted by 30°, the shape of the segments changes to hexagonal. (d) As it is further tilted by 30°, the shape of the segments returns to rhombohedral. Lattice-resolved TEM images for the (e) 0°, (f) 30°, and (g) 60°-tilt. The distance between the adjacent (111) planes is 2.7 Å. The zone axes of the corresponding SAED patterns are [011¯], [112¯], and [101¯], for the 0°, 30°, and 60°-tilt (insets), respectively. The wire axis is [111]. (f) Tomographic 3D reconstruction images: (i-iv) correspond to 0, 30, 60, and 90°-turns, showing the shape change of the segments from rhombohedral to hexagonal and vice versa, as the NW is tilted by 30°. The top-view (v) reveals its cross-section.

Supporting Information, movie S6. We concluded that the twin structure of the InAs NWs is virtually the same as that of the Zn3P2 NWs. Figure 2b displays the schematic model of the twinned InAs NWs. The top or bottom planes of the segment are hexagonal in shape and the side facets are enclosed by six equivalent {112} planes, as shown in scheme (i). The straight morphology at the 〈112〉 zone axis and the zigzagged morphology at the 〈011〉 zone axis are shown in schemes (ii) and (iii), respectively. This twin structure model is very consistent with that of other research groups.2,3,8,13,18 © 2010 American Chemical Society

CdO NWs were synthesized using the vapor transport method and their XRD pattern confirms the formation of cubic CdO (JCPDS No: 78-0653; a ) 4.725 Å), as shown in the Supporting Information, Figure S1. The SEM image shows high-density chain-like CdO NWs (Figure 4a). The TEM image shows that the zigzagged NW consists of connected rhombohedral segments (Figure 4b). The outer diameter is 180 nm. As this NW is tilted by 30°, the 2D shape of the segments changes to hexahedral, in which the two side planes are aligned with the growth direction (Figure 4c). When it is further tilted by 30°, the shape of the segments 1687

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changes to rhombohedral, in which the side planes are aligned zigzagged to the growth direction (Figure 4d). Their corresponding lattice-resolved TEM image and selected-area ED (SAED) pattern confirm their single-crystalline nature (Figure 4e-g). The zone axis of the SAED pattern changed from [011¯] f [112¯] f [101¯] for tilt of 0°, 30° and 60°, respectively (insets). The wire axis has the [111] direction. The distance between the adjacent (111) planes is 2.7 Å, which is consistent with that of bulk CdO. Figure 4h displays the tomographic 3D reconstruction images and the corresponding movies are supplied in Supporting Information, movies S7 and S8. Images (i-iv) correspond to 0, 30, 60, and 90° turns, respectively, showing the change of segments from rhombohedron f hexagon f rhombohedron f hexagon in the 2D projection view. The top-view (v) reveals the NW’s cross-section. Movie S9 in the Supporting Information provides the movie of the STEM images that used for this 3D reconstruction. A schematic model is shown in Figure 2c. The six apexes of the hexahedral blocks are directed toward the six equivalent 〈011〉 directions (at the [111] zone axis) and the two apexes along the [111] axial direction, as shown in scheme (i). The surface of the hexahedral blocks is enclosed by the {011} planes. At the 〈011〉 zone axis (symbolized as “s” in scheme (i)), the NW shows hexagonal segments, as shown in scheme (ii). As the NW is tilted by 30°, the zone axis becomes 〈112〉 (symbolized as “z” in scheme (ii)) and the shape of the segments changes to rhombohedral, as shown in scheme (iii). Then, the NW shows a zigzagged morphology. There have been some previous reports on polyhedron-shaped surface or straight nanowires both having the [111] axial direction.37,38 However, no 3D structure analysis of the zigzagged CdO NWs has been provided so far. The 3D structure of the zigzagged CdO NWs is exactly the same as that of the previously reported Zn2SnO4 NWs.26 It is noteworthy that the 3D structure of the zigzagged CdO NWs exhibits both some similarity and differences, when compared with that of the twinned Zn3P2 and InAs NWs. They are all grown along the [111] direction. The CdO NWs have the hexahedral segments enclosed by six {011} planes, while the twinned Zn3P2 and InAs NWs have the octahedral segments enclosed by six {112} planes. The relative surface energy of the {112} planes and {011} planes in a given growth condition would determine the shape of the segments and ultimately their 3D structure. Figure 5a shows the SEM image of the high-density TiO2 NBs synthesized using heated Ti foil under constant oxygen (O2) gas flow. The XRD pattern confirms the formation of rutile TiO2 (JCPDS No: 86-0147; tetragonal, a ) 4.594 Å, c ) 2.958 Å), as shown in the Supporting Information, Figure S1. The growth of the TiO2 NBs could be considered to follow the SLS growth mechanism; Ti melts to form nanoparticle droplets on the surface of the Ti foil, O diffuses into these nanoparticles from the gas phase, Ti and O saturate into the nanoparticles and then precipitate to form the TiO2 NBs. The © 2010 American Chemical Society

