Katsuhiro Nichogi

Structural control of evaporated lead-phthalocyanine films

Monoclinic lead-phthalocyanine crystal film having only (320) orientation and monoclinic crystal film composed of lower (200)-and upper (320)-rich orientation were obtained at a substrate temperature of 293 K, and deposition rates higher than 1.0 nm s−1 and from 0.8 to 0.2 nm s−1, respectively. A film consisting of mixed triclinic and monoclinic crystals in which the distribution was almost homogeneous was obtained at a substrate temperature of 293 K and a deposition rate of 0.1 nm s−1, and a pure triclinic crystal film was obtained at a substrate temperature of 373 K.

Lead phthalocyanine Langmuir-Blodgett films

Abstract Different orientations of the phthalocyanine ring were obtained in Langmuir-Blodgett (LB) films of tetrasubstituted phthalocyanine lead, depending on the properties of the substituents ( t -butyl, cumylphenoxy and pentoxy groups). This difference affected their electrical properties; tetrapentoxy phthalocyanine lead LB films exhibited high conductivity (10 −4 −10 −7 S cm −1 ) in the direction parallel to the substrate, while the others had insulating properties. This high conductivity was attributed to the formation of a face-to-face stack of lead phthalocyanine molecules in the LB film.

Mixed-stack charge-transfer films prepared by Langmuir-Blodgett technique and donor doping

Abstract The formation of mixed-stack charge-transfer (CT) films was studied. Spectroscopic analysis suggested that donor doping to a 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) film prepared by the Langmuir-Blodgett technique resulted in the formation of the mixed-stack CT film when 3,3′,5,5′-tetramethylbenzidine (TMB) was used as a donor, while degradation of the CT complex was observed when N,N,N′,N′-tetramethyl-p-phenylenediamine was used. Although X-ray diffraction measurements suggested that the order of the layered structure and crystallinity of the film were decreased by the doping, it was proved that the molecular columns of TMB and C18TCNQ were preferentially oriented along a substrate from polarized electronic spectra and scanning electron microscope observation. Therefore, non-linear electrical conduction as observed in mixed-stack CT crystals was expected. In the current density-electric field characteristic for the TMB-C18TCNQ film, the non-linearity was weaker than that observed in a TMB-TCNQ crystal. Furthermore, a temperature-induced neutral-ionic (N-I) phase transition was not observed from 4.2 K to room temperature in a spectroscopic measurement.

Structural Analysis of Langmuir-Blodgett Films of Alkylated Tetracyanoquinodimethanes

Structural characterization of alkylated tetracyanoquinodimethane (TCNQ) Langmuir and Langmuir-Blodgett (LB) films has been carried out. Scanning electron microscope (SEM) and atomic force microscope (AFM) observations reveal numerous platelet microcrystal domains in the alkylated TCNQ Langmuir films transferred onto a quartz substrate. The analysis of height distribution in the AFM data proves that these domains have periodic layered structure. The thickness of these layered structures agrees well with the d-values obtained by X-ray diffraction analysis of the alkylated TCNQ LB films. Moreover, the high-resolution AFM image and the d-value of the 2-dodecyl-TCNQ Langmuir film suggest that the structure of the second and the upper layers in the domains is identical to that of the single crystal. SEM and AFM measurements prove that the LB films consist of numerous platelet microcrystal domains which are superposed upon each other, resembling a mosaic structure. Therefore, the periodic structures observed in X-ray diffraction measurements of the alkylated TCNQ LB films are ascribed to the layered structure in these domains, although they are usually explained as well-defined multilayered structure.

Nonlinear Electrical Properties of Evaporated Lead Phthalocyanine Films

Current-voltage characteristics of Au/evaporated lead phthalocyanine film/Au sandwich-type cells showed nonlinear electrical electrical properties with asymmetric barriers. From analyses of crystal structures of PbPc films, these barriers were found to be attributable to nonuniformity of crystal structure and orientation along the film thickness

Molecular arrangements in Langmuir-Blodgett films of 2-dodecyl-, 2-pentadecyl- and 2-octadecyl-7,7,8,8-tetracyanoquinodimethanes

