Showing papers on "Bilayer graphene published in 2011"
TL;DR: In this paper, a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures is provided.
Abstract: We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.
2,930 citations
TL;DR: This work addresses the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moiré pattern periodicity leads to moirÉ Bloch bands.
Abstract: A moire pattern is formed when two copies of a periodic pattern are overlaid with a relative twist. We address the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moire pattern periodicity leads to moire Bloch bands. The two layers become more strongly coupled and the Dirac velocity crosses zero several times as the twist angle is reduced. For a discrete set of magic angles the velocity vanishes, the lowest moire band flattens, and the Dirac-point density-of-states and the counterflow conductivity are strongly enhanced.
2,323 citations
TL;DR: The intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices is reported, providing strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene.
Abstract: We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier–assisted thermoelectric technologies for efficient solar energy harvesting.
967 citations
TL;DR: Findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.
Abstract: In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.
793 citations
TL;DR: It is found that BN substrates result in extraordinarily flat graphene layers that display microscopic Moiré patterns arising from the relative orientation of the graphene and BN lattices.
Abstract: The use of boron nitride (BN) as a substrate for graphene nanodevices has attracted much interest since the recent report that BN greatly improves the mobility of charge carriers in graphene compared to standard SiO2 substrates. We have explored the local microscopic properties of graphene on a BN substrate using scanning tunneling microscopy. We find that BN substrates result in extraordinarily flat graphene layers that display microscopic Moire patterns arising from the relative orientation of the graphene and BN lattices. Gate-dependent dI/dV spectra of graphene on BN exhibit spectroscopic features that are sharper than those obtained for graphene on SiO2. We observe a significant reduction in local microscopic charge inhomogeneity for graphene on BN compared to graphene on SiO2.
586 citations
TL;DR: The growth of monolayer nitrogen-doped graphene in centimeter-scale sheets is demonstrated using a chemical vapor deposition process with pyridine as the sole source of both carbon and nitrogen.
Abstract: In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. Doping with electron-donor nitrogen heteroatoms can modulate the electronic properties of graphene to produce an n-type semiconductor. Here we demonstrate the growth of monolayer nitrogen-doped graphene in centimeter-scale sheets using a chemical vapor deposition process with pyridine as the sole source of both carbon and nitrogen. High-resolution transmission microscopy and Raman mapping characterizations indicate that the nitrogen-doped graphene sheets are uniformly monolayered. The existence of nitrogen-atom substitution in the graphene planes was confirmed by X-ray photoelectron spectroscopy. Electrical measurements show that the nitrogen-doped graphene exhibits an n-type behavior, different from pristine graphene. The preparation of large-area nitrogen-doped graphene provides a viable route to modify the properties of monolayer graphene and promote its applications in electronic devices.
576 citations
TL;DR: In this article, the authors investigate band-gap tuning in bilayer transition-metal dichalcogenides by external electric fields applied perpendicular to the layers, and show that the fundamental band gap of MoS, MoSe, MoTe, and WS bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic.
Abstract: We investigate band-gap tuning in bilayer transition-metal dichalcogenides by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS${}_{2}$, MoSe${}_{2}$, MoTe${}_{2}$, and WS${}_{2}$ bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the giant Stark effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices.
531 citations
TL;DR: High magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition observe an unexpected electron-hole asymmetry which is substantially larger than the asymmetry in either single or untwayer graphene.
Abstract: We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene
528 citations
TL;DR: In this article, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers of monolayer and trilayer graphene, and both of them can be controlled by an electric field.
Abstract: Monolayer graphene has no electronic band gap. Bilayer graphene does, and can be controlled by an electric field. And for trilayer graphene, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers.
457 citations
TL;DR: The distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.
Abstract: Graphene is an outstanding material with ultrahigh thermal conductivity. Its thermal transfer properties under various strains are studied by reverse nonequilibrium molecular dynamics. Based on the unique two-dimensional structure of graphene, the distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties are demonstrated under strains. For example, the corrugation under uniaxial compression and helical structure under light torsion, as well as tube-like structure under strong torsion, exhibit enormously different thermal conductivity. The important robustness of thermal conductivity is found in the corrugated and helical configurations of graphene nanoribbons. Nevertheless, thermal conductivity of graphene is very sensitive to tensile strain. The relationship among phonon frequency, strain and thermal conductivity are analyzed. A similar trend line of phonon frequency dependence of thermal conductivity is observed for armchair graphene nanoribbons and zigzag graphene nanoribbons. The unique thermal properties of graphene nanoribbons under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.
