Showing papers by "Andre K. Geim published in 2017"

Tunable sieving of ions using graphene oxide membranes

Abstract: Ion permeation and selectivity of graphene oxide membranes with sub-nm channels dramatically alters with the change in interlayer distance due to dehydration effects whereas permeation of water molecules remains largely unaffected. Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications1,2,3,4,5. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 A (ref. 4), which is larger than the diameters of hydrated ions of common salts4,6. The cutoff is determined by the interlayer spacing (d) of ∼13.5 A, typical for graphene oxide laminates that swell in water2,4. 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 tunable ion sieving. Membranes with d from ∼9.8 A to 6.4 A are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ∼10–100 kJ mol–1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules 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.

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

Abstract: Encapsulated few-layer InSe exhibits a remarkably high electronic quality, which is promising for the development of ultrathin-body high-mobility nanoelectronics. A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound1,2. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes3,4. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V−1 s−1 and 104 cm2 V−1 s−1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides5,6,7 and black phosphorus8,9,10,11.

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

Abstract: Highly laminar graphene oxide flakes (10 to 20 μm in diameter) are fabricated. Reducing flake thickness to 10 nm enables water and organic solvent permeation, enabling the flakes to act as a highly effective organic solvent membrane. Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation1,2,3,4,5,6,7,8,9. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents1, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10–20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports1,2,3,4. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

Size effect in ion transport through angstrom-scale slits

Abstract: In the field of nanofluidics, it has been an ultimate but seemingly distant goal 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 the effects of angstrom-scale confinement, which is important for the development of nanofluidics, molecular separation, and other nanoscale technologies.

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

Abstract: 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.

Superballistic flow of viscous electron fluid through graphene constrictions

Abstract: Graphene systems are clean platforms for studying electron–electron (e–e) collisions. Electron transport in graphene constrictions is now found to behave anomalously due to e–e interactions: conductance values exceed the maximum free-electron value. Electron–electron (e–e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways1,2,3,4,5,6. Despite strong interest, experiments on the subject proved challenging because of the simultaneous presence of different scattering mechanisms that suppress or obscure consequences of e–e scattering7,8,9,10,11. Only recently, sufficiently clean electron systems with transport dominated by e–e collisions have become available, showing behaviour characteristic of highly viscous fluids12,13,14. Here we study electron transport through graphene constrictions and show that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene15. Notably, the measured conductance exceeds the maximum conductance possible for free electrons16,17. This anomalous behaviour is attributed to collective movement of interacting electrons, which ‘shields’ individual carriers from momentum loss at sample boundaries18,19. The measurements allow us to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, our work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behaviour for designing graphene-based devices.

High-temperature quantum oscillations caused by recurring Bloch states in graphene superlattices

Abstract: Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization. The oscillations are extremely robust and persist well above room temperature in magnetic fields of only a few tesla. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work hints at unexplored physics in Hofstadter butterfly systems at high temperatures.

Scalable and efficient separation of hydrogen isotopes using graphene-based electrochemical pumping

Abstract: Thousands of tons of isotopic mixtures are processed annually for heavy-water production and tritium decontamination. The existing technologies remain extremely energy intensive and require large capital investments. New approaches are needed to reduce the industry’s footprint. Recently, micrometre-size crystals of graphene are shown to act as efficient sieves for hydrogen isotopes pumped through graphene electrochemically. Here we report a fully-scalable approach, using graphene obtained by chemical vapour deposition, which allows a proton-deuteron separation factor of around 8, despite cracks and imperfections. The energy consumption is projected to be orders of magnitude smaller with respect to existing technologies. A membrane based on 30 m2 of graphene, a readily accessible amount, could provide a heavy-water output comparable to that of modern plants. Even higher efficiency is expected for tritium separation. With no fundamental obstacles for scaling up, the technology’s simplicity, efficiency and green credentials call for consideration by the nuclear and related industries. Thousands of tons of water are processed every year for hydrogen isotope separation, using extremely costly technology. Here the authors demonstrate a fully-scalable graphene electrochemical pump, which promises to dramatically reduce the energy and capital costs.

Edge currents shunt the insulating bulk in gapped graphene.

