Single Layer Epitaxial Graphene Research Articles

The role of charge distribution on the friction coefficients of epitaxial graphene grown on the Si-terminated and C-terminated faces of SiC

The friction coefficients of single-layer epitaxial graphene grown on the Si-terminated and C-terminated faces of Silicon Carbide (SiC) substrate were measured under ambient conditions using Friction Force Microscope (FFM). The lateral friction force measurements acquired in the applied normal force range between 4.0 and 16.0 nN showed that the friction coefficient of graphene on the C-terminated face of SiC is about two times smaller than the one grown on its Si-terminated face. The lateral friction was found to be decreased as the average of root mean square roughness increases suggesting the observed difference in the friction coefficients cannot be related to the roughness of the graphene layers. DFT calculations demonstrated that the altered periodicity of charge distribution on graphene due to the specific interactions with two distinct polar faces of SiC substrate might explain the observed difference in the friction coefficients.

Large, non-saturating magnetoresistance in single layer chemical vapor deposition graphene with an h-BN capping layer

We report large, non-saturating magnetoresistance (MR) of ∼140% in single layer chemical vapor deposition (CVD) graphene with an h-BN capping layer at room temperature at B = 9 T. Based on the classical model developed by Parish and Littlewood, our results show that the MR is proportional to the average mobility <μ > and decreases with increasing temperature. In contrast, in a large-area, extremely homogenous single layer epitaxial graphene (EG) device, the MR is saturating and is inversely proportional to <μ>, which is consistent with the finite resistance network picture. By comparing the results obtained from CVD graphene with an h-BN capping layer with those from the EG device, we show that the non-saturating linear characteristics come from multi-channel current paths in a two-dimensional plane due to the intrinsic grain boundaries and domains of CVD graphene by capping an h-BN layer that increase the <μ> of CVD graphene. Our results on CVD graphene with an h-BN capping layer pave the way for industrial schemes of graphene-based and air-stable magnetic field sensors with a linear, large response at room temperature.

Hydrogen intercalation of epitaxial graphene and buffer layer probed by mid-infrared absorption and Raman spectroscopy

We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated single layer epitaxial graphene and buffer layer grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The Raman spectroscopy is used to show presence of buffer layer and single layer graphene prior to intercalation. We also present Raman spectra of quasi free standing monolayer and bilayer graphene after hydrogen intercalation at temperatures between 790 and 1510°C. We have found that although the Si-H bonds form at as low temperatures as 790°C, the well developed bond order has been reached only for samples intercalated at temperatures exceeding 1000°C. We also study temporal stability of hydrogen intercalated samples stored in ambient air. The optical spectroscopy shows on a formation of silyl and silylene groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process.

The transport properties of Dirac fermions in chemical vapour-deposited single-layer graphene

The electronic transport properties of Dirac fermions in chemical vapour-deposited single-layer epitaxial graphene on anSiO2/Si substrate have been investigated using the Shubnikov–de Haas (SdH) oscillations technique. The magnetoresistance measurements were performed in the temperature range between 1.8 and 43 K and at magnetic fields up to 11 T. The 2D carrier density and the Fermi energy have been determined from the period of the SdH oscillations. In addition, the in-plane effective mass as well as the quantum lifetime of 2D carriers have been calculated from the temperature and magnetic field dependences of the SdH oscillation amplitude. The sheet carrier density (1.42 × 1013 cm−2 at 1.8 K), obtained from the low-field Hall Effect measurements, is larger than that of 2D carrier density (8.13 × 1012 cm−2). On the other hand, the magnetoresistance includes strong magnetic field dependent positive, non-oscillatory background magnetoresistance. The strong magnetic field dependence of the magnetoresistance and the differences between sheet carrier and 2D carrier density can be attributed to the 3D carriers between the graphene sheet and the SiO2/Si substrate.

