This research program, supported by the Solid State and Materials Chemistry Program, aims at a molecular level understanding of chemical modulation of intermolecular electron transport using a specially designed model device containing pi-conjugated molecules such as oligothiophene based organic thin films. To circumvent contribution from surface roughness, Ultraflat NanoElectrodes (UNEs) with sub-nanometer roughness will be fabricated, onto which designed molecules will be placed using atomic force microscopy (AFM) based nanografting method. The multifunctional AFM enables in situ nanolithography, high-resolution structural characterization, and measurements of electron transport. The chemical and electronic properties of organic films will be controlled by varying their head group, intermolecular interactions and terminal group. The response of the electronic properties during and after exposure to chemical species will be studied, in correlation to structure in situ. The project combines the technical strength of nanolithography and imaging (PI), UNE fabrication and electron transport (co-PI), and organic synthesis (collaborator). With molecular level structure and packing engineered precisely on UNEs, the systematic investigation of intermolecular electron transport becomes feasible and reliable. This systematic investigation will review the correlation of intermolecular transport with the film morphology, reveal the role of structure, especially domain boundaries and molecular level packing, and examine the influence of analytes binding to the film termini.
NON-TECHNICAL SUMMARY:
The research aims at accurate measurements of electron transport along and in between molecules within model junctions of electronic devices. These measurements are technically very challenging, yet critical in order to realize the great potential of modern molecular electronic devices, which are smaller than state-of-the-art transistors, with properties tunable by varying the structure and packing of the molecules. The research team plans to take best advantage of today's technology, fabrication of ultraflat microelectrodes with sub-nanometer roughness, the ability to position molecules with high precision, and measurements of electron transport over these well-engineered junctions. This investigation should yield reliable and fundamental information in understanding organic semiconductor based devices, and guiding design of a next generation of molecular devices. The investigation is highly interdisciplinary, and requires close collaboration among Liu (AFM nanofabrication and high resolution imaging), Salmeron (UNE fabrication and device characterization), and Schore (synthesis of organic semiconductor materials), whose team members will attain advanced training in micro/nanofabrication, molecular resolution imaging, electron movement in confined space, and device making. The team has been working closely with local community college faculty (Miller, SacCity College) and students to expose them to the concept and experiments of these model devices. Being extremely proficient in both Spanish and English languages, Salmeron has participated and will continue to participate in Hispanic Radio Shows to attract Hispanic students to the forefront of modern devices and materials science.
With the rapid advances in nanotechnology during the past two decades, the potential of using nanotechnology to produce and utilize molecular devices is becoming a reality. To realize the potential of molecular devices, one must characterize the structure of these devices in correlation with their electronic properties. With molecular level information at hand, the research and development effort could be enabled to attain the designed devices and performances. The key challenges in obtaining molecular level information are the difficulty to visualize molecules and decouple electronic property with other factors such as surface roughness, molecular packing, and imperfection, just to name a few. Joining forces among three teams, organic synthesis, device fabrication, and molecular level imaging and measurement, this NSF supported project focuses on addressing these challenges and extracts molecular level information on structure and electronic property. The project outcomes are comprised of: Molecular level measurements: The team has synthesized a series of molecules that are good model systems for future molecular level devices. These molecules are named oligothiophene derivative, 4-(5’’’’-decyl-[2,2’;5’,2’’;5’’,2’’’;5’’’,2’’’’] pentathiophen-5-yl)-butyric acid (D5TBA). Different from past approaches, the synthetic route is green and enables a new concept of "molecular lego", i.e., putting various parts together to make the molecules of your design. In addition, the team has produced and characterized the molecular thin films on electrode surfaces, using atomic force microscopy, scanning tunneling microscopy, and transmission electron microscopy. Further, the team has performed a series of highly difficult experiments to extend the atomic force microscopy technology to measure the electron movement, in correlation with the structural information. The results provided new insights into the molecular level order and stability of these thin films, and the conductivity at molecular level. Publications and Presentations: This work has yielded 3 publications, 3 submitted manuscripts, and at least 1 more manuscript to be submitted soon. The team has also given over 20 seminars at scientific meetings. Broader Impacts: The information attained from this investigation provides molecular level insights into the nanomaterials, which shall guide future development and realization of their applications as electronic devices, biosensors and drug delivery vehicles. The technical training and hands-on projects have brought graduate students and postdoctoral researchers to the forefront of nanotechnology. Three students who worked on this project have attained their Ph.D. degrees. Two will work for nanotechnology companies, and one will be a postdoc in PNL. Postdoc who worked on this project will become a professor for academic institutions and continue his new research in nanoscience. We anticipate two more Ph.D. degrees (one from an under-represented minority group) before next March. The project has attracted great attention from undergraduate students. In the three grant years eight undergraduate students participated in the projects (three from under-represented minority groups). We anticipate two publications from them. This project has active participation from a local community college, Sacramento City College (SCC). There have been one faculty member and six students participating in measurements. Teaming up, we have introduced the concept of nanotechnology and molecular electronics to two classes at SCC, reaching over 250 students, many of whom are first generation college attendees.
