Crystal Structure of In2O3(ZnO)m Superlattice Wires and Their Photoluminescence Properties Xitian Zhang,*,† Huiqing Lu,† Hong Gao,† Xiaojing Wang,‡ Haiyang Xu,‡ Quan Li,‡ and Suikong Hark*,‡
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 364–367
Heilongjiang Key Laboratory for AdVanced Functional Materials and Excited State Processes, School of Physics and Electronic Engineering, Harbin Normal UniVersity, Harbin, 150025, P.R. China, and Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, N.T., Hong Kong, P.R. China ReceiVed May 27, 2008; ReVised Manuscript ReceiVed September 15, 2008
ABSTRACT: One-dimensional In2O3(ZnO)m superlattice wires were first synthesized on silicon substrates by evaporating a mixture of In and ZnO powders. The high-resolution transmission electron microscopy image indicates that the wires have a superlattice structure along their length, which consists of alternating stacks of an InO2- octahedral layer, as an inversion boundary, and an InO+(ZnO)m slab. Each slab is further separated by many triangular (zigzag) shape-contrast boundaries. They are secondary polarity inversion boundaries formed by 5-fold coordinated In and Zn atoms. The formation of such superlattice nanostructures is explained by the zigzag model. The PL properties of the superlattice nanostructures are discussed with regard to its temperature dependence for the first time. Transparent oxide semiconductor (TOS) thin films, which are used as transparent electrodes in optoelectronic devices, have been widely studied.1 Among the TOSs, the thin films of a homologous compound In2O3(ZnO)m, with a so-called superlattice structure have attracted considerable interest. Because of the spatial confinement of conductive electrons in the twodimensional layer, not only are In2O3(ZnO)m thin films a good TOS, but they also exhibit exceptional electron transport properties.2 Recently, K. Nomura et al. reported high performance transparent field-effect-transistors exhibiting good performances, such as normally off characteristics with on-to-off current ratios >106, using similar superlattice structure materials.3 On the other hand, with rapid developments in nanoscience and nanotechnology, the syntheses of one- and zero-dimensional semiconductor nanostructures have gained a tremendous amount of attention in recent years, due to their fascinating chemistry and size-, shape-, and material-dependent properties. Their interesting electronic, optical, and magnetic properties, along with small size and chemical reactivity, have led to a wide range of applications in nano-optoelectronics, medical diagnostics, catalysis, drug delivery, therapeutics, and chemical sensing,4,5 originating from the three- or two-dimensional spatial confinement effect of conductive carriers. Until now, research on the In2O3(ZnO)m system has been focused on its bulk and thin film forms; reports on one-dimensional nanostructure and their photoluminescence (PL) properties were rather rare.6,7 Inspired by the above information, we synthesized one-dimensional In2O3(ZnO)m wires by merely evaporating the mixture of ZnO and In powder, and no other precursor such as Sn7 or Co2O36 was introduced. Compared with the study in ref 6, our growth process is simpler and the as-synthesized products are easily reproduced. The diameter of the as-synthesized wires is uniform along the entire length. The microstructure of the wires is composed of an alternating InO2- (In-O) layer and an InO+(ZnO)m (Zn/In-O) slab along its length, in which there is a zigzag-shape-contrasted polarity inversion boundary, formed by 5-fold coordinated In and Zn atoms inside the Zn/In-O slab. * Corresponding authors: E-mail:
[email protected] (X.T.Z.);
[email protected] (S.K.H.) † Harbin Normal University. ‡ The Chinese University of Hong Kong.
