Robert Magerle

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.

Phase behavior in thin films of cylinder-forming ABA block copolymers: mesoscale modeling.

The phase behavior of cylinder-forming ABA block copolymers in thin films is modeled in detail using dynamic density functional theory and compared with recent experiments on polystyrene-block-polybutadiene-block-polystyrene triblock copolymers. Deviations from the bulk structure, such as wetting layer, perforated lamella, and lamella, are identified as surface reconstructions. Their stability regions are determined by an interplay between surface fields and confinement effects. Our results give evidence for a general mechanism governing the phase behavior in thin films of modulated phases.

The Effect of Diiodooctane on the Charge Carrier Generation in Organic Solar Cells Based on the Copolymer PBDTTT-C

Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network.

Role of dissimilar interfaces in thin films of cylinder-forming block copolymers.

We study the effect of dissimilar interfaces on the phase behavior of cylinder forming block copolymers in thin films by means of dynamic density-functional theory. In this article, we show that dissimilarity of the interfaces induces hybrid structures. These structures appear when the surface fields at the two interfaces stabilize different surface structures and/or reconstructions. We propose a general classification of hybrid structures and give an unifying description of phase behavior of cylinder forming block copolymer films. Our results are consistent with experimental observations.

Subsurface imaging of soft polymeric materials with nanoscale resolution.

Nondestructive depth-resolved imaging of ∼20-nm-thick surface layers of soft polymeric materials is demonstrated using amplitude modulation atomic force microscopy (AM-AFM). From a map of amplitude-phase-distance curves, the tip indentation into the specimen is determined. This serves as a depth coordinate for reconstructing cross sections and volume images of the specimens mechanical properties. Our method reveals subsurface structures which are not discernible using conventional AM-AFM. Results for surfaces of a block copolymer and a semicrystalline polymer are presented.

Nanotomography with enhanced resolution using bimodal atomic force microscopy

High resolution volume images of semicrystalline polypropylene were obtained by stepwise wet-chemical etching followed by atomic force microscopy of the specimen. Enhanced signal-to-noise ratio and spatial resolution were achieved by using the second flexural eigenmode of the cantilever for phase imaging while the amplitude of the first mode was used as feedback signal. The energy dissipated between the tip and the sample revealed characteristic differences between the crystalline and the amorphous regions of the polypropylene after etching, indicating the presence of a thin ( < 10 nm thick) amorphous layer on top of crystalline regions.

Nanoscale Swelling Heterogeneities in Type I Collagen Fibrils.

The distribution of water within the supramolecular structure of collagen fibrils is important for understanding their mechanical properties as well as the biomineralization processes in collagen-based tissues. We study the influence of water on the shape and the mechanical properties of reconstituted fibrils of type I collagen on the nanometer scale. Fibrils adsorbed on a silicon substrate were imaged with multiset point intermittent contact (MUSIC)-mode atomic force microscopy (AFM) in air at 28% relative humidity (RH) and in a hydrated state at 78% RH. Our data reveal the differences in the water uptake between the gap and overlap regions during swelling. This provides direct evidence for different amounts of bound and free water within the gap and overlap regions. In the dry state, the characteristic D-band pattern visible in AFM images is due to height corrugations along a fibrils axis. In the hydrated state, the fibrils surface is smooth and the D-band pattern reflects the different mechanical properties of the gap and overlap regions.

Thin polymer films on chemically patterned, corrugated substrates

We study the effect of a chemical pattern on the wetting and dewetting behaviour of thin polystyrene (PS) films on regularly corrugated silicon substrates. Our results reveal that the film preparation, annealing method, and confinement play a critical role in the final film structure. On evaporating gold on both sides of the facets (such that it covered the crests of the facets, and not the troughs), we observed dewetting, which proceeded to the gold, demonstrating an enthalpic effect contrary to the outcome previously observed when gold was only evaporated on one side of the facet. We also coated the substrate with octadecyltrichlorosilane (OTS); this led to a gold and OTS striped structure. PS films several nanometres thick dewet such substrates, with a preferential direction for dewetting in the direction of the stripes forming droplets of a considerably larger size than the stripes.

Automatization of nanotomography

An approach for automated nanotomography, a layer-by-layer imaging technique based on scanning probe microscopy (SPM), is presented. Stepwise etching and imaging is done in situ in a liquid cell of an SPM. The flow of etching and rinsing solutions after each etching step is controlled with solenoid valves which allow for an automated measuring protocol. The thermal drift and the drift of the piezo scanner is corrected by applying offsets calculated from the cross correlation coefficients between successive images. As an example, we have imaged human bone with approximately 10 nm resolution using tapping mode SPM and successive etching with hydrochloric acid.