No abstract

We demonstrate the control of resonance characteristics of a drum-type graphene mechanical resonator in a nonlinear oscillation regime using the photothermal effect, which is induced by a standing wave of light between graphene and a substrate. Unlike the resonance characteristics of a conventional Duffing-type nonlinearity, those of the nonlinear oscillation regime are modulated by the standing wave of light with a contribution of the scattered light of an actuation laser, despite a slight variation of amplitude. Numerical calculations conducted with a combination of equations of heat and motion with the Duffing-type nonlinearity explain this modulation: the photothermal effect delays the modulation of graphene stress or tension.

Because phonons are the main carriers for graphene heat transfer, modifying the dynamic properties of the crystal lattice by isotopes modulates the phonon behavior and alters the thermal properties. Here we demonstrate an artificially controlled texture synthesis of 12C‐graphene/13C‐graphene heterostructures via chemical vapor deposition and an O2 plasma etching. The electrical and thermal properties of the graphene across the heterojunction show that 12C‐graphene and 13C‐ graphene are electronically connected as resistors in series, while the thermal conductivity across the junction is dramatically reduced due to the suppressed phonon propagation, which causes the conductivity across the junction to be lower than that of graphene sheets with randomly mixed isotopes. These findings should help realize novel two‐dimensional graphene thermoelectric devices where phonon modulation controls the electrons and heat transport independently. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Graphene samples with or without isotope heterojunctions were synthesized by low-pressure thermal CVD using 12 CH 4 and 13 CH 4 as carbon sources. Details of the fabrication procedure for the heterojunctions are described elsewhere. [ 32 ] Before transferring graphene onto SiO 2 (500 nm)/Si substrates, Pt electrodes and a microfabricated heater were deposited on the substrate by electron beam evaporation and a liftoff technique. Graphene samples were then transferred onto the substrateThe thermoelectric properties of various graphene samples, including isotopically modifi ed heterostructures grown by chemical vapor deposition, are investigated from the viewpoint of thermoelectric device applications. The thermoelectric power of graphene varies from −80 to 90 µV K −1 , depending on the applied gate voltage. Similar to typical metals and semiconductors, the thermoelectric power of graphene decreases as the electrical conductivity increases, regardless if isotopes are present. The results follow a line with a slope of −198 µV K −1 in the Jonker plot, indicating that the maximum power factor is 5.29 × 10 −3 W m −1 K −2 irrespective of carbon isotope modulation in graphene. Because limiting phonon propagation independent of the electric properties can reduce the thermal conductivity of graphene containing carbon isotopes and isotopic heterojunctions, introducing carbon isotopes into the graphene structure improves the thermoelectric fi gure of merit without affecting the power factor.

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.