Buckling characterization of vertical ZnO nanowires using nanoindentation
Liang-Wen Jia兲
Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan, Republic of China
Sheng-Joue Young
Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
Te-Hua Fang and Chien-Hung Liu
Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan, Republic of China
共Received 3 August 2006; accepted 11 December 2006; published online 17 January 2007兲 Nanomechanical characterization of vertical well-aligned single-crystal ZnO nanowires on ZnO:Ga/ glass templates was performed by nanoindentation technique. The buckling loads were found to be 1465 and 215N for the ZnO nanowires of 100 and 30 nm diameters, respectively. Furthermore, the buckling energies for the ZnO nanowires of 100 and 30 nm diameters were 3.62⫻10−10and 3.69⫻10−11J, respectively. Based on the Euler buckling model, Young’s modulus of the individual ZnO nanowire has been derived from two possible modes in this work. © 2007 American Institute
of Physics. 关DOI:10.1063/1.2431785兴
One-dimensional 共1D兲 materials such as nanowires 共NWs兲, nanobelts, and nanorods have attracted considerable interest in recent years.1–3They present the utmost challenge to semiconductor technology, making possible fascinating novel devices. These 1D materials have been demonstrated to exhibit superior electrical, optical, mechanical, and ther-mal properties, and can be used as nanoscale interconnects, active components of optical electronic devices, and nano-electromechanical systems. However, it is important to un-derstand the mechanical characteristics of these nanowires prior to any feasible applications. For example, mechanical properties of carbon nanotubes have been extensively studied by tensile loading, bending, and buckling.2,3
1D oxide systems such as SnO2, SiO2, GeO2, indium tin oxide, Al2O3, and ZnO nanowires have also attracted much attention in recent years.4–12Among them, ZnO is a n type direct-gap semiconductor with a large exciton binding energy of 60 meV and wide band gap energy of 3.37 eV at room temperature. Hence, ZnO is regarded as a promising photo-nic material.13However, only few reports on the mechanical properties of ZnO nanowires can be found in the literature.4–8In this work, the buckling instabilities in verti-cal ZnO NWs have been characterized by nanoindentation tests. Based on Euler buckling model, we also estimated Young’s modulus共elastic modulus兲 of individual NW.
The ZnO NWs used in this study were grown on ZnO:Ga/glass templates, the synthesis of NWs was per-formed by a modified self-catalyzed vapor-liquid-solid method without any metal catalyst.12Detailed growth proce-dures can be found elsewhere.9,10 The photoluminescence 共PL兲 and x-ray diffraction 共XRD兲 were then used to charac-terize the optical and crystallographic properties of the as-grown ZnO NWs. Surface morphologies of samples and size distribution of the NWs were characterized by a LEO 1530
field-emission scanning electron microscope共FESEM兲, oper-ated at 5 keV. The investigation on the buckling behavior of the NWs was performed by means of a Hysitron nanoinden-tation system. Uniaxial compression on the exposed NWs was accomplished with a diamond indenter of 2m diameter.
The FESEM images of the as-grown ZnO NWs in samples A and B were shown in Fig.1共a兲and1共b兲. We found the typical diameter, length, and density of the ZnO NWs in sample A were approximately 100 nm, 2000 nm, and 8.2 ⫻109cm−2while the ones of sample B were estimated to be 30 nm, 800 nm, and 1.2⫻1010cm−2, respectively. Note that we can tune the oxygen stream during NW growth to obtain the different size of ZnO NWs. It was found that these ZnO NWs were distributed uniformly across the entire substrate and the tops of these NWs were hexagonal with the c axis perpendicular to the substrate surface.12 As shown in Fig. 1共a兲, the FESEM image reveals that some wires are stuck together for the 100-nm-diameter wires. For the 30-nm-diameter wires, the same phenomenon is observed, and the wires are also bending and not perfectly straight, as shown in Fig. 1共b兲. The stuck wires will cause smaller dis-placement than the single vertical wires under the same
load-a兲Author to whom correspondence should be addressed; electronic mail: [email protected] and [email protected]
FIG. 1. FESEM images of共a兲 sample A with the 100-nm-diameter ZnO NWs, and共b兲 sample B with the 30-nm-diameter ones.
