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2009, 113, 15480–15483 Published on Web 08/12/2009
Indium Oxide-zinc Oxide Nanosized Heterostructure and Whispering Gallery Mode Luminescence Emission N. W. Wang, Y. H. Yang, and G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics & Engineering, Zhongshan (Sun Yat-sen) UniVersity, Guangzhou 510275, Guangdong, People’s Republic of China ReceiVed: July 21, 2009; ReVised Manuscript ReceiVed: August 5, 2009
In2O3-ZnO one-dimensional nanosized heterostructures constructed by In2O3 quadrangular columns and ZnO hexagonal disks have been fabricated by thermal chemical vapor transport and condensation with Au catalysts, and the cathodeluminescence of the as-prepared heterostructures are characterized. It was found that there is basically no luminescence emission from In2O3 quadrangular columns, while there is the intense luminescence emission from ZnO hexagonal disks, which are attributed to the whispering gallery mode from the optical resonator of ZnO hexagonal disks. Fabrications of functional nanostructures as building blocks for optoelectronic nanodevices have been attracting considerable attention in recent years.1 One of the most fascinating fields is the binary compound with metal oxides and semiconductor nanostructures such as In2O3-SnO2 one-dimensional (1D) heterostructures,2 ZnO-SnO2 three-dimensional (3D) epitaxial heterostructure,3 SnO2 triangular nanoparticles on ZnO nanobelts,4 hierarchical organization of Ga2O3-In2O3 nanostructures,5 coaxial ZnO-Al2O3 heterostructures,6 and In2O3-ZnO films.7 Among these investigations above, a lot of works focus on the preparation of In2O3-ZnO nanosized heterostructures by doping, that is, Zn-doped In2O3 and In-doped ZnO. For example, In is doped in ZnO nanorods, and Zn is doped In2O3 nanowires.8,9 It is well known that ZnO is a direct wide-band-gap semiconductor with a large exciton binding energy of 60 meV and is considered as an ideal material for optoelectronic applications. In2O3 is also a wide-band-gap metal oxide semiconductor with a band gap of about 3.6 eV and is an efficient doped element to improve the optoelectronic properties of ZnO. However, there have not been any reports involving the fabrication of In2O3-ZnO 1D nanosized heterostructures and related optoelectronic properties. In this contribution, we report that In2O3-ZnO nanosized heterostructures constructed by In2O3 quadrangular columns and ZnO hexagonal disks have been fabricated by thermal chemical vapor transport and condensation with Au catalysts. Interestingly, we find intense luminescence emission with the whispering gallery mode (WGM) from the optical resonator of ZnO hexagonal disks. In2O3-ZnO nanosized heterostructures are fabricated by thermal chemical vapor transport and condensation with Au catalysts.10-12 In detail, In2O3 (99.9%), ZnO (99%), and graphite powders (99.9%) are mixed as the reaction source in a weight ratio of 3:3:2 and then loaded on one end of an alumina plate. Then, Si substrates covered with a Au layer are placed on another end of the alumina plate. The Au layer is deposited by * To whom correspondence should be addressed. E-mail: stsygw@ mail.sysu.edu.cn.
10.1021/jp906924w CCC: $40.75
sputtering for 40 s. Finally, the alumina plate is put inside of a small quartz tube, which is pushed into a horizontal quartz tube electric furnace. The system is first evacuated for 10 min and then rapidly heated to 1100 °C from room temperature. After maintaining at this temperature for 1 h, the system is cooled down naturally. During the whole process, nitrogen flow is introduced as the carrier gas at the rate of 100 sccm, and the pressure is 680 Torr. The light blue films are observed on substrates when substrates are moved out. Field emission scanning electron microscopy (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD) are employed to characterize the morphologies and structures of samples. Cathodeluminescence (CL) measurement of samples is carried out at room temperature using a Gatan Mono-CL system attached to a FESEM (spot size is about 10 nm) with the accelerating voltage of 25 kV. Bunchgrass-like nanostructures are prepared in our studies as shown in SEM images in Figure 1. The corresponding XRD pattern of samples in Figure 1e shows that the fabricated nanostructures are indexed to the hexagonal wurtzite ZnO and cubic In2O3, in which some small peaks are from SiO2 of the substrates. From Figure 1b, we can clearly see that the prepared nanostructures are constructed by quadrangular columns and hexagonal disks. By line-scanning EDS measurements in Figure 1c, we can find that the quadrangular columns are In2O3 and that the hexagonal disks are ZnO. Note that a relatively small portion of EDS data can come from a background substrate in Figure 1c. In order to avoid the influence of the substrate on EDS measurement of samples, we first peel the prepared nanostructures from the substrate, then move them onto a copper grid, and then characterize the composition of an individual nanostructure on a copper grid by line-scanning EDS measurements, as shown in Figure 1d. Definitely, the EDS measurement in Figure 1d confirms that the quadrangular columns are In2O3 and that the hexagonal disks are ZnO. In Figure 1b, the size of the In2O3 quadrangular columns is in the range of 500 nm to 1 µm, and the side length of the ZnO hexagonal disks is 2.5-5 2009 American Chemical Society
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Figure 1. SEM images of the fabricated nanosized heterostructures (a) and a single heterostructure (b). Line-scanning EDS of one as-prepared heterostructure (c), in which the red spectrum means the O element, blue one is the In element, and the green one is the Zn element. Line-scanning EDS of an individual heterostructure on a copper grid (d). The corresponding XRD pattern of the as-prepared nanostructures (e).
