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Sehun Kim

Publications and source records attributed to Sehun Kim.

14 recordsLinked to original sources
PubMed2006

Bond character of thiophene on Ge(100): Effects of coverage and temperature.

We have studied the adsorption and decomposition of thiophene (C4H4S) on Ge(100) using scanning tunneling microscopy (STM), high-resolution core-level photoemission spectroscopy (HRPES), and density functional theory (DFT) calculation. Analysis of S 2p core-level spectra reveals three adsorption geometries, which we assign to a Ge-S dative bonding state, a [4 + 2] cycloaddition bonding state, and a decomposed bonding state (desulfurization reaction product). Furthermore, we found that the number ratio of the three adsorption geometries depended on the molecular coverage and the annealing temperature. At low coverages, the kinetically favorable dative bonding state is initially formed at room temperature. As the molecular coverage increases, thermodynamically stable [4 + 2] cycloaddition reaction products are additionally produced. In addition, we found that as the surface temperature increased, the [4 + 2] cycloaddition reaction product either possibly desorbed as molecular thiophene or decomposed to form a metallocycle-like species (C4H4Ge2) and a sulfide (Ge2S). We systematically elucidate the changes in the bonding states of adsorbed thiophene on Ge(100) according to the thiophene coverage and annealing temperature.

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PubMed2006

Self-induced 1-D molecular chain growth of thiophene on Ge(100).

The adsorption of thiophene on Ge(100) has been studied using scanning tunneling microscopy (STM), high-resolution core-level photoemission spectroscopy (HRPES), and density functional theory (DFT) calculations. Until now, thiophene is known to react with the Ge(100) dimer through a [4 + 2] cycloaddition reaction at room temperature, similar to the case of thiophene on Si(100). However, we found that thiophene has two adsorption geometries on Ge(100) at room temperature, such as a kinetically favorable Ge-S dative bonding configuration and a thermodynamically stable [4 + 2] cycloaddition adduct. Moreover, our STM results show that under 0.25 ML thiophene molecules preferentially produce one-dimensional molecular chain structures on Ge(100) via the Ge-S dative bonding configuration.

Journal Article↗
PubMed2006

Self-assembled monolayers on Pt(111): molecular packing structure and strain effects observed by scanning tunneling microscopy.

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.

Journal Article↗
PubMed2006

Chemical reactions and adsorption geometries of pyrrole on Ge(100).

The adsorption structures of pyrrole (C(4)H(5)N) on a Ge(100) surface at various coverages have been investigated with both scanning tunneling microscopy (STM) and ab initio density-functional theory (DFT) calculations. Three distinct features are observed in the STM images at low coverages. The comparison of the STM images with the simulation reveals that the most dominant flowerlike feature with a dark side is that the adsorbed pyrrole molecules with H dissociated form bridges between two down Ge atoms of neighboring Ge dimer rows through N-Ge bonding and beta-carbon-Ge interaction. The flowerlike feature without a dark side is also observed as a minority, which is identified as nearly the same structure as the most dominant one where a dissociated H is out of the feature. The third feature showing bright protrusions may be due to a C- and N-end-on (CN) configuration, where the pyrrole molecule is located on one dimer row. At higher coverages, the number of localized configurations increases.

Adsorption↗
PubMed2006

Magnetic superlattices and their nanoscale phase transition effects.

The systematic assembly of nanoscale constituents into highly ordered superlattices is of significant interest because of the potential of their multifunctionalities and the discovery of new collective properties. However, successful observations of such superlattice-associated nanoscale phenomena are still elusive. Here, we present magnetic superlattices of Co and Fe(3)O(4) nanoparticles with multidimensional symmetry of either AB (NaCl) or AB(2) (AlB(2)). The discovery of significant enhancement (approximately 25 times) of ferrimagnetism is further revealed by forming previously undescribed superlattices of magnetically soft-hard Fe(3)O(4)@CoFe(2)O(4) through the confined geometrical effect of thermally driven intrasuperlattice phase transition between the nanoparticulate components.

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PubMed2006

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template.

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.

Crystallization↗
PubMed2005

Study of adsorption and decomposition of H2O on Ge(100).

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.

Journal Article↗
PubMed2005

Ethylenediamine on Ge(100)-2 x 1: the role of interdimer interactions.

We have investigated the reaction of the bifunctional molecule ethylenediamine on Ge(100)-2 x 1 using multiple internal reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Ethylenediamine exhibits different adsorption behavior than simple methylamines on the Ge(100)-2 x 1 surface. At low coverages, ethylenediamine undergoes dissociative chemisorption via an interdimer dual N-H dissociation reaction. As coverage increases, the N-H dissociation reaction is inhibited and formation of a Ge-N dative-bonded structure dominates.

