J. Phys. Chem. C 2010, 114, 18031–18036
18031
Growth and Valence Excitations of ZnO:M(Al, In, Sn) Hierarchical Nanostructures Cheng-Yu Wang,† Chuan-Pu Liu,*,† Hui-Wen Shen,† Yi-Ju Chen,† Chien-Lin Kuo,† Ting-Yu Wang,‡ Rong-Kun Zheng,‡ and Simon P. Ringer‡ Department of Materials Science and Engineering, National Cheng Kung UniVersity, Tainan, Taiwan 701, and Australian Key Center for Microscopy and Microanalysis, The UniVersity of Sydney, NSW, Australia 2006 ReceiVed: April 21, 2010; ReVised Manuscript ReceiVed: August 16, 2010
Comb- and fishbone-like doped ZnO nanostructures were synthesized by introducing different dopants in alloying vapor deposition process. Whereas, Al and Sn doping could induce comb-like structures, In doping introduced fishbone-like structures due to different chemical activity of dopants involved. While belts exhibit the growth direction of [011j0] for fishbone-like structures or [21j1j0] for comb-like structures, all branches grow only along the [0001] direction. However, the morphology of the belts and the nucleation of the branches are remarkably different among these three structures. Elemental mapping with electron energy loss spectra (EELS) indicated that all the dopants are incorporated rather uniformly into ZnO, consistent with the expansion of the (0001) lattice spacing. All the interband and intraband transitions have been probed by valence EELS. The results show that the projected band gap transitions vary with dopants, resulting in 2.03 eV for ZnO:Al, 2.3 eV for ZnO:In, and 2.7 eV for ZnO:Sn, realized by the shift of the green emission maximum arisen from impurity deep levels. 1. Introduction Being one of the most important semiconductors, zinc oxide (ZnO) thin film has been demonstrated in the optoelectronics for electron emitters, luminescent centers, and varistors.1 Recently, one-dimensional (1D) ZnO nanostructures have stimulated intensive research interest due to the potential to serve as building blocks for nanometer optoelectronic devices such as efficient emitters,2 waveguide lasers,3,4 piezoelectric effect transistors,5,6 and nanogenerators.7 In addition, ZnO itself can act as a promising candidate for ultraviolet luminescence and the ideal substrate for epitaxial growth of gallium nsitride (GaN).8 The performance of these devices is determined by field confinement, piezoelectric properties, and wide band gap together with high exciton binding energy of the ZnO nanostructures. Simultaneously, the research interest also reflects the essential importance of manipulating the electronic structure for nanometer devices. In addition to 1D nanostructures, ZnO shows diverse hierarchical structures and they are classified below based on production methods. According to the synthetic routes, when pure Zn and/or ZnO was used as the sources, ZnO nanocombs,9-15 nanosprings,16 and tetrapods17,18 have been synthesized by the vapor-transport-condensation mechanism. When ZnO with other oxides were used as the sources, nanocombs from the In2O3 backbone,19 nanorings accompanied by indium oxide layer,20,21 and In2O3(ZnO)m superlattice nanowires22 have also been reported. Furthermore, either Zn or Sn was also utilized to produce multibranched ZnO structures23,24 and In induces nanowires growing along the [112j0] direction.25 However, doped hierarchical structures have never been analyzed in detail, although Al-, Ga-, In-, and Sn-doped ZnO nanowires with [0001], [011j0], and [011j1] growth directions were demon* To whom correspondence should be addressed. E-mail: cpliu@ mail.ncku.edu.tw. Phone: 886-6-2757575 ext 62943. Fax: 886-6-2346290. Mail: No. 1, University Rd., Eastern Dist., Tainan, Taiwan 704. † National Cheng Kung University. ‡ The University of Sydney.
