A major research theme in the group is the use of spatial confinement of semiconducting polymers to produce new optical materials and understand how polymer conformation affects the underlying physics of polymer optical and electrical properties. Examples of work in this area include our recent accomplishments with highly polarized, optical quality thin films and our efforts to use pore size to selectively control polymer conformation.
In the first of these experiments, we take advantage of a very fruitful collaboration with Canon basic research in Japan. In the Canon labs, research scientists have discovered ways to produce hexagonal honeycomb surfactant-templated porous silicas with uniaxial (in-plane) alignment of the pores. Incorporation of polymer into these films produces highly anisotropic optical materials that show a strong polarization dependence in both absorption and emission.1 Moreover, the well-defined polymer geometry allows us to address some fundamental questions about polymer photophysics, such as the orientation of the dipole and polarons with respect to the chain, exciton annihilation and energy transfer. The process of polymer incorporation and conductivity of these highly oriented polymers is currently under investigation.
By varying the size of the pores in our hexagonal honeycomb-structured material, we can also determine how spatial confinement can be used to control polymer conformation.2 For example, we find that small pores (~2 nm diameter) produce isolated, straight chains, medium pores (~5 nm) allow for multiple chains per pore but keep the polymer chains extended and parallel, while large pores (> 8 nm) allow for multiple polymer chains per pore but now allow these chains to coil up as they do in a polymer film. This degree of control means that the same polymer can now be placed in many different conformations and the photophysics of that material can be examined. For example, we have used CW photo-induced absorption, light-induced ESR, and optically detected magnetic resonance (ODMR) to examine how polymer conformation controls the ability to produce free carriers upon photoexcitation. We find that single polymer chains produce free carriers with low probability; this may be because an interchain exciton that is delocalized across multiple chains is needed to facilitate the process of charge separation. Free carriers can be produced in samples with multiple, parallel polymer chains, but these carriers have short lifetimes. Once the chains are allowed to coil, long lived carriers are produced, indicating that kinks in the polymer chains serve as trap sites for polarons.
Complementary to these host/guest polymer experiments, we also have a variety of experiments in collaboration with both the Rubin and Wudl groups here at UCLA to use amphiphilic semiconducting polymers to directly template periodic inorganic phases.4 Such direct assembly removes many of the tedious and inefficient aspects of our polymer host/guest chemistry. Various experiments make use of both side chain amphiphiles and amphiphilic diblock copolymers. The goal of the two main projects involving direct templating by semiconducting amphiphilic polymers are to make nanostructured photovoltaic cells (using side chain amphiphiles) and batteries (using block copolymer amphiphiles).
Small metallic nanoparticles are potentially useful for magnetic recording and other nanoscale magnetic applications. Unfortunately, due to finite size effects in these nanocrystals, they become superparamagnetic at room temperature. However, we have shown that spatial confinement can be used to control interactions between nanoscale magnets.5 In this work, superparamagnetic cobalt nanocrystals are incorporated into the long straight pores of a hexagonal honeycomb-structured silica. By allowing the magnets to couple only in rows, we find that it is possible to produce pseudo-anisotropic magnetic nanocrystals which show much harder magnetic behavior than the starting nanocrystals. We are also interested in understanding how different confining geometries (e.g. cubic, hexagonal honeycomb), interparticle distances, and nanoparticle size, crystallographic structure, and shape affect magnetic coupling. For materials where the confining host is a semiconductor such as titania, we also want to examine if the nanocrystal guest species can couple with conduction electrons in the framework, producing magnetoresistive effects. The goal of all these experiments is to understand and show how nanoscale spatial confinement is a powerful method for controlling materials properties and producing anisotropic materials behavior.
1. W. Molenkamp, M. Watanabe, H. Miyata, and S.H. Tolbert, “Highly Polarized Luminescence from Optical Quality Films of a Semiconducting Polymer Aligned within Oriented Mesoporous Silica.” J. Am. Chem. Soc. in press.