****NON-TECHNICAL ABSTRACT**** Electronic charge transport is one of the most fundamental and central subjects in condensed matter physics. From a technological standpoint, the electronics and optoelectronics industries depend on a detailed understanding of, and the associated ability to control, charge transport. Conventional conductivity measurements, however, are limited to relatively low frequencies and samples for which satisfactory electrical contacts can be produced. This individual investigator award supports a project to pursue experiments that address previously intractable problems concerning charge transport and carrier dynamics in organic photoconducting materials and in nanometer-scaled materials. Using an experimental approach based on the capabilities of lasers that are able to provide very fast pulses of light (i.e. femtosecond pulsed lasers), measurements of the high-frequency conductivity of the materials will be made, without the need for electrical contacts. The scientific results will be of significance not only to the disciplines of condensed-matter and materials physics, but also have the potential to impact several important technologies such as organic electronic and optoelectronic devices and solar cells. This project will support the education of a Ph.D. student. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experimental research provides an ideal training ground for career paths from academia to high technology.
This individual investigator award supports a project to investigate electronic charge transport and carrier dynamics in an array of novel electronic materials using the ultrafast optical method of terahertz time-domain spectroscopy. With the new capabilities to be developed, the technique will permit sensitive measurements of the complex conductivity of materials up to the mid-infrared frequencies without the need of contacts and with femtosecond time resolution. The research program will address problems in charge transport properties of organic photoconductors and carrier multiplication in semiconductor nanocrystals. This project will support the education of a Ph.D. student. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in optics and lasers provides an ideal training ground for career paths from academia to industry.
The goal of this project was to investigate electronic charge transport and carrier dynamics in novel nanoscale materials based on advanced optical spectroscopic techniques. Semiconductor nanowires (such as silicon), graphene, and transition metal dichalcogenides (such as molybdenum disulfide) have been investigated as model one-dimensional and two-dimensional systems. Large-area arrays of silicon nanowires were grown via the vapor-liquid-solid process using metal catalysts. And the atomically thin samples of graphene or transition metal dichalcogenides were fabricated either by the mechanical exfoliation technique or chemical vapor deposition method since their bulk crystals are built up of van der Waals bonded layers which can be easily separated into stable units of atomic thickness. Our experimental investigations revealed the role of phonons, charge-charge interactions, and dimensionality on charge transport and dynamics of silicon nanowires and graphene. Our investigations also revealed the effect of sample thickness, mechanical strain and electron-hole interaction on the electronic structure and optical properties of atomically thin molybdenum disulfide samples. In addition, the program developed new approaches that can provide direct access to frequency dependent conductivity over a broad spectral window spanning from the far-infrared to the visible and ultraviolet range in micro-scale samples and devices. The broader impact of the program is several-fold. The scientific results are of significance not only in their immediate disciplines of condensed matter and materials physics, but also have the potential to impact several important technologies. Research on charge transport in nanoscale systems has the potential for significant future impact on the development of high efficiency solar cells and photodetectors. Investigation on two-dimensional crystals has the potential to impact on the development of novel spin- and valley-based electronic and optoelectronic devices. Research supported by this grant was also a crucial part of science education at both the graduate and undergraduate levels, and student training was an integral part of this program. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provided an ideal training ground. Collaboration with material scientists and researchers from engineering school also provided further opportunities for student scientific interaction in an inter-disciplinary environment.
