The interfacial regions between phases are sites of critical importance in many relevant processes and technologies. The catalysis of chemical reactions by metals, the corrosion of metals, the pollution of groundwater by toxic chemicals released from soil surfaces, the organization of surfactants at liquid-liquid interfaces important in phase-transfer catalysis, and the conversion of chlorofluorocarbons to reactive chlorine species which destroy ozone in the upper atmosphere are all examples of important chemical processes which occur at surfaces or within interfaces. Despite decades of intense study, our understanding of the chemistry of these and similar interfacial and surface processes at the molecular level is still only modestly developed. Thus, the development of adequate tools with which to study surface and interfacial chemistry and elucidation of the molecular details of such complex chemistry represent two of the most exciting frontiers of modern measurement science.
Our research seeks to develop an understanding of such chemistry in several technologically important areas including organic semiconductor-based photonics devices such as organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs), surface wetting and lubrication, organized assemblies including self-assembled layers and surfactant systems, and environmental systems. Methodologies employed for these efforts include surface vibrational spectroscopies, electrochemistry, surface electron spectroscopies, work function measurements, ellipsometry, fluorescence microscopy,electron microscopy, the scanning probe microscopies (AFM and STM), Langmuir trough methods. These methods are supplemented by more conventional chemical measurement tools (e.g. mass spectrometry, NMR spectroscopy, FTIR spectroscopy, fluorescence spectroscopy) as needed for complete characterization of relevant solution and interfacial systems.
Specific systems of current interest include:
The Chemistry of Organic Semiconductors and their Interfaces in Molecular Electronic & Photonic Devices
Solid-state organic semiconductor (OSC) materials are the essential active components of a variety of existing and emerging technologies, including LED lasers and organic-based electronic and photonic devices. These technologies suffer from limited lifetimes due to the inevitable degradation of OSCs through poorly defined chemical, photochemical and photophysical processes, especially in operando, or under conditions of operation. Addressing these limitations requires characterization of the molecular structural, electronic, and charge transport attributes of these OSC materials during and after degradation. Toward this end, in collaboration with Dr. Erin Ratcliff of the University of Arizona Department of Materials Science and AEngineering, we are employing a suite of state-of-the-art methods to study OSCs in environments that systematically increase in chemical and optoelectronic complexity, including in operando. Tools being used include surface vibrational spectroscopies (Raman, PM-IRRAS) in environments ranging from ambient to ultrahigh vacuum, x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and charge mobility measurements. Exceptional among these tools are our capabilities for UHV Raman spectroscopy for detailed chemical characterization, which are being implemented in operando. When coupled with UPS/IPES, for full electronic characterization, and correlated with results from traditional charge mobility measurements, the goal of this effort is to provide the most complete picture to date of degradation in OSC devices. The following three objectives, along with the relevant questions being addressed through this work, represent the foci of our efforts:
1. Intrinsic OSC chemistry. What roles do intrinsic chemical nature, degree and type of crystallinity, and frontier orbital energetics of the OSC play in dictating chemical, photochemical, and photophysical degradation of the OSC? How do different chemical penetrants contribute to or drive these degradation pathways as a function of time and exposure? What role do thermal versus photolytic pathways play in OSC degradation?
2. Effects of "intrinsic" catalysts and materials interfaces. What roles do catalyst species, introduced as impurities during the fabrication process, created through unintended chemical reactions during the fabrication process, or created through degradation reactions, play in initiating or accelerating degradation pathways? What role do contacts of the OSC with transparent conductive oxide or metallic electrodes play in initiating or accelerating degradation pathways?
3. In operando effects. What role does the presence of an applied electric field play in promoting or altering the chemical, photochemical, or photophysical degradation pathways elucidated above in Objectives 1 and 2, or in changing their mechanisms? What role does the presence of excess charge carriers near OSC-electrode contacts and interfaces with other active materials play in promoting or altering the chemical, photochemical, or photophysical degradation pathways elucidated above?
Green' Microbially-Produced and Bio-Inspired Surfactants
Surfactants are critical, ubiquitous chemicals in many consumer products areas, technologies, and industrial processes. The ubiquity of surfactants as essential chemicals and their massive industrial use, coupled with emerging concerns about their long-term environmental impacts, present a compelling opportunity for advances in molecular design and synthesis of green surfactants. Widespread interest in these issues supports exploration of all viable alternatives to meet the increasing demands of the global surfactant market, predicted to reach $40B annually by 2018.The toxicity of many conventional synthetic surfactants as well as their persistence in the environment is driving regulatory and consumer pressure to develop greener and more sustainable alternatives to these materials. This effort combines the scientific expertise of four chemists at the University of Arizona, Jeanne E. Pemberton, Robin L. Polt, Steven D. Scharwtz, and Hamish S. Christie, an environmental microbiologist in the UA Department of Soil, Water and Environmental Science, Raina M. Maier, and a toxicologist in the UA Department of Pharmcology and Toxicoogy Walter T. Klimecki, at the University of Arizona to explore the systematic design, synthesis, and characterization of a wide array of new glycolipid surfactants. This effort builds on our recent efforts to develop a versatile, green synthesis of glycolipids. We are further exploring this methodology through synthesis of target glycolipids identified by computational methods to possess excellent surfactant properties in solution and at surfaces. Tight interplay between molecular design based on computation, synthesis, and characterization, along with the versatility, ease and low cost of our synthetic approaches, is enabling a comprehensive determination of molecular structure-glycolipid function. We are improving our synthetic strategy to make it greener using univariate green metrics as quantitative benchmarks for assessment and are exploring the use of renewable reagents from natural resources for synthesis of glycolipids. Solution aggregation behavior of surfactants is characterized with a comprehensive suite of techniques to determine critical micelle concentration and aggregate shape and size. Surface adsorption and aggregation is also characterized (adsorption affinity, surface coverage, orientation, surface aggregate structure). Experimental solution and surface properties will be correlated with those predicted by computational approaches. Biodegradability of surfactants is assessed by measurement of CO2 production. Both eco- and cytotoxicity screening of desirable surfactant candidates is being undertaken as well using vibreofischeri and two mammalian cell lines to determine IC50 values. The ultimate goal of this workis to provide important new insight into surfactant structure-function relationships that can drive future advances in molecular design for effieicnet and green surfactants.