Takao Ochiai

Chemical coating of large-area Au nanoparticle two-dimensional arrays as plasmon-resonant optics

Innovative nanophotonic applications require a technique for generating not a nanometer-scale point but a large-area (mm2−m2) near-field light source. We succeeded in developing a large-area near-field light source that is densely constructed of uniform-size gold nanoparticles (AuNPs) two-dimensionally arrayed with regular interparticle gaps, which has tunable localized surface plasmon resonance bands (600–1100 nm). The near-field excitation properties based on the optical tunability of the AuNP two-dimensional arrays demonstrate that our chemical coating of large-area near-field light sources is widely applicable such as for high-sensitivity optical sensors and high-efficiency solar cells.

Enhancement of self-assembly of large (>10 nm) gold nanoparticles on an ITO substrate

Densely arraying gold nanoparticles (AuNPs) immobilized on substrates is still a challenge. For the arraying, surface modification of AuNPs with alkanethiols is a key technology. However, if the particle size is larger than 20 nm, short-chain alkanethiols have very weak interparticle interaction such that self-assembly is not realized, whereas long-chain alkanethiols have very strong interaction such that self-aggregation is induced in solution. One solution is their combination. Eventually, a mixture gave a dense array of 55 nm AuNPs on an ITO substrate with a coverage of approximately 80%. Our approach provides high-performance plasmonic optics with a resonance wavelength of more than 700 nm.

Plasmon-Resonant Optics on an Indium–Tin-Oxide Film for Exciting a Two-Photon Photochromic Reaction

Two-dimensional arrays of gold nanoparticles (AuNPs) as localized surface plasmon-resonant (LSPR) optics on a transparent conductive layer of indium–tin-oxide (ITO) were successfully fabricated. The typical surface coverage of the 10 nm sized AuNPs is over 87% for on ITO film roughness of ±0.2 nm. The LSPR wavelength is tunable in the range of 622–905 nm. By adjusting the LSPR wavelength to 905 nm at peak, two-photon photochromic reaction of diarylethene derivative in solution phase was demonstrated with irradiation by an incoherent near-infrared light in the range of 0.017–0.033 W/cm2 instead of a laser, thanks to the AuNP two-dimensional array.

Micromagnetic study of current-pulse-induced magnetization switching in magnetic tunnel junctions with antiferromagnetically and ferromagnetically coupled synthetic free layers

We investigated the dynamics of current-pulse-induced magnetization switching in magnetic tunnel junctions (MTJs) with antiferromagnetically and ferromagnetically coupled synthetic free layers through micromagnetic simulations. We found that a magnetic vortex is formed in thick upper ferromagnetic layers and plays an important role in magnetization switching in both types of synthetic free layers. Furthermore, higher thermal stability is observed in an MTJ with the ferromagnetically coupled free layer at an annealing temperature of 250 °C.

Correlation between microstructure and electromagnetic properties in magnetic tunnel junctions with naturally oxidized MgO barrier

We investigated the correlation between the microstructure and the electromagnetic properties of magnetic tunnel junctions (MTJs) with MgO barriers fabricated by direct sputtering and natural oxidation of the Mg layers. The MTJ with an oxidized Mg barrier contained amorphous phases in both the barrier and the CoFeB free layer and exhibited large interface fluctuations in both the CoFeB reference/MgO and MgO/CoFeB interfaces This had a significant influence on the low MR ratio and large switching current. Inserting a CoFe seed layer under the oxidized Mg barrier greatly improved the crystallization of the oxidized MgO which ultimately led to a higher MR ratio and lower switching current. In addition to the textured CoFeB/MgO/CoFeB structures, the sharp interface of the MgO barrier, low B content in the MgO barrier, and Fe-rich composition in the free layer are thought to be of key importance for ensuring high magneto-transport properties.