With this award from the Chemistry Research Instrumentation and Facilities: Departmental Multi-User Instrumentation program (CRIF:MU), Professors A. Graham Lappin, Gregory V. Hartland, Libai Huang, Prashant V. Kamat and Masaru K. Kuno of the University of Notre Dame will construct a laser spectrometer system and use it for ultrafast absorption and emission studies of single nanostructures including investigations of the photophysics of carbon nanotubes and studies of materials with low fluorescence quantum yields, such as semiconductor nanowires, CdTe NWs, and even non-fluorescent materials, such as metallic nanowires.
Ultrafast spectrometers use modern laser technology to study molecular phenomena on very short time scales such as femtoseconds. Very fundamental processes in chemistry, biology and materials science (e.g., bond breaking and formation, electron transfer, molecular dynamics and others) occur on ultrafast time scales. This acquisition will exploit the versatility of the instrument in the growing field of ultrafast spectroscopy applied to nanomaterials. Fundamental research will be performed by graduate students, select undergraduates, and postdocs as part of their academic experience. This will provide training and experience in the growing field of ultrafast methodology with state of the art instrumentation.
PIs: Graham Lappin, Gregory Hartland, Prashant Kamat, Masaru Kuno and Libai Huang, University of Notre Dame Single molecule/single particle studies are an active area of research in physical science. These measurements have the potential of uncovering interesting and new physics that is often hidden in ensemble measurements. In this field, dynamics studies are usually done through Time-Correlated Single Photon Counting (TCSPC), however, these measurements require nano-objects or molecules with high emission yields and are relatively slow (a few hundred picosecond time resolution at best). This limits the materials and process that can be studied. In this project the PIs constructed a transient absorption instrument that is sensitive enough to detect and interrogate single metal and semiconductor nano-objects, with dimensions on the order of 10 nm. This was achieved by coupling a Ti:Sapphire laser/optical parameter oscillator system with an inverted optical microscope to simultaneously achieve ultrafast time resolution (ca. 200 fs) and diffraction limited spatial resolution (300 nm). Laser pulses with wavelengths from the near-IR (3 μm) to the near-UV (350 nm) can be obtained from this system, which means that a wide range of different materials can be interrogated. The high time resolution of the laser system means that we can study a variety of different chemical and photophysical processes. For example, we have used this instrument to study charge carrier trapping in single semiconductor nanowires. For CdTe nanowires this process occurs on a picosecond time-scale, which is too fast to study by TCSPC. The results show that there are large differences in the trapping times for different nanowires, presumably due to differences in surface chemistry. This type of information is relevant to applications of these materials in solar cells, for example, and cannot be obtained by conventional ultrafast measurements. We have also used this instrument to examine energy dissipation in single metal nanostructures, as well as exciton dynamics in polymer blends. The flexibility to study different materials is an important aspect of the laser system, and a wide variety of users on the Notre Dame campus have conducted experiments using this instrument. The instrument is also easy to use, being based on a "turn-key" commercial laser system, and has helped train undergraduate and graduate students at the University of Noter Dame, as well as undergraduates from near-by four year colleges, in ultrafast measurements, optical microscopy and sample preparation. Representative publications: Shun S. Lo, Todd Major, Nattasamon Petchsang, Libai Huang, Masaru Kuno and Gregory V. Hartland, "Charge Carrier Trapping and Acoustic Phonon Modes in Single CdTe Nanowires", ACS Nano, 6 (6), 5274–5282 (2012). Shun Shang Lo, Todd A. Major, Mary Sajini Devadas, and Gregory V. Hartland, "Optical Detection of Single Nano-objects by Transient Absorption Microscopy", Analyst, 138, 25-31 (2013).