Each triangular area (so-called well layer in a quantum well structure) is surrounded by an In-O layer and two zigzag-shapecontrasted boundaries (so-called barrier layer in the quantum well structure) and is referred to as an InO+(ZnO)m quantumrod (IOZOQR). Especially, in this paper, we discuss the PL properties related to the superlattice nanostructures in details. The In2O3(ZnO)m superlattice wires were synthesized. An In/ ZnO (molar ratio 1:10) mixture was loaded into one end of an alumina boat and the Au-coated Si substrate was placed at the other end, downstream of the mixture. In order to synthesize In2O3(ZnO)m superlattice wires, Au is necessary, and temperature and pressure have to be also well controlled in our experiment. The previous research work8 has confirmed that one-dimensional ZnO nanostructures cannot be synthesized without Au coating. The boat was placed inside an alumina tube that was inserted into a horizontal furnace, and the mixture was located at the center of the furnace. The furnace was heated to a preset temperature (1400 °C) under a flow of 30 mL/min of nitrogen gas and a pressure of 50 Pa. The furnace temperature was kept for 20 min. Then, the furnace was naturally cooled to room temperature. The synthesized products were found covering the entire substrate. They were characterized by scanning electron microscopy (SEM, LEO, 1450 VP), and high-resolution transmission electron microscopy (HRTEM, Philips Tecnai F20), equipped with energy dispersive X-ray (EDX) spectrometer. Their photoluminescence (PL) spectra were taken using the 325 nm line of a He-Cd laser as the excitation source. A typical SEM image of the products is shown in Figure 1a. A high density of smooth wire-like nanostructures (WLNs) is clearly seen over the entire surface of the substrate. The length of the WLNs is about a few tens of micrometers. All the peaks of the WLNs in the X-ray diffraction pattern (Figure S1, Supporting Information) belong to the hexagonal wurtzite ZnO phase structure within the errors, but the peak widths are slightly broadened. Their morphology was further studied by TEM. A low-magnification image of which is given in Figure 1b, revealing their wire morphology. Their diameters are uniform and less than 100 nm. EDX analysis confirms that these WLNs are indeed composed of In, Zn, and O at atomic ratios of 5:39: 56 (Figure S2, Supporting Information). The Zn content (Zn/ [Zn+In]) is about 89 at.%. Their surfaces appear to have
10.1021/cg800554e CCC: $40.75 2009 American Chemical Society Published on Web 11/19/2008
Crystal Structure of In2O3(ZnO)m Superlattice Wires
Figure 1. (a) A SEM image of In2O3(ZnO)m wire-like superlattice nanostructures; (b) TEM image of more wires; (c) a TEM image of an individual In2O3(ZnO)m superlattice wire.
Figure 2. (a) HRTEM lattice image of the wire in Figure 1c; the left bottom arrows point to the In-O layer, that is the interface between two adjacent slabs; (b) the In mapping image; (c) a SAED pattern taken along the 〈21j1j0〉 zone axis; (d) an enlarged SAED pattern of c.
alternately bright and dark contrast lines along the length. For a better inspection of the morphologies, an individual wire is shown in Figure 1c. This is a typical superlattice feature for the In2O3(ZnO)m system, in which the bright contrast lines correspond to the In-O layers and the dark lines the In/Zn-O slab. The In atoms in the In-O layer occupy octahedral sites and are 6-fold coordinated by O atoms, while the In and Zn atoms in the In/Zn-O slab randomly occupy the tetrahedral or trigonal bipyramidal sites.9 A more detailed structure of the WLNs is shown in the HRTEM image, energy-filtered image using electron energy loss spectroscopy (EELS), and selected
Crystal Growth & Design, Vol. 9, No. 1, 2009 365