APPLIED PHYSICS LETTERS 90, 033109共2007兲
0003-6951/2007/90共3兲/033109/3/$23.00 90, 033109-1 © 2007 American Institute of Physics
column 共K=0.5兲 or fixed-pinned column 共K=0.7兲 mode. These results correspond with the work of molecular dynam-ics simulations performed by Kulkarni et al.,6as well as the experimental revelation of Chen et al.7This behavior can be attributed to high compressive internal stress levels resulting from the surface stress and high surface-to-volume ratios at the nanoscale.6,7 In other words, because our experimental NWs are high quality single crystals with few defects, it is expected that such a phenomenon of size dependence may originate from surface modification of NWs, as the surface effect becomes significant due to the large surface-to-volume ratio.
In summary, we report the experimental observations of buckling instabilities of vertical well-aligned single-crystal ZnO nanowires prepared on ZnO:Ga/glass templates. The critical buckling loads of the ZnO NWs are found to be 1465 and 215N for samples A共100 nm diameter兲 and B 共30 nm diameter兲, respectively. Furthermore, the buckling energy was 3.62⫻10−10 and 3.69⫻10−11J for samples A and B, respectively. Euler buckling model can be employed in evaluating Young’s modulus 共E兲 and the critical buckling strain共cr兲 of individual ZnO NW.
This work was supported by National Science Council of Taiwan under Contract No. NSC-95-2221-E-150-077-MY3.
The authors would like to thank the Advanced Optoelec-tronic Technology Center, National Cheng Kung University, Taiwan for the support through equipment and cooperation.
1L. W. Ji, T. H. Fang, S. C. Hung, Y. K. Su, S. J. Chang, and R. W. Chuang, J. Vac. Sci. Technol. B 23, 2496共2005兲.
2P. Poncharal, Z. L. Wang, D. Ugarte, and W. A. de Heer, Science 283, 1513共1999兲.
3M. F. Yu, O. Lourie, M. J. Dyer, K. Moloni, T. F. Kelly, and R. S. Ruoff, Science 287, 637共2000兲.
4H. Saitoh, Y. Namioka, H. Sugata, and S. Ohshio, Jpn. J. Appl. Phys., Part 1 40, 6024共2001兲.
5S. X. Mao, M. H. Zhao, and Z. L. Wang, Appl. Phys. Lett. 83, 993共2003兲. 6A. J. Kulkarni, M. Zhou, and F. J. Ke, Nanotechnology 16, 2749共2005兲. 7C. Q. Chen, Y. Shi, Y. S. Zhang, J. Zhu, and Y. J. Yan, Phys. Rev. Lett. 96,
075505共2006兲.
8Z. L. Wang and J. H. Song, Science 312, 242共2006兲.
9V. Valcarcel, A. Souto, and F. Guitian, Adv. Mater.共Weinheim, Ger.兲 10, 138共1998兲.
10Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947共2001兲. 11Z. R. Dai, Z. W. Pan, and Z. L. Wang, Adv. Funct. Mater. 13, 9共2003兲. 12C. L. Hsu, S. J. Chang, H. C. Hung, Y. R. Lin, C. J. Huang, Y. K. Tseng,
and I. C. Chen, IEEE Trans. Nanotechnol. 4, 649共2005兲.
13Z. X. Mei, X. L. Du, Y. Wang, M. J. Ying, Z. Q. Zeng, H. Zheng, J. F. Jia, Q. K. Xue, and Z. Zhang, Appl. Phys. Lett. 86, 112111共2005兲. 14S. P. Timoshenko and J. M. Gere, Theory of Elastic Stability
共McGraw-Hill, New York, 1961兲, p. 46.
15S. O. Kucheyev, J. E. Bradby, J. S. Williams, C. Jagadish, and M. V. Swain, Appl. Phys. Lett. 80, 956共2002兲.
033109-3 Ji et al. Appl. Phys. Lett. 90, 033109共2007兲
