Spontaneous orientation polarization of flavonoids.

Spontaneous orientation polarization (SOP) is macroscopic electric polarization that is attributed to a constant orientational degree of dipole moments of polar molecules on average. The phenomenon has been found in small molecules like H2O at low temperatures and π-conjugated molecules employed in organic light-emitting diodes. In this study, we demonstrate that a thin film of baicalein, a flavonoid compound found in natural products, exhibits SOP and resultant giant surface potential (GSP) exceeding 5500 mV at a film thickness of 100 nm. Vacuum-deposition of baicalein under high vacuum results in smooth and amorphous films, which enables the generation of GSP with a slope of 57 mV/nm in air, a value comparable to the representative of an organic semiconductor showing GSP, tris(8-hydroxyquinoline)aluminum(III) (Alq3). We also found the superior photostability of a baicalein film compared to an Alq3 film. These findings highlight the potential of baicalein in new applications to organic electronics.

Disentangling Origins of Adhesive Bonding at Interfaces between Epoxy/Amine Adhesive and Aluminum.

Joining metals by adhesive bonding is essential in widespread fields such as mobility, dentistry, and electronics. Although adhesive technology has grown since the 1920s, the roles of interfacial phenomena in adhesive bonding are still elusive, which hampers the on-demand selection of surface treatment and adhesive types. In the present study, we clarified how chemical interactions and mechanical interlocking governed adhesive bonding by evaluating adhesion properties at the interfaces between epoxy/amine adhesive and two kinds of Al adherends: a flat aluminum hydroxide (AlxOyHz) and technical Al plate with roughness. Spectroscopic and microscopical data demonstrate that the protonation of the amino groups in an amine hardener converts Al(OH)3 on the AlxOyHz surface to AlO(OH). The interfacial protonation results in an interfacial dipole layer with positive charges on the adhesive side, whose electrostatic interaction increases the interfacial fracture energy. The double cantilever beam tests for the flat AlxOyHz and technical Al substrates clarify that the mechanical interlocking originating from the surface roughness further increases the fracture energy. This study disentangles the roles of the chemical interactions and mechanical interlocking occurring at the epoxy adhesive/Al interface in the adhesion mechanism.

Chemical reactions of graphitic carbon nitride films with glass surfaces and their impact on photocatalytic activity.

Thin films of graphitic carbon nitride (g-CN), a visible-light-driven photocatalyst, have recently attracted interest for application in photoelectrochemical cells for water splitting and high-throughput photocatalysis. In typical syntheses, g-CN films are formed by heating the nitrogen-rich precursor and substrate to 500-600 °C. The heated substrate should affect the polycondensation of the precursor and thereby alter the properties of the g-CN film. In this paper, we demonstrate that soda-lime glass, such as commercial glass slides, modifies the chemical structure of g-CN. The terminal amino groups of g-CN are partially substituted with cyanamide and hydroxyl groups. The electron-withdrawing groups provide the energy offsets of the frontier orbitals between the modified and unmodified molecules, facilitating exciton dissociation. After alkali metals are removed, the modified g-CN film exhibits a faster photodegradation of methyl orange compared with a melon film. The simple protocol to activate a g-CN film without co-catalysts paves a new way to enhance photocatalytic activity via selections of substrates, including waste glass.

Bicyclic-ring base doping induces n-type conduction in carbon nanotubes with outstanding thermal stability in air.

The preparation of air and thermally stable n-type carbon nanotubes is desirable for their further implementation in electronic and energy devices that rely on both p- and n-type material. Here, a series of guanidine and amidine bases with bicyclic-ring structures are used as n-doping reagents. Aided by their rigid alkyl functionality and stable conjugate acid structure, these organic superbases can easily reduce carbon nanotubes. n-Type nanotubes doped with guanidine bases show excellent thermal stability in air, lasting for more than 6 months at 100 °C. As an example of energy device, a thermoelectric p/n junction module is constructed with a power output of ca. 4.7 µW from a temperature difference of 40 °C.

Distributions of Potential and Contact-Induced Charges in Conventional Organic Photovoltaics.

