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Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It is also often presumed that individual GO monolayers contain a large density of nanoscale pinholes that lead to considerable gas leakage across the GO basal plane. Here we show that relatively large, micrometer-scale areas of monolayer GO are impermeable to gases, including helium, while exhibiting proton conductivity through the basal plane which is nearly two orders of magnitude higher than that of graphene. These findings provide insights into the key properties of GO and demonstrate that chemical functionalization of 2D crystals can be utilized to enhance their proton transparency without compromising gas impermeability.

Development of stable room-temperature bright single-photon emitters using atomic defects in hexagonal-boron nitride flakes (h-BN) provides significant promises for quantum technologies. However, an outstanding challenge in h-BN is creating site-specific, stable, high emission rate single photon emitters with very low Huang-Rhys (HR) factor. Here, we discuss the photonic properties of site-specific, isolated, stable quantum emitter that emit single photons with a high emission rate and unprecedented low HR value of 0.6 at room temperature. Scanning confocal image confirms site-specific single photon emitter with a prominent zero-phonon line at ~578 nm with saturation photon counts of 105 counts/second. The second-order intensity-intensity correlation measurement shows an anti-bunching dip of ~0.25 with an emission lifetime of 2.46 ns. Low-energy electron beam irradiation and subsequent annealing are important to achieve stable single photon emitters.

Intelligent transport of molecular species across different barriers is critical for various biological functions and is achieved through the unique properties of biological membranes. An essential feature of intelligent transport is the ability to adapt to different external and internal conditions and also the ability to memorise the previous state. In biological systems, the most common form of such intelligence is expressed as hysteresis. Despite numerous advances made over previous decades on smart membranes, it is still a challenge for a synthetic membrane to display stable hysteretic behaviour for molecular transport. Here we show the memory effects and stimuli regulated transport of molecules through an intelligent phase changing MoS$_2$ membrane in response to external pH. We show that water and ion permeation through 1T' MoS$_2$ membranes follows a pH dependent hysteresis with a permeation rate that switches by a few orders of magnitude. We demonstrate that this phenomenon is unique to the 1T' phase of MoS$_2$ due to the presence of surface charge and exchangeable ions on the surface. We further demonstrate the potential application of this phenomenon in autonomous wound infection monitoring and pH-dependent nanofiltration. Our work significantly deepens understanding of the mechanism of water transport at the nanoscale and opens an avenue for developing neuromorphic applications, smart drug delivery systems, point-of-care diagnostics, smart sensors, and intelligent filtration devices.

Organic electrochemical transistors (OECTs) based on Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) are a benchmark system in organic bioelectronics. In particular, the superior mechanical properties and the ionic-electronic transduction yield excellent potential for the field of implantable or wearable sensing technology. However, depletion-mode operation PEDOT:PSS-based OECTs cause high static power dissipation in electronic circuits, limiting their application in electronic systems. Hence, having control over the threshold voltage is of utmost technological importance. Here we demonstrate PEDOT:PSS-based dual-gate OECTs with solid-state electrolyte where the threshold voltage is seamlessly adjustable during operation. We show that the degree of threshold voltage tuning linearly depends on the gate capacitance, which is a straightforward approach for circuit designers to adjust the threshold voltage only by the device dimensions. The PEDOT:PSS-based dual-gate OECTs show excellent device performance and can be pushed to accumulation-mode operation, resulting in a simplified and relaxed design of complementary inverters.

Van der Waals (vdW) heterostructures continue to attract intense interest as a route of designing materials with novel properties that cannot be found in naturally occurring materials. Unfortunately, this approach is currently limited to only a few layers that can be stacked on top of each other. Here we report a bulk material consisting of superconducting monolayers interlayered with monolayers displaying charge density waves (CDW). This bulk vdW heterostructure is created by phase transition of 1T-TaS2 to 6R at 800 \degC in an inert atmosphere. Electron microscopy analysis directly shows the presence of alternating 1T and 1H monolayers within the resulting bulk 6R phase. Its superconducting transition (Tc) is found at 2.6 K, exceeding the Tc of the bulk 2H phase of TaS2. The superconducting temperature can be further increased to 3.6 K by exfoliating 6R-TaS2 and then restacking its layers. Using first-principles calculations, we argue that the coexistence of superconductivity and CDW within 6R-TaS2 stems from amalgamation of the properties of adjacent 1H and 1T monolayers, where the former dominates the superconducting state and the latter the CDW behavior.

