Conducting Polymer Based Photonic Crystals
Yun-Ju Lee, Former Graduate Student
Department of Materials Science and Engineering
The general area of my research involves the synthesis and characterization of mesostructured materials inside a template. A template consists of two of more types of structured nanocompartments that are either chemically or physically distinct; examples include lyotropic liquid crystals formed by self-organization of a amphiphile-solvent mixture and colloidal crystals formed by close-packing of colloidal suspension during controlled drying. By synthesizing the target material only inside one type of nanocompartments, we can create materials with a three-dimensional array of pores that are adjustable in size and geometry (Figure 1). Such templated materials may exhibit improved and even novel transport, electronic, and optical properties that are not easily achieved with conventional lithography.
The current focus of my research is on synthesis of mesostructured conducting polymer films inside colloidal crystals using electrochemistry. Conducting polymers generally contain p-bonding and delocalized electrons along the backbone chains, yielding a semiconducting polymer that can be reversibly tuned through doping, an oxidation/reduction process where charge carriers are introduced to the polymeric backbone either chemically or electrochemically. Largely due to the reversibility of doping, conducting polymers may be suitable for applications such as chemical sensing, energy storage, and photonic switching. Electrochemical polymerization is an effective method of synthesizing mesostructured conducting polymer films since the material grows from the electrode/solution interface, leading to complete infilling of the interstitial space within the colloidal crystal, which can then be etched to generate the inverse opal structure. Introduction of mesoporosity into a conducting polymer film may improve its properties by enhancing mass transport of counter ions associated with the doping process. This effect has already been demonstrated in the case of randomly mesoporous conducting polymers, which exhibit capacitance values several times higher than equivalent bulk films [5,6]. Many theories have tried to explain this enhanced transport phenomenon in mesostructured conducting polymers. However, the exact effect of mesoporosity on conducting polymer properties remains unclear, because a conducting polymer in operation consists of complex ionic and electronic transports between three phases (electrode, conducting polymer, and electrolyte solution). It is hoped that by characterizing templated conducting polymer films with well controlled pore structure, we can help elucidate the properties of mesoporous conducting polymers. As a proof of concept, we have shown that poly(pyrrole) films electrochemically synthesized within silica colloidal crystals replicates the porosity of the colloidal crystals (Figure 2a) and exhibit higher charging capacities than their bulk counterparts (Figure 2b), and work is currently ongoing in this project. Conducting polymers synthesized inside low defect density colloidal crystals may also exhibit interesting novel properties. For example, colloidal crystals possess three-dimensional Bragg diffraction due to the periodic colloidal crystal structure and refractive index contrast between the colloids and the interstitial voids. By adjusting the refractive contrast either thermally or chemically, surface fictionalized colloidal crystals have been shown to demonstrate optical switching and chemical sensing capabilities [7,8]. Because the refractive index of a conducting polymer changes significantly during doping, mesoporous conducting polymers may also exhibit such qualities, and this is another area of current research.
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