Showing papers by "Kostya S. Novoselov published in 2016"
TL;DR: Two-dimensional heterostructures with extended range of functionalities yields a range of possible applications, and spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system.
Abstract: BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.
4,851 citations
TL;DR: Recent advances in the use of graphene and other 2D materials in catalytic applications are reviewed, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals).
Abstract: Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.
1,683 citations
TL;DR: Graphene hosts a unique electron system in which electron-phonon scattering is extremely weak but electron-electron collisions are sufficiently frequent to provide local equilibrium above the temperature of liquid nitrogen, under which electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids.
Abstract: Graphene hosts a unique electron system in which electron-phonon scattering is extremely weak but electron-electron collisions are sufficiently frequent to provide local equilibrium above the temperature of liquid nitrogen. Under these conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids. Here we report strong evidence for this transport regime. We found that doped graphene exhibits an anomalous (negative) voltage drop near current-injection contacts, which is attributed to the formation of submicrometer-size whirlpools in the electron flow. The viscosity of graphene’s electron liquid is found to be ~0.1 square meters per second, an order of magnitude higher than that of honey, in agreement with many-body theory. Our work demonstrates the possibility of studying electron hydrodynamics using high-quality graphene.
595 citations
TL;DR: High conductive, highly flexible, light weight and low cost printed graphene for wireless wearable communications applications and can be processed at low temperature so that it is compatible with heat-sensitive flexible materials like papers and textiles.
Abstract: In this paper, we report highly conductive, highly flexible, light weight and low cost printed graphene for wireless wearable communications applications As a proof of concept, printed graphene enabled transmission lines and antennas on paper substrates were designed, fabricated and characterized To explore its potentials in wearable communications applications, mechanically flexible transmission lines and antennas under various bended cases were experimentally studied The measurement results demonstrate that the printed graphene can be used for RF signal transmitting, radiating and receiving, which represents some of the essential functionalities of RF signal processing in wireless wearable communications systems Furthermore, the printed graphene can be processed at low temperature so that it is compatible with heat-sensitive flexible materials like papers and textiles This work brings a step closer to the prospect to implement graphene enabled low cost and environmentally friendly wireless wearable communications systems in the near future
176 citations
TL;DR: Scanning near-field optical microscopy is used to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium and reports on shape-dependent resonances that attest to low dielectric losses that permit coherent standing waves of the subdiffractional polaritons to form.
Abstract: We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of "hot" rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape. Both dependences are consistent with directional propagation of hyperbolic phonon-polariton rays that are launched at the edges and zigzag through the interior of the nanocones, sustaining multiple internal reflections off the sidewalls. Additionally, we observe a strong overall enhancement of the near-field signal at discrete resonance frequencies. These resonances attest to low dielectric losses that permit coherent standing waves of the subdiffractional polaritons to form. We comment on potential applications of such shape-dependent resonances and the field concentration at the hot rings.
127 citations
TL;DR: In this paper, photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2 have been reported, namely electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization.
Abstract: Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH...
105 citations
TL;DR: Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties as discussed by the authors, and have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation.
Abstract: Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.
71 citations
TL;DR: In this paper, a light-emitting device based on a monolayer WSe$_2$ and also comprising boron nitride tunnelling barriers and graphene electrodes is constructed.
Abstract: Recent developments in fabrication of van der Waals heterostructures enable new type of devices assembled by stacking atomically thin layers of two-dimensional materials. Using this approach, we fabricate light-emitting devices based on a monolayer WSe$_2$, and also comprising boron nitride tunnelling barriers and graphene electrodes, and observe sharp luminescence spectra from individual defects in WSe$_2$ under both optical and electrical excitation. This paves the way towards the realization of electrically-pumped quantum emitters in atomically thin semiconductors. In addition we demonstrate tuning by more than 1 meV of the emission energy of the defect luminescence by applying a vertical electric field. This provides an estimate of the permanent electric dipole created by the corresponding electron-hole pair. The light-emitting devices investigated in our work can be assembled on a variety of substrates enabling a route to integration of electrically pumped single quantum emitters with existing technologies in nano-photonics and optoelectronics.