Abstract Atomic force microscopy (AFM) on Langmuir–Blodgett (LB) films of alkyl-tetracyanoquinodimethanes (alkyl-TCNQs) have been carried out. The two-dimensional periodic molecular arrangements are observed on the surface of each LB films. The observed periodic structure due to the two-dimensional molecular arrangements in the LB films of C12- (a0=0.83±0.02 nm, b0=0.47±0.01 nm, and γ=95±2°) and C18TCNQ (a0=0.83±0.02 nm, b0=0.48±0.01 nm, and γ=94±2°) are almost identical with each other, while that of the C15TCNQ (a0=0.78±0.03 nm, b0=0.47±0.01 nm, and γ=101±2°) is different from those films. Since the alkyl-TCNQ LB films consists of bimolecular layers, the observed images are due to the two-dimensional arrangements of the TCNQ chromophores. Thus, the difference in the way of subcell packing of the alkyl chains must affect the packing of the TCNQ chromophores, and the difference of the molecular arrangements seems to depend on an even or odd number of the length of the alkyl chain. These results agree with the result of infrared spectroscopy: the alkyl chains in the LB films of C12- and C18TCNQ crystallize with orthorhombic subcell packing, while those of C15TCNQ are in the hexagonal or pseudohexagonal subcell packing.

Thermal behavior of one-layer Langmuir-Blodgett films of 2-pentadecyl-7,7,8,8-tetracyanoquinodimethane studied by ultraviolet-visible and infrared spectroscopies

Abstract Thermal Behavior of one-layer Langmuir-Blodgett (LB) films of 2-pentadecyl-7,7,8,8-tetra-cyanoquinodimethane (pentadecyl-TCNQ) has been investigated by use of infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopies. An IR transmission spectrum of the film shows gradual temperature-dependent changes below the melting point of the film, and then gives dramatic spectral changes near the melting point. Temperature-dependent UV-Vis spectral changes of the film indicate that the morphological changes occur in domains of the film, and that absorption bands in the 320–560 nm region arising from the π-π* transition of the chromophore consist of at least three components due to different forms of chromophore aggregation. More active state of the alkyl-chain, hexagonal state, seems to be a trigger for the morphological changes in the domains.

Highly oriented thin film of a mixed-stack charge-transfer complex

Abstract Doping of 5,10-dimethyl-5,10-dihydrophenazine ((Me) 2 P) to a 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C 18 TCNQ) LB film has been examined in a vapor phase, and it is found that the mixed-stack charge-transfer (CT) complex of (Me) 2 P and C 18 TCNQ in a quasi-ionic phase is formed by the doping. The (Me) 2 P-C 18 TCNQ LB film is found to consist of many platelet-like microcrystal domains with a well-ordered layered structure of thickness 3.31 nm. In the high-resolution atomic force microscopy measurements, a two-dimensional periodic molecular arrangement is observed. The period is 0.96 nm × 0.83 nm and the angle between the axes is 59°. By polarized electronic absorption spectroscopy it is confirmed that the molecular arrangement consists of the alternatively stacked array of (Me) 2 P and C 18 TCNQ. The doping in the vapor phase is very effective for the preparation of highly crystallized and oriented thin films of mixed-stack CT complexes.

An Infrared Study on Molecular Orientation in Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-Tetracyanoqui Noddvethane and 3,3′,5,5′-Tetramethylbenzjdine Prepared by the Langmuir-Blodgett Technique and Donor Doping

Abstract Molecular orientation in mixed-stack charge transfer (CT) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (acceptor, A) doped with 3,3′,5,5′-tet ramethylbenzidine (donor, D) has been investigated by infrared (IR) transmission and reflection absorption (RA) spectroscopy. A comparison of relative intensities of IR bands originated from chromophores of D and A reveals that the long axes of both D and A are almost parallel to the substrate in a one-layer CT film. A layer dependence of the molecular orientation has been explored by a comparison of IR transmission and RA spectra of multilayer mixed-stack CT films.

Pattern Classification in a Perceptron Circuit Using Lead Phthalocyanine Films

Evaporated films of lead phthalocyanine (PbPc) show a unique conductivity change after a very slight amount of gases are adsorbed. The change in the conductivity of the films depends on the polarity and the strength of input voltages, and therefore is analogous to the change of synaptic connections in nerve systems. Therefore, a perceptron circuit has been fabricated using PbPc films as synaptic connections. After learning via loading of input signals and teacher signals, this circuit can classify random inputs of eight-dimensional voltage patterns into two categories when the input patterns are linearly separable.