strated.26 Recently, the authors have developed the alloyingvapor-deposition method where Zn and foreign metal powders were mixed and heated to initiate eutectic reactions prior to the growth of ZnO hierarchical nanostructures. The mechanism was further employed to grow Al-doped cuboid arrays from rods.27 Finally, in the two-step growth strategy, liana-like structure by growing ZnO nanowires from SnO2 backbone was demonstrated.28 The possible applications of hierarchical ZnO structures, such as laser beam splitter by using ZnO combs10 and lasing from the ZnO@SnO2 liana,28 have been demonstrated. Though various hierarchical structures have been synthesized, growth mechanisms accounting for periodic comb formation in single crystalline ZnO belts remain unclear. In this report, we demonstrated that dopant species can manipulate the morphology of comb- and fishbone-like nanostructures. Interestingly, though the resulting nanostructures look similar to each other, they do differ in orientation, thickness variation, and microstructure in the belts. Most importantly, the characteristics of all the nanostructures are in big contrast to the common combor fishbone-like structures reported in the literature, suggesting the huge impact of dopants. As a result, dopant could induce hierarchical ZnO nanostructures accordingly, for various promising applications. Electron energy loss spectroscopy (EELS) has been employed to analyze excitations of crystal electrons, and valence EELS exploits band gap transitions between valence and conduction band together with plasmon excitation.29-31 Interband transitions of bulk ZnO were calculated, and peaks corresponding to Zn3d, O-2p, and O-2s to conduction band were identified.32 Such transitions in single ZnO nanowires were also probed by VEELS,33 wherein surface plasmon excitation of 9.45 eV and bulk plasmon of 18.1 eV was recognized. Meanwhile, excitonic fine structures of bulk ZnO can be probed in detail by optical pumping.34 In pure ZnO, UV emission corresponding to free exciton recombination is 3.377 eV at room temperature. Green
10.1021/jp103594m 2010 American Chemical Society Published on Web 10/05/2010
18032
J. Phys. Chem. C, Vol. 114, No. 42, 2010
Wang et al.
Figure 1. (a) A low magnification BF image of a ZnO:Al comb-like structure, (b) the energy-filtered image, (c) the thickness map with profile from the rectangle, (d) the Al elemental mapping, and (d together with e) high-resolution images from the first and second step along the rod. The inset in the fist row is the SAD from the marked area.
emission can be attributed to singly negative charged zinc vacancy, singly positive charged oxygen vacancy, and surface defect.35 In this study, the alloying-vapor-deposition method was employed further to dope ZnO with tin (Sn) or indium (In) and the comb-like together with the fishbone-like structures were produced. Transmission electron microscopy (TEM) and scanning TEM (STEM) EELS were employed to probe microstructure, elemental distribution, and band gap transitions of the hierarchical structures, which help to resolve growth mechanisms in doped hierarchical ZnO, and then to design ZnO nanostructure. 2. Experimental Methods Al-, Sn-, and In-incorporated ZnO (ZnO:M) nanostructures were synthesized by the alloying-vapor-deposition method with metals Al, Sn, In, and Zn powders as the sources on the substrate of Si (100). Al and Zn (7:93 by weight) were coheated to 500 °C, lasting 30 min for eutectic reaction and then heated to 650 °C, lasting 1 h for growth. In and Zn (1:9 by weight) were coheated to 500 °C, lasting 30 min and then heated to 550 °C, lasting 1 h for growth. In the case of Sn and Zn (the weight ratio of 3:7), it follows the route of In and Zn. As-grown nanostructures were scraped off the substrates and dispersed in ethanol ultrasonically. Then, suspensions were dropped on a lacey carbon-coated TEM grid for electron microscopy examination. The as-grown ZnO:M nanostructures were examined by a TEM JEOL-2100F operated at 200 kV, fitted with a field emission gun (FEG), a charge coupled device (CCD), camera, and a GIF system. Thickness maps were performed on this apparatus and calculated by intensity ratios of the unfiltered to filtered images at each pixel, acquiring total electron intensity and only elastic scattering intensity, respectively. Elemental maps were obtained by the standard three-window technique