Journal Article↗
PubMed2005

Langmuir monolayers of Co nanoparticles and their patterning by microcontact printing.

In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle solution over an air-water interface and then transferring it to a hydrophobic substrate by using the Langmuir-Blodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic I-V curve.

Journal Article↗
PubMed2005

Layer-by-layer growth on Ge(100) via spontaneous urea coupling reactions.

We have demonstrated the layer-by-layer growth, via a urea coupling reaction between two bifunctional molecules, ethylenediamine and 1,4-phenylene diisocyanate, to form an ultrathin film on Ge(100)-2 x 1 at room temperature under vacuum conditions. The initial adsorption and subsequent growth of each layer was studied with multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. Ethylenediamine reacts with Ge(100)-2 x 1 to produce a surface-bound amine group which is available for additional reaction. Subsequent exposure of 1,4-phenylene diisocyanate leads to a spontaneous urea coupling reaction between the surface-bound amine and the highly reactive isocyanate functional group. Three bands at 1665, 1512, and 1306 cm(-)(1) are characteristic of a urea linkage and provide evidence of the coupling reaction. The coupling procedure can be repeated in a binary fashion to create covalently bound ultrathin films at room temperature, and in the present work, we demonstrate the successful growth of four layers. In addition, we have found that an initial exposure of 1,4-phenylene diisocyanate to Ge(100)-2 x 1 produces an isocyanate-functionalized surface which, upon exposure to ethylenediamine, also forms urea linkages. This layer-by-layer deposition method provides a strategy with which to design and produce precisely tailored organic materials at semiconductor interfaces.

Journal Article↗
PubMed2005

Chiral attachment of styrene mediated by surface dimers on Ge100.

We have investigated the chiral adsorption configurations of styrene on Ge(100) using scanning tunneling microscopy at 300 K. The chemisorbed styrene on a single Ge dimer reduces the symmetry of the molecule, which produces a chiral center, and leads to the (S) or (R) chiral on-top configuration. We have found that the dimeric adsorption of styrene induced by the Ge surface dimer structure forms the enantiomeric and diastereomeric paired end-bridge configurations. We determine the absolute chirality of adsorbed styrene on Ge(100) and demonstrate a novel method for the achiral molecule to produce dimeric enantiomers and diastereomers attached to the semiconductor surface.

Adsorption↗
PubMed2005

Double dative bond configuration: pyrimidine on Ge(100).

The adsorption of pyrimidine onto Ge(100) surfaces has been investigated using real-time scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density-functional theory (DFT) calculations. Our results show that the adsorbed pyrimidine molecules are tilted about 40 degrees with respect to the Ge surface, and through a Lewis acid-base reaction form bridges between the down-Ge atoms of neighboring Ge dimer rows by double Ge-N dative bonding without loss of aromaticity. For coverages of pyrimidine up to 0.25 ML, a well-ordered c(4x2) structure results from states that appear in STM micrographs as oval-shaped protrusions, which correspond to pyrimidine molecules datively adsorbed on every other dimer. However, above 0.25 ML, the oval-shaped protrusions gradually change into brighter zigzag lines. At 0.50 ML, a p(2x2) structure results from the states that appear in STM as zigzag lines. The zigzag lines are formed by the attachment of pyrimidine molecules to the down-Ge atoms of every Ge dimer. However, the unstable p(2x2) structure eventually reconstructs into a c(4x2) structure due to steric hindrance between the adsorbed pyrimidine molecules after stopping the exposure of pyrimidine to the surface.

Adsorption↗
PubMed2003

Formation of highly ordered organic monolayers by dative bonding: pyridine on ge(100).

The adsorption of pyridine onto the Ge(100) surface has been studied using both real-time scanning tunneling microscopy (STM) and ab initio pseudopotential density functional calculations. The results show that pyridine molecules adsorb on the electron-deficient down-Ge atoms of the Ge=Ge dimers via Ge-N dative bonding, with the pyridine ring tilted to the surface. The electron-rich up-Ge atoms remaining after adsorption of pyridine induce an asymmetric dimer row, which is mainly reconstructed to the c(4 x 2) structure. At pyridine coverage of 0.25 ML, the adsorbed pyridine molecules form a perfectly ordered monolayer. The entire Ge substrate underlying this organic monolayer rearranges into the c(4 x 2) structure.

Journal Article↗