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We report an ellipsometric study of the monolayer graphene prepared by the chemical vapor deposition (CVD). The investigation was carried over a broad spectral range spanning the ultraviolet to the near-infrared at variable incidence angles. Comparison with graphite and previously published graphene data shows a high degree of similarity in the optical properties of these two materials.

During recent years, graphene has attracted growing interest for application in biology, particularly in the biosensing field for sensitivity enhancement. For the biosensing purpose, the graphene layers have to be uniform and have an accurately controlled thickness. Graphene quality may vary depending on the method of its production. Using Raman spectroscopy, an assessment was made of the quality and suitability of the use of CVD and exfoliated graphene for biosensors. The Raman spectra of graphene obtained by the chemical vapor deposition method were measured before and after transferring. We found that CVD graphene with single and bi-layer thickness has a uniform structure with lager area and a small number of defects. But the number of defects increases with an increase of the number of layers. Also, we found that for some samples with CVD graphene, sometimes it is difficult to determine the number of layers since the specific shape of the 2D peak is not always explicitly expressed. Furthermore, it was shown that the exfoliated graphene, although having fewer defects, have non-uniform structure, with smaller areas of monolayer graphene.

The relative orientation between atomic lattices in twisted bilayer graphene opens up a whole new field of rich physics. So, the study of self-assembled twisted bilayer graphene gives deep insight into its underlying growth process. Cuts in monolayer graphene via the atomic force microscopy technique are used to start self-assembly and to generate a folding process. The final configurations for this self-assembly process are investigated. Here, the focus is on structures that arise from one cut. During the self-assembly, these structures not only move forward but also rotate. As it turns out, the final positions for all studied structures can be assigned to commensurate interlayer configurations.

Twisted van der Waals systems have been receiving recent attention due to their potential for moire-induced band modulation and corresponding exotic correlated phases. Here, we present a Raman spectroscopic study of artificial trilayer graphene (3LG), represented by monolayer graphene (1LG) on top of Bernal-stacked bilayer graphene (2LG), as a function of the twist angle (θt) with respect to each other. The artificially twisted 3LG with θt >5° shows a distinctive 2D peak, which is literally composed of the typical 2D peak of 1LG and that of 2LG, without signatures of strong coupling between the 1LG and the 2LG. The overall trends of the relative Raman shift and the full width at half maxima of the 2D peak are also provided as a function of θt ranging from 0° to 30°. In particular, non-twisted 3LG shows 2D peak characteristics very similar to those of natural Bernal-stacked 3LG, revealing that the top 1LG and the bottom 2LG are translationally rearranged to be the most thermodynamically stable state. We also realized slightly twisted 3LG with a finite θt

The electron inelastic mean free path (IMFP) is an important quantity for electron spectroscopy and microscopy techniques. At present, there are very few data of IMFPs for two-dimensional (2D) materials. Here, we determine the IMFP at energies below 100 eV for monolayer graphene both experimentally and theoretically. The experimental determination is based on a data-driven spectral analysis technique, including the virtual substrate method and the reverse Monte Carlo method. The theoretical determination is performed within the dielectric formalism, using the energy-loss function calculated in the time-dependent density functional theory. The experimental and theoretical results show that the IMFP for monolayer graphene is almost constant (about 1 nm) in the energy range of 6–100 eV. This study suggests a general and reliable approach to determine low-energy IMFPs for 2D materials.

We report a proof-of-concept study of extraordinary magnetoresistance (EMR) in devices of monolayer graphene encapsulated in hexagonal boron nitride, having metallic edge contacts and a central metal shunt. Extremely large EMR values, $MR=(R(B) - R_0) / R_0\sim 10^5$, are achieved in part because $R_0$ approaches or crosses zero as a function of the gate voltage, exceeding that achieved in high mobility bulk semiconductor devices. We highlight the sensitivity, $dR/dB$, which in two-terminal measurements is the highest yet reported for EMR devices, and in particular exceeds prior results in graphene-based devices by a factor of 20. An asymmetry in the zero-field transport is traced to the presence of $pn$-junctions at the graphene-metal shunt interface.

