Research Areas

Extreme ultra-violet (EUV) lithography continues to make good progress in sensitivity, line-edge roughness (LER) and resolution goals but in many cases current photoresists have yet to achieve industry goals. A major challenge remains the issue of stochastics, a looming problem that has not yet been addressed in any resist system. Therefore, a new approach to low exposure dose, sensitive photoresist which do not suffer from low photon stochastics is needed and we feel that scissionable, depolymerizing photoresists are excellent candidates to fill that need. We plan to include photoactive compounds, control molecular weight and distribution and use these aspects of molecular structure control to address the problem of stochastics with these patterning materials.

Chain scissionable polymers are usually known to have a low ceiling temperature (T_c) and can undergo controlled degradation in response to various external stimulus such as light, heat, pH or oxidation conditions. They were mostly inspired by the analogous dendritic structures composed of "self-immolative spacers" that were developed for pro-drug chemistry. Their polymer chains usually contain weak chemical bonds, which can be cleaved by external triggers and thus starting the depolymerization process. Due to their interesting properties, they have been widely utilized as sacrificial components in many areas, such as lithography, drug delivery, self-healing materials and transient electronics. Our study currently focuses on both poly(phthalaldehyde) and poly(olefin sulfone) systems. As we investigate these materials, we will determine which is the best strategy based on variables such as lithographic performance, shelf stability, effect of stochastics, and the like.

Polymer brushes have recently attracted considerable interest for generating molecularly defined surfaces for applications in nanotechnology, molecular biology, and biomedical sciences. Two main advantages of using polymer brush systems are the ability to mitigate non-specific adsorption and the creation of tailor-made surfaces to control the immobilization of bioanalytes through specific receptor recognition interactions. The use of polymer brushes allows the formation of uniform surfaces with controlled chemical architecture that exhibit good chemical and thermal stability.

Currently we are in collaboration with other groups to develop a microfluidic electrochemical biosensor that utilizes polymer brushes for the detection of antibodies. The ability to detect selective antibodies is essential for diagnosing infectious diseases and advancing medical applications. Our device is tailored to eliminate non-specific absorption and other limitations associated with current assays, but can be modified for the identification of antibodies specific for any infectious agent.

Another area of polymer brushes being explored is that of patterned brushes, which can be used to create surfaces with tailored surface properties. Various patterning methods have been used to fabricate patterned polymer brushes, the most conventional methods involve patterning of surface immobilized initiator followed by surface initiated polymerization. To simplify the process, our group patterns features in the nanometer regime by direct e-beam lithography. Currently, we are investigating the responsive nature in different solvents of binary brush systems at selected feature sizes. Polymer brushes are chosen by their ability to swell in a good solvent and cover the neighboring brush which is in a collapsed state. Other patterning projects include exploring the morphology of e-beam patterned block copolymer brushes. Our results show that pattern size and solvent treatment influence the morphology of phase separated block copolymer brushes.

Fouling of marine surfaces is a historically difficult and costly economic problem; build-up of micro-organisms, algae, and calcareous species on the surface of ship hulls results in decreased fuel efficiency and maneuverability, increased maintenance frequency and costs, and increased spread of invasive species. Many commercially available antifouling paints incorporate harmful biocides that have detrimental side effects on the marine environment, so there is a growing need for benign alternatives.

There are currently over 4000 known marine fouling species that use a diverse set of chemical and physical mechanisms to adhere to marine surfaces, and to combat this requires complex, multifunctional materials. Our research focuses on the development of a biocide-free antifouling and fouling release coating (to prevent settlement and to enhance removal by shear forces, respectively) that is universally effective. Using a system of modifiable surface-active block copolymers (SABC), we have systematically screened and identified a variety of functionalities for their performance against marine organisms. Now, in collaboration with the Segalman group in Chemical Engineering at the University of California – Santa Barbara, we are using sequence-controlled chemistry to include a variety of hydrophobic, hydrophilic, and active chemistries that alter the SABC surface properties. This gives us the ability to precisely tune the surface characteristics of our coatings and to optimize their performance against a wide variety of marine species.

A start-up company, Nanosurfaces Inc., was set up to commercialize these polymer formulations for foul release coatings.

This program between Cornell, Chicago and U Washington is a joint experimental and computational study of the synthesis, self-assembly and electronic and ionic conduction characteristics of a series of new conjugated polymers and liquid crystals. As an example, in a study of a liquid crystal consisting of terminal tetraethyleneglycol groups on both ends of a quaterthiophene core molecular dynamic simulations done expressly predicted spontaneous formation of a highly ordered smectic phase in agreement with temperature dependent grazing-incidence wide angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD). Significantly, this ordered smectic phase is maintained upon blending with bis(trifluoro-methanesulfonyl)imide (LiTFSI) as ion source at a concentration ratio up to r = [Li+]/[EO] = 0.05. Nanosegregation between oligothiophene and PEO moieties and pi-pi stacking of thiophene rings lead to the formation of efficient 2D pathways for ion transport, resulting in thin film in-plane ionic conductivity as high as 5.2 x 10^-4 S/cm at 70C and r = 0.05 as measured by electrochemical impedance spectroscopy (EIS).

Upon heating the samples above the transition temperature, a significant loss of liquid crystal layer ordering and a pronounced drop in ionic conductivity is observed. Upon cooling, partial and local crystallization of the quaterthiophene core leads to misalignment of smectic domains, causing an ionic conductivity decrease compared to the as-cast state. Our results highlight the critical roles of pi-pi interactions in dictating the self-assembly behavior of this class of conjugated liquid crystals and the consequent implications for ionic transport properties. Doping with F4TCNQ led to electronic conductivity as long as the quaterthiophene retained pi-pi stacking of thiophene rings. Work in progress includes additional liquid crystal and ordered block copolymer materials with both ionic and electronic conductivity.

Polymer-grafted nanoparticles (PGNs) were synthesized by mini-emulsion polymerization using Activator Regenerated Electron Transfer (ARGET)/Atom Transfer Radical Polymerization (ATRP). Initially, the surface of mono-disperse silica nanoparticles (25 nm) were modified by an ATRP initiator and different polymers such as Poly(methyl methacrylate), Polystyrene and Poly(N-isopropylacrylamide) were grafted on the particle surface. Thereby, phase transfer agents, such as tetrabutylammonium bromide (TBAB), assists the transfer of monomer and the ATRP catalyst-complex to the particle surface for a locally controlled polymerization. Furthermore, the brush canopy size, graft density and molar mass of the grafted polymer chains can be controlled by changing the monomer feeding rate and the monomer concentration. The resulting well-defined PGNs are an important building block for the creation of functional superlattices by changing the entanglement and the interactions between the PGNs. This work opens the potential for photonic devices and sensors.