Single Molecule Scanning Tunneling Microscopy Studies of Dynamic Disorder in Organic Semiconductors

North Carolina State University, Raleigh NC

Investigators

Abstract

Nontechnical Description: A major challenge in the use of organic electronic materials in important applications like flat panel displays and solar cells is that these materials often lack good ordering of their molecules. Such molecular disorder often results in compromised performance of many organics-based devices. This project addresses the impact of disorder that arises from very fast motion of the molecules in thin films of organic materials. The research team uses a powerful microscope capable of observing and analyzing the rapid motion of single molecules. By carrying out these experiments on a range of important organic electronic materials, the project provides device engineers with information about how to choose appropriate molecules to minimize negative effects of molecular motion on device performance. The project supports training of graduate students in many aspects of organic electronics and scientific leadership within the infrastructure of the interdisciplinary Organic and Carbon Electronics Lab. This infrastructure includes a materials fabrication facility operated primarily by undergraduate students. It also includes educational initiatives such as the development of instructional software for understanding the principles of organic materials. Finally, the project also supports crystal growth competitions for grade and middle school students. Technical Description: In the past decade, dynamic disorder has come to the forefront of understanding of the fundamental limitations of performance in organic electronics materials. Fluctuations in position and geometry - mediated by phonons and intramolecular vibrations - can modulate electronic transfer integrals that control charge transport in these materials. This project applies tunneling current fluctuation spectroscopy in a variable temperature scanning tunneling microscope to observe dynamic disorder processes in real time with single molecule spatial resolution. By tuning sample temperature, the time scale of molecular fluctuations is controlled and eventually frozen out at low temperatures. By simultaneously adding local tunneling spectroscopy measurements, orbital specific dynamic effects are isolated. The project focuses on the newest classes of small molecule organic electronic materials such as side-arm functionalized pentacenes and anthradithiophenes. In addition, it uses laser illumination of organic single crystals to allow routine variable temperature scanning tunneling microscopy studies of these important materials class benchmarks.