area electron diffraction (SAED) patterns in Figure 2. The HRTEM image in Figure 2a reveals a layered structure and a wavy contrast forming a zigzag or triangular shaped boundary. For the layered structure, it is very clear that the image consists of lattice fringes periodically interleaved by bright lines. The space between the two adjacent lattice fringes is about 0.26 nm, corresponding to the (0002) plane spacing. It may be suggested that the bright line corresponds to the In-O octahedral layers. In addition, the lattice fringes are modulated, forming a zigzag contrast boundary inside the slab. Such a modulation structure is observed in one-dimensional nanostructures for the first time. According to our statistical results, m ) 10 - 16. Why are the zigzag or triangular contrast boundaries formed in the In/Zn-O slab? First, since indium ions in the slabs randomly occupy the tetrahedral or trigonal bipyramidal sites, they could produce local lattice distortions that result in strains. If the strains were relaxed by zigzag boundaries, the formed crystal structure could be the most stable. Second, Yan et al. recently proposed a zigzag modulated boundary model,10 based on first-principle densityfunctional theory calculations. They thought that the zigzag contrast boundaries consist of In and Zn atoms, which are 5-fold coordinated by O atoms. However, the O atoms are 4-fold coordinated. Because the In-O octahedral layers are inversion boundaries, the polarities of ZnO at its two sides are inverted. Consequently, the polarities must be inverted again inside the Zn/In-O slabs. In other words, a second polarity inversion boundary must exist. The zigzag boundaries provide the required second polarity inversion (head-to-head configuration).10 The model proposed by Yan et al. was also supported by a simulation of electron diffraction pattern and total-energy calculation. Similarly modulated structures were also observed in the InFeO3(ZnO)m system.7 In order to further interpret the observed contrast in the lattice image, we made an In element mapping of the local region shown in Figure 2a using EELS; the result of which is shown in Figure 2b. It confirms that the trivalent In3+ ions are mainly distributed within the interface between two adjacent slabs (marked by the left bottom arrows in Figure 2a) and along the zigzag boundaries. Figures 2c and 2d are two selected-area electron diffraction (SAED) patterns corresponding to Figure 2a taken along the 〈21j1j0〉 zone axis of the ZnO structure. Figure 2d is an enlarged from Figure 2c. In the patterns, some weak satellite spots are clearly observed, indicating the presence of a modulated structure. The periodicity of modulation along the 〈01j10〉 direction is estimated to be 5.5 nm by averaging the various measured periodicities, which vary slightly from one area to another. It is also characteristic that the phase of the modulation is coherent within the In/Zn-O slabs. The angle of the zigzag shape as shown in Figure 2a is found to be about 52°. The HRTEM image and SAED pattern show that the WLNs are of wurtzite structure and grow along the 〈0001〉 direction or c axis. According to the HRTEM results and Yan’s zigzag model, we imagine that (i) the WLN may be composed of IOZOQR arrays, (ii) there is a triangular crosssection for each quantum rod, and (iii) the quantum rods are separated by the In-O octahedral layers and the zigzag boundaries. The length of the rods equals the diameter of wires, and the isosceles length of the triangular sides is about 5 nm. A schematic illustration of the In2O3(ZnO)m superlattice wire is shown in Figure 3. The inset is a schematic diagram of a quantum rod with a triangular cross-section; one side corresponds to the In-O layer, and the other two sides the zigzag boundaries. Some twinned WLNs (TWLNs) (10%) were observed among the WLNs. All of the TWLNs have a clear boundary running
366 Crystal Growth & Design, Vol. 9, No. 1, 2009
Zhang et al.
Figure 5. (a) A low-magnification HRTEM image of the twin wire; (b) the corresponding EDX spectrum; (c) a HRTEM lattice image.
Figure 3. A schematic illustration of In2O3(ZnO)m superlattice wires; the inset shows a schematic diagram of a quantum rod with a triangular cross-section, which is enclosed by an In-O layer and the two zigzag contrast boundaries. The height equals the diameter of wires. Figure 6. Temperature-dependent PL spectra. The inset shows a model of conduction and valence bands of the barrier and well layers.
Figure 4. (a) A TEM image of a twin wire; (b) the corresponding SAED pattern.
Figure 7. PL spectrum of the superlattice nanostructures and ZnO nanowires taken at 9 K. The top inset is a photograph of the superlattice nanostructures emitting and bottom inset is a photograph of the ZnO nanowires emitting. The inset graph is an enlarged visible emission spectra.
