Technical: This project aims for measurement and understanding of charge transport in single molecules. The approach to achieve ohmic contact is to synthesize conductor-organic semiconducting molecule-conductor with nanoscale metallic or molecularly doped contacts self-assembled or templated by DNAs. The conducting contacts are self-aligned and make contacts with each end of the organic semiconductor molecule (OSM) with length scale ranges from 5-100 nm. Precisely fabricated, ultrasmall gaps are not needed since the overall hybrid structure will be much longer than the organic molecule of interest. Experiments on electrostatic modification of molecular electronic states via a nearby strongly coupled gate electrode are included. The methods developed are expected to lay the groundwork for developing useful molecular electronic devices and eventually integrating them into complex circuits.
The project addresses basic research issues in a topical area of materials science and macromolecular chemistry with technological relevance, and is expected to provide unique opportunities for graduate and undergraduate training in an interdisciplinary field. The proposed work will allow direct measurement of charge transport through single molecules with different chemical functionalities and length, providing critical information on whether organic molecules have sufficient performance for nanoelectronics. The PI will continue with her activities to reach out to a broad population ranging from K-12, community college, undergraduate, and graduate students as well as efforts to engage and prepare the teachers of tomorrow for new areas of science and technology. This project will expose both graduate students and undergraduates to organic chemistry, polymer chemistry, surface chemistry, materials and thin film characterization, device fabrication, and device characterization. Students will experience an interdisciplinary approach to problem solving and become equipped with a combination of technical engineering skills, basic scientific understanding, and communication skills. This project is co-supported by the DMR Electronic and Photonic Materials and CHE MSN (Macromolecular, Supramolecular and Nanochemistry) programs.
Intellectual Merit In this project, we aimed to develop an electronic component out of a single molecule. With such electronic components built, we can then conduct investigation on them in order to answer some of the most fundamental scientific questions yet to be answered: How does electricity flow through a molecule? How would the size, the shape and the structure of a molecule affect its conductivity? And will we be able to build functional devices, such as transistor or diode, based on just one single molecule? To tackle these challenges, we choose to take a bottom-up approach. In other words, we use chemical synthesis to arrange atoms to form the molecule with desired structure and size. Because the size of a typical organic molecule is less then 1 nanometer (e.g., a hair is about 100,000 nanometers wide), it is extremely difficult to build electronic devices at this scale. To solve this problem, we make polymers (i.e. very large molecules) with lengths of a few hundred nanometers. In order to investigate the electronic properties, the polymer we made must be conductive. There are two strategies to ensure molecular conductivity and we have synthesized conductive polymers using both methods. First, one can arrange many conductive sub-units onto a polymer chain so that these sub-units are close contact with one another. When a voltage is applied, the electrons or holes can hop between the units easily to conduct electricity. In this particular case, C60 fullerene (a molecule with soccer ball shape and 1 nm in diameter) was used to serve as the conductive sub-unit. We were able to make a polymer with ~800 buckyballs attached onto the chain. The conductivity of the polymer was enhanced dramatically compared to a thin film made of free buckyballs. This is due to our molecular design that forces a close contact between the buckyballs on the polymer chain. The second strategy to make a long polymer molecule conductive is to ensure the backbone is fully conjugated. A fully conjugated molecule would have all its atoms sharing the same electron orbital, so that the electrons in the gigantic orbital are all coupled. As a result, when voltage is applied on such molecule, the electrons can transport fast through the conjugated structure. We were able to synthesize such a polymer with a length over 200 nanometers. In addition to making long molecules, it is also essential that we are able to "see" the single molecules in order to build electronic device based on them. Normal polymers, however, cannot be seen under a microscopy even when the molecular length is above 100 nanometers, because the height and width of most molecules is less than 1 nanometer – too thin to be detected by even the most advanced microscopy. We designed the polymer molecules in a way that the thickness and width of the thread like molecule is much larger than normal polymers (>3 nm). As a result, we were able to "see" these single polymer chains using atomic force microscopy. These polymers lie on the surface in an extended manner instead of being coiled up. As a result, it is possible to place electrodes on the both ends of the polymer chain in order to measure the current flowing along the single polymer molecule. We are at the stage of fabricating these devices using tools provided by contemporary nanotechnology. Broader Impact This work resulted in a systematic understanding of molecular design for efficient charge transport in organic semiconductors. This is crucial for advancing the field of organic electronics. Our approach resulted in improved charge transport in organic semiconductors. Additionally, we were able to extend the use of the synthesized polymers from this work for efficient all-polymer solar cells. Since organic semiconductors are key components for organic light emitting diodes, organic solar cells, transistors and sensors, enhanced charge transport will result in better performance for these devices. We have been committed to work closely with our existing NSF centers and Office of Science Outreach on campus to reach out to a broad population ranging from K-12, community college, undergraduate, and graduate students as well as prepare the teachers of tomorrow for new areas of science and technology. This research trained graduate students, undergraduates and postdoctoral researchers to organic chemistry, surface chemistry, materials and thin film characterization, device fabrication and device characterization as well as a wide range of organic electronics technologies. Students received training on communicating project progress and directions by weekly meetings with the PI and presentations at group meetings and conferences. The students learned a multidisciplinary approach to problem solving, thus, obtaining an impressive combination of technical engineering, basic scientific understanding, and communication skills. In addition, various high school students, community college students participated in the research. Our work helped to broaden the participation in science and engineering.
