Caterina Minelli
This thesis describes some of three years'work carried at the Centre Suisse d'Electronique et de Microtechnique (Neuchâtel, Switzerland) under the supervision of Dr. Martha Liley and in collaboration with Prof. Horst Vogel of the Ecole Polytechnique Fédérale of Lausanne (Switzerland). The goal of this work is to contribute to the development of innovative technologies in the treatment of surfaces, for the lateral organization of various materials on the micrometer and nanometer scale. The aim is therefore to offer a valid alternative to common lithographic techniques that often require expensive capital equipment and infrastructures. In spite of the high-quality and precision of their products, these techniques often have a limit of ~70 nm on the resolution of the minimum feature size. In this work, techniques based on the ability of some systems to self-organize were used. In particular, the tendency of immiscible polymer mixtures and of block copolymers to form separate phases was exploited. Films made of different polymer phases formed templates for the organization of various materials, with length scale that varied from some 10 µm to some 10 nm. The materials that were most investigated were particles of the diameter of some nanometer, made of semiconductors (CdSe and ZnS) and of metals (Au, CoPt3 and Co). These nanoparticles have different properties from the corresponding bulk material. This fact is due to two main phenomena: in a nanoparticle the electronic energy levels of electrons are discrete because of their spatial confinement. Also, surface effects arise due to the high surface to volume atom ratio compared to the bulk material. As an example, semiconductor particles of the diameter of some nanometers are fluorescent. Semiconductor nanoparticle fluorescence is usually characterized by long fluorescence lifetimes. This fact promoted the investigation, in the framework of this thesis, of the fluorescence properties of Mn-doped ZnS nanoparticles. These particles turned out to have exceptionally long fluorescence lifetimes compared to the fluorophores commonly used as fluorescent markers in biological techniques. This singular property allowed the construction of a simple and cost effective instrument for time-resolved detection of the nanoparticle fluorescence. This technique allows a remarkable increase of the signal-to-noise ratio compared to conventional detection methods. Two approaches were explored to laterally organize nanoparticles on polymer surfaces. In the first method, the nanoparticles were added into a mixture of immiscible polymers and a film was formed from the solution by spin-coating, typically on silicon or glass slides. Numerical simulations by other authors indicate that if the particles have a higher affinity for one of the two polymers, they will be distributed in the corresponding phase. This behaviour was confirmed by our experiments. The lateral dimensions of the patterns in which the nanoparticles organized could be changed with continuity from some 10 µm to se sub-micrometer range. Their shape was modelled from a stochastic to an ordered type. The second approach explored for the organization of the nanoparticles consisted of the pre-formation of the polymer films and their subsequent decoration with the nanoparticles. Due of the different interactions of the two polymers with the particle-solvent system, different absorption behaviours of the nanoparticles were found on the two polymer phases. This technique allowed nanoparticle organization on homopolymer demixed films in patterns having typical sizes in the micron and in the submicron range. Alternatively, the nanoparticles were organized on block copolymer films in regular patterns having typical periodicities of the order of 100 nm. Nanoparticles organized on thin block copolymer films could be transferred on the hard substrate via removal of the polymer molecules by oxygen plasma etching. This process did not affect the nanoparticle organization. Under particular conditions, an aggregation of 10 nm gold nanoparticles was induced using oxygen plasma. This technique allowed the formation of gold nanowires and nanostructures both on polymer layers and on the hard substrate. Their width varied from about 25 nm to the micrometer, while their length extended for various micrometers. They presented a fingerprint like structure or, alternatively, quasi-parallel nanowires extended for several µm2, the typical periodicities being about 100 nm. The conductivity of these nanowires and nanostructures was demonstrated using SEM.
EPFL2004
