Showing papers by "Andre K. Geim published in 2013"

Van der Waals heterostructures

Abstract: Fabrication techniques developed for graphene research allow the disassembly of many layered crystals (so-called van der Waals materials) into individual atomic planes and their reassembly into designer heterostructures, which reveal new properties and phenomena. Andre Geim and Irina Grigorieva offer a forward-looking review of the potential of layering two-dimensional materials into novel heterostructures held together by weak van der Waals interactions. Dozens of these one-atom- or one-molecule-thick crystals are known. Graphene has already been well studied but others, such as monolayers of hexagonal boron nitride, MoS2, WSe2, graphane, fluorographene, mica and silicene are attracting increasing interest. There are many other monolayers yet to be examined of course, and the possibility of combining graphene with other crystals adds even further options, offering exciting new opportunities for scientific exploration and technological innovation. Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

Strong light-matter interactions in heterostructures of atomically thin films.

Abstract: 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%).

Vertical field-effect transistor based on graphene?WS2 heterostructures for flexible and transparent electronics

Abstract: The celebrated electronic properties of graphene(1,2) have opened the way for materials just one atom thick(3) to be used in the post-silicon electronic era(4). An important milestone was the creation of heterostructures based on graphene and other two-dimensional crystals, which can be assembled into three-dimensional stacks with atomic layer precision(5-7). Such layered structures have already demonstrated a range of fascinating physical phenomena(8-71), and have also been used in demonstrating a prototype field-effect tunnelling transistor(12), which is regarded to be a candidate for post-CMOS (complementary metal-oxide semiconductor) technology. The range of possible materials that could be incorporated into such stacks is very large. Indeed, there are many other materials with layers linked by weak van der Waals forces that can be exfoliated(3,13) and combined together to create novel highly tailored heterostructures. Here, we describe a new generation of field-effect vertical tunnelling transistors where two-dimensional tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or chemical vapour deposition-grown graphene. The combination of tunnelling (under the barrier) and thermionic (over the barrier) transport allows for unprecedented current modulation exceeding 1 x 10(6) at room temperature and very high ON current. These devices can also operate on transparent and flexible substrates.

Cloning of Dirac fermions in graphene superlattices

Abstract: Placing graphene on a boron nitride substrate and accurately aligning their crystallographic axes, to form a moire superlattice, leads to profound changes in the graphene’s electronic spectrum. In 1976 Douglas Hofstadter predicted that electrons in a lattice subjected to electrostatic and magnetic fields would show a characteristic energy spectrum determined by the interplay between two quantizing fields. The expected spectrum would feature a repeating butterfly-shaped motif, known as Hofstadter's butterfly. The experimental realization of the phenomenon has proved difficult because of the problem of producing a sufficiently disorder-free superlattice where the length scales for magnetic and electric field can truly compete with each other. Now that goal has been achieved — twice. Two groups working independently produced superlattices by placing ultraclean graphene (Ponomarenko et al.) or bilayer graphene (Kim et al.) on a hexagonal boron nitride substrate and crystallographically aligning the films at a precise angle to produce moire pattern superstructures. Electronic transport measurements on the moire superlattices provide clear evidence for Hofstadter's spectrum. The demonstrated experimental access to a fractal spectrum offers opportunities for the study of complex chaotic effects in a tunable quantum system. Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties1,2,3,4,5,6,7,8. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice1,2,3,4. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter’s butterfly9) has been limited to the observation of new low-field oscillations5 and an internal structure within Landau levels6,7,8. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate’s moire potential10,11,12 acts as a superlattice and leads to profound changes in the graphene’s electronic spectrum. Second-generation Dirac points13,14,15,16,17,18,19,20,21,22 appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene’s conduction and valence bands. Strong magnetic fields lead to Zak-type cloning23 of the third generation of Dirac points, which are observed as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems7,8,9,22,23,24 and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures25.

Resonant tunnelling and negative differential conductance in graphene transistors

Abstract: The chemical stability of graphene and other free-standing two-dimensional crystals means that they can be stacked in different combinations to produce a new class of functional materials, designed for specific device applications. Here we report resonant tunnelling of Dirac fermions through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. The resonance occurs when the electronic spectra of the two electrodes are aligned. The resulting negative differential conductance in the device characteristics persists up to room temperature and is gate voltage-tuneable due to graphene’s unique Dirac-like spectrum. Although conventional resonant tunnelling devices comprising a quantum well sandwiched between two tunnel barriers are tens of nanometres thick, the tunnelling carriers in our devices cross only a few atomic layers, offering the prospect of ultra-fast transit times. This feature, combined with the multi-valued form of the device characteristics, has potential for applications in high-frequency and logic devices.