A density-functional-theory based relativistic scattering formalism is used to study charge transport through thin Pt films with room temperature lattice disorder. A Fuchs-Sondheimer specularity coefficient $p \sim 0.5$ is needed to describe the suppression of the charge current at the surface even in the absence of surface roughness. The charge current drives a spin Hall current perpendicular to the surface. Analysing the latter with a model that is universally used to interpret the spin Hall effect in thin films and layered materials, we are unable to recover values of the spin-flip diffusion length $l_{\rm sf}$ and spin Hall angle $\Theta_{\rm sH}$ that we obtain for bulk Pt using the same approximations. We trace this to the boundary conditions used and develop a generalized model that takes surface effects into account. A reduced value of $\Theta_{\rm sH}$ at the surface is then found to describe the first-principles transport results extremely well. The in-plane spin Hall effect is substantially enhanced at the surface.

Little is known about the spin-flip diffusion length $l_{\rm sf}$, one of the most important material parameters in the field of spintronics. We use a density-functional-theory based scattering approach to determine values of $l_{\rm sf}$ that result from electron-phonon scattering as a function of temperature for all 5d transition metal elements. $l_{\rm sf}$ does not decrease monotonically with the atomic number Z but is found to be inversely proportional to the density of states at the Fermi level. By using the same local current methodology to calculate the spin Hall angle $\Theta_{\rm sH}$ that characterizes the efficiency of the spin Hall effect, we show that the products $\rho(T)l_{\rm sf}(T)$ and $\Theta_{\rm sH}(T)l_{\rm sf}(T)$ are constant.

We extract local current distributions from interatomic currents calculated using a fully relativistic quantum mechanical scattering formalism by interpolation onto a three-dimensional grid. The method is illustrated with calculations for Pt$|$Ir and Pt$|$Au multilayers as well as for thin films of Pt and Au that include temperature-dependent lattice disorder. The current flow is studied in the "classical" and "Knudsen" limits determined by the sample thickness relative to the mean free path $\lambda$, introducing current streamlines to visualize the results. For periodic multilayers, our results in the classical limit reveal that transport inside a metal can be described using a single value of resistivity $\rho$ combined with a linear variation of $\rho$ at the interface while the Knudsen limit indicates a strong spatial dependence of $\rho$ inside a metal and an anomalous dip of the current density at the interface which is accentuated in a region where transient shunting persists.

Experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals has been missing until a bilayer Cr2Ge2Te6, and a three-atom thick monolayer CrI3 are shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals type ultra-thin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111) oriented sheets are predominant and is supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111) oriented three-atom thick pyrite sheet has a stable ferromagnetic ground state different from its diamagnetic bulk counterpart. This theoretical finding is evaluated experimentally employing low temperature superconducting quantum interference device measurements and observed an anomalous ferromagnetic kind of behavior.

The surface free energy is one of the most fundamental properties of solids, hence, manipulating the surface energy and thereby the wetting properties of solids, has tremendous potential for various physical, chemical, biological as well as industrial processes. Typically, this is achieved by either chemical modification or by controlling the hierarchical structures of surfaces. Here we report a phenomenon whereby the wetting properties of vermiculite laminates are controlled by the hydrated cations on the surface and in the interlamellar space. We find that by exploiting this mechanism, vermiculite laminates can be tuned from superhydrophillic to hydrophobic simply by exchanging the cations; hydrophilicity decreases with increasing cation hydration free energy, except for lithium. Lithium, which has a higher hydration free energy than potassium, is found to provide a superhydrophilic surface due to its anomalous hydrated structure at the vermiculite surface. Building on these findings, we demonstrate the potential application of superhydrophilic lithium exchanged vermiculite as a thin coating layer on microfiltration membranes to resist fouling, and thus, we address a major challenge for oil-water separation technology.

