Arifumi Okada

Effect of solvent evaporation temperature on the structure of two-dimensional melamine networks on Au(111)

By scanning tunneling microscopy (STM), we investigated two-dimensional (2D) structures of melamine formed on Au(111) surfaces by solvent evaporation. By increasing the evaporation temperature, the well-known ordered honeycomb 2D molecular phase, in which all molecules are linked by hydrogen bonding, changes to four coexisting phases, i.e., a 2D network consisting of linear segments, 1D molecular rows, and hexagonal and distorted hexagonal structures. The first two phases are sometimes observed in ultrahigh vacuum (UHV) on metallic substrates other than Au. The last two phases have lattice parameters close to those of the well-known honeycomb structure. The structural change observed in this study is attributed to local temperature and concentration distributions of the solution and substrate surface during solvent evaporation. From the results, we found that the molecular nanostructures can be tailored by the solvent evaporation method with small changes in temperature.

Magnesiothermic reduction of silica glass substrate—Chemical states of silicon in the generated layers

Abstract We applied magnesiothermic reduction to silica glass substrates at various conditions including solid state or solid-Mg liquid reaction, and solid-Mg vapor reaction. Magnesium silicide with highly oriented to the 〈110〉 direction against the substrate surface was obtained in the solid state reaction at temperatures from 600 °C to 700 °C using Mg grains while Si crystallites were obtained in the reaction with Mg film deposited on the glass substrate at 560 °C. On the other hand, in the reduction with Mg vapor at 575 °C, brown and amorphous layer was formed on the surface of the silica glass substrate. The layer was transparent in the visible and near-infrared regions, and showed an interference pattern in the transmission spectra, indicating the homogeneity of the layer. The thickness and refractive index were estimated as 770 nm and 1.94, respectively. As the reaction with Mg vapor proceeded further, Mg2Si and MgO crystallites were formed. Oxidation states and their depth profiles of silicon atoms in the layers were investigated by X-ray photoelectron spectroscopy. The silicon atoms in the brown and amorphous layer existed in intermediate oxidation states, −2 and +3. The reaction proceeding, the formal charge of the silicon atoms varied to −4 corresponding to Mg2Si and +2.

Study of MgAl2O4(111) Spinel Surface Using Noncontact Atomic Force Microscopy

The surface structures of MgAl2O4(111) were investigated using noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS) attached with a scanning electron microscope (SEM). Two different specimens with different XPS Mg/Al intensity ratios were examined. The nc-AFM image of the sample with high Mg content shows formation of nanorods on the large steps and rough terraces. The nanorod is assumed to be MgO, which was precipitated from the MgO-rich solid solution. On the other hand, another sample (close to MgO:Al2O3=1:1) show flat terraces and relatively small step heights corresponding to one period of the (111) stacking sequence of MgAl2O4(111). Thus we found that the deviations of bulk compositions strongly affect the surface structures.

DFT study of the stereoselective formation of cis-W(CO)4(η2-C2H4)2 in the gas-phase consecutive coordination of C2H4 onto doubly coordinatively unsaturated W(CO)4.

B3LYP-based density functional theory (DFT) calculations are reported that provide insight into the stereoselective formation of cis-W(CO)(4)(η(2)-C(2)H(4))(2) observed in the pulsed 266 nm laser photolysis of tungsten hexacarbonyl (W(CO)(6)) in the presence of C(2)H(4) in the gas phase at room temperature (J. Phys. Chem. 1995, 99, 4558). W(CO)(4)(η(2)-C(2)H(4)) formed through the coordination of C(2)H(4) onto coordinatively unsaturated W(CO)(4) was found to have a pseudo-C(2v) symmetry (distorted trigonal bipyramid with an angle of ca. 90° between the two equatorial COs) with a bond dissociation enthalpy (BDE) of W-C(2)H(4) of 125 kJ mol(-1). In the subsequent coordination of C(2)H(4) onto the W(CO)(4)(η(2)-C(2)H(4)), having one vacant coordinatively unsaturated site, no barrier was found in the reaction path to cis-complex formation, while there was a barrier of about 89 kJ mol(-1) to the trans-complex. The calculations show that the stereoselective formation of cis-W(CO)(4)(η(2)-C(2)H(4))(2) is due to kinetic rather than thermodynamic control. The trans-W(CO)(4)(η(2)-C(2)H(4))(2) was calculated to be more stable than cis-W(CO)(4)(η(2)-C(2)H(4))(2) by about 10 kJ mol(-1). The BDE of W-C(2)H(4) in cis-W(CO)(4)(η(2)-C(2)H(4))(2) was estimated to be 61 kJ mol(-1).