398 citations
TL;DR: In this article, the authors present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride and extract mobilities as high as 125'000 cm2 V−1 s−1 at room temperature and 275'000cm2 V −1 s −1 at 4.2'K.
Abstract: We present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm2 V−1 s−1 at room temperature and 275 000 cm2 V−1 s−1 at 4.2 K. The excellent quality is supported by the early development of the ν = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a fast, simple, and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.
TL;DR: The epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition is reported, showing typical tunable transfer characteristics under varying gate voltages.
Abstract: We report the epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-by-layer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper.
01 Mar 2011
TL;DR: In this paper, high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition was performed. But the results were limited to the case of twisted bilayer bilayer graphene.
Abstract: We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.
TL;DR: Graphene spin relaxation in graphene spin valves is investigated and strongly contrasting behavior for single-layer graphene (SLG) and bilayer graphene (BLG) is observed, which indicates the dominance of Dyakonov-Perel spin relaxation at low temperatures.
Abstract: We investigate spin relaxation in graphene spin valves and observe strongly contrasting behavior for single-layer graphene (SLG) and bilayer graphene (BLG). In SLG, the spin lifetime (τ(s)) varies linearly with the momentum scattering time (τ(p)) as carrier concentration is varied, indicating the dominance of Elliot-Yafet (EY) spin relaxation at low temperatures. In BLG, τ(s) and τ(p) exhibit an inverse dependence, which indicates the dominance of Dyakonov-Perel spin relaxation at low temperatures. The different behavior is due to enhanced screening and/or reduced surface sensitivity of BLG, which greatly reduces the impurity-induced EY spin relaxation.
TL;DR: Results indicate that there is a correlation between SERS enhancement factor and the extent of the G band splitting, and the strongest interaction occurs between Ag and single-layer graphene, and that the Ag deposition on graphene can induce doping of graphene.
Abstract: The interaction between graphene and metal was investigated by studying the G band splitting in surface-enhanced Raman scattering (SERS) spectra of single-, bi-, and trilayer graphene. The Ag deposition on graphene induced large enhancement of the Raman signal of graphene, indicating SERS of graphene. In particular, the G band was split into two distinct peaks in the SERS spectrum of graphene. The extent of the G band splitting was 13.0 cm−1 for single-layer, 9.6 cm−1 for bilayer, and 9.4 cm−1 for trilayer graphene, whereas the G band in the SERS spectrum of a thick multilayer was not split. The average SERS enhancement factor of the G band was 24 for single-layer, 15 for bilayer, and 10 for trilayer graphene. These results indicate that there is a correlation between SERS enhancement factor and the extent of the G band splitting, and the strongest interaction occurs between Ag and single-layer graphene. Furthermore, the Ag deposition on graphene can induce doping of graphene. The intensity ratio of 2D an...
TL;DR: In this article, the gate-tunable Kondo effect in ion-beam-damaged graphene suggests that defects induce magnetism in graphite, but it is unclear whether this extends to graphene.
Abstract: Although evidence indicates that defects induce magnetism in graphite, it’s unclear whether this extends to graphene. An observation of the gate-tunable Kondo effect in ion-beam-damaged graphene suggests it does.
TL;DR: In this article, the authors investigated the mechanisms determining the growth of high-quality monolayer and bilayer graphene on Cu using chemical vapor deposition (CVD) and showed that graphene growth on Cu is not only determined by the process parameters during growth, but also substantially influenced by the quality of Cu substrate and how the Cu substrate is pretreated.
TL;DR: The synthesis of large-area monolayer and multilayer, particularly bilayer, graphene films on Cu-Ni alloy foils by chemical vapor deposition with methane and hydrogen gas as precursors is reported.
