Kenneth D. Harris

Large amplitude light-induced motion in high elastic modulus polymer actuators

Well-defined gradients in molecular alignment have been used as tools to generate large amplitude, light-induced deformations in stiff polymer networks. These systems are reversible, monolithic and based on a simple one-step self-assembly process. To fabricate the actuators, diacrylate dopants containing azobenzene moieties were blended with liquid crystalline diacrylate hosts and photopolymerized in a twisted configuration. The resulting twisted networks were heavily crosslinked with room temperature elastic moduli on the order of 1 GPa. Regardless of the temperature with respect to the glass transitions, subsequent exposure to UV radiation induced anisotropic expansion/contraction, and simple variations in geometry were used to generate uniaxial bending or helical coiling deformation modes. Because mechanical energy is directly related to elastic modulus, these systems are expected to provide significantly greater work output than contemporary polymer actuator materials.

Fast assembly of ordered block copolymer nanostructures through microwave annealing.

Block copolymer self-assembly is an innovative technology capable of patterning technologically relevant substrates with nanoscale precision for a range of applications from integrated circuit fabrication to tissue interfacing, for example. In this article, we demonstrate a microwave-based method of rapidly inducing order in block copolymer structures. The technique involves the usage of a commercial microwave reactor to anneal block copolymer films in the presence of appropriate solvents, and we explore the effect of various parameters over the polymer assembly speed and defect density. The approach is applied to the commonly used poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) and poly(styrene)-b-poly(2-vinylpyridine) (PS-b-P2VP) families of block copolymers, and it is found that the substrate resistivity, solvent environment, and anneal temperature all critically influence the self-assembly process. For selected systems, highly ordered patterns were achieved in less than 3 min. In addition, we establish the compatibility of the technique with directed assembly by graphoepitaxy.

Flexible electronics under strain: a review of mechanical characterization and durability enhancement strategies

Flexible electronics incorporate all the functional attributes of conventional rigid electronics in formats that have been altered to survive mechanical deformations. Understanding the evolution of device performance during bending, stretching, or other mechanical cycling is, therefore, fundamental to research efforts in this area. Here, we review the various classes of flexible electronic devices (including power sources, sensors, circuits and individual components) and describe the basic principles of device mechanics. We then review techniques to characterize the deformation tolerance and durability of these flexible devices, and we catalogue and geometric designs that are intended to optimize electronic systems for maximum flexibility.

Indium tin oxide nanopillar electrodes in polymer/fullerene solar cells

Using high surface area nanostructured electrodes in organic photovoltaic (OPV) devices is a route to enhanced power conversion efficiency. In this paper, indium tin oxide (ITO) and hybrid ITO/SiO(2) nanopillars are employed as three-dimensional high surface area transparent electrodes in OPVs. The nanopillar arrays are fabricated via glancing angle deposition (GLAD) and electrochemically modified with nanofibrous PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate)). The structures are found to have increased surface area as characterized by porosimetry. When applied as anodes in polymer/fullerene OPVs (architecture: commercial ITO/GLAD ITO/PEDOT:PSS/P3HT:PCBM/Al, where P3HT is 2,5-diyl-poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C(61)-butyric acid methyl ester), the air-processed solar cells incorporating high surface area, PEDOT:PSS-modified ITO nanoelectrode arrays operate with improved performance relative to devices processed identically on unstructured, commercial ITO substrates. The resulting power conversion efficiency is 2.2% which is a third greater than for devices prepared on commercial ITO. To further refine the structure, insulating SiO(2) caps are added above the GLAD ITO nanopillars to produce a hybrid ITO/SiO(2) nanoelectrode. OPV devices based on this system show reduced electrical shorting and series resistance, and as a consequence, a further improved power conversion efficiency of 2.5% is recorded.

Molecular electronics using diazonium-derived adlayers on carbon with Cu top contacts: critical analysis of metal oxides and filaments

Evaporation of Cu metal onto thin (less than 5xa0nm) molecular layers bonded to conductive carbon substrates results in electronic junctions with an ensemble of molecules sandwiched between two conductors. The resulting devices have previously been characterized through analysis of current density-voltage (j-V) curves for several different molecular layers and as a function of layer thickness. The approach represents an ensemble rather than single molecule technique, in which the electronic response represents that of a large number of molecules (10(6)-10(12)) in parallel as well as the conducting contacts contained in the molecular junction. In this paper, we extend a more detailed investigation of two critical issues: the possibility of conduction by metal filaments, and the potential role of top contact oxidation contributing to the electronic properties of the junctions. The results show that the conductance of the junctions can be modulated by changes in the deposition environment, but that the changes are not related to Cu oxide in the top contact. Based on these results, we propose that the conditions during top contact deposition change the way in which the molecules contact the metal, leading to differences in the effective junction area. Finally, through systematic studies using variation of the temperature, we show that metal filament conduction is distinct from that observed for the molecular junctions and that if the current observed experimentally passed through nanoscopic metal filaments the Joule heating would lead to rapid melting. For a series of junctions with structurally related aromatic molecules (including biphenyl, nitrobiphenyl, fluorene, and nitroazobenzene), the electron transfer mechanism is briefly investigated using area-independent analysis methods. It is shown that field emission and/or transport through bands formed by the molecular layer is likely, based on the weak temperature dependence of junction conductance.

