Graphene-metal Hybrid Research Articles

Graphene–Gold–Au@Ag NPs-PDMS Films Coated Fiber Optic for Refractive Index and Temperature Sensing

A novel fiber-optic refractive index (RI) and temperature sensor based on a multimode–single-mode–multimode fiber (MMF-SMF-MMF) structure coated with graphene–metal hybrid films was proposed and investigated. The surface plasmon resonance (SPR)-based RI sensitivity is improved by the addition of the graphene layer above or below the gold coating of the fiber and then further enhanced with the addition of gold–silver core–shell nanoparticles (Au@Ag NPs). Furthermore, the surface of graphene–Au–Au@Ag NPs film was modified with polydimethylsiloxane (PDMS) for temperature sensing, achieving a high sensitivity of −1.02 nm/°C with good repeatability. This new structure with graphene–gold–Au@Ag NPs-PDMS hybrid films shows applications in many fields, including environmental protection, clean energy, and nano-medicine.

Epitaxial and Atomically Thin Graphene–Metal Hybrid Catalyst Films—the Dual Role of Graphene As the Support and the Chemically-Transparent Protective Cap

Commercialization of polymer electrolyte membrane (PEM) fuel cell materials has been of a great interest over the past decade. At the cathode side, the oxygen reduction reaction (ORR) is the key reaction where sluggish kinetics is present, for which Pt remains the catalyst of choice. In order to leapfrog the current limits to Pt use, three main objectives have to be achieved. First, is a reduction of Pt loading; second, is an enhancement of the mass activity per Pt atom; and third is an increase the catalyst durability. Herein, we demonstrate the dual role of single graphene layer, both as a growth template and as a protective cap for 2D Pt monolayers catalysts, where all three objectives can be achieved.Using iterative under potential deposition (UPD), atomic layers of Pt catalyst are grown on top of a single layer graphene. X-ray absorption spectroscopic (XAS) and scanning transmission electron microscopy (STEM) analyses show that Pt growth is dictated by the graphene-templated epitaxy. Pt/graphene intimacy induces a localized compressive strain on Pt monolayers ranges 3-10%, owing to structural defects, with an overall compressive strain of 3.5% according to extended x-ray absorption fine structure (EXAFS) analysis. In addition, cyclic voltammetry (CV) analysis shows fully-wetted Pt monolayers coverage of graphene under-layer with only a ~1 nm ultra-thin layer of Pt, while ripening was suppressed as shown through STEM images. Atomic Force Microscopy (AFM) analysis demonstrates that Pt monolayers prefer to follow Frank-van der Merwe growth (i.e. layer-by-layer growth mode) rather than Volmer-Weber growth (i.e. island growth mode), where root mean square (rms) of surface roughness remains quite similar while increasing Pt loading is increased.Pt/graphene hybrid catalysts show superior catalytic activity for ORR relative to the graphene-free counterparts or state of the art Pt commercial catalyst. A combination of the graphene-imposed compressive strain and electron transfer, push the Pt d-band center up, lowering the overpotential needed for ORR to occur. Furthermore, the graphene/Pt cap hybrid shows the graphene protecting Pt MLs from both dissolution and from ripening, with no Pt loss after 1000 fuel cell operating cycles. Our demonstration of a graphene-Pt hybrid opens the door for graphene/metal or metal/graphene architectures with potential applications in, and not limited to, energy, thermo-electric and electronics field.

Tailoring slow light with a metal–graphene hybrid metasurface in the terahertz regime

A metal–graphene hybrid metasurface is proposed and numerically investigated for active tailoring of slow light in the terahertz (THz) regime. In the unit cell of the metasurface, four mirror-symmetrically arranged aluminum split ring resonators (SRRs) form the bright-dark-dark-bright mode coupling and exhibit a strong electromagnetically induced transparency (EIT) resonance characteristic. With monolayer graphene placed under the dark SRRs, both the transmission amplitude of EIT resonance and the group delay within the transparency window can be continuously tuned via shifting the Fermi level of graphene. Further theoretical analyses reveal that the active modulation arises from the increasing damping rate of the dark SRRs due to the recombination effect of the conductive graphene. The proposed metasurface provides an effective approach to tailoring the slow light effect and will find important applications in active functional devices for future THz wireless communications.

