Ryousuke Ishikawa

Doping graphene films via chemically mediated charge transfer

Transparent conductive films (TCFs) are critical components of a myriad of technologies including flat panel displays, light-emitting diodes, and solar cells. Graphene-based TCFs have attracted a lot of attention because of their high electrical conductivity, transparency, and low cost. Carrier doping of graphene would potentially improve the properties of graphene-based TCFs for practical industrial applications. However, controlling the carrier type and concentration of dopants in graphene films is challenging, especially for the synthesis of p-type films. In this article, a new method for doping graphene using the conjugated organic molecule, tetracyanoquinodimethane (TCNQ), is described. Notably, TCNQ is well known as a powerful electron accepter and is expected to favor electron transfer from graphene into TCNQ molecules, thereby leading to p-type doping of graphene films. Small amounts of TCNQ drastically improved the resistivity without degradation of optical transparency. Our carrier doping method based on charge transfer has a huge potential for graphene-based TCFs.

Electrophoretic deposition of high quality transparent conductive graphene films on insulating glass substrates

Graphene is a promising material for transparent conductive films (TCFs) because of its high electrical conductivity and excellent optical transparency over a wide spectral range. We have previously reported on an inexpensive means of producing graphene-based TCFs using chemically derived graphene flakes. However, the deposition of chemically derived graphene can yield poor stacking of graphene flakes, which degrades the electrical conductivity of the resulting films. Here, we describe an alternative approach for producing large areas of TCFs based on electrophoretic deposition of graphene onto glass substrates using charged graphene oxide flakes. This method enabled the deposition of highly stacked graphene films onto insulating glass substrates with potential for TCFs.

Solid-phase crystallization of amorphous silicon nanowire array and optical properties

An amorphous silicon nanowire (a-SiNW) array perpendicular to a glass substrate can be successfully obtained through the metal-assisted chemical etching of amorphous silicon (a-Si) thin films. The solid-phase crystallization of a-SiNWs was carried out by thermal annealing in a forming gas in the temperature range from 600 to 900 °C. The effects of hydrogen in the film and the film morphology on the crystallization of a-SiNWs were investigated by Raman spectroscopy and transmission electron microscopy. A higher hydrogen concentration of a-SiNWs reduced the crystallization temperature, as in a-Si thin films. It was also revealed that the large surface area of the a-SiNW array affected the crystallization process. We also studied the optical property of the fabricated SiNW array and demonstrated its high potential as an active layer in solar cells.

Layer-by-Layer Assembled Transparent Conductive Graphene Films for Silicon Thin-Film Solar Cells

The potential of chemically derived graphene as a solution-processable transparent conductive film has been explored. Synthesis of amine-functionalized graphene oxide was intended for its utilization in layer-by-layer (LBL) assembly. LBL assembly of graphene oxide was utilized to fabricate graphene-based thin films in a scalable and highly reproducible way. It was found that the optical transmittance and sheet resistance of the film decrease with an increase in the number of LBL cycles in a reproducible way. The sheet resistance of the LBL-assembled GO film was improved by an order of magnitude at the same optical transparency due to the good uniformity and stacking of graphene flakes. Furthermore, we demonstrated the potential for the window electrodes of silicon thin-film solar cells.

Radical-assisted chemical doping for chemically derived graphene.

Carrier doping of graphene is one of the most challenging issues that needs to be solved to enable its use in various applications. We developed a carrier doping method using radical-assisted conjugated organic molecules in the liquid phase and demonstrated all-wet fabrication process of doped graphene films without any vacuum process. Charge transfer interaction between graphene and dopant molecules was directly investigated by spectroscopic studies. The resistivity of the doped graphene films was drastically decreased by two orders of magnitude. The resistivity was improved by not only carrier doping but the improvement in adhesion of doped graphene flakes. First-principles calculation supported the model of our doping mechanism.