68 citations
TL;DR: Graphene is used as a transparent conductive coating on curved PMMA substrates in adaptive contact lenses based on liquid crystals, where simultaneously the high electrical conductivity, transparency, flexibility and elasticity of graphene are being utilised.
Abstract: The superlatives of graphene cover a whole range of properties: electrical, chemical, mechanical, thermal and others. These special properties earn graphene a place in current or future applications. Here we demonstrate one such application - adaptive contact lenses based on liquid crystals, where simultaneously the high electrical conductivity, transparency, flexibility and elasticity of graphene are being utilised. In our devices graphene is used as a transparent conductive coating on curved PMMA substrates. The adaptive lenses provide a + 0.7 D change in optical power with an applied voltage of 7.1 Vrms - perfect to correct presbyopia, the age-related condition that limits the near focus ability of the eye.
25 citations
16 Dec 2016
TL;DR: In this article, a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation is demonstrated.
Abstract: Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride-graphene tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.
19 citations
07 Dec 2016
TL;DR: In this paper, an epoxy-based micro-lens was used to enhance the photoluminescence of tungsten diselenide (WSe2) monolayers.
Abstract: The recent discovery of semiconducting two-dimensional materials is predicted to lead to the introduction of a series of revolutionary optoelectronic components that are just a few atoms thick. Key remaining challenges for producing practical devices from these materials lie in improving the coupling of light into and out of single atomic layers, and in making these layers robust to the influence of their surrounding environment. We present a solution to tackle both of these problems simultaneously, by deterministically placing an epoxy based micro-lens directly onto the materials’ surface. We show that this approach enhances the photoluminescence of tungsten diselenide (WSe2) monolayers by up to 300%, and nearly doubles the imaging resolution of the system. Furthermore, this solution fully encapsulates the monolayer, preventing it from physical damage and degradation in air. The optical solution we have developed could become a key enabling technology for the mass production of ultra-thin optical devices, such as quantum light emitting diodes.
TL;DR: In this article, a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation is demonstrated.
Abstract: Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.
TL;DR: In this article, the effect of Zeeman splitting on electron transport on hexagonal boron nitride (h-BN) was investigated using magnetotransport measurements and it was shown that a parallel magnetic field up to 30 Tesla does not affect the transport properties of graphene on h-BN.
Abstract: Graphene on hexagonal boron nitride (h-BN) is an atomically flat conducting system that is ideally suited for probing the effect of Zeeman splitting on electron transport. We demonstrate by magnetotransport measurements that a parallel magnetic field up to 30 Tesla does not affect the transport properties of graphene on h-BN even at charge neutrality where such an effect is expected to be maximal. The only magnetoresistance detected at low carrier concentrations is shown to be associated with a small perpendicular component of the field which cannot be fully eliminated in the experiment. Despite the high mobility of charge carriers at low temperatures, we argue that the effects of Zeeman splitting are fully masked by electrostatic potential fluctuations at charge neutrality.
TL;DR: In this article, the authors address the conflicting behavior of different pristine graphene dispersions through their careful preparation and characterization in aqueous media, and establish the influence of pre-association with different dispersants, including those likely to be encountered during the transport and excretion of graphene in vivo.
Abstract: Contradictions in the reported biocompatibility of graphene-related materials have been attributed to differences in their preparation. Herein, we address the conflicting behavior of different pristine graphene dispersions through their careful preparation and characterization in aqueous media. Although exfoliated in different media, all graphene dispersions were physically similar and comprised few-layer graphene flakes of 100 to 400 nm mean length with relatively defect-free basal planes. The dispersions were colloidally stable, including in physiological saline and when organic solvents were exchanged with water by dialysis, due to their negative zeta potentials (−28 mV to −60 mV) from interaction with water and the different dispersants. Thus, we have been able to establish the influence of pre-association with different dispersants, including those likely to be encountered during the transport and excretion of graphene in vivo. Shear-forced association with neutral phospholipids was transient, as the lipids desorbed to form liposomes, and left a hemolytic dispersion, whereas other dispersions were not hemolytic. High boiling point solvents widely used to exfoliate graphene are toxic and viewed difficult to remove, but were readily removed by simple dialysis. Human serum albumin readily and stably adsorbed in predominantly monomeric form to pristine graphene in physiological saline, which may be expected in vivo. This work shows the large influence that different adsorbates can have on the behaviour of otherwise physically-similar graphene.
