Yuki Anno

Highly photosensitive graphene field-effect transistor with optical memory function.

Graphene is a promising material for use in photodetectors for the ultrawide wavelength region: from ultraviolet to terahertz. Nevertheless, only the 2.3% light absorption of monolayer graphene and fast recombination time of photo-excited charge restrict its sensitivity. To enhance the photosensitivity, hybridization of photosensitive material and graphene has been widely studied, where the accumulated photo-excited charge adjacent to the graphene channel modifies the Fermi level of graphene. However, the charge accumulation process slows the response to around a few tens of seconds to minutes. In contrast, a charge accumulation at the contact would induce the efficient light-induced modification of the contact resistance, which would enhance its photosensitivity. Herein, we demonstrate a highly photosensitive graphene field-effect transistor with noise-equivalent power of ~3 × 10−15 W/Hz1/2 and with response time within milliseconds at room temperature, where the Au oxide on Au electrodes modulates the contact resistance because of the light-assisted relaxation of the trapped charge at the contact. Additionally, this light-induced relaxation imparts an optical memory function with retention time of ~5 s. These findings are expected to open avenues to realization of graphene photodetectors with high sensitivity toward single photon detection with optical memory function.

Photoresponse of graphene field-effect-transistor with n-type Si depletion layer gate

Graphene/semiconductor Schottky junctions are an emerging field for high-performance optoelectronic devices. This study investigates not only the steady state but also the transient photoresponse of graphene field-effect transistor (G-FET) of which gate bias is applied through the Schottky barrier formed at an n-type Si/graphene interface with a thin oxide layer, where the oxide thickness is sufficiently thin for tunneling of the charge carrier. To analyze the photoresponse, we formulate the charge accumulation process at the n-Si/graphene interface, where the tunneling process through the SiOx layer to graphene occurs along with recombination of the accumulated holes and the electrons in the graphene at the surface states on the SiOx layer. Numerical calculations show good qualitative agreement with the experimentally obtained results for the photoresponse of G-FET.

Effect of defect-induced carrier scattering on the thermoelectric power of graphene

The thermoelectric properties of graphene are strongly related to the defect density, and as such, these can be used to investigate carrier scattering. In this study, the defect density was controlled by the use of oxygen plasma treatment. Oxygen plasma introduces structural defects into graphene, initially introducing sp3 defects that transform into vacancy-type defects with further exposure, as indicated by XPS analysis, and these transitions cause substantial changes in both the electrical and thermoelectric properties of graphene. In this work, we estimate the effects of both defect density and species, analyzed by Raman spectroscopy, on the thermoelectric power of graphene, and find that the maximum thermoelectric power decreases with increasing defect density. We also find, from Ioffes semiclassical approximation, that at the lower defect densities, phonons are the predominant source of carrier scattering, while at higher defect densities, the scattering is mainly caused by charged impurities, whic...

Optical manipulation of nonlinear vibration of graphene mechanical resonator

We demonstrate manipulation of nonlinear vibration of graphene mechanical resonator (G-MR) optically by photothermal effects of laser. Different photothermal effects are induced by combining of scattering light and different standing waves of light, which have different effects on nonlinear vibration. Experimental results indicate that nonlinearity is suppressed or promoted for each photothermal effects without almost changing its amplitude. These changes cannot be explained by conventional nonlinear vibration that the nonlinearity increases with increasing amplitude. To reveal the principle of the modulation, we proposed novel vibration model including photothermal effects in nonlinear vibration. Numerical calculation from the model well fits experimental results and revealed the principle. We believe that these technics of controlling nonlinear vibration open the further applications of G-MR.

Effect of defects on graphene thermoelectric properties

The thermal conductivity must be reduced to apply graphene as thermoelectric devices. Because the main heat carrier in graphene is phonon, introducing defects into graphene affects the phonon propagation in graphene, resulting in the change in the thermal conductivity. Here we investigate the effect of defects induced by oxygen plasma treatment on the thermoelectric property of graphene. The thermoelectric power of graphene with defects gradually decreases with increasing the density of defects compared to that of pristine graphene. Although defects degrade the electronic transport, leading to the reduction in both the thermoelectric power and electrical conductivity, the thermal conductivity is more reduced. As a consequence, graphene with defects shows higher thermoelectric figure of merit. This suggests that the performance of graphene-based thermoelectric devices can be enhanced more effectively by introducing defects in graphene.

Phonon engineering of graphene by local strain

Due to its extremely high thermal conductivity, graphene is one of the potential candidates for thermal management materials. Because the main heat carrier is phonon, thermal conductivity of graphene can be modulated by inducing the local strain in graphene networks. Here we propose a device structure with which the in-plane thermal conductivity can be measured while inducing the strain into graphene. From atomic force microscopy under 7.3×103 Pa, the deflection at the center of the graphene membrane was 35 nm, indicating that the surface area increases by 0.14% from the original surface area under atmospheric pressure. Nanoindentation measurement shows 56% increase in the pretention of the graphene membrane under 7.3×103 Pa. Thus, the device structure can be used to measure the thermal conductivity while inducing strain by the pressure difference across the membrane.

Evaluation of Graphene Thin Films by Surface Plasmon Resonance

We have proposed the application of the surface plasmon resonance method to the measurement of the optical response of graphene thin films. A surface plasmon is a coherent electron oscillation that is excited at the interface between two materials when p-polarized light is incident. We revealed that the excitation angle shifts by about 0.1° in the case of monolayer graphene compared with the bare Au surface as determined by calculation. We also identified angle shifts of the excitation conditions for surface plasmon experimentally when graphene thin films fabricated by filtration exist on the Au surface, corresponding to 5–8 layers of graphene, as determined by the calculation.