A model of carrier density and drain current for monolayer graphene field-effect transistors (GFET) is proposed in this paper. In general, the carrier density is the numerical integration of the density of states (DOS) and Fermi-Dirac distribution. To avoid numerical solution, a physical-based and analytical calculation for carrier density and quantum capacitance is presented. Due to the intrinsic physical mechanism, the interface trap density is taken into account in the drain current model of GFET. Through the comparisons between model results and numerical iterations or experimental data, the validity of the proposed models is supported. The clear physical conception and simplicity of algorithm make our scheme suitable for compact modelling.

We fabricated an optically transparent dipole antenna based on chemical vapor deposition (CVD)-grown monolayer graphene on an optically transparent quartz substrate and characterized its properties in microwave bands. The measurements of the reflection coefficients for the dipole antenna revealed that ∼90% of the microwave power transmitted to the CVD monolayer graphene of the antenna element. By measuring transmission coefficients, we demonstrated that the graphene dipole antenna radiated microwave power around the operational frequency (∼20.7 GHz). The operational frequency of the graphene dipole antenna (∼20.7 GHz) shifted to a higher frequency than that of the Au dipole antenna with the same structure (∼9.2 GHz), which suggests that monolayer graphene behaves not as a metal but as a dielectric material.

A formalism describing the thermionic emission from a single layer graphene sheet operating at a finite temperature and the consequent formation of the thermionic sheath in its proximity has been established. The formulation takes account of two dimensional densities of state configuration, Fermi-Dirac (f-d) statistics of the electron energy distribution, Fowler's treatment of electron emission, and Poisson's equation. The thermionic current estimates based on the present analysis is found to be in reasonably good agreement with experimental observations (Zhu et al., Nano Res. 07, 1 (2014)). The analysis has further been simplified for the case where f-d statistics of an electron energy distribution converges to Maxwellian distribution. By using this formulation, the steady state sheath features, viz., spatial dependence of the surface potential and electron density structure in the thermionic sheath are derived and illustrated graphically for graphene parameters; the electron density in the sheath is see...

The graphene coplanar waveguide (CPW) has recently been found to have large insertion loss (typically larger than 50 dB/100 μm), which mainly results from the large resistance of graphene. The poor radio-frequency transmission property of graphene hampers its application in interconnect, a low loss material is thus required. In this paper, low-resistance graphite CPWs with effective graphite length up to 200 μm were fabricated. A record low insertion loss of graphite CPW (2.76 dB/100 μm) is demonstrated, and the average insertion loss of our graphite CPWs is only ∼1/5 of that of our monolayer graphene CPWs. Moreover, we find the insertion loss of graphite CPW may be even smaller at higher frequencies. Our investigation shows that graphite is a possible candidate for interconnect and may even be more applicable at ultra-high frequencies.

The performances of graphene sheet in micro- and nano-electronics and devices are significantly affected by its morphology, which depends on the surface features of the supporting substrate. The substrates with non-developable concave surface are widely used with graphene sheet in applications but rarely studied. Therefore, a theoretical model is established based on the energy analysis to explain the adhesion mechanisms and predict the morphology of the graphene sheet on a non-developable concave surface. Four different morphologies of the graphene sheet are revealed, and the critical conditions are established to predict which morphology the graphene/substrate system belongs to. For the monolayer graphene sheets much larger than the concave of substrate, the final equilibrium morphology is dominated by the half cone angle of the concave. The graphene sheet conforms completely to the SiO2 substrate if the half cone angle is less than 27.5° and spans over the concave if the angel is larger than 27.5°. For...

Graphene interfaced with a semiconductor forms a Schottky junction with rectifying properties. The inherent spatial inhomogeneity due to the formation of ripples and ridges in graphene can lead to fluctuations in the Schottky barrier height (SBH). The non-ideal behavior of the temperature dependent barrier height and ideality factor greater than 4 can be attributed to these spatial inhomogeneities. Assuming a Gaussian distribution of the barrier, mean SBHs of 1.30 ± 0.18 eV and 1.16 ± 0.16 eV are found for graphene/SiC junctions on the C- and Si-face, respectively. These findings reveal intrinsic spatial inhomogeneities in the SBHs in graphene based Schottky junctions.

