The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

The response of gold nanorods to both thermal and ultrafast laser-induced heating has been examined. The thermal heating experiments show structural changes that occur on timescales ranging from hours to days. At the highest temperature examined (250 degrees C) the nanorods are transformed into spheres within an hour. On the other hand, no structural changes are observed in the laser-induced heating experiments up to temperatures of 700 +/- 50 degrees C. This is attributed to thermal diffusion in the laser experiments. Measurements of the period of the extensional mode of the nanorods using time-resolved spectroscopy show a significant softening at high pump laser powers. However, the decrease in the period is less than expected from bulk Young's modulus vs. temperature data.

Time-resolved spectroscopy has been used to examine the elastic properties of single crystal gold nanorods with a [100] growth direction. These rods were produce by seed-mediated growth in the presence of silver ions, using both chemical and photochemical reduction. Analysis of the experimental data yields a value of Young's modulus for the nanorods of E NR = 31 ¡ 1 GPa. This is approximately 26% smaller than the value for bulk gold of E [100] = 42 GPa. The reduction in the size of Young's modulus is consistent with our previous studies of penta-twinned, [110] growth direction nanorods, where we found E NR = 64 ¡ 2 GPa compared to E [110] = 81 GPa for bulk gold. The fact that both the single crystal and the penta-twinned nanorods show a similar reduction in E compared to the bulk values (20%-30%) shows that this effect does not arise from the presence of twin planes in the nanorods. Our data show a weak correlation between the measured values of E NR for the [100] nanorods and the surface-to-volume ratio of the rods. The larger value of Young's modulus at small size is possibly due to defect elimination. These results underscore the importance of growth direction in determining the elastic properties of nanorods and nanowires.

The response of single crystal, cubic silver particles to ultrafast laser-induced heating has been examined experimentally and theoretically. The transient absorption traces display clear modulations due to coherently excited vibrational modes. Nanocube samples with edge lengths smaller than 50 nm show a single modulation, whereas samples larger than 50 nm show two vibrational modes. The results are compared to finite element calculations, where the cubes are modeled as having cubic crystal symmetry with the principal axes parallel to the sides of the particle. The action of the laser pulse is treated in two ways, first, as creating a uniform initial strain. In this case the predominant mode excited is the breathing mode. The period of this mode is in reasonable agreement with the vibrational periods measured for the smaller cubes and with the higher frequency modulation observed for the larger cubes. A nonuniform initial strain is also considered, which could arise from nonuniform heating for particles larger than the optical skin depth of the metal. In this case the predominant mode excited is a nontotally symmetric mode. The calculated periods from this analysis are in reasonable agreement with the lower frequency modulations observed for the larger samples. The results from this study show that, to within the accuracy of these measurements, the elastic constants of cubic silver nanoparticles are the same as bulk silver.

The optical properties of two Au-Ag nanobox samples with average edge lengths of 44 and 58 nm and wall thicknesses of 6 and 8 nm, respectively, have been studied by single particle spectroscopy. The measurements gave an average line width of Gamma = 306 +/- 7 meV with a standard deviation of sigma = 30 meV for the 44-nm boxes, and Gamma = 350 +/- 9 meV with sigma = 35 meV for the 58-nm boxes. These line widths are much broader than those of gold nanorods with comparable resonance energies. The increased broadening is attributed to a combination of surface scattering of electrons, as well as increased radiation damping for the nanoboxes. Discrete dipole approximation calculations have been performed with and without surface scattering of electrons to compare with the experimental spectra. The calculations confirm that both electron-surface scattering and radiation damping are important effects in this system.

Au nanocages were synthesized via a galvanic replacement reaction. The extinction peak of these hollow structured particles is shifted into the near-IR compared with the Ag nanocube templates. Energy transfer from the Au nanocages into the surrounding environment (water) as well as the coherently excited vibrational modes of the nanocages were studied by femtosecond pump-probe spectroscopy. The time scale for energy relaxation was found to increase with the size of the particles, with the relaxation time being independent of the laser intensity. The time scales for relaxation are comparable to those for solid spherical gold particles and are consistent with energy relaxation being controlled by heat dissipation in the solvent. The period of the coherently excited vibrational mode is proportional to the dimensions of the nanocages. Intensity-dependent measurements show that in solution the nanocages maintain their integrity up to lattice temperatures of 1100 +/- 100 K.

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Time-resolved spectroscopy has been used to investigate the vibrational properties of hollow cubic nanoparticles: Au-Ag nanoboxes and nanocages. In these experiments laser-induced heating was used to coherently excite the breathing vibrational modes of the particle. The vibrational periods scale with the edge length of the particle, and the nanocages and nanoboxes showing equivalent responses despite a large difference in their morphology. The measured vibrational periods are compared to finite element calculations, where the particles are modeled as a hollow cube, with the principle crystal axes parallel to the sides of the cube. Very good agreement is obtained between the calculations and the experimental data, with the experimental frequencies being slightly lower than the calculated values (by ∼ 7 %). These results demonstrate the importance of accurately modeling the particles in order to interpret experimental data.Advances in synthetic techniques have made it possible to produce high quality samples of metal and semiconductor nanoparticles in a variety of different shapes, such as rods, 1,2,3 triangles, 4,5 cubes and boxes, 6 and branched structures. 7,8 These particles have unique optical properties, and are finding use in a variety of applications, from molecular sensing 9 to biological labeling 10 and photothermal therapy. 11,12 For metal particles the optical response is dominated by the surface plasmon resonance (SPR), which is a coherent oscillation of the conduction electrons across the particle surface. 13 Recent research has focused on understanding how the position and width of the SPR depends on the size and shape of the particles. 14 In this regard, single particle spectroscopy has been particularly useful. 15,16 These experiments have shown, for example, how radiation damping, 15,17 electron-surface scattering, 17,18 and the "lightning-rod effect" 19 control the width of the plasmon resonance.Time-resolved experiments also provide information about the properties of nanoparticles. For metals, transient absorption experiments conducted on a sub-picosecond timescale give information about electron-electron 20 and electron-phonon coupling. 21 At longer timescales (tens to hundreds of picoseconds) transient absorption measurements give data about how energy relaxes from the particle to the environment. 22,23 This information is important to photothermal therapy, where laser-induced heating of nanoparticles is used to kill selected cells. 11,12 For high quality samples, modulations due to coherently excited vibrational modes To obtain quantitative information from the vibrational spectroscopy experiments, an expression is needed that relates the measured frequencies to the particle dimensions and elastic constants. For spheres (both homogeneous and core-shell) and rods, analytic expressions are available for the important modes observed in the transient absorption experiments. 24,25,26, 27 However, for other shapes the vibrational modes must be calculated numerically. 28 In our recent stud...