Tetsuo Akasaka

Mechanically Controlled Quantum Contact With On-Chip MEMS Actuator

We report fabrication and characterization of a mechanically controlled quantum point contact with on-chip electrostatic actuators. Two metallic tips separated with a nano gap are integrated with a micro mechanism, which can be actuated by applied voltage. The electrostatic actuation of the micro mechanism allows the control of separation gap at nanometer level. The gap variation at nanometer level is calibrated with the devices operated in transmission electron microscope. Furthermore, performance of the device is demonstrated by measuring conductance quantization while separating the tips in contact as well as electrical measurements by using tips coated with self assembled monolayers (1-4 benzene dithiol) and conjugated Ru complex

Photo-induced energy transfer and its switching in dyad and triad chromophore systems composed of coumarin, Ru(II) and Os(II) terpyridine-type complexes

A new dyad and two triad chromophore compounds containing coumarin, Ru(II) and Os(II) terpyridine-type complexes were synthesized. The dyad is composed of coumarin and Os(II) units, while the triads are composed of coumarin, Ru(II) and Os(II) units. One of the triads has a phenylene spacer to connect the Ru(II) unit and the Os(II) unit, while the other has an azo moiety. Energy transfer in these multichromophoric systems has been probed by electronic absorption and luminescence spectroscopy. The switching behavior of photo-induced energy transfer by redox stimuli in the latter triad has been examined. Photophysical and electrochemical analysis indicates that the contribution of the energy transfer from the coumarin chromophores and the Ru(II) center to the Os(II)-centered emission in the ‘switch-on state’ is estimated to be about 70%. This contribution is larger than that in the previously reported Ru(II)/Os(II) dyad system, which was evaluated to be 40%. Thus, it is concluded that an improved switching of directional energy transfer has been achieved from the coumarin moiety to the Os(II) center in this new triad chromophore system.

Novel Gold Nanoparticles/Conjugated Molecules Network Structures Fabricated By Self‐assembling Process

Self‐assembled network of gold nanoparticles connected by molecules on SiO2/Si surfaces over macroscopic distances was prepared by alternately dipping substrates into gold nanoparticle‐ and molecule‐solutions. A significant conductivity was observed even after 1 cycle of deposition, suggesting that a conductive pathway was formed between the electrodes through the gold nanoparticle‐molecule network. Furthermore, it should be noted that the present unique fabrication method is compatible with planar transistor geometry.

Atomistic Picture of Electromigration Process and its Application to High‐yield Fabrication of Nanogap Electrodes

Precisely controlled electromigration process was introduced as a high‐yield fabrication technique of nanogap electrodes. During the stress‐and‐relax voltage control we observed successive step‐like decreases in the conductance across the junction by one quantum of conductance 2e2/h, which corresponds to the one‐by‐one removal events of gold atoms. The yield of nanogaps showed great improvement; we have reproducibly obtained the nanogaps with resistance of ∼5 × 104 ∼ 1011 ohms (gap separation = 0.3 ∼ 1.5 nm) without any catastrophic breakage for almost all tested sample. Preliminary results on transport properties were also presented for nanogaps containing molecules.

Synthesis and properties of an efficient and switchable photosensitizing unit, [Ru(4,4′-diphenyl-2,2′-bipyridine)2(7-amino-dipyrido[3,2-a:2′,3′-c]phenazine)]2+, for a photo-induced energy transfer system

A Ru(II) complex containing the 4,4′-diphenyl-2,2′-bipyridine (Ph2bpy) and 7-amino-dipyrido[3,2-a:2′,3′-c]phenazine (dppz-NH2) ligands was synthesized and found to be an excellent switchable photosensitizing unit suitable for photo-induced energy transfer, which has large light absorptivity, superior excited state properties and an amino functional unit for connecting acceptor units. [Os(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) unit was connected to the amino group of this photosensitizing unit by an amide bond to construct a heterodinuclear Ru(II)/Os(II) photo-induced energy transfer system. This system showed stronger Os(II) centered emission compared to the similar heterodinuclear Ru(II)/Os(II) complex containing 2,2′-bipy (bpy) instead of Ph2bpy. Analysis of the photophysical processes of these complexes indicated that the stronger Os(II) emission was ascribed not only to the higher absorptivity but also to the increased energy transfer efficiency from the Ru(II) to the Os(II) center. It also demonstrated that photosensitization by energy transfer to the Os(II) center was switched on and off reversibly by protonation/deprotonation of the dppz moiety.