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August 2015
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  1. Center for Functional Nanomaterials Seminar

    2 pm, CFN, Bldg. 735, Conference Rm. A - 1st fl.

    Hosted by: Mingzhao Liu

    Center for Functional Nanomaterials Seminar Photoresponses in Vanadium Dioxide Nanowires Prof. Hanwei Gao Department of Physics, Florida State University Tuesday, August 11, 2015 2:00 p.m. Bldg. 735 " Conf. Rm. A, 1st floor Abstract: Vanadium dioxide (VO2) has drawn much attention for its unique metal-insulator transition near the room temperature. The high electrical resistivity below the transition temperature (about 64 °C) is a result of the strong electron-electron correlation. Such interactions can potentially lead to remarkable charge carrier multiplication under optical excitation, a process desirable for efficient optoelectronics and photovoltaics. However, because the resistivity is highly temperature-dependent, the observed light-induced conductivity in VO2 was often attributed to photothermal effects. By varying the chopping frequency of the optical illumination, we have distinguished the photothermal and photoconductive effects in VO2 nanowires. The frequency dependent measurements indicated that the relatively slow photothermal processes can be well suppressed with high chopping frequency, whereas the fast photo-excitation of charge carrier results in a frequency-independent photoconductivity in VO2. Resolving these coexisting processes paves the way for further studies of carrier dynamics under optical excitations in strong electron correlated materials. . Host: Mingzhao Liu Joann Tesoriero Center for Functional Nanomaterials P.O. Box 5000 Upton, NY 11973 Tel. (631) 344-7791 Tax: (631) 344-3093 Tesoriero@bnl.gov

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  1. Center for Functional Nanomaterials Seminar

    10 am, CFN, Bldg. 735, first fl. conference room A

    Hosted by: Anibal Boscoboinik

    Center for Functional Nanomaterials Seminar Caught in the Act! Live Observations of Catalysts Using High-pressure Scanning Probe Microscopy Irene M. N. Groot Leiden Institute of Physics and Leiden Institute of Chemistry, the Netherlands Friday, August 21, 2015 10:00 am Bldg. 735 " Conf. Rm. A Recently it has become clear that essential differences can exist between the behavior of catalysts under industrial conditions (high pressure and temperature) and the (ultra) high vacuum conditions of traditional laboratory experiments. Differences in structure, composition, reaction mechanism, activity, and selectivity have been observed. These observations indicated the presence of the so-called pressure gap, and made it clear that meaningful results can only be obtained at high pressures and temperatures. However, most of the techniques traditionally used to study catalysts and their reactions were designed to operate under (ultra) high vacuum conditions. To bridge the pressure gap, the last years have seen a tremendous effort in designing new instruments and adapting existing ones to be able to investigate catalysts in situ under industrially relevant conditions. This talk focuses on the development of scanning probe microscopy for operando observations of active model catalysts. In our group, we have developed set-ups that combine an ultrahigh vacuum environment for model catalyst preparation and characterization with a high-pressure flow reactor cell, integrated with either a scanning tunneling microscope or an atomic force microscope. With these set-ups we are able to perform atomic-scale investigations of well-defined model catalysts under industrial conditions. Additionally, we combine the structural information from scanning probe microscopy with time-resolved mass spectrometry measurements on the gas mixture that leaves the re

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  1. SEP

    30

    Wednesday

    CFN Proposal Deadline

    "CFN Proposal Deadline for January-April Cycle 2016"

    11:45 pm, CFN

    Wednesday, September 30, 2015, 11:45 pm

  2. OCT

    2

    Friday

    CFN Colloquium

    "In-situ XAS, TXM and RIXS experiments"

    Presented by Frank de Groot, Debye Institute of Nanomaterials Science, Utrecht University, Netherlands

    11 am, Bldg 735, Seminar Room, 2nd Floor

    Friday, October 2, 2015, 11:00 am

    Hosted by: Deyu Lu

    New developments in in-situ x-ray absorption (XAS), transmission x-ray microscopy (TXM) and resonant inelastic x-ray scattering (RIXS) will be discussed. A brief introduction is given of x-ray absorption spectroscopy, including the multiplet interpretation of XAS spectral shapes [1,2]. Nanoscale chemical imaging of catalysts under working conditions is possible with transmission x-ray microscopy. We have shown that TXM can image a catalytic system under relevant reaction conditions and provides detailed information on the morphology and composition of the catalyst material in situ [3]. The 20 nanometer resolution combined with powerful chemical speciation by XAS and the ability to image materials under reaction conditions opens up new opportunities to study many chemical processes. I will discuss the present status of in-situ TXM, with an emphasis on the abilities of the 10+ nm resolution TXM technique in comparison with 0.1 nm STEM-EELS [4,5]. Hard X-ray TXM allows the measurement of chemical images and tomographs under more realistic conditions, using a capillary reactor at 10 bar Fischer-Tropsch conditions [6]. The second part of the talk deals with resonant inelastic x-ray scattering (RIXS), In 2p3d RIXS one scans through the 2p XAS edge and measures the optical excitation range. As an example, the RIXS spectra of CoO will be discussed. The experimental resolution of 100 meV at ADRESS allows the detailed observation of the electronic structure. First-principle theoretical modelling was performed for the ground state and multiplet analysis for the RIXS experiments. The implications for measurements on coordination compounds (cobalt carboxylates) and cobalt nanoparticles is discussed, in particular the comparison with optical spectroscopy [7]. Related to the RIXS measurements is the analysis of Fluorescence yield (FY) detected x-ray absorption spectra (XAS), including the intrinsic deviations of the FY-XAS spectral shape from