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Employing the non-equilibrium Green's function with the third order tight binding Hamiltonian, we show that 12.7 nm long and 1.11 nm wide zigzag and armchair graphene nanoribbons (ZGNR and AGNR) in contact with asymmetric source and drain, consisting of graphene topped with Au and Ti, respectively, can operate as visible and solar blind photodetectors under zero externally applied biases. The carbon atoms at the channels' edges are considered to be hydrogen passivated. Numerical simulations show that the unbiased Au-ZGNR-Ti and Au-AGNR-Ti photodetectors can detect photons of energies 2.72 eV (456 nm, visible reign) and 5.02 eV (247 nm, solar blind reign), respectively, with the corresponding quantum efficiencies as high as 90% and 60% and responsivities of 93.8 and 13.4 A/mW. We also show that in Au-ZGNR-Ti the local photocurrents are distributed more around the ZGNR edges in regions near the source and drain sides of the channel, for the zero gate-source voltage (VGS=0), whereas in Au-AGNR-Ti for the same biasing condition, the local photocurrents are distributed far from the AGNR edges within regions near the source and drain sides of the channel. As the applied gate voltage moves away from the Dirac point, the peaks of the local photocurrents move away from the source and drain sides of the channel in opposite directions, in either asymmetric photodetector. Polarities of the local photocurrents are altered, as the gate-source voltage approaches the Dirac point, where the total photocurrent is maximized for both asymmetric photodetectors. [ABSTRACT FROM AUTHOR]

Characteristics of photodetectors with asymmetric source and drain contacts and armchair graphene nanoribbons (a-GNR) channel under monochromatic illuminations of various incident energies in the range of mid infrared (0.1 eV) to solar blind ultra violet (10 eV) are simulated. Simulations show the photocurrent spectrum for the device with an a-GNR of 30 unit cells long and 10 C-atoms wide connected between asymmetric leads made of Au-contacted and Ti-contacted graphenes, under monochromatic illumination of incident intensity 10 3 W/cm 2 , exhibits a peak current of 1.897 μA. This peak that is obtained in absence of any external biases, applied to the device terminals, occurs at the energy E = 5.02 eV and correspond to the quantum efficiency of 60%. The evaluated responsivity for this photodetector under zero applied biases equals 13.4 A/mW. The negative and positive local photocurrents are shown peak near the p-type source-channel and n-type drain-channel boundaries under negative gate to source voltages, respectively. Polarities of the local photocurrents alter when the gate-source voltage approaches to the Dirac point, for which the maximum total photocurrent is achieved. Finally, the total device current versus the drain-source voltages obtained under monochromatic illumination, for V GS = 0, is shown to shift toward negative values in comparison the dark current. This is because the dark current and photocurrents flow in opposite directions. Simulations are performed, using the non-equilibrium Green's function (NEGF) formalism coupled to Poisson solver. [ABSTRACT FROM AUTHOR]

We show an armchair graphene nanoribbon channel connected between asymmetric ferromagnetic source-drain structure--i.e., p-type Co/Au/graphene source and n-type Co/Cu/graphene drain--can operate as dual function spin-photodetector, under zero external biases at room temperature. It can function as an optical spin-valve with magnetoresistance of greater than 60% and responsivity as high as 25.12 A/mW, when irradiated by an un-polarized light of energy ~3.03 eV. Under a circularly polarized illumination, this optical spin-valve can also operate as a light helicity detector. The calculated magnetoresistances for right and left circularly polarized lights are both greater than 60%. [ABSTRACT FROM AUTHOR]