Akitaka Hoshino

Growth shape of isotactic polystyrene crystals in thin films

The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11 nm thick films. The it-PS crystals grown in the thin films show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a diffusion field appears: dendrites and compact seaweed. The branching morphology is formed through a morphological instability caused by the gradient of film thickness around a crystal; the thicker the film thickness, the larger is the lateral growth rate of crystals. Regardless of the morphological change, the growth rate as well as the lamellar thickness depends on the crystallization temperature as predicted by the surface kinetics.

SCANNING TUNNELING MICROSCOPE CONTRAST OF PERYLENE-3,4,9,10-TETRACARBOXYLIC-DIANHYDRIDE ON GRAPHITE AND ITS APPLICATION TO THE STUDY OF EPITAXY

Epitaxial films of perylene‐3,4,9,10‐tetracarboxylic‐dianhydride (PTCDA) on graphite (0001) were investigated by scanning tunneling microscopy. Molecular image contrast of PTCDA was found to depend strongly upon the molecular orientation and the position on graphite. In particular, the periodic discrepancy between PTCDA and graphite lattice points results in a modulation of contrast, which can be used to determine the epitaxial relation of PTCDA relative to the substrate accurately. By analyzing this modulation of contrast, we determined two kinds of epitaxial orientation of PTCDA. These orientations have no exact commensurate relation with graphite, but every lattice point of PTCDA lies on a lattice line parallel to the a axis (or b axis) of graphite. This specific feature contributes to decreasing the interfacial energy. The contrast mechanism of adsorbed molecules is also discussed.

Redetermination of the crystal structure of α-copper phthalocyanine grown on KCl

The crystal structure of a polymorph of copper phthalocyanine (CuPc) grown on a KCl substrate is redetermined by transmission electron diffraction. It has a triclinic unit cell containing one molecule; the crystal does not have a herringbone-type molecular arrangement, which is a common packing mode of planar phthalocyanines. The molecular packing is determined by the diffraction intensity with the aid of the calculation of molecular packing energy. One of the striking features of this polymorph is its stacking mode within a molecular column: the molecular stacking direction projected on a molecular plane is different by an angle of about 45° from that of the α-modifications of platinum phthalocyanine (PtPc) and metal-free phthalocyanine (H2Pc). A powder X-ray diffraction profile calculated for the polymorph agrees well with that of so-called α-CuPc and Rietveld analysis for α-CuPc indicates that the CuPc crystals grown on KCl are actually α-CuPc; hence, α-CuPc is not isostructural with either α-PtPc or α-H2Pc. On the basis of the present results and the reported crystal structures of the planar phthalocyanines that form molecular columns, the polymorphs of the phthalocyanines can be classified into four types distinguished by the molecular stacking mode within the column: α(×)-, α(+)-, β(×)- and β(+)-types.

Dynamics of alpha and beta processes in thin polymer films: poly(vinyl acetate) and poly(methyl methacrylate).

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) have been investigated by dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz at temperatures from 263 to 423 K. The alpha process, the key process of glass transition, is observed for thin films of PVAc and PMMA as a dielectric loss peak at a temperature T(alpha) in temperature domain with a fixed frequency. For PMMA, the beta process is also observed at a temperature T(beta). For PVAc, T(alpha) decreases gradually with decreasing thickness, and the thickness dependence of T(alpha) is almost independent of the molecular weight (Mw< or =2.4x10(5)). For PMMA, T(alpha) remains almost constant as thickness decreases down to a critical thickness dc, at which point it begins to decrease with decreasing thickness. Contrastingly, T(beta) decreases gradually as thickness decreases to dc, and below dc it decreases drastically. For both PVAc and PMMA, the broadening of the distribution of the relaxation times in thinner films is observed and this broadening is more pronounced for the alpha process than for the beta process. It is also observed that the relaxation strength is depressed as the thickness decreases for both the polymers.

CRYSTAL GROWTH OF ISOTACTIC POLYSTYRENE IN ULTRATHIN FILMS: FILM THICKNESS DEPENDENCE

The film thickness dependence of crystal growth is investigated for isotactic polystyrene (it-PS) in thin films for thicknesses from 20 down to 4 nm. The single crystals of it-PS grown at 180°C in the ultrathin films show a morphology typical of diffusion-controlled growth: dense branching morphology and fractal seaweed. The characteristic length of the morphology, i.e., the width of the branch, increases with decreasing film thickness. The thickness dependence of the crystal growth rate shows a crossover around the lamellar thickness of 8 nm. The thickness dependences of the growth rate and morphology are discussed in terms of the diffusion of chain molecules in thin films.

