Jae Ryang Hahn

Imaging and vibrational spectroscopy of single pyridine molecules on Ag(110) using a low-temperature scanning tunneling microscope

A scanning tunneling microscope (STM) was used to extract the images of single, isolated pyridine molecules adsorbed on Ag(110) and to record their vibrational spectrum at 13 K. On the STM image, the pyridine molecule appears as an elongated protrusion along the [001] direction on top of a silver atom, indicating that it is bonded through its nitrogen lone pair electrons. STM inelastic electron tunneling spectroscopy of the adsorbed pyridine revealed C-D and C-H stretch modes at 282 and 378 meV, respectively.

Binding characteristics of pyridine on Ag"110…

A combination of low-temperature scanning tunneling microscopy and density functional theory calculations was used to determine the binding characteristics of single pyridine molecules at a low coverage on a silver surface. The results indicated that pyridine binds to silver through the nitrogen atom in either a perpendicular or a parallel configuration with the latter structure being more prevalent. Both configurations are produced predominantly through electrostatic interaction between nitrogen and silver atoms. This is induced by charge redistribution in the pyridine molecule and nearby silver atoms upon pyridine adsorption.

Adsorption structures of benzene on a Si(5512)-2×1 surface: A combined scanning tunneling microscopy and theoretical study

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2x1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-sigma bonds with the substrate and of two C[Double Bond]C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03-1.20 eV on the adatoms and 0.22 eV on the honeycomb chains.

Vibrational mode specific bond dissociation in a single molecule

Tunneling electrons from a scanning tunneling microscope were used to image and dissociate single O(2)-water-O complexes adsorbed on a Ag(110) surface at 13 K. The dissociation rate was measured as a function of the energy and current of the tunneling electrons; an increase was found in this rate by a factor of approximately 100 at an electron energy equivalent to that of the O-H (D) stretch vibration. These results indicate that the rate of bond dissociation is competitive with the other energy dissipation pathways of the stretch vibration. The barrier to the dissociation of the water molecules is lowered by the formation of hydrogen bonds with oxygen species.

Di–σ and Dative Binding of Benzene and Pyridine on a Si(5,5,12)-2 ×1

We investigated the adsorption of benzene and pyridine on Si(5,5,12)-(2×1) at 80 K by using variable-low temperature scanning tunneling microscopy and density functional theory calculations. The benzene molecule most strongly binds to two adatoms on the D3 and D2 units in a tilted butterfly configuration, which consists of di–σ bonds between C atoms and Si adatoms and two C=C double bonds in the benzene molecule. Pyridine molecules interact with adatom(s) on the D2 and D3 units through both Si–N dative bonding and di–σ bonds. The dative bonding through the lone pair electrons of N atom produces a vertical configuration (pyridine-like), which is more stable than di–σ bonds. Di–σ bonds can be formed either through Si–N1 and Si–C4 or Si–C2 and Si–C5.

Hot carrier-selective chemical reactions on Ag(110)

Here, we show that the pathways, products, and efficiencies of reactions occurring on a metal surface can be spatially modulated by varying the type and energy of hot carriers produced by injecting tunneling electrons or holes from a scanning tunneling microscope tip into the metal surface. Control over the metal surface reactions was demonstrated for the large-scale dissociation reaction of O2 molecules on a Ag(110) surface. Hot electrons (or holes) transported through the metal surface to chemisorbed O2 selectively dissociated the molecule into two oxygen atoms separated along the [110] (or [001]) lattice direction. The reaction selectivity was enhanced compared to the selectivity of a direct reaction involving tunneling carriers.

The preserved aromaticity of aniline molecules adsorbed on a Si(5 5 12)−2×1 surface

We present a scanning tunneling microscopy and first-principles calculations study of the adsorption structures of aniline on a Si(5 5 12)-2x1 surface. Dissociation from the aniline molecules of one or two H atom(s) bonded to N is favored, and then adsorption onto adatom, tetramer, and dimer rows of Si(5 5 12)-2x1 occurs in several distinct configurations. On the adatom row, aniline binds to an adatom in a tilted configuration, which is formed via a sigma bond between the adatom and N, with one dissociated H atom adsorbed on a nearby adatom. No further hydrogen dissociation occurs. On the tetramer and dimer rows, the structures with two dissociated hydrogens and upright configurations are the most stable. Aniline does not adsorb onto the honeycomb chains; this adsorption configuration has a low adsorption energy. In all the adsorption configurations of aniline on this surface, the molecules aromaticity is preserved. Thus Si-N bonding of aromatic amine molecules provides a strategy for the homogeneous aromatic functionalization of high index Si surfaces.

Binding structures of propylene glycol stereoisomers on the Si(001)-2×1 surface: a combined scanning tunneling microscopy and theoretical study.

The binding configuration of propylene glycol stereoisomer molecules adsorbed on the Si(001)-2×1 surface was investigated using a combination of scanning tunneling microscopy (STM) and density functional theory calculations. Propylene glycol was found to adsorb dissociatively via two hydroxyl groups exclusively as a bridge between the ends of two adjacent dimers along the dimer row. The chirality was preserved during bonding to Si atoms and was identifiable with STM imaging. The large number of propylene glycol conformers in the gas phase was reduced to a single configuration adsorbed on the surface at low molecular coverage.