Abstract: Controlling the thickness and uniformity during growth of multilayer graphene is an important goal. Here we report the synthesis of large-area monolayer and multilayer, particularly bilayer, graphene films on Cu–Ni alloy foils by chemical vapor deposition with methane and hydrogen gas as precursors. The dependence of the initial stages of graphene growth rate on the substrate grain orientation was observed for the first time by electron backscattered diffraction and scanning electron microscopy. The thickness and quality of the graphene and graphite films obtained on such Cu–Ni alloy foils could be controlled by varying the deposition temperature and cooling rate and were studied by optical microscopy, scanning electron microscopy, atomic force microscopy, and micro-Raman imaging spectroscopy. The optical and electrical properties of the graphene and graphite films were studied as a function of thickness.
TL;DR: In this paper, the authors present experimental results that folded structures in graphene, termed grafold, exist, and their formations can be controlled by introducing anisotropic surface curvature during graphene synthesis or transfer processes.
Abstract: The folding of paper, hide, and woven fabric has been used for millennia to achieve enhanced articulation, curvature, and visual appeal for intrinsically flat, two-dimensional materials. For graphene, an ideal twodimensional material, folding may transform it to complex shapes with new and distinct properties. Here, we present experimental results that folded structures in graphene, termed grafold, exist, and their formations can be controlled by introducing anisotropic surface curvature during graphene synthesis or transfer processes. Using pseudopotential-density-functional-theory calculations, we also show that double folding modifies the electronic band structure of graphene. Furthermore, we demonstrate the intercalation of C60 into the grafolds. Intercalation or functionalization of the chemically reactive folds further expands grafold’s mechanical, chemical, optical, and electronic diversity.
TL;DR: In this paper, the authors present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride and extract mobilities as high as 125 000 cm^2/V/s at room temperature and 275 000 cm 2/V /s at 4.2 K. The excellent quality is supported by the early development of the nu = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4 2 K.
Abstract: We present electronic transport measurements of single- and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm^2/V/s at room temperature and 275 000 cm^2/V/s at 4.2 K. The excellent quality is supported by the early development of the nu = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a new and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique is simple, fast and yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.
TL;DR: A general transfer-free method to directly grow large areas of uniform bilayer graphene on insulating substrates from solid carbon sources such as films of poly(2-phenylpropyl)methysiloxane, poly(methyl methacrylate), polystyrene, and poly(acrylonitrile-co-butadiene- co-styrene) and randomly imaged graphene edges by high-resolution transmission electron microscopy is demonstrated.
Abstract: Here we demonstrate a general transfer-free method to directly grow large areas of uniform bilayer graphene on insulating substrates (SiO(2), h-BN, Si(3)N(4), and Al(2)O(3)) from solid carbon sources such as films of poly(2-phenylpropyl)methysiloxane, poly(methyl methacrylate), polystyrene, and poly(acrylonitrile-co-butadiene-co-styrene), the latter leading to N-doped bilayer graphene due to its inherent nitrogen content. Alternatively, the carbon feeds can be prepared from a self-assembled monolayer of butyltriethoxysilane atop a SiO(2) layer. The carbon feedstocks were deposited on the insulating substrates and then caped with a layer of nickel. At 1000 °C, under low pressure and a reducing atmosphere, the carbon source was transformed into a bilayer graphene film on the insulating substrates. The Ni layer was removed by dissolution, affording the bilayer graphene directly on the insulator with no traces of polymer left from a transfer step. The bilayer nature of as-grown samples was demonstrated by I(G)/I(2D) Raman mapping, the statistics of the full-width at half-maximum of the Raman 2D peak, the selected area electron diffraction patterns over a large area, and randomly imaged graphene edges by high-resolution transmission electron microscopy.
TL;DR: A transfer-free method of synthesizing bilayer graphene directly on SiO(2) substrates by carbon diffusion through a layer of nickel is reported, eliminating any transfer process.
Abstract: Here we report a transfer-free method of synthesizing bilayer graphene directly on SiO2 substrates by carbon diffusion through a layer of nickel. The 400 nm nickel layer was deposited on the top of SiO2 substrates and used as the catalyst. Spin-coated polymer films such as poly(methyl methacrylate), high-impact polystyrene or acrylonitrile–butadiene–styrene, or gas-phase methane were used as carbon sources. During the annealing process at 1000 °C, the carbon sources on the top of the nickel decomposed and diffused into the nickel layer. When cooled to room temperature, bilayer graphene was formed between the nickel layer and the SiO2 substrates. The nickel films were removed by etchants, and bilayer graphene was then directly obtained on SiO2, eliminating any transfer process. The bilayer nature of the obtained graphene films on SiO2 substrates was verified by Raman spectroscopy and transmission electron microscopy. The Raman spectroscopy mapping over a 100 × 100 μm2 area indicated that the obtained graph...
