Showing papers by "James Hone published in 2010"

Atomically thin MoS2: a new direct-gap semiconductor

Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Boron nitride substrates for high-quality graphene electronics

Abstract: Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.

Anomalous lattice vibrations of single- and few-layer MoS2.

Abstract: Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

Frictional Characteristics of Atomically Thin Sheets

Abstract: Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length-dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets' increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Probing Strain-Induced Electronic Structure Change in Graphene by Raman Spectroscopy

Abstract: Two-phonon Raman scattering in graphitic materials provides a distinctive approach to probing the material's electronic structure through the spectroscopy of phonons. Here we report studies of Raman scattering of the two-dimensional mode of single-layer graphene under uniaxial stress and which implicates two types of modification of the low-energy electronic structure of graphene: a deformation of the Dirac cone and its displacement away from the K point.

Frictional Characteristics of Atomically-Thin Sheets

Abstract: Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Substrate effect on thickness-dependent friction on graphene

Abstract: Using friction force microscopy, we have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells. Both graphene on SiO 2 / Si substrates and graphene freely suspended over the wells showed a trend of increasing friction with decreasing number of atomic layers of graphene. However, this trend with thickness was absent for graphene deposited on mica, where the graphene is strongly bonded to the substrate. Measurements together with a mechanics model suggest that mechanical confinement to the substrate plays an important role in the frictional behavior of these atomically thin graphite sheets. Loosely bound or suspended graphene sheets can pucker in the out-of-plane direction due to tip-graphene adhesion. This increases contact area, and also allows further defonnation of the graphene when sliding, leading to higher friction. Since thinner samples have lower bending stiffness, the puckering effect and frictional resistance are greater. However, if the graphene is strongly bound to the substrate, the puckering effect will be suppressed and no thickness dependence should be observed. The results can provide potentially useful guidelines in the rational design and use of graphene for nano-mechanical applications, including nano-lubricants and components in micro- and nanodevices.

Graphene field-effect transistors based on boron nitride gate dielectrics

Abstract: Graphene field-effect transistors are fabricated utilizing single-crystal hexagonal boron nitride (h-BN), an insulating isomorph of graphene, as the gate dielectric The devices exhibit mobility values exceeding 10,000 cm2/V-sec and current saturation down to 500 nm channel lengths with intrinsic transconductance values above 400 mS/mm The work demonstrates the favorable properties of using h-BNas a gate di-electric for graphene FETs

Force generated by actomyosin contraction builds bridges between adhesive contacts.

Abstract: Extracellular matrices in vivo are heterogeneous structures containing gaps that cells bridge with an actomyosin network. To understand the basis of bridging, we plated cells on surfaces patterned with fibronectin (FN)-coated stripes separated by non-adhesive regions. Bridges developed large tensions where concave cell edges were anchored to FN by adhesion sites. Actomyosin complexes assembled near those sites (both actin and myosin filaments) and moved towards the centre of the non-adhesive regions in a treadmilling network. Inhibition of myosin-II (MII) or Rho-kinase collapsed bridges, whereas extension continued over adhesive areas. Inhibition of actin polymerization (latrunculin-A, jasplakinolide) also collapsed the actomyosin network. We suggest that MII has distinct functions at different bridge regions: (1) at the concave edges of bridges, MIIA force stimulates actin filament assembly at adhesions and (2) in the body of bridges, myosin cross-links actin filaments and stimulates actomyosin network healing when breaks occur. Both activities ensure turnover of actin networks needed to maintain stable bridges from one adhesive region to another.

Radio frequency electrical transduction of graphene mechanical resonators

Abstract: We report radio frequency (rf) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer, employing a local gate to minimize parasitic capacitance. A resist-free doubly clamped sample with resonant frequency ∼34 MHz, quality factor ∼10 000 at 77 K, and signal-to-background ratio of over 20 dB is demonstrated. In addition to being over two orders of magnitude faster than the electrical rf mixing method, this technique paves the way for use of graphene in rf devices such as filters and oscillators.

Cell spreading as a hydrodynamic process

Abstract: Many cell types have the ability to move themselves by crawling on extra-cellular matrices Although cell motility is governed by actin and myosin filament assembly, the pattern of the movement follows the physical properties of the network ensemble average The first step of motility, cell spreading on matrix substrates, involves a transition from round cells in suspension to polarized cells on substrates Here we show that the spreading dynamics on 2D surfaces can be described as a hydrodynamic process In particular, we show that the transition from isotropic spreading at early time to anisotropic spreading is reminiscent of the fingering instability observed in many spreading fluids During cell spreading, the main driving force is the polymerization of actin filaments that push the membrane forward From the equilibrium between the membrane force and the cytoskeleton, we derive a first order expression of the polymerization stress that reproduces the observed behavior Our model also allows an interpretation of the effects of pharmacological agents altering the polymerization of actin In particular we describe the influence of Cytochalasin D on the nucleation of the fingering instability

Excitons and high-order optical transitions in individual carbon nanotubes: A Rayleigh scattering spectroscopy study

Abstract: We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the line shape is consistent with an excitonic model, but not one of free carriers. For semiconducting species, sidebands are observed at $\ensuremath{\sim}200\text{ }\text{meV}$ above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such sidebands are not apparent for metallic nanotubes, as expected from the reduced strength of excitonic interactions in these systems.

Radio frequency electrical transduction of graphene mechanical resonators

Abstract: We report radio frequency (rf) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer, employing a local gate to minimize parasitic capacitance. A resist-free doubly clamped sample with resonant frequency ~ 34 MHz, quality factor ~ 10000 at 77 K, and signal-to-background ratio of over 20 dB is demonstrated. In addition to being over two orders of magnitude faster than the electrical rf mixing method, this technique paves the way for use of graphene in rf devices such as filters and oscillators.

Electronic compressibility of gapped bilayer graphene

Abstract: We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a 1-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while D < 0 the peak appears only for holes. This asymmetry that can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.

Infrared spectra of individual semiconducting single-walled carbon nanotubes: Testing the scaling of transition energies for large diameter nanotubes

Abstract: We have measured the low-energy excitonic transitions of chiral assigned individual large-diameter semiconducting single-walled nanotubes using a high-resolution Fourier transform photoconductivity technique. When photoconductivity is complemented by Rayleigh scattering spectroscopy, as many as five optical transitions can be identified on the same individual nanotube over an energy range of 0.3-2.7 eV. We find that well-established energy scaling relations developed for nanotubes of smaller diameter are not consistent with the measured low-energy transitions in large (1.8-2.3 nm) diameter nanotubes.

Systems And Methods For Integrating A Single DNA Molecule Into A Molecular Electronic Device

Abstract: The disclosed subject matter provides a techniques for precisely and/or functionally cutting carbon nanotubes, e.g., single walled carbon nanotubes (“SWNTs”) and integrating a single nucleic acid molecule (e.g., a DNA molecule) into a gap formed into the carbon nanotubes. In one aspect, a method of fabricating a molecular electronic device includes disposing a SWNT on a base layer, forming a gap in the SWNT using a lithographic process, and disposing a single DNA strand across the gap so that each end of the nucleic acid contacts a gap termini. The disclosed subject matter also provides techniques for measuring the electrical properties (charge transport) of a DNA molecule which is integrated into an SWNT. Furthermore, a molecular electronic device including an SWNT with an integrated nucleic acid molecule is disclosed.

Anomalous Lattice Vibrations of Single and Few-Layer MoS2

Abstract: Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the 3-dimensional to the 2-dimensional regime.