Browsing by Author "Morgenstern, Karina"

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Determination of dissociative fragment-adsorbate interaction energy during chemisorption of the diatomic molecule HCl on Si(100)
(2010-01-15)
Hsieh, Ming-Feng
;
Cheng, Jen-Yin
;
Yang, Jenq-Cheng
;
Lin, Deng-Sung
;
Morgenstern, Karina
This study investigates the surface chemistry and the ordering characteristics of coadsorbed hydrogen and chlorine atoms, generated by the exposure of the Si(100) surface to gas-phase HCl molecules at various substrate temperatures, by scanning tunneling microscopy (STM), core-level photoemission spectroscopy, and Monte Carlo simulation. Experimental results show that saturation exposure to HCl causes all surface dangling bonds to be terminated by the two fragments H and Cl atoms and that the number of H-terminated sites exceeds that of Cl-terminated ones by more than 10%. This finding suggests that, in addition to the dominant dissociative chemisorption, atomically selective chemisorption or atom abstraction occurs. STM images reveal that some Cl-terminated sites form patches with a local 2 x 2 structure at 110 K and that the degree of ordering is reduced as the substrate temperature increases. Results of Monte Carlo simulations demonstrate the importance of including dissociative fragment-adsorbate interactions during the random adsorption of diatomic molecules. Comparing the correlations between Cl-terminated sites identified from STM images and those predicted by simulation reveals two effective interaction energies of 8.5 +/- 2.0 and 3.5 +/- 2.0 meV between a dissociative fragment Cl atom and a nearest neighboring Cl adsorbates in the same dimer row and in the adjacent row, respectively.
Formation, Binding, and Stability of O-Ag-CO(2)-Ag-O Compounds on Ag(100) Investigated by Low Temperature Scanning Tunneling Microscopy and Manipulation
(2010-08-26)
Hsieh, Ming-Feng
;
Li, Hong-Doa
;
Lin, Deng-Sung
;
Morgenstern, Karina
;
物理研究所
The understanding of reaction intermediates in heterogeneous catalysis has important implications for the design of novel catalysts. We investigate the adsorption of CO(2) on oxygen precovered Ag(100) at low temperature (17 K) by scanning tunneling microscopy and inelastic electron tunneling manipulation at 5 K. On the terraces, the adsorption leads to O-Ag-CO(2)-Ag-O compounds with reduced binding of the oxygen to the surface as compared to the separately adsorbed molecules. The compound can be either dissociated into a bistable O-Ag-CO(2) compound at 1.6 V, dissociated into its constituents at 2.2 V, or reacted at 6.5 V into a species, which we tentatively attribute to CO(3). The thus obtained carbon trioxide or carbonate is an intriguing reaction intermediate, because it is not stable in the gas phase. Our detailed study of coadsorbed species outlines a possibility to investigate precursors of reactions that involve the substrate atoms.
Formation, Binding, and Stability of O-Ag-CO2-Ag-O Compounds on Ag(100) Investigated by Low Temperature Scanning Tunneling Microscopy and Manipulation
(2010-08-26)
Hsieh, Ming-Feng
;
Li, Hong-Doa
;
Lin, Deng-Sung
;
Morgenstern, Karina
;
物理研究所
The understanding of reaction intermediates in heterogeneous catalysis has important implications for the design of novel catalysts. We investigate the adsorption of CO2 on oxygen precovered Ag(100) at low temperature (17 K) by scanning tunneling microscopy and inelastic electron tunneling manipulation at 5 K. On the terraces, the adsorption leads to O-Ag-CO2-Ag-O compounds with reduced binding of the oxygen to the surface as compared to the separately adsorbed molecules. The compound can be either dissociated into a bistable O-Ag-CO2 compound at 1.6 V, dissociated into its constituents at 2.2 V, or reacted at 6.5 V into a species, which we tentatively attribute to CO3. The thus obtained carbon trioxide or carbonate is an intriguing reaction intermediate, because it is not stable in the gas phase. Our detailed study of coadsorbed species outlines a possibility to investigate precursors of reactions that involve the substrate atoms.
Hard repulsive barrier in hot adatom motion during dissociative adsorption of oxygen on Ag(100)
(2009-11-07)
Hsieh, Ming-Feng
;
Lin, Deng-Sung
;
Gawronski, Heiko
;
Morgenstern, Karina
;
物理研究所
Random pairing simulation and low temperature scanning tunneling microscopy (STM) are used to investigate the detailed O(2) dissociative adsorption processes at 200 K for various coverages. The distribution of oxygen adatoms shows a strong repulsion between the adsorbates with a radius of similar to 0.8 nm. The comparison between STM results and simulation reveals two prominent pairing distances of 2 and 4 nm and their branching ratio is about 2:1. These findings shed new light on the origin of the large intrapair distances found and on the process behind the empirical "eight-site rule.".