We describe a process for the growth of a single, electronically decoupled graphene layer on SiC(0001). The method involves annealing in disilane to (1) prepare flat, clean substrates, (2) grow a single graphene layer, and (3) electronically decouple the graphene from the substrate. This approach uses a single process gas, at μTorr pressures, with modest substrate temperatures, thus affecting a drastic simplification over other processes described in the literature.

We investigate a seed layer-free growth of HfO2 on commercially available chemical vapor deposited (CVD) graphene from various suppliers. It is revealed that the samples of monolayer graphene transferred from Cu to SiO2/Si substrates have different coverage with bi- and multi-layer graphene islands. We find that the distribution and number of such islands impact the nucleation and growth of HfO2 by CVD. In particular, we show that the edges and surface of densely distributed bi-layer graphene islands provide good nucleation sites for conformal CVD HfO2 layers. Dielectric constant of 16 is extracted from measurements on graphene-HfO2-TiN capacitors.

We report here the buckling phenomenon of a monolayer graphene with four free edges subjected to uniaxial tension by molecular dynamics simulations. We find that the buckling occurs when tension force is applied and is sufficiently large for both armchair and zigzag single layer graphene sheets. The maximum amplitude is increased and the maximum wavelength is decreased with increase in temperature.

Raman characterizations show the G peak of an unintentional doped single crystal bilayer graphene (BLG) splits into two peaks: S and AS peaks. From the relative shift between S and AS peaks, the doping concentration is estimated to be from 8.8 × 1012 cm−2 to 2 × 1013 cm−2, as in the same order of that in monolayer graphene prepared under the same condition. The dopants distribute relatively homogeneously in a 0.7 mm× 0.3 mm large BLG judged through the G peak splitting. The 2D peak of heavy doped BLG can only be deconvoluted into three peaks, corresponding to 2D1B, 2D1A, and 2D2A peaks.

The strain dependence of conductance of monolayer graphene has been studied experimentally here. The results illustrate the notable transitions: the slight increase, the dramatic decrease, and the sudden dropping of the conductance by gradually increasing the uniaxial strain. The graphene conductance behaves reversibly by tuning of the elastic tensile strain up to 4.5%, while it fails to recover after the plastic deformation at 5%. The change in conductance due to strain is surprisingly high, which indicates the potential applications in electromechanical devices.

The existence of surface steps was found to have significant function and influence on the growth of graphene on copper via chemical vapor deposition. The two typical growth modes involved were found to be influenced by the step morphologies on copper surface, which led to our proposed step driven competitive growth mechanism. We also discovered a protective role of graphene in preserving steps on copper surface. Our results showed that wide and high steps promoted epitaxial growth and yielded multilayer graphene domains with regular shape, while dense and low steps favored self-limited growth and led to large-area monolayer graphene films. We have demonstrated that controllable growth of graphene domains of specific shape and large-area continuous graphene films are feasible.

We demonstrated doping in monolayer graphene via local electrical stressing. The doping, confirmed by the resistance-voltage transfer characteristics of the graphene system, is observed to continuously tunable from N-type to P-type as the electrical stressing level (voltage) increases. Two major physical mechanisms are proposed to interpret the observed phenomena: modifications of surface chemistry for N-type doping (at low-level stressing) and thermally-activated charge transfer from graphene to SiO2 substrate for P-type doping (at high-level stressing). The formation of P-N junction on two-dimensional graphene monolayer is demonstrated with complementary doping based on locally applied electrical stressing.

In this letter, we demonstrate that a graphene monolayer, over which three metallic electrodes forming a coplanar waveguide are patterned, acts as a frequency multiplier and generates frequencies at least up to 40 GHz. These results show that monolayer graphene is a natural frequency multiplier.