Amplification of direct current magnetic responses of magnetic nanobeads due to induced self-assembly of magnetic microbeads

Detection of small concentrations of sub-200-nm-sized SPBs (superparamagnetic beads with sizes similar to target molecules) used as ‘magnetic labels’ is critical for the development of rapid, highly sensitive, and portable point of care treatment (POCT) systems. Currently, magnetoresistive (MR) biosensors are used for the detection of large concentrations of SPBs but such an approach is not suitable for monitoring small numbers of sub-200-nm SPBs due to the intrinsic noise of these electronic devices. In order to overcome this limitation of conventional MR sensors, we have developed a simple procedure for detecting small concentrations of sub-200-nm-diameter SPBs for biosensing by exploiting magnetically induced self-assembly of micrometer-sized SPBs onto nanometer targets. Here, our approach enables the physical amplification of the signal from otherwise undetectable nano-SPB targets using Hall biosensors without using the application of ac, magnetic fields or lock-in detection, thereby enabling the prod...

Preparation of p-type NiO films by reactive sputtering and their application to CdTe solar cells

Transparent p-type NiO films were prepared by reactive sputtering using the facing-target system under Ar-diluted O2 gas at Tsub of 30 and 200 °C. The increasing intensity of dominant X-ray diffraction (XRD) peaks indicates improvements in the crystallinity of NiO films upon Cu doping. In spite of the crystallographic and optical changes after Cu-doping, the electrical properties of Cu-doped NiO films were slightly improved. Upon Ag-doping at 30 °C under low O2 concentration, on the other hand, the intensity of the dominant (111) XRD peaks was suppressed and p-type conductivity increased from ~10−3 to ~10−1 S cm−1. Finally, our Ag-doped NiO films were applied as the back contact of CdTe solar cells. CdTe solar cells with a glass/ITO/CdS/CdTe/NiO structure exhibited an efficiency of 6.4%, suggesting the high potential of using p-type NiO for the back-contact film in thin-film solar cells.

Properties of Si/SiO2 superlattice nanodisc array prepared by nanosphere lithography

We fabricated nanostructured Si/SiO2 superlattice films for solar cell appliactions. The Si/SiO2 superlattice films were fabricated by thermal annealing of a-Si/SiO2 superlattice films. TEM observations revealed the existence of nanocrystalline Si in the Si layer. This sample showed photoluminescence spectrum with peak energy at around 1.5 eV. It was also found that the defect density in the superlattice was reduced by using forming gas annealing. Applying nanosphere lithography and reactive ion etching, we successfully prepared nanostructures on the surface of the superlattice. We also compared the optical properties with the simulation results using rigorous coupled wave analysis.

Porous Silicon Based Protocol for the Rapid and Real-Time Monitoring of Biorecognition Between Human IgG and Protein A Using Functionalized Superparamagnetic Beads

Optical interferometer biosensors based on porous silicon (PSi) are being studied for chemical and biological sensor applications. In particular, single PSi is a promising sensing platform for biomolecules and based on monitoring changes in the optical thickness (2nL) from Fabry-Pérot fringes due to magnetic particle-labels covering PSi surfaces. These methods offer a fast, and one-step method for immunoassaying by combining nano-sized superparamagnetic beads (SPBs) with interferometer PSi platforms. Furthermore, SPBs covered with biomolecules have a higher reflective index than the biomolecules alone, which results in larger shifts of the optical thickness (2nL) by the penetration of SPBs inside pore walls of PSi. In this work, we immobilized protein A onto macropore PSi and used optical reflection to detect human IgG immobilized onto nano-sized SPBs by measuring changes of optical thickness (2nL). Furthermore, the optical thickness (2nL) was proportional to mass of the biomolecules, thus the Δ2nL corresponded to the mass fraction of active IgG with SPBs inside PSi pores. Therefore, we quantified the changes of optical thickness (2nL) to enable the detection of SPBs functionalized human IgG based on protein-A modified macropore sized PSi platform.

Patterning of Two-Dimensional Graphene Oxide on Silicon Substrates

Chemically synthesized graphene is promising for device applications because the chemical approach enables ease of mass production and chemical modification of its properties. However, a major drawback of graphene based devices is that it is difficult to integrate the small flakes of graphene into device architectures. In order to overcome this limitation, we describe a simple procedure for patterning graphene oxide (GO) flakes onto predefined locations of silicon substrates. We exploited the negatively charged surface of GO flakes, and successfully patterned GO flakes onto photolithographically defined positively charged regions on silicon substrates. We demonstrate the simultaneous fabrication of multiple GO flakes device structures by controlling the surface chemistry of substrates. Our procedure for the precise positioning of GO flakes will be an important step in the fabrication of graphene devices.