LSM - Seminars: March 2012 Archives

Seminars Archives

February 2012 | March 2012 | April 2012

Monday, March 05, 2012
Electron Spin-Polarized 4He+ ion Scattering for Surface Magnetism Study Taku Suzuki National Institute for Materials Science Tsukuba, Japan 12:00 Noon, CCR 201 A 4He+ ion has either up or down spin, like electrons. The spin of a 4He+ ion beam can be polarized by optical pumping. The spin-polarized4He+ ion beam has an unique character, that is a sensitivity to surface magnetism. We have developed a novel analytical method for surface magnetic structure using low energy polarized 4He+ ion beam (spin-polarized ion scattering spectroscopy (SP-ISS)). SP-ISS is capable of analyzing surface spin of outermost surfaces with element selectivity. I will discuss our recent results by SP-ISS [1-3]. [1] T. Suzuki, H. Kuwahara, Y. Yamauchi, Surf. Sci. 605 (2011) 1197. [2] T. Suzuki, Y. Yamauchi, Phys. Rev. A 82 (2010) 042709. [3] T. Suzuki, Y. Yamauchi, S. Hishita, Phys. Rev. Lett. 107 (2011) 176101.

Monday, March 05, 2012

Electron Spin-Polarized 4He+ ion Scattering for Surface Magnetism Study
Taku Suzuki
National Institute for Materials Science
Tsukuba, Japan
12:00 Noon, CCR 201

A 4He+ ion has either up or down spin, like electrons. The spin of a 4He+ ion beam can be polarized by optical pumping. The spin-polarized4He+ ion beam has an unique character, that is a sensitivity to surface magnetism. We have developed a novel analytical method for surface magnetic structure using low energy polarized 4He+ ion beam (spin-polarized ion scattering spectroscopy (SP-ISS)). SP-ISS is capable of analyzing surface spin of outermost surfaces with element selectivity. I will discuss our recent results by SP-ISS [1-3].

[1] T. Suzuki, H. Kuwahara, Y. Yamauchi, Surf. Sci. 605 (2011) 1197.
[2] T. Suzuki, Y. Yamauchi, Phys. Rev. A 82 (2010) 042709.
[3] T. Suzuki, Y. Yamauchi, S. Hishita, Phys. Rev. Lett. 107 (2011) 176101.

Thursday, March 22, 2012
Internal State Resolved Studies of Phonon-Meidated Surface Chemistry Art Utz Tufts University 12:00 Noon, CHEM 260 Sub-ps timescales and sub-Ångstrom length scales complicate experimental efforts to probe transition state access for activated chemical reactions on surfaces. By selectively preparing reagents in distinct vibrational states and experimentally limiting the time for the gas-surface encounter, we are able to follow these dynamics and build a molecular level picture of energy flow, redistribution, and activation. Highlighted studies explore methane activation on Ni and Ir surfaces. We find evidence for mode-selective chemistry, where the identity of methane's vibrational state, and not just its internal energy, dictates reactivity, bond selective chemistry, in which vibrational excitation can selectively activate dissociative chemisorption in one of several chemical bonds, and phonon-mediated chemistry, where excitation of surface phonons opens new channels to chemical reactivity. Taken together, these studies provide molecular level insight into the nature of the transition state and suggest new strategies for exerting control over the outcome of gas-surface reactions.

Thursday, March 22, 2012

Internal State Resolved Studies of Phonon-Meidated Surface Chemistry
Art Utz
Tufts University
12:00 Noon, CHEM 260

Sub-ps timescales and sub-Ångstrom length scales complicate experimental efforts to probe transition state access for activated chemical reactions on surfaces. By selectively preparing reagents in distinct vibrational states and experimentally limiting the time for the gas-surface encounter, we are able to follow these dynamics and build a molecular level picture of energy flow, redistribution, and activation. Highlighted studies explore methane activation on Ni and Ir surfaces. We find evidence for mode-selective chemistry, where the identity of methane's vibrational state, and not just its internal energy, dictates reactivity, bond selective chemistry, in which vibrational excitation can selectively activate dissociative chemisorption in one of several chemical bonds, and phonon-mediated chemistry, where excitation of surface phonons opens new channels to chemical reactivity. Taken together, these studies provide molecular level insight into the nature of the transition state and suggest new strategies for exerting control over the outcome of gas-surface reactions.

Thursday, March 29, 2012
Understanding the Complex Growth of Silicon Nanowires Jerry Tersoff IBM 12:00 Noon, CHEM 260 Semiconductor nanowires can be readily grown with diameters down to 10 nm or less, using the Vapor-Liquid-Solid (VLS) process. They are being intensely studied for a variety of applications, ranging from transistors to sensors to photovoltaics. However, depending on growth conditions, nanowires can grow in different orientations, or can kink from one direction into another. Therefore reliable fabrication requires an understanding of the essential growth processes and failure mechanisms. A new theoretical approach makes it possible to model the growth of nanowires and understand many of the puzzling aspects of growth. Simulation results will be shown illustrating several important phenomena in nanowire growth. These include orientation selection, kinking, and jog formation, as well as sawtooth patterning of the wire sidewall. We also show how failure to initiate a freestanding wire can lead to “lateral nanowires”, where the catalyst crawls along the surface. Because of the competition between different growth modes, small variations in the conditions can lead to quite different nanowire morphologies. Finally, we find novel oscillatory growth modes. Comparison between simulations and experiments confirms that the simulations capture the most important features of nanowire growth.

Thursday, March 29, 2012

Understanding the Complex Growth of Silicon Nanowires
Jerry Tersoff
IBM
12:00 Noon, CHEM 260

Semiconductor nanowires can be readily grown with diameters down to 10 nm or less, using the Vapor-Liquid-Solid (VLS) process. They are being intensely studied for a variety of applications, ranging from transistors to sensors to photovoltaics. However, depending on growth conditions, nanowires can grow in different orientations, or can kink from one direction into another. Therefore reliable fabrication requires an understanding of the essential growth processes and failure mechanisms. A new theoretical approach makes it possible to model the growth of nanowires and understand many of the puzzling aspects of growth. Simulation results will be shown illustrating several important phenomena in nanowire growth. These include orientation selection, kinking, and jog formation, as well as sawtooth patterning of the wire sidewall. We also show how failure to initiate a freestanding wire can lead to “lateral nanowires”, where the catalyst crawls along the surface. Because of the competition between different growth modes, small variations in the conditions can lead to quite different nanowire morphologies. Finally, we find novel oscillatory growth modes. Comparison between simulations and experiments confirms that the simulations capture the most important features of nanowire growth.

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