ARTICLE pubs.acs.org/JPCC
Elucidation of the Complex Structure of Nanoparticles Composed of Bismuth, Antimony, and Tellurium Using Scanning Transmission Electron Microscopy Derrick M. Mott,* Nguyen T. Mai, Nguyen T. B. Thuy, Teruyoshi Sakata, Koichi Higashimine, Mikio Koyano, and Shinya Maenosono School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
bS Supporting Information ABSTRACT: In this Article, we report on a study of the atomic structure of nanoparticles composed of bismuth, antimony, and tellurium with both wire and disk shape through use of the scanning transmission electron microscopy-high angle annular dark field detector technique. Recent advances in scanning transmission electron microscopy, such as aberration correction, have enabled a detailed structure analysis for materials with true atomic level resolution. Such ability is important for elucidating the underlying mechanisms, which give rise to the enhanced and novel properties displayed by nanoparticles. The atomic scale analysis of the particles revealed important insight into the nanoparticle structure. It was found that the nanowires are composed of both hexagonal tellurium and rhombohedral Bi2Te3 phases where the Bi2Te3 appears to have grown from the tips of the tellurium wires. The nanodiscs were found to be entirely rhombohedral in nature and were composed of both Bi2Te3 and Sb2Te3 phases with Bi2Te3 found at the particle center and Sb2Te3 appearing to subsequently grow at the periphery of the particles. The in-depth atomic structural characterization is expected to lead to a greater understanding of how to synthesize these thermoelectric type materials with more controllable properties such as size, shape, structure, and composition.
’ INTRODUCTION Transmission electron microscopy (TEM) has been one of the most important and widely employed tools for scientists working in the field of nanotechnology because it provides direct visual information about nanoparticle size and shape.1 TEM data have become indispensable because the nanoparticle size (between 1 and 100 nm) is smaller than the wavelength of visible light, which makes them unable to be visualized with optical microscopes.1 Over time, TEM technology improved, with the resolution and magnification power of the instruments increasing over time. High resolution transmission electron microscopy (HRTEM) was soon achieved, which allowed visualization of the atomic lattice in a crystalline material, further increasing the power and significance of the technique. However, one disappointing aspect revolving around the current TEM technology stems from the fact that the imaging resolution achieved has never approached that theoretically predicted, which originates from challenges in controlling the image forming electron beam in the instrument, level of vacuum, or stability of the high voltage source for the filament.1,2 As a result, until now, true atomic resolution images have been elusive. Recently, however, new advances in TEM technology have increased the resolution limit to the subangstrom scale.3 By using aberration corrected scanning transmission electron microscopy (STEM), which relies on rastering a focused electron beam over the sample,4 and a high angle annular dark field (HAADF) detector, which provides r 2011 American Chemical Society
enhanced Z contrast,5,6 electron microscopy images with an atomic resolution (nominally 0.8 Å) have been achieved.2,3 Such ability opens the door to studying the atomic level structure of a wide range of materials, including nanoparticles, whose properties may be strongly dependent on a unique atomic structure.2,3,5,6 One interesting area of research now focuses on nanostructured thermoelectric (TE) materials.7 11 In this field, the nanoscale particle size leads to enhanced TE efficiency for the overall material, primarily because of enhanced scattering of the heat carrying phonon at the crystal grain interface.8 Materials such as Bi2Te3 or (Bi0.5Sb0.5)2Te3 have been demonstrated to be potentially powerful thermoelectric materials7,12 with incorporation of nanostructuring increasing the efficiency even more.10,11 However, several factors, including the nanoscale particle properties (size, shape), specific composition, and atomic level structure, dictate the resulting TE efficiency. The atomic structure for the nanoparticles is an especially challenging area to address. However, by using the STEM-HAADF technique, the atomic structure for this class of materials can now be definitively determined. In our own previous research, we successfully synthesized TE type nanoparticles composed of Bi, Sb, and Te with both wire Received: June 14, 2011 Revised: July 30, 2011 Published: August 02, 2011 17334
dx.doi.org/10.1021/jp205588e | J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
ARTICLE
(NW) and disk (ND) shape.13 The synthesis technique was demonstrated to be very versatile with the nanoparticle morphology varying by simply changing the identity of the organic capping species used in the synthesis. It was found that the NWs have a micrometer length (∼3 5 μm) scale with a diameter ranging from about 20 to 50 nm. The aspect ratio of the NWs is quite high (∼100) as well as the shape monodispersity with only NWs observed. The NDs have a platelet-like morphology with a diameter of about 100 nm and thickness of about 25 nm. The resulting particles have a uniform morphology; however, they exhibit phase segregation as evidenced by XRD analysis.13 It was found that the NWs are composed of primarily hexagonal Te and rhombohedral Bi2Te3 phases, while the NDs are composed of Bi2Te3 and Sb2Te3 phases. It is a particular challenge to address whether the nanoparticles are composed of a single phase (multiple single particle compositions) or if the single particles are themselves phase segregated. The aim of this study is to gain insight into the phase segregated properties of the nanoparticles as well as to gain an expanded understanding of the atomic structure for these materials.