Photopatterned liquid crystalline polymers for microactuators

We have investigated the properties of thermally-responsive, patterned liquid crystalline polymers as their dimensions are scaled to a size suitable for use in microelectromechanical systems. All samples were fabricated using surface alignment and photopatterning techniques that can be used to produce integrated devices anchored on a substrate. The thermomechanical properties of free-standing macroscopic samples with varying concentration of crosslinking molecules were investigated in order to optimize the thermal response of the material. It was found that samples containing 12% crosslinker were able to expand by up 19% when heated. The thermomechanical properties of surface-anchored films were also investigated, and it was found that by employing a polymerized cholesteric structure of the liquid crystalline units a thermal expansion of up to 11% could be achieved when the sample was heated to 200 °C. Patternability was demonstrated using a simple photopatterning process that was used to fabricate samples consisting of lines of cholesteric material on a bare substrate, or alternating regions of liquid crystalline polymer in the isotropic and cholesteric phases. Actuation of these films was also demonstrated.

Physical Properties of Anisotropically Swelling Hydrogen-Bonded Liquid Crystal Polymer Actuators

Glassy polymeric actuators are described which reversibly deform in response to changes in pH and/or the presence of water. Hydrogen-bonded liquid crystalline monomers act as precursors, and these materials are photopolymerized (without mechanical drawing) into monodomain nematic networks. We discuss the influence of film composition, polymerization conditions, and chemical treatments on the properties of these anisotropic networks. We show that large-amplitude reversible motion can be generated in response to small changes in pH. Pretreatment in basic solutions also activates a response to water, and this effect is investigated in detail. Elastic moduli, which are directly related to the generation of work, have not been reported previously, and therefore particular attention is given to mechanical properties. A marked anisotropy is observed in the moduli parallel and perpendicular to the nematic director. We find that pretreatment in basic solutions reduces this anisotropy yet preserves magnitudes on the order of 2 GPa. Employing complex director profiles (such as the twisted configuration), extremely large amplitude bending-mode deformations are induced in response to small pH changes, immersion in water, or variations in the relative humidity. Since the elastic moduli are large, work output significantly greater than conventional liquid crystal elastomers is predicted

Microchannel Surface Area Enhancement Using Porous Thin Films

Microchannels having highly enhanced surface area have been fabricated by a thin-film deposition technique known as glancing angle deposition. Porosimetry and simulation of the microstructures was used to estimate the surface area enhancement of the new microchannels. Two distinct types of microchannels have been created. In the first, a pre-existing microchannel is coated with a film of SiO 2 deposited at a highly oblique angle. The resulting structure consists of the original channel filled with slanted columns of SiO 2 . An example of this type of channel was estimated by porosimetry to have surface area of 517 cm 2 /cm 2 . The second microchannel type is produced by lithographic methods and consists of films of SiO 2 with helical microstructure bounded by walls of photoresist. Simulation of one example of this type of channel led to an estimate of 42 cm 2 /cm 2 . This estimate, however, ignores the mesoscopic scale surface roughness of the microstructures.

Automated Defect and Correlation Length Analysis of Block Copolymer Thin Film Nanopatterns

Line patterns produced by lamellae- and cylinder-forming block copolymer (BCP) thin films are of widespread interest for their potential to enable nanoscale patterning over large areas. In order for such patterning methods to effectively integrate with current technologies, the resulting patterns need to have low defect densities, and be produced in a short timescale. To understand whether a given polymer or annealing method might potentially meet such challenges, it is necessary to examine the evolution of defects. Unfortunately, few tools are readily available to researchers, particularly those engaged in the synthesis and design of new polymeric systems with the potential for patterning, to measure defects in such line patterns. To this end, we present an image analysis tool, which we have developed and made available, to measure the characteristics of such patterns in an automated fashion. Additionally we apply the tool to six cylinder-forming polystyrene-block-poly(2-vinylpyridine) polymers thermally annealed to explore the relationship between the size of each polymer and measured characteristics including line period, line-width, defect density, line-edge roughness (LER), line-width roughness (LWR), and correlation length. Finally, we explore the line-edge roughness, line-width roughness, defect density, and correlation length as a function of the image area sampled to determine each in a more rigorous fashion.

Large-area microfabrication of three-dimensional, helical polymer structures

A technique has been developed to fabricate polymeric helices with sub-micron dimensions. These helices are made using a double-templating process, in which an inorganic thin film deposited using glancing angle deposition acts as the master. The shape, pitch, handedness and number of turns of the polymer helices can be tuned by altering the deposition parameters of the master film. The structure of this positive master is copied into a negative intermediate template of photoresist, which itself acts as a master for the templating of polymer helices. This process is demonstrated with four multifunctional acrylates. The master, intermediate template and polymer helices are characterized using scanning electron microscopy, and the polymer helices are characterized using energy dispersive x-ray spectroscopy. It is shown that a large number of polymer helical microstructures, which are anchored to both a thick substrate and a thin capping layer, can be made in parallel over areas of mm2 to cm2.