Takahisa Ohno

Ultrafast permeation of water through protein-based membranes

Pressure-driven filtration by porous membranes is widely used in the production of drinking water from ground and surface water. Permeation theory predicts that filtration rate is proportional to the pressure difference across the filtration membrane and inversely proportional to the thickness of the membrane. However, these membranes need to be able to withstand high water fluxes and pressures, which means that the active separation layers in commercial filtration systems typically have a thickness of a few tens to several hundreds of nanometres. Filtration performance might be improved by the use of ultrathin porous silicon membranes or carbon nanotubes immobilized in silicon nitride or polymer films, but these structures are difficult to fabricate. Here, we report a new type of filtration membrane made of crosslinked proteins that are mechanically robust and contain channels with diameters of less than 2.2 nm. We find that a 60-nm-thick membrane can concentrate aqueous dyes from fluxes up to 9,000 l h(-1) m(-2) bar(-1), which is approximately 1,000 times higher than the fluxes that can be withstood by commercial filtration membranes with similar rejection properties. Based on these results and molecular dynamics simulations, we propose that protein-surrounded channels with effective lengths of less than 5.8 nm can separate dye molecules while allowing the ultrafast permeation of water at applied pressures of less than 1 bar.

Density functional theory investigation of benzenethiol adsorption on Au(111)

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

Self-Consistent Calculation of the Band Structure of C8K Including the Charge Transfer Effect

The method of the band structure calculation for layer-type materials is presented with use of atomic pseudopotentials and the Madelung-type potential due to the charge transfer. A simple physical model is proposed in order to estimate the amount of charge transfer, which has been a matter of controversy in graphite intercalation compounds. The band structure of C 8 K is calculated self-consistently by the present method and the amount of the charge transfer is determined non-empirically to be 0.6. The obtained band structure is almost the same as that calculated by the tight binding method.

Passivation of GaAs(001) surfaces by chalcogen atoms (S, Se and Te)

Abstract In order to elucidate the passivating effects of the chalcogenide solution treatment on GaAs surfaces, we have performed first-principles Pseudopotential calculations of the GaAs(001) surfaces adsorbed with a monolayer of chalcogen atoms. It is shown that chalcogen atoms adsorb in the bridge site on both the Ga-terminated and the As-terminated GaAs surfaces and form covalent bonds with Ga or As atoms. The chalcogen-Ga bond is found to be stronger than the chalcogen-As bond. We have shown that the chalcogen-Ga bond remarkably reduces the surface state density in the GaAs mid-gap region, while the chalcogen-As bond does not. It is suggested that the chalcogen-Ga bonds are dominant on the chalcogen-treated GaAs surface and are responsible for the passivation of the surface. In addition, we infer that Se atoms can passivate GaAs(001) surfaces as effectively as S atoms, but that the passivating effect of Te atoms is weak in comparison with S and Se atoms.

Electronic coupling assembly of semiconductor nanocrystals: self-narrowed band gap to promise solar energy utilization

To promote solar energy conversion into clean energy, hydrogen fuel and electrical power, photoactive nanostructures, e.g.cadmium sulfide nanotubes and nanocages, have been developed from electronic coupling assembly of nanocrystals. The inter-particle electronic coupling provides the nanostructures with self-narrowed band gap even less than that of bulk material. Consequently, the nanostructures achieve superior aptitude for solar energy utilization.

Band Structures and Charge Distributions along the c-Axis of Higher Stage Graphite Intercalation Compounds

Self-consistent non-empirical calculations of band structures of higher stage graphite intercalation compounds have been performed up to the 6-th stage by using the numerical-basis-set LCAO method. The calculations are carried out for a thin film of n contiguous graphite layers bounded by two ionized intercalant layers. From these calculations, the charge distribution along the c -axis and the stage dependence of the Fermi-level density of states are determined. It is clarified that the c -axis charge distribution is extremely inhomogeneous and that the effect of the σ bands is essentially important in determining the charge distribution along the c -axis. The measured stage dependence of the orbital contribution in the total magnetic susceptibility of alkali-metal compounds is discussed on the basis of the calculated results of the c -axis charge distribution.

Theoretical investigation on electron transport through an organic molecule: Effect of the contact structure

Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.

Dependence of the conduction of a single biphenyl dithiol molecule on the dihedral angle between the phenyl rings and its application to a nanorectifier

The transport properties of a biphenyl dithiol (BPD) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Greens function method based on the density functional theory. In particular, their dependence on the dihedral angle (phi=90 degrees -180 degrees ) between two phenyl rings is investigated. While the dihedral-angle dependence of the density of states projected on the BPD molecular orbitals is small, the transport properties change dramatically with phi. The transmission at the Fermi energy exhibits a minimum at phi=90.0 degrees and greatly increases with phi. The ratio of the maximum obtained at phi=180 degrees to the minimum exceeds 100. As an application of this characteristic transport behavior, a BPD molecule functionalized with NH(2) and NO(2) groups is considered. It is found that this molecule works as a nanorectifier.

Carbon-Doped Silicon Oxide Films with Hydrocarbon Network Bonds for Low-k Dielectrics: Theoretical Investigations

We have computationally explored the chemical structures of carbon-doped silicon oxide (SiOCH) films that give the smallest dielectric constant (k) under the required mechanical strength for low-k dielectrics. The focus of this study is on the SiOCH structures that have hydrocarbon components in the polymer network as cross-links. It has been found that SiOCH films of small dielectric constants can have improved mechanical strengths if the hydrocarbon components form cross-links, instead of the terminal methyl groups in the conventional structure. The calculated results suggest that SiOCH films of ideal structures can have substantially smaller dielectric constants than films of current interconnect technology with the same mechanical strengths.

Effect of vacancy-type oxygen deficiency on electronic structure in amorphous alumina

Electronic and atomic structures associated with a vacancy-type oxygen deficiency in an amorphous alumina model are studied by first-principles calculations. The energy levels of the oxygen defects significantly shift depending on their charge states because of remarkable changes of local atomic structures. That is different in character from the α crystal case. We discuss a possibility of the oxygen defects as a conductive path and present an atomistic mechanism of the resistive switching effects in the memory devices.