Showing papers by "Kostya S. Novoselov published in 2012"
TL;DR: This work reviews recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.
Abstract: Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.
7,987 citations
TL;DR: A detailed analysis of the Raman spectra of graphene containing different type of defects is presented, finding that the intensity ratio of the D and D' peak is maximum for sp(3)-defects, it decreases for vacancy-like defects, and it reaches a minimum for boundaries in graphite.
Abstract: Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D′ peak is maximum (∼13) for sp3-defects, it decreases for vacancy-like defects (∼7), and it reaches a minimum for boundaries in graphite (∼3.5). This makes Raman Spectroscopy a powerful tool to fully characterize graphene.
1,716 citations
TL;DR: In this paper, the preparation and characterisation of different forms of graphene are reviewed and different techniques that have been employed to prepare graphene such as mechanical and solution exfoliation, and chemical vapour deposition are discussed briefly.
1,101 citations
TL;DR: In this paper, a cross sectional TEM view of several graphene and boron nitride heterostructures is presented, showing that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean.
Abstract: Heterostructures of very thin films have been used for decades in research and industry. Now a transmission electron microscopy study demonstrates the possibility of realizing perfect structures built by piling up one-atom-thick layers of graphene and boron nitride. By stacking various two-dimensional (2D) atomic crystals1 on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties2,3,4,5. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging6,7,8,9,10. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces. Here we employ cross sectional TEM to take a side view of several graphene–boron nitride heterostructures. We find that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean. Moreover, we observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene. This work proves the concept of heterostructures assembled with atomic layer precision and provides their first TEM images.
827 citations
TL;DR: The results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field and offers great potential for applications in tunnel devices and in field-effect transistors with ahigh carrier density in the conducting channel.
Abstract: We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.
812 citations
TL;DR: Graphene is just one example of a large class of two-dimensional crystals as mentioned in this paper, which can either be extracted from layered three-dimensional materials or grown artificially by several different methods.
Abstract: Graphene is just one example of a large class of two-dimensional crystals. These crystals can either be extracted from layered three-dimensional materials or grown artificially by several different methods. Furthermore, they present physical properties that are unique because of the low dimensionality and their special crystal structure. They have potential for semiconducting behavior, magnetism, superconductivity, and even more complex many-body phenomena. Two-dimensional crystals can also be assembled in three-dimensional heterostructures that do not exist in nature and have tailored properties, opening an entirely new chapter in condensed matter research.
314 citations
TL;DR: It is demonstrated that, in general, multilayer graphene will give rise to higher levels of reinforcement than monolayer material, with the optimum number of layers depending upon the separation of the graphene flakes in the nanocomposite.
Abstract: The stress transfer between the internal layers of multilayer graphene within polymer-based nanocomposites has been investigated from the stress-induced shifts of the 2D Raman band. This has been undertaken through the study of the deformation of an ideal composite system where the graphene flakes were placed upon the surface of a polymer beam and then coated with an epoxy polymer. It is found that the rate of band shift per unit strain for a monolayer graphene flake is virtually independent of whether it has one or two polymer interfaces (i.e., with or without an epoxy top coating). In contrast, the rate of band shift is lower for an uncoated bilayer specimen than a coated one, indicating relatively poor stress transfer between the graphene layers. Mapping of the strain in the coated bilayer regions has shown that there is strain continuity between adjacent monolayer and bilayer regions, indicating that they give rise to similar levels of reinforcement. Strain-induced Raman band shifts have also been eva...
273 citations
TL;DR: It is reported that boron atoms can be efficiently substituted for carbon in graphene and Boron-doped graphene appears to be a useful tool for engineering the physical and chemical properties of graphene.
Abstract: The introduction of foreign atoms, such as nitrogen, into the hexagonal network of an sp2-hybridized carbon atom monolayer has been demonstrated and constitutes an effective tool for tailoring the intrinsic properties of graphene. Here, we report that boron atoms can be efficiently substituted for carbon in graphene. Single-layer graphene substitutionally doped with boron was prepared by the mechanical exfoliation of boron-doped graphite. X-ray photoelectron spectroscopy demonstrated that the amount of substitutional boron in graphite was ∼0.22 atom %. Raman spectroscopy demonstrated that the boron atoms were spaced 4.76 nm apart in single-layer graphene. The 7-fold higher intensity of the D-band when compared to the G-band was explained by the elastically scattered photoexcited electrons by boron atoms before emitting a phonon. The frequency of the G-band in single-layer substitutionally boron-doped graphene was unchanged, which could be explained by the p-type boron doping (stiffening) counteracting the...
233 citations
TL;DR: In this paper, the authors describe electron transport in suspended devices with carrier mobilities of several 106 cm2 V1 s−1 and with the onset of Landau quantization occurring in fields below 5 mT.
Abstract: The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 106 cm2 V–1 s–1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as ≈108 cm–2, allowing a neutral state with a few charge carriers per entire micrometer-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap.
180 citations
Journal Article•
TL;DR: A correlated-electron phase was observed at low temperatures in suspended graphene bilayers with high carrier mobilities, which represents a new class of strongly correlated electronic ground states.