’ EXPERIMENTAL SECTION Chemicals. Bismuth trichloride (BiCl3) 98%, antimony trichloride (SbCl3) 99%, tellurium tetrachloride (TeCl4) 99%, oleic acid (OAC) 90%, oleylamine (OAM) 70%, 1,2-hexadecanediol 90%, 1-decanethiol (DT) 96%, and dioctylether 99% as well as common solvents were obtained from Aldrich. Synthetic Technique. First, 1.67 10 4 mol each of BiCl3, SbCl3, and TeCl4 were mixed with 25 mL of dioctylether, and then 1.5 10 3 mol of 1,2-hexadecanediol was added along with the capping species, the identity of which was used to manipulate the morphology of the resulting nanostructures. For the synthesis of nanowires, 0.16 mL of OAC and 0.17 mL of OAM were used. For the synthesis of nanodiscs, 1.5 mL of DT was used as capping agent. The mixture was purged with argon under vigorous stirring. At this point, the flask was heated to 105 °C for 10 min to remove water, which also caused the reactants to completely dissolve in the solvent (a light gray color in the solution). After this, the temperature was increased to 200 °C and was held for 1 h. The formation of particles within this time was evidenced by the solution color change from light gray to dark gray or black depending on the capping species used. After reaction, the NP solution was cooled to room temperature, and the particles were purified by precipitation in ethanol. The materials could be briefly resuspended (precipitation occurs within 1 day) in hexane with additional OAC/OAM and/or DT. The resulting NPs were then characterized in terms of morphology, composition, and structure.13 Instrumentation and Measurements. Techniques included transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Eenergy dispersive spectroscopy (EDS) to analyze the nanoparticles. TEM and HR-TEM analysis was performed on Hitachi H-7100 (100 kV) and H-9000NAR (300 kV) instruments, respectively. XRD patterns were collected in reflection geometry using a Rigaku RINT2000 X-ray diffractometer at room temperature with Cu KR radiation (wavelength 1.542 Å). STEM analysis and EDS elemental mapping were performed on a Jeol JEM-ARM200F instrument operated at 200 kV with a spherical aberration corrector; the nominal resolution is 0.8 Å.
Figure 1. TEM images of NWs synthesized with (A) binary OAC/ OAM (2000 nm scale bar), (B) a magnified view of the NWs (100 nm scale bar), (C) a HR-TEM image of a single NW (5 nm scale bar), (D) NDs synthesized with DT (100 nm scale bar), (E) HR-TEM image of a ND on-edge (5 nm scale bar), and (F) HR-TEM image of a ND on-face (5 nm scale bar).
TEM and STEM samples were prepared by dropping suspended nanoparticles onto a carbon-coated copper grid and drying in air.