Most of the studied two-dimensional (2D) materials have been obtained by exfoliation of van der Waals crystals. Recently, there has been growing interest in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum. Here, we report an approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we use 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF3), which has a non-layered, rhombohedral structure and therefore cannot be possibly obtained by exfoliation. The conversion of InSe into InF3 is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF3 layers are doped with selenium. We study this new 2D material by optical, electron transport and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation into InF3 films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently-bonded non-cleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals.

The pH dependence of emission from graphene oxide is believed to be due to the protonation of surface functional groups. In this study we use transient absorption spectroscopy to study the sub-picosecond charge dynamics in graphene oxide over a range of pH values, observing dynamics consistent with an excited state protonation step for pH < 9.3. The timescale of this process is ~ 1.5 ps, and a corresponding change in recombination dynamics follows. A broad photo-induced absorption peak centred at 530 nm associated with excited state protonation is also observed.

Developing 'smart' membranes that allow precise and reversible control of molecular permeation using external stimuli would be of intense interest for many areas of science: from physics and chemistry to life-sciences. In particular, electrical control of water permeation through membranes is a long-sought objective and is of crucial importance for healthcare and related areas. Currently, such adjustable membranes are limited to the modulation of wetting of the membranes and controlled ion transport, but not the controlled mass flow of water. Despite intensive theoretical work yielding conflicting results, the experimental realisation of electrically controlled water permeation has not yet been achieved. Here we report electrically controlled water permeation through micrometre-thick graphene oxide (GO) membranes. By controllable electric breakdown, conductive filaments are created in the GO membrane. The electric field concentrated around such current carrying filaments leads to controllable ionisation of water molecules in graphene capillaries, allowing precise control of water permeation: from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies and can revolutionize the field of artificial biological systems, tissue engineering and filtration.

Graphene oxide relative dielectric permittivity, both its real and imaginary parts, have been measured under various humidity conditions at GHz. It is demonstrated that the relative dielectric permittivity increases with increasing humidity due to water uptake. This electrical property of graphene oxide was used to create a battery-free wireless radio-frequency identification (RFID) humidity sensor by coating printed graphene antenna with the graphene oxide layer. The resonance frequency as well as the backscattering phase of such graphene oxide/graphene antenna become sensitive to the surrounding humidity and can be detected by the RFID reader. This enables batteryless wireless monitoring of the local humidity with digital identification attached to any location or item and paves the way for low-cost efficient sensors for Internet of Things applications.

Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation rates. However, the membranes' use has been limited mostly to aqueous solutions because GO membranes appear to be impermeable to organic solvents, a phenomenon not fully understood yet. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from flakes with large sizes of ~ 10-20 micron. Without sacrificing their sieving characteristics, such membranes can be made exceptionally thin, down to ~ 10 nm, which translates into fast permeation of not only water but also organic solvents. We attribute the organic solvent permeation and sieving properties of ultrathin GO laminates to the presence of randomly distributed pinholes that are interconnected by short graphene channels with a width of 1 nm. With increasing the membrane thickness, the organic solvent permeation rates decay exponentially but water continues to permeate fast, in agreement with previous reports. The application potential of our ultrathin laminates for organic-solvent nanofiltration is demonstrated by showing >99.9% rejection of various organic dyes with small molecular weights dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification, filtration and related technologies.

It has been an ultimate but seemingly distant goal of nanofluidics to controllably fabricate capillaries with dimensions approaching the size of small ions and water molecules. We report ion transport through ultimately narrow slits that are fabricated by effectively removing a single atomic plane from a bulk crystal. The atomically flat angstrom-scale slits exhibit little surface charge, allowing elucidation of the role of steric effects. We find that ions with hydrated diameters larger than the slit size can still permeate through, albeit with reduced mobility. The confinement also leads to a notable asymmetry between anions and cations of the same diameter. Our results provide a platform for studying effects of angstrom-scale confinement, which is important for development of nanofluidics, molecular separation and other nanoscale technologies.