Tunable unidirectional light transmission in a graphene–metal hybrid metamaterial

ABSTRACTGraphene’s unique properties, such as strong plasmonic response and electrostatic doping, enable control over the properties of light in an active way. In this paper, we propose a graphene–metal hybrid metamaterial, which exhibits tunable wideband unidirectional light transmission. The hybrid metamaterial consists of a complementary split-ring graphene and a metallic grating. Unidirectional optical transmission with a wide bandwidth of 14.8% of the central frequency at 29.3 THz and a large tuning range of 6.6 THz is found to be achievable in simulations. The light reflection and graphene absorption are shown to be the major factors limiting the efficiency of unidirectional transmission. A large electron scattering time of graphene is beneficial for improving the transmission efficiency. The graphene–metal hybrid metamaterial enables active control over the propagation of light, which could be of interest for infrared isolation, polarization transformation, etc.

Controllable linear asymmetric transmission and perfect polarization conversion in a terahertz hybrid metal-graphene metasurface.

We propose a three layered metal-graphene-metal metasurface to investigate the controllable linear asymmetric transmission and perfect polarization conversion in THz regime, by using the finite-difference time-domain (FDTD) method. An on-to-off control of asymmetric transmission and perfect polarization conversion is achieved by changing the Fermi energy of graphene from 0.8 eV to 0 eV. We present the electric field distribution and Fabry-Perot-like cavity model to analyze the working mechanisms. By gradually shifting the Fermi energy of graphene, two functions are realized, i.e., controllable linear asymmetric transmission and controllable total transmission with near perfect polarization conversion.

Dynamically Tunable Plasmon-Induced Transparency Based on Radiative–Radiative-Coupling in a Terahertz Metal–Graphene Metamaterial

New technologies and materials with superior characteristics impel great development of functional devices in the terahertz field. The dynamically tunable plasmon-induced transparency (PIT) based on radiative–radiative-coupling in terahertz hybrid metal–graphene metamaterial is numerically investigated in this paper. For the active manipulation of the PIT device, the single-layer graphene is integrated into the proposed structure consisting of the split-ring-resonator (SRR) and the closed-ring-resonator (CRR). Dynamically adjusting Fermi energy in graphene leads to modulation of the PIT window, allowing for the active control of the group delay. From the simulated electrical field distributions and effective circuit model to analyze, the transmission spectrum modulation can be attributed to the altering in the energy loss of the dark mode resonator through the conduction effect of the graphene layer. Our work offers theoretical references for the development of slow light terahertz devices in the future.

A Short Review on the Role of the Metal-Graphene Hybrid Nanostructure in Promoting the Localized Surface Plasmon Resonance Sensor Performance.

Localized Surface Plasmon Resonance (LSPR) sensors have potential applications in essential and important areas such as bio-sensor technology, especially in medical applications and gas sensors in environmental monitoring applications. Figure of Merit (FOM) and Sensitivity (S) measurements are two ways to assess the performance of an LSPR sensor. However, LSPR sensors suffer low FOM compared to the conventional Surface Plasmon Resonance (SPR) sensor due to high losses resulting from radiative damping of LSPs waves. Different methodologies have been utilized to enhance the performance of LSPR sensors, including various geometrical and material parameters, plasmonic wave coupling from different structures, and integration of noble metals with graphene, which is the focus of this report. Recent studies of metal-graphene hybrid plasmonic systems have shown its capability of promoting the performance of the LSPR sensor to a level that enhances its chance for commercialization. In this review, fundamental physics, the operation principle, and performance assessment of the LSPR sensor are presented followed by a discussion of plasmonic materials and a summary of methods used to optimize the sensor’s performance. A focused review on metal-graphene hybrid nanostructure and a discussion of its role in promoting the performance of the LSPR sensor follow.

