Wataru Mizutani

NMR studies of structure and dynamics of liquid molecules confined in extended nanospaces.

We fabricated an NMR cell equipped with 10-100 nm scale spaces on a glass substrate (called extended nanospaces), and investigated molecular structure and dynamics of water confined in the extended nanospaces by (1)H NMR chemical shift (delta(H)) and (1)H and (2)H NMR spin-lattice relaxation rate ((1)H- and (2)H-1/T(1)), (1)H NMR spin-spin relaxation rate ((1)H-1/T(2)), and (1)H NMR rotating-frame spin-lattice relaxation rate ((1)H-1/T(1rho)) measurements of H(2)O and (2)H(2)O. The delta(H) and (1)H- and (2)H-1/T(1) results showed that size-confinement produces slower translational motions and higher proton mobility of water, but does not affect the hydrogen-bonding structure and rotational motions. Such unique phenomena appeared in the space size of 40 to 800 nm. However, the (1)H-1/T(1) value at 40 nm was still different from that in 4 nm porous nanomaterial, because translational and rotational motions were inhibited for H(2)O molecules in the nanomaterial. By examining temperature- and deuterium-dependence of the (1)H-1/T(1) values, the molecular translational motions of the confined water were found to be controlled by protonic diffusion invoking a proton hopping pathway between adjacent water rather than hydrodynamic translational diffusion. Furthermore, we clarified that proton exchange between adjacent water molecules in extended nanospaces could be enhanced by the chemical exchange of protons between water and SiOH groups on glass surfaces, ( identical with SiO(-)...H(+)...H(2)O) + H(2)O --> triple bond SiO(-) + (H(3)O(+) + H(2)O) --> triple bond SiO(-) + (H(2)O + H(3)O(+)), based on (1)H-1/T(2) measurements. An enhancement of proton exchange rate of water due to the reduction of space sizes was verified from the results of (1)H-1/T(1rho) values, and the rate of water in the 100 nm sized spaces is larger by a factor of more than ten from that of bulk water. Such size-confinement effects were distinctly observed for hydrogen-bond solvents with strong proton-donating ability, while they did not appear for aprotic and nonpolar solvent cases. Based on these NMR results, we suggested that an intermediate phase, in which protons migrate through a hydrogen-bonding network and the water molecules are loosely coupled within 50 nm from the surface, exists mainly in extended nanospaces. This model could be supported by a three-phase theory based on the weight average of three phases invoking the bulk, adsorbed, and intermediate phases.

Voltage‐dependent scanning tunneling microscopy images of liquid crystals on graphite

Voltage‐dependent images of liquid crystals on graphite were observed in air by scanning tunneling microscopy (STM). Molecular rows of liquid crystals and the atomic pattern of the graphite substrate were imaged with high (above 1 V) and low (below 0.1 V) bias voltages, respectively. Patterns of molecules, grain boundaries, and distinguishable defects of the liquid crystal arrangement were reproduced even after imaging the substrate in the same area. This indicates that the graphite lattice can be seen by STM without touching or disturbing the adsorbed molecules on it. A resonant tunneling model is proposed to explain the phenomenon.

Nanometer-scale patterning of self-assembled monolayer films on native silicon oxide

A nanoscale-patterning method on silicon oxide using a self-assembled monolayer (SAM) was developed. The silicon surface with native oxide was additionally oxidized locally in dry nitrogen atmosphere by the field-induced oxidation (FIO) technique using an atomic force microscope with a conductive cantilever, and then immersed in octadecyltrichlorosilane (OTS) solution. The contact angle and topography image revealed that the OTS layer was formed only on the native oxide. In contrast, when FIO was performed under the humidity of 88%, OTS SAM was formed on both FIO and native oxide. These results indicate that SAM formation on silicon oxides can be locally suppressed by FIO in a dry environment. By using this technique, we could fabricate a line structure of OTS SAM as narrow as 22 nm.

Local elasticity measurement on polymers using atomic force microscopy

Local elasticity was measured on polymers by modifying atomic force microscopy (AFM) in which the sample height was modulated and the response of the cantilever was detected. In a case of polystyrene (PS) on mica, there existed many circular holes in the film that were stiffer than the rest parts. The holes were identified as the substrate mica surface and the rest as PS films. In the case of a polyethyleneoxide surface on mica, a crystalline nature was observed. In addition, the surface was modified by scanning an AFM tip at large forces of 20 nN. The scanned area exhibited an increase in elasticity as well as formation of a striped structure.

