.3

Nanoparticle ArraysThe unique optical property of a metallic nanoparticle is the localized surface plasmon resonance (LSPR). This resonance occurs when the correct wavelength of light strikes a metallic nanoparticle, causing the plasmon of conduction electrons to oscillate collectively. The term “LSPR” is used because this collective oscillation is localized within the near surface region of the nanoparticle. The two consequences of exciting the LSPR are (1) selective photon absorption and (2) generation of locally enhanced or amplified electromagnetic (EM) fields at the nanoparticle surface. The LSPR for noble metal nanoparticles in the 20-to a few-hundred-nanometer-size regime occurs in the visible and IR regions of the spectrum and can be measured by ultraviolet (UV)-visible-IR extinction spectroscopy. Works by Van Duyne et al. show that Ag nanoparticle arrays from colloidal lithography exhibited important LSPR properties [54]. The LSPR of the array is correlated with the shape and size of the building blocks in the array. Our work has revealed that the morphologies of the periodic nanoparticle arrays can be modified by annealing or laser irradiation [55]. The former will spheroidize the particles of the whole sample, and the particles are easily aggregated during heating. The latter will modify the morphology of the array in a selected area, as required. As an example, the Nd:YAG laser is used, operating at 1 Hz at the third harmonic wavelength of 355 nm with a nominal pulse width of 7 ns. The laser pulses were unfocused with an energy density of 15 mJ/cm2. The incident light beam is perpendicular to the substrate of samples. It can be found that the morphology of the nanoparticle in the array evolved from truncated triangle (from the top view) to a completely spherical shape with an increase in the number of laser pulses. Figure 8.34 is the corresponding result for Au nanoparticle

arrays. After about 40 laser pulses, three sharp corners of each particle become separated from the main body of the particle and three nanogaps of about 30 nm are formed in each particle, as demonstrated in Fig. 8.34A. This morphology is particularly intriguing because it might be possible for such an array to be used as a substrate for molecular-switching devices. As the number of laser pulses is increased to 100, the nanoparticles at the corners become smaller and almost disappear, while the main body of the particle evolves from a polyhedron to a rounded and finally to a nearly spherical shape, as shown in Fig. 8.34B-D. This demonstrates that the morphology of the nanostructured arrays can be manipulated by laser radiation through the appropriate number of pulses. Applying more than 100 laser pulses did not induce any further changes but complete disappearance of the nanoparticles at the corners and the edge sides of the original particles. Figure 8.34E shows that a sample has been irradiated by more than 500 pulses, and its morphology is similar to that of the sample irradiated for about 100 pulses, indicating that the particle has reached its equilibrium shape after 100 pulses. Further, tilted observation has shown that the final particles are nearly spherical shaped, as demonstrated in the inset of Fig. 8.34E.