****NON-TECHNICAL ABSTRACT**** Graphene is a novel carbon material consisting of one-atom thick sheets of carbon. Discovered in 2004, this material holds great promise for electronic applications ranging from transparent conducting films (used in solar cells) to ultra-high-speed transistors. This individual investigator award supports a research project at a predominately undergraduate institution that will investigate how rapidly the conductivity of graphene can be varied, which is relevant to the development of high-speed graphene-based electronics. It is expected that the conductivity of graphene can change on very fast timescales. To investigate the fast change in conductivity, the graphene sample will be excited with an ultra-short pulse of light and subsequently the resulting change in conductivity will be measured with an ultra-short pulse of terahertz radiation. Conductivity measurements under a range of conditions will enable the isolation of the fundamental processes, which control the electronic properties of graphene. This research will also serve to train undergraduate science students in Photonics and experimental Condensed Matter Physics. During the past 15 years, the investigator's laboratory has trained 35 advanced undergraduates, more than two thirds of whom have pursued graduate degrees in Science, Mathematics or Engineering.
Graphene shows exceptional promise as an electronic material for nanoscale transistors and high-speed electronics. This individual investigator award supports a project that will study carrier dynamics in graphene and ultra-thin graphite using time-resolved THz spectroscopy. Conducting films of graphene flakes can now be easily produced from graphene oxide and from graphene solutions. Time resolved THz and infrared spectroscopy are well-suited tools for studying the electronic properties of these flakes. These measurements use an optical or infrared pump pulse to excite a sample and a delayed THz pulse to probe the resulting transient change in conductivity. The research is expected to determine the electron and hole scattering rates, lifetimes and mobilities in graphene flakes formed by mechanical or chemical exfoliation of graphite. In this way the measurements will help probe the suitability of these materials for a variety of applications from transparent conducting films to transistors. The research program will also provide education and research training opportunities for advanced undergraduate students. During the past 15 years, the investigator's laboratory has trained 35 advanced undergraduates, more than two thirds of whom have pursued graduate degrees in Science, Mathematics or Engineering.
The scientific goal of this project was to investigate electrical conduction in graphene on femtosecond timescales in order to determine the suitability of this material for high-speed electronic and optoelectronic applications. A broader goal was to support the training of scientists and engineers by providing research assistantships for undergraduate Physics Majors at Macalester College. Graphene is a one-atom-thick sheet of carbon. It possesses exceptional mechanical and electronic properties that make it a promising material for a new generation of high-speed transistors and optical devices, including graphene-based optical detectors and terahertz lasers. Interest in graphene made by Chemical Vapor Deposition (CVD graphene) has exploded because this material can now be grown in large-area sheets and used to coat almost any substrate, making it straightforward to integrate into current semiconductor manufacturing processes. We investigated CVD graphene using all-optical time-resolved conductivity measurements. In these experiments the conductivity of a material is determined by measuring the transmission of a pulse of terahertz radiation. We look for changes in the conductivity when the THz pulse is made to arrive before, during or after the material is excited with a femtosecond pulse of optical radiation. At Macalester College, we set-up and performed measurements which probed graphene either at ambient conditions or in a strong magnetic field. Conductivity measurements in a magnetic field can separately probe the density of free electrons and the electron mobility. At the University of Minnesota we performed time-resolved conductivity measurements in which the wavelength of the light used to excite the graphene was varied. Our results show that the conductivity of CVD graphene can either increase or decrease when the material is photoexcited. The change occurs due to two competing optical processes: an increase in the density of free electrons that increases conductivity, and an increase in the scattering rate of the electrons, which decreases conductivity. The sign of the conductivity change depends on which of these processes is dominant. Our magnetic field dependent measurements can separate these processes. After photoexcitation the conductivity recovers to its original value with a characteristic time of 3 picoseconds. Our results support a model in which photoexcited electrons very rapidly interact with each other to reach a common electron temperature. This change in electron temperature is what drives the change in carrier density and scattering rate. A characteristic feature of this model is that the light-induced conductivity change should depend on the power absorbed by the graphene, but not on the wavelength of the excitation, and this is what our pump-wavelength resolved measurements show. Scientific Impact: These results show that graphene photodetectors are similar to a class of infrared detectors known as hot electron bolometers. Graphene photodectors can be expected to have only low/moderate sensitivity but to be able to respond to very rapidly changing light signals. They also suggest that graphene is not well suited to making terahertz lasers. We also performed terahertz and infrared transmission measurements of a high-performance thermoelectric material containing tellurium nanowires in a conducting polymer matrix. Introduction of tellurium nanowires boosts the DC conductivity of the material by x100, and our measurements probe the mechanism behind the increase. We conclude that the increased DC conductivity of the hybrid material results linkage between PEDOT grains or filaments by the tellurium nanowires. Broader Impact: This project provided 10-week, paid summer research experiences to twelve undergraduate students. It also provided a full-time academic year research assistantship to one post-baccalaureate student. The students involved in this project include seven Americans, five (black) students from Africa and the Caribbean and one Asian student. Two of the American students were female. These experiences will contribute to the students' development as scientists or engineers. Of the eight participants who have graduated, three are currently in graduate programs in Physics or Engineering and two are employed in information technology. The project also supported science education at Macalester College by providing equipment that is used in demonstrations and instructional labs in four courses.
