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

Figure 10 shows HR-TEM images of gold nanorods before (Fig. 10a)and after (Fig. 10b–10d) exposure to femtosecond laser pulses having anincreasing amount of energy [29]. The as-prepared nanorod in Fig. 10a showsno defects sites, as explained earlier. The nanorods in Figs. 10b and 10c showpoint defects and twins, respectively. Finally, the nanodot obtained after thecomplete shape transformation shows multiple twinning (Fig. 10d). Apossible mechanism of the rod-to-sphere shape transformation process isthe following [29]. The as-prepared nanorods are defect-free single crystals.The sides of the rod are enclosed by {110} and {100} facets and its growthdirection is [001]. The small {111} facets are present, but only at the corners.While being illuminated by laser pulses, point defects are first created in thebody of the nanorods and they serve as the nuclei for the formation of twinsand stacking faults. The twin is formed by two crystals with a specificorientation, which is only possible if local melting takes place. This suggeststhat the melting first takes place at the defect sites in order to form a twinnedcrystal. Then, surface diffusion must occur simultaneously in order toenhance the growth of the twinned crystal. This process is thermodynamicallydriven by the fact that the lower surface energy {111} face gains the area whilethe area of the {110} face slowly decreases. A continuing growth of thetwinned crystal finally eliminates the unstable {110} surface. Finally, theentire particle is enclosed by the more stable {111} and/or {100} faces.Melting, therefore, occurs not only on the surface of the nanorods but alsoinside the particle by creating defect sites. This is a mechanism which is quitedifferent from what one would expect for melting of bulk materials.So far, we have reviewed the results on the rod-to-sphere shape transformationinduced by femtosecond laser pulses. However, interesting differencesare observed when nanosecond pulses are used instead [26,27]. Inparticular, it was found that the energy threshold for the complete meltingof gold nanorods was lower for femtosecond pulses than for pulses of a nanosecondduration [27]. This result is illustrated in Figs. 11a and 11b, whichshow nanoparticles produced by irradiation with femtosecond (Fig. 11a) andnanosecond (Fig. 11b) laser pulses. In this experiment, the same pulse energies,beam sizes, and excitation wavelengths (f800 nm) were used to allow adirect comparison. The dominant particle shape after nanosecond irradiationFigure 10 Series of TEM images of gold nanorods taken at different stages of therod-to-sphere shape transformation. The amount of laser energy absorbed by theparticles increases from top to bottom. The image in (a) shows a defect-free goldnanorod before laser irradiation. The particle in (b) has point defects inside the particleand (c) shows an example of a nanorod with a twin defect. The spherical nanoparticlesobtained after the shape transformation show multiple twinning (d).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.