Abstract: An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures. This long-standing puzzle is usually explained by charge inhomogeneity. Here we revisit the issue by investigating proximity-induced superconductivity in gapped graphene and comparing normal-state measurements in the Hall bar and Corbino geometries. We find that the supercurrent at the charge neutrality point in gapped graphene propagates along narrow channels near the edges. This observation is corroborated by using the edgeless Corbino geometry in which case resistivity at the neutrality point increases exponentially with increasing the gap, as expected for an ordinary semiconductor. In contrast, resistivity in the Hall bar geometry saturates to values of about a few resistance quanta. We attribute the metallic-like edge conductance to a nontrivial topology of gapped Dirac spectra.

Imaging resonant dissipation from individual atomic defects in graphene

Abstract: Conversion of electric current into heat involves microscopic processes that operate on nanometer length scales and release minute amounts of power. Although central to our understanding of the electrical properties of materials, individual mediators of energy dissipation have so far eluded direct observation. Using scanning nanothermometry with submicrokelvin sensitivity, we visualized and controlled phonon emission from individual atomic-scale defects in graphene. The inferred electron-phonon “cooling power spectrum” exhibits sharp peaks when the Fermi level comes into resonance with electronic quasi-bound states at such defects. Rare in the bulk but abundant at graphene’s edges, switchable atomic-scale phonon emitters provide the dominant dissipation mechanism. Our work offers insights for addressing key materials challenges in modern electronics and enables control of dissipation at the nanoscale.

Unraveling the 3D Atomic Structure of a Suspended Graphene/hBN van der Waals Heterostructure

Abstract: In this work we demonstrate that a free-standing van der Waals heterostructure, usually regarded as a flat object, can exhibit an intrinsic buckled atomic structure resulting from the interaction between two layers with a small lattice mismatch. We studied a freely suspended membrane of well-aligned graphene on a hexagonal boron nitride (hBN) monolayer by transmission electron microscopy (TEM) and scanning TEM (STEM). We developed a detection method in the STEM that is capable of recording the direction of the scattered electron beam and that is extremely sensitive to the local stacking of atoms. A comparison between experimental data and simulated models shows that the heterostructure effectively bends in the out-of-plane direction, producing an undulated structure having a periodicity that matches the moire wavelength. We attribute this rippling to the interlayer interaction and also show how this affects the intralayer strain in each layer.

Intercalant-independent transition temperature in superconducting black phosphorus

Abstract: Research on black phosphorus has been experiencing a renaissance over the last years, after the demonstration that few-layer crystals exhibit high carrier mobility and a thickness-dependent bandgap. Black phosphorus is also known to be a superconductor under high pressure exceeding 10 GPa. The superconductivity is due to a structural transformation into another allotrope and accompanied by a semiconductor-metal transition. No superconductivity could be achieved for black phosphorus in its normal orthorhombic form, despite several reported attempts. Here we describe its intercalation by several alkali metals (Li, K, Rb and Cs) and alkali-earth Ca. All the intercalated compounds are found to be superconducting, exhibiting the same (within experimental accuracy) critical temperature of 3.8±0.1 K and practically identical characteristics in the superconducting state. Such universal superconductivity, independent of the chemical composition, is highly unusual. We attribute it to intrinsic superconductivity of heavily doped individual phosphorene layers, while the intercalated layers of metal atoms play mostly a role of charge reservoirs. No superconductivity could so far be achieved in black phosphorus in its normal orthorhombic form. Here, the authors demonstrate that intercalation with alkali metals makes black phosphorus superconducting with intercalant-independent transition temperature and near-identical superconducting characteristics.

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

Abstract: Graphene is hailed as an ideal material for spintronics due to weak intrinsic spin–orbit interaction that facilitates lateral spin transport and tunability of its electronic properties, including a possibility to induce magnetism in graphene. Another promising application of graphene is related to its use as a spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive devices, the most prominent class of spintronic devices widely used as magnetic sensors. In particular, few-layer graphene was predicted to act as a perfect spin filter. Here we show that the role of graphene in such devices (at least in the absence of epitaxial alignment between graphene and the FMs) is different and determined by proximity-induced spin splitting and charge transfer with adjacent ferromagnetic metals, making graphene a weak FM electrode rather than a spin filter. To this end, we report observations of magnetoresistance (MR) in vertical Co-graphene-NiFe junctions with 1–4 graphene layers separating the ferromagnets, and demonstrate that the dependence of the MR sign on the number of layers and its inversion at relatively small bias voltages is consistent with spin transport between weakly doped and differently spin-polarized layers of graphene. The proposed interpretation is supported by the observation of an MR sign reversal in biased Co-graphene-hBN-NiFe devices and by comprehensive structural characterization. Our results suggest a new architecture for vertical devices with electrically controlled MR.