’ RESULTS AND DISCUSSION The results are discussed primarily in terms of studying the atomic structure for the two different shapes/structures of nanoparticles composed of bismuth, antimony, and tellurium. EDS-elemental mapping is first discussed in regards to the compositional/structural analysis followed by the atomic level structural characterization using the STEM-HAADF technique. The analysis provides definitive characterization of the atomic level structure for these TE type nanoparticles. Transmission Electron Microscopy and High ResolutionTEM. To illustrate the general particle morphology, TEM and
HR-TEM images are first shown. Figure 1 shows the images collected for the two different types of NPs studied. Figure 1A is a TEM image for nanowires (NWs) synthesized using OAC/OAM binary capping species, while Figure 1B shows a zoomed-in view of the wires with uniform morphology. The alternating light and 17335
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
Figure 2. XRD patterns of nanowires (A) and nanodiscs (B). The symbols correspond to Te with hexagonal structure ([), and Bi2Te3 (b), Sb2Te3 (9), and (Bi0.5Sb0.5)2Te3 (1) with rhombohedral structure.
dark striations observed in the NWs arise as a result of physical stress (bending or twisting) in the particles.14 Figure 1C shows the HR-TEM image of a single nanowire where the atomic lattice can be visualized. Figure 1D shows the TEM image collected for the NDs synthesized using DT capping species. In this image, some discs can be observed sitting on face or on edge. Figure 1E shows the HR-TEM image for a single disk sitting on edge, while Figure 1F shows a disk on face. In the high resolution images, the atomic lattice for the particles can be observed and indicates the monocrystalline nature of the individual particles. However, in past research (and shown in the XRD result), it was established that these materials are phase segregated in nature. The NWs exhibit Te and Bi2Te3 phases, and the NDs contain Bi2Te3 and Sb2Te3 phases.13 Measurement of the atomic lattice spacing among different particles or different places within a single particle does not show a significant deviation, and so is not a useful indicator for where the different phases exist in the different types of particles. As a result, it is a particular challenge to address whether the nanoparticles are composed of a single phase (multiple single particle compositions) or if the single particles are themselves phase segregated. X-ray Diffraction Crystallography. As illustrated in our previous work,13 XRD analysis elucidates the general composition of the resulting nanoparticles after synthesis. Figure 2 shows the XRD patterns collected for the wire and disk shaped particles. In each sample, several phases can be identified. For the nanowires, both hexagonal tellurium ([) and rhombohedral phase Bi2Te3 (b) can be identified. In the nanodisc sample, both Bi2Te3 and Sb2Te3 (9) phases are found. Each sample also seems to contain a minor amount of (Bi0.5Sb0.5)2Te3 (1). The XRD results illustrate the complex nature of the nanoparticle composition and structure.13 However, in combination with the TEM results, the location of the different phases within the particles cannot be determined. Energy Dispersive Spectroscopy Elemental Mapping. To address the question of whether the nanoparticles are composed of single or multiple phases, EDS-mapping was performed. This technique allows the visualization of the relative location of the different elements in the sample (a two-dimensional map). The color intensity serves as a tool to judge the relative amount of element present. Figure 3 shows the EDS-mapping images for the NWs. In this figure, a dark field image as well as elemental maps for Bi (M line) and Te (L line) are included along with an
ARTICLE
overlay of the individual maps. Sb was not mapped in this analysis because of the relative instability of this element under the electron beam, which would cause a significant error in the analysis. The dark field image shows the wire morphology of the particles and serves as a reference for particle location when compared to the elemental maps. The map for Te indicates that Te is present at all locations within the NWs; however, for Bi, the composition is not uniform, with Bi existing only at certain isolated areas within the wires. The overlay of the Bi and Te maps further elucidates this. The data indicate that the NWs are composed of a predominantly monometallic Te phase along with segregated areas of Bi2Te3. Figure 4 shows the EDS-mapping result for a single ND sitting on face. In this analysis, the dark field image along with mapping of Bi and Te areas is also shown, in addition to an overlay of the elemental maps. As above, Sb was not mapped because of the instability under the electron beam. The dark field image shows a single disk that may have a nonuniform surface, as indicated by the variation in contrast throughout the disk. As can be observed, there seems to be little inhomogeneity