Showing papers by "Andre K. Geim published in 2015"

Light-emitting diodes by band-structure engineering in van der Waals heterostructures

Abstract: The advent of graphene and related 2D materials has recently led to a new technology: heterostructures based on these atomically thin crystals. The paradigm proved itself extremely versatile and led to rapid demonstration of tunnelling diodes with negative differential resistance, tunnelling transistors, photovoltaic devices and so on. Here, we take the complexity and functionality of such van der Waals heterostructures to the next level by introducing quantum wells (QWs) engineered with one atomic plane precision. We describe light-emitting diodes (LEDs) made by stacking metallic graphene, insulating hexagonal boron nitride and various semiconducting monolayers into complex but carefully designed sequences. Our first devices already exhibit an extrinsic quantum efficiency of nearly 10% and the emission can be tuned over a wide range of frequencies by appropriately choosing and combining 2D semiconductors (monolayers of transition metal dichalcogenides). By preparing the heterostructures on elastic and transparent substrates, we show that they can also provide the basis for flexible and semi-transparent electronics. The range of functionalities for the demonstrated heterostructures is expected to grow further on increasing the number of available 2D crystals and improving their electronic quality.

Square ice in graphene nanocapillaries

Abstract: Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology Theory suggests many possible phases for adsorbed and confined water, but it has proved challenging to assess its crystal structure experimentally Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement The observations show that the nanoconfined water at room temperature forms 'square ice'--a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules Square ice has a high packing density with a lattice constant of 283 A and can assemble in bilayer and trilayer crystallites Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature

Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere

Abstract: Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decompose in air, which has severely hindered their investigation and potential applications. Here we introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly ex...

WSe2 Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature

Abstract: Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for optoelectronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQWs) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2- and MoSe2-based LEQWs shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250× more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such different temperature dependences to the inverted sign of spin–orbit splitting of conduction band states in tungste...

Nonlocal transport and the hydrodynamic shear viscosity in graphene

Abstract: Motivated by recent experimental progress in preparing encapsulated graphene sheets with ultrahigh mobilities up to room temperature, we present a theoretical study of dc transport in doped graphene in the hydrodynamic regime. By using the continuity and Navier-Stokes equations, we demonstrate analytically that measurements of nonlocal resistances in multiterminal Hall bar devices can be used to extract the hydrodynamic shear viscosity of the two-dimensional (2D) electron liquid in graphene. We also discuss how to probe the viscosity-dominated hydrodynamic transport regime by scanning probe potentiometry and magnetometry. Our approach enables measurements of the viscosity of any 2D electron liquid in the hydrodynamic transport regime.

Graphene-protected copper and silver plasmonics.

Abstract: Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, and increase sensitivity of bio-detection A widespread use of plasmonic devices is notably hindered by high losses and the absence of stable and inexpensive metal films suitable for plasmonic applications To this end, there has been a continuous search for alternative plasmonic materials that are also compatible with complementary metal oxide semiconductor technology Here we show that copper and silver protected by graphene are viable candidates Copper films covered with one to a few graphene layers show excellent plasmonic characteristics They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances Our results are likely to initiate wide use of graphene-protected plasmonics

Binder-free highly conductive graphene laminate for low cost printed radio frequency applications

Abstract: In this paper, we demonstrate realization of printable radio frequency identification (RFID) antenna by low temperature processing of graphene ink. The required ultra-low resistance is achieved by rolling compression of binder-free graphene laminate. With compression, the conductivity of graphene laminate is increased by more than 50 times compared to that of as-deposited one. Graphene laminate with conductivity of 4.3 × 104 S/m and sheet resistance of 3.8 Ω/sq (with thickness of 6 μm) is presented. Moreover, the formation of graphene laminate from graphene ink reported here is simple and can be carried out in low temperature (100 °C), significantly reducing the fabrication costs. A dipole antenna based on the highly conductive graphene laminate is further patterned and printed on a normal paper to investigate its RF properties. The performance of the graphene laminate antenna is experimentally measured. The measurement results reveal that graphene laminate antenna can provide practically acceptable retur...

Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures

Abstract: Understanding magnetoresistance, the change in electrical resistance under an external magnetic field, at the atomic level is of great interest both fundamentally and technologically. Graphene and other two-dimensional layered materials provide an unprecedented opportunity to explore magnetoresistance at its nascent stage of structural formation. Here we report an extremely large local magnetoresistance of ∼2,000% at 400 K and a non-local magnetoresistance of >90,000% in an applied magnetic field of 9 T at 300 K in few-layer graphene/boron-nitride heterostructures. The local magnetoresistance is understood to arise from large differential transport parameters, such as the carrier mobility, across various layers of few-layer graphene upon a normal magnetic field, whereas the non-local magnetoresistance is due to the magnetic field induced Ettingshausen-Nernst effect. Non-local magnetoresistance suggests the possibility of a graphene-based gate tunable thermal switch. In addition, our results demonstrate that graphene heterostructures may be promising for magnetic field sensing applications.