The interfaces of dissimilar materials play central roles in photophysical events in organic photovoltaics (OPVs). Depth profiles of electrostatic potential and contact-induced charges determine the energy-level lineup of the frontier orbitals at electrode/organic and organic heterointerfaces. They are critical for the elementary processes in an OPV cell, such as generation and diffusion of free carriers. A simple electrostatic model describes the energetics in organic heterojunctions supported by an electrode, and experiments via photoelectron spectroscopy and the Kelvin probe method validate the potential distribution in the stacking direction of the device. A comparative study has clarified the significance of Fermi-level pinning and resulting electrostatic fields in determining the energy-level alignment. In this review, we discuss how parameters of device constituents affect the distributions of potential and the dark charges in conventional OPVs comprising metallophthalocyanine and C60 as donor and acceptor, respectively. The results of previous studies, together with additional numerical simulations, suggest that a number of the factors influence the depth profiles of the dark charge and potential, such as the work function of bottom materials, layer thickness, structural inhomogeneity at interfaces, top electrode, and stacking sequence.

An autonomous actuator driven by fluctuations in ambient humidity.

Devices that respond to negligibly small fluctuations in environmental conditions will be of great value for the realization of more sustainable, low-power-consumption actuators and electronic systems. Herein we report an unprecedented film actuator that seemingly operates autonomously, because it responds to the adsorption and desorption of a minute amount of water (several hundred nanograms per 10 mm(2)) possibly induced by fluctuations in the ambient humidity. The actuation is extremely rapid (50 ms for one curl) and can be repeated >10,000 times without deterioration. On heating or light irradiation, the film loses adsorbed water and bends quickly, so that it can jump vertically up to 10 mm from a surface or hit a glass bead. The film consists of a π-stacked carbon nitride polymer, formed by one-pot vapour-deposition polymerization of guanidinium carbonate, and is characterized by a tough, ultralightweight and highly anisotropic layered structure. An actuator partially protected against water adsorption is also shown to walk unidirectionally.

Energy-level alignment at organic heterointerfaces.

Today's champion organic (opto-)electronic devices comprise an ever-increasing number of different organic-semiconductor layers. The functionality of these complex heterostructures largely derives from the relative alignment of the frontier molecular-orbital energies in each layer with respect to those in all others. Despite the technological relevance of the energy-level alignment at organic heterointerfaces, and despite continued scientific interest, a reliable model that can quantitatively predict the full range of phenomena observed at such interfaces is notably absent. We identify the limitations of previous attempts to formulate such a model and highlight inconsistencies in the interpretation of the experimental data they were based on. We then develop a theoretical framework, which we demonstrate to accurately reproduce experiment. Applying this theory, a comprehensive overview of all possible energy-level alignment scenarios that can be encountered at organic heterojunctions is finally given. These results will help focus future efforts on developing functional organic interfaces for superior device performance.

Propeller-shaped fused oligothiophenes: a remarkable effect of the topology of sulfur atoms on columnar stacking.

Propeller-shaped regioisomers of fused oligothiophenes F9T(endo), F9T(anti), and F9T(exo) were successfully synthesized. DFT calculations indicated that their core parts are distorted from planarity due to intramolecular steric repulsions involving large sulfur atoms. In contrast with soft crystalline F9T(anti) and F9T(exo), F9T(endo) self-assembles into a hexagonal columnar liquid crystal (Col(h) LC), displaying a clear X-ray diffraction (XRD) due to its stacked π-conjugated core. In each LC column, well-organized intermolecular S-S contacts are developed triple-helically along the columnar axis with a helical pitch of 4.04 nm. Among LC semiconductors reported to date, Col(h) LC F9T(endo) displays a top-class charge-carrier mobility (0.18 cm(2) V(-1) s(-1)) with a distinct ambipolar character featuring well-balanced hole and electron mobilities. A thin film, prepared by mixing F9T(endo) with soluble fullerene PCBM, shows a photovoltaic response, when the fullerene content is large enough to compensate a small absorptivity of F9T(endo) for visible light.

Accessing surface Brillouin zone and band structure of picene single crystals.

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility µ(h)≥9.0 cm(2)/V s (298 K) were deduced in the Γ-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.