CRYSTAL GROWTH & DESIGN
Visible Photoluminescence of Ultrathin ZnO Nanowire at Room Temperature Lin,*,†
Yang,†,‡
Yan-Ru Shang-Shian Song-Yeu Shinn-Tyan Wu,‡ and I-Cherng Chen§
Tsai,†
Hao-Cheng
2006 VOL. 6, NO. 8 1951-1955
Hsu,‡
PhotoVoltaics Technology Center, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan, Republic of China, Department of Materials Science & Engineering, National Tsing-Hua UniVersity, Hsinchu 30043, Taiwan, Republic of China, and Material and Chemical Research Laboratories, Industrials Technology Research Institute, Chutung, Hsinchu 31040, Taiwan, Republic of China ReceiVed August 13, 2005; ReVised Manuscript ReceiVed May 23, 2006
ABSTRACT: Zinc oxide (ZnO) nanowire (NW) was grown on a ZnO-buffered silicon substrate by a hydrothermal method in an aqueous solution that contained methenamine (C6H12N4) and zinc nitrate hexahydrate (Zn(NO3)2‚6H2O). The concentration of the zinc nitrate is the main factor that governed the morphology of the NW. As the concentration was reduced gradually, the diameter of the NW became smaller, and an ultrathin (e10 nm) single crystal of ZnO NW emerged. The sample of ultrathin NW contains many dislocations and exhibits an unusual characteristic photoluminescence (PL) spectrum in the visible light region. One-dimensional (1D) nanomaterials have attracted considerable interest because of their potential applications.1,2 Among 1D oxide nanosystems, zinc oxide (ZnO) is one of the most promising materials for fabricating optoelectronic devices that operate in the ultraviolet (UV) region, owing to its large exciton binding energy of 60 meV and wide band gap energy of 3.37 eV at room temperature. 3 ZnO nanowires (NWs) can be synthesized by numerous methods, such as chemical vapor deposition (CVD),4 vapor-liquid-solid (VLS) mechanism,5-7 vapor phase transport deposition,8,9 and aqueous-solution growth methods.10-13 Although ZnO is a wide band gap material, its nanostructure has been shown to emit broad yellow-orange photoluminesence (PL) at low temperature.12,13 However, in the authors’ experience, the ZnO nanostructure does not have a characteristic PL spectrum in the visible light region at room temperature. This investigation reports the synthesis of ultrathin (e10 nm) ZnO NW by the aqueous-solution growth method and their deformation by the formation of defects. PL measurement shows strong characteristic peaks of ultrathin ZnO NW in the visible light region at room temperature. Prior to the deposition of ZnO nanowires, a ZnO film of thickness 125 nm ((6 nm) was deposited directly on Si(111) substrates using RF sputtering. ZnO NW was grown on the buffered silicon substrate by a hydrothermal method in an aqueous solution of methenamine (C6H12N4) and zinc nitrate hexahydrate (Zn(NO3)2‚6H2O) at 85 °C. The growth method was similar to the hydrothermal process that was developed by Vayssieres et al.10,11 Table 1 gives the growth conditions of the samples. The grown samples were dipped in deionized water and dried in an oven at 120 °C for 0.5 h. The crystallography and structure of the as-grown samples were identified by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-4000EX). The surface mor* To whom all correspondence should be addressed. E-mail: yanrulin@ itri.org.tw. † Photovoltaics Technology Center, Industrials Technology Research Institute. ‡ National Tsing-Hua University. § Material and Chemical Research Laboratories, Industrial Technology Research Institute.
Table 1. Growth Conditions of the NWsa sample
[Zn(NO3)2‚6H2O] (M)
temp (°C)
time (h)
A B C D
1 × 10 -2 2 × 10 -3 5 × 10 -4 5 × 10 -4
85 85 85 85
5 5 5 14
a
The concentration of C6H12N4 was 1 × 10-2 M in all cases.
phologies of the samples and the size distribution of the nanowires were characterized using a JEOL JSM-6500F microscope and a LEO-1530 field emission scanning electron microscope (FESEM), operated at 15 and 5 keV, respectively. The PL properties of the samples were also characterized using a Jobin Yvon-Spex Fluorolog-3 spectrophotometer. A Xe lamp, emitting at 254 nm, was the excitation source when the PL measurements were made. Figure 1 shows the X-ray diffraction (XRD) θ-2θ scan data and TEM image of the buffer film. Figure 1 indicates that the buffer film had a columnar structure and was preferably oriented in the ZnO [0002] direction. Parts a-d of Figure 2 present the FESEM images of samples A-D in Table 1. Figure 2 clearly shows that the concentration of the zinc nitrate was the main factor that governed the diameter of the NWs, and the results are similar to those of Vayssieres.10,11 Diameters of the NWs of sample A are approximately 50-140 nm, and the NWs have straight morphology. As the diameter is reduced, the NWs become too thin to resist the deformation, and they become tangled with each other. The entanglement of the thin NWs may be caused by capillarity, i.e., by the surface tension of the water, during the drying in the oven. As the concentration of Zn(NO3)2‚6H2O is reduced to 5 × 10-4 M, it is difficult to form the NW at a finite time. However, ultrathin (e10 nm) ZnO NW emerged after extending the reaction time. Parts a-c of Figure 3 plot the room-temperature PL spectra of samples A, B, and D in Table 1, respectively. The PL spectrum of sample A exhibits a narrow peak at around 390 nm and a broad peak located between 450 and 700 nm. The narrow and broad PL peaks of sample A are familiar in the PL spectrum of the ZnO nanostructure or the ZnO single crystalline
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Figure 1. XRD θ-2θ scan data (a) and cross-sectional TEM image (b) of the buffer film.
