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ZnO nanotubes, these variables must all be optimized with respect to the .... (2009). 12D. Wang, H. W. Seo, C. C. Tin, M. J. Bozack, J. R. Williams, M. Park, and ... M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl.
APPLIED PHYSICS LETTERS 95, 073114 共2009兲

Trimming of aqueous chemically grown ZnO nanorods into ZnO nanotubes and their comparative optical properties M. Q. Israr,1,a兲 J. R. Sadaf,1 L. L. Yang,1 O. Nur,1 M. Willander,1 J. Palisaitis,2 and P. O. Å. Persson2 1

Department of Science and Technology (ITN), Linköping University, SE-60174 Norrköping, Sweden Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-58183, Linköping, Sweden

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共Received 11 June 2009; accepted 29 July 2009; published online 21 August 2009兲 Highly oriented ZnO nanotubes were fabricated on a silicon substrate by aqueous chemical growth at low temperature 共⬍100 ° C兲 by trimming of ZnO nanorods. The yield of nanotubes in the sample was 100%. Photoluminescence spectroscopy of the nanotubes reveals an enhanced and broadened ultraviolet 共UV兲 emission peak, compared with the initial nanorods. This effect is attributed to whispering gallery mode resonance. In addition, a redshift of the UV emission peak is also observed. Enhancement in the deep defect band emission in the nanotubes compared to nanorods was also manifested as a result of the increased surface area. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3211124兴 ZnO is a promising material due to the wide band gap 共3.37 eV兲 at room temperature and large exciton binding energy 共60 meV兲. In addition, ZnO possesses a large number of deep level defects 共intrinsic and extrinsic兲 that emit light covering the whole visible range.1 For a large number of applications, shape and size of the material plays a key role for the performance of the device. One-dimensional nanostructures of ZnO, for instance, nanoscale rods, wires, needles, rings, tubes, whiskers2–6 etc., are currently subject to intense scientific investigations due to their excellent piezoelectric, chemical, physical, and optical properties. Among these structures, ZnO tubular structures have attracted much interest due to the porous structure and large surface area, and a variety of methods are used to grow these.7–10 In the present study, the fabrication of ZnO nanotubes 共ZNTs兲 from ZnO nanorods, with corresponding properties, is reported. This method is advantageous by being a cost efficient, simple with high yield, low temperature deposition process and also proves to be less hazardous compared to other methods. The comparative optical properties yield an increased optical response in the ultraviolet 共UV兲 and visible emission bands for the ZNTs as compared to ZnO nanorods. The fabrication of vertically well-aligned hexagonal ZnO nanorods consists of deposition of a seed layer on the silicon 共100兲 substrate and annealed at 250 ° C for 30 min. ZnO nanorods were grown by placing the precoated Si substrate in the solution containing zinc nitrate hexahydrate 关Zn共NO3兲2 · 6H2O, 99.9% purity兴 and methamine 共C6H12N4, 99.9% purity兲 with equimolar concentration of 0.1M. The beaker was placed in a preheated oven at 93 ° C for 3 h.11 The fabrication of ZnO nanotubes was carried out by suspending the sample with the ZnO nanorods upside down in 100 ml aqueous solution of potassium chloride 共KCl兲. The experiment was repeated multiple times while varying temperature, concentration of precursor 共KCl兲 as well as etching time. For complete etching and fine structure control of the ZnO nanotubes, these variables must all be optimized with respect to the dimensions of the original ZnO nanorods. In a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: ⫹46762319966.

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the present experiments, the best results were obtained from immersion of the ZnO nanorods for 10 h in a 5M concentration of KCl solution at 95 ° C. The etching mechanism is closely related to the initial crystal structure of ZnO nanorods and can be explicated by the difference in stability between the polar and nonpolar planes of ZnO nanorods. The distinctive ZnO crystal habit ¯ 兲 plane and shows two polar planes: the basal 共0001兲 / 共0001 ¯ six low-indexed nonpolar planes, 共1010兲, parallel to the c-axis. The nonpolar planes are the most stable planes by exhibiting lower surface energy than polar planes which are metastable. This is the reason why etching rate is faster along polar plane than nonpolar planes.9 In the present experiment, the structural instability of the polar planes enables etching of the nanorod core. This is accomplished by the adsorption of chloride ions 共Cl−兲 onto the 共0001兲 ZnO plane and hence the adsorption of Cl− progressively dissolves the entire nanorod core from top to bottom by the formation of an extremely water soluble complex.10 Scanning electron microscopy 共SEM兲 images were recorded to examine the morphology, dimensions, and density of the ZnO nanotubes. Figure 1 shows the SEM images of the ZnO nanorods prior to 共a兲 and after etching 共b兲. ZnO nanotubes with 125–275 nm diameter, 1 – 2 ␮m length, and 20–35 nm wall thickness were obtained with an apparent

