Taishi Kuri

Direct observation of defect-diminished fatty acid monolayers and their optical applications

Abstract The aggregation structure of fatty acid monolayers on the water surface have been classified with respect to thermal ( T sp , T αc , T m ) and chemical (the degree of ionic dissociation of hydrophilic group) factors. In the case of amphiphiles with a non-ionic hydrophilic group, at T sp below T m , the monolayer is in a crystalline phase which is designated “the crystalline monolayer”. The crystalline monolayer is further classified into “the fusing-oriented crystalline monolayer” and “the randomly assembled crystalline monolayer” at T sp below and above T αc , respectively. At T sp above T m , the monolayer is in an amorphous phase which is designated “the amorphous monolayer”. In the case of amphiphiles with an ionic hydrophilic group, at T sp below T m , amphiphile molecules form “the compressing crystallized monolayer”, and T sp above T m , the monolayer is not crystallized by compression. Molecular-resolution images of molecules in the monolayers were successfully observed with an atomic force microscope (AFM) for the first time. A high mechanical stability of the monolayer is inevitably required for the non-destructive AFM observation of the fatty acid monolayer. For the preparation of the mechanically stable monolayer, the continuous compression method up to a low surface pressure and the multi-step creep method were used. Further, the Langmuir-Blodgett (LB) films with a homogeneous surface morphology could be constructed by the mechanically stable monolayers, and the morphological homogeneity was necessary for the construction of low-propagation loss LB film optical waveguides.

Construction of low-loss optical waveguide from Langmuir-Blodgett films

The relationship between waveguide properties and their morphological regularity is investigated for the LB films prepared by two preparation methods. The optical propagation loss of the LB film prepared by the multi-step creep method was 2.5 dB(DOT)cm-1, whereas that of the LB film prepared by the continuous compression method was 8.4 dB (DOT)cm-1. The lower propagation loss of the LB film prepared by the multi-step creep method results from a remarkable reduction of the surface light scattering due to the flat surface morphology of the LB film.

Novel concept for the aggregation structure of fatty acid monolayers on the water surface and direct observation .of molecular arrangements in their monolayers

The aggregation structure of fatty acid monolayers on water subphases of different pH’s was investigated by means of transmission electron microscopy. Fatty acid monolayers exhibited the phase transition from an amorphous state to a crystalline one by surface compression in the case of a highly dissociated state of hydrophilic groups, whereas they did not show the phase transition in the case of a slightly dissociated state. The aggregation structure of monolayers on the water surface was systematically classified into “the crystalline monolayer”, “the amorphous monolayer” and “the compressing crystallized monolayer” with respect to thermal and chemical (intermolecular repulsive) factors.Molecular-resolution images of fatty acid molecules in the monolayers on mica substrate were successfully observed with an atomic force microscope (AFM) for the first time. The AFM image of a lignoceric acid monolayer prepared at a surface pressure of 5mNm−1 showed a two-dimensional periodic structure with locally disordered molecular arrangements. Also, the nondestructive AFM image observation was successful for a stearic acid monolayer which was prepared by a multistep creep method, indicating that a high mechanical stability of the monolayer is inevitably required for the nondestructive AFM observation.

Construction of defect-diminished crystalline monolayer by area-creep method

The structural relaxation mechanism of stearic acid crystalline monolayer on the water surface was investigated on the basis of area-creep measurements and transmission electron microscopic observations. The relaxation behavior of the crystalline monolayer was classified into the behaviors at high and low surface pressure regions. A multi-step creep method, which was proposed on the basis of the relaxation mechanism of monolayer, allowed us to construct a mechanical stable and defect-diminished crystalline monolayer.

Direct observation of molecular arrangement in lignoceric acid monolayer with atomic force microscope

A molecular resolution image of lignoceric acid monolayer, which was prepared at a subphase temperature of 293 K on the water surface and transferred onto a mica substrate, was successfully observed in air with an atomic force microscope(AFM) for the first time. The AFM image of the monolayer showed a flat surface in a large area of 500 × 500 nm2 and two-dimensional regular array with locally disordered molecular arrangement in a small area of 8 × 8 nm2.