Bilayer Graphene Flakes Research Articles

Theoretical Prediction of Capacitance of Bilayer Graphene Flakes

AbstractElectric double layer capacitors (EDLCs) have been known as energy storage device and different materials have been used as EDLC electrode materials. Recently, graphene has been considered as one of the most promising materials for use in EDLC structure. In this research, a theoretical study on bilayer graphene (as an EDLC with vacuum dielectric) in two symmetries (D2h and D6h) with different distances and applied voltages is done, where the charge separation among a bilayer graphene structure is calculated by electron deformation orbital theory. Although it was found that capacitance is almost independent of applied voltage in constant distances, the relationship between capacitance and inverse of distance was nonlinear in the cases of constant voltages. The structure change from D2h to D6h did not affect the specific capacitances, significantly, although the areal capacitance of D2h structure was greater than that of D6h structure.

Rotational stability of twisted bilayer graphene

Moir\'e superlattices form in twisted graphene bilayers due to periodic regions of commensurability, but truncation of the moir\'e patterns affects the rotational stability of finite-sized sheets. Here, we report the stepwise untwisting of nanometer-sized bilayer graphene flakes at elevated temperatures, each step corresponding to a potential energy barrier formed by changes to the commensurability between the moir\'e superlattice and flake size with twist angle. The number of locally stable energy states and their barrier energies scale with the flake size, allowing twisted graphene flakes of several tens of nanometers to remain thermally stable even at chemical vapor deposition temperatures.

Direct growth of mm-size twisted bilayer graphene by plasma-enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD) techniques have been shown to be an efficient method to achieve single-step synthesis of high-quality monolayer graphene (MLG) without the need of active heating. Here we report PECVD-growth of single-crystalline hexagonal bilayer graphene (BLG) flakes and mm-size BLG films with the interlayer twist angle controlled by the growth parameters. The twist angle has been determined by three experimental approaches, including direct measurement of the relative orientation of crystalline edges between two stacked monolayers by scanning electron microscopy, analysis of the twist angle-dependent Raman spectral characteristics, and measurement of the Moiré period with scanning tunneling microscopy. In mm-sized twisted BLG (tBLG) films, the average twist angle can be controlled from 0° to approximately 20°, and the angular spread for a given growth condition can be limited to < 7°. Different work functions between MLG and BLG have been verified by the Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Electrical measurements of back-gated field-effect-transistor devices based on small-angle tBLG samples revealed high-quality electric characteristics at 300 K and insulating temperature dependence down to 100 K. This controlled PECVD-growth of tBLG thus provides an efficient approach to investigate the effect of varying Moiré potentials on tBLG.

Highlighting the Dynamics of Graphene Protection toward the Oxidation of Copper Under Operando Conditions.

We performed spatially resolved near-ambient-pressure photoemission spectromicroscopy on graphene-coated copper in operando under oxidation conditions in an oxygen atmosphere (0.1 mbar). We investigated regions with bare copper and areas covered with mono- and bi-layer graphene flakes, in isobaric and isothermal experiments. The key method in this work is the combination of spatial and chemical resolution of the scanning photoemission microscope operating in a near-ambient-pressure environment, thus allowing us to overcome both the material and pressure gap typical of standard ultrahigh-vacuum X-ray photoelectron spectroscopy (XPS) and to observe in operando the protection mechanism of graphene toward copper oxidation. The ability to perform spatially resolved XPS and imaging at high pressure allows for the first time a unique characterization of the oxidation phenomenon by means of photoelectron spectromicroscopy, pushing the limits of this technique from fundamental studies to real materials under working conditions. Although bare Cu oxidizes naturally at room temperature, our results demonstrate that such a graphene coating acts as an effective barrier to prevent copper oxidation at high temperatures (over 300 °C), until oxygen intercalation beneath graphene starts from boundaries and defects. We also show that bilayer flakes can protect at even higher temperatures. The protected metallic substrate, therefore, does not suffer corrosion, preserving its metallic characteristic, making this coating appealing for any application in an aggressive atmospheric environment at high temperatures.

