Terahertz Studies of Transient Photoconductivity in Quantum Dots and Electron Transfer in Bacterial Reaction Centers

Yale University, New Haven CT

Investigators

Abstract

Dr. Charles Schmuttenmaer of Yale University is funded for his research in terahertz studies of transient photoconductivity in quantum dots and electron transfer in bacterial reaction centers by a grant in the Physical Chemistry Program of the Chemistry Division. The PI will measure time-dependent photoconductivity in isolated quantum dots (QDs) as well as in arrays of QDs. This information is of importance in the design of optoelectronics devices based on QD technology, where it is essential to understand the timescales that carriers remain mobile after absorption of a photon, and the actual value of the mobility. The PI also will develop a new and direct method for monitoring charge transfer events. The motion of the electron itself, rather than a change in absorption or fluorescence spectrum, provides the measured signal. This occurs because accelerating charges generate electromagnetic pulses, and if the charge transfer and/or solvent reorientation timescale is on the order of 100 fs to 10 ps, then a THz pulse is generated. This methodology has been benchmarked on dye molecules oriented in strong electric fields, and will be used to study the primary step of charge transfer in bacterial reaction centers that are spatially oriented by using a poly-histidine tag that anchors a specific residue of the protein to a functionalized quartz slide. Over the last several years, the PI's group at Yale University has learned how to carry out and properly interpret a new class of experiments using short pulses of far-infrared light. For technical reasons involving the intensity of light sources and sensitivity of detectors, the far-infrared region of the spectrum has been historically neglected for spectroscopic experiments compared to other regions of the spectrum. With regard to time-resolved spectroscopic experiments, wherein one "excites" or changes a sample with one pulse of light, and then monitors the change in the sample with a second pulse of light (which is essentially the type of work for which the Nobel Prize in Chemistry was awarded to Ahmed Zewail in 1999), there has been essentially no representation in the far-infrared region of the spectrum. The PI and his group will use these new pulse techniques to elucidate two important scientific issues. First, they will investigate transient photoconductivity in quantum dots. Quantum dots are a new type of material that is radically changing materials science because their properties depend not only on their composition, but also on their size (once they are small enough that quantum effects become important). The PI will assess how well and for how long these particles conduct electricity after absorbing photons of light. At a very basic and fundamental level, there are many important applications with respect to new types of optoelectronic devices and high speed communications. Second, they will focus on the primary charge separation step in photosynthesis. Photosynthesis is arguably the most important process that transpires on this planet. It provides the oxygen we breath, and equally important, it is the source of all fossil fuels - the energy which is so important to our society. The PI will monitor this primary charge transfer event in a much more direct and unambiguous manner than heretofore possible, and provide further characterization of the mechanisms of charge transfer. There is an urgent need for scientific personnel trained in nanotechnology, and the students and post-docs working on QDs will be trained in a wide variety of techniques in this emerging and rapidly changing field. Students and post-docs working on the photosynthesis project will receive training in ultrafast spectroscopy and will learn about electron transfer in photosynthetic systems.