Showing papers by "Kostya S. Novoselov published in 2014"
TL;DR: Three-dimensionally confined 'hyperbolic polaritons' in boron nitride nanocones that support four series (up to the seventh order) modes in two spectral bands are reported on.
Abstract: Strongly anisotropic media, where the principal components of the dielectric tensor have opposite signs, are called hyperbolic. Such materials exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional length scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material hexagonal boron nitride is an attractive alternative. Here we report on three-dimensionally confined 'hyperbolic polaritons' in boron nitride nanocones that support four series (up to the seventh order) modes in two spectral bands. The resonant modes obey the predicted aspect ratio dependence and exhibit high-quality factors (Q up to 283) in the strong confinement regime (up to λ/86). These observations assert hexagonal boron nitride as a promising platform for studying novel regimes of light-matter interactions and nanophotonic device engineering.
754 citations
TL;DR: This work reports on the search for alternative substrates for making quality graphene heterostructures using atomically flat crystals and attributes the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN, and transition metal dichalcogenides.
Abstract: Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micrometer-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulfides and hBN are found to exhibit consistently high carrier mobilities of about 60 000 cm(2) V(-1) s(-1). In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide, and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ∼1000 cm(2) V(-1) s(-1). We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN, and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.
476 citations
TL;DR: A design for artificially structured materials to rival the present record for high-critical temperature cuprate superconductors by exhibiting macroscopic occupation of a quantum state and vanishing viscosity at high temperatures.
Abstract: All known superfluid and superconducting states of condensed matter are enabled by composite bosons (atoms, molecules and Cooper pairs) made of an even number of fermions. Temperatures where such macroscopic quantum phenomena occur are limited by the lesser of the binding energy and the degeneracy temperature of the bosons. High-critical temperature cuprate superconductors set the present record of ~100 K. Here we propose a design for artificially structured materials to rival this record. The main elements of the structure are two monolayers of a transition metal dichalcogenide separated by an atomically thin spacer. Electrons and holes generated in the system would accumulate in the opposite monolayers and form bosonic bound states--the indirect excitons. The resultant degenerate Bose gas of indirect excitons would exhibit macroscopic occupation of a quantum state and vanishing viscosity at high temperatures.
427 citations
TL;DR: Three layers of complexity are outlined that need to be considered carefully in the development of graphene for use in biomedical applications: material characteristics; interactions with biological components (tissues, cells, and proteins); and biological activity outcomes.
Abstract: Graphene is highly conductive, flexible, and has controllable permittivity and hydrophilicity, among its other distinctive properties ( 1 , 2 ). These properties could enable the development of multifunctional biomedical devices ( 3 ). A key issue for such applications is the determination of the possible interactions with components of the biological milieu to reveal the opportunities offered and the limitations posed. As with any other nanomaterial, biological studies of graphene should be performed with very specific, well-designed, and well-characterized types of materials with defined exposure. We outline three layers of complexity that are interconnected and need to be considered carefully in the development of graphene for use in biomedical applications: material characteristics; interactions with biological components (tissues, cells, and proteins); and biological activity outcomes.
273 citations
TL;DR: In this article, the authors show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.
Abstract: The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.
163 citations
TL;DR: It is shown that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.
Abstract: The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in the device fabrication.
150 citations
TL;DR: In this article, a 10-fold PL lifetime shortening was achieved, as a consequence of Purcell enhancement of the spontaneous emission rate of photoluminescence (PL).
Abstract: Integration of quasi-two-dimensional (2D) films of metal–chalcogenides in optical microcavities permits new photonic applications of these materials. Here we present tunable microcavities with monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; a 10-fold PL lifetime shortening is achieved, a consequence of Purcell enhancement of the spontaneous emission rate.
148 citations
TL;DR: In this article, the performance of an ideal mechanically exfoliated single crystal graphene flake compared to a scalable CVD graphene film was compared using hand-held Raman spectroscopy and the chemical vapor deposition (CVD) of graphene films.
Abstract: Functional graphene optical sensors are now viable due to the recent developments in hand-held Raman spectroscopy and the chemical vapor deposition (CVD) of graphene films. Herein, the strain in graphene/poly (methyl methacrylate) sensor coatings is followed using Raman band shifts. The performance of an "ideal" mechanically-exfoliated single crystal graphene flake is compared to a scalable CVD graphene film. The dry-transferred mechanically exfoliated sample has no residual stresses, whereas the CVD sample is in compression following the solvent evaporation during its transfer. The behavior of the sensors under cyclic deformation shows an initial breakdown of the graphene-polymer interface with the interface then stabilizing after several cycles. The Raman 2D band shift rates per unit strain of the exfoliated graphene are approximate to 35% higher than CVD graphene making the former more strain sensitive. However, for practical wide-area applications, CVD graphene coatings are still viable candidates as a Raman system can be used to read the strain in any 5 m diameter spot in the coating to an absolute accuracy of approximate to 0.01% strain and resolution of approximate to 27 microstrains (mu s), which compares favorably to commercial photoelastic systems.