Capacitance of graphene films: effect of the number of layers of the constituent graphene flakes

Assembly of graphene flakes into a film for an electric double-layer capacitor electrode causes restacking and aggregation of the constituent flakes that severely reduces the specific capacitance of the electrode. An understanding of different factors affecting aggregation will lead to strategies for improving the specific capacitance of graphene-based electrodes. In this work, we show that the number of layers of the constituent graphene flakes strongly affects the specific capacitance of the electrodes. For this, we prepared films from completely trilayer- and bilayer-rich graphene suspensions. The average thickness was considerably less, and the electrochemical surface area was considerably higher for the film of bilayer graphene flakes compared with the film of trilayer graphene flakes. The specific capacitance of the film of bilayer graphene (373 F/g) was at least 1.5 times higher than the value (238 F/g) for the film of trilayer graphene. An empirical approach using cyclic voltammetry showed that the amount of film surface easily accessible to the electrolyte was also higher for the film of bilayer flakes compared with the film of trilayer flakes.

Plasmon modes in double bilayer graphene heterostructures

We investigate the plasmon frequency and decay rate in a double bilayer graphene (DBLG) heterostructure made of two bilayer graphene (BLG) flakes separated by a spacer of thickness d. Calculations are carried out at zero temperature, within the random phase approximation, taking into account the inhomogeneity of the dielectric background of the system. Analytical expressions for plasmon frequencies are obtained by using the long wavelength expansions of the response and bare Coulomb interaction functions. We find that at long-wavelength limit, the acoustic (optical) plasmon mode shows the linear (square-root) energy dispersion and the inhomogeneity of dielectric background decreases the frequencies of both plasmon branches as in the case of DLG. The carrier density, spacer thickness and background dielectric constant affect remarkably on plasmon frequency and decay rate.

On the Development of a Simulation Strategy to Model the Behavior of Graphene-Based Devices in Electromagnetic Simulators

This paper presents a new simulation strategy to be implemented in electromagnetic simulators in order to calculate the level of the induced high-order harmonic components in mono- and bi-layer graphene flakes when they are driven by an input electromagnetic field and to evaluate the frequency response of structures, including graphene. The technique is applied to the design and analysis of a single-stage high-order frequency multiplier capable of generating an output signal in the 220–330-GHz range from an input signal in the 26–40-GHz bands, whose topology is based on a structured graphene sheet enclosed in a waveguide resonant cavity that maximizes the incident electromagnetic field. A prototype was implemented to validate the method, obtaining a good agreement with the simulation results. Furthermore, the prototype was also used to experimentally characterize the performance of the multi-layer graphene sheets. In this case, the developed model is used to calculate the frequency response of the structure, but it is not able to predict the output power since the mathematical model describing the frequency conversion phenomena cannot be extrapolated to the multi-layer graphene. Several configurations were tested in order to determine the influence of the graphene sheet’s thickness and shape on the output power. Finally, a −33-dBm level output signal at 280 GHz was generated as the seventh-harmonic component of an input signal with a frequency of 40 GHz, showing that the presented prototype can be used as a signal generator in practical submillimeter-wave applications.

Bilayer-rich graphene suspension from electrochemical exfoliation of graphite

The superior electronic properties of graphene change with number of layers, making different layered flakes suitable for different applications. Bilayer has a tunable band gap and is promising material for electronic applications. Liquid phase synthesis methods are scalable, can provide flakes with size suitable for microelectronic fabrication technologies and allow easy, defect-free transfer of flakes to the application substrate. However, these methods yield flakes with different number of layers. Here, we show a modified electrochemical exfoliation of graphite that gives a bilayer-rich graphene suspension. The method first obtains particles of stage-2 graphite intercalation compound. These particles have alternating weak and strong attraction between the two nearest neighboring layers which allowed the tuning of ultrasonic agitation intensity, such that cleavages occurred at weak attraction sites yielding primarily bilayer graphene flakes. Graphene flakes were produced only on ultrasonic cleaving and were done in DMF, a medium favorable for graphene dispersion thus avoiding aggregation of the freshly generated bilayer flakes. In principle, our method can be tuned for the liquid phase synthesis of layer-specific graphene flakes for any given number of layers. The method could therefore provide a general synthesis process for graphene required in different end-use.

Spin and Charge Caloritronics in Bilayer Graphene Flakes with Magnetic Contacts

We investigate the coupling of spin and thermal currents as a means to rise the thermoelectric efficiency of nanoscale graphene devices. We consider nanostructures composed of overlapping graphene nanoribbons with ferromagnetic contacts in different magnetic configurations. Our results show that the charge Seebeck effect is greatly enhanced when the magnetic leads are in an antiparallel configuration, due to the enlargement of the transport gap. However, for the optimization of the charge figure of merit ZT it is better to choose a parallel alignment of the magnetization in the leads, because the electron-hole symmetry is broken in this magnetic configuration. We also obtain the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the spin ZT can double its value with respect to the charge ZT for a wide temperature range, above 300 K. These findings suggest the potential value of graphene nanosystems as energy harvesting devices employing spin currents.

