Materials Science and Technology of Polymers at the University of Twente

Poly[ferrocenylsilanes] in Micro- and Nanofabrication

The main goal of this thesis is to explore novel approaches to self- and directed assembly of iron-containing macromolecules, poly(ferrocenylsilanes) (PFS), for applications in lithography and catalysis. Iron present in the main chain of PFS is responsible for the high resistance of this polymer to reactive ion etching. In order to utilize this singularity in lithographic applications on micro- and nanometer scales, the polymer has to be arranged on a surface in a predefined pattern with desired characteristic sizes and periodicity. Both top-down and bottom-up pattern fabrication approaches are presented. In all of the cases the polymer is confined in ultra-thin films where the presence of interfaces has a dominating effect on the PFS behavior. In general, this thesis presents a case for materials science of PFS at interfaces. Various PFS containing polymers were synthesized and used in surface decoration at different lengthscales. PFS homopolymers were patterned on the micrometer and sub-micrometer scale by soft lithography methods (top-down). Phase separation in block copolymers was used to create periodic patterns on a nanometer scale (bottom-up). The functionality of PFS-containing block copolymers was further illustrated by the utility of the organometallic domains in a catalytic application. Four novel aspects of PFS materials chemistry are presented in this thesis. Firstly, the structure of PFS homopolymers is optimized for temperature assisted soft lithographic applications. Secondly, a new synthetic route to PFS block copolymers is presented. This route combines anionic and living radical polymerizations and enables preparation of incompatible, amphiphilic block copolymers (e.g. PFS-b-PMMA). Such polymers can self-assemble in selective solvents into long cylindrical micelles. Furthermore, addition of hydrophilic components to PFS block copolymers can lead to the formation of stable Langmuir films on the air-water interface. Block copolymer blends were used to form patterns on multiple length-scales followed by Langmuir-Blodgett transfer from water onto silicon surfaces. Finally, phase separated PFS block copolymers were used in the preparation of iron-containing catalytic domains for the growth of carbon nanotubes. Chapters 1 and 2 give a general introduction to the research goals of the thesis, PFS macromolecules and phase behavior of polymers. The following chapters describe various methods of PFS assemblage in solutions and on surfaces for lithographic and catalytic purposes. First, patterns on micron and sub-micron scales are presented, followed by fabrication of nano-assemblies. The potential of poly(ferrocenylsilane) as an ink and resist in soft lithography has been demonstrated in Chapters 3 and 4. Different soft molding approaches were used in Chapter 3 to structure PFS on micron and sub-micron scales. The patterns obtained depend not only on the geometry of the stamp but also on the molding conditions. It has been shown that a stamp with one geometry can be used to give three different line patterns. The mobility of PFS ink required to form patterns is achieved either by solvent or temperature assisted patterning strategies. Especially, temperature-assisted capillary force lithography (CFL) provides additional control over the size and spacing of the PFS lines by changing the amount of the PFS ink used for patterning (i.e. varying the initial polymer film thickness). In all of the cases, substrate areas with sizes of several cm2 can be structured. Circular patterns, which are not easily accessible by other methods, were also produced. All the polymer structures were transferred into silicon substrates by plasma etching. This lithographic approach can also be scaled down to the submicrometer scale provided that stamps with desired geometries are available. Chapter 4 deals in detail with capillary force lithography. The influence of polymer structure on the patterning process was investigated. Three principle parameters have a direct impact on the quality of CFL polymer patterns: molar mass of the polymer, processing temperature (and time) and initial polymer film thickness. It was found that the right balance between glass transition temperature (Tg) and processing conditions (viscosity) is essential for successful patterning. On the one hand, polymers with too low a viscosity will easier dewet, on the other hand polymers with too high a viscosity will not yield patterns within the experimental time scale. Thus, the zero-shear-rate viscosity of a polymer melt is a very important factor influencing the quality of the patterns. A range of optimal viscosities for the pattern formation was established. Since polymer viscosity is a product of polymer molar mass and temperature, this information was used to optimize the patterning conditions and polymer structure for successful CFL pattern transfer. Chapter 5 presents a novel synthetic route to 2-bromoisobutyryl end-functionalized poly(ferrocenyldimethylsilane), allowing essentially quantitative end-capping of the PFS chain. Following the formation of the organometallic block by anionic polymerization, a