Kazuma Tsuboi

Spontaneous buildup of giant surface potential by vacuum deposition of Alq3 and its removal by visible light irradiation

We observed high and persistent spontaneous buildup of the surface potential (SP) upon vacuum deposition of tris(8-hydroxyquinolinato) aluminum(III) (Alq3) on an Au substrate under dark conditions. SP determined by the Kelvin probe method reached 28 V at a thickness of 560 nm and the surface of the Alq3 film was positively charged. We propose a model in which preferential orientation of the dipole moments of Alq3 molecules is the origin of this buildup of the SP. The intensity of second-harmonic generation was also dramatically increased by the deposition of Alq3 under dark conditions, which supports the notion of a buildup of dipole layers. This giant surface potential was almost completely removed by irradiation of Alq3 molecules with visible light, and irradiation during deposition also prevented the buildup of SP.

Top-Down Process Based on Electrospinning, Twisting, and Heating for Producing One-Dimensional Carbon Nanotube Assembly

Multiwalled carbon nanotube (MWNT)/poly(vinyl butyral) (PVB) composite nanofibers were prepared by electrospinning, successive twisting and heat treatment. The MWNTs were highly oriented in an electrified thin jet during electrospinning. The heat treatment of the twisted electrospun nanofiber yarns produced the characteristics of the CNT in the composite nanofiber yarns and enhanced their electrical properties, mechanical properties, and thermal properties. The electrical conductivity of the heated yarn was significantly enhanced and showed the maximum value of 154 S cm(-1) for the yarn heated at 400 °C. It is an order of magnitude higher than other electrospun CNT composite materials. These results demonstrated that the novel top-down process based on electrospinning, twisting, and heat treatment provide a promising option for simple and large-scale manufacture of CNT assemblies.

Control over Internal Structure of Liquid Crystal Polymer Nanofibers by Electrospinning

Liquid crystal polymer nanofibers with a diameter ranging from 0.13 to 4.71 µm were prepared by electrospinning from a main-chain liquid crystalline polyester, BB-5(3-Me). WAXD measurements showed that the formation and orientation of the ordered structure in the electrospun fibers were controlled by the fiber diameter formed during electrospinning. For BB-5(3-Me), the SmA structure with two layer spacings was formed in the fiber during the electrospinning. Under optimal spinning conditions, the SmA structure is highly oriented in the fiber. In addition, annealing transformed the metastable SmA structure in the BB-5(3-Me) fiber into stable SmCA one.

Multichannel biosensing platform of surface-immobilized gold nanospheres for linear and nonlinear optical imaging

What we believe to be a new label-free multichannel biosensing platform is proposed. It is composed of surface-immobilized gold nanospheres (SIGNs) above a gold surface with a nanogap supported by a merocyanine self-assembled monolayer. The circular SIGN spots with a diameter of 120 microm were arrayed for multichannel biosensing on a glass slide. Two kinds of sensing methods were examined: One is a reflectivity measurement of a blue ray and the other is a second-harmonic generation measurement. It was found that the SIGN system can be used as a promising platform for multichannel biosensing in both sensing methods.

Inkjet Printing of Graphene Nanoribbons for Organic Field-Effect Transistors

An unzipped graphene oxide nanoribbon is patterned by surface selective deposition or inkjet printing, and its successive reduction to a graphene nanoribbon by vacuum annealing leads to highly conducting transparent graphene nanoribbon films. Such graphene nanoribbon films are used as source and drain electrodes in bottom-contact organic transistors based on pentacene and sexithiophene as well as C60.

Reflection spectroscopy of merocyanine self-assembled monolayer on a gold substrate

Abstract Isomerization of merocyanine chromophores in a self-assembled monolayer (SAM) on a gold substrate was examined by means of visible-UV reflection spectroscopy. Anomalous peaks and/or dips observed in the spectra were ascribed to the loss of the metallic character of gold at the wavelength region shorter than 500nm. Spectral simulations based on the Lorentz oscillator model revealed that the merocyanine SAM undergoes isomerization even in a densely packed monolayer.

Nonlinear optical detection of proteins based on localized surface plasmons in surface immobilized gold nanospheres.

A new nonlinear optical method is presented to detect proteins binding to a gold surface without using fluorescent-dye labeling. After exposure of the protein-binding surface to a gold nanosphere solution, the nanospheres are immobilized above a gold surface with a nanogap supported by the protein. The gold nanospheres immobilized on the gold surface show strong localized surface plasmon (LSP) resonance, and the formation of this structure results in a marked increase in the optical second harmonic (SH) activity of the gold surface arising from a large enhancement of the electric field localized adjacent to the nanospheres on the LSP resonance. The SH image, therefore, gives a high contrast ratio, 7.0:1, of protein-binding spots to control spots. The contrast ratio is much greater than those obtained by linear reflectivity imaging.

Light scattering assisted surface plasmon resonance at electrospun nanofiber-coated gold surfaces

This letter reports the polarized optical response of nanofiber-coated gold surfaces that originated from surface plasmon resonance (SPR). The reflection spectra of the poly(vinylpyrrolidone) nanofiber-coated gold surfaces prepared by electrospinning showed a clear polarization dependence. This indicates that the SPR at the air/gold interface was assisted by light scattering of the nanofibers. In addition, the optical response of the gold surface depends on the density of the coated nanofibers. This can be explained by the scattering and the interference of the surface plasmons in the pores between the nanofibers.

Arrays of microdots of gold nanoparticles immobilized above gold surface probed by optical second-harmonic microscopy

Gold nanoparticle arrays are fabricated on a planar gold surface obtained from UV-photopatterned self-assembled monolayer (SAM) films. The arrays are composed of microdots in which spherical gold nanoparticles are immobilized above a gold surface with a gap distance of a few nanometers supported by SAM films used as a spacer. Two kinds of preparation methods were examined: (1) to pattern a spacer SAM film that makes cross-linkage between the gold surface and the gold nanoparticles, and (2) to form ordered microholes in a SAM film, followed by deposition of the spacer SAM film that makes the cross-linkage in the microholes. Both methods provide us with the ordered microdots of gold nanoparticles on a gold surface. The formation of the microdot arrays were probed by optical second-harmonic microscopy.

Scanning electro-optic microscope based on surface plasmon resonance.

An optical scanning probe microscope, based on surface plasmon resonance (SPR) in the attenuated total reflection geometry, is shown to successfully image the polarization structures in self-assembled monolayers of hemicyanine adsorbed on a gold surface. Application of an ac field to the tip gives rise to the linear electro-optic effect in the monolayer just below the tip and the local change in the refractive index of the monolayer was detected via the ac component of the reflected light intensity, in which SPR-based detection intensifies the ac component. Polarization structures in a monolayer can be clearly imaged by this technique.