163 citations
TL;DR: This work presents the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension, providing an alternative route to induce the formation of a band gap.
Abstract: We present the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension. Apart from a purely mechanical behavior in flake regions where both layers are strained evenly, certain effects stem from inhomogeneous stress distribution across the layers. These phenomena such as the removal of inversion symmetry in bilayer graphene may have important implications in the band gap engineering, providing an alternative route to induce the formation of a band gap.
TL;DR: In this paper, a plasmonic nanoarray coupled to a single layered graphene sheet transferred on the top of the nanoarray is shown to enhance the resonant Raman scattering from a graphene layer generated by near fields of a PLASMIC nanoarray.
Abstract: We report properties of a plasmonic nanoarray coupled to a single layered graphene sheet transferred on the top of the nanoarray. We observe significant (up to 1000 times) enhancements of resonant Raman scattering from a graphene layer generated by near fields of a plasmonic nanoarray. This enhancement is quantitatively explained by the electromagnetic mechanism. We experimentally demonstrate a significant spectral shift of diffractive coupled plasmon resonances induced by the graphene presence and show that the collective plasmon resonances can be used to study surface chemistry of graphene. Our results emphasize prospective capabilities of graphene-functionalized plasmonics.
TL;DR: The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics, and an experimental answer is offered by describing electron transport in suspended devices with carrier mobilities of several 10(6) cm(2) V(-1) s(-1), allowing a neutral state with a few charge carriers per entire micrometer-scale device.
Abstract: The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state with a few charge carriers per entire micron-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap.
Patent•
22 Mar 2012TL;DR: In this article, the authors present an application related to graphene based heterostructures and methods of making graphene-based HetNets, which include a first encapsulation layer, a second one, and a third one.
Abstract: This application relates to graphene based heterostructures and methods of making graphene based heterostructures. The graphene heterostructures comprise: i) a first encapsulation layer; ii) a second encapsulation layer; and iii) a graphene layer. The heterostructures find application in electronic devices.
TL;DR: In this article, a side view of several graphene-boron nitride heterostructures was taken using cross sectional transmission electron microscopy (TEM) to take a view of the interface roughness and electronic quality of encapsulated graphene.
Abstract: By stacking various two-dimensional (2D) atomic crystals [1] on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties [2-5]. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging [6-10]. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces. Here we employ cross sectional TEM to take a side view of several graphene-boron nitride heterostructures. We find that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean. Moreover, we observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene. This work proves the concept of heterostructures assembled with atomic layer precision and provides their first TEM images.
TL;DR: In this paper, a scanning gate microscopy (SGM) measurements on a graphene point contact was carried out under ambient conditions, which showed that the resistance of the point contact is strongly affected by the electric field of the tip at the position of the QPC.
Abstract: In this paper, we present scanning gate microscopy (SGM) measurements on a graphene quantum point contact, carried out under ambient conditions. Images obtained in SGM display two-dimensional maps of sample resistance, as a function of the position of a biased AFM tip and the voltage applied to the tip. In our studies, a graphene quantum point contact was fabricated using local anodic oxidation. Performing SGM imaging on the resulting device structure shows that the resistance through the graphene QPC is strongly affected by the electric field of the AFM tip at the position of the QPC.
TL;DR: In this paper, the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension is presented, showing that the removal of inversion symmetry may have important implications in the band gap engineering providing an alternative route to induce the formation of a band-gap.
Abstract: We present the first Raman spectroscopic study of Bernal bilayer graphene flakes under uniaxial tension. Apart from a purely mechanical behavior in flake regions where both layers are strained evenly, certain effects stem from inhomogeneous stress distribution across the layers. These phenomena such as the removal of inversion symmetry in bilayer graphene may have important implications in the band-gap engineering providing an alternative route to induce the formation of a band-gap.
01 Jan 2012
TL;DR: In this article, the mechanical response of single layer graphene is monitored by simultaneous Raman measurements through the shift of either the G or 2D optical phonons, for low levels of tensile and compressive strain.
Abstract: The mechanical response of single layer graphene is monitored by simultaneous Raman measurements through the shift of either the G or 2D optical phonons, for low levels of tensile and compressive strain. In tension, important physical phenomena such as the G and 2D band splitting are discussed. The results can be used to quantify the amount of uniaxial strain, providing a fundamental tool for graphene-based nanoelectronics. In compression, graphenes of atomic thickness embedded in plastic beams are found to exhibit remarkable high compression failure strains. The critical buckling strain for graphene appears to be dependent on the flake size and geometry with respect to the strain axis. It is shown that the embedded flakes can be treated as ideal plates and their behavior can be described by Euler mechanics.
28 Jan 2012
TL;DR: In this paper, the authors have published more than 20 research papers including 2 Science papers, 4 papers in Nature Physics and Nature Communications and 5 Phys. Rev. Letters, and other high-quality research journals.
