Susumu Ikeda

Reorientation of pentacene molecules from the flat-lying manner to the standing one which occurs at the early stage of thin film growth by physical vapor deposition was investigated by molecular dynamics simulations with a surface-modified amorphous SiO₂ substrate. A molecular cluster comprising 50 standing pentacene molecules was placed on the substrate together with lying molecules and clusters, and the standing cluster acted as a trigger for the reorientation of lying molecules and clusters. Some lying molecules and clusters adjacent to the standing cluster stood up probably by the short-range van der Waals interaction. Some lying clusters apart from the standing cluster also stood up, and this phenomenon was possibly caused by the effect of the long-range Coulomb interaction between clusters. The number of standing molecules on the substrate increased with increasing temperature, and the temperature dependence of the orientational transition rate was discussed in view of thermal fluctuation.

In the author’s last paper [Ikeda, Appl. Phys. Express 13, 015508 (2020)], it was suggested that the number of molecules that constitute a critical nucleus of pentacene is about ten based on the results of molecular dynamics (MD) simulations performed to investigate the stability of clusters comprising standing pentacene molecules on substrate surfaces. In this study, the author carried out additional MD simulations of clusters comprising ten pentacene molecules and found that these molecular clusters of critical size show stochastic behavior, which can be understood based on curves of free energy change. The discrepancy in the number of molecules that constitute a critical nucleus of pentacene between previous reports and this study is also discussed.

Water permeation pathways in electronic devices should be eliminated for the suppression of operational instabilities. We investigated possible pathways in field-effect transistors based on a laminated single crystal (SC) of an organic semiconductor, rubrene. Water-induced instabilities were found to be more obvious with a thicker rubrene SC. Furthermore, under our simulation conditions, molecular dynamics calculations of water diffusion on a rubrene SC showed that no water molecules penetrated the SC. These findings indicate that a space at the SC/substrate interface is a dominant pathway. The present study clearly shows the importance of conformality of SC lamination onto the underlying substrate.

Water permeation pathways in electronic devices should be eliminated for the suppression of operational instabilities. We investigated possible pathways in field-effect transistors based on a laminated single crystal (SC) of an organic semiconductor, rubrene. Water-induced instabilities were found to be more obvious with a thicker rubrene SC. Furthermore, under our simulation conditions, molecular dynamics calculations of water diffusion on a rubrene SC showed that no water molecules penetrated the SC. These findings indicate that a space at the SC/substrate interface is a dominant pathway. The present study clearly shows the importance of conformality of SC lamination onto the underlying substrate.

Molecular dynamics (MD) simulations were applied to the study of nucleation and growth of pentacene thin films, and stability of clusters (nuclei) comprising standing molecules was investigated. In simulations, the clusters consisting of more than ten standing molecules could stably exist on hydrophobic surfaces, while several tens of molecules were necessary for stabilization on hydrophilic surfaces. Furthermore, the stabilized clusters could grow by incorporating additional molecules in MD simulations. These results suggest that nucleation occurs on hydrophobic surfaces easier than on hydrophilic surfaces and the critical size of the nuclei of “standing” pentacene is about ten molecules on hydrophobic surfaces.

A new class of Pt-free catalysts was designed that included molecular iron phthalocyanine (FePc) derivatives, namely, iron azaphthalocyanine (FeAzPc) unimolecular layers (Fe AzULs) adsorbed on oxidized multiwall carbon nanotubes (oxMWCNTs). FeAzPcs were dissolved in organic solvents such as dimethyl sulfoxide (DMSO), and catalytic electrodes modified with molecularly adsorbed FeAzPcs were successfully prepared. The optimized composition of the catalytic electrodes was determined, and the electrodes exhibited superior activity for the oxygen reduction reaction (ORR) and better durability than conventional FePc catalytic electrodes and commercial Pt/C due to the electron-withdrawing properties of the pyridinic nitrogen in FeAzPcs. The catalytic electrodes that were molecularly modified with FeAzPcs have higher activities than those composed of FeAzPc crystals and oxMWCNTs. To the best of our knowledge, among all of the conventional catalysts based on modified MWCNTs and oxMWCNTs, this catalyst exhibits the highest activity. Unlike other Pt-free catalytic electrodes, the Fe AzUL catalytic electrodes can be prepared by low-cost processing without pyrolysis and are therefore promising catalytic electrode materials for applications, such as polymer electrolyte fuel cells and metal–air batteries.

