Hiroaki Azehara

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.

Distinct Chemical Contrast in Adhesion Force Images of Hydrophobic–Hydrophilic Patterned Surfaces Using Multiwalled Carbon Nanotube Probe Tips

In this paper, we describe a fabrication procedure for large-diameter carbon nanotube probe tips (CNT tips) for atomic force microscopy, the tip-end chemistry of the CNT tips, and their advantage drawn from the study of adhesion force imaging in an ambient atmosphere on a patterned hydrophobic and hydrophilic self-assembled monolayer, which has been prepared by a microcontact printing method. Force titration measurements in phosphate buffer solutions reveal that the CNT tip has retained carboxyl groups at its end. In adhesion force imaging, a distinct chemical contrast is obtained for the patterned surfaces as compared to a case using a silicon nitride tip. The origin of the distinct contrast is discussed in terms of the tip-end chemistry featured by carboxyl groups and a possible weakening of capillary forces of water caused at around the tip–sample interface because of the intrinsically hydrophobic nature of CNTs.

Difference in Self-Assembling Morphology of Peptide Nanorings

We synthesized the peptide nanorings of cyclo[-(D-Ala-L-Gln)3], cyclo[-(D-Cys-L-Gln)3], cyclo[-D-Cys-L-His-D-Ala-L-Asn-Gly-L-Gln-] and cyclo[-(L-Gln)5], and studied the way in which the difference in the type and/or number of component amino acid residues changes the self-assembling morphology of the nanorings on gold substrates by atomic force microscopy. The study revealed that cyclo[-(D-Ala-L-Gln)3] formed nanotube bundles through inter-ring hydrogen bonds, while the nanorings of cyclo[-(D-Cys-L-Gln)3] adhered to the gold surface directly due to the high affinity of thiol to gold. In contrast, a random amino acid sequence of cyclo[-D-Cys-L-His-D-Ala-L-Asn-Gly-L-Gln-] resulted in many isolated nanotubes, which were first observed in the present study. While the D,L-peptide nanotubes have very straight forms, the homo-L-peptide of cyclo[-(L-Gln)5] formed interesting randomly branching nanotubes that were entwined and grew on the substrate. Scanning tunneling microscopy was also performed and high-resolution images of both the peptide nanotubes and the nanotube bundles were obtained.

Chemical Force AFM with CNT Tips

In this paper, we focus on preparation of CNT-AFM tips, their chemical modification, and their application. The authors have been trying to use a carbon nanotube (CNT) as an AFM probe tip because of its nanoscale dimension, mechanical and chemical robustness, high aspect ratio, and so on. The first step toward this issue, we have prepared the checker pattern surface by self-assembling molecules of hexadecanethiol (-CH3 at the end, hydrophobic) and of mercaptoundecanol (-OH, hydrophilic) on to mica by microcontact printing method. Then we measured the friction forces as well as adhesion forces on this surface. The paper shows the force mapping images as well as force histograms which clearly discriminate the hydrophilic and hydrophobic parts of the sample surface: the former parts are higher than the latter ones in both friction and adhesion.

FORMATION OF SELF-ASSEMBLED MONOLAYERS ON GOLD SURFACES BY LUMINESCENT OLIGO (PARA-PHENYLENE-VINYLENE)-METHANETHIOL

Oligo(para-phenylene-vinylene)-methanethiol of trimer and dimer have been synthesized to prepare monolayers onto Au(111). The trimer diluted in dichloromethane glows with a brilliant blue fluorescence when excited with UV light (365 nm). The absorption and emission spectra of the dimer diluted in CH2Cl2 show a blue shift. The molecular level morphologies of the monolayers prepared on gold surfaces were probed by scanning tunneling microscopy or atomic force microscopy (AFM). The dimer is entire trans isomer, which formed highly ordered self-assembled monolayers (SAMs) on Au. The trimer is a mixture of cis–trans isomers, and did not form ordered monolayers on Au. However, the trimer molecules could be embedded in dodecanethiol SAM, and showed higher conductance than oligo(para-phenylene) molecules characterized using a conductive AFM.