Technical: This project uses interface-selective vibrational sum frequency generation spectroscopy, a nonlinear optics technique, to study molecular vibrations and their orientations at the active interface in organic field-effect transistors. The object of the project is to characterize the interface prior to and during device operation to gain insight into the relationships between interfacial molecular structure and function. It aims to advance the level of understanding not only in the fields of materials science and engineering where the devices are developed and tested, but also at the level of molecular design and synthesis by prescribing the features that need to be built into next generation molecules for efficient materials performance. The in-situ nature of experiments performed during organic field-effect transistor activation will provide a new perspective on the materials response from inside the functioning device, with the potential to describe not only what changes, but also what the timescales of change are and how these properties relate back to the macroscopic electrical response.
The project addresses basic research issues in a topical area of materials science with high technological relevance. The interdisciplinary nature of this project creates a training environment for undergraduate and graduate students, who are skilled in not only materials processing, but also electrical and spectroscopic techniques. The PI and his research team are actively involved in outreach activities. The PI, for example, is an active presenter on concepts of climate change for the FIRST LEGO League. These outreach activities not only seek to instill excitement about science in general, but to also make connections between the materials properties observed by the participants and the real-world technologies they are familiar with, such as plastic containers and cell phones.
Electronics are ubiquitous in the modern world, from sophisticated mobile devices to simple wires and switches. At the heart of these devices are materials with the ability to transmit, store, or otherwise manipulate electrical charges. One such component, the field-effect transistor, is at the heart of every digital electronic device, from laptops to digital alarm clocks. Typically, the active material in transistors is an inorganic semiconductor, such as silicon. However, there is a strong drive to replace these heavy, expensive materials with cheap, lightweight organic (carbon-based) semiconductors. During operation, a voltage applied to an organic field-effect transistor (oFET) accumulates charges at the interface between an organic layer and an insulating material. When the oFET is turned on, electricity flows through the molecules that coat this interface. The ways that molecules are aligned and packed together have important implications for how well the oFET functions, but measuring this information without destroying the device is challenging. In this project, a laser technique called vibrational sum frequency generation, or VSFG, was used to study molecules in oFETs. The advantage of this approach is that it is "surface specific" - it only produces signals from interfaces. This enabled measurement of the molecular architecture at interfaces from within oFETs that were being electrically controlled from the outside. Early studies in the project found that electrical charges were accumulating at transistor interfaces even when electrical measurements reported that they were not. This highlighted a fundamental difference between these two approaches - electrical measurements require that charges are mobile to produce current, whereas VSFG can optically observe charges accumulating at an interface even if the cannot move once they arrive. Following these studies, a series of experiments were initiated to look at molecular orientation on different types of surfaces. The chemistry of a surface can profoundly affect the orientation and behavior of molecules that stick to it. It was found that a particular semiconducting polymer would align to conduct charge very efficiently on highly hydrophobic ("water-hating") surfaces. The molecular orientation measured by VSFG tracked well with the electrical conduction. These results to motivated a more ambitious study of the evolution of interfacial molecules on these surfaces while they were annealed to high temperatures. It was shown that the annealing treatment caused changes in molecular orientation and ordering at the electrical interface. More importantly, this confirmed that it was the ordering that was the most effective at improving charge conduction, rather than average orientation. The Intellectual Merit of the measurements supported by this award is grounded in a new perspective on an old but persistent problem – improving electrical performance in organic electronics. The work opened a new window to characterizing charge conduction in transistors. The approach was novel in that it provided information that was interface specific and non-destructive so that the samples could be studied while they were in operation. The methods developed in this project can give feedback to benchmark and improve new materials. The orientation measurements showed that the interfacial chemistry was an important factor in directing the packing of molecules in the electrically active region of the transistor. This information at the level of detail that this project provided was not obtainable by other methods. Combined with the thermal annealing work, these measurements provided channels to understanding conduction in transistors at a fundamental molecular level. The Broader Impacts of these studies are that they may allow for new efficient materials to be improved and developed, which may lead to stronger and lighter electronics in societally important roles. In general, society benefits from more efficient technology, and understanding conduction through molecules is a crucial piece of that puzzle. Outside of the Broader Impacts of the scientific outcomes described above, this project contributed to the education of five undergraduate researchers. The PI and group members traveled to national meetings and conferences to disseminate the results of the experimental studies and published regularly in high quality journals. The PI and his group also participated in local outreach while supported by this award. The PI was a presenter in the Chemistry Department’s Energy and U show, which brings exciting science demonstrations to nearly 10,000 students per year at the University of Minnesota. Furthermore, the PI and a large fraction of the group directed the University on the Prairie program at the U of MN Southwest Regional Outreach Center (SWROC) every summer. In this annual event, the PI and group members travel to rural Minnesota for a three-day long event working with 60 participants in grades 7-12 from rural, geographically isolated portions of the state. Approximately half of the participants are female, and rural area junior high school teachers participate as well. The themes of these events reflect topics of societal importance, such as the chemistry of fuels, energy, climate change, and the environment.
