Kotaro Kajikawa

p-Sexiphenyl/metal interfaces studied by photoemission and metastable atom electron spectroscopy

Abstract p -Sexiphenyl(6P)/metal (Au, Mg) interfaces were studied by ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The vacuum level shifts ( Δ ) at the interface were observed in both 6P/Au ( Δ =−0.8±0.1 eV) and 6P/Mg ( Δ =−0.4±0.1 eV) systems. The MAES spectra indicate that the 6P molecules are on the substrate with their mean molecular plane parallel to the substrate surface in both systems, and that the 6P film on Mg substrate has some disorder. Thickness dependence of UPS, MAES, and XPS spectra suggests no significant chemical interaction for 6P on Au, Mg as well as Au, Mg on 6P interfaces. In the system of Au on 6P/Au, the spectral feature of 6P in MAES spectra did not disappear even when the thickness of evaporated gold reached 20 nm, indicating a penetration of Au atoms (or the clusters) into the 6P film.

Exchange kinetics of alkanethiol self-assembled monolayers probed by attenuated total reflection with enhancement of surface plasmon resonance

Exchange kinetics for octanethiol ( C8H17SH) self-assembled monolayers (SAMs) by octadecanethiol ( C18H37SH) in solution are studied by in situ measurements of reflectivity in the attenuated total reflection (ATR) geometry with surface plasmon resonance (SPR) enhancement. While rapid exchange kinetics are found for the octanethiol SAM prepared by 1-day exposure to the ethanol solution, the octanethiol SAM prepared by 30-minute exposure exhibits only slow exchange kinetics. This fact may originate from different adsorption states, due to the gradual dimerization of the octanethiol monomers during their exposure to the solution, with dimers being less strongly adsorbed than the monomers.

A differential thermal analysis and ultraviolet photoemission study on surface freezing of n-alkanes

The surface-freezing effect of pentacontane (n-C50H102) and tetratetracontane (n-C44H90) films evaporated on a copper substrate has been investigated by differential thermal analysis (DTA) simultaneously with measurements of surface-specific ultraviolet (UV) photoemission. Two anomalies in the DTA curve were observed near the bulk melting temperature, one of which has been attributed to bulk melting. Since the temperature dependence of the surface-specific UV photoemission measurement showed a corresponding stepwise increase and decrease in the photoemission current at the two anomalies observed in the DTA, we have concluded that the other phase transition peak originates from surface freezing.

Monomolecular layer formation of amphiphilic cyclodextrin derivatives at the air/water interface

Abstract Amphiphilic heptakis(6-alkylthio-6-deoxy)-β-cyclodextrin (CD) derivatives were synthesized and their monolayer behavior on a water surface was studied on the basis of surface pressure–molecular area (π–A) isotherms. The modification involved substitution at the C-6 position in the CD molecules with S-hexyl, S-decyl, S-tetradecyl, and S-octadecyl chains. The π–A isotherms for these derivatives were recorded under various conditions, varying the compression rate, the relaxation time before and after compression. The introduction of alkyl chains changed the water-soluble character of CD molecules to amphiphilic. As amphiphilic characters of the CD derivatives differ depending on the alkyl chain length, the stability of CD derivatives on the water surface was influenced by the experimental conditions such as the compression rate and the relaxation time. Therefore, the π–A isotherms for CD derivatives, with the exception of those which were octadecylated, were not very reproducible. This may be associated with dissolution of the CD derivatives into the water subphase. This phenomenon was conspicuous when the chain length was shorter. In contrast, the monolayer of the octadecylated derivative was stable enough to maintain the surface pressure for a long time.

Reflection spectroscopy of merocyanine self-assembled monolayer on a gold substrate

Abstract Isomerization of merocyanine chromophores in a self-assembled monolayer (SAM) on a gold substrate was examined by means of visible-UV reflection spectroscopy. Anomalous peaks and/or dips observed in the spectra were ascribed to the loss of the metallic character of gold at the wavelength region shorter than 500nm. Spectral simulations based on the Lorentz oscillator model revealed that the merocyanine SAM undergoes isomerization even in a densely packed monolayer.