Atom-scale covalent electrochemical modification of single-layer graphene on SiC substrates by diaryliodonium salts

Owing to its high conductivity, graphene holds promise as an electrode for energy devices such as batteries and photovoltaics. However, to this end, the work function and doping levels in graphene need to be precisely tuned. One promising route for modifying graphene’s electronic properties is via controlled covalent electrochemical grafting of molecules. We show that by employing diaryliodonium salts instead of the commonly used diazonium salts, spontaneous functionalization is avoided. This allows for precise tuning of the grafting density. By employing bis(4-nitrophenyl)iodonium(III) tetrafluoroborate (DNP) salt calibration curves, the surface functionalization density (coverage) of glassy carbon was controlled using cyclic voltammetry in varying salt concentrations. These electro-grafting conditions and calibration curves translated directly over to modifying single layer epitaxial graphene substrates (grown on insulating 6H-SiC (0001)). In addition to quantifying the functionalization densities using electrochemical methods, samples with low grafting densities were characterized by low-temperature scanning tunneling microscopy (LT-STM). We show that the use of buffer-layer free graphene substrates is required for clear observation of the nitrophenyl modifications. Atomically-resolved STM images of single site modifications were obtained, showing no preferential grafting at defect sites or SiC step edges as supposed previously in the literature. Most of the grafts exhibit threefold symmetry, but occasional extended modifications (larger than 4nm) were observed as well.

Epitaxial graphene contact electrode for silicon carbide based ultraviolet photodetector

We present the fabrication and characterization of graphene-semiconductor-graphene ultraviolet photodetector based on the rectifying character of Schottky junction at the interface between epitaxial graphene and SiC semiconductor. As-grown single layer epitaxial graphene is interdigitated as transparent conductive electrode to probe photo-generated charge carriers in a semi-insulating 4H-SiC substrate. The fabricated device exhibits the typical current-voltage characteristics of a conventional metal-semiconductor-metal type photodetector with low leakage current. Time-resolved photocurrent measurements suggest an excellent photocurrent reversibility and high response speed of the device. The measurements performed for different illumination wavelengths showed that the sample reveals higher responsivity values when it is exposed to the light with 254 nm wavelength. The obtained results imply that epitaxial graphene can be used readily as transparent conductive electrode for SiC based optoelectronic device applications.

Nonlinear terahertz field-induced carrier dynamics in photoexcited epitaxial monolayer graphene

Optical measurements on highly doped single-layer epitaxial graphene reveal that the photoexcitation of charge carriers leads to an enhancement of the THz transmission. In this paper, the results are explained in terms of the change in the carrier scattering rate due to the optical pump fluence and the THz electric field.

Thermoelectric effect in single layer epitaxial graphene formed on semiconductor substrate. Simple analytical model

In this paper we have investigated thermoelectric effect in the epitaxial graphene on a semiconductor substrate using a simple model. We have obtained the expressions for static conductance and thermopower of the epitaxial graphene. The thermopower of the epitaxial graphene can be abnormally large near the edges of the semiconductor band gap.

Van der Waals interactions mediating the cohesion of fullerenes on graphene

Fullerenes on single-layer epitaxial graphene are a model system to study very faint interactions at a molecular level. By a variable temperature scanning tunneling microscope we have been able to study ordered fullerene layers at 40 K, exclusively bound by van der Waals interactions. The experimentally determined adsorption geometry of the molecules is computationally confirmed only if van der Waals interactions are included in the calculation formalism. The relative orientation of fullerenes in their close-packed arrangement is found to be the crucial factor for determining the total energy. Observation of collective movements of fullerene islands points out the weak coupling to the substrate and the important role of the van der Waals cohesion forces within.

Bilayer graphene by bonding CVD graphene to epitaxial graphene

A novel method for creating bilayer graphene is described where single-layer CVD graphene grown on Cu is bonded to single-layer epitaxial graphene grown on Si-face SiC. Raman microscopy and x ray photoelectron spectroscopy demonstrate the uniqueness of this bilayer, as compared to a naturally formed bilayer, in that a Bernal stack is not formed with each layer being strained differently yet being closely coupled. Electrical characterization of Hall devices fabricated on the unusual bilayer show higher mobilities, and lower carrier concentrations, than the individual CVD graphene or epitaxial graphene layers.

Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.

Thermoelectric effect in high mobility single layer epitaxial graphene

The thermoelectric response of high mobility (∼20 000 cm2/V s at 4 K) single layer epitaxial graphene on silicon carbide substrates has been experimentally investigated. The temperature dependence of the thermopower displays a strong deviation from the Mott relation at a carrier density of 1 × 1012 cm−2, reflecting the importance of the screening effect. In the quantum Hall regime, the amplitude of the thermopower peaks is lower than a quantum value, despite the high mobility of the sample. In addition, a temperature dependence for the amplitude is observed, unexpected by theories. The Nernst signal changes it sign as the magnetic field increases.

Epitaxial Graphene Growth on &lt;I&gt;α&lt;/I&gt;-SiC: Probing the Effect of Surface Orientation

We review the results of growing few layer graphene on alpha-SiC with different surface orientations. To this end we have used successively a pure {000-1} C face, a 8 degrees-off one, a nonpolar {11-20} surface and, finally, a 8 degrees-off Si face and a pure (exactly {0001}-oriented) Si one. Without any need for complex hydrogen annealing, we have shown that a pure C or a 8 degrees-off C or even a pure {11-20} surface allows the growth of single layer epitaxial graphene sheets that are strain-free, p-type, low doped and exhibit reasonably high carrier mobility. This is not the case on pure Si or 8 degrees-off Si faces on which strained material is constantly found with heavy n-type doping and low carrier mobility.

Observation of the integer quantum Hall effect in high quality, uniform wafer-scale epitaxial graphene films

We report the observation of the integer quantum Hall states at Landau level fillings of ν=2, 6, and 10 in a Hall bar device made of a single-layer epitaxial graphene film on the silicon-face of silicon-carbide prepared via argon-assisted graphitization. The two-dimensional electron gas exhibits a low-temperature (at 4 K) carrier mobility of ∼14 000 cm2/V s at the electron density of 6.1×1011 cm−2. Furthermore, the sheet resistance obtained from four-probe measurements across the whole area (12×6 mm2) of another specimen grown under similar condition displays roughly uniform values (∼1600 Ω/square), suggesting that the macroscopic steps and accompanying multilayer graphene domains play a minor role in the low-temperature electronic transport.

Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N 2 pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e 2/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.

Half integer quantum Hall effect in high mobility single layer epitaxial graphene

The quantum Hall effect, with a Berry’s phase of π is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is ∼20 000 cm2/V⋅s at 4 K and 15 000 cm2/V⋅s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO2 and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics.

Plasmon dispersion on epitaxial graphene studied using high-resolution electron energy-loss spectroscopy

We have investigated the momentum-space-dependent behavior of plasmons on epitaxial graphene (EG) using high-resolution electron energy-loss spectroscopy. There are significant differences in the $\ensuremath{\pi}$ plasmon behavior for single, bilayer, and 3--4 layer graphene which originate from differences in the in-plane and out-of-plane modes, as well as the different band structures between single-layer and few-layer graphene. The $\ensuremath{\pi}$ and $\ensuremath{\sigma}+\ensuremath{\pi}$ surface plasmon modes in single-layer EG are recorded at 5.1 and 14.5 eV at small momentum transfer $(q)$; these are redshifted from the values in multilayer EG. In single-layer graphene, a linear dispersion of the plasmon mode is observed, in contrast to the parabolic dispersion in multilayer EG. The overall linear $\ensuremath{\pi}$ plasmon dispersion between 4.8--6.7 eV is attributed to the mixing of electronic transitions caused by local field effects, which includes the linear dispersion features resulting from transitions within the ``Dirac cone.'' We also observe that the intensity of the Fuches-Kliewer phonon of SiC and loss continuum of EG varies with the thickness of epitaxial graphene.