in the Bi and Te compositions (the lower right quadrant in the Te map may indicate a heightened amount of the metal, which could arise because of a deviation in particle thickness). This result is consistent for particles composed of Bi2Te3; however, there were no areas observed that were devoid of bismuth. EDS-mapping was also performed for the NDs lying on edge; however, no inhomogeneity in the composition was observed. In light of these results, the NDs appear to be composed mostly of Bi2Te3; however, the location of antimony within the particles could not be discerned because of the instability under the electron beam. Atomic Structural Characterization Using the STEMHAADF Technique. To more closely study the nanoparticle structure, aberration corrected scanning transmission electron microscopy coupled with a high angle annular dark field detector (STEM-HAADF) was used. This technique has recently emerged as a highly unique and useful tool because of the high level of resolution that can be achieved (nominal resolution of 0.8 Å), and because the dark field detector provides enhanced contrast as a function of the atomic number (Z).5,6 This allows atomic level imaging, the ability to differentiate between different elements in the image, and the ability to definitively identify the materials atomic structure. Figure 5A shows a STEM-HAADF image in roughly the center of a typical NW, with the wire running left to right. The white dots in the image represent individual atoms in the NW. The dark line running through the center of the wire is where two crystalline planes in the particle meet (a twin plane). The inset to the image shows the derived expanded unit cell for the material, which has a hexagonal structure. The unit cell c-axis length can be measured (5.93 Å) providing a good indication that this section of the wire is composed of monoelemental tellurium. This is further supported by the fact that each atom appears to have the same degree of contrast, indicating that this section of the particle has a uniform monoelemental composition. Figure 5B shows the crystal structure for hexagonal Te,15 while Figure 5C shows the corresponding expanded unit cell derived from the STEM-HAADF image, along with the corresponding reference and measured c-axis values.16 Figure 6A shows the STEM-HAADF image of the tip of a single NW. The NW tip appears to have a different atomic structure/composition as compared to the center area of the NW. Figure 6B shows a zoomed-in view of the area of the tip indicated by the box in Figure 6A. In this image, two different 17336
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
ARTICLE
Figure 3. EDS mapping of NWs synthesized with binary OAC/OAM. From left to right, dark field image, Bi map, Te map, and an overlay of the Bi and Te maps (250 nm scale bar for all).
Figure 4. EDS mapping of a single ND synthesized with DT. From left to right, dark field image, Bi map, Te map, and an overlay of the Bi and Te maps (20 nm scale bar for all).
Figure 5. STEM-HAADF image of (A) a nanowire in a Te monometallic area (1 nm scale bar). The accompanying schemes show (B) the crystal structure for hexagonal Te15 and (C) the corresponding unit cell derived from the STEM image.
atomic structures can be observed. On the left side of the image, the tellurium hexagonal structure can be visualized (the same as in Figure 5), while on the right-hand side of the image, a different atomic structure is observed. While it is difficult to definitively address the crystal structure in this region of the NW (primarily because the crystal orientation plays a large role in the image obtained),5 this structure is certainly not hexagonal in nature. The fact that the NW tip is darker than the side walls is likely because the tip is angled away from the viewing plane and is not a result of Z-contrast in the sample. Figure 6C shows the STEMHAADF image of a separate NW displaying a clearer image of the
Figure 6. STEM-HAADF image of (A) a nanowire tip (10 nm scale bar), (B) a magnified view of the tip (2 nm scale bar), (C) an area exhibiting the rhombohedral structure (1 nm scale bar), and (D) a scheme illustrating the rhombohedral Bi2Te3 structure observed in the sample.15
more complex crystal structure. On the basis of the earlier EDSmapping experiment, these areas seem to be composed of Bi2Te3 with rhombohedral structure. It is especially apparent in Figure 6C that the contrast varies markedly among different atoms in the image, which could arise as a result of Z-contrast (Bi Z = 83, 17337
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
ARTICLE
Figure 8. (A) STEM-HAADF image near the periphery of a nanodisc on face (1 nm scale bar) and (B) a scheme showing the measured lattice values taken from the image. Figure 7. STEM-HAADF image of (A) a nanodisc on face (1 nm scale bar). The accompanying schemes show (B) the crystal structure for rhombohedral Bi2Te3/Sb2Te315 and (C) the corresponding unit cell (viewed down the c axis) derived from the STEM image.