Prediction of the Epitaxial Orientation of Ultrathin Organic Films on Graphite

A method of predicting the epitaxial orientation of organic monolayer on graphite substrate was proposed together with a misfit defined appropriately for the prediction. This method is based on a lattice matching mode, point-on-line coincidence, that has been found through our scanning tunneling microscopy (STM) investigations of organic epitaxy on graphite. In order to examine efficiency of the method, epitaxial orientations of bis-1,2,5-thiadiazolo-tetracyano-quinodimethane (BTDA-TCNQ) and bis-1,2,5-selenadiazolo-tetracyano-quinodimethane (BSDA-TCNQ) were predicted and the predictions were compared with the orientations observed by STM. In both cases, the orientation observed was found to be exactly one of the predicted orientations, which suggests not only the efficiency of the prediction method but also the importance of the point-on-line coincidence in organic epitaxy.

Relaxation dynamics in thin supported polymer films

Abstract The glass transition and relaxation dynamics of thin polymer films supported on a substrate have been investigated for three different polymers, atactic polystyrene, poly(vinyl acetate), and atactic poly(methyl methacrylate) by using dielectric relaxation and thermal expansion spectroscopy. The temperature, T α , at which the imaginary component of the susceptibility has a peak due to the α-process at a fixed frequency, is measured as a function of thickness and the molecular weight of the polymers. The observed thickness and molecular weight dependence of T α is discussed in comparison with that of T g observed in the freely standing films of polystyrene.

Precise determination of the epitaxial orientation of a monomolecular layer organic film by scanning tunneling microscopy

Abstract Epitaxial films of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole (DTPP) on graphite (0001) were investigated by scanning tunneling microscopy (STM). In the STM images, one-dimensional periodic modulation of image contrast was observed in addition to each molecular image, which was interpreted as moire fringe caused by the interaction with a graphite lattice. Following to mathematical analysis, such one-dimensional periodic modulation was concluded to appear only under the condition that every lattice point of the deposited film is on an ( m , n )-line of the substrate lattice for certain integers m and n . Using this property, the epitaxial orientation of DTPP with respect to the substrate lattice was determined precisely. The resulting orientation exhibits no exact commensurate relation with graphite, but every lattice point of DTPP was found to lie on a lattice-line parallel to the b -axis ((1,0)-line) of graphite.

Epitaxial growth of organic crystals on organic substrates — polynuclear aromatic hydrocarbons

Abstract The epitaxial growth of organic crystals on some organic substrates was studied in various combinations of the deposit and the substrate, especially in the case of polynuclear aromatic hydrocarbons. Crystals of polynuclear aromatic hydrocarbons are known to be classified into the following four types on the basis of molecular packing mode in the crystal: herringbone (HB)-, sandwich herringbone (SHB)-, γ- and β-types [Desiraju and Gavezotti, Acta Cryst. B 45 (1989) 473]. The HB- or SHB-type crystal was found to grow epitaxially on an organic substrate having the same packing type. The a - and b -axes of the deposit are parallel to the a - (or b - and b - (or a -) axes of the organic substrates, respectively. In the combination of different packing types, e.g. the HB-type on the SHB-type substrate (HB/SHB) and the γ-type on the SHB-type (γ/SHB), the deposit does not always exhibit well-defined epitaxy, but in some cases, the deposit grows epitaxially as polymorphs. This suggests the possibility of controlling the stacking of organic thin films by careful selection of organic substrates.

Point-on-line coincidence in epitaxial growth of CuPcCl16 on graphite

Abstract Epitaxial growth of perchloro-copper-phthalocyanine (CuPcCl 16 ) on graphite was studied by STM and TEM. The crystals grew epitaxially with two slightly different orientations on the graphite surface. The corresponding orientations were also observed in a monolayer of CuPcCl 16 by STM. By detailed analysis of the moire-like pattern in the STM images, these orientations are interpreted as being consistent with a ‘point-on-line coincidence’ epitaxy. In order to realize the exact point-on-line coincidence for the respective orientation, the unit cell dimensions of the monolayer were slightly distorted at each orientation, although they are almost the same as the monoclinic structure of the thin film determined by TEM.