Singular phase nano-optics in plasmonic metamaterials for label-free single-molecule detection

Abstract: The non-trivial behaviour of phase is crucial for many important physical phenomena, such as, for example, the Aharonov-Bohm effect and the Berry phase. By manipulating the phase of light one can create 'twisted' photons, vortex knots and dislocations which has led to the emergence of the field of singular optics relying on abrupt phase changes. Here we demonstrate the feasibility of singular visible-light nano-optics which exploits the benefits of both plasmonic field enhancement and the peculiarities of the phase of light. We show that properly designed plasmonic metamaterials exhibit topologically protected zero reflection yielding to sharp phase changes nearby, which can be employed to radically improve the sensitivity of detectors based on plasmon resonances. By using reversible hydrogenation of graphene and binding of streptavidin-biotin, we demonstrate an areal mass sensitivity at a level of fg mm(-2) and detection of individual biomolecules, respectively. Our proof-of-concept results offer a route towards simple and scalable single-molecule label-free biosensing technologies.

Interaction phenomena in graphene seen through quantum capacitance

Abstract: Capacitance measurements provide a powerful means of probing the density of states. The technique has proved particularly successful in studying 2D electron systems, revealing a number of interesting many-body effects. Here, we use large-area high-quality graphene capacitors to study behavior of the density of states in this material in zero and high magnetic fields. Clear renormalization of the linear spectrum due to electron–electron interactions is observed in zero field. Quantizing fields lead to splitting of the spin- and valley-degenerate Landau levels into quartets separated by interaction-enhanced energy gaps. These many-body states exhibit negative compressibility but the compressibility returns to positive in ultrahigh B. The reentrant behavior is attributed to a competition between field-enhanced interactions and nascent fractional states.

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping.

Abstract: Control of magnetism by applied voltage is desirable 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 attracts interest because of 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 the magnetism of adatoms in graphene is itinerant and can be controlled by doping, so that magnetic moments are switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions; one from itinerant magnetism and the other from dangling bonds. Our work suggests that graphene’s spin transport can be controlled by the field effect, similar to its electronic and optical properties, and that spin diffusion can be significantly enhanced above a certain carrier density.

Generic miniband structure of graphene on a hexagonal substrate

Abstract: Using a general symmetry-based approach, we provide a classification of generic miniband structures for electrons in graphene placed on substrates with the hexagonal Bravais symmetry. In particular, we identify conditions at which the first moire miniband is separated from the rest of the spectrum by either one or a group of three isolated mini Dirac points and is not obscured by dispersion surfaces coming from other minibands. In such cases, the Hall coefficient exhibits two distinct alternations of its sign as a function of charge carrier density.

Raman fingerprint of aligned graphene/h-BN superlattices

Abstract: Graphene placed on hexagonal-boron nitride (h-BN) experiences a superlattice (Moire) potential, which leads to a strong reconstruction of graphene’s electronic spectrum with new Dirac points emerging at sub-eV energies. Here we study the effect of such superlattices on graphene’s Raman spectrum. In particular, the 2D Raman peak is found to be exquisitely sensitive to the misalignment between graphene and h-BN lattices, probably due to the presence of a strain distribution with the same periodicity of the Moire potential. This feature can be used to identify graphene superlattices with a misalignment angle smaller than 2°.

Giant magnetodrag in graphene at charge neutrality

Abstract: We report experimental data and theoretical analysis of Coulomb drag between two closely positioned graphene monolayers in a weak magnetic field. Close enough to the neutrality point, the coexistence of electrons and holes in each layer leads to a dramatic increase of the drag resistivity. Away from charge neutrality, we observe nonzero Hall drag. The observed phenomena are explained by decoupling of electric and quasiparticle currents which are orthogonal at charge neutrality. The sign of magnetodrag depends on the energy relaxation rate and geometry of the sample.

