Showing papers by "Kostya S. Novoselov published in 2013"
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TL;DR: In this paper, the authors review the field emerging at the intersection of graphene physics and plasmonics and review the applications of graphene-based plasmons for optical devices with extremely high speed, low driving voltage, low power consumption and compact sizes.
Abstract: Two rich and vibrant fields of investigation, graphene physics and plasmonics, strongly overlap Not only does graphene possess intrinsic plasmons that are tunable and adjustable, but a combination of graphene with noble-metal nanostructures promises a variety of exciting applications for conventional plasmonics The versatility of graphene means that graphene-based plasmonics may enable the manufacture of novel optical devices working in different frequency ranges, from terahertz to the visible, with extremely high speed, low driving voltage, low power consumption and compact sizes Here we review the field emerging at the intersection of graphene physics and plasmonics
2,475 citations
TL;DR: In this article, the effect of strain on the phonon modes of monolayer and few-layer MoS was investigated by observing the strain-induced shifts of the Raman-active modes.
Abstract: The effect of strain on the phonon modes of monolayer and few-layer MoS${}_{2}$ has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS${}_{2}$ sandwiched between two layers of optically transparent polymer. The resulting band shifts of the ${E}_{2g}^{1}$ ($\ensuremath{\sim}$$385.3\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) and ${A}_{1g}$ ($\ensuremath{\sim}$$402.4\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) Raman modes were found to be small but observable. First-principles plane-wave calculations based on density functional perturbation theory were used to determine the Gr\"uneisen parameters for the ${E}_{1g}$, ${E}_{2g}^{1}$, ${A}_{1g}$, and ${A}_{2u}$ modes and predict the experimentally observed band shifts for the monolayer material. The polymer--MoS${}_{2}$ interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS${}_{2}$ into devices.
543 citations
TL;DR: The mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering, are identified, which gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.
Abstract: Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrodinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.
462 citations
TL;DR: Large-area high-quality graphene capacitors are used to study behavior of the density of states in this material in zero and high magnetic fields, revealing a number of interesting many-body effects.
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.
256 citations
TL;DR: In this article, the authors demonstrate that encapsulation of molybdenum disulfide (MoS2) between two layers of graphene allows for a dramatic improvement in stability of the studied 2D crystal, and permits careful control over the defect nature and formation in it.
Abstract: Recent dramatic progress in studying various two-dimensional (2D) atomic crystals and their heterostructures calls for better and more detailed understanding of their crystallography, reconstruction, stacking order, etc. For this, direct imaging and identification of each and every atom is essential. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) are ideal, and perhaps the only tools for such studies. However, the electron beam can in some cases induce dramatic structure changes and radiation damage becomes an obstacle in obtaining the desired information in imaging and chemical analysis in the (S)TEM. This is the case of 2D materials such as molybdenum disulfide MoS2, but also of many biological specimens, molecules and proteins. Thus, minimizing damage to the specimen is essential for optimum microscopic analysis. In this letter we demonstrate, on the example of MoS2, that encapsulation of such crystals between two layers of graphene allows for a dramatic improvement in stability of the studied 2D crystal, and permits careful control over the defect nature and formation in it. We present STEM data collected from single layer MoS2 samples prepared for observation in the microscope through three distinct procedures. The fabricated single layer MoS2 samples were either left bare (pristine), placed atop a single-layer of graphene or finally encapsulated between single graphene layers. Their behaviour under the electron beam is carefully compared and we show that the MoS2 sample 'sandwiched' between the graphene layers has the highest durability and lowest defect formation rate compared to the other two samples, for very similar experimental conditions.
171 citations
TL;DR: 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 Moiré potential.
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°.
109 citations
TL;DR: In this article, a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer molybdenum disulfide (MoS2) is presented.
Abstract: We present a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer molybdenum disulfide (MoS2). From first-principles simulations, we find electron doping of graphene due to the presence of rhenium impurities in MoS2. Furthermore, we show that MoS2 edges give rise to charge reordering and a potential shift in graphene, which can be controlled through external gate voltages. The interplay of edge and impurity effects allows the use of the graphene-MoS2 hybrid as a photodetector. Spatially resolved photocurrent signals can be used to resolve potential gradients and local doping levels in the sample.
105 citations
TL;DR: In this paper, the plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene.
Abstract: The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of ${k}_{B}T$ at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies.
88 citations
TL;DR: It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two and that this change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking during deformation.
Abstract: The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (~0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.
81 citations
TL;DR: In this article, the authors 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.
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.
39 citations
TL;DR: P polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E(2g) phonons.
Abstract: We perform polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T. This reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E2g phonons. This is explained with a model of Raman scattering taking into account the effects of spatially inhomogeneous carrier densities and strain. Random fluctuations of strain-induced pseudomagnetic fields lead to increased scattering intensity inside the anticrossing gap, consistent with the experiments.
TL;DR: In this paper, the quantum capacitance of graphene is also strongly affected by its dielectric polarizability, which in a magnetic field is wave-vector dependent, and the authors study this effect both theoretically and experimentally.
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.
TL;DR: In this paper, a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer MoS2 is presented, where the interplay of edge and impurity effects allows the use of the graphene-MoS2 hybrid as a photodetector.
Abstract: We present a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer MoS2. From first-principles simulations we find electron doping of graphene due to the presence of rhenium impurities in MoS2. Furthermore, we show that MoS2 edges give rise to charge reordering and a potential shift in graphene, which can be controlled through external gate voltages. The interplay of edge and impurity effects allows the use of the graphene-MoS2 hybrid as a photodetector. Spatially resolved photocurrent signals can be used to resolve potential gradients and local doping levels in the sample.
TL;DR: In this paper, the authors proposed an explanation for the many-body correlations in bilayer graphene, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains (solitons').
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.
TL;DR: In this paper, temperature-dependent magneto-transport experiments around the charge neutrality point in graphene were conducted to determine the amplitude of potential fluctuations s responsible for the formation of electron-hole puddles.
Abstract: We present temperature-dependent magneto-transport experiments around the charge neutrality point in graphene and determine the amplitude of potential fluctuations s responsible for the formation of electron-hole puddles. The experimental value s � 20 meV is considerably larger than in conventional semiconductors which implies a strong localization of charge carriers observable up to room temperature. Surprisingly, in the quantized regime, the Hall resistivity overshoots the highest plateau values at high temperatures. We demonstrate by model calculations that such a peculiar behavior is expected in any system with coexisting electrons and holes when the energy spectrum is quantized and the carriers are partially localized.
01 May 2013
TL;DR: In this article, the plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene.
Abstract: The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of kBT at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies.
09 Jun 2013
TL;DR: In this paper, the authors demonstrate that monolayer graphene can control surface-plasmon modes in terahertz laser systems, by integrating a graphene-coated, multi-band filter, varied in situ by electrochemical gating.
Abstract: For the first time, we demonstrate that monolayer graphene can control surface-plasmon modes in terahertz lasers. Integrating a graphene-coated, multi-band filter, varied in situ by electrochemical gating, individual modes can be switched on and off.
01 Mar 2013
TL;DR: In this paper, the ultrafast dynamics of non-thermal electron relaxation in graphene upon impulsive excitation were studied and the 10-fs resolution two color pump-probe was used to reveal the non-equilibrium electron gas decay at early times.
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.