TL;DR: The thermal conductivity of two bilayer graphene samples each suspended between two microresistance thermometers was measured and the lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayers by a residual polymeric layer that was clearly observed by transmission electron microscopy.
Abstract: The thermal conductivity (κ) of two bilayer graphene samples each suspended between two microresistance thermometers was measured to be 620 ± 80 and 560 ± 70 W m−1 K−1 at room temperature and exhibits a κ ∝ T1.5 behavior at temperatures (T) between 50 and 125 K. The lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy.
TL;DR: A study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented and the peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayers graphene.
Abstract: When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moire patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.
TL;DR: Interestingly, electrons in the upper valence band can be directly excited from graphene to the conduction band, that is, the 3d orbitals of titania, under visible light irradiation, which should yield well-separated electron-hole pairs.
Abstract: We demonstrated for the first time by large-scale ab initio calculations that a graphene/titania interface in the ground electronic state forms a charge-transfer complex due to the large difference of work functions between graphene and titania, leading to substantial hole doping in graphene. Interestingly, electrons in the upper valence band can be directly excited from graphene to the conduction band, that is, the 3d orbitals of titania, under visible light irradiation. This should yield well-separated electron−hole pairs, with potentially high photocatalytic or photovoltaic performance in hybrid graphene and titania nanocomposites. Experimental wavelength-dependent photocurrent generation of the graphene/titania photoanode demonstrated noticeable visible light response and evidently verified our ab initio prediction.
TL;DR: This work reports a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films and demonstrates the first example of monolayer and bilayer graphene wafers using Cu-Ni alloy by combining the distinct segregation behaviors of Cu and Ni.
Abstract: Graphene has been attracting wide interests owing to its excellent electronic, thermal, and mechanical performances. Despite the availability of several production techniques, it is still a great challenge to achieve wafer-size graphene with acceptable uniformity and low cost, which would determine the future of graphene electronics. Here we report a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films. Without any extraneous carbon sources, 4 in. graphene wafers have been obtained from Ni, Co, Cu−Ni alloy, and so forth via thermal annealing with over 82% being 1−3 layers and excellent reproducibility. We demonstrate the first example of monolayer and bilayer graphene wafers using Cu−Ni alloy by combining the distinct segregation behaviors of Cu and Ni. Together with the easy detachment from growth substrates, we believe this facile segregation technique will offer a great driving force for graphene research.
TL;DR: This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.
Abstract: The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that anenergy gap in Bernal-stacked bilayer graphene can begeneratedbyapplyinganelectricdisplacement field.Moleculardopinghasalsobeenproposedto open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayergraphene,whichefficientlyblockstheaccessibledopingsitesandpreventsambientp-doping onthetoplayer.Thechargedistributionasymmetrybetweenthetopandbottomlayerscanthenbe enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.
TL;DR: In this paper, the epitaxial growth of graphene on Pt(111) surface was investigated and it was found out that the proportion of different rotational domains varies with growth temperature and the graphene quality can be improved by adjusting both the growth temperatures and ethylene exposure.
Abstract: We report on epitaxial growth of graphene on Pt(111) surface. It was found out that the proportion of different rotational domains varies with growth temperature and the graphene quality can be improved by adjusting both the growth temperature and ethylene exposure. Rippled and unrippled domains of high quality graphene are observed. The adhesive energy and electronic structure of two models, representing rippled and unrippled graphene, are obtained with density functional theory calculation, which shows that the interaction between graphene and Pt(111) surface is very weak and the electronic structure is nearly the same as that of a free standing graphene.
TL;DR: The charge carriers in single-layer graphene are effectively massless, while in bilayer graphene, they are massive as mentioned in this paper, which leads to an unusual quantum Hall response in which the Landau levels of massless and massive charge carriers repeatedly cross.
Abstract: The charge carriers in single-layer graphene are effectively massless. In bilayer graphene, they are massive. In trilayer graphene, the two types coexist, which leads to an unusual quantum Hall response in which the Landau levels of massless and massive charge carriers repeatedly cross.