Figure 2. Plane view FESEM images of the samples given in Table 1: (a) sample A; (b) sample B; (c) sample C (d) sample d.
film and result from free exciton emission and deep level emission, respectively.12,14 The PL spectrum of sample B has two new unusual peaks at 594 nm (∼2.09 eV) and 620 nm (∼2 eV). As the diameter of the NWs decreases to about 10 nm (sample D), the relative intensities of these two unusual peaks (594 and 620 nm) increase sharply without any shift, and two more peaks appear at 696.4 nm (∼1.78 eV) and 704 nm (∼1.76 eV). Recently, Lin et al. described several possible levels of defects in ZnO crystals,15 but the four characteristic PL peaks in the visible light region of the spectrum of the sample of ultrathin NW herein do not match their results. Figure 4a shows the cross-sectional TEM image of sample B. Comparing Figure 4a with Figure 2b shows that the
entanglement of the NWs is released because the NWs are filled with glue during the making of the TEM sample. Figure 4c shows an HRTEM image of the ZnO NW of the area denoted with a black rectangle in Figure 4b, and Figure 4d presents the Fourier transform (FFT) pattern of the area denoted with a black square in Figure 4c. Figure 4 clearly reveals that the NW is a single crystal with the wurtzite structure and grows in the [0002] direction. Figure 5 shows the cross-sectional HRTEM images of sample C. Figure 5b displays a highly magnified image of the area denoted with white rectangle in Figure 5a. Figure 5 clearly reveals that the ZnO grows epitaxially from the columnar grains of the buffer layer in the dilute zinc nitrate solution to form a
Visible Photoluminescence of Ultrathin ZnO Nanowire
Figure 3. Room-temperature PL spectra of the samples given in Table 1: (a) sample A; (b) sample B; (c) sample D.
continuous film. In fact, ZnO forms a continuous film at the beginning of the growth in the aqueous solution, no matter what the concentration of zinc nitrate is. Afterward, the NWs grow homoepitaxially along the columns of the buffer and the grains of the continuous film. Figure 6a shows the TEM image of ultrathin NW. Figure 6b displays an HRTEM image of the area covered by the black rectangle in Figure 6a. Fourier filtering was applied to the area outlined by a black dashed line in Figure 6b, as shown in Figure 6c, revealing only the ZnO (101h0) plane. Pairs of edge
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dislocations in the ultrathin NWs are easily identified and marked in Figure 6c. Each pair of dislocations might be formed by the breaking of the ZnO (101h0) plane. Wang et al. very recently reported several kinds of intrinsic defects and PL properties of ZnO nanobelts of 6 nm width. They reported the blue shift of the free-exciton PL peak of the nanobelts caused by the ultrasmall nanostructure.16 They also proved that nanobelts with single-crystal structures can vary across the intrinsic defects. However, they did not find pairs of dislocations, nor did they observe any characteristic PL peak in the visible light region. Figure 6d shows the low-magnification TEM image of the ultrathin NWs. The entanglement of the ultrathin NWs is serious and cannot be released even by filling with glue. It is impossible to vary single crystal NW just on the basis of the intrinsic defects in such serious entanglement. The ultrathin NWs must include a large number of broken crystal planes that may be sufficient to create a new defect level. In summary, ultrathin ZnO NWs with an average diameter of approximately 10 nm can be synthesized by the aqueoussolution growth method. The strength of such small ZnO NWs is very low, and the surface tension of the water can deform them. The single-crystal ultrathin NWs can be varied on the basis of the intrinsic defects and broken crystal planes. Four unusual PL peaks from ultrathin NWs in the visible light region were observed at room temperature. However, the emission mechanism is still unclear. Although many studies have focused on the “blue shift” caused by the ultrasmall nanostructure, the
Figure 4. Cross-sectional TEM images of sample B. Part d presents the Fourier transform (FFT) pattern of the area denoted by the black square in part c.
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Figure 5. Cross-sectional HRTEM images of sample C.
Figure 6. Cross-sectional TEM images of ultrathin NW. Part b is the HRTEM image of the area denoted by the black rectangle in part a. Fourier filtering was applied to the area outlined by a black dashed line in part b, as shown in part c, revealing only the ZnO (101h0) plane.
Visible Photoluminescence of Ultrathin ZnO Nanowire
small size can induce defects easily and defects may be the principal cause of changing the material properties. Supporting Information Available: Figures giving cross-sectional HRTEM images of sample B. This material is available free of charge via the Internet at http://pubs.acs.org.
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Crystal Growth & Design, Vol. 6, No. 8, 2006 1955 (8) Hsu, C. L.; Yang, S. S.; Tseng, Y. K.; Chen, I. C.; Lin, Y. R.; Chang, S. J.; Wu, S. T. J. Phys. Chem. B 2004, 108, 18799. (9) Lin, Y. R.; Tseng, Y. K.; Yang, S. S.; Wu, S. T.; Hsu, C. L.; Chang, S. J. Cryst. Growth Des. 2005, 5, 579. (10) Vayssieres, L.; Keis, K.; Lindquist, S. E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3350. (11) Vayssieres, L. AdV. Mater. 2003, 15, 464. (12) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031. (13) Qian, H. S.; Yu. S. H.; Gong, J. Y.; Luo, L. B.; Wen, L. L. Cryst. Growth Des. 2005, 5, 935. (14) Chen, Y.; Bagnall, D. M.; Koh, H. J.; Park, K. T.; Hiraga, K.; Zhu, Z.; Yao, T. J. Appl. Phys. 1998, 84, 3912. (15) Lin, B.; Fu, Z.; Jia, Y.; Liao, G. J. Electrochem. Soc. 2001, 148, G110. (16) Wang, X.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 8773.
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