for Al L-edge (73 eV), In M-edge (443 eV), and Sn M-edge (485 eV). For the Al L-edge, two pre-edge energy windows were acquired at 65 and 70 eV, and a postedge energy window was at 75 eV with the window width of 5 eV. For the In and Sn M-edges, the corresponding energy positions were selected to be (393, 423) and (450, 470) eV for the pre-edge windows and 468 and 505 eV for the postedge window with the energy width of 30 and 20 eV, respectively. STEM-EELS was performed on a STEM VG-HB601 operated at 100 kV, fitted with a cold FEG, a CCD camera, and an ENFINA system. The probe size was 1 nm and the EELS energy resolution was characterized by the full width at half-maximum, fwhm, of the zero-loss peak (ZLP), which was 0.7 eV for the ZnO:Al and 0.5 eV for the ZnO:In and ZnO:Sn. The convergent angle was 11 mrad and the collection angle was 4 mrad. Raw EELS spectra in the data cube were calibrated and then the ELF signals were obtained by the following equation: counts ) a exp(-b · E) + c, where a, b, and c are fitting variables and E is energy loss. The ELF is acceptable only when the function is positive and the intensity before intra/interband transition threshold approximates zero. The ELF signals are employed to evaluate intra- and interband transitions, which are dominated by defect energy levels introduced by dopants. The energy threshold is characterized by fitting to an equation expressed as y ) a · (E - Eb)n proposed by Rafferty,39 where y is ELF counts, a is counts, n is a coefficient, and Eb is the band gap threshold of interest. A low-pass filter (numerical filter function in DigitalMicrograph) with a width of 0.8 eV is employed to smooth the spectra for clear presentation. 3. Results A low magnification bright field (BF) image as shown in Figure 1a reveals that the morphology of product out of the ZnO:Al growth is comb-like and the inset is a selected area diffraction pattern (SADP) derived from the dotted circle area,
ZnO:M(Al, In, Sn) Hierarchical Nanostructures
J. Phys. Chem. C, Vol. 114, No. 42, 2010 18033
Figure 2. (a) A low magnification BF image of a ZnO:Sn comb-like structure, (b) the energy-filtered image, (c) the thickness map with profile from the rectangle, (d) the Sn elemental mapping, and (d together with e) high-resolution images from the belt matrix and from the interface between matrix and branch, respectively. The inset in the fist row is the SAD from the marked area.
indicating the growth directions of the belt and branches are [011j0] and [0001], respectively. Panels b and d of Figure 1 are the BF and Al elemental map of the same region, suggesting Al is incorporated rather uniformly into ZnO although some residual bending contours with sharp contrast is hardly eliminated completely. The thickness map of Figure 1b together with an intensity line profile from the rectangle area is shown in Figure 1c, demonstrating that thickness is increased from 0.3λmfp in the belt to 0.7λmfp in the branch, where λmfp is the mean free path of inelastic electron scattering in ZnO, which is calculated to be around 70 nm.40 Two high-resolution TEM (HRTEM) images shown in Figure 1e,f are representatives of the upper narrower and lower wider regions along the branch, respectively, revealing that the (0001) lattice spacing is the same of 0.536 nm and the (011j0) spacing of 0.286 nm without observable stacking faults and defects. However, the lattice spacings are larger than the respective spacings of 0.521 and 0.281 nm from ZnO JCPDS data,41 which coincides with the successful substitution of Zn with Al in the lattices. A low magnification BF image shown in Figure 2a with the inset of SADP also reveals that the results from the ZnO:Sn growth with directions of the belt and branches are [21j1j0] and [0001], respectively, with sharper interfaces between the belt matrix and the branches and no shrinkage of the branches toward the top. Panels b and d of Figure 2 are the BF image and the corresponding Sn elemental maps of the same area, also indicating that Sn is incorporated into ZnO. The thickness map of Figure 2b together with a profile from the rectangle is shown in Figure 2c, exhibiting that thickness is increased from 0.3λmfp in the belt to 1.2λmfp in the branch, much taller than the ZnO: Al branches. The HRTEM images from the belt in Figure 2e and from the branch in Figure 2f reveal that both areas have the same (0001) spacing of 0.542 nm and (21j1j0) spacing of 0.286 nm, which are also larger than those of bulk ZnO, caused by the Sn doping the same as in the Al doping case.
