Showing papers by "Kostya S. Novoselov published in 2009"
TL;DR: In this paper, the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations, are discussed.
Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.
20,824 citations
TL;DR: This work illustrates the concept of graphene as a robust atomic-scale scaffold on the basis of which new two-dimensional crystals with designed electronic and other properties can be created by attaching other atoms and molecules.
Abstract: Although graphite is known as one of the most chemically inert materials, we have found that graphene, a single atomic plane of graphite, can react with atomic hydrogen, which transforms this highly conductive zero-overlap semimetal into an insulator. Transmission electron microscopy reveals that the obtained graphene derivative (graphane) is crystalline and retains the hexagonal lattice, but its period becomes markedly shorter than that of graphene. The reaction with hydrogen is reversible, so that the original metallic state, the lattice spacing, and even the quantum Hall effect can be restored by annealing. Our work illustrates the concept of graphene as a robust atomic-scale scaffold on the basis of which new two-dimensional crystals with designed electronic and other properties can be created by attaching other atoms and molecules.
3,735 citations
TL;DR: In this paper, the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain was uncovered.
Abstract: We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E(2g) optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G(+) and G(-), by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D(') bands also redshift but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D, and D(') peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance.
1,762 citations
TL;DR: In this paper, a cantilever-beam arrangement was used to examine the structural properties of graphene flakes under both tension and compression using two sets of samples, one consisting of flakes just supported on a plastic bar and the other consisting of flake embedded within the substrate.
Abstract: Themechanical behaviorof grapheneflakesunderboth tension and compression is examined using a cantilever-beam arrangement. Twodifferent sets of samples are employed.One consists of flakes just supported on a plastic bar. The other consists of flakesembeddedwithin theplastic substrate.Bymonitoring the shift of the 2DRaman linewith strain, information on the stress transfer efficiency as a function of stress sign and monolayer support are obtained. In tension, the embedded flake seems to sustain strains up to 1.3%, whereas in compression there is an indication of flake buckling at about 0.7% strain. The retainment of such a high critical buckling strain confirms the relative high flexural rigidity of the embedded monolayer. The mechanical strength and stiffness of crystalline materials are normally governed by the strength and stiffness
425 citations
TL;DR: The retainment of such a high critical buckling strain confirms the relative high flexural rigidity of the embedded monolayer.
Abstract: The mechanical behaviour of graphene flakes under both tension and compression is examined using a cantilever-beam arrangement. Two different sets of samples were employed involving flakes just supported on a plastic bar but also embedded within the plastic substrate. By monitoring the shift of the 2D Raman line with strain, information on the stress transfer efficiency as a function of stress sign and monolayer support were obtained. In tension, the embedded flake seems to sustain strains up to 1.3%, whereas in compression there is an indication of flake buckling at about 0.7% strain. The retainment of such a high critical buckling strain confirms the relative high flexural rigidity of the embedded monolayer.
397 citations
TL;DR: No significant changes in carrier mobility is found either for different substrates or by using glycerol, ethanol, and water as a top dielectric layer, suggesting that Coulomb impurities are not the scattering mechanism that limits the mean free path attainable for graphene on a substrate.
Abstract: It is widely assumed that the dominant source of scattering in graphene is charged impurities in a substrate. We have tested this conjecture by studying graphene placed on various substrates and in high-κ media. Unexpectedly, we have found no significant changes in carrier mobility either for different substrates or by using glycerol, ethanol, and water as a top dielectric layer. This suggests that Coulomb impurities are not the scattering mechanism that limits the mean free path attainable for graphene on a substrate.
336 citations
TL;DR: In this article, the authors present a compelling evidence for the opening of a bandgap in exfoliated bottom-gated bilayer graphene by fitting the gatevoltage-modulated infrared reflectivity spectra in a large range of doping levels with a tight-binding model and the Kubo formula.
Abstract: We present a compelling evidence for the opening of a bandgap in exfoliated bottom-gated bilayer graphene by fitting the gate-voltage-modulated infrared reflectivity spectra in a large range of doping levels with a tight-binding model and the Kubo formula. A close quantitative agreement between the experimental and calculated spectra is achieved, allowing us to determine self-consistently the full set of Slonczewski-Weiss-McClure tight-binding parameters together with the gate-voltage-dependent bandgap. The doping dependence of the bandgap shows a good agreement with the existing calculations that take the effects of self-screening into account. We also identify certain mismatches between the tight-binding model and the data, which can be related to electron-electron and electron-phonon interactions.
252 citations
TL;DR: In this paper, the charge inhomogeneity induced by spurious chemical doping or metal contacts can lead to large systematic errors in assessing graphene's transport properties and, in particular, its minimal conductivity.
218 citations
TL;DR: In this paper, the authors present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare it with tight-binding calculations.
Abstract: We present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare it with tight-binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A strong gate voltage dependence of these structures and a significant electron-hole asymmetry are observed that we use to extract several band parameters. The structures related to the gate-induced band gap are less pronounced in the experiment than predicted by the tight-binding model that uses parameters obtained from previous experiments on graphite and recent self-consistent band-gap calculations.