Dynamically Tunable Resonant Strength in Electromagnetically Induced Transparency (EIT) Analogue by Hybrid Metal-Graphene Metamaterials

In this paper, a novel method to realize a dynamically tunable analogue of EIT for the resonance strength rather than the resonance frequency is proposed in the terahertz spectrum. The introduced method is composed of a metal EIT-like structure, in which a distinct EIT phenomenon resulting from the near field coupling between bright and dark mode resonators can be obtained, as well as an integrated monolayer graphene ribbon under the dark mode resonator that can continuously adjust the resonance strength of transparency peak by changing the Fermi level of the graphene. Comparing structures that need to be modulated individually for each unit cell of the metamaterials, the proposed modulation mechanism was convenient for achieving synchronous operations for all unit cells. This work demonstrates a new platform of modulating the EIT analogue and paves the way to design terahertz functional devices which meet the needs of optical networks and terahertz communications.

Terahertz electric field modulated mode coupling in graphene-metal hybrid metamaterials.

Taking advantage of the tunable conductivity of graphene under high terahertz (THz) electric field, a graphene-metal hybrid metamaterial consisting of an array of three adjoined orthogonally oriented split-ring resonators (SRRs) is proposed and experimentally demonstrated to show a maximum modulation depth of 23% in transmission when the THz peak field reaches 305 kV/cm. The transmission of the sample is dominated by the antisymmetric and symmetric resonant modes originating from the strong magneto-inductive and conductive coupling among the three SRRs, respectively. Numerical simulations and model calculations based on a coupled oscillator theory were performed to explain the modulation process. It is found that the graphene coating impairs the resonances by increasing the damping of the modes and decreasing the coupling between the SRRs whereas the strong THz field restores the resonances by decreasing the conductivity of graphene.

One-Step Photochemical Synthesis of Transition Metal–Graphene Hybrid for Electrocatalysis

For widespread use of renewable energy such as water splitting, the development of electrocatalysts on a large-scale at a low-cost that remains safe and environmentally friendly is still a great challenge. Here, we report the use of α-aminoalkyl radicals in a one-step procedure that synthesizes transition metal nanoparticle–graphene composites via photoreduction. The organic photocatalyst 2-methyl-1-[4-(methylthio)phenyl]-2-(morpholinyl)phenyl]-1-butanone (I-907) undergoes Norrish Type I photocleavage to generate strongly reducing α-aminoalkyl radicals, when exposed to UVA. For the first time, we demonstrate its ability to reduce graphene oxide (GO) and successfully synthesize Co3O4 nanoparticles decorated on graphene (Co3O4NP-rGO). The α-aminoalkyl radicals simultaneously reduce GO and Co2+ salts, which nucleates on the negatively charged GO sheets and grows to form nanoparticles. The resulting Co3O4NP-rGO showed decent catalytic activity and stability for the oxygen evolution reaction. Our work introduc...

Tunable filter based on hybrid metal-graphene structures over an ultrawide terahertz band using an improved Wave Concept Iterative Process method

An improved Wave Concept Iterative Process method is used to simulate a tunable filter based on hybrid metal-graphene over an ultrawide terahertz band. To accurately model the graphene, both the interband and intraband conductivities are incorporated into the WCIP method by using the wave concept and the boundary conditions at the graphene sheet. The WCIP method reduces the time consumption and memory size since it was applied to the surface conductivity graphene directly without involving volumetric discretization. Simulation results show that the transmission and reflection coefficients of the proposed filter can be adequately adjusted by changing the chemical potential of graphene, and operating temperature. It also shows that the interband conductivity has an important effect on the performance of the graphene device, especially at the frequency band larger than 8 THz.

Electrical Manipulation of Electromagnetically Induced Transparency for Slow Light Purpose Based on Metal-Graphene Hybrid Metamaterial

A terahertz metamaterial is presented and numerically investigated to achieve tunable electromagnetically induced transparency (EIT) for slow light. The unit cell consists of cut-wire pairs and U-shaped ring resonators with graphene strips placed between the metal film and the SiO2/Si substrate. Through bright-dark mode coupling, the radiative resonance induced by the U-shaped ring is suppressed, and then the typical EIT effect is realized. The transparency window and the accompanied group delay can be electrically manipulated with different Fermi energy of the graphene. By analyzing the surface distribution, the underlying tuning mechanism of this hybrid metamaterial is investigated in detail. Moreover, the transparency peak decreases slightly with the increasing strip width of the graphene layer but completely vanishes as the strip width exceeds the length of the covered U-shaped ring. The influence of the critical index of graphene quality, i.e., carrier mobility on the EIT effect, is considered. The results of this study may provide valuable guidance in designing and analyzing tunable EIT structures based on a metal-graphene hybrid structure for slow light purposes.

