chapter  8
Advances in the Fabrication of Plasmonic Nanostructures: Plasmonics Going Down to the Nanoscale
Pages 52

Precisely, a nano-object is so called when its properties differ from the bulk ones. The size reduction indeed makes appear a threshold below which the physical and chemical properties of the nanomaterial vary from the bulk ones. The modification of the material properties in fact stems from the drastic increase of the ratio between the surface and the volume of the material. Below the size threshold, the investigated properties become size-dependent. The size threshold of course depends on both the nature of the material and of the investigated properties and will differ depending if we pay attention to mechanical, chemical, optical, or magnetic features of nanoparticles. In the beginning of the 1980s, the field of plasmonics emerged when Nylander, Liedberg, and Lind demonstrated that surface plasmon resonance (SPR) of plasmonic thin films (≈50 nm) could lead to gas sensing [1]. Plasmonic thin films were extensively studied during the last three decades [2]. Therefore, they will not be included in the scope of the present chapter. More recently, a turning point was marked in the development of plasmonics via the progress in nanofabrication. Indeed, nanostructuration of noble metals allowed inducing interaction between light and the collective coherent oscillation of the metal free electrons without the need of prism or complex setup [3]. Such effect has been involved through centuries by artisans fabricating coloring glasses or church windows without understanding that they were synthesizing gold or copper nanoparticles [4]. The possibility of controlling the size and shape of metal nanoparticles boosted the field of plasmonics at the end of the 1990s and made it very attractive both from a fundamental and applied point of view.In nanofabrication, there is no technique prevailing over the others. The targeted applications and the plasmonic structure size or organization will drive the choice of the fabrication process. Two approaches have been developed to synthesize nano-objects: the top-down and the bottom-up ones. The top-down approach consists of taking and etching a massive material down to the desired shape and size. The processes associated to it take advantage of their high reproducibility and find some immediate applications in the industry. After many improvements, this approach nowadays allows to produce structures as small as 20 nm in size. Semiconductor companies even project reaching the 14 nm node technology via lithography or etching techniques [5].