We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in concentrations up to 10(13) per square centimeter and with room-temperature mobilities of approximately 10,000 square centimeters per volt-second can be induced by applying gate voltage.

Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmology and from astrophysics to quantum chemistry. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic Schrödinger equation. Here we report an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective 'speed of light' c* approximately 10(6) m s(-1). Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have observed the following: first, graphene's conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when concentrations of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass m(c) of massless carriers in graphene is described by E = m(c)c*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top experiment.

We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.graphene ͉ layered material

The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

Graphene -a monolayer of carbon atoms densely packed into a hexagonal lattice (1)-

We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Our [3,7]. Both sources of disorder can in principle be eliminated or reduced significantly. There are however intrinsic scatterers such as phonons that cannot be eliminated at room T and, therefore, set a fundamental limit on electronic quality and possible performance of graphene-based devices. How high is the intrinsic mobility μ in for graphene at 300K? This is one of the most important figures of merit for any electronic material but it has remained unknown.In this Letter, we show that electron-phonon scattering in graphene and its bilayer is so weak that, if the extrinsic disorder is eliminated, room-T mobilities ∼200,000 cm 2 /Vs are expected over a technologically relevant range of carrier concentration n. This value exceeds μ in known for any other semiconductor [8]. In particular, our measurements show that away from the neutrality point (NP) resistivity ρ of SLG has two components: in addition to the well-documented contribution ρ L =1/neμ due to long-range disorder [6,7], we have identified a small but notable n-independent resistivity ρ S indicating the presence of short-range scatterers [6,7,9]. We have also found that ρ L does not depend on T below 300K, whereas ρ S exhibits a sharp rise above ∼200K [10]. The latter contradicts to the existing theories [11] that expect a linear T dependence. We attribute this behavior to flexural (out-of-plane) phonons [12] that are excited inside ripples. Bilayer graphene (BLG) samples exhibited no discernible T dependence of μ away from NP, yielding even higher μ in . These findings provide an important benchmark for the research area and indicate that μ in graphene systems can be orders of magnitude higher than the values achieved so far. The reported measurements are also important for narrowing dominant scattering mechanisms in graphene, which remain hotly debated [3][4][5][6][7]11,13]. The studied devices were prepared from graphene obtained by micromechanical cleavage of graphite on top of an oxidized Si wafer (usually, 300nm of SiO 2 ) [14]. Single-and bi-layer crystallites were initially identified by their optical contrast [15], verified in some cases by Raman and atomic-force microscopy [2,14,16] and always crosschecked by measurements in high magnetic fields B, where SLG and BLG exhibited two distinct types of the quantum Hall effect [2,17]. To improve homogeneity, our standard Hall bar devices [1][2][3] were annealed at 200C in a H 2 -Ar mixture [18] and, then, inside a measurement cryostat at 400 K in He. To avoid accidental breakdown, gate voltages V g were limited to ±50V (n ∝ αV g with α ≈7.2x10

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.

The isolation of various two-dimensional (2D) materials, and the possibility to combine them in vertical stacks, has created a new paradigm in materials science: heterostructures based on 2D crystals. Such a concept has already proven fruitful for a number of electronic applications in the area of ultrathin and flexible devices. Here, we expand the range of such structures to photoactive ones by using semiconducting transition metal dichalcogenides (TMDCs)/graphene stacks. Van Hove singularities in the electronic density of states of TMDC guarantees enhanced light-matter interactions, leading to enhanced photon absorption and electron-hole creation (which are collected in transparent graphene electrodes). This allows development of extremely efficient flexible photovoltaic devices with photoresponsivity above 0.1 ampere per watt (corresponding to an external quantum efficiency of above 30%).