LSM - Seminars: September 2005 Archives

Seminars Archives

August 2005 | September 2005 | October 2005

Thursday, September 15, 2005
"Regionally specific hyperfine polarization of Rb atoms in the vicinity (~10e-5 cm) of Pyrex glass surfaces." Zhen Wu Department of Physics Rutgers-Newark Abstract: Hyperfine polarization < S l > of alkali-metal atoms has been studied for decades, partly because of the important role it plays in atomic frequency standards. The physical quantity measured in those studies was the bulk hyperfine polarization. However, due to surface interactions, the hyperfine polarizations in the vicinity of cell surfaces can be significantly different from that in the bulk, and information about hyperfine polarization very close to the cell surfaces is important in, e.g., designing miniature atomic clocks. We made regionally specific measurement of the hyperfine polarization of Rb atoms in the vicinity (~10-5 cm) of coated and uncoated Pyrex glass surfaces in optical pumping cells. We probe the hyperfine polarization of the Rb atoms in the vicinity of cell surfaces using the evanescent wave of a weak laser beam. We find that the polarization in the vicinity of uncoated surfaces is significantly lower than that in the bulk. The polarization decreases rapidly with decreasing distance from the surface. By contrast, the polarization in the vicinity of a silicone coated Pyrex glass surface is independent of the distance from the cell surface and is equal to the bulk polarization. Regionally specific measurement of the hyperfine polarization as a function of the penetration depth of the evanescent wave allows us to deduce the hyperfine polarization, its normal gradient, as well as the normal gradient coefficient at the cell surface. The hyperfine polarization in the vicinity of cell surfaces can also be used as a new way to quantify and map the regional surface property in coated optical pumping cells.

Thursday, September 15, 2005

"Regionally specific hyperfine polarization of Rb atoms in the vicinity (~10e-5 cm) of Pyrex glass surfaces."
Zhen Wu
Department of Physics
Rutgers-Newark

Abstract: Hyperfine polarization < S l > of alkali-metal atoms has been studied for decades, partly because of the important role it plays in atomic frequency standards. The physical quantity measured in those studies was the bulk hyperfine polarization. However, due to surface interactions, the hyperfine polarizations in the vicinity of cell surfaces can be significantly different from that in the bulk, and information about hyperfine polarization very close to the cell surfaces is important in, e.g., designing miniature atomic clocks. We made regionally specific measurement of the hyperfine polarization of Rb atoms in the vicinity (~10-5 cm) of coated and uncoated Pyrex glass surfaces in optical pumping cells. We probe the hyperfine polarization of the Rb atoms in the vicinity of cell surfaces using the evanescent wave of a weak laser beam. We find that the polarization in the vicinity of uncoated surfaces is significantly lower than that in the bulk. The polarization decreases rapidly with decreasing distance from the surface. By contrast, the polarization in the vicinity of a silicone coated Pyrex glass surface is independent of the distance from the cell surface and is equal to the bulk polarization. Regionally specific measurement of the hyperfine polarization as a function of the penetration depth of the evanescent wave allows us to deduce the hyperfine polarization, its normal gradient, as well as the normal gradient coefficient at the cell surface. The hyperfine polarization in the vicinity of cell surfaces can also be used as a new way to quantify and map the regional surface property in coated optical pumping cells.

Thursday, September 22, 2005
"A theoretical study of the surface chemistry of atomic layer deposition and organic functionalization of semiconductors." Charles B. Musgrave Department of Chemical Engineering Stanford University Abstract:ALD is a deposition process capable of depositing uniform and conformal ultra thin films over large areas. These characteristics are the result of the self-limiting nature of reactions between the precursor and the surface. Although ALD is rapidly becoming a process technology for the fabrication of future integrated circuits many issues remain to be understood. Unfortunately, the development of new ALD processes has been largely empirical with little fundamental surface science or theoretical chemical simulations employed to study its underlying chemistry. Our motivation is to employ quantum chemistry to study the surface reactions involved in ALD to provide a rational, fundamental basis for ALD chemistry from which new process conditions and chemistries can be developed. We have used DFT to predict the chemical mechanisms of several important ALD systems to establish a fundamental framework of principles governing ALD. The systems we have investigated include various important electronic materials including; HfO2, ZrO2, SiO2, Al2O3, Si3N4, AlN and HfN, each deposited using various precursors and substrates. Our results predict important ALD characteristics for each chemical system from which trends are uncovered and the viability of processes determined. The governing ALD principles and specific trends we can be used to screen, select and optimize ALD process choices. Furthermore, we and others are developing new processes, such as area-selective ALD, ALD on organic materials and ALD on high-mobility substrates guided by the results of this work. We have also investigated the chemistry of various organic compounds on silicon and germanium surfaces. I will discuss the reactions of some novel functional groups on Si which exhibit unique properties relative to other systems previously explored.