A simple fabrication method of nanogap electrodes for top-contacted geometry: application to porphyrin nanorods and a DNA network

We have developed a fabrication method for nanogap electrodes without employing photo- or electron-beam lithography to measure the electrical characteristics of nanostructured molecules. This angle-controlled shadow-masking method enables us to construct nanogap electrodes without a wet process after the molecules are positioned on the substrate. The proposed method makes it possible to measure electrical characteristics without structurally deforming or denaturing the molecules due either to the step edge of an electrode or to the organic solvents used in the wet process. The results demonstrate that a gap length between the electrodes of less than 100 nm can be fabricated reproducibly. We have measured the electrical characteristics of lambda DNA (λ-DNA) networks and molecular nanorods made of porphyrin-derivative molecules (TPPS: 5,10,15,20-tetraphenyl-21H,23H-porphyrine tetrasulfonic acid) in which J-aggregates are formed inside. Experimental findings reveal that the electrical conductivity of λ-DNA decreased under a vacuum condition, whereas that of TPPS nanorods decreased under oxygen and nitrogen gas-purged conditions.

Molecular arrangement and electrical conduction of self-assembled monolayers made from terphenyl thiols

We investigated molecular arrangement change of self-assembled monolayers (SAMs) made from [1,1:4,1-terphenyl]-4-thiol (TP0) on Au(111) using scanning tunneling microscopy. In an ethanol solvent, lying down phase of TP0 molecules where the conjugated rings are expected to contact directly the Au surface, were observed at the initial stage of TP0 SAM growth (at 1 min immersion). After 24 h immersion, a molecular lattice, which is similar to (√3 × √3)R30° structure (perpendicular arrangement) was observed. We further investigated the molecular arrangement dependence of electrical conduction of the TP0 SAMs using conductive atomic force microscopy technique. The resistances of the TP0 SAMs were 10 4 Ω (at parallel arrangement) and 10 8 Ω (at perpendicular arrangement). The resistance of the lying down TP0 SAM was lower than that of benzylmercaptane SAM where the thickness was identical to lying down TP0 SAM. The result indicated that the direct contact of the conjugated ring to the Au surface could further reduces the tunneling resistance.

Effect of Amines on Single-Walled Carbon Nanotubes in Organic Solvents: Control of Bundle Structures

We have investigated the control of bundle structures of single-walled carbon nanotubes (SWNTs) in organic solvents using atomic force microscopy (AFM) and transmission electron microscopy (TEM). We have reported herein that organic amines are effective in untangling the bundled SWNTs in N,N-dimethylformamide (DMF). When Bucky paper was sonicated in DMF with 1% organic amine for 1 h, AFM images clearly showed mainly individual SWNTs. The time-dependent TEM observation revealed that the role of amine is to prevent rebundling of SWNTs in organic solvents.

Measuring Molecular Conductivities Using Single Molecular-Sized Gap Junctions Fabricated without Using Electron Beam Lithography

We demonstrate a procedure for fabricating back-gate-type sub 5 nm gap junctions without using electron beam lithography. The fabricating procedure contains two photolithographic processes followed by shadow evaporation and oxygen plasma ashing. Oligo(para-phenylenevinylene) trimers (OPV3) attached to methylthiol at both ends were bridged between these junctions. The temperature dependence of OPV3 conductance indicated the characteristic behavior of hopping conduction. The results indicate that these junctions can be used in the study of conductance measurements through molecular wires.

Conductive Probe AFM Measurements of Conjugated Molecular Wires

Abstract: The electrical conduction of self‐assembled monolayers (SAMs) made from conjugated molecules was measured using conductive probe atomic force microscopy (CP‐AFM), with a focus on the molecular structural effect on conduction. First, the electrical conduction of SAMs made from phenylene oligomer SAMs was measured. The resistances through the monolayers increased exponentially with an increase in molecular length and the decay constants of transconductance β were about 0.45 to 0.61 Å−1 measured at lower bias region. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with an increase in the applied load up to 14 nN. Second, using an insertion technique into insulating alkanethiol SAMs, the electrical conduction of single conjugated terphenyl methanethiol and oligo(para‐phenylenevinylene) (OPV) molecules embedded into insulating alkanethiol SAMs were measured. Electrical currents through these single molecules of OPVs were estimated to be larger than those through single terphenyl molecules, suggesting that the OPV structure can increase the electrical conduction of single molecules. Third, apparent negative differential resistance (NDR) was observed at higher bias measurements of SAMs. The appearance of NDR might be related to roughness of SAM surface, because apparent NDR was often observed on rough surfaces. In any case, the tip‐molecule contact condition strongly affected carrier transport through metal tip/SAM/metal junction.

Theoretical Prediction and Atomic Force Microscope Observations of the Protein Nanotube Consisting of Homo-L-Amino Acid Penta-Peptide Nanorings

An unusual penta-peptide nanotube was synthesized by a solid-phase method using Fmoc chemistry. This nanotube consists of the natural homo-L-amino acid sequence (cyclo[-(L-Gln)5]), which is different from the already-known dl-peptide nanotubes having an even number of residues. We also observed the morphology by atomic force microscope (AFM) and found meandering tubular structures on the substrate. This result is consistent with our ab initio energy calculations, which show that the penta-peptide nanorings stabilize by breaking the C5 symmetry and stack themselves to form a meandering nanotube through the inter-ring hydrogen bonds.