Graphene oxide membranes show exceptional molecular permeation properties, with a promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ~9 Angstrom, which is larger than hydrated ion diameters for common salts. The cutoff is determined by the interlayer spacing d ~13.5 Angstrom, typical for graphene oxide laminates that swell in water. Achieving smaller d for the laminates immersed in water has proved to be a challenge.Here we describe how to control d by physical confinement and achieve accurate and tuneable ion sieving. Membranes with d from ~ 9.8 Angstrom to 6.4 Angstrom are demonstrated, providing the sieve size smaller than typical ions' hydrated diameters.In this regime, ion permeation is found to be thermally activated with energy barriers of ~10-100 kJ/mol depending on d. Importantly, permeation rates decrease exponentially with decreasing the sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for water molecules entry and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.

Van der Waals assembly of two-dimensional (2D) crystals continue attract intense interest due to the prospect of designing novel materials with on-demand properties. One of the unique features of this technology is the possibility of trapping molecules or compounds between 2D crystals. The trapped molecules are predicted to experience pressures as high as 1 GPa. Here we report measurements of this interfacial pressure by capturing pressure-sensitive molecules and studying their structural and conformational changes. Pressures of 1.2 +/- 0.3 GPa are found using Raman spectrometry for molecular layers of one nanometer in thickness. We further show that this pressure can induce chemical reactions and several trapped salts or compounds are found to react with water at room temperature, leading to 2D crystals of the corresponding oxides. This pressure and its effect should be taken into account in studies of van der Waals heterostructures and can also be exploited to modify materials confined at the atomic interfaces.

Graphene, a zero-gap semimetal, can be transformed into a metallic, semiconducting or insulating state by either physical or chemical modification. Superconductivity is conspicuously missing among these states despite considerable experimental efforts as well as many theoretical proposals. Here, we report superconductivity in calcium-decorated graphene achieved by intercalation of graphene laminates that consist of well separated and electronically decoupled graphene crystals. In contrast to intercalated graphite, we find that Ca is the only dopant that induces superconductivity in graphene laminates above 1.8 K among intercalants used in our experiments such as potassium, caesium and lithium. Ca-decorated graphene becomes superconducting at ~ 6 K and the transition temperature is found to be strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration. In addition to the first evidence for superconducting graphene, our work shows a possibility of inducing and studying superconductivity in other 2D materials using their laminates.

Adsorbed layers of water are ubiquitously present at surfaces and fill in microscopic pores, playing a central role in many phenomena in such diverse fields as materials science, geology, biology, tribology, nanotechnology. Despite such importance, the crystal structure of nanoconfined water remains largely unknown. Here we report high-resolution electron microscopy of mono- and few- layers of water confined between two graphene sheets, an archetypal example of hydrophobic confinement. Confined water is found to form square ice at room temperature - a phase with symmetry principally different from the conventional tetrahedral geometry of hydrogen bonding. The square ice has a high packing density with a lattice constant of 2.83 angstrom and during TEM observation assembles in bi- and tri- layer crystallites exhibiting AA stacking. Our findings are important for understanding of interfacial phenomena and, in particular, shed light on ultrafast transport of water through hydrophobic nanocapillaries. Our MD simulations show that square ice is likely to be common inside hydrophobic nanochannels, independent of their exact atomic makeup.

Graphene is impermeable to all gases and liquids, and even such a small atom as hydrogen is not expected to penetrate through graphene's dense electronic cloud within billions of years. Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of about 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10^-3 Ohm per cm2 above 250 C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen-based technologies.

One of the most desirable goals of graphene research is to produce ordered 2D chemical derivatives of suitable quality for monolayer device fabrication. Here we reveal, by focal series exit wave reconstruction, that C2F chair is a stable graphene derivative and demonstrates pristine long-range order limited only by the size of a functionalized domain. Focal series of images of graphene and C2F chair formed by reaction with XeF2 were obtained at 80 kV in an aberration-corrected transmission electron microscope. EWR images reveal that single carbon atoms and carbon-fluorine pairs in C2F chair alternate strictly over domain sizes of at least 150 nm^2 with electron diffraction indicating ordered domains >/= 0.16 square micrometer. Our results also indicate that, within an ordered domain, functionalization occurs on one side only as theory predicts. Additionally we show that electron diffraction provides a quick and easy method for distinguishing between graphene, C2F chair and fully fluorinated stoichiometric CF 2D phases.