Similarly, a SEM image in Figure 3a reveals the morphology of the product from the ZnO:In growth is fishbone-like with two-layered and two-sided branches. However, the backbone of the belt is wavy and too thick to obtain high-resolution images. The SADP in the inset from the dotted circle area in Figure 3b also confirms that the belt and branches are grown along ZnO [011j0] and [0001], respectively. In addition, thin shells grow on the branches. Figure 3b is the DF image with an arrowhead indicating the defect band in the belt. Figure 3d is the corresponding In elemental map, demonstrating that In is incorporated into ZnO branches as well. The thickness map together with the profile from the rectangular area is shown in Figure 3c, showing that thickness is increased from 0.2λmfp in the shell to 0.8λmfp in the branch. The HRTEM images of the tip, in Figure 3e, and the middle of a branch, in Figure 3f, reveal that the interfaces between shells and rods are coherent. Both areas exhibit the same (0001) lattice spacing of 0.544 nm and (011j0) of 0.292 nm, larger than those of ZnO bulk confirming the successful In substitution for Zn. Any types of dopants used here would always cause ZnO lattice dilation in all directions though to a slightly different degree. The raw valence EEL spectra (VEELS) are shown in Figure 4 with the characteristic peaks indicated, which were identified with the help of numerical second difference. For the peaks in the VEELS, theoretical calculations and experimental results of bulk ZnO32 show that O-2p is at ∼9.5 eV, Zn-3d is at ∼13.5 eV, and O-2s is at ∼21.8 eV. For ZnO nanowires, all the transitions should be the same as bulk except for the O-2s transition, which is reported to be split into 20.8 and 22.1 eV33 when the diameter is smaller than 20 nm. The bulk plasmon peak is at ∼18.1 eV. In our case of the ZnO:Al combs, the peaks located at ∼9.3, ∼12.6, ∼19.2, and 22 eV correspond to O-2p, Zn-3d overlapping with Al-3p, bulk plasmon, and O-2s transition, respectively. In the ZnO:In case, the peaks located at ∼9.3, ∼12.6, ∼18.3, 19.2, and ∼21.7 eV correspond to O-2p,
18034
J. Phys. Chem. C, Vol. 114, No. 42, 2010
Wang et al.
Figure 3. (a) A SEM image of a ZnO:In fishbone-like structure revealing two-layered branches grown on one side, (b) the dark field image of a fishbone with an arrowhead indicating a defect band, (c) the thickness map with profile from the rectangle, (d) the In elemental mapping, and (d together with e) high-resolution images from the interface between branch and shell and from the end of the same branch. The inset in the fist row is the SAD from the marked area.
Figure 5. Illustration of energy levels of pure-ZnO, ZnO:Al, ZnO:In, and ZnO:Sn from VEEL spectra. Theoretically calculated value of transition due to oxygen vacancy is included for comparison.43 Figure 4. VEEL spectra of pure-ZnO (from bottom), ZnO:In@shell, ZnO:In@branch, ZnO:Sn@initial belt, ZnO:Sn@branch, ZnO:Al@initial belt, and ZnO:Al@branch (top). Band gap thresholds, O-2p, Zn-3d, O-2s, and bulk plasmon are indicated.
Zn-3d, In-5p, bulk plasmon, and O-2s transition, respectively. In the ZnO:Sn case, the peaks located at ∼9.3, ∼13.2, ∼17.8, ∼19.2, and 22.4 eV correspond to O-2p, Zn-3d, Sn-5p, bulk plasmon, and O-2s, respectively. The O-2p and O-2s transitions are consistent with aforementioned results for ZnO:Al. and ZnO: In whereas O-2s transition is blue-shifted for ZnO:Sn. The variation of Zn-3d transition can be attributed to the broad and unoccupied states in the conduction band. Bulk plasmon energies are all blue-shifted relative to pure ZnO, which are attributed to more free carriers, dopant atoms, and interband coupling with bulk plasmon.29,42 Results of the interband transitions are shown in Figure 5 together with the energies of pure ZnO band gap and theoretically calculated green emission due to oxygen vacancy.43 In this curve-fitting procedure, the error of Eb is smaller than 0.025 eV, which indicates goodness of fit reaching 98%. Depending on dopants, the interband transition change from 3.11 eV for pure ZnO nanowires to 2.03 eV for ZnO:Al, 2.4 eV for ZnO:
