Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

laser pulses while being mixed in the rotating cylindrical cell. A partial meltingof the nanorods only slightly reduces their longitudinal plasmon absorptionand, hence, the absorption of another laser pulse is possible.This partial melting of gold nanorods is illustrated in Fig. 9, whichshows the TEM images before (Fig. 9a) and after (Fig. 9b) irradiation withlow-energy femtosecond pulses [27]. These images indicate that mostlyshorter and wider nanorods are present in the final solution. This is confirmedquantitatively by the statistical analysis of the nanorod distributions before(gray columns) and after (black columns) laser exposure, as illustrated in Fig.9c. The mean aspect ratio of the nanorods decreases from 4.1 to 2.6. Althoughsome nanorods are transformed into nanodots (complete particle melting),the main effect is a decrease in the nanorod length (from 44 to 35 nm) and asimultaneous increase in the nanorod width (from 11 to 13 nm). This resultindicates a gentle reshaping of the gold nanorods into shorter and widerparticles by partial surface melting and surface reconstruction [27]. Femtosecondlaser pulses with appropriate energies and wavelengths can, therefore,be used to narrow a relatively wide nanorod size distribution typical forexisting chemical procedures.In order to understand the mechanism of the shape transformation, wehave performed high-resolution TEM (HR-TEM) studies [29]. Colloidal goldnanorods were exposed to laser pulses with pulse energies below the thresholdneeded for a complete melting of the nanorods. The laser-induced structuralchanges were then followed by HR-TEM. Freshly prepared gold nanorodsbefore laser irradiation were found to have the {100}, {111}, and {110} facetsand contain no volume dislocations, stacking faults, or twins [56]. Therelatively unstable {110} facet is, however, usually absent in the sphericalnanodots prepared either by chemical methods or by photothermal melting ofthe nanorods. Spherical nanoparticles are dominated by {111} and {100}facets with shapes of truncated octahedral, icosahedral, and decahedral [56].The {111} and {100} facets are the lower-energy faces of gold. In order toreduce the strain associated with the sphericallike particle shape and toaccommodate the presence of only {111} and {100} facets, the nanodotsmust contain planar defects. An example of a multiple twinned particle is anicosahedral, which consists of 20 tetrahedra with {111} facets.Figure 9 Transmission electron microscopic images taken before (a) and after (b)exposure to low-energy femtosecond laser pulses (0.5 AJ). (c) Comparison of the aspectratio distribution of gold nanorods before (gray columns) and after (black columns)laser irradiation showing that the mean aspect ratio of the nanorods decreases from 4.1to 2.6.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.