Technical: This project is to study the synthesis of two types of optoelectronic nanostructures, carbon nanotubes and semiconductor nanowires, and to develop a convergent characterization platform, with a goal of simultaneous electrical and optical characterization with nanometer spatial resolution, nanosecond time resolution, as well as energy resolution. The platform will allow detailed investigation of the relation between nanomaterials synthesis parameters and their physical properties. In carbon nanotubes, the effect of the growth temperature and use of different reactive gases on the diameter, chirality and defect density will be studied. In semiconductor nanowires, Si/Ge nanowires with continuously varying dopant and composition level will be synthesized and their electron band structure will be determined with nanometer scale resolution. In addition, massively parallel characterization of individual nanostructures, for a rapid feedback with good statistical information for optimizing materials synthesis, is planned.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide new experimental techniques that could have a major impact on the understanding of electronic and optical properties of nanomaterials. It provides interdisciplinary training for both graduate and undergraduate students in important areas of science and nanotechnology. In addition, the project plans to develop outreach programs that are targeted for high school teachers and journalists, whose understanding of nanoscale science can greatly affect that of students and public in much larger scale, and to develop new courses and a summer school that are designed to engage undergraduate and graduate students in their early career in nanomaterials research.
Absorption of light is a key parameter essential for understanding and controlling the performance of optical and optoelectronic devices that we encounter in our daily lives. These devices include photovoltaic cells, photo detectors,CCD arrays, and lasers. One of the most exciting challenges of nanoscience is building these devices from nanometer scale structures. It has attracted wide attention as it makes possible new functionalities as well as high sensitivity. Examples for such new functionalities include light-emitting devices with higher effiencies, new power limiting devices, and antennas for light. Therefore, accurate measurements of optical absorption in nanostructures is one of the most important quantities necessary for the design and optimization of these nanostructure based optoelectronic devices. What our work has shown in the past year is that there is an important correction factor as much as 20% that needs to be considered depending on how the light absorption measurements are done. According to our study, 'one dimensional' (wire-like) nanostructures absorb light wave oscillating parallel to its long directions while 'two dimensional' (sheet-like) materials does not absorb light wave oscillaing perpendiculr to their surface. Without using the correction factor we propose, one might get the absorption value lower than the correct one and our work showed how to measure and model this effect carefully. During the entire period of this research project (2008-2013), we have developed new characterization methods, or 'new eyes', with better spatial, temporal and energy resolution in order to better control the properties of nanomaterials. Over the past five years, we have used our new characterization techniques for the growth and characterization of carbon nanotubes and graphene, one- and two-dimensional carbon nanostructures. This is important because, despite remarkable developments in this field, our ability to control their heterogeneity is still limited, a problem made more difficult by the lack of imaging tools with necessary resolution and speed. Significant accomplishments of our work in this area now include rapid and parallel identification of the 'chiral indices' (equivalent of atomic number for each atom) of many individual carbon nanotubes on a substrate, and the first comprehensive identification of defects in this material. These studies provided new insights that allow better control of the growth of carbon nanotubes, and they will lead to developments of new ways to use them to collect more light than normally possible. Furthermore, these well-defined carbon nanostructures will lead to exciting new device applications in energy conversion and energy transfer as well as applications in nanoscale imaging.
