Yuhsuke Yasutake

Simultaneous fabrication of nanogap gold electrodes by electroless gold plating using a common medical liquid

We report a simple and high yield method for fabricating multiple nanogaps simultaneously by an electroless gold plating technique using electroless gold plating solution which consists of common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C). The distance between the gold electrodes (33nm in average) on the SiO2∕Si substrate was decreased by selective deposition of gold onto the surface of the gold electrodes. By electroless gold plating, we fabricated nanogaps below 5nm in width with a 41% process yield. We also demonstrated the Coulomb blockade effect in octanethiol(C8)-protected Au nanoparticles by using such a fabricated nanogap.

Single-Electron Transistor Fabricated by Two Bottom-Up Processes of Electroless Au Plating and Chemisorption of Au Nanoparticle

Coulomb diamonds were clearly observed on single-electron transistors (SETs) fabricated by bottom-up processes of electroless plating of Au nanogap electrodes and chemisorption of a Au nanoparticle at 80 K. In the drain current–drain voltage characteristics, Coulomb staircases were modulated by the side gate voltage. Tunneling resistances and source/drain/gate capacitances of the SET were evaluated by fitting the theoretical Coulomb staircase determined on the basis of the full orthodox theory in a double-barrier tunneling junction to the experimental results of Coulomb blockade under the application of side gate voltages. The theoretical results for the Coulomb diamond are in good agreement with the experimental results.

Bias Stress Induced Threshold Voltage Shift in Pentacene Thin-Film Transistors

Threshold voltage instabilities in SiO2/polyimide dual-gate dielectric pentacene thin-film transistors are investigated as a function of bias stress time for 1000 s at temperatures between 260 and 340 K in nitrogen atmosphere. Field-effect mobility maintains constant values at every measurement temperature during the application of constant bias stress voltage. The threshold voltage shift at all measurement temperatures is described by the stretched exponential stress time dependence of ΔVth(t) = ΔVth0{1-exp [-(t/τ)β]}. These experimental results suggest that our threshold voltage shift can be interpreted as carrier injection from the pentacene channel into traps located at the channel/gate dielectric interface.

High-density G-centers, light-emitting point defects in silicon crystal

We propose a new method of creating light-emitting point defects, or G-centers, by modifying a silicon surface with hexamethyldisilazane followed by laser annealing of the surface region. This laser annealing process has two advantages: creation of highly dense G-centers by incorporating carbon atoms into the silicon during heating; freezing in the created G-centers during rapid cooling. The method provides a surface region of up to 200 nm with highly dense carbon atoms of up to 4 × 1019 cm−3 to create G-centers, above the solubility limit of carbon atoms in silicon crystal (3 × 1017 cm−3). Photoluminescence measurement reveals that the higher-speed laser annealing produces stronger G-center luminescence. We demonstrate electrically-driven emission from the G-centers in samples made using our new method.

Au Nanoparticles Chemisorbed by Dithiol Molecules Inserted in Alkanethiol Self-Assembled Monolayers Characterized by Scanning Tunneling Microscopy

Alkanethiol-protected Au nanoparticles have been chemically immobilized on Au(111) substrates by alkanedithiol and alkanethiol mixed self-assembled monolayers (SAMs). Octanethiol SAMs on Au(111) have been used as host matrices for molecular insertion by 1,8-octanedithiol (C8S2) and 1,10-decanedithiol (C10S2). The dithiol molecules in the mixed SAMs and the Au nanoparticles on the mixed SAMs were observed by scanning probe microscopy. The densities of C8S2 and C10S2 mixed SAMs were controlled by varying the dithiol concentration from 0.5 to 50 mM. The densities of Au nanoparticles were almost equal to those of the dithiol molecules, indicating that the Au nanoparticle chemisorbed through the protruding sulfur of the dithiol molecules displaced the protecting group of the Au core. The chemisorbed Au nanoparticles were stable for scanning tunneling spectroscopy (STS) measurements. The resistance of the chemisorbed nanoparticles between the Au core and the Au(111) substrate is discussed.

Boron Difluoride Complexes of Expanded N-Confused Calix[n]phyrins That Demonstrate Unique Luminescent and Lasing Properties

Complexation of novel multiply N-confused expanded calix[n]phyrins with boron difluoride afforded a new class of cyclic BODIPY (boron-dipyrromethene) arrays. The structures of circularly arranged BODIPY subunits linked in an N-confused fashion give rise to such photophysical properties unique to the macrocycles as redshifted emission wavelengths along with apparent large Stokes shifts, long emission lifetimes, and solid-state lasing. The DFT calculations support the size-dependent excited-state dynamics of the macrocycles.

Strain-induced direct band gap shrinkage in local Ge-on-insulator structures fabricated by lateral liquid-phase epitaxy

Direct band gap shrinkage in Ge-on-insulator (GOI) structures fabricated by lateral liquid-phase epitaxy (LLPE) was investigated by means of micro(μ)-photoluminescence and μ-Raman spectroscopy. The LLPE method, based on the rapid thermal process, was found to be an effective and feasible way to produce highly strained local GOI structures. We observed a significant redshift of direct gap emission amounting to 45 meV from the tensile-strained GOI layer. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. The local optical properties along the LLPE-grown GOI wire were examined and discussed on the basis of μ-PL mapping images.

Simultaneous Measurements of Drain-to-Source Current and Carrier Injection Properties of Top-Contact Pentacene Thin-Film Transistors

Drain-to-source current (IDS ¼ð IDISÞ=2) and displacement current (Idis ¼ IS þ ID) are evaluated using the simultaneous measurements of source (IS) and drain (ID) currents during the application of a constant drain voltage and a triangular-wave gate voltage (VGS) to top-contact pentacene thin-film transistors. The carrier mobility, threshold voltages and mean potential drops at the source/channel and channel/drain interface are simultaneously obtained from IDS-VGS and Idis-VGS relationships. Carrier injection properties, namely, the carrier injection voltage at the source electrode and the mean potential drop-VGS relathionship are discussed on the basis of results of the simultaneous measurements of IDS and Idis. (DOI: 10.1143/JJAP.46.390)

Near-Infrared Phosphorescent Iridium(III) Benzonorrole Complexes Possessing Pyridine-based Axial Ligands

Novel near-infrared phosphorescent iridium(III) complexes based on benzo-annulated N-linked corrole analogue (termed as benzonorrole) were synthesized. The structures of the complexes revealed octahedral coordination geometries involving an organometallic iridium-carbon bond with two external axial ligands. Interestingly, the iridium(III) complex exhibits near-infrared phosphorescence at room temperature at wavelengths beyond 900 nm. The significant redshift of the emission, as compared to the corrole congener, is originated from the ligand-centered triplet character. The fine-tuning of the photophysical properties of the complexes was achieved by introducing electron-donating and electron-withdrawing substituents on the axial pyridine ligands.

Time-resolved electroluminescence of bulk Ge at room temperature

Time-resolved spectroscopy of bulk Ge at room temperature allows to differentiate direct-gap electroluminescence (EL) from indirect-gap EL. The results indicate that there exists strong coupling between the eight-fold indirect L-valleys and the zone-center Γ-valley due to intervalley phonon scattering at room temperature in the absence of built-in strain. Pulse modulation of EL at 10-MHz is consistent with the relevance of the direct-gap transition.