Chain polymerization of diacetylene compound multilayer films on the topmost surface initiated by a scanning tunneling microscope tip

Chain polymerizations of diacetylene compound multilayer films on graphite substrates were examined with a scanning tunneling microscope (STM) at the liquid/solid interface of the phenyloctane solution. The first layer grew very quickly into many small domains. This was followed by the slow formation of the piled up layers into much larger domains. Chain polymerization on the topmost surface layer could be initiated by applying a pulsed voltage between the STM tip and the substrate, usually producing a long polymer of submicrometer length. In contrast, polymerizations on the underlying layer were never observed. This can be explained by a conformation model in which the polymer backbone is lifted up.     

Surface-enhanced ATR-IR spectroscopy with interface-grown plasmonic gold-island films near the percolation threshold

Flat nano-island films prepared by wet-chemical deposition were investigated with attenuated total reflection infrared (ATR-IR) spectroscopy and scanning electron microscopy (SEM) in order to analyze the correlation between film morphology and optical properties. Here we choose Au as representative coinage metal (Au, Ag, Cu) that shows strong structure-dependent surface-enhanced infrared absorption (SEIRA). Infrared spectra of octadecanethiol monolayers on films of different stages of morphologic development show effects that are characteristic for SEIRA, such as enhanced vibrational signals, Fano-type line shape, and adsorbate induced baseline shifts. Their extent was found to be strongly dependent on the structural details and the strongest enhancement occurs at the percolation threshold of the two-dimensional island system. Also films beyond percolation show significant enhancement due to residual nanoholes that are acting as hotspots.     

Solution reaction space Hamiltonian based on an electrostatic potential representation of solvent dynamics

Quantum chemical solvation models usually rely on the equilibrium solvation condition and is thus not immediately applicable to the study of nonequilibrium solvation dynamics, particularly those associated with chemical reactions. Here we address this problem by considering an effective Hamiltonian for solution-phase reactions based on an electrostatic potential (ESP) representation of solvent dynamics. In this approach a general ESP field of solvent is employed as collective solvent coordinate, and an effective Hamiltonian is constructed by treating both solute geometry and solvent ESP as dynamical variables. A harmonic bath is then attached onto the ESP variables in order to account for the stochastic nature of solvent dynamics. As an illustration we apply the above method to the proton transfer of a substituted phenol-amine complex in a polar solvent. The effective Hamiltonian is constructed by means of the reference interaction site model self-consistent field method (i.e., a type of quantum chemical solvation model), and a mixed quantum/classical simulation is performed in the space of solute geometry and solvent ESP. The results suggest that important dynamical features of proton transfer in solution can be captured by the present approach, including spontaneous fluctuations of solvent ESP that drives the proton from reactant to product potential wells.     

Synthesis and luminescence properties of lead-halide based organic–inorganic layered perovskite compounds (CnH2n+1NH3)2PbI4 (n=4, 5, 7, 8 and 9)

This paper describes the synthesis and characterization of organic–inorganic layered perovskite compounds, (C_nH_2n+1NH₃)₂PbI₄ (n=4, 5, 7, 8 and 9). The effect of the number of carbon atoms on luminescence properties has been examined. Thin films of microcrystalline (C_nH_2n+1NH₃)₂PbI₄ fabricated by spin-coating are highly oriented, with the c-axis perpendicular to the substrate surface. Temperature-dependent optical absorption spectra reveal that (C_nH_2n+1NH₃)₂PbI₄ films (n=4, 7, 8 and 9) show the structural phase transitions. The excitonic structures of (C_nH_2n+1NH₃)₂PbI₄ vary with the number of carbon atoms of the alkyl chain length. At low temperatures below 100 K, the lowest-energy free-exciton band of (C_nH_2n+1NH₃)₂PbI₄ (n=7, 8 and 9) split into three fine-structure levels. In contrast to (C_nH_2n+1NH₃)₂PbBr₄ films, (C_nH_2n+1NH₃)₂PbI₄ (n=7, 8 and 9) shows no triplet exciton emission, but it shows the Stokes-shifted emission from bound excitons.     

Electrical conduction of organic ultrathin films evaluated by an independently driven double-tip scanning tunneling microscope

The electrical transport properties of organic thin films within the micrometer scale have been evaluated by a laboratory-built independently driven double-tip scanning tunneling microscope, operating under ambient conditions. The two tips were used as point contact electrodes, and current in the range from 0.1 pA to 100 nA flowing between the two tips through the material can be detected. We demonstrated two-dimensional contour mapping of the electrical resistance on a poly(3-octylthiophene) thin films as shown below. The obtained contour map clearly provided an image of two-dimensional electrical conductance between two point electrodes on the poly(3-octylthiophene) thin film. The conductivity of the thin film was estimated to be (1-8) × 10(-6) S cm(-1). Future prospects and the desired development of multiprobe STMs are also discussed.     

Cationic gel crystals of lightly cross-linked poly(2-vinylpyridine) spheres (170∼180 nm in diameter) in the deionized aqueous suspension

Colloidal crystallization of deionized suspensions of the cationic gel spheres of lightly cross-linked poly(2-vinylpyridine), AIBA-P2VP (170～180 nm in diameter) has been studied from the microscopic observation, morphology, phase diagram, and elastic property. Critical concentrations of melting that coexisted with ion-exchange resins were low compared with those without resins and decreased as the degree of cross-linking decreased. The density of a gel sphere in suspension state (ρ), i.e., weight percent of the gel spheres divided by the corresponding volume percent, was between 0.5 and 0.8, and decreased as the degree of cross-linking of the spheres decreased. The ρ values also decreased with decreasing size of gel spheres, which supports the small P2VP gel spheres being softer than the large ones. The closest intersphere distances of the crystals were much longer than the hydrodynamic diameters of the gel spheres especially at low sphere concentrations. Fluctuation parameters evaluated from the rigidities of the crystals of AIBA-P2VP (0.05～0.09) were similar to those of gel crystals of cationic gel spheres of lightly cross-linked poly(2-vinylpyridine) spheres coated with poly(ethylene glycol), 400 nm in diameter, and thermo-sensitive gel spheres of pNIPAm, poly(N-isopropylacrylamide), but larger than those of typical colloidal hard spheres. The stable crystal phase formed beyond the adsorbed monolayer of cationic gel spheres on the surface of the substrate. These experimental findings support important role of the extended electrical double layers around the cationic gel spheres in addition to the excluded volume effect of the sphere themselves on the crystallization.     

Optical Spectroscopic Studies on Structural Changes of Helical-rich Proteins in Aqueous Solutions of Ionic Liquids

We have investigated structural changes of myoglobin and cytochrome c, which are helical-rich proteins, in aqueous 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) solutions by Fourier transform infrared and circular dichroism spectroscopy. At low [bmim][Cl] concentrations {X (mol% IL) < 10}, both proteins unfold. Remarkably, at high [bmim][Cl] concentrations (X > 10), myoglobin aggregates whereas cytochrome c refolds its α-helical structure. The tertiary structures of both proteins are disrupted over the entire range of studied [bmim][Cl] concentrations. Our results suggest that, in aqueous solutions at high [bmim][Cl] concentrations, the differences in structural transitions between myoglobin and cytochrome c might be due to the difference in hydration between these proteins.