Te Z = 52), or because of the particle structure (the dimer atoms would be farther from the viewing plane than the bright ones). The rhombohedral structure is further confirmed by comparing the observed atomic lattice to a modeled one. Figure 6D shows a model of the Bi2Te3 crystal with rhombohedral structure.15 The model was aligned to correspond with the orientation of the NW observed in the STEM image observed in Figure 6C. The resulting pattern that is formed shows rows of atoms that form a zigzag pattern, similar to that observed in the STEM image. The bright atoms that are observed in the STEM image (represented by the white circles in Figure 6D) may cause increased scattering of the electron beam, decreasing the penetration depth (as a result of particle structure or the identity of these atoms), which could explain why rows of zigzagging atoms are not observed for the bright atoms. Overall, the images indicate that the bulk of the NWs are composed of Te, which has a hexagonal structure, while Bi2Te3 with rhombohedral structure seems to have grown from the tips of the wires. The results indicate that the NWs contain two phases and suggest that the phase segregation is a result of the particle growth mechanism, where Te wires formed initially, followed by growth of Bi2Te3 at the nanowire tip. For the NDs, Figure 7A shows the STEM-HAADF image of a single particle on face. The atomic structure of this particle is much different from that observed for the NWs. In this image, three different types of atoms can be observed with different contrast including relatively bright, dim, and intermediate contrast. The inset to the image in Figure 7A represents a top down view of the expanded unit cell for the material where the a-axis value (4.33 Å) and the 101 crystalline plane spacing (3.75 Å) indicate that the material has a rhombohedral structure and is likely composed of Bi2Te3 or Sb2Te3.16 Figure 7B shows the rhombohedral reference structure for Bi2Te3/Sb2Te3 as viewed down the c-axis,15 while Figure 7C shows the measured values for the a-axis and 101 plane spacing. These values are also consistent with (Bi0.5Sb0.5)2Te3, which could exist as a minor phase in the sample. The difference in contrast could arise as a result of the identity of the atoms in the particle (i.e., Bi, Sb, or Te), but in this case it may also be that the contrast arises as a result of the atomic structure. In the rhombohedral structure, each crystalline plane is offset from the one above it, resulting in three repeating layers (if you view the structure along the c-axis). In this case, the top
layer would be the brightest, with the middle layer being dimer, and the final layer being the dimmest. It is likely a combination of these two factors that causes the difference in the observed contrast among the different atoms in this sample. Figure 8A shows the STEM-HAADF image for another area near the periphery of a ND. This area also seems to have a structure similar to that observed in Figure 7; however, in this case, there is little difference in contrast between the atoms in the structure. In addition, Figure 8B shows that the measured a-axis value in this area of the disk is 4.45 Å, while the 101 interplanar spacing is 3.84 Å, both of which correspond closely to values for Te with hexagonal structure (the 101 planes for the rhombohedral material would correspond to the 100 planes in the hexagonal structure for Te).16 These two observations seem conflicting; however, it is important to note that the resolution of the STEM-HAADF technique used here is nominally 0.8 Å, which is larger than the difference in the atomic spacing values for Te, Bi2Te3, Sb2Te3, and (Bi0.5Sb0.5)2Te3, making it a challenge to discriminate between these materials based solely on the interatomic spacing. In light of this, the aggregate data (including XRD, variation in contrast of different atoms among the samples, and the observed atomic structure) indicate that this area of