Elastic Property of Vertically Aligned Nanowires Jinhui Song, ² Xudong Wang, ² Elisa Riedo,* ,‡ and Zhong L. Wang* ,² School of Materials Science and Engineering and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 Received July 12, 2005; Revised Manuscript Received August 22, 2005 ABSTRACT An atomic force microscopy (AFM) based technique is demonstrated for measuring the elastic modulus of individual nanowires/nanotubes aligned on a solid substrate without destructing or manipulating the sample. By simultaneously acquiring the topography and lateral force image of the aligned nanowires in the AFM contacting mode, the elastic modulus of the individual nanowires in the image has been derived. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire where the AFM tip was scanning over the surface in contact mode. For the [0001] ZnO nanowires/nanorods grown on a sapphire surface with an average diameter of 45 nm, the elastic modulus is measured to be 29 ± 8 GPa. Characterizing the mechanical properties of nanowiress/ nanotubes/nanorods (NWs/NTs/NRs) is of great importance for their applications in electronics, optoelectronics, sensors, and actuators. There are several techniques that have been developed for measuring the elastic properties of individual NTs. The technique demonstrated by Lieber et al. 1 was based on quantifying the deflection of a carbon NT that was affixed at one end and the other end was free to be deflected by an atomic force microscope tip. The NT was laid in parallel to a solid substrate, and the elastic modulus of a carbon NT was calculated from the force-deflection curve. A technique by Wang et al. 2,3 relied on the electromechanical resonance of a NT/NW by in situ transmission electron microscopy (TEM). The resonance was stimulated by applying ac voltage across two electrodes, one of which was a carbon NT that was glued to a metal tip affixed on a specimen holder. The resonance frequency together with the geometrical parameters of the NT provided by TEM yielded the elastic modulus. The technique of Yu et al. 4 used two atomic force microscope tips to stretch a carbon NT that was glued at both ends to the two tips, respectively; the stretching force-displacement curve gave tensile strength and elastic modulus. A technique developed by Salvetat et al. 5,6 used an atomic force micro- scope tip to bend a NT or a bundle of single-walled NTs lying across a hole in a solid substrate. Quantifying the thermal vibration amplitude of a NT in TEM also yielded its elastic modulus. 7 Atomic force microscopy (AFM) has also been applied to study the transversal elasticity of NTs. 8 For all of these techniques, the NTs have to be removed from the substrate used in the growth and are manipulated for the measurements. Growth of aligned NTs/NWs is of great importance for many technological applications. Elastic properties of densely aligned carbon NTs have been measured by an “indentation” method, 9 in which a tip was pushed downward against the aligned NTs so that many NTs are in contact with the tip and its side surface. By measurment of the force-displace- ment curve, and the average number of NTs that were in contact with the tip and the contacting area of the NTs with the tip, an average elastic modulus of the NTs was received. This method requires that the density of the nanotubes is high and all of the nanotubes have the same size and length, and the measured result is a statistical average of all the NTs. In this paper, we demonstrate an alternative AFM based technique for measuring the elastic properties of individual aligned ZnO NWs in scanning area. By simultaneously recording the topography and lateral force image in AFM contact mode when the AFM tip scans across the aligned nanowire arrays, the elastic modulus of individual NWs is determined. This technique allows a measurement of the mechanical properties of individual NWs of different lengths in an aligned array without destructing or manipulating the sample. The aligned ZnO NW arrays were grown using gold as catalyst by a vapor-liquid-solid method (VLS) process, as reported previously. 10 By choice of an appropriate substrate, the epitaxial growth of ZnO on the single-crystal substrate, such as R-Al 2 O 3 , GaN, AlN, or Al 0.5 Ga 0.5 N, 11 yields aligned ZnO nanowires. Figure 1a shows a scanning electron microscopy (SEM) image of the as-grown ZnO nanowires on an R-Al 2 O 3 substrate, showing well-aligned distribution * Corresponding authors. E-mail: zhong.wang@mse.gatech.edu (Z.L.W.); elisa.riedo@physics.gatech.edu (E.R.). ² School of Materials Science and Engineering. ‡ School of Physics. NANO LETTERS 2005 Vol. 5, No. 10 1954-1958 10.1021/nl051334v CCC: $30.25 © 2005 American Chemical Society Published on Web 09/01/2005