Showing papers by "Kostya S. Novoselov published in 2010"

Fluorographene: A Two‐Dimensional Counterpart of Teflon

Abstract: A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives Fluorographene is a high-quality insulator (resistivity >10(12) Omega) with an optical gap of 3 eV It inherits the mechanical strength of graphene, exhibiting a Young's modulus of 100 N m(-1) and sustaining strains of 15% Fluorographene is inert and stable up to 400 degrees C even in air, similar to Teflon

Spectroscopic ellipsometry of graphene and an exciton-shifted van Hove peak in absorption

Abstract: We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the K point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is asymmetric and downshifted by 0.5 eV probably due to excitonic effects.

Surface-Enhanced Raman Spectroscopy of Graphene

Abstract: Surface-enhanced Raman scattering (SERS) exploits surface plasmons induced by the incident field in metallic nanostructures to significantly increase the Raman intensity Graphene provides the ideal prototype two-dimensional (2d) test material to investigate SERS Its Raman spectrum is well-known, graphene samples are entirely reproducible, height controllable down to the atomic scale, and can be made virtually defect-free We report SERS from graphene, by depositing arrays of Au particles of well-defined dimensions on a graphene/SiO(2) (300 nm)/Si system We detect significant enhancements at 633 nm To elucidate the physics of SERS, we develop a quantitative analytical and numerical theory The 2d nature of graphene allows for a closed-form description of the Raman enhancement, in agreement with experiments We show that this scales with the nanoparticle cross section, the fourth power of the Mie enhancement, and is inversely proportional to the tenth power of the separation between graphene and the center of the nanoparticle One important consequence is that metallic nanodisks are an ideal embodiment for SERS in 2d

Limits on charge carrier mobility in suspended graphene due to flexural phonons.

Abstract: The temperature dependence of the mobility in suspended graphene samples is investigated. In clean samples, flexural phonons become the leading scattering mechanism at temperature $T\ensuremath{\gtrsim}10\text{ }\text{ }\mathrm{K}$, and the resistivity increases quadratically with $T$. Flexural phonons limit the intrinsic mobility down to a few ${\mathrm{m}}^{2}/\mathrm{V}\text{ }\mathrm{s}$ at room $T$. Their effect can be eliminated by applying strain or placing graphene on a substrate.

Compression behavior of single-layer graphenes.

Abstract: Central to most applications involving monolayer graphenes is its mechanical response under various stress states. To date most of the work reported is of theoretical nature and refers to tension and compression loading of model graphenes. Most of the experimental work is indeed limited to the bending of single flakes in air and the stretching of flakes up to typically ∼1% using plastic substrates. Recently we have shown that by employing a cantilever beam we can subject single graphenes to various degrees of axial compression. Here we extend this work much further by measuring in detail both stress uptake and compression buckling strain in single flakes of different geometries. In all cases the mechanical response is monitored by simultaneous Raman measurements through the shift of either the G or 2D phonons of graphene. Despite the infinitely small thickness of the monolayers, the results show that graphenes embedded in plastic beams exhibit remarkable compression buckling strains. For large length (l)-...

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—a 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 four-band and two-band continuum approximations, and the four-band model is shown to always be a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, made out of either 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 for understanding the electronic properties of graphene bilayer. Also, we study the effect of electron–hole asymmetry terms, such as the second-nearest-neighbour hopping energies t' (in-plane) and γ4 (inter-layer), and the on-site energy Δ.

On Resonant Scatterers As a Factor Limiting Carrier Mobility in Graphene

Abstract: We show that graphene deposited on a substrate has a non-negligible density of atomic scale defects. This is evidenced by a previously unnoticed D peak in the Raman spectra with intensity of ∼1% with respect to the G peak. We evaluated the effect of such impurities on electron transport by mimicking them with hydrogen adsorbates and measuring the induced changes in both mobility and Raman intensity. If the intervalley scatterers responsible for the D peak are monovalent, their concentration is sufficient to account for the limited mobilities currently achievable in graphene on a substrate.

Generating quantizing pseudomagnetic fields by bending graphene ribbons

Abstract: We analyze the mechanical deformations that are required to create uniform pseudomagnetic fields in graphene. It is shown that, if a ribbon is bent in-plane into a circular arc, this can lead to fields exceeding 10 T, which is sufficient for the observation of pseudo-Landau quantization. The arc geometry is simpler than those suggested previously and, in our opinion, has much better chances to be realized experimentally soon. The effects of a scalar potential induced by dilatation in this geometry is shown to be negligible.

Compression Behavior of Single-layer Graphene

Abstract: Central to most applications involving monolayer graphene is its mechanical response under various stress states. To date most of the work reported is of theoretical nature and refers to tension and compression loading of model graphene. Most of the experimental work is indeed limited to bending of single flakes in air and the stretching of flakes up to typically ~1% using plastic substrates. Recently we have shown that by employing a cantilever beam we can subject single graphene into various degrees of axial compression. Here we extend this work much further by measuring in detail both stress uptake and compression buckling strain in single flakes of different geometries. In all cases the mechanical response is monitored by simultaneous Raman measurements through the shift of either the G or 2D phonons of graphene. In spite of the infinitely small thickness of the monolayers, the results show that graphene embedded in plastic beams exhibit remarkable compression buckling strains. For large length (l)-to-width (w) ratios (> 0.2) the buckling strain is of the order of -0.5% to -0.6%. However, for l/w <0.2 no failure is observed for strains even higher than -1%. Calculations based on classical Euler analysis show that the buckling strain enhancement provided by the polymer lateral support is more than six orders of magnitude compared to suspended graphene in air.

