Showing papers by "Kostya S. Novoselov published in 2008"

Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

Abstract: The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on $SiO_2$ back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to $5\times 10^{13} cm^{-2}$, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used $SiO_2$ back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.

Chaotic Dirac billiard in graphene quantum dots.

Abstract: The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nanometers), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nanometers, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 electron volt, demonstrating the possibility of molecular-scale electronics based on graphene.

Graphene-based liquid crystal device.

Abstract: Graphene is only one atom thick, optically transparent, chemically inert, and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter, we demonstrate liquid crystal devices with electrodes made of graphene that show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally used metal oxides in terms of low resistivity, high transparency and chemical stability.

Molecular doping of graphene

Abstract: Graphene is considered as one of the most promising materials for post silicon electronics, as it combines high electron mobility with atomic thickness [Novoselov et al. Science 2004, 306, 666−669. Novoselov et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453]. The possibility of chemical doping and related excellent chemical sensor properties of graphene have been demonstrated experimentally [Schedin et al. Nat. Mater. 2007, 6, 652−655], but a microscopic understanding of these effects has been lacking, so far. In this letter, we present the first joint experimental and theoretical investigation of adsorbate-induced doping of graphene. A general relation between the doping strength and whether adsorbates are open- or closed-shell systems is demonstrated with the NO2 system: The single, open shell NO2 molecule is found to be a strong acceptor, whereas its closed shell dimer N2O4 causes only weak doping. This effect is pronounced by graphene's peculiar density of states (DOS), which provides an id...

Macroscopic graphene membranes and their extraordinary stiffness

Abstract: The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal, and other characteristics of this one-atom-thick material. Here, we describe a new and highly reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by only one side do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.

Plasmon spectroscopy of free-standing graphene films

Abstract: Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms: graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe pi and pi+sigma-surface plasmon modes in free-standing single sheets at 4.7 and 14.6 eV, which are substantially redshifted from their values in graphite. These modes are in very good agreement with the theoretical spectra, which find the pi- and pi+sigma in-plane modes of graphene at 4.8 and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV.

Quantum resistance metrology in graphene

Abstract: We performed a metrological characterization of the quantum Hall resistance in a 1 μm wide graphene Hall bar. The longitudinal resistivity in the center of the ν=±2 quantum Hall plateaus vanishes within the measurement noise of 20 mΩ up to 2 μA. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5 μA current) equal to that in conventional semiconductors. The principal limitation of the present experiments is the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied.

Electronic properties of a biased graphene bilayer

Abstract: We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system -- \emph{biased bilayer}. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its 4-band and 2-band continuum approximations, and the 4-band model is shown to be always a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, either made out of SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point to understand the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, as the second-nearest-neighbor hopping energies $t'$ (in-plane) and $\gamma_{4}$ (inter-layer), and the on-site energy $\Delta$.

Graphene in Multilayered CPP Spin Valves

Abstract: In this paper, we have sandwiched a single-atom-thick sheet of graphene, a 2-D allotrope of carbon that is exciting tremendous research interest, between two ferromagnetic electrodes. Graphene has already been shown to support spin polarized conduction, when spin polarized electrons were injected into graphene sheets along the plane using magnetic electrodes. Here, we study spin polarized conduction perpendicular the plane of the graphene in conduction perpendicular to plane (CPP) geometry. It was found that graphene was sufficient to reduce the exchange coupling between the magnetic electrodes. We also found that in a NiFe/Au/Graphene/NiFe stack, the graphene channels the spin current perpendicular to the plane and thus produces an enhanced MR effect compared to a simple stack without the graphene. A significant anisotropy was found in the observed magnetoresistance in the cross electrode geometry employed.

STEM plasmon spectroscopy of free standing graphene

Abstract: Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms, graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe pi- and pi+ sigma-surface plasmon modes in free-standing single sheets at 4.7 eV and 14.8 eV which are substantially red-shifted from their values in graphite. These modes are in very good agreement with the theoretical spectra which find the pi- and pi + pi in-plane modes of graphene at 4.5 eV and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV. (C) 2008 WILEY-NCH Verlag GmbH & Co. KGaA, Weinheim.

Cyclotron resonance of electrons and holes in graphene monolayers

Abstract: We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonances are detected by observing changes in the photoconductive response of the sample. An electron velocity at the Dirac point of 1.093 x 10(6) m s(-1) is obtained, which is the fastest velocity recorded for all known carbon materials. In addition, a significant asymmetry exists between band structure for electrons and holes, which gives rise to a 5% difference between the velocities at energies of 125 meV away from the Dirac point.

Graphene-Based Liquid Crystal Device

Abstract: Graphene is only one atom thick, optically transparent, chemically inert and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter we demonstrate liquid crystal devices with electrodes made of graphene which show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally-used metal oxides in terms of low resistivity, high transparency and chemical stability.

Macroscopic graphene membranes and their extraordinary stiffness

Abstract: The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal and other characteristics of this one-atom-thick material. Here we describe a new and highly-reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microns in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by one side only do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.

Quantum hall effect in graphene

Abstract: Graphene is the first example of truly two-dimensional crystals - it's just one layer of carbon atoms. It turns out that graphene is a gapless semiconductor with unique electronic properties resulting from the fact that charge carriers in graphene obey linear dispersion relation, thus mimicking massless relativistic particles. This results in the observation of a number of very peculiar electronic properties - from an anomalous quantum Hall effect (QHE) to the absence of localization. It also provides a bridge between condensed matter physics and quantum electrodynamics and opens new perspectives for carbon-based electronics.