TL;DR: In this article, the resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and two gates has been investigated, and the discovery step structure of the current patterns with plateaus and abrupt jumps between them is caused by pinning of chemical potentials to the Landau levels in two graphene sheets.
Abstract: Resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and two gates has been investigated. The design has allowed studying transitions between individual Landau levels of different graphene sheets bounded by a narrow conductivity window with a width controlled by a bias voltage. Three-dimensional plots of the equilibrium tunneling conductivity against both gate voltages reflecting the displacement of resonances between various combinations of the individual Landau levels in the top and bottom sheets have been drawn and identified. The discovered step structure of the current patterns with plateaus and abrupt jumps between them is caused by pinning of chemical potentials to the Landau levels in two graphene sheets. The presence of negative differential conductivity regions in the current–voltage characteristics in the magnetic field with the peak-to-valley current ratio Ip/Iv ~ 2 indicates a high degree of the conservation of an in-plane momentum component at tunneling.
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TL;DR: In this article, it was shown that the pseudo-spin of a fermion in a magnetic field can be oriented by a strain induced pseudo-magnetic field through the Zeeman term.
Abstract: One of the fundamental properties of spin 1/2 particles is their interaction with magnetic fields. The exploration of this coupling can be quite elusive, for example in the case of neutrinos. Graphene has been shown to be a condensed matter platform for the study of such ultrarelativistic particles, with its neutrino-like charge carriers having a spin-like degree of freedom called pseudospin. Here we show that in analogy to the spin alignment of a fermion in a magnetic field, the pseudo-spin of graphene can be oriented by a strain induced pseudo-magnetic field through the Zeeman term. We reveal this pseudo-spin polarization as a sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunnelling microscope. The observed pseudo-spin polarization scales with the lifting height of the strained deformation and therefore with the pseudo-magnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models. This adds a key ingredient to the celebrated analogy of graphene's charge carriers to ultra-relativistic Dirac fermions. Furthermore, the deduced fields of about 1000 T could provide an effective THz valley filter, as a basic element of valleytronics.
01 Jan 2016
TL;DR: In this article, the first direct observation of clustering of covalently bonded impurities on graphene surface was made, and the authors interpreted this observation as formation of low conducting clusters of fluorinated graphene separated by areas of pristine material.
Abstract: Partially fluorinated graphene samples were studied by conductive AFM. The conductivity is found to be strongly non-uniform with low conductivity patches of about 10 nm. We interpret this observation as formation of low conducting clusters of fluorinated graphene separated by areas of pristine material. This is the first direct observation of clustering of covalently bonded impurities on graphene surface.
05 Jun 2016
TL;DR: In this paper, the electric field and reflection gain for a graphene-incorporated THz semiconductor laser waveguide was calculated by controlling the Fermi level of graphene and modifying the lasing modes of the waveguide via graphene plasmons.
Abstract: Here we calculate the electric field and reflection gain for a graphene-incorporated THz semiconductor laser waveguide. By controlling the Fermi level of graphene, the lasing modes of the waveguide are modified via graphene plasmons.
05 Jun 2016
TL;DR: In this article, tunable graphene plasmons were integrated into a THz laser waveguide with an aperiodic hologram lattice to control the cavity photon lifetime and modulate round trip gain by doping the graphene layer.
Abstract: We integrate tunable graphene plasmons into a THz laser waveguide with an aperiodic hologram lattice. This allows us to control the cavity photon lifetime and hence modulate round trip gain by doping the graphene layer.
TL;DR: In this paper, the authors review the properties of novel 2D crystals and how their properties are used in new heterostructure devices, such as tunneling transistors, resonant tunneling diodes, and light emitting Diodes.
Abstract: The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With new 2D materials, truly 2D physics has started to appear (e.g. absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc). Novel heterostructure devices are also starting to appear - tunneling transistors, resonant tunneling diodes, light emitting diodes, etc. Composed from individual 2D crystals, such devices utilize the properties of those crystals to create functionalities that are not accessible to us in other heterostructures. We review the properties of novel 2D crystals and how their properties are used in new heterostructure devices.