Energy level diagram of HC(NH2)2PbI3 single crystal evaluated by electrical and optical analyses.

Recently, organic-inorganic halide perovskites have received attention for applications in solar cells. Measurements of high-quality single crystals reveal lower defect densities and longer carrier lifetimes than those of conventional thin films, which result in improved electrical and optical properties. However, single crystal surfaces are sensitive to exposure to ambient conditions, and degrade under long-term storage in air. The surface also shows differences from the bulk in terms of its optical and electronic characteristics. For a heterojunction device, the interface at the single crystal is important. Understanding the difference between the surface and bulk properties offers insights into device design. Here, we prepared non-sliced and sliced formamidinium lead iodide (FAPbI3; FA+ = HC(NH2)2+) single crystals with a bandgap of 1.4 eV, which matches well with the requirements for solar cell photoabsorption layers. We evaluate the energy level diagrams of the surface and bulk regions, respectively. Our data indicate that the valence band maximum of the surface region is at a higher energy level than that of the bulk region. We also discuss hypotheses for the well-known and unexplained phenomena (multiple bandgaps and bandgap narrowing) seen in the absorption and photoluminescence spectra of single crystals. We conclude that these effects are likely caused by a combination of the degraded surface, Rashba-splitting in bulk, and self-absorption by the single crystal itself.

Requirement of LIM domains for the transient accumulation of paxillin at damaged stress fibres.

Cells recognize and respond to changes in intra- and extracellular mechanical conditions to maintain their mechanical homeostasis. Linear contractile bundles of actin filaments and myosin II known as stress fibres (SFs) mediate mechanical signals. Mechanical cues such as excessive stress driven by myosin II and/or external force may damage SFs and induce the local transient accumulation of SF-repair complexes (zyxin and VASP) at the damaged sites. Using an atomic force microscope mounted on a fluorescence microscope, we applied mechanical damage to cells expressing fluorescently tagged cytoskeletal proteins and recorded the subsequent mobilization of SF-repair complexes. We found that a LIM protein, paxillin, transiently accumulated at the damaged sites earlier than zyxin, while paxillin knockdown did not affect the kinetics of zyxin translocation. The C-terminal half of paxillin, comprising four-tandem LIM domains, can still translocate to damaged sites on SFs, suggesting that the LIM domain is essential for the mechanosensory function of paxillin. Our findings demonstrate a crucial role of the LIM domain in mechanosensing LIM proteins.

Direct detection of cellular adaptation to local cyclic stretching at the single cell level by atomic force microscopy.

The cellular response to external mechanical forces has important effects on numerous biological phenomena. The sequences of molecular events that underlie the observed changes in cellular properties have yet to be elucidated in detail. Here we have detected the responses of a cultured cell against locally applied cyclic stretching and compressive forces, after creating an artificial focal adhesion under a glass bead attached to the cantilever of an atomic force microscope. The cell tension initially increased in response to the tensile stress and then decreased within ∼1 min as a result of viscoelastic properties of the cell. This relaxation was followed by a gradual increase in tension extending over several minutes. The slow recovery of tension ceased after several cycles of force application. This tension-recovering activity was inhibited when cells were treated with cytochalasin D, an inhibitor of actin polymerization, or with (-)-blebbistatin, an inhibitor of myosin II ATPase activity, suggesting that the activity was driven by actin-myosin interaction. To our knowledge, this is the first quantitative analysis of cellular mechanical properties during the process of adaptation to locally applied cyclic external force.

Highest-occupied-molecular-orbital band dispersion of rubrene single crystals as observed by angle-resolved ultraviolet photoelectron spectroscopy.

The electronic structure of rubrene single crystals was studied by angle-resolved ultraviolet photoelectron spectroscopy. A clear energy dispersion of the highest occupied molecular orbital-derived band was observed, and the dispersion width was found to be 0.4 eV along the well-stacked direction. The effective mass of the holes was estimated to be 0.65(+/-0.1)m0. The present results suggest that the carrier conduction mechanism in rubrene single crystals can be described within the framework of band transport.

Development of loading system for liquid hydrogen into diamond-anvil cells under low temperature.

A loading system for hydrogen gas into the diamond-anvil cell has been developed. The loading of hydrogen gas is performed under low temperature by using liquid helium as a cooling medium. Also, a compression apparatus has been developed to load gaseous materials into various diamond-anvil cells. The present loading system and compression apparatus have been used successfully to form hydrogen hydrate. The present loading system can also be used to load other gaseous materials as a pressure medium.

Structural changes of filled ice Ic structure for hydrogen hydrate under high pressure.

High-pressure experiments of hydrogen hydrate, filled ice Ic structure, were performed using a diamond-anvil cell in the pressure range of 0.1-80.3 GPa at room temperature. In situ x-ray diffractometry (XRD) revealed that structural changes took place at approximately 35-40 and 55-60 GPa, and that the high-pressure phase of hydrogen hydrate survived up to at least 80.3 GPa. Raman spectroscopy showed that the changes in vibrational mode for the hydrogen molecules in hydrogen hydrate occurred at around 40 and 60 GPa, and these results were consistent with those of the XRD. At about 40 GPa, the intermolecular distance of host water molecules consisting the framework attained the critical distance of symmetrization of the hydrogen bond for water molecules, which suggested that symmetrization of the hydrogen bond occurred at around 40 GPa. The symmetrization might introduce some structural change in the filled ice Ic structure. In addition, the existence of the high-pressure phase above 55-60 GPa implies that a denser structure than that of filled ice Ic may exist in hydrogen hydrate.

Purely site-specific chemisorption and conformation of trimethylamine on Si(100)c(4 x 2).

One of the fundamental points of interest on the Si(100) surface is how the spatial localization of electron density on the buckled silicon dimer controls the site-specific reaction toward different Lewis acid and Lewis base molecules. We have investigated the adsorption of trimethylamine (TMA) on Si(100)c(4x2) using scanning tunneling microscopy (STM) at 80 K. The adsorbed TMA appears as a triangle-shaped bright protrusion in the occupied-state STM image. The triangle-shaped protrusion is ascribed to three methyl groups in the adsorbed TMA. The center of the protrusion is located on the down atom site, which indicates that the adsorption of TMA occurs only on the down dimer atom. Thus, TMA adsorption on Si(100)c(4x2) is found to be purely site-specific on the down dimer atom and can be categorized in Lewis acid-base reaction.