midway along their length as shown in Figure 4a. In addition to this, their appearance is decorated by the modulated waves and layered structures. Their corresponding SAED pattern is shown in Figure 4b, which was obtained along the [011j0]direction of the wurtzite ZnO structure. It consists of two sets of diffraction patterns. Spots with (21j1j2j) g vectors are common to both patterns. The angle between the (0001) planes in the two crystals is 117°. The growth direction of the TWLNs is [2j113j], which is normal to the [011j0]zone axis and in the (21j1j2j) plane. The twin plane is one of the {21j1j2j} family. It is not the same as a general (011j3) twin plane observed in ZnO nanowires.11 Figures 5a and 5b show an enlarged TEM image and corresponding EDX spectrum of the TWLN, which is composed
of Zn, O, and In. The Cu signals are from the TEM grids. Figure 5c shows the lattice image of the TWLN. Both HRTEM examinations and SAED patterns suggest that both the TWLNs and WLNs have the wurtzite structure. In order to investigate the PL properties of the superlattice nanostructures, we measured their spectra at different temperatures, which are shown in Figure 6. All spectra consist of a broad ultraviolet (UV) peak, while the visible emission associated with oxygen defects, which is usually found in ZnO nanowires, is not detected. As the temperature rises, the full width at half-maximum (FWHM) of the UV emission increases from 63 to 135 meV, and it is twice as broad as the FX peak in high quality nanocrystalline ZnO thin films at room tempera-
Crystal Structure of In2O3(ZnO)m Superlattice Wires
ture.12 The normalized spectra at 9 K of the In2O3(ZnO)m WLNs and the ZnO nanowires are compared in Figure 7. The former has only one broad UV emission peak centered at 3.32 eV, with a FWHM of 63 meV, while the latter is composed of a few UV emission peaks and a visible one. The FWHMs of the free exciton (FX) and bound exciton emissions (BX) centered at 3.36 and 3.34 eV are about 9 and 15 meV, respectively, obtained by fitting to the PL spectrum. The results are (1) the two PL features are completely different and (2) the UV peak position of the WLNs is slightly red-shifted and the FWHM is much broader than those of the BX emission of the ZnO nanowires. The top inset is a photograph, taken with a digital camera, showing the superlattice nanostructures emitting UV light; while the bottom inset is a photograph showing the ZnO nanowires emitting green light, taken with the same camera. In the experiment, we found that the former emits much more intense light than the latter. In order to explain the experimental results observed, we propose a band model, which is shown in the inset of Figure 6, of conduction and valence bands between the barrier and well layers. The justifications for the model are as follows: (i) A random distribution of Ga3+ and Zn2+ ions in the GaO+(ZnO)5 slab of InGaO3(ZnO)5 thin films modulates the electronic structure around the conduction band edge and forms a statistical distribution of localized potential tail states just below the conduction band edge.13 The In2O3(ZnO)m system is homologous to InGaO3(ZnO)m, consisting of an alternative stack of a InO2- (In-O) layer and an InO+(ZnO)m (In/Zn-O) slab along the c axis. (ii) We measured the time-resolved PL spectrum of the WLNs. The photon lifetime is less than 20 ps, which is the time resolution limited by our streak camera. The result nevertheless confirms that the emission is not from the recombination of FX or BX. From the band model, the depth of the wells is different. It could be attributed to a random distribution of In3+ and Zn2+ ions in the (In/Zn-O) slab, resulting in the statistical potential distribution. In such a band model, the PL features observed are easily understood. Conclusions In summary, one-dimensional In2O3(ZnO)m superlattice nanostructures were synthesized by evaporating a mixture of In/ZnO powder. HRTEM and SAED patterns show the formation of In2O3(ZnO)m superlattice nanostructures, which have a layered structure of alternating stacks of an In-O layer and an In/Zn-O slab along the c axis. In the slab, the zigzag-shape boundary is formed by 5-fold coordinated Zn and In atoms. Therefore, an IOZOQR with a triangular cross-section is interleaved with an In-O layer and a zigzag-shape boundary. In PL measurements, we found spectra consist of only one intense UV peak, which shows an obvious red shift, in the temperature range of 9 to 290 K. These features are attributed to the emission of the quantum wells whose well depths follow a statistical potential distribution.