Sub-bandgap Voltage Electroluminescence and Magneto-oscillations in a WSe2 Light-Emitting van der Waals Heterostructure.

Abstract: We report on experimental investigations of an electrically driven WSe2 based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe2. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B periodicity. This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy.

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

Abstract: Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started, yet without control on the single electron level. Here, we show that van-der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunneling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.

Graphene-based tunable SQUIDs

Abstract: The superconducting proximity effect in graphene can be used to create Josephson junctions with critical currents that can be tuned using local field-effect gates. These junctions have the potential to add functionality to existing technologies; for example, superconducting quantum interference device (SQUID) magnetometers with adaptive dynamic range and superconducting qubits with fast electrical control. Here, we present measurements of graphene-based superconducting quantum interference devices incorporating ballistic Josephson junctions that can be controlled individually. We investigate the magnetic field response of the SQUIDs as the junctions are gated and as the device is tuned between symmetric and asymmetric configurations. We find a highest transfer function 300 lV/U0, which compares favorably with conventional, low temperature DC SQUIDs. With low noise readout electronics and optimised geometries, devices based on ballistic graphene Josephson junctions have the potential to match the sensitivity of traditional SQUIDs while also providing additional functionality.

Imaging resonant dissipation from individual atomic defects in graphene

Abstract: Conversion of electric current into heat involves microscopic processes that operate on nanometer length-scales and release minute amounts of power. While central to our understanding of the electrical properties of materials, individual mediators of energy dissipation have so far eluded direct observation. Using scanning nano-thermometry with sub-micro K sensitivity we visualize and control phonon emission from individual atomic defects in graphene. The inferred electron-phonon 'cooling power spectrum' exhibits sharp peaks when the Fermi level comes into resonance with electronic quasi-bound states at such defects, a hitherto uncharted process. Rare in the bulk but abundant at graphene's edges, switchable atomic-scale phonon emitters define the dominant dissipation mechanism. Our work offers new insights for addressing key materials challenges in modern electronics and engineering dissipation at the nanoscale.

When lost in a multiverse

Abstract: Imagine you are completely lost, as occasionally I am at meetings, wondering whether your colleagues belong to the same universe. Under such circumstances, graphene can come to the rescue. Yes, graphene again... After 15 years of working with this material, I understand and share the sentiment. If you do find yourself lost at one of such meetings, take a small piece of graphene deposited on a transparent plastic film and have a look at the Sun through it. By evaluating the contrast between areas covered and not covered with graphene, one can estimate how much light it absorbs. The difference in contrast should be 2.3% — at least in our Universe. This may sound little but in fact graphene is one of the most light-absorbing materials per unit of thickness. More importantly, 2.3% is not just a random number: it is the value obtained when multiplying π by the fine-structure constant α = e/2ε0hc ≈ 1/137, with e the elementary charge, ε0 the electric constant, h the Planck constant and c the speed of light in vacuum. The first experiment to determine graphene’s optical transparency was done exactly as described above, by looking at white light through a small graphene membrane, taking a photograph and performing a simple image analysis1. The picture shows Rahul Nair, a PhD student at the time, posing with a device used in ref. 1 to measure graphene’s optical transparency. Within good accuracy, graphene was found to absorb a fraction πα of visible light. Nowadays it is possible to buy A4-size graphene deposited on a transparent substrate to do this experiment at home or at school. Unfortunately, graphene cannot offer metrological accuracy for α. Calculations showed that the absorption value was actually not as accurate as one would hope for, because graphene’s electronic structure actually deviates slightly from the Dirac (linear) spectrum often associated with the material2. Moreover, many-body effects (excitonic corrections) lead to additional deviations at short wavelengths. Nonetheless, graphene samples are perhaps adequate for accidental multiverse travellers who can be satisfied with a rough rather than metrological assessment of their sanity. But what if a visited universe differs from ours by only a tiny bit, say by one part per billion? With a new twist and a larger piece of equipment, graphene can help again. One could then use the quantum Hall effect (QHE) to ensure that α has not changed. The QHE allows measurements of the socalled von Klitzing constant or the resistance quantum h/e = 1/2ε0αc with such extreme accuracy that there is no reference left to check against3. QHE standards are now compared against each other using different materials exhibiting this phenomenon4,5. Graphene is one of them and the latest favourite of metrologists worldwide4,5, thanks to an extremely robust QHE, enabling metrology at higher temperatures, lower magnetic fields and smaller probing currents than any other material. In graphene, quantum Hall plateaux can survive up to room temperature6. They are also incredibly long as a function of magnetic field, if graphene grown on SiC is used5. The latter feature makes it easier to measure h/e2 accurately. In fact, the plateaux are so long that researchers initially thought that their magnet stopped sweeping because a QHE plateau continued beyond the horizon, with no changes in the signal over 80% of the available (14-tesla) field range5. Also, graphene allows extremely high probing currents, before deviations from the ideal QHE become discernible. This places weaker constraints on what type of measurement equipment can be used. The von Klitzing constant in graphene was compared with those observed in other materials. No deviations were observed between different systems4, proving that the resistance standard is independent of the material used for its realization. High-cost dedicated magnet systems with complex measurement setups are currently essential to provide QHE metrology. Thanks to graphene, these can soon be compressed into simpler, tabletop and cryogen-free systems. As the rumour goes, soon you will be able to buy a personal resistance standard from Oxford Instruments5, for just a few bitcoins. From a multiverse traveller’s perspective, such a tabletop standard is obviously less comfortable than having one von Klitzing in your pocket — a resistor of 25.8 kΩ, of course — but it can probably be fit into a (large) suitcase and, for instance, shipped to the International Space Station to check for generaland special-relativity corrections to h/e2, if any. ❐

