Abdeladim Guermoune

110 GHz measurement of large-area graphene integrated in low-loss microwave structures

We report high-frequency scattering parameter measurement of large-area monolayer graphene integrated on low-loss quartz substrates. High-quality graphene was grown by chemical vapour deposition on copper, chemically hole doped, and transferred to quartz. Microwave measurements were performed from 0.01 to 110 GHz. Simple microwave models were used to extract graphene impedance parameters. We find that contact resistance is effectively shunted above 3 GHz. Atomically thin large area graphene behaves as a wideband resistor with negligible kinetic inductance and negligible skin effect.

Faraday rotation in magnetically biased graphene at microwave frequencies

Faraday rotation is experimentally observed at microwave frequencies in a large-area graphene sheet biased with a static magnetic field, and interrogated by polarized fields in a hollow circular waveguide. A Faraday rotation of up to 1.5° and an isolation of more than 30 dB is observed, suggesting possible applications to graphene based isolators, circulators, and other non-reciprocal devices. An analytic model is developed for the scattering parameters of the measured structure. The model shows excellent agreement with the measurements and is used to extract the graphene conductivity, carrier density, and mobility.

Charge transfer hysteresis in graphene dual-dielectric memory cell structures

We report controlled charge transfer between large-area graphene and a dual-dielectric, silicon nitride/silicon oxide substrate. Graphene was grown on copper by chemical vapour deposition, transferred to the nitride substrates, and patterned into test structures. Hysteresis in conductance with varying gate voltage is easily understood in terms of electron transfer between graphene and nitride traps. Increased hysteresis with temperature suggests thermally activated charge transfer of a Poole-Frenkel or Schottky nature. A 7.3× change in graphene sheet resistance is observed at room temperature with the nitride in a charged and discharged state.

Functionalized CVD monolayer graphene for label-free impedimetric biosensing

Recent advances in large area graphene growth have led to many applications in different areas. In the present study, chemical vapor deposited (CVD) monolayer graphene supported on glass substrate was examined as electrode material for electrochemical biosensing applications. We report a facile strategy for covalent functionalization of CVD monolayer graphene by electrochemical reduction of carboxyphenyl diazonium salt prepared in situ in acidic aqueous solution. The carboxyphenyl-modified graphene is characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), as well as electrochemical impedance spectroscopy (EIS). We also show that the number of grafted carboxyphenyl groups on the graphene surface can be controlled by the number of cyclic voltammetry (CV) scans used for electrografting. We further present the fabrication and characterization of an immunosensor based on immobilization of ovalbumin antibody on the graphene surface after the activation of the grafted carboxylic groups via EDC/NHS chemistry. The binding between the surface-immobilized antibodies and ovalbumin was then monitored using Faradaic EIS in [Fe(CN)6]3−/4− solution. The percentage change of charge transfer resistance (Rct) after binding exhibited a linear dependence for ovalbumin concentrations ranging from 1.0 pg·mL−1 to 100 ng·mL−1, with a detection limit of 0.9 pg·mL−1. Our results indicate good sensitivity of the developed functionalized CVD graphene platform, paving the way for using CVD monolayer graphene in a variety of electrochemical biosensing devices.

Organic-free suspension of large-area graphene

We report an entirely organic-free method to suspend monolayer graphene grown by chemical vapour deposition over 10–20 μm apertures in a Cu substrate. Auger electron spectroscopy, Raman spectroscopy, scanning electron microscope, and transmission electron microscope measurements confirm high quality graphene with no measurable contamination beyond that resulting from air exposure. This method can be used to prepare graphene for fundamental studies and applications where the utmost cleanliness and structural integrity are required.

Contactless impedance measurement of large-area high-quality graphene

We present experimental work on the contactless measurement of graphene sheet impedance at frequencies up to 110 GHz in different waveguide geometries. Low-loss coplanar waveguides in series and shunt configuration have been demonstrated. A new coaxial waveguide coupled Corbino disk geometry with facile fabrication is introduced. Critical to the success of these measurements is a low contact impedance at high-frequencies, wherein the dc contact resistance is shunted by a contact capacitance that ultimately enables contactless measurement. The quasi-optic nature of waveguide measurements minimizes the effect of inevitable cracks in the graphene sheet, in contrast with typical transport measurements. We have applied our technique to the characterization of the sheet impedance and contact impedance of large-area, high-quality graphene grown by chemical vapour deposition.

Fabrication and characterization of suspended graphene membranes for miniature Golay cells

The development of miniaturized Golay cell arrays would enable the combination of the high sensitivity of a Golay cell with the imaging capability of focal plane arrays. The critical component of a miniaturized Golay cell is the deflecting membrane, which must simultaneously have a high breaking strength and a low flexural rigidity. Graphene suits this purpose ideally on account of its high strength and atomic thickness, in contrast with thicker polymeric membranes. Low flexural rigidity is critical to deflection sensitivity in response to temperature changes of the gas enclosed within a Golay cell scaled to the 10 μm to 100 μm scale. We report here a simple method for fabrication of suspended graphene membranes suitable for Golay cells. The technique is based on chemical vapour deposition of graphene on copper, followed by a sacrificial etch of the copper substrate. By this organic-free technique, graphene can be suspended over 10 - 20 μm apertures in copper thin films free of surface contamination and with high structural integrity. The cavities are sealed on the back-side with an indium film to produce proof-of-principle miniature cells with a flexible, suspended graphene membrane. Atomic force microscopy enables the force versus deflection curve of a graphene-enclosed cell to be characterized. We further report the temperature dependent equilibrium deflection (up to 60°C) of a graphene-enclosed cell by atomic force microscopy measurements taken with heat directly applied to the cell substrate.

Application of graphene membrane in micro-Golay cell array

We report the design, simulation, and fabrication of a miniaturized Golay cell array, implemented with monolayer graphene suspended over a TEM grid as the deflecting membrane. Currently, ultra-thin membranes for Golay cell applications suffer diminishing responsivity as the lateral dimensions are reduced to the microscopic scale. We propose graphene as the ideal membrane material for micro-Golay cell arrays, whereby the minimal elastic stiffness of atomically thin graphene allows membranes to be scaled to microscopic dimensions. We examine how graphenes unique material parameters, such as high mobility, negligible gas permeability, and supreme strength, offer ease of fabrication and improved performance over existing technology. Simulations of graphene membrane deflection versus temperature are presented, with an analysis of the optimal geometry for maximum sensitivity. Cavities with all spatial dimensions under 100 μm are predicted to provide sensitivities of hundreds of nanometres per Kelvin, in good competition with existing research on devices many times larger. Up to a four-fold increase in responsivity of 400 nm/K is predicted for a graphene cell of the same dimensions as current technology, and a three-fold increase for a cell one quarter the diameter. These predictions permit an increased detector density in a focal plane array application while still providing improved responsivity. Furthermore, our fabrication method permits the construction of arrays consisting of thousands of devices, avoiding individual cell assembly and including built-in electrical contacts due to the conductive nature of graphene. We also present a theoretical analysis of interferometric optical read-out of membrane deflection.