Abstract: : During the last year, we have published more than 20 research papers including 2 Science papers, 4 papers in Nature Physics and Nature Communications and 5 Phys. Rev. Letters. The most important technological result probably was the development of fabrication procedures to encapsulate graphene between boron-nitride crystals, which allows us to routinely achieve mobilities above 100,000 cm2/Vs and demonstrate room-temperature ballistic transport at micron scale (Nano Lett. 11, 2396, 2011). This development also led to the first double-layer graphene heterostructures, in which we reported interesting interaction phenomena (Nature Phys. online 2011) and which continue to be in the focus of our attention offering a wealth of new physics and potential applications. Interaction phenomena have also been studied in suspended devices made from graphene and its bilayer (Science 333, 860, 2011; Nature Phys. 7, 701, 2011) and by using the nonlocal geometry (Science 332, 328, 2011). As a result we can now routinely make and investigate complex graphene-BN heterostructures with mobilities 10 times higher than for graphene on the standard Si substrates. For suspended graphene devices, we achieve mobilities well above a million, that is, 100 times higher than for graphene on a Si substrate. Another important development over the last year was the demonstration of a graphene-based derivative, fluorographene that is a two-dimensional version of Teflon (Small 6, 2877, 2010). In all these publications, the PIs have gratefully acknowledged the AFOSR support. As concerns the entire 3-year project, it resulted in 4 Science research papers and dozens of reports in Nature series magazines, Phys. Rev. Letters, Nano Letters and other high-quality research journals. Despite the short reported period, our research has already caused very high impact that can be quantified by the number of citations for the papers supported by the AFOSR grant.
TL;DR: In this paper, the results of the study of graphene nanoablation under mechanical stress of an ultrasharp tip of a scanning probe microscope (SPM) were presented, and it was found that the SPM probe contact with graphene results in average removal of 7 · 10−3−5 · 10 −2 nm of film per scan, i.e., only a few carbon atoms or clusters, in the impact area.
Abstract: We present the results of the study of graphene nanoablation under mechanical stress of an ultrasharp (the rounding radius is ∼2 nm) tip of a scanning probe microscope (SPM)). It was found that the SPM probe contact with graphene results in average removal of 7 · 10−3−5 · 10−2 nm of film per scan, i.e., only a few carbon atoms or clusters, in the impact area. The capability of this precision nanoablation process was shown in developing graphene nanoislands and nanoribbons ∼1 µm long and ∼10 nm wide.
01 Jan 2012
TL;DR: In this paper, the authors used the Raman G peak to assess the response to strain carbon fibres with reference to graphene itself and derived a universal value of average phonon shift rate with axial stress, where is the G peak position at zero stress.
Abstract: Carbon fibres (CF) represent a significant volume fraction of modern structural airframes. Embedded into polymer matrices, they provide significant strength and stiffness gains over unit weight as compared to other competing structural materials. Most CF are known to be fully turbostratic, consisting of graphene layers slipped sideways relative to each other, which leads to an inter-graphene distance much greater than graphite. Here, we use the Raman G peak to assess the response to strain CF with reference to graphene itself [1]. Our results highlight the predominance of the in-plane graphene properties in all complex graphitic structures examined. A universal master plot relating the G peak strain sensitivity to tensile modulus of all types of CF, as well as graphene, is presented. We derive a universal value of average phonon shift rate with axial stress , where is the G peak position at zero stress, for both graphene and CF with annular morphology [1]. The use of this for stress measurements in a variety of applications is discussed.
02 Jul 2012
TL;DR: The structural and topographic properties of doped boron-nitride have been studied by aberration corrected scanning transmission electron microscopy (STEM) as mentioned in this paper, which revealed that metal atoms cluster preferentially in/on contaminated areas.
Abstract: Metal impurities, gold and nickel, have been deliberately introduced into boron-nitride (BN) sheets. The structural and topographic properties of doped BN have been studied by aberration corrected scanning transmission electron microscopy (STEM). Analysis revealed that metal atoms cluster preferentially in/on contaminated areas. The metal coverage on BN is almost the same for the same evaporated amount of 1 A.
02 Jul 2012
TL;DR: In this article, the structural and topographic properties of doped graphene have been studied by using conventional transmission electron and aberration corrected scanning transmission electron microscopy, which revealed that metal atoms cluster preferentially in/on contaminated areas.
Abstract: Metal impurities, gold, chromium and titanium, have been deliberately introduced into graphene. The structural and topographic properties of doped graphene have then been studied by using conventional transmission electron and aberration corrected scanning transmission electron microscopy. Analysis revealed that metal atoms cluster preferentially in/on contaminated areas. Contrarily to observations that gold atoms do not adhere to clean patches of monolayer graphene, chromium and titanium were found to be more reactive with clean monolayer graphene, and the coverage was higher than for gold for the same evaporated amount.
06 May 2012
TL;DR: In this article, the relaxation mechanism mediated by phonons has been extensively investigated, while the initial stages, ruled by fundamental electron-electron (e-e) interactions still pose a challenge.
Abstract: The impulsive optical excitation of carriers in graphene creates an out-of-equilibrium distribution, which thermalizes on an ultrafast timescale [1-4]. This hot Fermi-Dirac (FD) distribution subsequently cools via phonon emission within few hundreds of femtoseconds. While the relaxation mechanisms mediated by phonons have been extensively investigated, the initial stages, ruled by fundamental electron-electron (e-e) interactions still pose a challenge.