Photodetection Based on Ionic Liquid Gated Plasmonic Ag Nanoparticle/Graphene Nanohybrid Field Effect Transistors

Abstract

729 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.denanostructures and the complicated ligand exchange process required present challenges for practical applications. [ 14,17 ] This motivates our work in exploring new mechanisms on graphene that do not deal with the above mentioned diffi culties for photodetection.The charge carrier density and hence electrical conductivity of graphene is highly susceptive to, and can be tuned by, gate electrostatic fi eld E G . The on/off ratio is over 5 in double-layer graphene fi eld-effect transistors (FETs) at E G ∼3 MV/cm. [ 18 ] Higher gating effi ciency, defi ned as the change of graphene channel conductivity per applied gate voltage V G , can be obtained at larger gate capacitance, which can be achieved by using thinner high-κ dielectrics. A promising alternative is to use ionic liquid gate, which is highly effi cient due to the formation of an electric double-layer with extremely small spacing on the order of 1 nm (the Debye length) near the graphene/ion liquid interface. [ 19 ] This renders a high gating efficiency on graphene. [ 20,21 ] A recent work on ionic liquid gated graphene FETs reveals the double-layer capacitance is as high as 9.2-21 µF/cm 2 , [ 22,23 ] approximately 2-3 orders of magnitude higher than the back-gate capacitance using a layer of 90 nm SiO 2 (∼38 nF/cm 2 ). In addition, the ionic liquid-gated graphene has shown to reach a high doping level of 5 × 10 13 /cm 2 , [ 24 ] mainly due to the ultrahigh breakdown fi eld on the order of 1 MV/cm. [ 25 ] The high-effi ciency electrical gating of the ionic liquid may be correlated to an incident light through incorporation of plasmonic nanostructures on graphene FETs. It is known that an evanescent fi eld can be generated near the nanostructure surface as a consequence of localized surface plasmonic resonance (LSPR) upon absorption of light with matching frequency to LSPR of the nanostructure. [26][27][28][29] Setting up a correlation between LSPR and the graphene FET gating may provide a highly effi cient way to generate large photoconductivity and hence a new scheme for photodetection with high sensitivity. [ 30 ] An additional advantage of this scheme is the wavelength tunability determined by the LSPR frequency of the plasmonic nanostructure. By selecting plasmonic nanostructures of desired LSPR frequencies, photodetection at the targeted frequency range may be realized in a spectrum determined by the materials and geometric parameters of the plasmonic nanostructures. [ 31 ] Herein, we report a novel graphene photodetector based on plasmonic Ag-NP/graphene nanohybrids FETs with an ionic liquid top gate. The fabrication process of the ionic liquid gated Ag-NP/graphene nanohybrid FETs is depicted schematically in Figure 1 , consisting of six steps: CVD graphene sheet transferred onto substrate (Figure 1 a), electrode deposition on graphene (Figure 1 b), graphene FET fabrication (Figure 1 c), Ag-NP selfassembly on graphene FET channel (Figure 1 d), ionic liquid gate Graphene...

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