Thursday, September 22, 2005

"A theoretical study of the surface chemistry of atomic layer deposition and organic functionalization of semiconductors."
Charles B. Musgrave
Department of Chemical Engineering
Stanford University

Abstract:ALD is a deposition process capable of depositing uniform and conformal ultra thin films over large areas. These characteristics are the result of the self-limiting nature of reactions between the precursor and the surface. Although ALD is rapidly becoming a process technology for the fabrication of future integrated circuits many issues remain to be understood. Unfortunately, the development of new ALD processes has been largely empirical with little fundamental surface science or theoretical chemical simulations employed to study its underlying chemistry. Our motivation is to employ quantum chemistry to study the surface reactions involved in ALD to provide a rational, fundamental basis for ALD chemistry from which new process conditions and chemistries can be developed.

We have used DFT to predict the chemical mechanisms of several important ALD systems to establish a fundamental framework of principles governing ALD. The systems we have investigated include various important electronic materials including; HfO2, ZrO2, SiO2, Al2O3, Si3N4, AlN and HfN, each deposited using various precursors and substrates. Our results predict important ALD characteristics for each chemical system from which trends are uncovered and the viability of processes determined. The governing ALD principles and specific trends we can be used to screen, select and optimize ALD process choices. Furthermore, we and others are developing new processes, such as area-selective ALD, ALD on organic materials and ALD on high-mobility substrates guided by the results of this work.

We have also investigated the chemistry of various organic compounds on silicon and germanium surfaces. I will discuss the reactions of some novel functional groups on Si which exhibit unique properties relative to other systems previously explored.

Thursday, September 29, 2005
"Nanocatalysis at Brookhaven." Alex Harris Chair, Dept. of Chemistry Brookhaven National Laboratory 12:00 noon, Room 260, Wright-Rieman Chemistry Laboratory Abstract: While dispersion of catalytic particles at the nanometer scale has long been of importance in heterogeneous catalysis, there is a growing interest in controlled manipulation and characterization of structure and reactivity at atomic to nanometer scales that can be termed ‘nanocatalysis.’ I will describe three areas where advances are being pursued at BNL. (1) Platinum monolayer core-shell nanoparticles are dramatically improving fuel cell electrocatalysts for the challenging cathode oxygen reduction reaction by systematic variation of reactivity through d-band manipulation and other tricks. (2) Surface deposited nanostructures of transition metal carbides and oxides prepared through a new technique of Reactive Layer Assisted Deposition (RLAD) show unique selectivity in reactions such as partial dydrogenation to benzene and provide a route to a range of dispersed, supported nanoparticles for basic studies. (3) Mass-selected cluster sources provide well-defined clusters of metal compounds such as metal sulfides and carbides for reactivity studies and ultimately for connection of reactivity to structure in exactly determined cluster structures. I will also briefly mention links to the planned Center for Functional Nanomaterials at BNL which will enhance nanocatalysis research and provide a setting for stronger external interactions through collaboration and a user program.

Thursday, September 29, 2005

"Nanocatalysis at Brookhaven."
Alex Harris
Chair, Dept. of Chemistry
Brookhaven National Laboratory
12:00 noon, Room 260, Wright-Rieman Chemistry Laboratory

Abstract: While dispersion of catalytic particles at the nanometer scale has long been of importance in heterogeneous catalysis, there is a growing interest in controlled manipulation and characterization of structure and reactivity at atomic to nanometer scales that can be termed ‘nanocatalysis.’ I will describe three areas where advances are being pursued at BNL. (1) Platinum monolayer core-shell nanoparticles are dramatically improving fuel cell electrocatalysts for the challenging cathode oxygen reduction reaction by systematic variation of reactivity through d-band manipulation and other tricks. (2) Surface deposited nanostructures of transition metal carbides and oxides prepared through a new technique of Reactive Layer Assisted Deposition (RLAD) show unique selectivity in reactions such as partial dydrogenation to benzene and provide a route to a range of dispersed, supported nanoparticles for basic studies. (3) Mass-selected cluster sources provide well-defined clusters of metal compounds such as metal sulfides and carbides for reactivity studies and ultimately for connection of reactivity to structure in exactly determined cluster structures. I will also briefly mention links to the planned Center for Functional Nanomaterials at BNL which will enhance nanocatalysis research and provide a setting for stronger external interactions through collaboration and a user program.

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