In, and 2.7 eV for ZnO:Sn. Areas from both the belts and branches were selected to analyze variations and computation converges to the same results. The energy transition threshold of pure ZnO is lower than its band gap, related to band tail states caused by intrinsic complex donor levels, high carrier density, and electron momentum transferred to pump valence electrons. Moreover, these hierarchical ZnO structures show the band gap transition thresholds follow the sequence Ed,pure > Ed,In > Ed,Sn > Ed,Al, indicative of extra energy states created by dopants with the relative position determined by the type of dopants. 4. Discussion In this study, Al, Sn, and In were employed in the alloyingvapor-deposition method to synthesize the comb-like (Figures 1 and 2) and the fishbone-like (Figure 3) ZnO nanostructures. According to the elemental mapping shown in Figures 1d, 2d, and 3d, Al, Sn, and In atoms were successfully incorporated into ZnO structures. On the basis of the high-resolution images from all of the as-grown nanostructures, the (0001) lattice spacing is around 0.54 nm, and is larger than 0.52 nm from
ZnO:M(Al, In, Sn) Hierarchical Nanostructures ZnO bulk. Hence, all dopants are proposed to substitute for Zn sites resulting in (0001) lattice plane expansion. On the basis of the first-principle method, the configuration of Al substituting for Zn sites attains minimum formation energy43,44 for Al impurities, which is also expected to be applied for dopants of In and Sn in the branches. In our study, we obtain two distinct morphologies of doped ZnO nanostructures, namely the comb-like structures by doping with Al and Sn and the fishbone-like ones by doping with In. There have been many mechanisms proposed to explain ZnO fishbone-like and comb-like structures.9-15 Among them, the most common one involves a two-step process where the first step is to grow an initial belt, on which the secondary branches are nucleated in the second step. Due to chemically active (0001)-Zn plane and inert (0001j)-O plane on the initial belt, only the active (0001)-Zn planes induce branches growing, resulting in combs. However, the morphology of the doped combs structures in our case is distinct from that of the typical combs in that the thickness of the belt is not uniform and varies in accord with the branches as described earlier, which should be related with the effect of dopants. We propose a growth mechanism as follows. First, owing to the presence of the dopants, the surface energy of ZnO {0001} polar planes can be stabilized, which facilitate the formation of comb- or fishbonelike nanostructures. Second, since the doped lattice spacings are larger than those in the bulk rendering the initial belt unstable, we thus have proposed that dopant concentration fluctuation has occurred to modulate the lattice mismatch,27 where the higher dopant concentration area would preferably induce nucleation of branches and continued thickening process of the belt. The thickness of the belt reflects the catalytical power of the respective dopants. As to the fishbone-like structure, there are two possible mechanisms to enable both sides to be active. First, polarconversion domains could be present inside the thick initial belt resulting in an active (0001)-Zn surface existing on both sides. In the case of In-doped ZnO, in order to accommodate the highest lattice mismatch in the thicker belt, additional twin defects were created as indicated in Figure 3b, resulting in both sides being active. The other mechanism is involved with wavy sides of the initial belt possibly providing active nucleation sites equally. Two-layered branches as shown in Figure 3a provide evidence of multiple nucleation sites. Moreover, lateral growth on branches, as shown in Figure 3, revealing that In may enhance in-plane catalytic growth of ZnO, is consistent with previous results.25 Besides, the highest variation from the branches to the initial belt also suggests that the ZnO:Sn system is the most unstable. Hence, we have demonstrated that dopants help produce ZnO nanostructures of different morphology. According to the expression for bulk plasmon blue shift in semiconductors and insulators as29 (Eip)2 = (ni/n)E2p + E2i , where Ei is the binding energy of carriers (for intrinsic carriers, Ei g Eg, the band gap energy). Ep is bulk plasmon energy, Ep ) p(ne2/ ε0m0), by Drude model where n is the free electron density. For insulators or semiconductors, ni ) n. In our case, in addition to extrinsic carriers, dopant atoms also participate in the collective oscillation causing Ep to be blue shifted due to the increase of n. Interband transition energies of dopants smaller than bulk plasmon energy also makes plasmon energy increase due to coupling.42 The emission spectra of pure and doped ZnO nanostructures by optical pumping reveal the UV emission red shifts to 3.34 eV with a green emission maximum at 2.56 eV27 for Al-doped nanowires. For In-doped nanobelts and superlattice nanowires,