124 citations
TL;DR: It is argued, 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).
Abstract: Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few millielectronvolts. This behavior has been interpreted as the result of an intrinsic electronic ins...
74 citations
TL;DR: In this article, the authors report on the search for alternative substrates for making quality graphene heterostructures, using molybdenum or tungsten disulphides and hBN.
Abstract: Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micron-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulphides and hBN are found to exhibit consistently high carrier mobilities of about 60,000 cm$^{2}$V$^{-1}$s$^{-1}$. In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ~ 1,000 cm$^{2}$ V$^{-1}$s$^{-1}$. We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.
66 citations
01 Jan 2014
TL;DR: In this article, the authors showed that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1-2 nm.
Abstract: Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of –0.60% corresponding to a yield stress up to -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1–2 nm. These results were compared with DFT computations performed on analogue coronene/ PMMA oligomers and a reasonable agreement was obtained.
TL;DR: The results show that WS2 is an attractive option for applications in electronic and optoelectronic devices and pave the way for further studies in this two-dimensional material.
Abstract: Atomically thin layers of materials, which are just a few atoms in thickness, present an attractive option for future electronic devices. Herein we characterize, optically and electronically, atomically thin tungsten disulphide (WS2), a layered semiconductor. We provide the distinctive Raman and photoluminescence signatures for single layers, and prepare field-effect transistors where atomically thin WS2 serves as the conductive channel. The transistors present mobilities μ = 10 cm2 V−1 s−1 and exhibit ON/OFF ratios exceeding 100 000. Our results show that WS2 is an attractive option for applications in electronic and optoelectronic devices and pave the way for further studies in this two-dimensional material.
TL;DR: In this paper, a microinjection-micromanipulator system was employed to deposit droplets of aqueous solutions containing redox-active species directly on selected micro-scale areas of mechanically exfoliated graphene layers on polymer coated silicon wafers.
Abstract: A micro apparatus for electrochemical studies on individual high quality graphene flakes is presented. A microinjection-micromanipulator system has been employed to deposit droplets of aqueous solutions containing redox-active species directly on selected micro-scale areas of mechanically exfoliated graphene layers on polymer coated silicon wafers. This approach allows the clear distinction between the electrochemical activity of pristine basal planes and the edges (defects) or steps to be measured. Voltammetric measurements were performed in a two-electrode configuration, and the standard heterogeneous electron transfer rate (k°) for reduction of hexachloroiridate (IrCl62−) was estimated. The kinetics of electron transfer were evaluated for several types of graphene: mono, bi, and few layer basal planes, and the k° was estimated for an edge/step between two few layer graphene flakes. As a comparison, the kinetic behaviour of graphite basal planes was measured for the deposited aqueous droplets. The appearance of ruptures on the graphene monolayer was observed after deposition of the aqueous solution for the case of graphene on a bare silicon/silicon oxide substrate.
TL;DR: A simple method of modulating the electrical conductivity of graphene by dual-side molecular n-doping with diethylenetriamine on the top and amine-functionalized self-assembled monolayers (SAMs) at the bottom is reported.
Abstract: Doping is an efficient way to engineer the conductivity and the work function of graphene, which is, however, limited to wet-chemical doping or metal deposition particularly for n-doping, Here, we report a simple method of modulating the electrical conductivity of graphene by dual-side molecular n-doping with diethylenetriamine (DETA) on the top and amine-functionalized self-assembled monolayers (SAMs) at the bottom. The resulting charge carrier density of graphene is as high as −1.7 × 1013 cm−2, and the sheet resistance is as low as ∼86 ± 39 Ω sq−1, which is believed to be the lowest sheet resistance of monolayer graphene reported so far. This facile dual-side n-doping strategy would be very useful to optimize the performance of various graphene-based electronic devices.
TL;DR: It is suggested that less-prestigious universities in China should focus on offering vocational education programmes or training students for high-level positions outside academia to restore the balance.
TL;DR: In this article, the authors present tunable microcavities with embedded monolayer MoS2 or few monoline GaSe films, and observe significant modification of spectral and temporal properties of photoluminescence.