Anomalous Coulomb drag between bilayer graphene and a GaAs electron gas

We report on Coulomb drag experiments between a bilayer graphene flake and a GaAs two-dimensional electron gas, where the charge-carrier densities of both systems can be tuned independently. For both p- and n-type graphene charge carriers, we observe that the Coulomb drag unexpectedly changes direction when the temperature is lowered. We find this phenomenon to be dominant when the Fermi wave vector in graphene is larger than in GaAs. At temperatures above , the drag signal is consistent with momentum exchange. In all discussed regimes, the Onsager relation is respected.

Graphene-Embedded Hydrogel Nanofibers for Detection and Removal of Aqueous-Phase Dyes.

A facile route to graphene/polymer hydrogel nanofibers was developed. An aqueous dispersion of graphene (containing >40% bilayer graphene flakes) stabilized by a functionalized water-soluble polymer with phenyl side chains was successfully electrospun to yield nanofibers. Subsequent vapor-phase cross-linking of the nanofibers produced graphene-embedded hydrogel nanofibers (GHNFs). Interestingly, the GHNFs showed chemical sensitivity to the cationic dyes methylene blue (MB) and crystal violet (CV) in the aqueous phase. The adsorption capacities were as high as 0.43 and 0.33 mmol g-1 s-1 for MB and CV, respectively, even in a 1.5 mL s-1 flow system. A density functional theory calculation revealed that aqueous-phase MB and CV dyes were oriented parallel to the graphene surface and that the graphene/dye ensembles were stabilized by secondary physical bonding mechanisms such as the π-π stacking interaction in an aqueous medium. The GHNFs exhibited electrochemical properties arising mainly from the electric double-layer capacitance, which were applied in a demonstration of GHNF-based membrane electrodes (5 cm in diameter) for detecting the dyes in the flow system. It is believed that the GHNF membrane can be a successful model candidate for commercialization of graphene due to its easy-to-fabricate process and remarkable properties.

Electro-graphitization and exfoliation of graphene on carbon nanofibers

We present electrical current assisted graphitization as an alternative to conventional high temperature annealing of carbon nanofibers. In-situ experiments were performed on individual vapor grown carbon nanofibers inside a transmission electron microscope to measure the changes in resistance as a function of current density while observing the microstructural changes in real time. About 1000 times decrease in resistivity was measured at current density below 106 A/cm2. Further increase in current density leads to the uniform exfoliation of mostly bi-layer graphene flakes from the skin of the graphitic nanofibers, which leads to further reduction of the electrical resistance. The uniformity of the graphene flake growth over the nanofiber surface area achieved in this study is difficult to achieve with conventional approaches. Further experiments on networked nanofibers suggest that the graphene can remarkably reduce the electrical Kapitza resistance of the nanofiber junctions. The demonstrated processing of such hierarchical nanostructured fibers lead to high surface area and high conductivity carbon nanofibers that can impact flexible electrodes in electrochemical energy conversion or high specific strength composites applications.

Photon-assisted transport in bilayer graphene flakes

The electronic conductance of graphene-based bilayer flake systems reveal different quantum interference effects, such as Fabry-P\'erot resonances and sharp Fano antiresonances on account of competing electronic paths through the device. These properties may be exploited to obtain spin-polarized currents when the same nanostructure is deposited above a ferromagnetic insulator. Here we study how the spin-dependent conductance is affected when a time-dependent gate potential is applied to the bilayer flake. Following a Tien-Gordon formalism we explore how to modulate the transport properties of such systems via appropriate choices of the $ac$-field gate parameters. The presence of the oscillating field opens the possibility of tuning the original antiresonances for a large set of field parameters. We show that interference patterns can be partially or fully removed by the time-dependent gate voltage. The results are reflected in the corresponding weighted spin polarization which can reach maximum values for a given spin component. We found that differential conductance maps as functions of bias and gate potentials show interference patterns for different $ac$-field parameter configurations. The proposed bilayer graphene flake systems may be used as a frequency detector in the THz range.

Contact Resistance and Channel Conductance of Graphene Field-Effect Transistors under Low-Energy Electron Irradiation.