FIG. 1. 共a兲 SEM image of the as grown ZnO nanorods 共reference sample兲 and 共b兲 SEM image of ZNTs for the immersion time of 10 h in 5M concentration of KCl aqueous solution at 95 ° C. The top right corner insets show high magnification SEM images of single 1D ZnO nanostructures.

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© 2009 American Institute of Physics

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Israr et al.

Appl. Phys. Lett. 95, 073114 共2009兲

FIG. 3. XRD pattern of 共a兲 ZnO nanorods showing a sharp diffraction 共0002兲 peak which indicates growth along the c-axis direction and 共b兲 the 共0002兲 peak observed from ZnO nanotubes.

FIG. 2. 共a兲 TEM image of single ZnO nanotube with variation in diameter and walls of nanotube from top to bottom and the associated electron diffraction pattern. 共b兲 Bottom image of ZnO nanotube. 共c兲 Corresponding lattice resolved image.

alignment perpendicular to the substrate. It was also observed that all the ZnO nanorods are trimmed into ZNTs. The hollow structure of ZnO nanotubes was further investigated by transmission electron microscopy 共TEM兲. The sample preparation for TEM characterization was carried out by scratching the ZnO nanotubes from the Si substrate followed by ultrasonication in ethanol to disperse the nanotubes. A drop of the ZnO nanotube containing liquid was deposited on a standard holey carbon grid. Figure 2共a兲 shows the TEM image of the hollow structure from a single ZnO nanotube. The diameter is slowly increasing toward the root of the nanotube along with wall thickness. A nanotube is also viewed along the tube axis in Fig. 2共b兲. High resolution TEM of the tube, as shown in Fig. 2共c兲, reveals the ZnO wurtzite hexagonal structure with the 共0002兲 atomic planes oriented perpendicular to the growth direction of the nanotube. The electron diffraction pattern in Fig. 2共a兲 indicates that ZnO nanotubes are single crystals which can also be deduced from the smoothly varying diffraction contrast in images of inset of Figs. 2共a兲 and 2共c兲. The orientation and crystal structure of ZnO nanotubes and ZnO nanorods grown on silicon wafer were carefully analyzed by x-ray diffraction 共XRD兲 using Cu K␣ x rays. Acquisition parameters were kept the same while measuring both the ZNTs and the ZnO nanorods. Figures 3共a兲 and 3共b兲 show the corresponding XRD pattern for ZnO nanorods and ZnO nanotubes with the 共0002兲 peaks at 2␪ position of 34.43°. The reduced intensity of the ZNT peak is associated with the reduction in volume after etching. A silicon peak is also observed at 2␪ position of 33.17°. No other characteristic peaks were obtained in the XRD pattern which indicates the absence of impurities and high crystalline quality of ZNTs. Figure 4 shows the comparison of emission characteristics of ZnO nanorods 共a兲 and ZnO nanotubes 共b兲 from photoluminescence 共PL兲 measurements spectra at room tempera-

ture. All measurements were performed under the same conditions with excitation wavelength of 266 nm. It is well known that the ZnO material reveals two emission bands: UV and the visible band. Ultraviolet emission is generally considered to be related to the radiative recombination of free excitons, and the visible emission peaks are attributed to deep level defects. Figure 4共b兲 shows a significant change in the UV emission peak. For ZnO tubular structure, the walls of nanotubes are suggested to behave like resonant cavities. The circular optical mode endorsed by total internal reflections from the walls of ZNTs, may cause strong intensification in the spontaneous emission supported by whispering gallery mode resonances.12–14 A redshift is also observed of the UV emission peak which is the result of different resonant reflected wavelengths and/or the heat due to total internal reflection of the light within the ZNTs.15 The wavelength of these emission peaks were found to be strongly related to the diameter of the ZNTs, therefore, the resonant reflected wavelength from each nanotube will have slightly different wavelength. Since the distribution of diameters, and hence wavelengths, is rather narrow, the reflected wavelengths overlap to form a single broad Gaussian peak.16 The defect emission peaks observed at 542 and 692 nm from the nano-

FIG. 4. 共Color online兲 Room temperature photoluminescence spectra of 共a兲 ZnO nanorods and 共b兲 ZnO nanotubes.