Size effect in ion transport through angstrom-scale slits

Abstract: 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.

A new detection scheme for van der Waals heterostructures, imaging individual fullerenes between graphene sheets, and controlling the vacuum in scanning transmission electron microscopy

Abstract: This presentation contains three topics that are of current interest in particular for low-dimensional materials. First, I will describe a new detection scheme that allows us to unveil the deformations in a graphene-hBN heterostructure [1], where the two layers have only a very small lattice mismatch. We use a pixelated detector for recording the scattered intensity in the angular range of the annular darkfield detector. The asymmetry in the scattered intensity turns out to be very sensitive to the local projected displacement between the two layers (Fig. 1). By comparing the experimental signal with simulations for different atomistic models, we establish the 3D structure of the sample [1].

Extremely sensitive magnetoresistance sensors using few-layer graphene/boron-nitride

Abstract: Understanding magnetoresistance, the change in electrical resistance upon an external magnetic field, at the atomic level is of great interest both fundamentally and technologically.

Indirect excitons in van der Waals heterostructures at room temperature

Abstract: Indirect excitons (IXs) in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by a high binding energy making them stable at room temperature and giving the opportunity for exploring fundamental phenomena in excitonic systems and developing excitonic devices operational at high temperatures. We present the observation of IXs at room temperature in van der Waals TMD heterostructures based on monolayers of MoS$_2$ separated by atomically thin hexagonal boron nitride. The IXs realized in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD, and their energy is gate controlled.

Tunable giant valley splitting in edge-free graphene quantum dots on boron nitride

Abstract: Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene, the first atomically thin 2D material, offers two binary degrees: the electron spin and the valley degree of freedom. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started without control for single electrons. Here, we show that van-der Waals stacking of 2D materials offers a natural platform for valley control due to the relatively strong and spatially varying atomic interaction between adjacent layers. We use an edge-free quantum dot, induced by the tip of a scanning tunneling microscope into graphene on hBN. We demonstrate a valley splitting, which is tunable from -5 meV to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts controlled valley splitting of single electrons by more than an order of magnitude, which will probably enable robust spin qubits and valley qubits in graphene.

Understanding 2D Crystal Vertical Heterostructures at the Atomic Scale Using Advanced Scanning Transmission Electron Microscopy

Abstract: The emerging area of two dimensional (2D) materials has attracted a great deal of scientific attention in recent years. Like graphene, these materials can be exfoliated to single atom thickness and can then be mechanically layered together to create new van der Waals crystals with bespoke properties. However the performance of such materials is strongly dependent on the quality of the crystals and the interfaces between different crystals. Cross sectional imaging using atomic resolution transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is the only technique able to characterize the nature of buried interfaces in these engineered van der Waals crystals [1].