May 22, 2014

Paul Weiss and colleagues have significantly improved organic electronic devices by employing mixed SAMs of the symmetric cage molecules carboranethiols to tune work function of gold and silver contacts with little alteration in surface energy.

Full Story: A team of researchers led by Paul S. Weiss, Fred Kavli Chair in NanoSystems Sciences, a UCLA distinguished professor of chemistry and biochemistry and materials science and engineering, and director of the California NanoSystems Institute (CNSI), and Yang Yang, the Carol and Lawrence E. Tannas Jr. Endowed Chair in Engineering, a UCLA professor of materials science and engineering, and director of the CNSI Nano Renewable Energy Center, have significantly improved organic electronic devices. To do so, they have employed mixed self-assembled monolayers (SAMs) of the symmetric cage molecules carboranethiols to tune the work function of gold and silver contacts with little alteration in surface energy. Their findings indicate that mixed monolayers of carboranethiol isomers provide an ideal platform for the study and fabrication of solution-processed organic field-effect transistors (OFETs).

Abstract figure from the resulting publication in ACS Nano Letters:
Interface Control in Organic Electronics Using mixed Monolayers of Carboranethiol Isomers

Solution-processed OFETs are the subject of great interest worldwide because they can be manufactured at large scales at low temperatures using cost-effective, high-throughput techniques. Recent advances have led to significant improvements in OFET device performance. Notably, using mixed self-assembled monolayers (SAMs) in OFETs can be used to align the electronic energies between the metal contacts and the active polymer layers above. Earlier attempts to align these energies disrupted the structure of the polymer and thus device performance. In the new work, two or more thiol molecules with distinctly oriented dipole moments, and depending on which molecules were added and the interface energies within the OFET could be tuned to match for optimum performance without changing the wetting and structure of the polymer. 

Weiss Group
This independent tuning is made possible by the symmetric cage structures of carboranethiols, which enable control of dipole moments without introducing additional chemical functionality. The symmetry of these compact, upright cage molecules leads to simply prepared, high-quality SAMs that have few and small defects. The authors achieved continuous energy tuning over a considerable range with small changes in wetting and no significant changes in the overlying polymer structure, by co-depositing two carboranethiol isomers with different dipole directions. 
In addition to Paul Weiss and Yang Yang, study authors include postdoctoral researchers Jaemyung Kim, You Seung Rim, and Yongsheng Liu; and Ph.D. candidates Andrew Serino, John Thomas, and Huajun Chen.
The study was supported by the National Science Foundation, and the Kavli Foundation and is published in the American Chemical Society (ACS) journal Nano Letters. It has been made freely available and open access here through ACS Editors’ Choice as the top paper of the day published by the ACS journals.
The California NanoSystems Institute is an integrated research facility located at UCLA and UC Santa Barbara. Its mission is to foster interdisciplinary collaborations in nanoscience and nanotechnology; to train a new generation of scientists, educators, and technology leaders; to generate partnerships with industry; and to contribute to the economic development and the social well-being of California, the United States and the world. The CNSI was established in 2000 with $100 million from the state of California. The total amount of research funding in nanoscience and nanotechnology awarded to CNSI members has risen to over $1 billion. UCLA CNSI members are drawn from UCLA's College of Letters and Science, the David Geffen School of Medicine, the School of Dentistry, the School of Public Health and the Henry Samueli School of Engineering and Applied Science. They are engaged in measuring, modifying, and manipulating atoms and molecules—the building blocks of our world. Their work is carried out in an integrated laboratory environment. This dynamic research setting has enhanced understanding of phenomena at the nanoscale and promises to produce important discoveries in health, energy, the environment, and information technology.