Density of States and Zero Landau Level Probed through Capacitance of Graphene

Abstract: We report capacitors in which a finite electronic compressibility of graphene dominates the electrostatics, resulting in pronounced changes in capacitance as a function of magnetic field and carrier concentration. The capacitance measurements have allowed us to accurately map the density of states D, and compare it against theoretical predictions. Landau oscillations in D are robust and zero Landau level (LL) can easily be seen at room temperature in moderate fields. The broadening of LLs is strongly affected by charge inhomogeneity that leads to zero LL being broader than other levels.

On resonant scatterers as a factor limiting carrier mobility in graphene

Abstract: We show that graphene deposited on a substrate has a non-negligible density of atomic scale defects. This is evidenced by a previously unnoticed D peak in the Raman spectra with intensity of about 1% with respect to the G peak. We evaluated the effect of such impurities on electron transport by mimicking them with hydrogen adsorbates and measuring the induced changes in both mobility and Raman intensity. If the intervalley scatterers responsible for the D peak are monovalent, their concentration is sufficient to account for the limited mobilities achievable in graphene on a substrate.

Tearing Graphene Sheets From Adhesive Substrates Produces Tapered Nanoribbons

Abstract: Graphene is a truly two-dimensional atomic crystal with exceptional electronic and mechanical properties. Whereas conventional bulk and thinfilm materials have been studied extensively, the key mechanical properties of graphene, such as tearing and cracking, remain unknown,partly due to its two-dimensional nature and ultimate single-atom-layer thickness, which result in the breakdown of conventional material models. By combining first-principles ReaxFF molecular dynamics and experimental studies, a bottom-up investigation of the tearing of graphene sheets from adhesive substrates is reported, including the discovery of the formation of tapered graphene nanoribbons. Through a careful analysis of the underlying molecular rupture mechanisms, it is shown that the resulting nanoribbon geometry is controlled by both the graphene‐substrate adhesion energy and by the number of torn graphene layers. By considering graphene as a model material for a broader class of two-dimensional atomic crystals, these results provide fundamental insights into the tearing and cracking mechanisms of highly confined nanomaterials.

Graphene as a transparent conductive support for studying biological molecules by transmission electron microscopy

Abstract: We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any submicrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.

Graphene as a transparent conductive support for studying biological molecules by transmission electron microscopy

Abstract: We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any sub-micrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.

Hunting for Monolayer Boron Nitride: Optical and Raman Signatures

Abstract: We describe the identification of single- and few- layer boron nitride. Its optical contrast is much smaller than that of graphene but even monolayers are discernable by optimizing viewing conditions. Raman spectroscopy can be used to confirm BN monolayers. They exhibit an upshift in the fundamental Raman mode by up to 4 cm-1. The number of layers in thicker crystals can be counted by exploiting an integer-step increase in the Raman intensity and optical contrast.

From One Electron to One Hole: Quasiparticle Counting in Graphene Quantum Dots Determined by Electrochemical and Plasma Etching

Abstract: Graphene – a monolayer of carbon atoms packed into a hexagonal lattice – is widely considered to be a promising material for future electronics. Many unusual properties of this two-dimensional crystal originate from the linear, gapless spectrum of its quasiparticles. At the same time, many electronics applications require the presence of an energy gap. To this end, considerable efforts have been applied to create nanostructured devices out of graphene sheets (such as nanoribbons, quantum point contacts (QPC), single electron transistors, and quantum dots (QD)), in which a gap can be opened due to quantum confinement of the charge carriers. In most cases the formation of such graphene nanostructures relies on the removal of unwanted areas of graphene by reactive plasma etching (usually in oxygen plasma). The performance of such nanostructured devices is expected to depend strongly on the quality and chemical nature of the sample edges. Therefore, it is crucially

Direct determination of the crystallographic orientation of graphene edges by atomic resolution imaging

Abstract: In this letter, we show how high-resolution scanning tunneling microscopy (STM) imaging can be used to reveal that certain edges of micromechanically exfoliated single layer graphene crystals on silicon oxide follow either zigzag or armchair orientation. Using the cleavage technique, graphene flakes are obtained that very often show terminating edges seemingly following the crystallographic directions of the underlying honeycomb lattice. Performing atomic resolution STM-imaging on such flakes, we were able to directly prove this assumption. Raman imaging carried out on the same flakes further validated our findings.

Fluorinated graphene: Fluorographene: A Two-Dimensional Counterpart of Teflon (Small 24/2010)

Abstract: A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >1012Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Young’s modulus of 100 N m−1 and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.

Scanning probe lithography on graphene

Abstract: In this paper, we want to outline the principles of how scanning probe lithography on graphene is performed. We will show several examples of structures etched in graphene using this technique, including an example of a nanoelectronic device. In the last part, we present data regarding the oxidation kinetics when performing scanning probe lithography on graphite (HOPG). [GRAPHICS] Two lines oxidized in graphene using scanning probe lithography. The left line (indicated by the arrow) is only 30 nm wide, indicating that this technique has good enough resolution to create nanoelectronic device structures. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optics of flat carbon - spectroscopic ellipsometry of graphene flakes

Abstract: We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasi-particle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the K point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is downshifted by 0.5 eV probably due to excitonic effects.

Fluorographene: mechanically strong and thermally stable two-dimensional wide-gap semiconductor

Abstract: We report a stoichiometric derivative of graphene with a fluorine atom attached to each carbon. Raman, optical, structural, micromechanical and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >10^12 Ohm per square) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting Young's modulus of 100 N/m and sustaining strains of 15%. Fluorographene is inert and stable up to 400C even in air, similar to Teflon.

Preparation and characterisation of exfoliated graphene for quantum resistance metrology

Abstract: Exfoliated graphene samples have been prepared for use in quantum resistance metrology. Good progress is recently made in achieving contact resistances to graphene of less than 50 Ω. Details are presented on the handling and measurement of graphene samples.