129 citations
TL;DR: In this article, a simple model involving a field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements, was proposed to reproduce both the increase in low-temperature resistivity and the anomalous 0-value plateau in the Hall conductivity.
Abstract: We have measured a strong increase of the low-temperature resistivity ${\ensuremath{\rho}}_{xx}$ and a zero-value plateau in the Hall conductivity ${\ensuremath{\sigma}}_{xy}$ at the charge neutrality point in graphene subjected to high magnetic fields up to 30 T. We explain our results by a simple model involving a field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements. The model reproduces both the increase in ${\ensuremath{\rho}}_{xx}$ and the anomalous $\ensuremath{
u}=0$ plateau in ${\ensuremath{\sigma}}_{xy}$ in terms of coexisting electrons and holes in the same split zero-energy Landau level.
121 citations
TL;DR: It is suggested that the intensity growth and the softening originate from the coupling of the phonon mode to the narrow electronic transition between parallel bands of the same character, while the asymmetry is due to the interaction with the continuum of transitions between the lowest hole and electron bands.
Abstract: We observe a giant increase of the infrared intensity and a softening of the in-plane antisymmetric phonon mode E(u) ( approximately 0.2 eV) in bilayer graphene as a function of the gate-induced doping. The phonon peak has a pronounced Fano-like asymmetry. We suggest that the intensity growth and the softening originate from the coupling of the phonon mode to the narrow electronic transition between parallel bands of the same character, while the asymmetry is due to the interaction with the continuum of transitions between the lowest hole and electron bands. The growth of the peak can be interpreted as a "charged-phonon" effect observed previously in organic chain conductors and doped fullerenes, which can be tuned in graphene with the gate voltage.
01 May 2009
TL;DR: In this article, the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain was uncovered.
Abstract: We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E(2g) optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G(+) and G(-), by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D(') bands also redshift but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D, and D(') peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance.
TL;DR: In this article, the scaling behavior of the quantum Hall plateau-plateau transitions in the recently discovered new type of 2DES, graphene, was investigated, and the scaling exponent = 0.40 0.02 was consistent with universal scaling theory.
Abstract: The integer quantum Hall effect in two-dimensional electron systems 2DESs is caused by localized states in the tails of individual Landau levels which give rise to quantized plateaus in the Hall resistance. The states in the center of the Landau levels are extended; their wave functions are delocalized. Their delocalization is governed by a localization length, which decays exponentially away from the Landau level centers 1,2 with a universal critical scaling exponent related to this decay. 3 In this Rapid Communication we investigate the scaling behavior of the quantum Hall plateau-plateau transitions in the recently discovered new type of 2DES, graphene. 4,5 When changing the carrier concentration n at a constant field, the peak width of the longitudinal conductivity for higher Landau levels N 1 and the inverse slope of the Hall conductivity scale as T. Our experimentally measured scaling exponent = 0.40 0.02 is consistent with universal scaling theory. 1–3,6–8 The transition through the zeroth Landau level, however, shows no clear scaling behavior which we explain by a different scaling mechanism governed by a temperature independent intrinsic length scale. Our sample was made by micromechanical exfoliation of natural graphite and subsequently contacted by gold contacts and patterned into a 1-m-wide Hall bar by electron-beam lithography and reactive plasma etching. 9 The structure was deposited on a 300 nm Si/ SiO2 substrate thereby forming a graphene ambipolar field effect transistor. Prior to the measurements the sample was annealed at 400 K, placing its charge neutrality point CNP at zero gate voltage with a mobility of = 1.0 m 2 Vs �1 .
TL;DR: For example, a pair of carbon atoms bonded together can accept one, two or three hydrogen atoms, forming ethyne, ethene and ethane, chemicals used in welding, anaesthesia and vodka-making, respectively.
Abstract: When nature had to choose an element as the basis for life, it chose carbon. If I had to guess why, I would say the reason was carbon's extraordinary versatility. Bonding between carbon atoms is exceptionally strong; indeed, the strongest materials on Earth are all made of carbon. However, bonding between carbon and other elements, though stable, can easily be changed by chemical reactions. The resulting compounds are often surprisingly different from one another. For example, a pair of carbon atoms bonded together can accept one, two or three hydrogen atoms, forming ethyne, ethene and ethane – chemicals used in welding, anaesthesia and vodka-making, respectively.
TL;DR: The observation of Hall quantization and complete lifting of the degeneracy in bilayer graphene at magnetic fields an order of magnitude lower than previously reported has important implications for an understanding of the role of many-body interactions in the exotic behaviour of bi-and monolayer graphene as discussed by the authors.
Abstract: The observation of Hall quantization and complete lifting of the degeneracy in bilayer graphene at magnetic fields an order of magnitude lower than previously reported has important implications for an understanding of the role of many-body interactions in the exotic behaviour of bi- and monolayer graphene.
TL;DR: In this article, the transverse spin transport properties of a single layer graphene flake was found to break the exchange coupling between magnetic films and also to enhance the magnetoresistance effect.
Abstract: In this paper we report transverse spin transport properties of graphene in a device, where for the first time a mono-atomically thin atomic fabric was sandwiched between magnetic thin films. We found that a single layer graphene flake was sufficient to break the exchange coupling between magnetic films and also to enhance the magnetoresistance effect.