Showing papers by "Andre K. Geim published in 2012"
TL;DR: Submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H2O permeates through the membranes at least 1010 times faster than He).
Abstract: Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H 2 O permeates through the membranes at least 10 10 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.
2,602 citations
TL;DR: A bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness is reported, which has potential for high-frequency operation and large-scale integration.
Abstract: An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively Such devices have potential for high-frequency operation and large-scale integration
2,401 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
01 Jan 2012
500 citations
TL;DR: Graphene bubbles are used to study the Raman spectrum of graphene under biaxial (e.g., isotropic) strain and the Gruneisen parameters are in excellent agreement with the theoretical values.
Abstract: We use graphene bubbles to study the Raman spectrum of graphene under biaxial (e.g., isotropic) strain. Our Gruneisen parameters are in excellent agreement with the theoretical values. Discrepancy in the previously reported values is attributed to the interaction of graphene with the substrate. Bilayer balloons (intentionally pressurized membranes) have been used to avoid the effect of the substrate and to study the dependence of strain on the interlayer interactions.
473 citations
TL;DR: In this paper, the Coulomb drag in the regime of weak to intermediate (d l) coupling was studied experimentally, where d is the interlayer separation and l is the characteristic distance between charge carriers, where the two Dirac liquids effectively nest within the same plane but can still be tuned and measured independently.
Abstract: C oulomb drag is a frictional coupling between electric currents flowing in spatially separated conducting layers. It is caused by interlayer electron‐electron interactions. Previously, only the regime of weak (d l) to intermediate (d l) coupling could be studied experimentally, where d is the interlayer separation and l is the characteristic distance between charge carriers. Here we use graphene‐boron-nitride heterostructures withd down to 1 nm to probe Coulomb drag in the limitd l such that the two Dirac liquids effectively nest within the same plane, but can still be tuned and measured independently. The strongly interacting regime reveals many unexpected features. In particular, although drag vanishes because of electron‐hole symmetry when either layer is neutral, we often find drag strongest when both layers are neutral. Under this circumstance, drag is positive in zero magnetic field but changes its sign and rapidly grows in strength with field. The drag remains strong at room temperature. The broken electron‐hole symmetry is attributed to mutual polarization of closely spaced interacting layers.
318 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
TL;DR: In this paper, an extensive investigation to figure out the origin of room temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly oriented pyrolytic graphite (HOPG) was conducted.
Abstract: We report on an extensive investigation to figure out the origin of room temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray microanalysis revealed the presence of micron-size magnetic clusters (predominantly Fe) that are rare and would be difficult to detect without careful search in a scanning electron microscope in the backscattering mode. The clusters pin to crystal boundaries and their quantities match the amplitude of typical ferromagnetic signals. No ferromagnetic response is detected in samples where we could not find such magnetic inclusions. Our experiments show that the frequently reported ferromagnetism in pristine HOPG is most likely to originate from contamination with Fe-rich inclusions introduced presumably during crystal growth.
55 citations
TL;DR: In this paper, an extensive investigation to figure out the origin of room-temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly oriented pyrolytic graphite (HOPG) was conducted.
Abstract: We report on an extensive investigation to figure out the origin of room-temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray microanalysis revealed the presence of micron-size magnetic clusters (predominantly Fe) that are rare and would be difficult to detect without careful search in a scanning electron microscope in the backscattering mode. The clusters pin to crystal boundaries and their quantities match the amplitude of typical ferromagnetic signals. No ferromagnetic response is detected in samples where we could not find such magnetic inclusions. Our experiments show that the frequently reported ferromagnetism in pristine HOPG is most likely to originate from contamination with Fe-rich inclusions introduced presumably during crystal growth.
50 citations
Patent•
22 Mar 2012TL;DR: In this article, the authors proposed an application related to graphene based heterostructures and transistor devices comprising graphene, which consisted of a first graphene layer, a spacer layer, and a third graphene layer.
Abstract: This application relates to graphene based heterostructures and transistor devices comprising graphene. The heterostructures comprise i) a first graphene layer; ii) a spacer layer and iii) a third graphene. The transistors comprise (i) an electrode, the electrode comprising a graphene layer, and (ii) an insulating barrier layer.
TL;DR: In this paper, the polarization-resolved Raman-scattering response due to E-2g phonons in monolayer graphene has been investigated in magnetic fields up to 29 T.
Abstract: The polarization-resolved Raman-scattering response due to E-2g phonons in monolayer graphene has been investigated in magnetic fields up to 29 T. The hybridization of the E-2g phonon is only observed with the fundamental inter-Landau-level excitation (involving the n = 0 Landau level) and in just one of the two configurations of the circularly cross-polarized excitation and scattered light. This polarization anisotropy of the magnetophonon resonance is shown to be inherent to relatively strongly doped graphene samples with carrier concentrations typical for graphene deposited on Si/SiO2 substrates.
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: Just one atom thick, graphene can be folded like plastic film, yet it is stronger than diamond and conducts electricity better than copper or gold as mentioned in this paper, and it can be traced back to the formation of the atom.
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.
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.
TL;DR: In this paper, the authors used graphene bubbles to study the Raman spectrum of graphene under biaxial (e.g. isotropic) strain and the results were in excellent agreement with the theoretical values.
Abstract: In this letter we use graphene bubbles to study the Raman spectrum of graphene under biaxial (e.g. isotropic) strain. Our Gruneisen parameters are in excellent agreement with the theoretical values. Discrepancy in the previously reported values is attributed to the interaction of graphene with the substrate. Bilayer balloons (intentionally pressurized membranes) have been used to avoid the effect of the substrate and to study the dependence of strain on the inter-layer interactions.
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.