Separation of water using a membrane

Abstract: This invention relates to uses of graphene oxide, and in particular graphene oxide on a porous support, and a membrane comprising these materials. This invention also relates to methods of dehydration, which include vapour phase separation and pervaporation. Pervaporation is a method of separating mixtures of liquids using a membrane. Pervaporation consists of two basic steps: permeation of the permeate through the membrane and evaporation of the permeate from the other side of the membrane. Pervaporation is a mild which can be used to separate components which would not survive the comparatively harsh conditions needed for distillation (high temp, and/or low pressure).

Field-effect control of tunneling barrier height by exploiting graphene's low density of states

Abstract: We exploit the low density of electronic states of graphene to modulate the tunnel current flowing perpendicular to the atomic layers of a multi-layer graphene-boron nitride device. This is achieved by using the electric field effect to raise the Fermi energy of the graphene emitter layer and thereby reduce the effective barrier height for tunneling electrons. We discuss how the electron charge density in the graphene layers and the properties of the boron nitride tunnel barrier determine the device characteristics under operating conditions and derive expressions for carrier tunneling in these highly anisotropic layered heterostructures.

Effect of dielectric response on the quantum capacitance of graphene in a strong magnetic field

Abstract: The quantum capacitance of graphene can be negative when the graphene is placed in a strong magnetic field, which is a clear experimental signature of positional correlations between electrons. Here we show that the quantum capacitance of graphene is also strongly affected by its dielectric polarizability, which in a magnetic field is wave-vector dependent. We study this effect both theoretically and experimentally. We develop a theory and numerical procedure for accounting for the graphene dielectric response, and we present measurements of the quantum capacitance of high-quality graphene capacitors on boron nitride. Theory and experiment are found to be in good agreement.

Stacking boundaries and transport in bilayer graphene

Abstract: Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few meV. This behaviour has been interpreted as the result of an intrinsic electronic instability induced by many-body correlations. Intriguingly, however, some samples of similar mobility exhibit good metallic properties, with a minimal conductivity of the order of $2e^2/h$. Here we propose an explanation for this dichotomy, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains (`solitons'). We argue, using a numerical analysis, that the hallmark features of the previously inferred many-body insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing.

Ultrafast Non-Thermal Electron Dynamics in Single Layer Graphene

Abstract: We study the ultrafast dynamics of non-thermal electron relaxation in graphene upon impulsive excitation. The 10-fs resolution two color pump-probe allows us to unveil the non-equilibrium electron gas decay at early times.

Cross-sectional imaging of graphene-based devices

Abstract: The 2D structure and excellent electronic properties of graphene have generated worldwide interest, and offer the potential to overcome the limitations of silicon electronics.1 Early graphenebased devices consisted of a single atomic layer, but the possibility of tailoring electronic properties by stacking several different 2D crystals atop each other has since been discovered.2–5 In the assembly of these multilayer heterostructures, unwanted materials can become trapped between layers, adversely affecting the device quality. Understanding the cause of performance variations and thereby optimizing the properties of these devices is an extremely challenging task. Using high-resolution transmission electron microscopy imaging, a top view of the atomic structure of free-standing graphene sheets has been revealed.6–12 However, as the number of layers increases, these images become difficult or impossible to interpret due to the uppermost layers obscuring the view of lower layers. Characterizing the uppermost layer of a device is possible using a wide range of surface-sensitive analytical techniques, but these approaches are insensitive to buried interfaces. To characterize the structure of graphene devices, we have pioneered an unusual approach that involves the extraction of cross sections.13, 14 Focused ion beam milling is a technique in which a beam of ions (we use gallium) is focused on the sample, locally eroding the surface in a highly controlled fashion. This method has been used to image a wide range of devices, including 3D nanostructures, and has also found application in the preparation of thin sections of material suitable for transmission electron microscopy imaging. The major benefit of this approach, compared with conventional sample preparation routes, is the ability to Figure 1. (a) Schematic image showing the device structure and intended cross-sectional slice geometry. The heterostructure incorporates layers of boron (BN) and graphene (G) with gold (Au) contacts on an oxidized silicon (Si) substrate. (b) Optical image of actual device structure, with different layers indicated. Scanning electron microscopy images show (c) top-view image of device and (d) the process of sample preparation. (d)(i) The surface of the coated sample, (ii) after depositing protective platinum strip, (iii) after etching around all sides of the slice with a gallium ion beam, and (iv) a slice removed from the device using a nanomanipulator needle. (v) The slice is ready for transmission electron microscopy imaging after being attached to the grid and polished to reach a thickness of 20nm.