Masaaki Shimatani

Acoustic carrier transportation induced by surface acoustic waves in graphene in solution

The acoustic charge transportation induced by surface acoustic wave (SAW) propagation in graphene in solution was investigated. The sign of acoustic current (I A) was found to switch when crossing the Dirac point because the major carrier was transitioned from holes to electrons by the change in electrolyte-gate voltage. I A also exhibited a peak value under conditions of both hole and electron conduction. These results can be explained on the basis of a change in the type of major carrier in graphene, as well as a change in the carrier mobility of graphene.

Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

Graphene is a promising new material for photodetectors due to its excellent optical properties and high-speed response. However, graphene-based phototransistors have low responsivity due to the weak light absorption of graphene. We have observed a giant Dirac point shift upon white light illumination in graphene-based phototransistors with n-doped Si substrates, but not those with p-doped substrates. The source-drain current and substrate current were investigated with and without illumination for both p-type and n-type Si substrates. The decay time of the drain-source current indicates that the Si substrate, SiO2 layer, and metal electrode comprise a metal-oxide-semiconductor (MOS) capacitor due to the presence of defects at the interface between the Si substrate and SiO2 layer. The difference in the diffusion time of the intrinsic major carriers (electrons) and the photogenerated electron-hole pairs to the depletion layer delays the application of the gate voltage to the graphene channel. Therefore, th...

Graphene Surface Acoustic Wave Sensor for Simultaneous Detection of Charge and Mass

We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA-VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus, we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing.

Photocurrent enhancement of graphene phototransistors using p-n junction formed by conventional photolithography process

A p–n junction was developed in a graphene transistor by a simple photolithography process used in typical semiconductor processes. The p- and n-type regions were formed by coating photoresist on part of the graphene channel and immersion of the uncovered graphene region in alkali developer, respectively. A 3-fold enhancement of the photocurrent was observed at the maximum field effect mobility. It is therefore important to maximize the field effect mobility by doping to maximize the photocurrent. The results obtained here are an important step toward the production of high-sensitivity graphene-based phototransistors compatible with conventional industrial procedures.

Broadband photoresponse of graphene photodetector from visible to long-wavelength infrared wavelengths

Graphene, which is carbon arranged in atomically thin sheets, has drawn significant attention in many fields due to its unique electronic and optical properties. Photodetectors are particularly strong candidates for graphene applications due to the need for a broadband photoresponse from the ultraviolet to terahertz regions, high-speed operation, and low fabrication costs, which have not been achieved with the present technology. Here, graphene-based transistors were investigated as simple photodetectors for a broad range of wavelength. The photoresponse mechanism was determined to be dependent on factors such as the operation wavelength, the components near the graphene channel of the photodetector, and temperature. Here, we report the detailed mechanism that defines the photoresponse of graphene-based transistors. Graphene transistors were prepared using doped silicon (Si) substrates with a SiO2 layer, and source and drain electrodes. Single-layer graphene was fabricated by chemical vapor deposition, transferred onto the substrates, and the graphene channel region was then formed. The photoresponse was measured in the visible, near-infrared (NIR), and mid- and long-wavelength IR (MWIR and LWIR) regions. The results indicated that the photoresponse was enhanced by the Si substrate gating at visible wavelengths. Cooling was required at wavelengths longer than NIR due to thermal noise. Enhancement by the thermal effect of the insulator layer becomes dominant in the LWIR region, which indicates that the photoresponse of graphene-based transistors can be controlled by the surrounding materials, depending on the operation wavelength. These results are expected to contribute to the development of high-performance graphenebased photodetectors.

Effect of insulator layer in graphene plasmonic metamaterials for infrared detection

Graphene is an atomically thin carbon layer with a two-dimensional hexagonal lattice structure and has rich optoelectronic properties well suited to a wide range of applications. Graphene is considered to be a promising material for photodetectors because it exhibits excellent properties such as broadband absorption covering at least the ultraviolet to terahertz frequencies. However, the low optical absorption of graphene, at ca. 2.3%, still remains an important problem. Plasmonic metamaterial structures are good candidates to address this challenge. Metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) are highly suitable for the introduction and application of graphene. MIM-PMAs have a multilayer structure that includes plasmonic micropatches, an insulator, and a metal reflector layer. MIM-PMAs exhibit wavelength-selective absorption according to the micropatch size. Our previous research has demonstrated that the optical absorption of graphene is enhanced only by the main plasmonic resonance mode, and the plasmonic resonance modes in MIM-MPAs are strongly influenced by the insulator material. Therefore, the insulator layer plays an important role in graphene-coated MIM-PMAs. In this study, we have investigated the effect of the insulator layer in graphene-covered MIM-PMAs. The graphene was fabricated by chemical vapor deposition and transferred onto MIM-PMAs with different insulator thicknesses. Reflectance measurements demonstrated that varying the insulator thickness had a significant effect on the absorbance of graphene and resulted in modulation of the absorption wavelength. These results indicate that the plasmonic resonance localized at graphene near the plasmonic micropatches is modulated by the waveguide mode in the insulator layer. We believe that the present study will lead to significant improvements in graphene-based infrared detectors.

Graphene on plasmonic metamaterials for infrared detection

Graphene consists of a single layer of carbon atoms with a two-dimensional hexagonal lattice structure. Recently, it has been the subject of increasing interest due to its excellent optoelectronic properties and interesting physics. Graphene is considered to be a promising material for use in optoelectronic devices due to its fast response and broadband capabilities. However, graphene absorbs only 2.3% of incident white light, which limits the performance of photodetectors based on it. One promising approach to enhance the optical absorption of graphene is the use of plasmonic resonance. The field of plasmonics has been receiving considerable attention from the viewpoint of both fundamental physics and practical applications, and graphene plasmonics has become one of the most interesting topics in optoelectronics. In the present study, we investigated the optical properties of graphene on a plasmonic metamaterial absorber (PMA). The PMA was based on a metal-insulator-metal structure, in which surface plasmon resonance was induced. The graphene was synthesized by chemical vapor deposition and transferred onto the PMA, and the reflectance of the PMA in the infrared (IR) region, with and without graphene, was compared. The presence of the graphene layer was found to lead to significantly enhanced absorption only at the main plasmon resonance wavelength. The localized plasmonic resonance induced by the PMA enhanced the absorption of graphene, which was attributed to the enhancement of the total absorption of the PMA with graphene. The results obtained in the present study are expected to lead to improvements in the performance of graphene-based IR detectors.