ruthenium-mediated controlled radical polymerization of methyl methacrylate was employed to grow the second block. Well-defined PFS-b-PMMA block copolymers with low polydispersities (Mw/Mn < 1.1) and controlled compositions were obtained. The use of ATRP to form the second block implies that a wide variety of (meth)acrylic and other vinyl monomers can be used, thus opening up the way to novel hybrid organic-organometallic block copolymers and amphiphilic organometallic block copolymers. Self assembly of these block copolymers in a selective solvent is investigated in Chapter 6. We have shown that PFS-b-PMMA block copolymers self-assemble into micelles in acetone. The size and shape of the micelles depended on the length of the PFS block and its amount relative to the PMMA block. In case of the block copolymer with the longest PMMA block, the block copolymers tend to form monomolecular micelles with only a small fraction of polymer associating into cylindrical micelles. As the PFS fraction increases, more and more copolymer chains aggregate into cylindrical micelles. Block copolymers with 22 % of PFS formed well-defined cylindrical micelles exclusively, with a diameter of 18 nm and a length of over 3 m. On the other hand, when PFS was in the majority, only crew-cut spherical aggregates were formed. Micelle formation was studied by means of dynamic light scattering. Experiments performed in depolarized and polarized configurations did not show any rotational diffusion contribution to fluctuations of scattered light usually associated with rod-like molecules and their optical anisotropy. Lack of rotation can be explained by the extreme length of the micelles and by their flexibility in solution. The micellar solutions exhibit a critical micelle temperature at, or slightly above the solvent boiling point. At the CMT, long cylindrical micelles transform into spherical micelles with a hydrodynamic radius on the order of 30 nm as calculated using the Stokes-Einstein equation. Cryo-TEM measurements on frozen block copolymer solutions identical to those used in the DLS measurements showed that long, cylindrical micelles were formed in the block-selective solvent for the PFS-b-PMMA block copolymers with 18 and 26 wt% PFS, which agrees well with the DLS results. High-resolution 1H NMR studies of the micellar solutions in deuterated acetone recorded at temperatures varying from 25 to 55 ºC revealed that the PFS block is not in a ‘glassy’ state and solvent molecules penetrate inside of the micellar core. The contrast in polarity between the two blocks “amplified” by a polar nonsolvent is the principle driving force for micelle formation. Wide angle X-ray scattering experiments performed on our cylindrical micelles showed no evidence of crystallization in these samples. Cylindrical micelles were also deposited on silicon substrates via dip-coating and characterized with AFM and SEM. The deposition process can be optimized and controlled by changing the polymer concentration and/or dipping velocity. Such micelles could be used as lithographic masks in dry etching of silicon substrates. Cylindrical micelles with high Tg shells such as PMMA constitute a very good choice for surface deposition due to the higher stability of the cylinders. Furthermore, the polarity of PMMA can also play a role especially when the substrate shows an affinity towards this block. In Chapter 7 we have demonstrated that a blend of two block copolymers with a common block can form patterns on multiple length-scales at the air-water interface. Addition of a hydrophilic-hydrophobic block copolymer to a hydrophobic block copolymer causes formation of surface micelles with ‘micelles-in-a-micelle’ morphology already for a 25% blend. Further addition of PS-b-PVP block copolymer forces the system into a layered film when there is enough hydrophilic block copolymer to form a continuous monolayer with a second blend component phase separating on top of the brush. Upon further blending of the hydrophilic component, stable surface micelles are formed with a diameter of 80 nm (as estimated from AFM measurements) with a discrete pattern inside of the core consisting of 3 to 5 dots with diameters on the order of 10 nm. The micelles can be transferred onto various solid substrates using the Langmuir-Blodgett technique. Such transferred films with hierarchical morphologies can be of potential interest for lithographic and catalytic applications. The organization of blends at the air-water interface provides access to new, interesting morphologies, which would be difficult or impossible to achieve otherwise. Finally, Chapter 8 presents another, novel application for PFS block copolymers. We have demonstrated that an effective, nanostructured, well-ordered growth catalyst for carbon nanotubes can be obtained easily from self-assembled PS-b-PFS block copolymers. The as-prepared catalyst is mostly in the form of iron(III) species. After annealing and during the CVD process, lower-valence iron species are formed. Several interesting perspectives are offered by this novel catalyst system: the size of the catalyst domains as well as their spacing are tunable via the composition of the polymer, opening a way of controlling the spacing and the diameter of the tubes.