| Graphene is a single-atom-thick planar sheet of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice. Its unique electronic structure impacts itself many excellent properties, such as high carrier mobility (200000cm2(V-1s-1)), ultrahigh Fermi velocity (-c/300, c is the light velocity), large surface area, high transmittance, high Young’s modulus, high thermal conductivity and low resistivity, which has sparked tremendous research interests ranging from physics, chemistry and material to biology.In visible and near-infrared regions, the interband transition dominates, leading to an absorbance of～2.3%for monolayer graphene. In mid-infrared, far-infrared and terahertz (THz) regions, the intraband transition contributes, allowing for the existence of graphene plasmonic waves via electrostatic doping or gating graphene. Thus, the absorbance is much larger than2.3%, which broke the limitations and restrictions for many optical applications due to the inherently weak absorption of graphene. With much smaller plasmon wavelength, strong field confinement, low loss and good tunability, graphene can be a promising plasmonic material alternative to the conventional noble metals. Moreover, graphene has the excellent properties of atomic thickness (the thickness for monolayer graphene is0.34nm), atomic uniformity, delocalized%bond, good biological compatibility and chemical inertness, which make it a good candidate for sensing applications.This article aims to construct graphene-metal nanoparticles (NPs) hybrid systems, explore their plasmonic properties and the potential applications in surface-enhanced Raman scattering (SERS). By designing simple and novel structures, plasmons have been excited in mid-infrared and the sensing properties has been researched. The details are summarized as follow:1. Metal NPs were prepared by depositting and annealing metal films, graphene was grown on Cu foils by chemical vapor deposition (CVD) using methane as source, graphene-metal NPs hybrid systems were designed and prepared. In the work, Au NPs/graphene/Au NPs sandwiched hybrid films were prepared. The plasmonic properties (resonant wavelength and transmittance) of the hybrid film have been effectively tuned by the thickness of Au films deposited and the number of graphene layers sandwiched between Au NPs. Compared to the Au NPs/Au NPs direct stacking structure, a suppressed transmission of～16%accompanied with a14nm red-shift of the resonant wavelength were observed by embedding a monolayer graphene between two layers of vertically stacked Au NPs. Finite element simulations have demonstrated that the strong coupling between graphene and the two layers of Au NPs leads to an electric field enhancement of up to88times in graphene defined gaps. When the Au NPs/graphene/Au NPs sandwiched hybrid films were used as SERS substrates, the Raman enhancement factors of up to108for Rhodamine B (RhB) and Rhodamine6G (R6G), and a detection limit of0.1nM for methylene blue and Sudan Ⅲ molecules have been achieved. The plasmonic resonant wavelength of the hybrid films can be readily tuned by using different metals. In the work,8nm Au NPs/monolayer graphene/8nm Ag NPs has been prepared to match the wavelength of the excitation laser (532nm), and a SERS enhancement factor of over109for both RhB and R6G molecules has been demonstrated. Furthermore, when the8nm Au NPs/monolayer graphene/8nm Ag NPs hybrid film was fabricated on Ag substrate, an ultrasensitive SERS detection with a limit of down to10-13M was achieved due to the hybridization between the localized surface plasmon (LSP) of metallic NPs and the propagating surface plasmon polaritons (SPP) on the Ag film, which enlarge the application of nanosphere or similar to nanosphere metal NPs.2. Raman spectroscopy is a simple and efficient method to probe the structural and physical properties of graphene, such as the presence of defects, strain and charge. Unfortunately, the Raman signal of graphene grown on Cu foils is weak with a strong fluorescence background due to the strain in graphene on Cu foils and the plasmon emission of Cu. The plasmonic effect of the Au NPs self-assembeled in Au films leads to strong absorption at the resonant wavelength. The Raman signals of as-grown graphene on Cu foils was enhanced49folds by depositing a4nm-thick Au film on its surface. Finite element simulations demonstrated that the enhancement is related to the coupling between graphene and the plasmon modes of Au NPs. The hybrid systems are reliable SERS substrates with detection limit as low as0.1nM for dye molecules RhB, R6G, Sudan Ⅲ and Sudan Ⅳ, which open up opportunities in developing potential applications of as-grown graphene on Cu foils.3. Plasmonic waves in graphene can be excited by guided-mode resonances through setting graphene on a dielectric gratings. By numerical simulations we have demonstrated that, after adding a low-permittivity insulator between graphene and silicon gratings, the hybrid structure has a blue-shift in the resonant wavelength and a higher quality factor (Q-factor) and the sensitivity improves45.13%and figure of merit (FOM) increases～190%when used as plasmonic sensor for detecting a20nm-thick sensing medium on graphene. It was found that the confined graphene plasmons could also be excited in graphene embedded between silicon gratings and SiO2substrates. Plasmonic waves in graphene can also be excited by setting graphene nanoribbons on dielectric substrate. The plasmon properties can be tuned with Fermi energy and carrier mobility of graphene, structure of graphene nanoribbons and substrates. Finite element simulations demonstrated that the plasmonic wavelength in graphene is one or two orders of magnitude smaller than its vacuum wavelength, leading to extremely high field confinement. The sensitivity can be as high as1697nm/RIU when detecting the shift of the plasmonic resonant wavelength through adsorbing biomolecules on its surface. The excellent features of the structures may have implications on the development of active plasmonic sensing devices. |
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