Publications

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Electro-optical organic materials hold great promise for the development of high-efficiency devices based on exciton formation and dissociation, such as organic photovoltaics (OPV) and organic light-emitting devices (OLEDs). However, the external quantum efficiency (EQE) of both OPV and OLEDs must be improved to make these technologies economical. Efficiency rolloff in OLEDs and inability to control morphology at key OPV interfaces both reduce EQE. Only by creating materials that allow manipulation and control of the intimate assembly and communication between various nanoscale excitonic components can we hope to first understand and then engineer the system to allow these materials to reach their potential. The aims of this proposal are to: 1) develop a paradigm-changing platform for probing excitonic processes composed of Crystalline Nanoporous Frameworks (CNFs) infiltrated with secondary materials (such as a complimentary semiconductor); 2) use them to probe fundamental aspects of excitonic processes; and 3) create prototype OPVs and OLEDs using infiltrated CNF as active device components. These functional platforms will allow detailed control of key interactions at the nanoscale, overcoming the disorder and limited synthetic control inherent in conventional organic materials. CNFs are revolutionary inorganic-organic hybrid materials boasting unmatched synthetic flexibility that allow tuning of chemical, geometric, electrical, and light absorption/generation properties. For example, bandgap engineering is feasible and polyaromatic linkers provide tunable photon antennae; rigid 1-5 nm pores provide an oriented, intimate host for triplet emitters (to improve light emission in OLEDs) or secondary semiconducting polymers (creating a charge-separation interface in OPV). These atomically engineered, ordered structures will enable critical fundamental questions to be answered concerning charge transport, nanoscale interfaces, and exciton behavior that are inaccessible in disordered systems. Implementing this concept also creates entirely new dimensions for device fabrication that could both improve performance, increase durability, and reduce costs with unprecedented control of over properties. This report summarizes the key results of this project and is divided into sections based on publications that resulted from the work. We begin in Section 2 with an investigation of light harvesting and energy transfer in a MOF infiltrated with donor and acceptor molecules of the type typically used in OPV devices (thiophenes and fullerenes, respectively). The results show that MOFs can provide multiple functions: as a light harvester, as a stabilizer and organizer or the infiltrated molecules, and as a facilitator of energy transfer. Section 3 describes computational design of MOF linker groups to accomplish light harvesting in the visible and facilitate charge separation and transport. The predictions were validated by UV-visible absorption spectroscopy, demonstrating that rational design of MOFs for light-harvesting purposes is feasible. Section 4 extends the infiltration concept discussed in Section to, which we now designate as "Molecule@MOF" to create an electrically conducting framework. The tailorability and high conductivity of this material are unprecedented, meriting publication in the journal Science and spawning several Technical Advances. Section 5 discusses processes we developed for depositing MOFs as thin films on substrates, a critical enabling technology for fabricating MOF-based electronic devices. Finally, in Section 6 we summarize results showing that a MOF thin film can be used as a sensitizer in a DSSC, demonstrating that MOFs can serve as active layers in excitonic devices. Overall, this project provides several crucial proofs-of- concept that the potential of MOFs for use in optoelectronic devices that we predicted several years ago [ 3 ] can be realized in practice.

Electrostatics plays an important role in the self-assembly of amphiphilic peptides. To develop a molecular understanding of the role of the electrostatic interactions, we develop a coarse-grained model peptide and apply self-consistent field theory to investigate the peptide assembly into a variety of aggregate nanostructures. We find that the presence and distribution of charged groups on the hydrophilic branches of the peptide can modify the molecular configuration from extended to collapsed. This change in molecular configuration influences the packing into spherical micelles, cylindrical micelles (nanofibers), or planar bilayers. The effects of charge distribution therefore have important implications for the design and utility of functional materials based on peptides. © 2014 American Chemical Society.

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Controlling the size and shape of nanopores in polymer membranes can significantly impact transport of molecular or ionic species through these membranes. Here we describe a facile method to controllably form conical nanopores in ion-tracked polycarbonate membranes. Commercial polycarbonate ion-tracked membranes were placed between a concentrated alkaline solution and an acidic solution. By varying the height of the acidic solution, the hydrostatic pressure was controlled, regulating the acid flux through the nanopores. The resulting asymmetric etching of the membrane produced conical pores with controllable aspect ratios. Scanning electron microscopy of both the pores and nickel nanostructures electrolessly templated in the pores confirms their conical shape. This safe, straightforward approach obviates the need to use large voltages, currents, and/or plasma etching equipment traditionally employed to create conical nanopores. © 2014 The Royal Society of Chemistry.

Controlling the materials chemistry of the solid-state ion conductor NaSICON is key to realizing its potential utility in emerging sodium-based battery technologies. We describe here the influence of excess sodium on phase evolution of sol-gel synthesized NaSICON. Alkoxide-based sol-gel processing was used to produce powders of Na3Zr2PSi2O12 NaSICON with 0-2 atomic % excess sodium. Phase formation and component volatility were studied as a function of temperature. NaSICON synthesis at temperatures between 900-1100C with up to 2% excess sodium significantly reduced the presence of zirconia, sodium phosphate, and sodium silicate secondary phases in fired NaSICON powders. Insights into the role of sodium on the phase chemistry of sol-gel processed NaSICON may inform key improvements in NaSICON development.

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