Nozomu Matsukawa

Electrostatic placement of single ferritin molecules

We electrostatically placed a single ferritin molecule on a nanometric 3-aminopropyltriethoxysilane (APTES) pattern that was on an oxidized Si substrate. The numerical analysis of the total interaction free energy for ferritin predicted that a quadrilateral array of 15nm diameter APTES nanodisks placed at intervals of 100nm would accommodate a single molecule of ferritin in each disk under a Debye length of 14nm. The experiments we conducted conformed to theoretical predictions and we successfully placed a single ferritin molecule on each ATPES disk without ferritin adsorbing on the SiO2 substrate surface.

High-Density and Highly Surface Selective Adsorption of Protein-Nanoparticle Complexes by Controlling Electrostatic Interaction

High-density cage-shaped proteins with inorganic cores were selectively adsorbed as a monolayer onto a 3-aminopropyl-triethoxysilane (APTES) layer on a Si substrate. The electrostatic interaction between the protein and substrate surface was studied and it was proven that protein adsorption density depends on the quantitative balance of surface charge on the substrate and protein. The combination of a highly positive APTES layer and moderately negative ferritin, Fer-4, achieved an adsorption density of 7.6×1011 cm-2 and the combination of the APTES layer and Listeria ferritin (Lis-fer) reached an adsorption density of 1.3×1012 cm-2. The adsorption process including the reduced charge of Lis-fer due to denaturation further enhanced the adsorption density up to 1.5×1012 cm-2, whereas no Lis-fer was adsorbed onto the SiO2 surface under the same conditions. This new technique makes it possible to produce a nanodot monolayer with a density higher than 1×1012 cm-2, which can be applied to floating nanodot gate memories.

Making Monolayer of Inorganic Nanoparticles on Silicon Substrate

A monolayer of inorganic nanoparticles (NPs) was fabricated on a silicon wafer using a cage-shaped protein, ferritin, which can sequester several kinds of inorganic NP in their cavities. Ferritins were bound electrostatically in aqueous condition to the silicon wafer which was modified with aminosilane molecules. The obtained sample was heat-treated at 500°C under oxygen gas, and the protein moiety and aminosilane were completely eliminated. The obtained NP monolayer showed no aggregation or sintering. This new method can be used to produce a dispersed inorganic NP monolayer on a silicon substrate as designed, which could be used as a nanodot array in floating nanodot gate memories.

Suppression of Inhomogeneous Segregation in Graphene Growth on Epitaxial Metal Films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.

Hexagonal Close-Packed Array Formed by Selective Adsorption onto Hexagonal Patterns

A patterned two-dimensional hexagonally ordered array of ferritin molecules, the outer surfaces of which had been genetically modified by titanium (Ti) specific binding peptides (minT1-LF), was realized in a self-assembling manner on a hexagonal Ti thin film island made on a silicon substrate. The optimum degree of order was realized at the pH with the maximum selectivity of minT1-LF adsorption on the Ti surface with respect to the silicon dioxide (SiO2) surface. Quartz crystal microbalance (QCM) measurement revealed that minT1-LF adsorbed onto the Ti surface strongly and irreversibly, but adsorbed onto the silicon dioxide surface weakly and reversibly. It was suggested that the concentration of minT1-LF on the Ti pattern promotes hexagonal close-packed ordering and axis aligning.

Self-aligned placement of biologically synthesized Coulomb islands within nanogap electrodes for single electron transistor

Biological synthesis and self-aligned placement of a Coulomb island was demonstrated for single electron transistor (SET) fabrication using a cage-shaped protein, apoferritin. Homogenous ϕ7 nm Co3O4 and In oxide nanoparticles (NPs) were synthesized utilizing the apoferritin cavity as a spatially restricted chemical reaction chamber. Apoferritin accommodating a NP (Co3O4, In oxide) showed specific affinity to a Ti surface and self-aligned itself between a pair of Au/Ti nanogap electrodes. After the protein cage was eliminated, two tunnel junctions between the NP and each electrode had the same gap, thereby forming an ideal SET structure. The produced SET exhibited a Coulomb-staircase/oscillation at 4.2 K.

Direct production of a two-dimensional ordered array of ferritin-nanoparticles on a silicon substrate

We have proposed a Bio Nano process that is a bottom-up technique using protein supramolecules for the fabrication of nanoelectronic devices that exceed the limits imposed by conventional lithography. One of its features is a nanostructure, for example, a two-dimensional (2D) ordered array using protein self-assembly. In this study, we show that a 2D ordered array with nanoparticles forms on a thermally-oxidized silicon substrate by using a solution without metals or sodium ions, which cause malfunctions of semiconductor devices. Furthermore, after protein elimination by UV-ozone treatment, a 2D ordered array was confirmed. The clarification of the principle of a 2D ordered array will be promoted in the future.

Fabrication and Magnetoresistance Properties of Spin-Dependent Tunnel Junctions Using an Epitaxial Fe3O4 Film

Magnetoresistance of spin-dependent tunnel junctions has been studied using a high-quality epitaxial Fe3O4 film. The bottom magnetic electrodes of epitaxial Fe3O4 were grown onto the TiN-buffered (110) surface of MgO single-crystal substrates, and trilayer junctions of Fe3O4/AlOx/CoFe mesa were fabricated by sequential sputtering and Ar ion etching. The junctions showed the magnetoresistance (MR) ratio of more than 10% at room temperature with butterfly-like hysteresis which arose from the different coercive fields between Fe3O4 and CoFe when the field was applied along the easy axis of the epitaxial Fe3O4 layer. The MR ratio remained almost constant against the temperature down to nearly 100 K. Below 100 K, the decrease of MR and the increase of junction resistance were observed, which may be related to the Verwey transition that inevitably occurs in the characteristic of high-quality Fe3O4 samples.

Electrostatic Placement of Nanodots onto Silicon Substrate Using Ferritin Protein Supramolecules with Control of Electrostatic Interaction in Solution

The behavior of the electrostatic adsorption of a single ferritin protein supramolecule, which formed a nanodot in its inner cavity, on a nanometric 3-aminopropyltriethoxysilane (APTES) pattern made on an oxidized Si substrate was studied using a numerical calculation. The total interaction free energy of the system, which included a ferrin, a substrate with an APTES nanopattern and a buffer solution, was calculated. The obtained distribution of the interaction potential that ferritin experiences can be used to explain theoretically the ferritin adsorption onto a quadrilateral array of 15-nm-diameter APTES nanodisks placed at intervals of 100 nm under a Debye length of 14 nm. This numerical calculation method described here can be applied to the estimation of the electrostatic adsorption behavior of nanometer-sized material as well as proteins.

The characterization of a single discrete bionanodot for memory device applications

We investigated electronic properties of a biochemically synthesized cobalt oxide bionanodot (Co-BND) by means of scanning tunneling microscopy/spectroscopy (STM/STS) and Kelvin-probe force microscopy (KFM). Experimentally obtained I-V characteristics and numerically obtained dI/dV and (dI/dV)/(I/V) from I-V revealed the band gap energy, band position of valence and conduction band of the Co-BND. KFM observation shows that bias polarity dependent surface potential change after charge injection. The observed surface potential change indicates that the Co-BND has a charge storage capability. We demonstrated the application of Co-BNDs for electronic devices by choosing flash memory as the example device. The fabricated Co-BND embedded MOS memory showed clear memory operation due to the charge confinement in the embedded Co-BNDs.