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rods are of relatively low intensity, but have increased significantly for the ZNTs. The increased intensity reveals more intrinsic 共Vzn, Vo, or Zni兲 and extrinsic 共acceptors兲 defects.17,18,1 in the nanotubes as compared to the nanorods. Despite etching of the nanorod core and associated reduction in volume, these defects are predominantly associated with the surface region in ZnO.19 In summary, we demonstrated that the fabrication of ZnO nanotube arrays with 100% yield by etching of ZnO nanorods grown by aqueous chemical growth can effectively be achieved. It was found that the optical emission from ZNTs was enhanced in UV as well as in visible band and that a redshift of the UV emission is also observed. The enhancement in UV emission was attributed to the presence of whispering gallery modes optical resonances within the walls of ZNTs. In comparison, the visible light emission enhancement is suggested to occur due to the increase in surface area in ZNTs. The improved emission properties of nanotubes compared to nanorods emphasize their potential for UV and visible light-emitting diode applications. C. Klingshirn, Phys. Status Solidi B 244, 3027 共2007兲. H. Zhou, J. Fallert, J. Sartor, R. J. B. Dietz, C. Klingshirn, H. Kalt, D. Weissenberger, D. Gerthsen, H. Zeng, and W. Cai, Appl. Phys. Lett. 92, 132112 共2008兲. 3 J. Zúñiga-Pérez, A. Rahm, C. Czekalla, J. Lenzner, M. Lorenz, and M. 1 2

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Israr et al.

Grundmann, Nanotechnology 18, 195303 共2007兲. D. Park, Y. Tak, J. Kim, and K. Yong, Surf. Rev. Lett. 14, 1061 共2007兲. 5 Z. L. Wang, ACS Nano. 2, 1987 共2008兲. 6 J. Liu and X. Huang, J. Solid State Chem. 179, 843 共2006兲. 7 G. Zhang, M. Adachi, S. Ganjil, A. Nakamura, J. Temmyo, and Y. Matsui, Jpn. J. Appl. Phys., Part 1 46, L730 共2007兲. 8 X. Kong, X. Sun, X. Li, and Y. Li, Mater. Chem. Phys. 82, 997 共2003兲. 9 L. Vayssieres, K. Keis, A. Hagfeldt, and S. E. Lindquist, Chem. Mater. 13, 4395 共2001兲. 10 J. Elias, R. T. Zaera, G. Y. Wang, and C. L. Clement, Chem. Mater. 20, 6633 共2008兲. 11 L. L. Yang, Q. X. Zhao, and M. Willander, J. Alloys Compd. 469, 623 共2009兲. 12 D. Wang, H. W. Seo, C. C. Tin, M. J. Bozack, J. R. Williams, M. Park, and Y. Tzeng, J. Appl. Phys. 99, 093112 共2006兲. 13 M. Tomita, K. Totsuka, H. Ikari, K. Ohara, H. Mimura, H. Watanabe, H. Kume, and T. Matsumoto, Appl. Phys. Lett. 89, 061126 共2006兲. 14 Y. Zhang, W. Zhang, and C. Peng, Opt. Express 16, 10696 共2008兲. 15 D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl. Phys. Lett. 70, 2230 共1997兲. 16 C. Kim, Y. J. Kim, E. S. Jang, G. C. Yi, and H. H. Kim, Appl. Phys. Lett. 88, 093104 共2006兲. 17 Q. X. Zhao, P. Klason, M. Willander, H. M. Zhong, W. Lu, and J. H. Yang, Appl. Phys. Lett. 87, 211912 共2005兲. 18 K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, Appl. Phys. Lett. 68, 403 共1996兲. 19 A. B. Djurišić, W. C. H. Choy, V. A. L. Roy, Y. H. Leung, C. Y. Kwong, K. W. Cheah, T. K. Gundu Rao, W. K. Chan, H. Fei Lui, and C. Surya, Adv. Funct. Mater. 14, 856 共2004兲. 4

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