Abstract: Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic properties. However, photonic applications of thin films may be limited owing to weak light absorption and surface effects leading to reduced quantum yield. Integration of 2D films in optical microcavities will permit these limitations to be overcome owing to modified light coupling with the films. Here we present tunable microcavities with embedded monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; PL life-time shortening by a factor of 10 is achieved, a consequence of Purcell enhancement of the spontaneous emission rate. This work has potential to pave the way to microcavity-enhanced light-emitting devices based on layered 2D materials and their heterostructures, and also opens possibilities for cavity QED in a new material system of van der Waals crystals.
TL;DR: In this article, the authors showed that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1-2 nm.
Abstract: Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of -0.60 % corresponding to a yield stress of -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1-2 nm. These results were compared with DFT computations performed on analogue coronene/ PMMA oligomers and a reasonable agreement was obtained.
01 Apr 2014
Posted Content•
TL;DR: In this paper, the authors proposed a design for artificially structured materials to rival this record, which consists of two monolayers of a transition metal dichalcogenide (TMD) and an atomically thin hexagonal boron nitride spacer.
Abstract: All known superfluid and superconducting states of condensed matter are enabled by composite bosons (atoms, molecules, Cooper pairs) made of an even number of fermions. Temperatures where such macroscopic quantum phenomena occur are limited by the lesser of the binding energy and the degeneracy temperature of the bosons. High critical temperature cuprate superconductors set the present record of ~100 K. Here we propose a design for artificially structured materials to rival this record. The main elements of the structure are two monolayers of a transition metal dichalcogenide (TMD) and an atomically thin hexagonal boron nitride (hBN) spacer. Electrons and holes generated in the system would accumulate in the separate TMD layers and form bosonic bound states --- the indirect excitons. The resultant degenerate Bose gas of excitons would exhibit macroscopic occupation of a quantum state, vanishing viscosity, and superconductivity at high temperatures.
Posted Content•
TL;DR: In this article, the effect of aspect ratio on the critical strain to failure of 1LG flakes with respect to axial compression was evaluated under an external compression strain of approximately -1 %.
Abstract: The mechanical behavior of embedded monolayer graphene in a polymer matrix under axial compression is examined here by monitoring the shifts of the 2D Raman phonons under an incremental applied strain. In order to establish the effect of aspect ratio upon the critical strain to failure a wide range of length-to-width ratios of almost rectangular 1LG flakes were tested up to an external compression strain of approximately -1 %. Care was taken to define the position of zero strain due to the presence-in some cases-of a residual stress and to assess the effect of transfer length upon the efficiency of stress transfer. The obtained critical strain values for first failure- after transfer length correction in short flakes- were found to be independent of flake size with a mean value of - 0.60 + 0.11%. By combining Euler mechanics with a Winkler type of approach, both the modulus of interaction between graphene and polymer, as well as, the buckling wavelength could be established. The results show clearly that unlike buckling in air, the presence of a constraint such as a polymer matrix induces graphene buckling of very short wavelength of the order of 1-2 nm. Finally, by conducting calculations of the interaction between an analogue of monolayer graphene (coronene) and PMMA oligomers within the framework of density functional theory (DFT), the effect of lateral constrain provided by the polymer, upon the out-of-plane buckling of graphene has been assessed.
11 Jun 2014
TL;DR: In this paper, a comparison of two-dimensional (2D) materials, including graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMD), in terms of their structure and durability under intense electron beams is presented.
Abstract: Two-dimensional (2D) materials, graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMD) have been investigated by means of Scanning Transmission Electron Microscopy (STEM), in particular via High Angle Annular Dark Field (HAADF) imaging technique. They are compared in terms of their structure and durability under intense electron beams.
01 Jan 2014
TL;DR: In this article, the authors examined the mechanical behavior under compression of graphene monolayers embedded in polymer matrices and found that the critical strain to failure was universal for all dimensions ratio of the flakes with mean value of -0.6±0.11%.
Abstract: In the present study the mechanical behaviour under compression of graphene monolayers embedded in polymer matrices is examined. Using four-point-bending apparatus compressive strains of -1% were applied gradually to the sample and Raman measurements were taken insitu at every loading step. By monitoring the shift of the 2D Raman band with the applied strain the critical strain to failure is estimated. The critical strain was found to be universal for all dimensions ratio of the flakes with mean value of -0.6±0.11%. Using Winkler’s model, the interaction between graphene-polymer is simulated with linear elastic springs. An analytical formula for estimating the critical strain to buckling is obtained, as well as the buckling wavelength. Finally, DFT analysis was performed to gain more insight for the estimation of the analytical parameters.