Orientational structure in hemicyanine self-assembled films studied by absorption spectroscopy and optical second-harmonic generation

Abstract Self-assembled monolayers (SAMs) of the hemicyanine dyes, 2-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (MeHEMI) and 4-[4[(dimethylamino)styryl]-1-docosylpyridinium bromide (C22HEMI), were successfully deposited on silica substrates by drawing a substrate from their ethanol solutions. Both hemicyanine SAMs on a hydrophilic substrate generate a strong second-harmonic (SH) light due to their polar ordering. Interferometry of the SH lights revealed that the dimethylamino group is attached to the surface in the C22HEMI SAM, while the pyridinium ion part is attached to the hydrophilic surface in the MeHEMI SAM. These results indicate that the polar orientation of the C22HEMI SAM is strongly influenced by the presence of a long alkyl chain in the chemical structure.

Spectral properties of aggregates in adsorbed dye monolayer films

Abstract Adsorbed dye monolayer films are prepared by the complexation of a cyanine dye dissolved in the aqueous subphase to stearic acid (SA), oleic acid (OA) and dihexadecyl phosphate (DHP) monolayers at the air-water interface. Using UV-Vis spectroscopy, a wide range of aggregation behaviour of the dye was observed with variation of the subphase pH and pre-treatment of the quartz substrate onto which the monolayers were transferred from the water surface. The dye complexed to a 1:1 molar mixture of SA and OA showed both monomer and J-aggregate on a hydrophobic substrate and only monomer when transferred onto a hydrophilic substrate. For the DHP/dye monolayers, changing the subphase pH and the surface pressure at transfer yielded differing amounts of monomer and aggregate, with the greatest amount of aggregate being obtained when transferring from the condensed phase. Furthermore, two different types of J-aggregate for the DHP/dye systems were observed depending upon the substrate pre-treatment.

J aggregates in a Langmuir-Blodgett monolayer probed by scanning near-field optical microscopy

Domains consisting of J aggregates in a single monolayer deposited on a silica substrate were probed by scanning near-field optical microscopy. Since the fluorescence intensity changes smoothly from the center to the edge of a domain, it is likely that the J aggregates are homogeneously distributed in each domain of the monolayer.

Differential thermal analysis and ultraviolet photoemission study on the surface freezing effect of n-alkane

Abstract We have performed differential thermal analysis (DTA) and surface-specific ultraviolet (UV) photoemission measurements to investigate the surface freezing effect of n -alkanes. The samples used in the experiments were pentacontane ( n -C 50 H 102 : 650 A) and tetratetracontane ( n -C 44 H 90 : 600A) films evaporated on a copper substrate. We have observed a monolayer phase transition by DTA and proved that the transition is of first order. Since the temperature dependence of the surface-specific UV photoemission measurement showed a stepwise increase and decrease of the photoemission current at the transitions observed in DTA, we could conclude that the monolayer phase transition peak originates from the surface freezing effect.

Ultrathin-film Local Oscillator for Determination of Complex Components of Second-Order Nonlinear Susceptibility

The second-harmonic generation (SHG) interferometry technique has to date been used to determine the phase of second-harmonic (SH) light generated in a single polarization combination of a polarizer and an analyzer. However, it is not easy to compare phases of the SH light generated by a plural polarization combination, because a thick local oscillator, such as an α-quartz single crystal, has several disadvantages for this purpose. The birefringence and optical rotatory power of the quartz crystal often require further consideration on the phase relation of the SH lights with different polarizations. Thus, the full determination of the complex χ-components of an organic ultrathin film by the use of the quartz crystal is impractical. After surveying SH active systems, we found that a 2-docosylamino-5-nitropyridine (DCANP) Langmuir-Blodgett (LB) film is the most applicable for the phase measurements. A few nanometers-thick DCANP LB film, which is much thinner than the wavelength of the SH light, yields a negligible phase delay even if birefringence is present. The SHG efficiency of the film can be easily controlled by changing the number of layers. Some experimental results are addressed to demonstrate the advantage of the DCANP LB film as a local oscillator.