Graphene-Supported Thin Metal Films for Nanophotonics and Optoelectronics.

Graphene-metal hybrid nanostructures have attracted considerable attention due to their potential applications in nanophotonics and optoelectronics. The output characteristics of devices based on such nanostructures largely depend on the properties of the metals. Here, we study the optical, electrical and structural properties of continuous thin gold and copper films grown by electron beam evaporation on monolayer graphene transferred onto silicon dioxide substrates. We find that the presence of graphene has a significant effect on optical losses and electrical resistance, both for thin gold and copper films. Furthermore, the growth kinetics of gold and copper films vary greatly; in particular, we found here a significant dependence of the properties of thin copper films on the deposition rate, unlike gold films. Our work provides new data on the optical properties of gold and copper, which should be considered in modeling and designing devices with graphene-metal nanolayers.

Design of Tunable Multiband Hybrid Graphene Metal Antenna in Microwave Regime

<p>Graphene is an <a title="Allotrope" href="https://en.wikipedia.org/wiki/Allotrope">allotrope</a> (form) of carbon consisting of a single layer of carbon atoms arranged in an <a title="Hexagonal lattice" href="https://en.wikipedia.org/wiki/Hexagonal_lattice">hexagonal lattice</a>. It is the basic structural element of many other allotropes of carbon, such as <a title="Graphite" href="https://en.wikipedia.org/wiki/Graphite">graphite</a>, <a title="Charcoal" href="https://en.wikipedia.org/wiki/Charcoal">charcoal</a>, <a title="Carbon nanotube" href="https://en.wikipedia.org/wiki/Carbon_nanotube">carbon nanotubes</a> and <a title="Fullerene" href="https://en.wikipedia.org/wiki/Fullerene">fullerenes</a>. In this paper, a tunable hybrid metal-graphene antenna in the microwave regime is proposed. This antenna composed of the copper patch and four graphene strips. The antenna designs used for the cellular long-term evolution system and the operating frequency bands of 1.8, 2.5, 2.6, and 3.6 GHz, are evaluated to demonstrate the working principle and the performance tradeoffs. Furthermore, the proposed antenna can be tuned by varying applied DC voltage on the graphene which leads to change in the chemical potential of the graphene and hence the surface conductivity and electrical properties are changed. The simulation results reveal that the antenna operates in multi-band where scattering factor S<sub>11</sub>< -10 dB. In addition, the results show that hybrid metal-graphene frequency reconfigurable antennas can, at the same time, provide a tunable bandwidth and antenna matching.</p>

Free-standing Three Dimensional Graphene Incorporated with Gold Nanoparticles as Novel Binder-free Electrochemical Sensor for Enhanced Glucose Detection

The electrochemical sensing performance of metal-graphene hybrid based sensor may be significantly decreased due to the dissolution and aggregation of metal catalyst during operation. For the first time, we developed a novel large-area high quality three dimensional graphene foam-incorporated gold nanoparticles (3D-GF@Au) via chemical vapor deposition method and employed as free-standing electrocatalysis for non-enzymatic electrochemical glucose detection. 3D-GF@Au based sensor is capable to detect glucose with a wide linear detection range of 2.5 μM to 11.6 mM, remarkable low detection limit of 1 μM, high selectivity, and good stability. This was resulted from enhanced electrochemical active sites and charge transfer possibility due to the stable and uniform distribution of Au NPs along with the enhanced interactions between Au and GF. The obtained results indicated that 3D-GF@Au hybrid can be expected as a high quality candidate for non-enzymatic glucose sensor application. Keywords: Gold nanoparticles, Graphene foam, Nanohybrid, Catalyst, Glucose sensor

Tunable hybrid metal–graphene frequency selective surfaces based on split‐ring resonators by leapfrog ADI‐FDTD method