the ND is likely composed of Sb2Te3. Sb and Te only differ by one atomic number, so the difference in contrast between these two atoms would be negligible, whereas Te (Z = 52) and Bi (Z = 83) have a very different Z number. As a result, the area of the ND imaged in Figure 8 is likely composed of Sb2Te3, while that imaged in Figure 7 is composed of Bi2Te3. These observations suggest that in the synthetic approach the Bi2Te3 formed first, followed by the growth of the Sb2Te3 on the outside edge of the ND. Figure 9A shows the STEM-HAADF image of a ND on edge. The image shows an atomic structure consistent with the rhombohedral structure, with differing contrast among the atoms in the sample. The c-axis value of the material was measured and found to be 30.38 Å, corresponding closely to that for Bi2Te3 (30.497 Å) or Sb2Te3 (30.450 Å). Given the observed variation in contrast among the different atoms in the sample, this section of the particle is likely composed of Bi2Te3. Figure 9B shows the accompanying scheme of the unit cell for Bi2Te3 and the measured c-axis value, while Figure 9C shows the model rhombohedral crystal structure as viewed down the a-axis with the reference c-axis values listed.15 General Proposed Formation Mechanism. Considering the phase segregated structure of both the wire and the disk shaped NPs observed within individual particles at the atomic level, 17338
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
ARTICLE
intriguing building blocks for thermoelectric materials with enhanced properties and are a subject of continuing investigation.
Figure 9. STEM-HAADF image of (A) a nanodisc on edge (1 nm scale bar); the accompanying schemes show (B) a side view of the lattice cell of Bi2Te3 along with the measured c-axis value and (C) the reference Bi2Te3/Sb2Te3 lattice as viewed down the a-axis15 with the reference c-axis values for Bi2Te3 and Sb2Te3.16
Scheme 1. Proposed Formation Mechanism Leading To the Observed Particle Phase Segregationa
’ CONCLUSIONS Nanoparticles that contain Bi, Sb, and Te with wire and disk shapes that display phase segregation were studied using the STEM-HAADF technique along with EDS-elemental mapping. The results indicate that individual nanowires are composed of hexagonal phase tellurium with rhombohedral phase Bi2Te3 appearing to grow from the tellurium nanowire tips. The phase segregation of individual NWs is observed directly via the EDSmapping images, while the STEM-HAADF images provide atomic level structural characterization. For the NDs, the individual particles are composed of two phases as well, with Bi2Te3 existing near the particle center and Sb2Te3 near the periphery of the NDs, as evidenced by the differing contrast of the individual atoms in the particles. This suggests that the Bi2Te3 particle center forms first, followed by growth of the Sb2Te3 on the outside of the particle. The results provide definitive atomic level structural characterization of these complex nanoparticles, ultimately giving insight into the relationship between the synthetic conditions and the resulting particle composition/structure properties. The results are also expected to lead to a better understanding for how the particle formation mechanism can be manipulated, leading to particles with novel and complex structures. Such ability is expected to lead to nanoparticle-based thermoelectric materials with unique and enhanced properties. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional energy dispersive spectroscopy-elemental mapping data for nanodiscs laying on edge. This material is available free of charge via the Internet at http://pubs.acs.org.
a
Pathway A illustrates the initial formation of a Te NW growing along the c-axis followed by subsequent growth of Bi2Te3 at the NW tip, continuing along the c-axis. Pathway B illustrates the initial formation of a Bi2Te3 ND followed by the subsequent growth of Sb2Te3 at the periphery of the disc along the a- and b-axes directions.
