Controlled Nanostructuring of Ag Films by Ion Beam Sputter Deposition for Surface Plasmon Resonance Applications

The growth of Ag nanostrucutres on borosilicate glass substrates by ion beam sputter deposition in the Ar ion energy range from 150 to 600 eV is demonstrated. Rates of deposition as low as 0.01 nm/s are achieved at an Ar ion energy of 150 eV. This leads to the formation of a random array of nearly spherical Ag particles with a mean size of similar to 100 nm, separated by distances of similar order of magnitude. The particles organize themselves into arrays over lengths of at least 10 pm. As the thickness is increased from 3 to 18 nm there is a transition in morphology from an array to linear chains and finally a dense continuous film. There is a similar microstructural evolution as a function of increasing ion energy. The plasmon resonances can be tuned depending on shape, size and interparticle distances. As the thickness of the films increase, the main plasmon peaks can be tuned from 380 to 680 nm. The spheroidal shape of the particles induces additional peaks (localized surface plasmons) centered around 430 +/- 10 nm. Detailed simulations have been carried out based on Maxwell Garnett theory to distinguish the effects of shape and size on plasmon resonances. It is demonstrated that shape rather than the size of the particles has a stronger influence on the shift in plasmon resonances.