X-ray reflectometry (XRR), a surface-sensitive technique widely used for characterizing surfaces, buried interfaces, thin films, and multilayers, enables determination of the electron density distribution perpendicular to a well-defined surface specularly reflecting X-rays. However, the electron density distribution parallel to the surface cannot be determined from an X-ray reflectivity curve. The electron density correlation in the lateral direction is usually probed by measuring the grazing-incidence small-angle X-ray scattering (GISAXS). GISAXS measurement, however, typically requires using a collimated X-ray point beam to distinguish the GISAXS from the specularly reflected X-rays, and so the sample must be scanned in the lateral direction with the point beam to investigate variations in the surface and interface morphology for a region larger than the size of the beam. In this paper, we report a new approach based on X-ray grating interferometry: an X-ray sheet beam is used instead of an X-ray point beam. A method using this approach can simultaneously provide one-dimensional real-space images of X-ray reflectivity, surface curvature, and ‘dark-field’ contrast with a field-of-view of more than a few millimetres. As a demonstration, a sample having a 400 nm line and space SiO₂ pattern with a depth of 10 nm on its surface was used, and the dark-field contrast due to the unresolved line and space structure, creating GISAXS in the lateral direction, was successfully observed. Quantitative analysis of these contrasts provided the real-space distribution of the structural parameters for a simple model of the grating structure. Our study paves the way to a new approach to structure analysis, providing a quantitative way to investigate real-space variations in surface and interface morphology through wavefront analysis.

The three–dimensional (3–D) structure of crystals around the interface between massive potassium (K)–feldspar and quartz–feldspar intergrowth (graphic granite) of two samples collected from a pegmatite body in Ishikawa town, Fukushima Prefecture, Japan, was determined by X–ray computed tomography (CT). Based on the results and additional information obtained by serial thin section and polarizing observations, the formation processes of graphic granite and pegmatite bodies are discussed. The most important finding of the 3–D observation using X–ray CT is the very flat spatial distribution of the tips of the quartz crystals. The sharp interface between the graphic and single–phase parts suggests that the change from graphic granite to massive K–feldspar crystallization corresponds to the transition of the thermodynamic conditions at the growth front from non–equilibrium to equilibrium. This indication leads to a new model for the pegmatite genesis, that is, graphic granite forms at the marginal zone under non–equilibrium conditions shortly after the intrusion of magma due to rapid cooling by the cold host rock body and, subsequently, the crystallization of massive K–feldspar starts when the conditions at the crystal growth front reenter the ‘feldspar + melt’ field in the phase diagram due to the relaxation of the conditions. Other results related to the conditions of the growth environment with coexisting aqueous fluid, such as the connectivity of the quartz crystals, are also discussed, for example, the influence of the supercritical state on the nucleation of quartz crystals.

Molecular dynamics (MD) simulations of the organic semiconductors α-sexithiophene (6T) and pentacene were carried out to clarify the mechanism of organic graphoepitaxy at the molecular level. First, the models of the grooved substrates were made and the surfaces of the inside of the grooves were modified with –OH or –OSi(CH₃)₃, making the surfaces hydrophilic or hydrophobic. By the MD simulations of 6T, it was found that three stable azimuthal directions exist (0, ∼45, and 90°; the angle that the c-axis makes with the groove), being consistent with experimental results. MD simulations of deposition processes of 6T and pentacene were also carried out, and pentacene molecules showed the spontaneous formation of herringbone packing during deposition. Some pentacene molecules stood on the surface and formed a cluster whose a-axis was parallel to the groove. It is expected that a deep understanding of the molecular-scale mechanisms will lead graphoepitaxy to practical applications, improving the performance of organic devices.

The three-dimensional distribution of melt in partially molten synthetic samples compositionally corresponding to diopside (90 wt.%)–anorthite (10 wt.%) and doped with PbO, WO₃, MoO₃, or Cs ₂ O to enhance contrast was studied by X-ray computed tomography (CT) with synchrotron radiation. The heavy elements were strongly concentrated in the melt and contributed to an increase of the X-ray linear attenuation coefficient (LAC) of it. PbO was found to be compatible with silicate melt (>20 wt.% in solution) and incompatible with diopside crystals. Other oxides WO₃ (∼10 wt.%), MoO₃ (∼5 wt.%) and Cs₂O (< 5 wt.%) are also soluble only in the melt. Such doping is useful not only for LAC control in X-ray CT measurements, but also for systematic control of the structure (wetting properties, distribution and connectivity) of partial melt. This technique gives basic information for discussion of the 3D distribution of partial melt having different wetting properties. As PbO was most effective in visualization of the diopside–anorthite partially molten system, CT images of the PbO-bearing sample were used for further 3D investigation of distribution. A distribution of dihedral angles at solid-melt-solid triple junctions ranging from 22 to 55° was observed with the 3D data. This range in angle distribution was probably caused by anisotropy of crystals and the result supports the argument that there is some limitation in a theoretical framework of stereology which estimates the 3D structure based on 2D observations. Investigators have begun to apply X-ray CT to the study of the 3D distribution of partial melts in rocks using synchrotron radiation. Our study on the effect of doping is one approach for developing a technique to investigate 3D melt distribution.