J. Phys. Chem. C, Vol. 114, No. 42, 2010 18035 the UV emission red shifts to 3.315-3.327 eV36-38 with the similar green emission maximum at 2.25 eV. For Sn-doped nanowires, UV emission red shifts to 3.25 eV26 with the similar green emission maximum at 2.48 eV. It seems to suggest that the relative position of various impurity energy levels with respect to valence band maximum derived from the defect bands follows Ed,In > Ed,Sn > Ed,Al. In comparison, in the defect emission regime,35 the energy level due to Al dopants is in good agreement qualitatively, but the order reverses between In and Sn. Valences of foreign atoms would further complicate the defect levels due to variations in chemical activity. 5. Conclusion Hierarchical ZnO nanostructures are synthesized successfully by introducing the dopants Al, Sn, and In to the Zn source in the alloying-vapor-deposition process. Utilizing Al and Sn induces the comb-like structures with belts growing in the [21j1j0] or [011j0] directions and branches growing along the [0001] direction. However, utilizing In induces the fishbone-like structure with belts growing in the [011j0] direction. Lattice distortion and chemical activity of dopants causes periodic thickness variations in the belts leading to branches growing. Dopants modulate projected interband transition thresholds with 2.03 eV for ZnO:Al, 2.4 eV for ZnO:In, and 2.7 eV for ZnO: Sn. Hence, dopants modulate ZnO morphology and interband transitions, enabling doped ZnO nanostructures candidates for solar cell scaffolds. Acknowledgment. The authors are grateful for the NSC Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pintung Area. The authors also acknowledge the facilities as well as scientific and technical assistance from staff in the AMMRF (Australian Key Center for Microscopy & Microanalysis), The University of Sydney. References and Notes (1) Mishra, K. C.; Schmidt, P. C.; Johnson, K. H.; de Boer, B. G.; Berkowitz, J. K.; Dale, E. A. Phys. ReV. B 1990, 42, 1423–1430. (2) Wang, R. C.; Liu, C. P.; Huang, J. L.; Chen, S. J.; Tseng, Y. K.; Kung, S. C. Appl. Phys. Lett. 2005, 87, 013110-1-013110-3. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. (4) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. D. Science 2004, 305, 1269–1273. (5) Wang, X. D.; Zhou, J.; Song, J. H.; Xu, N. S.; Wang, Z. L. Nano Lett. 2006, 6, 2768–2772. (6) He, H., Jr.; Hsin, C. L.; Lih, J. L.; Chen, L. J.; Wang, Z. L. AdV. Mater. 2007, 19, 781–784. (7) Wang, Z. L.; Song, J. H. Science 2006, 312, 242–246. (8) Cole, J. J.; Wang, X.; Knuesel, R. J.; Jacobs, H. O. Nano Lett. 2008, 8, 1477–1481. (9) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett 2003, 91, 185502-1–185502-4. (10) Pan, Z. W.; Mahurin, S. M.; Dai, S.; Lowndes, D. H. Nano Lett. 2005, 5, 723–727. (11) Zhang, Y. H.; Liu, J.; Liu, T.; You, L. P.; Li, X. G. J. Cryst. Growth 2005, 285, 541–548. (12) Zhang, Y.; Song, X.; Zheng, J.; Liu, H.; Li, X.; You, L. Nanotechnology 2006, 17, 1916–1921. (13) Bai, W.; Yu, K.; Zhang, Q.; Xu, F.; Peng, D.; Zhu, Z. Appl. Surf. Sci. 2007, 253, 6835–6839. (14) Zhuo, R. F.; Feng, H. T.; Liang, Q.; Liu, J. Z.; Chen, J. T.; Yan, D.; Feng, J. J.; Li, H. J.; Cheng, S.; Geng, B. S.; Xu, X. Y.; Wang, J.; Wu, Z. G.; Yan, P. X.; Yue, G. H. J. Phys. D: Appl. Phys 2008, 41, 1854051–185405-14. (15) Li, C.; Fang, G.; Su, F.; Li, G.; Wu, X.; Zhao, X. Cryst. Growth Des. 2009, 6, 2588–2591. (16) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625–1631. (17) Dai, Y.; Zhang, Y.; Li, Q. K.; Nan, C. W. Chem. Phys. Lett. 2002, 358, 83–86. (18) Dai, Y.; Zhang, Y.; Wang, Z. L. Solid State Commun. 2003, 126, 629–633.
