Georg Krausch

Surface-induced structure formation of polymer blends on patterned substrates

Phase separation in bulk mixtures commonly leads to an isotropic, disordered morphology of the coexisting phases. The presence of a surface can significantly alter the phase-separation process, however,. Here we show that the domains of a phase-separating mixture of polymers in a thin film can be guided into arbitrary structures by a surface with a prepatterned variation of surface energies. Such a pattern can be imposed on a surface by using printing methods for depositing microstructured molecular films, thereby allowing for such patterns to be readily transferred to a two-component polymer film. This approach might provide a simple means for fabricating polymer-based microelectronic circuits or polymer resists for lithographic semiconductor processing.

Wetting at polymer surfaces and interfaces

Abstract Experimental research on wetting in polymer films is a subject that is reaching maturity. We review progress from the past few years in research into the influence of a boundary in polymer blends, concentrating largely on the wetting transition, and the growth of wetting layers, where we pay particular attention to blends in which hydrodynamic flow plays a dominant role. A summary of work over the same period concerning the dewetting of polymer films is also included, along with a discussion of the role of pattern formation caused by dewetting and topographically and chemically patterned substrates. We conclude by summarising some experiments that we believe may inspire future research.

Nanostructured Thin Films via Self‐Assembly of Block Copolymers

Thin films of incompatible block copolymers self-assemble into highly regular supramolecular structures with characteristic dimensions in the 10-100 nm regime. There is increasing interest in controlling the resulting structures and utilizing them, for instance in the area of nanotechnology. So far, research has concentrated mainly on exploiting the melt structure of diblock copolymers. Recent work on block copolymer solutions and more complex co- and terpolymer architectures has revealed a rich variety of novel thin-film structures, some of which exhibit high complexity and order. In addition, by use of mean field dynamic density functional theory along with well-controlled experiments, the fundamentals of thin film structure formation have been elucidated. We highlight some aspects of these studies and point to future directions in this lively field of materials science.

Phase behavior in thin films of cylinder-forming ABA block copolymers: Experiments

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than approximately 1.5 domain spacings.

Surface induced self assembly in thin polymer films

Abstract The effect of boundary surfaces on phase separation and microphase separation in binary polymer blends and block copolymers respectively, has gained increasing attention over the last 5 years. It has been realized that the complex interplay between wetting and phase separation may severely influence the phase separation process thereby leading to near-surface domain structures, which differ distinctly from the respective bulk morphologies. In the present article, we try to summarize the basic features of surface directed (micro-) phase separation in immiscible polymer systems. For both polymer blends and block copolymers a brief review of the historical development is given, followed by a list of selected examples representing the large number of current research activities in these fields. Particular attention is given to the possibility to actively control the domain morphologies via surface interactions in view of possible technological applications.

Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?

We have investigated the optical resolution of a scanning near-field optical microscope in reflection collection mode using an uncoated fiber tip. We demonstrate that the apparent resolution in the optical signal (better than 70 nm) is a topography-induced effect. We believe that the purely optical resolution is only of the order of λ/2 and diffraction limited.

Scanning near-field optical lithography

Abstract We discuss the influence of lateral fiber vibrations on the lateral resolution in scanning near field optical lithography using a tapered optical fiber. The fiber tip acts as a sub-wavelength sized light source to perform high-resolution photolithography in a commercial linear photoresist. After exposure and development, the structures are investigated by atomic force microscopy and a mechanical replica of the optical near field is revealed. We were able to create structures with lateral dimensions below λ 5 . The aspect ratio and the overall shape of the lines are found to depend on the lateral vibrations of the fiber tip needed to perform the shear-force distance measurement.

Near field microscopy and lithography with uncoated fiber tips: a comparison

We show that lateral resolution well beyond 100 nm can be obtained in scanning near-field optical microscopy (SNOM) using uncoated glass fiber tips in the internal reflection mode. Exposure of a photosensitive layer through the tip of a tapered optical fiber reveals that the illuminated area underneath the fiber tip is far larger than the best resolution obtained in microscopy. This finding points to the importance of the particular beam path in an internal reflection mode SNOM experiment. Lithographic experiments with metal coated fiber tips show that in this case, the light field emitted by the tip is indeed confined to an area below 100 nm.

Structure at polymer interfaces determined by high‐resolution nuclear reaction analysis

We describe a method based on nuclear reaction analysis, using the reaction 2H(3He, 4He)1H, (Q=18.352 MeV) to determine composition profiles of deuterated polymer chains in thin films. By detecting the emitted α particles (4He) at forward angles (30°) we are able to achieve a spatial resolution of 7 nm half width at half maximum (HWHM) at the deuterated sample surface, and 15 nm HWHM at a depth of some 130 nm. We use our method to probe initial diffusional broadening at the interface between deuterated and protonated polystyrene films. Our measured profiles are in close agreement with earlier measurements (over larger spatial scales) and with mean field models for the diffusional process in this system.

Self‐assembly of a homopolymer mixture via phase separation

We propose a mechanism for creating lateral order in thin films of a homopolymer mixture via spinodal decomposition in the presence of a laterally structured substrate surface. The system under consideration is a mixture of deuterated polystyrene and partially brominated polystyrene (PBrxS, x=0.5) spuncast onto different substrates from toluene solution. We demonstrate that periodic stripe‐like domain structures can be observed in the polymer films after spinodal decomposition on a periodic array of Cr lines on a H‐terminated Si surface. The effect vanishes when the chemistry of the underlying structure is changed, strongly indicating an interface‐driven ordering of the polymer film.