Ryota Negishi

Fabrication of nanoscale gaps using a combination of self-assembled molecular and electron beam lithographic techniques

We have developed and tested a new method of fabricating nanogaps using a combination of self-assembled molecular and electron beam lithographic techniques. The method enables us to control the gap size with an accuracy of approximately 2nm and designate the positions where the nanogaps should be formed with high-resolution patterning by using electron beam lithography. We have demonstrated the utility of the fabricated nanogaps by measuring a single electron tunneling phenomenon through dodecanethiol-coated Au nanoparticles placed in the fabricated nanogap.

I-V characteristics of single electron tunneling from symmetric and asymmetric double-barrier tunneling junctions

I-V characteristics of single electron tunneling from a symmetric and an asymmetric double-barrier tunneling junction (DBTJ) were examined. A single Au nanoparticle was trapped in nanogap whose size was precisely controlled using a combination of electron beam lithography and molecular ruler technique. Though the symmetric junction showed a monotonic rise with a bias beyond the Coulomb gap voltage, the asymmetric junction showed Coulomb staircases. The capacitance of the junction estimated from the fitting curves using the Coulomb conventional theory was consistent with the capacitance calculated from the observed structure. The authors quantitatively found the correlation between the electrical and structural properties of DBTJ.

Method for Controlling Electrical Properties of Single-Layer Graphene Nanoribbons via Adsorbed Planar Molecular Nanoparticles

A simple method for fabricating single-layer graphene nanoribbons (sGNRs) from double-walled carbon nanotubes (DWNTs) was developed. A sonication treatment was employed to unzip the DWNTs by inducing defects in them through annealing at 500 °C. The unzipped DWNTs yielded double-layered GNRs (dGNRs). Further sonication allowed each dGNR to be unpeeled into two sGNRs. Purification performed using a high-speed centrifuge ensured that more than 99% of the formed GNRs were sGNRs. The changes induced in the electrical properties of the obtained sGNR by the absorption of nanoparticles of planar molecule, naphthalenediimide (NDI), were investigated. The shape of the I-V curve of the sGNRs varied with the number of NDI nanoparticles adsorbed. This was suggestive of the existence of a band gap at the narrow-necked part near the NDI-adsorbing area of the sGNRs.

Band-like transport in highly crystalline graphene films from defective graphene oxides

The electrical transport property of the reduced graphene oxide (rGO) thin-films synthesized from defective GO through thermal treatment in a reactive ethanol environment at high temperature above 1000 °C shows a band-like transport with small thermal activation energy (Ea~10 meV) that occurs during high carrier mobility (~210 cm2/Vs). Electrical and structural analysis using X-ray absorption fine structure, the valence band photo-electron, Raman spectra and transmission electron microscopy indicate that a high temperature process above 1000 °C in the ethanol environment leads to an extraordinary expansion of the conjugated π-electron system in rGO due to the efficient restoration of the graphitic structure. We reveal that Ea decreases with the increasing density of states near the Fermi level due to the expansion of the conjugated π-electron system in the rGO. This means that Ea corresponds to the energy gap between the top of the valence band and the bottom of the conduction band. The origin of the band-like transport can be explained by the carriers, which are more easily excited into the conduction band due to the decreasing energy gap with the expansion of the conjugated π-electron system in the rGO.

Extraordinary suppression of carrier scattering in large area graphene oxide films

In this study, we find that thermal treatment in ethanol vapor has a remarkable suppression effect of carrier scattering occurring between reduced graphene oxide (rGO) flakes in large area films. We observe excellent electrical properties such as high carrier mobility (∼5 cm2/Vs) and low sheet resistance (∼40 KΩ/□) for the rGO films. From the electrical conductivity analysis of large area rGO films using two-dimensional variable range hopping model and structural analysis using Raman spectra measured from the rGO films, we reveal that the significant effect is caused by the expansion of conjugated π-electron system in rGO flake due to the efficient restoration of graphitic structure.

Thickness Control of Graphene Overlayer via Layer-by-Layer Growth on Graphene Templates by Chemical Vapor Deposition

We examined graphene growth on graphene templates prepared by the mechanical exfoliation of graphite crystals using a sloped-temperature chemical vapor deposition (CVD) apparatus with ethanol. The structural characterization of the grown graphene layers in detail by atomic force microscopy and Raman spectroscopy reveals that the film thickening of graphene overlayers proceeds by the layer-by-layer growth mode, and that film thickness increases linearly as a function of growth time. This result indicates that a graphene growth technique using the CVD apparatus is a potential approach to precisely controlling the number of graphene layers, which is one of the important subjects for fabricating practical devices with uniform electrical properties using graphene as the channel material, such as field effect transistors.

Study of photoelectron spectroscopy from extremely uniform Si nanoislands on Si(111) 7×7 substrate

The electronic and structural properties of self-assembled Si nanoislands on a Si(111) 7×7 dimer-adatom-stacking fault substrate are investigated by photoelectron spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Uniform Si nanoislands are formed on the Si(111) 7×7 substrate by control of the growth conditions. For the nanoislands fabricated on the substrate, the photoelectron spectrum shows a significant peak shift of ≈0.1eV, which is caused by a surface state related to a dangling bond at the nanoisland.

Interrelations between the local electronic states and the atomic structures in the Si nanoscale island on Si(111)-(7×7) surface

We have investigated local electronic states and atomic structures of a self-assembled Si nano-island on Si(111)-(7×7) dimer-adatom-stacking fault (DAS) substrate by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. The normalized differential conductivity (dI/dV)/(I/V) spectra show several peaks, whose energies depend on each individual dangling bond site on the island, and these states are different from dangling bond states on the (7×7) DAS substrate. STM images at the edge of the island also show some interesting variations as a function of the sample bias voltage. The variations are explained by modifications of dangling bond states on T4 site atoms and on buckled dimer atoms in the vicinity of the edge. From these results, we find a detailed behavior of a redistribution of the electron charge to stabilize the atomic structure of the nano-island.

The fabrication and single electron transport of Au nano-particles placed between Nb nanogap electrodes.

We have fabricated Nb nanogap electrodes using a combination of molecular lithography and electron beam lithography. Au nano-particles with anchor molecules were placed in the gap, the width of which could be controlled on a molecular scale (approximately 2 nm). Three different anchor molecules which connect the Au nano-particles and the electrodes were tested to investigate their contact resistance, and a local gate was fabricated underneath the Au nano-particles. The electrical transport measurements at liquid helium temperatures indicated single electron transistor (SET) characteristics with a charging energy of about approximately 5 meV, and a clear indication of the effect of superconducting electrodes was not observed, possibly due to the large tunnel resistance.

Strain induced modification of quasi-two-dimensional electron gas state on √3×√3-Ag structure

To establish fundamental understanding of the influence of lattice strain to a quasi-two-dimensional electron gas state (2DEG), the both effects of compressive and tensile strains induced in the √3×√3-Ag structure formed on the Ge/Si(111) and the Si/Ge(111) surfaces were investigated with scanning tunneling microscopy and angle resolved ultraviolet photoelectron spectroscopy. The effective mass of the 2DEG is decreased by the compressive strain and that is increased by the tensile strain. The results indicate the dispersion of the electronic state will be modified by controlling the lattice strain.