Rate-determining process in MISIM photocells for optoelectronic conversion using photo-induced pure polarization current without carrier transfer across interfaces

Recently, we proposed a [metal|insulator|semiconductor|insulator|metal] (MISIM) photocell, as a novel architecture for high-speed organic photodetectors. The electric polarization in the S layer, induced by modulated light illumination, propagates into the outside circuit as a polarization current through the I layers, without any carrier transfer across the interfaces. In the present work, we examined the MISIM photocells consisting of zinc-phthalocyanine(ZnPc)-C60 bilayers for the S layer and Parylene C for the two I layers, to understand the fundamental aspects of the MISIM photocells, such as current polarity and modulation-frequency dependence. It was found that, in such devices, the current polarity was primarily determined by the polarization in the S layer, which was induced by the donor–acceptor charge-transfer upon illumination. Furthermore, the ON and OFF current, which appeared in the periods of illumination-on and -off, respectively, exhibited significantly different dependence on the modulation frequency. This was well-explained by an imbalance between a quick polarization in the S layer during illumination and its slow relaxation in the dark

Growth of Pentacene Crystals by Naphthalene Flux Method

We report the crystal growth of pentacene from a solution of naphthalene. The solubility of pentacene in naphthalene was evaluated by optical absorption at elevated temperature. The crystal growth was performed in an H-shaped sealed glass tube or metal vessels sealed with ultrahigh-vacuum compatible flanges placed in heated two-zone aluminum blocks. The obtained crystals had a single-crystal-like appearance and flat surface. They were made of aligned microtwins of the “bulk type” (interlayer spacing 14.5 Å) polymorph

Molecular Dynamics Simulation of Poly(Ether Ether Ketone) (PEEK) Polymer to Analyze Intermolecular Ordering by Low Wavenumber Raman Spectroscopy and X-ray Diffraction

Poly(ether ether ketone) (PEEK) is an important engineering plastic and evaluation of its local crystallinity in composites is critical for producing strong and reliable mechanical parts. Low wavenumber Raman spectroscopy and X-ray diffraction are promising techniques for the analysis of crystal ordering but a detailed understanding of the spectra has not been established. Here, we use molecular dynamics combined with a newly developed approximation to simulate local vibrational features to understand the effect of intermolecular ordering in the Raman spectra. We found that intermolecular ordering does affect the low wavenumber Raman spectra and the X-ray diffraction as observed in the experiment. Raman spectroscopy of intermolecular vibration modes is a promising technique to evaluate the local crystallinity of PEEK and other engineering plastics, and the present technique offers an estimation without requiring heavy computational resources

Pitched π‑Stacking Crystal Structure and Two-Dimensional Electronic Structure of Acenaphtho[1,2-k]fluoranthene Analogues with Various Substituents

Electronic properties of organic semiconductors are governed by their crystal structures. Rubrene, a high-mobility organic semiconductor, forms a pitched π-stacking structure. We here focused on 7,14-diphenylacenaphtho[1,2-k]fluoranthene (DPAF), which gives a crystal isomorphic to rubrene. In addition to DPAF, we newly synthesized 7,14-dithienyl-AF (DTAF) and obtained three types of pitched π-stacking structures: the previously reported orthorhombic DPAF (DPAF-O), a new polymorphic monoclinic DPAF (DPAF-M), and DTAF. To investigate factors causing these molecules to form pitched π-stacking structures, the intermolecular interactions of the face-to-face molecular pairs were calculated for these molecules, nonsubstituted AF and butyl-substituted AF molecules. Unsubstituted AF and butyl-substituted AF have the most stable π-stacking structure with a small misalignment, whereas DPAF and DTAF have the most stable π-stacking structure with a large misalignment along the molecular long axis. Such misaligned π-stacking structures are essential for pitched π-stacking structures. Theoretical calculations of the mobility anisotropy based on the hopping model suggested that DPAF-M and DTAF form one-dimensional electronic states while DPAF-O forms two-dimensional electronic states. Actually, isotropic mobility was observed in SC-FETs with DPAF-O. Collectively, our results indicated that molecular design that introduces broad π-conjugated moieties at the ends of molecules is effective for enhancing the two-dimensionality of electronic states of pitched π-stacking structures

Ultrahigh pressure-induced modification of morphology and performance of MOF-derived Cu@C electrocatalysts

We report the pyrolysis of copper-containing metal-organic frameworks under high pressure and the effect of the applied pressure on the morphology and electrocatalytic performance toward the oxygen-related reactions of the products. The high-pressure and high-temperature (HPHT) syntheses were performed under 5, 2.5, 1, and 0.5 GPa, and the Cu@C products were obtained except for the 2.5 GPa experiment. Copper formed a shell-like nanostructure on the carbon matrices during the 0.5 GPa experiment, whereas copper formed sub-nanometer sized particles in the carbon matrices with the increasing pressure. It is considered that the transportation of copper atoms by outgassing during the pyrolysis affects the morphology. Electrochemical measurements revealed that all samples exhibited activity for the oxygen reduction reaction (ORR). The 0.5 GPa-treated product also exhibited the oxygen evolution reaction (OER). The overall ORR/OER performance of this product was excellent among Cu-based bifunctional materials even though it did not contain cocatalysts such as nitrogen-doped carbon or other metal elements. The Cu(iii) species in the nano-thick copper shell structure provided the active sites for the OER

Synthesis of Epitaxial MoS2/MoO2 Core-Shell Nanowires by Two- Step Chemical Vapor Deposition with Turbulent Flow and Their Physical Properties

MoO2 nanowires (NWs), MoO2/MoS2 core-shell NWs, and MoS2 nanotubes (NTs) were synthesized by the turbulent flow chemical vapor deposition of MoO2 using MoO3, followed by sulfurization in the sulfur gas flow. The involvement of MoOx suboxide is suggested by density functional theory (DFT) calculations of the surface energies of MoO2. The thickness of the MoS2 layers can be controlled by precise tuning of sulfur vapor flow and temperatures. MoS2 had an armchair-type winding topology due to the epitaxial relation with the MoO2 NW surface. A single similar to few-layer MoO2/MoS2 core-shell structure showed photoluminescence after the treatment with a superacid. The resistivities of an individual MoO2 NW and a MoS2 NT were measured, and they showed metallic and semiconducting resistivity-temperature relationships, respectively

Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport

Electrically conducting TTF–TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed-stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF–TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF–TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interactions in the substrate, and the sheet resistance also exhibits a systematic change. The resistance drop is rapid and remarkable when the donor of the substrate CT complex is weak. The difference in the morphology and the properties is considered to arise from the difference in the ease of nucleus formation and the rate of crystal growth of the TTF–TCNQ nanocrystals