Barrier films preventing permeation of gases and moistures are important for many industries ranging from food to medical and from chemical to electronic. From this perspective, graphene has recently attracted particular interest because its defect free monolayers are impermeable to all gases and liquids. However, it has proved challenging to develop large-area defectless graphene films suitable for industrial use. Here we report barrier properties of multilayer graphitic films made by chemical reduction of easily and cheaply produced graphene oxide laminates. They are found to provide a practically perfect barrier that blocks all gases, liquids and aggressive chemicals including, for example, hydrofluoric acid. In particular, if graphene oxide laminates are reduced in hydroiodic acid, no permeation of hydrogen and water could be detected for films as thin as 30 nm, which remain optically transparent. The films thicker than 100 nm become completely impermeable. The exceptional barrier properties are attributed to a high degree of graphitization of the laminates and little structural damage during reduction. This work indicates a close prospect of thin protective coatings with stability and inertness similar to that of graphene and bulk graphite, which can be interesting for numerous applications.

Irradiation with high-energy ions has been widely suggested as a tool to engineer properties of graphene. Experiments show that it indeed has a strong effect on its transport, magnetic and mechanical characteristics. However, to use ion irradiation as an engineering tool requires understanding of the type and detailed characteristics of the produced defects which is still lacking, as the use of high-resolution transmission electron microscopy (HRTEM) - the only technique allowing direct imaging of atomic-scale defects - often modifies or even creates defects during imaging, thus making it impossible to determine the intrinsic atomic structure. Here we show that encapsulating the studied graphene sample between two other (protective) graphene sheets allows non-invasive HRTEM imaging and reliable identification of atomic-scale defects. Using this simple technique, we demonstrate that proton irradiation of graphene produces reconstructed monovacancies, which explains the profound effect that such defects have on magnetic and transport properties. This finding resolves the existing uncertainty with regard to the effect of ion irradiation on the electronic structure of graphene.

There has been intense interest in filtration and separation properties of graphene-based materials that can have well-defined nanometer pores and exhibit low frictional water flow inside them. Here we investigate molecular permeation through graphene oxide laminates. They are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves blocking all solutes with hydrated radii larger than 4.5A. Smaller ions permeate through the membranes with little impedance, many orders of magnitude faster than the diffusion mechanism can account for. We explain this behavior by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The ultrafast separation of small salts is attributed to an 'ion sponge' effect that results in highly concentrated salt solutions inside graphene capillaries.

A possibility to control magnetic properties by using electric fields is one of the most desirable characteristics for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and offers a hope due to its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that adatoms magnetism in graphene is itinerant and can be controlled by doping, so that magnetic moments can be switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions: one coming from the same itinerant magnetism and the other due to broken bonds. Our work suggests that magnetic response of graphene can be controlled by the field effect, similar to its transport and optical properties, and that spin diffusion length can be significantly enhanced above a certain carrier density.

Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic, and nanophotonic materials. The interaction of light with carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools via phonon emission. Here we combine pump-probe spectroscopy, featuring extreme temporal resolution and broad spectral coverage, with a microscopic theory based on the quantum Boltzmann equation, to investigate electron-electron collisions in graphene during the very early stages of relaxation. We identify the fundamental physical mechanisms controlling the ultrafast dynamics in graphene, in particular the significant role of ultrafast collinear scattering, enabling Auger processes, including charge multiplication, key to improving photovoltage generation and photodetectors.

Polarization resolved, Raman scattering response due to E$_{2g}$ phonon in monolayer graphene has been investigated in magnetic fields up to 29 T. The hybridization of the E$_{2g}$ phonon with only the fundamental inter Landau level excitation (involving the n=0 Landau level) is observed and only in one of the two configurations of the circularly crossed polarized excitation and scattered light. This polarization anisotropy of the magneto-phonon resonance is shown to be inherent to relatively strongly doped graphene samples, with carrier concentration typical for graphene deposited on SiO$_2$.

