Acoustic Oscillations and Elastic Moduli of Single Gold Nanorods Peter Zijlstra, † Anna L. Tchebotareva, ‡ James W. M. Chon,* ,† Min Gu, † and Michel Orrit* ,‡ Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne UniVersity of Technology, P.O. Box 218, Hawthorn, 3122, VIC Australia, and MoNOS Huygens Laboratorium, UniVersiteit Leiden, 2300 RA Leiden, The Netherlands Received August 13, 2008; Revised Manuscript Received September 2, 2008 ABSTRACT We present the first acoustic vibration measurements of single gold nanorods with well-characterized dimensions and crystal structure. The nanorods have an average size of 90 nm × 30 nm and display two vibration modes, the breathing mode and the extensional mode. Correlation between the dimensions obtained from electron microscope images and the vibrational frequencies of the same particle allows us to determine the elastic moduli for each individual nanorod. Contrary to previous reports on ensembles of gold nanorods, we find that the single particle elastic moduli agree well with bulk values. Metal nanowires and nanorods open attractive avenues in photonics 1 and quantum electronics. 2 Understanding their structure and elastic properties is crucial for many of their future uses. In contrast to a polycrystalline bulk metal, nanoparticles often are single crystals or are composed of very few single crystals. Defects are rare or absent because they easily anneal at nearby surfaces. As size decreases, the surface-to-volume ratio increases and surface effects (such as surface tension or melting) become significant. At small enough scales, the mechanical properties are expected to differ from those of bulk materials. Electronic confinement and atomic discreteness may also affect elastic moduli. The elasticity of gold and silver nanoparticles has been studied on ensembles through laser-induced acoustic oscil- lations. 3-5 It appears that vibrational modes depend in unique ways on the size, shape, 3 crystal structure, 5,6 and local environment 7,8 of a nanoparticle. In ensemble measurements, however, differences from bulk elastic properties are likely to be screened by the differences between the individual particles. Recent ultrafast spectroscopic measurements on single gold nanospheres 7 and single silver nanocubes 8 demonstrated how to remove this heterogeneity. They revealed differences in the local environment, but they did not directly yield the mechanical properties. This can only be achieved when the precise morphology and dimensions of each individual particle are known, which requires electron microscopy and ultrafast spectroscopy of the same single nanoparticle. 9 Here, we measure the acoustic responses and electron micrographs of the same single gold nanorods, from which we derive their elastic constants. Contrary to previous reports 5,10 on ensembles of gold nanorods, we find the elastic moduli of single nanorods to agree well with bulk values, confirming theoretical estimates. 11,12 The gold nanorods used in this study were prepared by silver-assisted seed-mediated growth. 13 This yielded nano- particles with an average length of 92 ( 9 nm and an average width of 33 ( 3 nm. After preparation, the remaining solutes were diluted by 4 orders of magnitude through centrifugation. This dilution ensured a minimum of surfactant remaining on the surface of the rod without inducing any noticeable aggregation. After this, the nanorod solution was spin-coated on a clean fused silica glass coverslip. The coverslip was marked by femtosecond laser writing to help locate the same nanorods for spectroscopy and electron microscopy. The sample surface was imaged by both scanning electron microscopy (SEM) and single-particle white-light scatter- ing. 9,14,15 Measuring the plasmon energy and line width facilitates the identification of isolated rods. For single- particle white-light microscopy, 14 the reflected light from a high-power Xenon lamp was collected by the focusing objective and directed to an avalanche photodiode (APD). The spot was raster-scanned over the sample surface. To increase the visibility of the rods, the reflected light was bandpass-filtered (638 ( 45 nm) before detection. Some nanorods appear dimmer because part of their scattering spectrum falls outside the filter band. The unfiltered scattering * Corresponding authors. E-mail: orrit@molphys.leidenuniv.nl. E-mail: jchon@groupwise.swin.edu.au. † Swinburne University of Technology. ‡ Universiteit Leiden. NANO LETTERS 2008 Vol. 8, No. 10 3493-3497 10.1021/nl802480q CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008