We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownian motion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substrate surface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerably improved alignment.

We demonstrate a simple and versatile way to achieve high yield synthesis of shape- and size-controlled multi-branched gold nanoparticles (MBNPs). Control over the shape of the MBNPs was achieved by varying the ratio of gold to the mild reducing agent ascorbic acid, using a seed-mediated growth approach. Higher ascorbate concentrations resulted in the smoothing of branches, leading to the yield of relatively more isotropic particles. Furthermore, we found that using much higher silver concentrations in the growth solution resulted in the formation of rod-shaped micro-features together with MBNPs; we postulate them to be cetyltrimethyl ammonium silver bromide crystals. The as-prepared MBNPs show interesting tunable optical properties that are strongly influenced by the particle shape. The results are discussed in terms of plasmon coupling between the core and branches of the MBNPs.

For the entire range of spheroidal noble metal nanoparticles, including spheres, rods, and disks, the optical properties are investigated as a function of their geometry and size by means of extinction spectra obtained by numerical methods. For spherical silver and gold particles, Mie theory is used to identify multipole plasmon resonances up to hexadecupole order as a function of particle radius. With increasing particle dimensions, higher-order multipole modes become more important. Moreover, the similarity in dielectric functions of silver and gold gives rise to almost identical optical properties at wavelengths exceeding 700 nm. For oblate and prolate nanoparticles, the dipole, quadrupole, and octupole plasmon resonances for aspect ratios up to 8 are considered. As with the nanospheres, extinction spectra for small nanodisks and nanorods are in agreement with the quasi-static approximation, whereas for larger dimensions, a red shift is observed. The plasmon peak positions are analyzed in terms of a material-independent factor that only depends on the geometry. We attempt to obtain a generalized description of the plasmon energies as a function of particle geometry and size. Finally, the size-dependent red shift is compared to surface plasmon dispersion relations and discussed in terms of surface curvature of the nanoparticle edge.

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