This article is a reply to the Comment by D. Spemann et al (arXiv:1204.2992) in response to our paper 'Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite' (EPL, 97 (2012) 47001).

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.

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.

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 submicron-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors and gases, including helium, but allow unimpeded permeation of water (H2O 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.

Using magnetization measurements, we show that point defects in graphene - fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene's structural stability.

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.

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3 degrees the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20 degrees the layers effectively decouple and the electronic properties are indistinguishable from those in single layer graphene, while for smaller angles we observe a slow-down of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist induced Van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

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.

We have studied magnetization of graphene nanocrystals obtained by sonic exfoliation of graphite. No ferromagnetism is detected at any temperature down to 2 K. Neither do we find strong paramagnetism expected due to the massive amount of edge defects. Rather, graphene is strongly diamagnetic, similar to graphite. Our nanocrystals exhibit only a weak paramagnetic contribution noticeable below 50K. The measurements yield a single species of defects responsible for the paramagnetism, with approximately one magnetic moment per typical graphene crystallite.

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.

Local laser excitation and temperature readout from the intensity ratio of Stokes to anti-Stokes Raman scattering signals are employed to study the thermal properties of a large graphene membrane. The concluded value of the heat conductivity coefficient \kappa ~ 600 W/m ⋅K is smaller than previously reported but still validates the conclusion that graphene is a very good thermal conductor.

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.

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.

We show that strong photoluminescence (PL) can be induced in single-layer graphene on using an oxygen plasma treatment. PL characteristics are spatially uniform across the flakes and connected to elastic scattering spectra distinctly different from those of gapless pristine graphene. Oxygen plasma can be used to selectively convert the topmost layer when multi-layer samples are treated

Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. Since its experimental discovery, graphene continues to attract enormous interest, in particular as a new kind of matter, in which electron transport is governed by a Dirac-like wave equation, and as a model system for studying electronic and phonon properties of other, more complex, graphitic materials[1-4]. Here, we uncover the constitutive relation of graphene and probe new physics of its optical phonons, by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E2g optical mode splits in two components, one polarized along the strain and the other perpendicular to it. This leads to the splitting of the G peak into two bands, which we call G+ and G-, by analogy with the effect of curvature on the nanotube G peak[5-7]. Both peaks red shift with increasing strain, and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The singly degenerate 2D and 2D' bands also red shift, but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D and D' peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance[8, 9]

Graphene - a monolayer of carbon atoms densely packed into a hexagonal lattice - has one of the strongest possible atomic bonds and can be viewed as a robust atomic-scale scaffold, to which other chemical species can be attached without destroying it. This notion of graphene as a giant flat molecule that can be altered chemically is supported by the observation of so-called graphene oxide, that is graphene densely covered with hydroxyl and other groups. Unfortunately, graphene oxide is strongly disordered, poorly conductive and difficult to reduce to the original state. Nevertheless, one can imagine atoms or molecules being attached to the atomic scaffold in a strictly periodic manner, which should result in a different electronic structure and, essentially, a different crystalline material. A hypothetical example for this is graphane, a wide-gap semiconductor, in which hydrogen is bonded to each carbon site of graphene. Here we show that by exposing graphene to atomic hydrogen, it is possible to transform this highly-conductive semimetal into an insulator. Transmission electron microscopy reveals that the material retains the hexagonal lattice but its period becomes markedly shorter than that of graphene, providing direct evidence for a new graphene-based derivative. The reaction with hydrogen is found to be reversible so that the original metallic state and lattice spacing are restored by annealing and even the quantum Hall effect recovers. Our work proves the concept of chemical modification of graphene, which promises a whole range of new two-dimensional crystals with designed electronic and other properties.

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.

We show that the optical transparency of suspended graphene is defined by the fine structure constant, alpha, the parameter that describes coupling between light and relativistic electrons and is traditionally associated with quantum electrodynamics rather than condensed matter physics. Despite being only one atom thick, graphene is found to absorb a significant (pi times alpha=2.3%) fraction of incident white light, which is a consequence of graphene's unique electronic structure. This value translates into universal dynamic conductivity G =e^2/4h_bar within a few percent accuracy.

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.