insight into the general formation mechanism is obtained. Scheme 1 shows the general proposed formation mechanism for both the NWs (Scheme 1A) and the NDs (Scheme 1B). In the proposed formation mechanism for the NWs, first a tellurium wire forms with a growth direction along the c-axis. After the initial wire formation, Bi2Te3 grows from the nanowire tip, continuing along the c-axis. This is supported by the observation of hexagonal phase tellurium in the body of the wires with Bi2Te3 found at only the wire tips, as well as a clear boundary between the two phases. For the NDs, the Bi2Te3 phase forms first with a discrete disk shape, growing along the a- and b-axes directions. The Sb2Te3 subsequently grows form the side walls of the disk, continuing along the a- and b-axes directions, forming the observed phase segregated particle with Bi2Te3 at the particle center and Sb2Te3 at the periphery. The ability to manipulate such formation routes opens the door to creating particles with well-defined and novel structures, for instance, nanoscale wires with alternating tellurium and Bi2Te3 phases, or discs with alternating Bi2Te3 and Sb2Te3 layering. Such particles are highly
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +81-761-51-1587. Fax: +81-761-51-1625. E-mail: derrickm@ jaist.ac.jp.
’ ACKNOWLEDGMENT This work was supported by the Grant-in-Aid for Scientific Research (C), and partly by the Comprehensive Support Programs for Creation of Regional Innovation: “Practical Application Research”. D.M.M. gratefully acknowledges support by the Japan Society for the Promotion of Science (JSPS) fellowship. ’ REFERENCES (1) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science; Springer: New York, 2009. (2) Mayoral, A.; Mejía-Rosales, S.; Mariscal, M. M.; Perez-Tijerina, E.; Jose-Yacaman, M. Nanoscale 2010, 2, 2647–2651. (3) Van Aert, S.; Batenburg, K. J.; Rossell, M. D.; Erni, R.; Van Tendeloo, G. Nature 2011, 470, 372–375. (4) M€uller, S. A.; M€uller, D. J.; Engel, A. Micron 2011, 42, 186–195. (5) Krumeich, F.; M€uller, E.; Wepf, R. A.; Nesper, R. J. Phys. Chem. C 2011, 115, 1080–1083. (6) Hernandez-Garrido, J. C.; Yoshida, K.; Gai, P. L.; Boyes, E. D.; Christensen, C. H.; Midgley, P. A. Catal. Today 2011, 160, 165–169. 17339
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
The Journal of Physical Chemistry C
ARTICLE
(7) Zhao, Y.; Burda, C. ACS Appl. Mater. Interfaces 2009, 6, 1259–1263. (8) Kovalenko, M. V.; Spokoyny, B.; Lee, J.-S.; Scheele, M.; Weber, A.; Perera, S.; Landry, D.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132, 6686–6695. (9) Ma, Y.; Hao, Q.; Poudel, B.; Lan, Y.; Yu, B.; Wang, D.; Chen, G.; Ren, Z. Nano Lett. 2008, 8, 2580–2584. (10) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B Nature 2001, 413, 597–602. (11) Zhao, Y.; Dyck, J. S.; Hernandez, B. M.; Burda, C. J. J. Phys. Chem. C 2010, 114, 11607–11613. (12) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320, 634–638. (13) Mott, D.; Mai, N. T.; Thuy, N. T. B.; Maeda, Y.; Linh, T. P. T.; Koyano, M.; Maenosono, S. Phys. Status Solidi A 2011, 208, 52–58. (14) Fan, X. A.; Yang, J. Y.; Chen, R. G.; Zhu, W.; Bao, S. Q. Mater. Sci. Eng., A 2006, 438 440, 190–193. (15) Reference crystal structures and images accessed from the American Mineralogist Crystal Structure Database (http://rruff.geo. arizona.edu/AMS/amcsd.php). Accessed on April 18th, 2011. (16) Reference data taken from the International Centre for Diffraction Data, PDF card numbers: tellurium, 00-036-1452; Bi2Te3, 01-0892009; Sb2Te3, 00-015-0874; and (Bi0.5Sb0.5)2Te3, 01-072-1835.
17340
dx.doi.org/10.1021/jp205588e |J. Phys. Chem. C 2011, 115, 17334–17340
