ELECTROCHEMICAL REPLICATION OF SELF-ASSEMBLED BLOCK COPOLYMER NANOSTRUCTURES
Particular emphasis is given to the patterning of quasi 1D nanowire and bicontinuous gyroid arrays, both of which are of particular interest for device applications. The first study of these highly ordered electrochemically patterned semiconductor arrays used in a real device application is described in a dye-sensitized bulk heterojunction solar cell. 2.1 INTRODUCTIONThe engineering of material architectures and composite structures on the nanometer length scale presents an extremely versatile route to creating enhanced and novel functionalities. Spontaneous self-assembly of constituent parts offers a rather attractive way around the ever-increasing demands of patterning from the top down on ever-smaller length scales. Block copolymers (BCPs), consisting of block sequences of two or more distinct monomer units, are a class of macromolecule that exhibit such self-assembly on the 10 nm scale. The assembly is readily tunable; depending on molecular characteristics (chemical composition and molecular weights) and processing parameters (temperature or application of external fields), the copolymer molecules organize themselves into a rich spectrum of highly ordered, chemically heterogeneous phases on the scale of a single polymer chain. The topology of these microphases ranges from close-packed spheres, hexagonally packed cylinders and bicontinuous gyroids to sheet-like lamellae and perforated lamellae.Many proposed applications exploiting BCP patterns, for example high-density memory, photovoltaics, battery electrodes, and sensor technologies, are surface-supported, that is, in a film geometry. Low-cost, large-area solution processing of films is no problem for many polymers. We must, however, contend with the additional complication of interactions with external interfaces, which strongly influence the behavior of the molecular assembly. The grand challenge to the application of BCP patterning in novel film devices lies in simultaneous control of the microphase, coupled with a way of transferring such model structures into active materials and
composites with desirable intrinsic properties. This is rather more difficult than it sounds; despite over three decades of development, there remain surprisingly few examples of functioning device applications that truly exploit ordered BCP assemblies. Broadly speaking, there are three approaches to overcome this challenge:I Direct synthesis: a BCP made by chemically tethering two or more electronically functional polymers. II Sacrifical templating: a BCP in which one component can be selectively removed after phase separation to leave a mesoporous matrix of the second. This material acts as a
template to be subsequently filled with a chosen functional material. The template can be removed later to leave a freestanding replicate of the BCP morphology. III Structure directing agent: a blend of the BCP with chemical precursors of functional materials, relying on microphase separation as an in situ structure director. While route I is perhaps the most intuitive and direct approach, these copolymers are generally rather difficult to synthesize, which limits the available choice of composite materials to synthentically compatible copolymers. Route II, on the other hand, is more within the grasp of today's materials and allows us to concentrate efforts to control self-assembly on relatively well-understood model copolymers, while significantly expanding the selection of functional materials that can be synthesized in the template. Electrochemical deposition, already well developed in the replication of other nano and microscale porous systems such as anodized alumina and track-etched membranes, is ideally suited to reproducing the extremely small and high-aspect-ratio pores typical of BCP structures. Figure 2.1 outlines the concept of sacrificial copolymer templating.Electrochemical replication of porous copolymer templates was first demonstrated by Thurn-Albrecht et al. in 2000. They produced a hexagonally ordered array of 10 nm diameter cobalt nanowires, standing vertically on a substrate some 25 nm apart with a density in excess of 1.9 × 1011 cm-2. This work not only demonstrated the great potential of porous BCP templates but also highlighted the main practical difficulties of this method and how they can be overcome. First, the BCP must be selectively degradable, that is, one block
can be completely removed in an etching process, which leaves the second component mechanically sound, acting as a self-supporting mesoporous matrix. Second, the pore structure left by etching must span the full thickness of the template from surface to substrate, such that any electrolyte flowing into the pores is able to achieve electronic contact with an underlying working electrode. That relatively few further examples of high-aspect-ratio electrochemical BCP replication exist owes much to the difficulties in satisfying this condition. The following discussion is, therefore, largely dedicated to describing important aspects of porous template formation in diblock copolymers — the characteristics of thin film self-assembly, microphase alignment techniques, selective degradation, and suitability to large area electrochemical replication. Progress in the patterning of standing and lying nanowire arrays and recent replication of spontaneously bicontinuous networks using this technique are reviewed. Finally, the first application of such highly ordered nanostructures in functioning photovoltaic devices is described in nanostructured solar cells based on electrochemically synthesized titanium dioxide arrays.