18036
J. Phys. Chem. C, Vol. 114, No. 42, 2010
(19) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287–1291. (20) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348–1351. (21) Ding, Y.; Kong, X. Y.; Wang, Z. L. Phys. ReV. B 2004, 70, 2354081–235408-7. (22) Zhang, X.; Lu, H.; Gao, H.; Wang, X.; Xu, H.; Li, Q.; Hark, S. Cryst. Growth Des. 2009, 9, 364–367. (23) Gao, P.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 12653–12658. (24) Gao, P. X.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1315–1320. (25) Fan, H. J.; Fuhrmann, B.; Scholz, R.; Himcinschi, C.; Berger, A.; Leipner, H.; Dadgar, A.; Krost, A.; Christiansen, S.; Gosele, U.; Zacharias, M. Nanotechnology 2006, 17, S231–S239. (26) Bae, S. Y.; Na, C. W.; Kang, J. H.; Park, J. J. Phys. Chem. B 2005, 109, 2526–2531. (27) Kuo, C. L.; Wang, R. C.; Liu, C. P.; Huang, J. L. Nanotechnology 2008, 19, 035605-1-035605-5. (28) Cheng, C.; Liu, B.; Yang, H.; Zhou, W.; Sun, L.; Chen, R.; Yu, S. F.; Zhang, J.; Gong, H.; Sun, H.; Fan, H. J. ACS Nano 2009, 3, 3069– 3076. (29) Egerton, R. F. Electron energy loss spectroscopy in the electron microscope; Plenum Press: New York, 1996. (30) Keast, V. J.; Bosman, M. Microsc. Res. Tech. 2007, 70, 211–219. (31) de Abajo, F. J. G. ReV. Mod. Phys. 2010, 82, 209–275. (32) Dorn, R.; Luth, H.; Buchel, M. Phys. ReV. B 1977, 16, 4675–4683.
Wang et al. (33) Wang, J.; An, X.; Li, Q.; Egerton, R. F. Appl. Phys. Lett. 2005, 86, 201911-1–201911-3. (34) Teke, A.; Ozgur, U.; Dogan, S.; Gu, X.; Mokoc, H. Phys. ReV. B 2004, 70, 195207-1–195207-10. (35) Djurisic, A. B.; Leung, Y. H.; Tam, K. H.; Hsu, Y. F.; Ding, L.; Ge, W. K.; Zhong, Y. C.; Wong, K. S.; Chan, W. K.; Tam, H. L.; Cheah, K. W.; Kwok, W. M.; Philips, D. L. Nanotechnology 2007, 18, 095702. (36) Jie, J.; Wang, G.; Han, X.; Yu, Q.; Liao, Y.; Li, G.; Hou, J. G. Chem. Phys. Lett. 2004, 387, 466–470. (37) Jie, J.; Wang, G.; Han, X.; Hou, J. G. J. Phys. Chem. B 2004, 108, 17027–17031. (38) Chen, Y. W.; Liu, Y. C.; Lu, S. X.; Xu, C. S.; Shao, C. L.; Wang, C.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Chem. Phys. 2005, 123, 134701-1–134701-5. (39) Rafferty, B.; Brown, L. M. Phys. ReV. B 1998, 58, 10326–10337. (40) Iakoubovskii, K.; Mitsuishi, K.; Nanakyama, Y.; Furuya, K. Microsc. Res. Tech. 2008, 71, 626–631. (41) Sawada, H.; Wang, R.; Sleight, A. W. J. Solid State Chem. 1996, 122, 148–150. JCPDS No. 890510. (42) Strum, K. AdV. Phys. 1982, 31, 1–31. (43) Janotti, A.; van de Walle, C. Rep. Prog. Phys. 2009, 72, 1265011–126501-29. (44) Lany, S.; Zunger, A. Phys. ReV. Lett. 2007, 98, 045501-1-045501-4.
JP103594M
