TWO DIMENSIONAL SELF ASSEMBLY OF NANOSPHERES, A VERSATILE METHOD FOR NANOFABRICATION

Nanotechnology is one of the most innovative and multidisciplinary fields in modern research. Techniques to manipulate and control matter at the nanometric scale, giving the possibility to change the morphology and the physical and chemical properties are growing in number. At the nano scale, changing the morphology implies changing also the properties of matter: many properties are no longer intrinsic but they depend on the size, shape, and even on the environment. One of the most striking example is given by gold and silver colour when they are in the nanoparticle form. Gold, for example ould be vine-red or green, or bluish or black simply by changing the morphology of the particles. Appropriate manipulation of the matter could also give birth at new proprieties such as the transmission of light through holes much smaller that the light wavelength, allowing the possibility to control at a very intimate scale the propagation of light. It is clear that to exploit the great potential of nanotechnology it is important to have nanofabrication techniques that have a very precise control on the production of nanometric materials or materials with nanometric structures. There are many technologies that can produce structures with an outstanding resolution of just a few nanometer (Electron Beam Lithography, Focused Ion Beam). These technologies are ”serial” fabrication tool, they can produce one object at time and this means high costs and low throughput. On the other hand parallel technologies derive from the semiconductor industry and are mostly optical lithographic methods that are limited by the diffraction limit of light (200nm). In this thesis work the need for a nanofabrication tool that can allow the production of smaller nanostructures will be addressed by using a nanofabrication tool that meets the following criteria: • parallel processing • low cost • large area processing (cm2) • scalability • reproducibility • easy implementation We choose to exploit the ability of self aggregation of matter in ordered structures. In particular we exploited the tendency of spherical monodisperse nanoparticles to assemble in ordered, close packed structures known as colloidal crystals. One monolayer of such colloidal crystal is a very interesting structure because it has a well ordered array of pores among the particles that have a well defined size and shape, that could be tuned by simply changing the size of the self assembled colloidal particles. A simple and easy method to create and deposit on different substrate these self assembled monolayer of polystyrene nanoparticles will be presented. Monolayers will be used to synthesize arrays of monodisperse plasmonic nanoparticles with a very good control on their size and shape, allowing the tuning of the plasmonic proprieties on the desired application. We will use the array of plasmonic nanoparticles to realize molecular sensors and to amplify the Raman signal by the Surface Enhanced Raman Scattering effect. We will study the rise in temperature induced by illumination with a laser light resonant with the nanoparticle’s plasmonic transition. This information could be very interesting for the biological application of the nanoparticles arrays since temperature variation in such a very complex environment could have relevant effect. Moreover we will use these 2D colloidal crystal to synthesize different kinds of nanostructures like an array of holes in a metal film. This nanostructure is very interesting since the discovery of its ability to transmit light even if the hole size is much smaller than the light’s wavelength. A synthesis method based on self assembled nanospheres could be useful for the fabrication of such nanostructures because of its high flexibility in changing the nanoparticles size and so the array geometric parameters like hole size and the lattice period. Self assembled monolayers will be used as a template for the synthesis of nanostructured thin films of TiO2. Titania is a semiconductor of great technological interest in many different fields: catalysis, energy conversion, gas sensing. We will fabricate using the same technology two different nanostructured thin film: a macroporous thin films and a surface patterned with a nanobowl pattern. Finally we will demonstrate the use of self assembled monolayers coupled with a standard technology used in the semiconductor industry such the ion implantation. Nanometric patterns will be produced on Si wafers using the ordered monolayer as a mask for the ion beam.