Christopher E. D. Chidsey - US grants

9412720 Chidsey This research project addresses the structural origins of the rates of long distance electron transfer across interfaces. It is supported by the Analytical and Surface Chemistry program, with a goal of understanding the role of intervening and surrounding material in the transfer of electrons between a metal electrode and a precisely positioned electroactive species. The continuum dielectric model will be examined as a basis for understanding and predicting the activation barrier to electron transfer, the role of electroactive species located between the metal and the polar electrolyte will be examined, and the effect of electronic states of the spacer material on electron transfer will be probed in these studies. Thiol monolayers on gold electrodes form the basis for these electron transfer studies. These measurements will have impact on the development of new sensor and display technologies, and will help to develop an understanding of interfacial charge transfer in applications ranging from biology to microelectronics. %%% In order to understand the rates and mechanisms of charge transfer between electrodes and electroactive species, a number of model approaches have been developed. One very promising approach uses self assembled monolayer technology to position the electroactive species and to control the structure and electronic properties of the medium through which electron transfer occurs. The work supported here uses this approach to examine the structural origins of the rates of charge transfer across interfaces. Knowledge in this area is crucial to understanding the mechanisms of biological membrane charge transfer, and in the development of new sensing and display technologies. The work supported here will contribute to the development of this understanding.

This project aims to synthesize metal-organic semiconducting molecule-metal structures with nanoscale metallic contacts pre-assembled or templated by DNAs, or directly connected to the molecule as chemisorbed gold nanoparticles/nanowires. The metallic contacts will form ohmic contacts to molecular devices for circuits from DC to microwave frequencies. Precisely fabricated, ultrasmall gaps are not needed since the overall hybrid structure will be much longer than the organic molecule of interest. At optical frequencies, the metallic contacts will form an electromagnetic cavity around the molecular device, enhancing optical fields to be utilized in single-molecule spectroscopic measurements. Self-assembly of these new nanoscale objects will be investigated both theoretically and experimentally. Electrical devices will be fabricated to study charge transport through single molecules. New electrical, optical and physical phenomenon may arise from these unique nanoscale structures. The open planar geometry formed in this work is expected to allow electrostatic modification of electronic states using a nearby strongly-coupled gate electrode, and will reduce fluorescence quenching by nearby metallic electrodes. Single-molecule based transistors and light-emitting diodes may be generated from the proposed structures. The methods developed will lay the groundwork for developing molecular electronic and optical devices and integrating them into complex circuits.
Intellectual Merit. Fundamental advances across disciplines are essential to the advancement of nanoscale devices and to understanding their behavior. Molecular synthesis, self-assembly, and charge transport are essential components for realizing nanoscale devices with organic molecules. A coordinated team attack on such issues can advance the state of single-molecule devices. This project will be carried out by a team of two chemists, one solid-state physicist, one spectroscopist, and one theorist together with collaborators from industry, national labs, and foreign universities with expertise in polymer synthesis, surface chemistry, biochemistry, DNA self-assembly, DNA metallization, spectroscopy, charge transport, fluid dynamics simulation, and device fabrication.
Broader Impacts. This project may result in a new approach to make electrical contacts to single molecules, which will allow study of charge transport through single molecules with different chemical functionalities and length as well as measurement of unique optical properties arising from a single molecule confined in a nanogap. The proposed work will not only answer fundamental questions of intramolecular charge transport mechanisms in molecules with length scale of 5-100 nm, it will also provide answers to technological questions of whether organic molecules have sufficient performance for nanoelectronics and whether the mobility of molecular devices will be dramatically increased by alignment of organic semiconducting molecules between electrodes. This project also utilizes methods to self- assemble DNA-polymer-DNA and nanoparticle-molecule-nanoparticle structures using electrophoresis and dielectrophoresis to allow electrical connections to be made to single organic semiconducting molecules. The PIs will work closely with existing NSF centers and the Stanford Office of Science Outreach to reach a broad population ranging from K-12, community college, undergraduate, and graduate students as well as to prepare teachers of tomorrow for new areas of science and technology. Two internship positions every year for minority and/or women community college students are integral to the project. One research position per year will also be provided to a middle school or high school teacher during the summer; PIs will continue to work with them to develop their education plans after their summer research. PIs will also reach out to the general public through a public website and participation in various community events. The graduate students and postdoc involved in the project will actively interact with each other and have the opportunity to interact with researchers from industry, national labs, and international collaborators. They will be well equipped with a combination of technical engineering skills, basic scientific understanding, and communication skills, and poised to contribute to nanoscience and nanotechnology.

With this award from the Chemistry Research Instrumentation and Facilities: Multi User program (CRIF:MU), the Chemistry Department at Stanford University will upgrade their infra red and fluorescence spectrometry equipment for teaching and research with acquisition of an infrared microscope with spatial resolution, FTIR monolayer analysis and diffuse reflectance accessories, and upgrade a fluorimeter with a stopped-flow module for rapid mixing capabilities and time resolution measurement of reaction rates down to the millisecond regime. The instruments will be used in various research projects including the study of protein conformational changes using single molecule fluorescence spectroscopy to make comparisons with ensemble measurements; investigations of a wide range of biological systems include green fluorescent protein (GFP) variants, vibrational Stark effect (VSE) spectroscopy, and the analysis of model membrane surfaces with spatially resolved FTIR; investigations to spatially resolve carbon nanotube hydrogenation chemistry at the ends and sidewalls with potential applications in hydrogen storage and nanotube nansensors; studies to develop surface functionalization chemistry for carbon, including graphite and carbon nanotubes, important to fuel cells, batteries and biosensor developments. The instruments will be used to introduce a nanotechnology element into undergraduate teaching in the chemistry department with experiments planned for a laboratory course. The instruments will be used by undergraduate, graduate students and postdoctoral fellows in their research and will benefit various outreach programs such as RET, REU and SURE, and the research of NSF-funded centers at Stanford including CPIMA and NSEC.