µm. In the inserted figure, the side length and thickness of the ZnO hexagonal disks are 3.3-5 µm and 174 nm, respectively, and the In2O3 quadrangular columns are in the range of 520-870 nm. Therefore, we can conclude that the In2O3-ZnO nanosized heterostructures constructed by In2O3 quadrangular columns with cubic phase and ZnO hexagonal disks with wurtzite structure are fabricated in our case. CL images of two individual In2O3-ZnO nanosized heterostructures are shown Figure 2c-f, and the corresponding SEM images are shown in Figure 2a,b. Meanwhile, the corresponding CL spectra of two samples are shown in Figure 3, in which two main peaks are at 379 and 499 nm, respectively, for the first sample and 379 and 496 nm, respectively, for the second sample. These peaks originate from the band edge emission and the deep level emissions affiliated with defects, respectively. In the first sample shown in Figure 2a, the side lengths of the top two hexagonal disks (No. 1 and No. 2) are about 1.94 and 3.04 µm, respectively. The panchromatic CL image of the first sample shows that In2O3 quadrangular columns are dark and ZnO hexagonal disks exhibit inhomogeneous luminescence emission in Figure 2c, and the luminescence emission from the boundary is stronger than that from the center region of the ZnO hexagonal disks. Further, the corresponding monochromatic CL image at a wavelength of 499 nm shows that the main luminescence emission seems to be from the hexagonal boundary in Figure 2e. In the second sample shown in Figure 2b, the side lengths of No. 3 and No. 4 hexagonal disks are about 3.30
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Figure 2. SEM images (a,b), the corresponding panchromatic CL images (c,d), and the corresponding monochromatic CL images (e,f) (wavelengths at 499 and 496 nm, respectively) of two samples.
and 4.10 µm, respectively. Interestingly, the panchromatic CL image of the second sample is similar to that of the first sample in Figure 2d. Moreover, the corresponding monochromatic CL image at a wavelength of 496 nm shows that the strong luminescence emission is from the hexagonal boundary. Importantly, this type of spatial distribution of the luminescence intensity is always attributed to the WGM in optical resonant cavities.13,14 Therefore, the intense luminescence located near the side of the ZnO hexagonal disks originates from the WGM enhanced emission of the optical resonator of the ZnO hexagonal disks. We exploit the following equation to check whether it is identified with the WGM-like enhanced luminescence14
nR )
λ 6 N + arctan(β√3n2 - 4) π 3√3
[
]
(1)
where R is the side length of the hexagonal resonant cavity, n is the refractive index, λ is the light wavelength, the integer N is the interference order of the resonance, and β represents the polarization that is equal to n-1 and used for the transverse magnetic (TM).14 If the total phase shift of the wave along the path is an integral number of 2π in the entire wave trains, the enforced constructive interference will generate the standing wave.14,15 Therefore, we choose the value 2.5 as the refractive index at 379 nm and 2.0 at 499 and 496 nm.14-16 According to eq 1, the resonant wavelength of 379 nm fulfills the constructive interference condition, and the integer N is 64 for the first ZnO hexagonal disk (No. 1) and 102 for the second one (No. 2) in Figure 2a. These results mean that the phase shifts are, respectively, 64 and 102 multiples of 2π when the wave transports one circle along the resonator cavity for the top two
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Figure 4. Schematic illustration of a simple growth model of In2O3-ZnO 1D nanosized heterostructures. (a) ZnO nucleation on the top of the In2O3 column, (b) the ZnO disk and In2O3 column continuous growth and ZnO nucleation on top of the In2O3 column again, and (c) 1D In2O3-ZnO nanoheterostructure formation.
Figure 3. CL spectra of the two samples in Figure 2. The CL spectrum in (a) is relevant to the sample in Figure 2a, and the CL spectrum in (b) corresponds to the sample in Figure 2b. The CL measurements of samples are carried out at room temperature using a Gatan Mono-CL system attached to a FESEM (the spot size is about 10 nm) with the accelerating voltage of 25 kV.
