Nanomanufacturing of Ordered Plasmonic Nanostructures: Effect of Metal-Substrate Interactions

Ordered metal nanoparticles have gained great importance in recent research activities due to their potential applications in nonlinear optics and nanophotonics below the diffraction limit. These enhanced optical properties arise from the interactions between the photons and the resonant localized surface plasmons, i.e., the collective oscillation of the free electron gas density at the metal/dielectric interface. An important example of one of the many applications that these nanoparticle arrays is that they can act as broadband antennas for trapping sunlight in thin film solar cells. These thin film solar cells have the potential to reduce the cost of photovoltaics while increasing light absorption leading to an increase in efficiency. A route to make such ordered structures, while still being practical and cost effective, can be through instabilities of an ultra thin (1-10 nm) fluid film leading to spatial patterns with length and time scales that are dependent on properties such as interfacial tension, contact angle with the substrate, and long range dispersion forces such as the van der Waals interaction [Favazza et al. Nanotechnology 17 (2006) 4229-4234, Trice et. al. Physical Review Letters 101, 017802 (2008)]. In such a process a metallic film is deposited on a substrate (e.g. silica) by physical deposition and subsequently the metal-substrate bilayer system is irradiated with a nanosecond or femtosecond pulsed laser. The laser fluence is selected to be above the melting threshold of the metal. During the pulse, the liquid film undergoes dewetting, resulting in the formation of a variety of spatially ordered nanostructures with a broad range of plasmon responses. In this work we study the added effects of a diffusion potential on the stability of the thin film. This potential represents the chemical affinity of the metal to the substrate such as the one that exists between metals and Si. Significant increases of the diffusion potential through the film have the ability to damp perturbations caused by forces such as the van der Waals interactions and increase the length scales between the particles. A new parameter is identified to quantify the effect of film-substrate interactions on the dampening of the perturbations. Implications to plasmon based solar energy harvesting applications will be discussed.