From nanocolumns to nanowires: Self-assembled metal nanostructures for optical applications produced by pulsed laser deposition.

Abstract

Metal-dielectric nanostructured films and surfaces present unique optical properties that make them ideal candidates for the development of optical and opto-electronic nanodevices, due to the possibility of generating a large electromagnetic field enhancement in the vicinity of the metal nanostructures (NSs) through localized surface plasmon resonances. However, most practical applications require a precise control of the morphology and organization of the metal NSs, since these characteristics define their optical behavior. In particular, the presence of non-spherical, ordered distributions of metal NSs allow a wider range of applications in fields other than optics, such as sensing or biomedicine among others. One interesting aspect for applications is that the achievement of nanostructures oriented in the direction perpendicular (nanocolumns, NCls) or parallel (nanowires, NWs) to the plane of the substrate makes controlled polarization effects possible, while a large aspect ratio (AR) intensifies the relevant mode of the plasmonic resonances and spectrally shifts it towards the optical communication wavelength range. In this work we apply sequential pulsed laser deposition (PLD) to the lithography-free synthesis of either NCls or NWs in a single-step. PLD allows a precise control over the amount of deposited metal, as well as reduced NSs size dispersion, yet the intrinsic characteristics of PLD make difficult the fabrication of metal NSs with well-defined shape and symmetry. We will show that the pulsed character of PLD, combined with an appropriate design of the deposition sequence and geometry, and the use of rippled substrates enable to produce self-assembled oriented Ag NCls and Au NWs having small diameters (DNCl¿3 nm, DNW ¿12 nm) and large aspect ratios (ARNCl¿20, ARNW ¿50). Optical characterization of the as-deposited nanostructures confirms a clear polarization dependence of the plasmonic resonant modes that is determined by the NSs¿ morphology and distribution. We will finally show the potential application of these NSs as omnidirectional polarizers.