Detecting topological currents in graphene superlattices

Abstract: Making use of graphene's valleys Graphene has two distinct valleys in its electronic structure, in which the electrons have the same energy. Theorists have predicted that creating an asymmetry between the two valleys will coax graphene into exhibiting the so-called valley Hall effect (VHE). In this effect, electrons from the two valleys move across the sample in opposite directions when the experimenters run current along the sample. Gorbachev et al. achieved this asymmetry by aligning graphene with an underlying layer of hexagonalboron nitride (hBN) (see the Perspective by Lundeberg and Folk). The authors measured the transport characteristics of the sample, which were consistent with the theoretical predictions for the VHE. The method may in the future lead to information processing using graphene's valleys. Science, this issue p. 448; see also p. 422 Graphene is aligned with a layer of hexagonal boron nitride to achieve the valley Hall effect. [Also see Perspective by Lundeberg and Folk] Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene’s two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several micrometers away from the nominal current path. Locally, topological currents are comparable in strength with the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by means of gate voltage can be exploited for information processing based on valley degrees of freedom.

Resonant tunnelling between the chiral Landau states of twisted graphene lattices

Abstract: A class of multilayered functional materials has recently emerged in which the component atomic layers are held together by weak van der Waals forces that preserve the structural integrity and physical properties of each layer. An exemplar of such a structure is a transistor device in which relativistic Dirac fermions can resonantly tunnel through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. An applied magnetic field quantizes graphene’s gapless conduction and valence band states into discrete Landau levels, allowing us to resolve individual inter-Landau-level transitions and thereby demonstrate that the energy, momentum and chiral properties of the electrons are conserved in the tunnelling process. We also demonstrate that the change in the semiclassical cyclotron trajectories, following an inter-layer tunnelling event, is analogous to the case of intra-layer Klein tunnelling. For small twist angles, electrons can resonantly tunnel between graphene layers in a van der Waals heterostructure. It is now shown that the tunnelling not only preserves energy and momentum, but also the chirality of electronic states.

Landau level spectroscopy of electron-electron interactions in graphene.

Abstract: We present magneto-Raman scattering studies of electronic inter-Landau level excitations in quasineutral graphene samples with different strengths of Coulomb interaction. The band velocity associated with these excitations is found to depend on the dielectric environment, on the index of Landau level involved, and to vary as a function of the magnetic field. This contradicts the single-particle picture of noninteracting massless Dirac electrons but is accounted for by theory when the effect of electron-electron interaction is taken into account. Raman active, zero-momentum inter-Landau level excitations in graphene are sensitive to electron-electron interactions due to the nonapplicability of the Kohn theorem in this system, with a clearly nonparabolic dispersion relation.

Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators

Abstract: We assess the potential of two-terminal graphene-hexagonal boron nitride-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.

Nonlocal Response and Anamorphosis: The Case of Few-Layer Black Phosphorus.

Abstract: Few-layer black phosphorus was recently rediscovered as a narrow-bandgap atomically thin semiconductor, attracting unprecedented attention due to its interesting properties. One feature of this material that sets it apart from other atomically thin crystals is its structural in-plane anisotropy which manifests in strongly anisotropic transport characteristics. However, traditional angle-resolved conductance measurements present a challenge for nanoscale systems, calling for new approaches in precision studies of transport anisotropy. Here, we show that the nonlocal response, being exponentially sensitive to the anisotropy value, provides a powerful tool for determining the anisotropy in black phosphorus. This is established by combining measurements of the orientation-dependent nonlocal resistance response with the analysis based on the anamorphosis relations. We demonstrate that the nonlocal response can differ by orders of magnitude for different crystallographic directions even when the anisotropy is a...

High thermal conductivity of hexagonal boron nitride laminates

Abstract: Two-dimensional materials are characterised by a number of unique physical properties which can potentially make them useful to a wide diversity of applications. In particular, the large thermal conductivity of graphene and hexagonal boron nitride has already been acknowledged and these materials have been suggested as novel core materials for thermal management in electronics. However, it was not clear if mass produced flakes of hexagonal boron nitride would allow one to achieve an industrially-relevant value of thermal conductivity. Here we demonstrate that laminates of hexagonal boron nitride exhibit thermal conductivity of up to 20 W/mK, which is significantly larger than that currently used in thermal management. We also show that the thermal conductivity of laminates increases with the increasing volumetric mass density, which creates a way of fine-tuning its thermal properties.