We studied the effects of low-energy electron beam irradiation up to 10 keV on graphene-based field effect transistors. We fabricated metallic bilayer electrodes to contact mono- and bi-layer graphene flakes on SiO2, obtaining specific contact resistivity and carrier mobility as high as 4000 cm2·V−1·s−1. By using a highly doped p-Si/SiO2 substrate as the back gate, we analyzed the transport properties of the device and the dependence on the pressure and on the electron bombardment. We demonstrate herein that low energy irradiation is detrimental to the transistor current capability, resulting in an increase in contact resistance and a reduction in carrier mobility, even at electron doses as low as 30 e−/nm2. We also show that irradiated devices recover their pristine state after few repeated electrical measurements.

Giant Frictional Drag in Double Bilayer Graphene Heterostructures.

We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10 K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T=1.5 K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

Supercurrent reversal in Josephson junctions based on bilayer graphene flakes

We investigate the Josephson effect in a bilayer graphene flake contacted by two monolayer sheet deposited by superconducting electrodes. It is found that when the electrodes are attached to the different layers of the bilayer, the Josephson current is in a $\pi$ state when the bilayer region is undoped and in the absence of vertical bias. Applying doping or bias to the junction reveals $\pi-0$ transitions which can be controlled by varying the temperature and the junction length. The supercurrent reversal here is very different from the ferromagnetic Josephson junctions where the spin degree of freedom plays the key role. We argue that the scattering processes accompanied by layer and sublattice index change give rise to the scattering phases which their effect varies with doping and the bias. Such scattering phases are responsible for the $\pi-0$ transitions. On the other hand if both of the electrodes are coupled to the same layer of the flake or the flake has AA stacking instead of common AB, the junction will be always in $0$ state since layer or sublattice index is not changed.

Electronic Conductance of Twisted Bilayer Nanoribbon Flakes

We study the transport properties of a twisted bilayer graphene flake contacted by two monolayer nanoribbons which act as leads. We analyze the conductance in terms of the spectra of the bilayer nanoribbon and the monolayer contacts. The low-energy transport properties are governed by the edge states with AB stacking. Remarkably, the electronic conductance in this energy region does not depend much on the relative position of the leads, in contrast with that of bilayer flakes with more symmetric stackings. We attribute this feature to the localization of these low-energy states in the AB edge regions of the flake, having a much smaller weight at the junctions between the flake and the nanoribbon leads.

Percolation transitions in bilayer graphene encapsulated by hexagonal boron nitride

We studied the plateau-plateau transitions that characterize the electrical transport in the quantum Hall regime in a high mobility bilayer graphene flake encapsulated by hexagonal boron nitride at magnetic fields up to 9 T and temperatures above 300 mK. We measured independently the exponent $\ensuremath{\kappa}$ of the temperature-induced transition broadening, the critical exponent $\ensuremath{\gamma}$ of the localization length, and the exponent $p$ ruling the temperature scaling of the coherence length, finding consistency with the relation $\ensuremath{\gamma}=p/2\ensuremath{\kappa}$. The observed value of $\ensuremath{\kappa}=0.30(0.28,0.32)$ deviates from that of the quantum Hall critical point. The obtained $\ensuremath{\gamma}=1.25(0.96,1.54)$ questions the validity of a pure Anderson transition, and reveals percolation as the underlying driving mechanism.

Fabry-Pérot interference in gapped bilayer graphene with broken anti-Klein tunneling.

We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device's electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

Characterizing Edge and Stacking Structures of Exfoliated Graphene by Photoelectron Diffraction

The two-dimensional C 1s photoelectron intensity angular distributions (PIADs) and spectra of exfoliated graphene flakes and crystalline graphite were measured using a focused soft X-ray beam. Suitable graphene samples were selected by thickness characterization using Raman spectromicroscopy after transferring mechanically exfoliated graphene flakes onto a 90-nm-thick SiO2 film. In every PIAD, a Kagomé interference pattern was observed, particularly clearly in the monolayer graphene PIAD. Its origin is the overlap of the diffraction rings formed by an in-plane C–C bond honeycomb lattice. Thus, the crystal orientation of each sample can be determined. In the case of bilayer graphene, PIAD was threefold-symmetric, while those of monolayer graphene and crystalline graphite were sixfold-symmetric. This is due to the stacking structure of bilayer graphene. From comparisons with the multiple scattering PIAD simulation results, the way of layer stacking as well as the termination types in the edge regions of bilayer graphene flakes were determined. Furthermore, two different C 1s core levels corresponding to the top and bottom layers of bilayer graphene were identified. A chemical shift to a higher binding energy by 0.25 eV for the bottom layer was attributed to interfacial interactions.