Single semiconductor nanostructure extinction spectroscopy
Abstract: There has been thirty years of emission-based single particle microscopy and spectroscopy since Moerner’s seminal single molecule study. While highly successful in revealing the properties of matter hidden by ensemble averages, the limits of emission-based microscopies have now become apparent. To address recognized future needs and, in particular, the need to go beyond fluorescent specimens, single particle extinction techniques have been developed. Motivating this has been the desire to acquire information about the electronic structure of nanoscale materials difficult to obtain otherwise using either ensemble or emission-based single particle measurements. Circumstances where single particle extinction measurements offer superior alternatives to traditional microscopies/spectroscopies include situations where the material of interest is non-emissive or where current syntheses yield ensembles with large size and/or compositional distributions that hide the underlying spectral response of a material. Most relevant, though, are cases where information about the underlying electronic structure of a material -something directly encoded in its absorption- is less forthcoming and is at best inferred indirectly using emission-based approaches. While single particle extinction measurements are inherently difficult, a number of strategies have since been developed to overcome the practical issues of measuring the ~0.0001% extinction of light by an analyte. Photothermal heterodyne imaging (PHI) and spatial modulation spectroscopy (SMS) are two of the most popular of these newly developed techniques. Through their use, the physics of individual semiconductor nanostructures such as CdSe nanowires/nanorods and single-walled carbon nanotubes (SWCNTs) have been explored at an unprecedented level. This talk describes the inherent problems associated with measuring the extinction of low dimensional semiconductors. It simultaneously describes the fundamental operating principles of PHI and SMS while highlighting their achievements. It then reviews what exactly we have learned about the fundamental physics of CdSe, a model nanosystem. The talk ends by describing the future development of new single particle extinction methodologies such as infrared photothermal heterodyne imaging, which portend future successes in revealing the detailed physics of nanostructures beyond both ensemble averages and corresponding single particle, emission-based insights. Bio: Masaru Kuno did his undergraduate work at Washington University in St. Louis, finishing in 1993. He then did his PhD (1993-1998) at MIT with Moungi Bawendi, working on the band edge fine structure of CdSe quantum dots. This was followed by a NRC postdoctoral appointment at JILA/NIST/University of Colorado, Boulder with David Nesbitt and Alan Gallagher. At JILA, Dr. Kuno worked on understanding the fluorescence intermittency of individual quantum dots and their unusual power law blinking kinetics. Dr. Kuno then worked at the US Naval Research Laboratory in Washington DC between 2001-2003. While at NRL, he developed syntheses for mercury chalcogenide quantum dots. Since 2003, Dr. Kuno has been at the University of Notre Dame. He is currently a Professor of Chemistry and Biochemistry. His research interests include the synthesis of low dimensional materials, their optical characterization at the single particle level and their use within the context of renewable energy applications. Molecular Engineering and Sciences Seminar Series This weekly seminar brings together students, faculty and invited guests from various disciplines across campus to explore current trends in molecular engineering and nanotechnology. It is a forum for active interdisciplinary discussions. These talks are open to the public and attract a diverse audience of students and faculty. |