Lifting of the Landau level degeneracy in graphene devices in a tilted magnetic field

Abstract: We report on transport and capacitance measurements of graphene devices in magnetic fields up to 30 T. In both techniques, we observe the full splitting of Landau levels and we employ tilted field experiments to address the origin of the observed broken symmetry states. In the lowest energy level, the spin degeneracy is removed at filling factors $ u=\pm1$ and we observe an enhanced energy gap. In the higher levels, the valley degeneracy is removed at odd filling factors while spin polarized states are formed at even $ u$. Although the observation of odd filling factors in the higher levels points towards the spontaneous origin of the splitting, we find that the main contribution to the gap at $ u= -4,-8$, and $-12$ is due to the Zeeman energy.

Proton conducting membrane comprising monolithic 2d material and ionomer, a process for preparing same and use of same in fuel cell and hydrogen gas sensor

Abstract: The present invention relates to a graphene-based or other 2-D material membrane which allows the passage of protons and deuterons and to a method of facilitating proton or deuteron permeation through such a membrane. Monocrystalline membranes made from mono-and few-layers of graphene, hBN, molybdenum disulfide (MoS2), and tungsten disulfide (WS2)etc. are disclosed. In effect, the protons or deuterons are charge carriers that pass through the graphene or other 2-D material membrane. This process can be contrasted with the passage of gaseous hydrogen. Hydrogen is an uncharged gaseous species which is diatomic. In other words, the gas is in molecular form when considering the normal barrier properties whereas in the case of the present invention, the species which is being transported through the membrane is a charged ion comprising a single atom. Membranes of the invention find use in a number of applications such as fuel cells.

Reduced graphene oxide barrier materials

Abstract: This invention relates to barrier materials comprising reduced graphene oxide, methods of making said materials and their uses. The reduced graphene oxide is preferably formed from the reduction of graphene oxide by HI, HBr or ascorbic acid.

Sieving hydrogen isotopes through two dimensional crystals

Abstract: One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen ions (thermal protons) penetrate through them. We show that monolayers of graphene and boron nitride can be used to separate hydrogen ion isotopes. Employing electrical measurements and mass spectrometry, we find that deuterons permeate through these crystals much slower than protons, resulting in a separation factor of ~10 at room temperature. The isotope effect is attributed to a difference of about 60 meV between zero-point energies of incident protons and deuterons, which translates into the equivalent difference in the activation barriers posed by two dimensional crystals. In addition to providing insight into the proton transport mechanism, the demonstrated approach offers a competitive and scalable way for hydrogen isotope enrichment.

Graphene-hBN resonant tunneling diodes as high-frequency oscillators

Abstract: We assess the potential of two-terminal graphene-hBN-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.

Synthesis, Modification and Characterization of Nanocarbon Electrodes for Determination of Nucleic Acids

Abstract: Unique mechanical, electronic, chemical, optical, and electrochemical properties of nanosized carbon materials predestine them for numerous potential applications including photocatalysis, electrochemistry, electronics, and optoelectronics. Carbon nanotubes and graphene are some of the most intensively explored carbon allotropes in materials science. The possibility to translate the individual properties of these monodimensional (carbon nanotubes SWCN,MWCN) and bidimensional (graphene) building units into two-dimensional free-standing thick and thin films has paved the way to use these allotropes in a number of the mentioned applications. Moreover, the possibility to conjugate carbon nanomaterials with biomolecules has received particular attention with respect to the design of chemical sensors and biosensors. In this paper, we reviewed types, structure, and especially different methods of synthesis (preparation) of carbon nanomaterials including arc discharge, laser ablation, and chemical vapor deposition. Moreover, we mentioned some rarely used ways of arc discharge deposition, which involves arc discharge in liquid solutions in contrary to standard used deposition in a gas atmosphere. Besides synthesis, modifications of carbon nanomaterials with biologically important molecules for biosensing of DNA and RNA are discussed.

Control of excitons in multi-layer van der Waals heterostructures

Abstract: We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of \Mo* and hexagonal boron nitride (hBN). The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.

Cross sectional STEM imaging and analysis of multilayered two dimensional crystal heterostructure devices

Abstract: 1. School of Materials, University of Manchester, Manchester, M13 9PL, UK 2. School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK 3. Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK 4. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 5 Manchester Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester, M13 9PL, UK

Matériaux barrières d'oxyde de graphène réduit

Abstract: Cette invention concerne des materiaux barrieres comprenant de l'oxyde de graphene reduit, des procedes de fabrication desdits materiaux, et leurs utilisations. L'oxyde de graphene reduit est, de preference, forme a partir de la reduction d'oxyde de graphene par HI, HBr ou l'acide ascorbique.