Tunable surface nanostructures from Block copolymer self-assembly

Nanostructures are known to exhibit novel optical, electrical, and magnetic and many other interesting physical properties and a lot of research has been invested on the synthesis and study of such structures in the recent years. The massive drive towards miniaturization of devices heavily relies on the successful functioning of nanostructures. Self-assembly (or the bottom-up approaches) is emerging as an attractive tool for creating functional nanostructures owing to its simplicity and relatively low cost approaches. Nanostructures with high density and periodicity on surface are important in applications like high density data storage, sensors, catalyst support surfaces and other functional interfaces. Reverse micelles formed by the self-organization of amphiphilic diblock copolymer molecules have been used to create an array of polymer nanoparticles on silicon surface by spin coating . The formation of reverse micelles, the means of tuning their sizes and systematically varying the periodicities using solvent quality, solution concentration, deposition conditions and polymer molecular weight as handles will be presented. The hydrophilic core of the reverse micelles is capable of binding metal ions and so, inorganic nanoparticle arrays can be derived from these micelle arrays. The size of the nanoparticles in the array is further tunable by working on the loading conditions. The tunability of nanoparticle sizes will be presented as well.

Bottom-up approaches for organizing nanoparticles with polymers

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

Luminescent nanocrystal modified photoresin for fabrication of high-aspect ratio three-dimensional microstructures

Jürgen Brugger, Vahid Fakhfouri

The organic polymers incorporating inorganic nanoparticles can result in material possessing with unique size-dependent properties, such as mechanical, optical, electrical, thermal and magnetic, that can be istinctively conveyed into the composite by the suitable nanoparticles. When nano-objects such as luminescent nanocrystals (NCs) particles are embedded into photostructurable polymer materials, they provide a new, highly processable material, which enables the engineering of novel, highly integrated micro/nanosystems (MEMS/NEMS) with enhanced intrinsic functional properties. The current effort aims, on one side, to the incorporation of the functional nanoparticles into the polymer matrix and, on the other side, to preserve the characteristic structurability of photosensitive polymers by UV lithography. In this paper we present a novel composite material obtained from the combination of luminescent nanocrystals (NCs) of cadmium selenide coated by a shell of zinc sulfide (CdSe@ZnS) with epoxy-based photosensitive resins. This class of material is well-known for polymer MEMS applications due to its unique structurability by UV lithography and mechanical properties. Luminescent CdSe@ZnS NCs with tuneable size, exhibiting size-dependent optoelectronic properties have been synthesized by means of colloidal routes methods. The presence of an organic ligand shell makes the NCs soluble in different types of solvents and thus enables their processing and incorporation into polymers. For this purpose a range on different solvents were tested in order to optimize the incorporation of the luminescent NCs (4.7 nm) in the epoxy-based resin. Spectroscopical and microscopical investigation were performed on the prepared composite samples and the effects of the NC concentration, size, surface chemistry and solvent composition were investigated in order to evaluate their role on the spectroscopical and structural behaviour of the nanocomposite properties. The NC-doped resin was then spin-coated on a silicon wafer, baked, exposed to UV light through a mask and developed. The results confirm the overall UV-structuring capability of the modified polymer and the NC emission properties were demonstrated to be retained. Furthermore, the luminescent properties of the NCs were retained after patterning by UV photolithography, although some aggregation of NCs was observed. The presence of organic ligand shell at the colloidal NC surface shows to be relevant for the subsequent processing of the doped photo-resin. The presented route looks promising for the microfabrication of polymer MEMS with inherent functionalities. Further optimization needs to be implemented also in view of a possible extension to incorporation of different types of colloidal NCs.

2006

Surface Nanostructuring through block copolymer self-assembly

Jürgen Brugger, Harry Heinzelmann, Sivashankar Krishnamoorthy, Raphaël Pugin

Synthesis of nanostructures using polymer self-assembly methods have gained widespread attention due to the multitude of possibilities they offer and their simple and cost-effective approaches. Self-assembly of block copolymers has been recognized as a promising means of creating functional nanostructures in both bulk and on surfaces. The nanostructures formed by block copolymers have been useful for their chemistries, morphology and dimensions. Block copolymers form micelles when dissolved in selective solvents. Several morphologies have been reported for micelles like spherical, cylindrical, and worm-like. Spherical micelles form an ordered array on surface under appropriate conditions of preparation and coating. Their orderly arrangement along with their dimensions offers a simple way of achieving high density nanoscale patterned surfaces. This presentation will highlight our recent work on the use of block copolymer micellar self-assembly as a novel platform to create functional interfaces. The development of a tool box consisting of micellar arrays with tunable micelle dimensions and array periodicities on surface and the application of micellar arrays for both its topography and chemistry will be discussed.