ZnO disks of the first sample. At the same time, the wavelength of 499 nm satisfies the constructive interference condition, and the integer N is 38 and 61 for No. 1 and No. 2, respectively. Moreover, for the top two ZnO hexagonal disks (No. 3 and No. 4) of the second sample in Figure 2b, the wavelengths coincide with the constructive interference condition when the total phase shift along its path is 111 (for No. 3) and 138 (for No. 4) multiples of 2π for the wavelength of 379 nm and 67 (for No.3) and 84 (for No. 4) multiples of 2π for the wavelength of 496 nm. Therefore, these results strongly support that the intense luminescence located near the side of the ZnO hexagonal disks originates from the WGM enhanced emission. Importantly, In2O3-ZnO nanosized heterostructures fabricated in our studies could be expected to be applicable to optoelectronic nanodevices. Note that Kim et al.14 reported whispering gallery modelike enhanced emission from an individual ZnO nanodisk. Then, in this study, we fabricated the unique 1D array of micro- and nanodisks of ZnO with WGM enhanced emission by In2O3-ZnO nanosized heterostructures. Therefore, this 1D array of nanodisks seems more advanced for application of ZnO
nanostructures in optoelectronic nanodevices than that of the previous similar reports above. Actually, from the viewpoint of nanodevice application, one In2O3-ZnO heterostructures in our case can fabricate many ZnO nanodisks on 1D nanostructures in series. Thus, one In2O3-ZnO heterostructure could be regarded as one functional unit of nanodevices, in which ZnO nanodisks with WGM enhanced emission are connected in series with each other. We propose a simple model for the fabrication of In2O3-ZnO nanosized heterostructures based on our experimental observations as shown in Figure 4. First, ZnO and In2O3 powders are reduced by the graphite powders in the high-temperature region in a horizontal quartz tube electric furnace. Then, Zn and In vapors are transported to the surface of the substrates in the relatively low-temperature region by the carried gas. Then, both ZnO and In2O3 nanostructures can form by the vapor-liquidsolid (VLS) process assisted by the Au catalyst. Our experiments clarify that the background nanostructures are short rods of ZnO and In2O3 and small disks of ZnO in Figure 1b. Maybe, the temperature is favorable for In2O3 nanowires grown in our case. Therefore, many In2O3 nanowires would grow up quickly from the background nanostructures. However, Zn could deposit and then nucleate on the top of In2O3 nanowires, as shown in Figure 4a. Meanwhile, In2O3 would continually deposit and nucleate on ZnO nuclei, as shown in Figure 4b. Finally, ZnO nuclei embedded in In2O3 nanowires develop into hexagonal disks, as shown in Figure 4c. In summary, In2O3-ZnO 1D nanosized heterostructures constructed by In2O3 quadrangular columns and ZnO hexagonal disks have been fabricated using a simple thermal evaporation, and the intense luminescence emission with the WGM from the optical resonator of ZnO hexagonal disks has been observed by CL spectroscopy. The formation of In2O3-ZnO heterostructures was pursued. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50525206 and U0734004). References and Notes (1) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 1247.
Letters (2) Vomiero, A.; Ferroni, M.; Comini, E.; Faglia, G.; Sberveglieri, G. Nano Lett. 2007, 7, 3553. (3) Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Lin, S. C.; Lin, Z. W.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2005, 127, 11777. (4) Wang, J. X.; Sun, X. W.; Xie, S. S.; Yang, Y.; Chen, H. Y.; Lo, J. Q.; Kwong, D. L. J. Phys. Chem. C 2007, 111, 7671. (5) Xu, L.; Su, Y.; Li, S.; Chen, Y. Q.; Zhou, Q. T.; Yin, S.; Feng, Y. J. Phys. Chem. B 2007, 111, 760. (6) Yang, Y.; Kim, D. S.; Knez, M.; Scholz, R.; Berger, A.; Pippel, E.; Hesse, D.; Gosele, U.; Zacharias, M. J. Phys. Chem. C 2008, 112, 4068. (7) Gupta, R. K.; Ghosh, K.; Patel, R.; Mishra, S. R.; Kahol, P. K. J. Cryst. Growth 2008, 310, 3019. (8) Chen, Y. W.; Liu, Y. C.; Lu, S. X.; Xu, C. S.; Shao, C. L. J. Chem. Phys. 2005, 123, 134701. (9) Hsin, C. L.; He, J. H.; Chen, L. Appl. Phys. Lett. 2006, 88, 063111.
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15483 (10) Wang, B.; Yang, Y. H.; Wang, C. X.; Yang, G. W. Chem. Phys. Lett. 2005, 407, 347. (11) Wang, B.; Yang, Y. H.; Wang, C. X.; Xu, N. S.; Yang, G. W. J. Appl. Phys. 2005, 98, 124303. (12) Wang, B.; Ouyang, G.; Yang, Y. H.; Yang, G. W. Appl. Phys. Lett. 2007, 90, 121905. (13) Wiersig, J. Phys. ReV. A 2003, 67, 023807. (14) Kim, C.; Kim, Y. J.; Jang, E. S.; Yi, G. C.; Kim, H. K. Appl. Phys. Lett. 2006, 88, 093104. (15) Nobis, T.; Kaidashev, E. M.; Rahm, A.; Lorenz, M.; Grundmann, M. Phys. ReV. Lett. 2004, 93, 103903. (16) Schmidt, R.; Rheinlander, B.; Schubert, M.; Spemann, D.; Butz, T.; Lenzner, J.; Kaidashev, E. M.; Lorenz, M.; Rahm, A.; Semmelhack, H. C.; Grundmannet, M. Appl. Phys. Lett. 2003, 82, 2260.
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