Crystal Growth & Design, Vol. 9, No. 1, 2009 367
Acknowledgment. The work in this paper was partially supported by the National Natural Science Foundation of China under Grant No. 60776010; the Natural Science Foundation of Heilongjiang (A2007-03), the Excellent Leader of Subjects, the Bureau of Science and Technology of Harbin (2007RFXXG028); the Education Bureau of Heilongjiang Province (11531225 and 11531227); and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 2150521). Supporting Information Available: Expermental details. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Shah, A.; Torres, P.; Tscharner, R.; Wyrsch, N.; Keppner, H. Science 1999, 285, 692–8. (b) Kim, H.; Lee, K.; Kwon, J. H. Appl. Phys. Lett. 2006, 88, 012103–5. (c) Ramamoorthy, K.; Kumar, K.; Chandramohan, R.; Sankaranarayanan, K. Mater. Sci. Eng., B 2006, 126, 1–15. (d) Ginley, D. S.; Bright, C. MRS Bull. 2000, 25, 15–18. (e) Franken, R. H.; van der Werf, C. H. M.; Lo¨ffler, J.; Rath, J. K.; Schropp, R. E. I. Thin Solid Films 2006, 501, 47–50. (f) Tak, Y.; Kim, K.; Park, H.; Lee, K.; Lee, J. Thin Solid Films 2002, 411, 12– 16. (2) (a) Nomura, K.; Ohta, H.; Ueda, K.; Kamiya, T.; Orita, M.; Hirano, M.; Suzuki, T.; Honjyo, C.; Ikuhara, Y.; Hosono, H. Appl. Phys. Lett. 2004, 95, 5532–4. (b) Dupon, L.; Maugy, C.; Naghavi, N.; Guery, C.; Tarascon, J. M. J. Solid State Chem. 2001, 158, 119–33. (c) Uchida, N.; Bando, Y.; Nakamura, M.; Kimizuka, N. J. Electron Microsc. 1994, 43, 146–50. (d) Li, C.; Bando, Y.; Nakamura, M.; Kimizuka, N. Micron 2000, 31, 543–50. (e) Li, C. F.; Bando, Y.; Nakamura, M.; Onoda, M.; Kimizuka, N. J. Solid State Chem. 1998, 139, 347–55. (f) Hao, Y. F.; Meng, G. W.; Wang, Z. L.; Ye, C. H.; Zhang, L. D. Nano Lett. 2006, 6, 1650–5. (g) Geng, B. Y.; Liu, X. W.; Du, Q. B.; Wei, X. W.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 163104–6. (3) Nomura, K.; Ohta, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Science 2003, 300, 1269–72. (4) (a) Suh, K.; Shin, J. H.; Park, O. H.; Bae, B. S.; Lee, J. C.; Choi, J. H. Appl. Phys. Lett. 2005, 86, 053101–3. (b) Ryan, K. M.; Erts, D.; Olin, H.; Morris, M. A.; Holmes, J. D. J. Am. Chem. Soc. 2003, 125, 6284–8. (c) Lu, X.; Hanrath, T.; Johnston, K. P.; Jorgel, B. A. Nano Lett. 2003, 3, 93–9. (5) Mieszawska, A. J.; Jalilian, R.; Sumanasekera, G. U.; Zamborini, F. P. Small 2007, 3, 722–56. (6) Jie, J.; Wang, G.; Han, X.; Hou, J. G. J. Phys. Chem. B 2004, 108, 17027–31. (7) Na, C.; Bae, S. Y.; Park, J. J. Phys. Chem. B 2005, 109, 12785–90. (8) 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, 1897–9. (9) Ohta, H.; Nomura, K.; Orita, M.; Hirano, M.; Ueda, K.; Suzuki, T.; Ikuhara, Y.; Hosono, H. AdV. Funct. Mater. 2003, 13, 139–44. (10) Yan, Y. F.; Da Silva, J. L. F.; Wei, S. H.; Al-Jassim, M. Appl. Phys. Lett. 2007, 90, 261904–6. (11) Ding, Y.; Wang, Z. L.; Sun, T. J.; Qiu, J. S. Appl. Phys. Lett. 2007, 90, 153510–2. (12) Zhang, X. T.; Liu, Y. C.; Zhi, Z. Z.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Xu, W.; Fan, X. W.; Kong, X. G. J. Lumin. 2002, 99, 149–54. (13) Nomura, K.; Kamiya, T.; Ohta, H.; Ueda, K. Appl. Phys. Lett. 2004, 85, 1993–5.
CG800554E