presence of the catalytic Au nanoparticles would help to produce the nanoparticles by forming a eutectic mixture. The Chen group reported the synthesis of TiO2 NWs by oxidizing Ti foil using O2 or organic molecules, such as acetone, ethanol, etc.39 Our synthesis method adopted the use of Au nanoparticles and O2 gas, which reduced the reaction time to less than 1 h. Figure 5b corresponds to the TEM image of the NB having a width of 300 nm and a flat side edge. There are periodic dark/bright strips at the horizontal direction over the whole nanobelt. As it is tilted around the axial direction, the diameter is reduced by 1/6, confirming its belt structure. Figure 5c corresponds to the TEM image for its ∼40° tilt, showing a twisted form that consists of uniform 30 nm-thick strips tilted with an angle of ∼50° to the long axis. The average diameter was reduced to 130 nm. A 90° tilt produces a straight morphology with a smooth surface and a diameter of 50 nm (Figure 5d). The lattice-resolved TEM images of the 0, 40, and 90°-turn morphologies are shown in Figure 5e-g, respectively, and their corresponding FFT ED patterns are shown in the insets. The zone axis is [100] for the 0° turn, [011] for the 60° turn, and [001] for the 90° turn. The [010] axial direction remains the same during the 90° tilt. The distance between the adjacent (200) or (020) planes is 2.3 Å, which is consistent with that of bulk rutile TiO2. At the [100] zone axis (0° turn, Figure 5e), the FFT ED pattern of the dark strip part (as shown in the top inset) shows more significant stacking faults (superlattice spots) along the [011] direction than that of the brighter part (as shown in the bottom inset). In the 40°-turn image (Figure 5f), the twist direction matches the [011] direction. At the [001] zone axis (90° turn, Figure 5g), the FFT ED pattern shows no significant stacking faults. Figure 5h corresponds to a series of TEM images for the -60, -30, 0, +30, and +60° tilt of another NB. The tilt of the flat TiO2 NB induces the twisted form, followed by the straight form and oppositedirection twisted form, and finally the flat belt form. The corresponding movie of STEM images is supplied in the Supporting Information, movie S10. Another series of TEM images clearly demonstrates that the twisted form changes to the opposite-direction twisted one through the straight one, with the continuous change of the twist angle (Supporting Information, Figure S3). There are unfortunately no ED patterns revealing the twin segments; the shared direction cannot intrinsically appear at the three zone axes, [100], [011], and [001]. Nevertheless, the tilt produces periodically the straight and zigzagged forms and the superlattice stacking faults along the [011] direction, which suggests the alternating stacking of the twin segments. Therefore, based on the ED pattern, we built a possible model, as shown in Figure 6. The octahedral slice segments have six apexes oriented in the 〈011〉/〈001〉 directions, i.e., [011], [01¯1], [01¯1¯], [011¯], [001], and [001¯] (scheme (i)). The twinned octahedral slice segments stack along the [100] axial direction. These segments consist of 1688

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FIGURE 5. (a) SEM image of the high-density TiO2 NBs on the Ti substrates. (b) TEM image of the NBs (width ) 300 nm). TEM images for their (c) 40° tilt, showing the zigzagged structure, and (d) 90° tilt, producing the straight morphology. Lattice-resolved images of the (e) 0, (f) 40, and (g) 90°-turn morphologies and their corresponding FFT ED patterns (insets). The zone axis is indexed as [100] for the 0° turn, [011] for the 40° turn, and [001] for the 90° turn. The [010] axial direction remains the same for each tilt. The distance between the adjacent (100) planes is 2.3 Å. (h) A series of TEM images of another NB showing the morphology change for the -60, -30, 0, +30, and +60°-tilt.

