Browsing by Author "Lin, Deng-Sung"

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Apparent topographic height variations measured by noncontact atomic force microscopy
(2007-07-01)
Yang, Kai-Ming
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Chung, Jen-Yang
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Hsieh, Ming-Feng
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Lin, Deng-Sung
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物理研究所
The topographic height measurement on a sample consisting of domains of different materials in noncontact atomic force microscopy (NC-AFM) is typically incorrect owing to the variation in electrostatic force between a tip and a sample. The tip-sample electrostatic force is owing to the difference in effective contact potential between a tip and a sample. This study demonstrates that the error in height strongly depends on the bias applied between the tip and the sample, the radius of the tip apex. the work function difference, and the frequency shift. Experimental results are well explained by integrated model calculations and by including the van der Waals and electrostatic forces between the tip and the sample in the analysis. When the simultaneous compensation of contact potentials during imaging is not performed, the errors occurring in the height measurement can be estimated from the tip-sample distance vs the bias curves obtained in situ.
Correlation of reaction sites during the chlorine extraction by hydrogen atom from Cl/Si(100)-2X1
(2007-07-21)
Hsieh, Ming-Feng
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Chung, Jen-Yang
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Lin, Deng-Sung
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Tsay, Shiow-Fon
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物理研究所
The Cl abstraction by gas-phase H atoms from a Cl-terminated Si(100) surface was investigated by scanning tunneling microscopy (STM), high-resolution core level photoemission spectroscopy, and computer simulation. The core level measurements indicate that some additional reactions occur besides the removal of Cl. The STM images show that the Cl-extracted sites disperse randomly in the initial phase of the reaction, but form small clusters as more Cl is removed, indicating a correlation between Cl-extracted sites. These results suggest that the hot-atom process may occur during the atom-adatom collision. (C) 2007 American Institute of Physics.
Determination of dissociative fragment-adsorbate interaction energy during chemisorption of the diatomic molecule HCl on Si(100)
(2010-01-15)
Hsieh, Ming-Feng
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Cheng, Jen-Yin
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Yang, Jenq-Cheng
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Lin, Deng-Sung
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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
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Li, Hong-Doa
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Lin, Deng-Sung
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Morgenstern, Karina
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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.
Growth behaviour of Ge nano-islands on the nanosized Si{111} facets bordering on two {100} planes
(2006-10-28)
Ferng, Shyh-Shin
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Yang, Tsung-Hsi
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Luo, Guangli
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Yang, Kai-Ming
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Hsieh, Ming-Feng
Si( 100) substrates were used to fabricate various nanosized {111} facets between the ( 100) planes using photolithography and anisotropic wet chemical etching. Following simultaneous Ge chemical vapour deposition on the neighbouring ( 100) and {111} facets, the Ge nano-island formation and distribution was observed on both the ( 100) terraces and the {111} side walls using a dynamical atomic force microscope. The nano-island formation on the nanosized {111} strip facets was found to be strongly suppressed upon reducing the strip width due primarily to the interaction of adatoms on the neighbouring facets. Specifically, the difference in the effective chemical potential of Ge adatoms on the two neighbouring facets leads to the depletion of nano-islands on the {111} strip with width < 500 nm under the growth condition used in this study.
Hard repulsive barrier in hot adatom motion during dissociative adsorption of oxygen on Ag(100)
(2009-11-07)
Hsieh, Ming-Feng
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Lin, Deng-Sung
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Gawronski, Heiko
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Morgenstern, Karina
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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.".
Mediation of chain reactions by propagating radicals during halogenation of H-masked Si(100): Implications for atomic-scale lithography and processing
(2009-04-28)
Ferng, Shyh-Shin
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Wu, Shiao-Ting
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Lin, Deng-Sung
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Chiang, Tai C.
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物理研究所
Scanning tunneling microscopy reveals a free radical-induced surface chain reaction in the chlorination of nanoscale patterns on an otherwise H-passivated (masked) Si (100). While scanning probe methods can be used to pattern active surface regions with single-bond precision, follow-up selective chemical vapor deposition with polyatomic molecules can produce various filling characteristics. On active surface regions, molecular Cl(2) undergoes an atom abstraction reaction in which a Si dangling bond abstracts one atom of the incident Cl(2) molecule while the complementary Cl atom is scattered away from the initial abstraction site either back into the vacuum or to be captured by a second dangling bond and adsorbed there, or to react with a nearby adsorbed H atom to form volatile HCl. In contrast, I(2) undergoes only dissociative adsorption on two immediately neighboring dangling bonds, whereby two I-Si bonds are formed simultaneously upon cleavage of the I(2) bond. The different chemisorption processes of the two model diatomic molecular gases place intrinsic limitations on atomic-scale lithography and processing: Adsorption of Cl(2) results in spillage over the prepatterned regions of active bonds. In contrast, adsorption of I(2) is a pair process and results in under-filling. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3122987]
Possibility of direct exchange diffusion of hydrogen on the Cl/Si(100)-2X1 surface
(2009-07-01)
Hsieh, Ming-Feng
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Lin, Deng-Sung
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Tsay, Shiow-Fon
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物理研究所
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Institute of Physics
The diffusion behavior of hydrogen substitutional sites on the chlorine-terminated Si(100) surface was investigated at elevated temperatures using time-lapse scanning tunneling microscopy (STM). STM movies show that each hydrogen atom undergoes Brownian motion within a monochloride dimer row. The position of a hydrogen substitutional site is exchanged directly with that of an immediate neighboring chlorine atom in either the same dimer (intradimer diffusion) or in one of the two adjacent dimers in the same row (intrarow diffusion). Accordingly, conceptual direct exchange diffusion (DED) in a two-dimensional lattice was experimentally observed. Analysis of STM movies at various temperatures yielded rather low attempt frequencies and energy barriers, leading to the suggestion that the diffusion mechanism involves an intermediate low-energy molecular state. Density-functional theory (DFT) calculations were also performed and provided partial support for the proposed diffusion mechanism.
Sodium chloride on Si(100) grown by molecular beam epitaxy
(2011-02-25)
Chung, Jen-Yang
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Li, Hong-Dao
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Chang, Wan-Heng
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Leung, T. C.
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Lin, Deng-Sung
Sodium chloride (NaCl) films were grown on an Si(100)-(2 x 1) surface at near room temperature by molecular beam epitaxy (MBE). The atomic structure and growth mode of the prototypical ionic materials on the covalent bonded semiconductor surface is examined by synchrotron core-level x-ray photoemission spectrum (XPS), scanning tunneling microscopy (STM), and first-principles calculations. The Si 2p, Na 2p, and Cl 2p core-level spectra together indicate that adsorbed NaCl molecules at submonolayer coverage [i.e., below 0.4 monolayer (ML)] partially dissociate and form Si-Cl species, and that a significant portion of the dangling-bond characteristics of the clean surface remains after NaCl deposition of 1.8 MLs. The deposition of 0.65-ML NaCl forms a partially ordered adlayer, which includes NaCl networks, Si-Cl species, adsorbed Na species, and isolated dangling bonds. The STM results revealed that the first adlayer consists of bright protrusions which form small c(2 x 4) and (2 x 2) patches. Above 0.65 ML, the two-dimensional NaCl double-layer growth proceeds on top of the first adlayer.