A tunable hybrid metal–graphene frequency selective surface (FSS) based on split-ring resonators at mid-infrared frequencies is proposed and numerically investigated by an improved leapfrog alternating direction implicit (ADI)-finite-difference time-domain (FDTD) method. The graphene conductivity described by a new closed-form approximate expression is incorporated into the leapfrog ADI-FDTD through the vector-fitting technique and auxiliary differential equation approach. Numerical simulations show that the proposed method has good computational accuracy and higher efficiency compared to conventional FDTD method and analytical solutions. Besides, the transmission and reflection coefficients of the FSS can be effectively adjusted by changing the chemical potential of graphene, the layer number of graphene patch and operating temperature. This work provides a novel method for designing tunable devices for optical plasmonic applications.

Graphene–metal hybrid metamaterials for strong and tunable circular dichroism generation

A strong and dynamically controlled circular dichroism (CD) effect has aroused great attention due to its desirable applications in modern chemistry and life sciences. In this Letter, we propose a graphene-metal hybrid chiral metamaterial to generate mid-infrared CD with an intensity of more than 10%, which can be actively controlled over a wide wavelength range. In addition to the strong tunability, the CD signal intensity of our nanostructure is drastically larger than that of the purely graphene-based chiroptical nanostructures. Our design offers a new strategy for developing tunable chiral metadevices, which could be used in various applications, such as biochemical detection and information processing.

Tunable resonators based on hybrid metal–graphene structures at mid-infrared frequencies using an improved Leapfrog WCS-FDTD method

In this paper, tunable resonators based on hybrid metal–graphene structures at mid-infrared frequencies are investigated using an improved Leapfrog weakly conditional stable finite-difference time-domain (WCS-FDTD) method. Simulation results reveal that the transmission responses of the proposed devices can be predicted efficiently and controlled conveniently by means of changing chemical potentials and operating temperatures. Moreover, the structures with symmetry and asymmetry metal ribbons can be used as high-sensitivity sensors and switches, respectively. This work may provide a novel method for designing ultrafast active optoelectronic devices in optical communication.

Dynamic electromagnetically induced transparency based on a metal-graphene hybrid metamaterial

A novel design of a dynamic electromagnetically induced transparency (EIT) metamaterial is proposed. The metamaterial consists of two kinds of cut-wire metal resonators with a graphene strip placed between them. The destructive interference between the two resonators gives rise to a transparency window. By varying the Fermi energy of the graphene through external gating, the EIT effect can be manipulated dynamically and the maximum modulation depth can reach up to 81%. The efficient modulation is controlled only in the EIT window with slight changes in transmission dips, which may avert the additional noises at adjacent frequencies in the modulation process. Moreover, the actively controlled slow-light effect and sensing performances can also be realized as the corresponding EIT window is modulated. This work provides a strategy to achieve a tunable EIT effect in a metal-graphene hybrid structure and exhibits potential applications in designing terahertz modulators, environmental sensors and slow-light devices.

Nanoparticle activated and directed assembly of graphene into a nanoscroll

Graphene nanoscrolls (GNS) have emerged as an important class of materials with applications in the field of lubrication, energy storage devices, and catalysis. However, the formation of GNS even with existing advanced experimental techniques could be very challenging. Here, we investigate the ability of diamond, nickel (Ni), platinum (Pt), and gold (Au) nanoparticles (NPs) in activating, guiding, and stabilizing the GNS by performing reactive molecular dynamics (RMD) simulations. NPs of certain diameters, when placed on a free-standing graphene sheet, can activate either wrapping or scrolling of the graphene. This activation is due to non-bonded interactions between graphene and NPs. However, for a complete wrap and scroll formation and its stabilization both graphene-graphene and NPNP interactions were also critical. Surface atoms of Ni and Pt NPs undergo reconstruction during the wrapping or scrolling of graphene, suggesting that the NP-graphene interactions are more favorable as compared to NPNP and graphene-graphene interactions. We have identified new energy criteria which must be satisfied to facilitate a complete GNS formation for the graphene-NP systems investigated in the present study. Overall, our study can provide a new method to design metal-graphene hybrid materials that can be used for a variety of applications.