the twinned hexagonal subunits separated along the 〈011〉 directions. When the incident electron beam is projected at the 〈100〉 zone axis (symbolized as “s” in scheme (i)), the [011]/[01¯1] (and [01¯1¯]/[011¯]) apexes become collinear, so that the wide belt shape with straight sides comes into view, as shown in scheme (ii). The twin planes between the hexagonal subsegments would have significant stacking faults along the 〈011〉 directions, which appear as dark stripes in the TEM images. When the NB is tilted to the 〈011〉 zone axis (symbolized as “z” in scheme (i)), as shown in scheme (iii), the zigzagged 〈011〉 directions appear, which explain the twisted shape along the [011] direction. A 90° tilt results in the 〈001〉 zone axis (symbolized as “s” in scheme (i)), producing a straight morphology with a narrower width, due to the collinear [01¯1]/[01¯1¯] ([011]/[011¯]) © 2010 American Chemical Society

apexes, as shown in scheme (iv). This tentative model may need further confirmation in the future. In summary, tomographic reconstruction and HRTEM images were used to characterize the unique 3D structure of twinned superlattice Zn3P2 NWs and InAs NWs, synthesized by the vapor transport method. Zn3P2 belongs to a unique tetragonal system, in which its lattice constant (c/a ) 1.414) leads to its having pseudo cubic symmetry. The Zn3P2 NWs consequently adopt a unique superlattice structure that consists of twinned octahedral slice segments having alternating orientations along the axial [111] direction of the pseudo cubic unit cell. Each octahedral slice segment has its apexes indexed as the six equivalent 〈112〉 directions at the [111] zone axis. After each 30° turn, the straight and zigzagged morphologies appear repeatedly at 1689

DOI: 10.1021/nl1000168 | Nano Lett. 2010, 10, 1682-–1691

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FIGURE 6. Schematic model constructed for the TiO2 NB at the (i) [100], (ii) [010], (iii) [011], and (iv) [001] zone axes. (i) The octahedral blocks have six apexes at the 〈011〉/〈001〉, i.e., [011¯], [01¯1], [011¯], [011], [001], and [001¯], directions, at the [100] zone axis. (ii) At the [010] zone axis, the [011] and [01¯1] (and [01¯1¯]/[011¯]) apexes become collinear, producing the straight morphology (symbolized as “s” in (i)). (iii) When tilted, the zigzagged [011] direction appears at the side (symbolized as “z” in (i)), when projected at the [011] zone axis. (iv) When tilted up to 90°, the [01¯1]/[01¯1¯] ([011]/[011¯]) apexes becomes collinear when projected at the [001] zone axis, producing the straight morphology (symbolized as “s” in (i)).

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the 〈112〉 and 〈011〉 zone axes, respectively. The 3D structure of the twinned Zn3P2 NWs is virtually the same as that of the twinned ZB InAs NWs. The zigzagged CdO NWs consist of hexahedral segments, whose six apexes are matched to the 〈011〉 directions linked along the [111] axial direction. The rutile (tetragonal) TiO2 NBs, grown along the [010] direction, exhibit a unique twisted form that appears at the [011] zone axis, while the wide- and narrow-width straight morphologies appear at the [100] and [001] zone axes, respectively. We suggested a twinned superlattice structure that consist of twinned octahedral slices, whose six apexes are indexed by the 〈011〉/〈001〉 directions, stacked along the axial [100] direction. The method of tomographic 3D reconstruction, combined with the high-resolution TEM images, enables the precise structural analysis of various twinned and zigzagged 1D nanostructures.

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Acknowledgment. This study was supported by the NRF (R01-2008-000-10825-0; 2008-02364; 2008-314-C00175), Korea University Grant and MKE under the auspices of the ITRC support program supervised by the IITA (2008-C10900804-0013). This research was also supported by the WCU (World Class University) program through the NRF funded by the Ministry of Education, Science and Technology (R3110035). The HVEM (Daejeon), XRD (Taegu), and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.

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Supporting Information Available. Movies S1-S10, XRD, FFT ED pattern, and TEM images of TiO2 NBs. This material is available free of charge via the Internet at http://pubs. acs.org.

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DOI: 10.1021/nl1000168 | Nano Lett. 2010, 10, 1682-–1691