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 [011 j 0] for fishbone-like structures or [21 j 1 j 0] 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 nano- structures. 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 hier- archical 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 tetrapods 17,18 have been synthesized by the vapor-transport-condensation mechanism. When ZnO with other oxides were used as the sources, nanocombs from the In 2 O 3 backbone, 19 nanorings accompanied by indium oxide layer, 20,21 and In 2 O 3 (ZnO) m superlattice nanowires 22 have also been reported. Furthermore, either Zn or Sn was also utilized to produce multibranched ZnO structures 23,24 and In induces nanowires growing along the [112 j 0] direction. 25 However, doped hierarchical structures have never been analyzed in detail, although Al-, Ga-, In-, and Sn-doped ZnO nanowires with [0001], [011 j 0], and [011 j 1] growth directions were demon- strated. 26 Recently, the authors have developed the alloying- vapor-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 SnO 2 backbone was demon- strated. 28 The possible applications of hierarchical ZnO struc- tures, such as laser beam splitter by using ZnO combs 10 and lasing from the ZnO@SnO 2 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. Interest- ingly, 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 comb- or 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 prom- ising 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 Zn- 3d, 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 * 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. J. Phys. Chem. C 2010, 114, 18031–18036 18031 10.1021/jp103594m  2010 American Chemical Society Published on Web 10/05/2010