Technical: The goal of this project is to develop systematic surface modification strategies to improve the interrelated issues of morphology, energetics, and charge transport across organic/inorganic interfaces. The approach is based on difunctional molecular layers. One end is designed for surface attachment while the other end imparts a special chemical or electronic characteristic to the modified interface. Research will be directed toward developing surface molecular layers on semiconducting oxides, with a special emphasis on ZnO/polymer blends. The overall focus of the work is on identifying design rules that could be applied to a broader class of oxide interfaces and organic constituents, which would simplify future development of organic/inorganic composites. Advanced optical, structural, surface, and electronic characterization techniques will be applied to these systems to develop an understanding of the properties of the specific functionalized surfaces under study, and define a systematic approach to uncovering the interfacial morphological and electronic properties of organic/inorganic hybrid materials.
The project addresses basic research issues in a topical area of materials science having high technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new understanding and capabilities for potential next generation electronic/photonic devices. Organic/inorganic composites have the potential for creating electronic materials with new functionality in energy efficiency, biological, medical, and environmental applications. This project will accelerate the development of these materials by providing fundamental understanding of one of the key issues limiting performance. The proposed project directly integrates research with education. Students and faculty will work with an existing outreach program at the Colorado School of Mines (CSM) aimed at Denver elementary and middle schools with large populations of underrepresented groups. Modules, which connect basic science concepts to the impact that advanced materials are having on the world, will be introduced into these schools. Approaches developed during the project will also be included in a teacher recertification program taught by one of the Co-PIs. The project includes significant undergraduate research involvement through summer REU opportunities and an international student exchange with NTNU (Norwegian University of Science and Technology). All of the students will work on a daily basis as part of an integrated team with their advisors, collaborators at the National Renewable Energy Laboratory (NREL) which is located in close proximity to CSM, with scientists at NTNU, and with colleagues at the University of Arizona. A focus on diversity, communication skills, and developing skills for work in a team environment are all integral parts of the proposed program. This project is co-funded by the DMR Solid State and Materials Chemistry program, and the DMR Electronic and Photonic Materials program.
The central goal of this project has been to understand and control the interface between metal oxides and organic electronic materials through molecular modifications of the oxide surface. Organic/inorganic hybrid materials are actively researched for applications extending from organic photovoltaics and light emitting diodes to bioinorganic sensors. These are often nanostructured materials with large interfacial area. The properties of the interface between the organic and inorganic constituents can both limit performance and enable properties neither material possesses alone. Through this project, modifications of a prototypical thin film electron accepter, ZnO, and hole acceptor, NiOx, were developed that allowed interfacial properties between the organic and metal oxide to be tuned and optimized. Of particular importance was the ability of dipolar molecules to tune the work function of the metal oxides over wide ranges and effect charge transport energetics at metal oxide/organic interfaces. A broad range of structural, optical, and electronic characterizations were utilized to develop a fundamental picture of monolayer attachment chemistries and interfacial energetics. Central scientific outcomes of the project included: 1) A comprehensive comparison of different attachment families (e.g. thiols, silanes, carboxylic acids, and phosphonic acids) including the quality of the layers they form, the nature of their bonding, and undesirable effects like etching. In particular, development of an attachment chemistry to both ZnO and NiOx for covalently bonded triethoxysilanes molecular layers is a first in a field which has tended to emphasize organic acids. 2) Demonstration of continuous tuning of the work functions of ZnO and NiOx over more than 500meV using mixed monolayers of two triethoxysilanes with different dipolar end groups. 3) When ZnO is treated with organic acids, there is often unwanted etching of the surface that occurs in parallel with molecular attachment. This project has provided the most comprehensive picture to date of the competition between bonding and dissolution in this system. The work defines how to characterize and control attachment of organic acids and associated dissolution and helps explain inconsistencies in prior published studies of organic acid treatments of ZnO. Alloying ZnO with more than 10% MgO was shown to greatly enhance etch resistance which has implications for dye sensitized solar cell fabrication. 4) Development of "designer" molecules with end groups that have strong electron donating and accepting character which lead to large dipole moments and shifts in metal oxide work function of up to 1eV. Density Functional Theory simulations of dipole moments of molecules of interest guided this work. 5) Four archival journal articles have been published. A fifth has been submitted with two others are in preparation. Over the course of the project, five invited presentations and 20 contributed presentations directly discussing project research results have been made by the co-PIs, graduate, and undergraduate students involved in the project. Human resource development was integral to the project. Four graduate students were directly involved in cutting edge research. They have become proficient in surface and bulk structural and electronic characterization techniques and became experts in molecular bonding of monolayers to oxide surfaces and the properties of oxide films. They learned how to synthesize organic solar cells and interpret solar cell metrics. They attended seminars at the National Renewable Energy Laboratory (NREL) and within the Renewable Energy Materials Research Science and Engineering Center at the Colorado School of Mines (CSM), which expanded their understanding of materials science and renewable energy. They collaborated with scientists at NREL and used NREL facilities. They assisted with K-12 outreach and in mentoring undergraduate research projects. One student has graduated and is employed at a university. The second and third will graduate soon. One has secured a prestigious post doc overseas. The fourth will continue thesis work jointly with NREL. Involvement of undergraduates in cutting edge research was an area of emphasis. Over the course of the project, eight summer Research Experience for Undergraduate (REU) students participated in their first "open ended" research experience. Two were international REUs with collaborators in Norway. An additional 14 undergraduates participated through senior thesis projects. Many have been co-authors on publications. Their written and oral presentation skills significantly improved as a result of their involvement. Those that have already graduated have gone on to graduate schools or technical positions in the private sector. The co-PIs and their students participated in a summer outreach program for elementary and middle school teachers from disadvantaged school districts. Graduate students in the project developed and taught a week long experiential module introducing basic concepts at the heart of solar energy to middle school students in CSMs Multicultural Engineering Program. The PIs believe strongly in the benefit that diversity brings to the research enterprise. Cultural diversity through the Norwegian collaborators benefitted all of the students in each group. Women and minorities were well represented in the program and contributed significantly to the overall accomplishments.
