We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is ...interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m⁻¹) and -690 Nm⁻¹, respectively. The breaking strength is 42 N m⁻¹ and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of σint = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
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Two-dimensional van der Waals materials have demonstrated fascinating optical and electrical characteristics. However, reports on magnetic properties and spintronic applications of van der Waals ...materials are scarce by comparison. Here, we report anomalous Hall effect measurements on single crystalline metallic Fe3GeTe2 nanoflakes with different thicknesses. These nanoflakes exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, the coupling length between van der Waals atomic layers in Fe3GeTe2 is estimated to be ~5 van der Waals layers. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into van der Waals magnetic heterostructures, paving the way for spintronic research and applications based on these devices.
The substitutional doping method is ideally suited to generating doped two-dimensional (2D) materials for practical device applications as it does not damage or destabilize such materials. However, ...recently reported substitutional doping techniques for 2D materials have given rise to discontinuities and low uniformities, which hamper the extension of such techniques to large-scale production. In the current work, we demonstrated uniform substitutional doping of monolayer MoS2 in a 2 in. wafer of area >13 cm2. The devices based on doped MoS2 showed extremely high uniformity and stability in electrical properties in ambient conditions for 30 days. The photodetectors based on the doped MoS2 samples showed an ultrahigh photoresponsivity of 5 × 105 A/W, a detectivity of 5 × 1012 Jones, and a fast response rate of 5 ms than did those based on undoped MoS2. This work showed the feasibility of real-life applications based on functionalized 2D semiconductors for next-generation electronic and optoelectronic devices.
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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, E1 2g 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.
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This research explores the nonlinear elastic properties of two-dimensional molybdenum disulfide. We derive a thermodynamically rigorous nonlinear elastic constitutive equation and then calculate the ...nonlinear elastic response of two-dimensional MoS sub(2) with first-principles density functional theory (DFT) calculations. The nonlinear elastic properties are used to predict the behavior of suspended monolayer MoS sub(2) subjected to a spherical indenter load at finite strains in a multiple-length-scale finite element analysis model. The model is validated experimentally by indenting suspended circular MoS sub(2) membranes with an atomic force microscope. We find that the two-dimensional Young's modulus and intrinsic strength of monolayer MoS sub(2) are 130 and 16.5 N/m, respectively. The results approach Griffith's predicted intrinsic strength limit of sigma sub(int) ~ E/9, where E is the Young's modulus. This study reveals the predictive power of first-principles density functional theory in the derivation of nonlinear elastic properties of two-dimensional MoS sub(2). Furthermore, the study bridges three main gaps that hinder understanding of material properties: DFT to finite element analysis, experimental results to DFT, and the nanoscale to the microscale. In bridging these three gaps, the experimental results validate the DFT calculations and the multiscale constitutive model.
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We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized ...optical microscopy with high-resolution transmission electron microscopy analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag(1) and Ag(2) modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to the birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy.
Frictional Characteristics of Atomically Thin Sheets Lee, Changgu; Li, Qunyang; Kalb, William ...
Science (American Association for the Advancement of Science),
04/2010, Volume:
328, Issue:
5974
Journal Article
Peer reviewed
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.
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Although hexagonal boron nitride (h-BN) is a good candidate for gate-insulating materials by minimizing interaction from substrate, further applications to electronic devices with available ...two-dimensional semiconductors continue to be limited by flake size. While monolayer h-BN has been synthesized on Pt and Cu foil using chemical vapour deposition (CVD), multilayer h-BN is still absent. Here we use Fe foil and synthesize large-area multilayer h-BN film by CVD with a borazine precursor. These films reveal strong cathodoluminescence and high mechanical strength (Young's modulus: 1.16 ± 0.1 TPa), reminiscent of formation of high-quality h-BN. The CVD-grown graphene on multilayer h-BN film yields a high carrier mobility of ∼ 24,000 cm(2) V(-1) s(-1) at room temperature, higher than that (∼ 13,000 (2) V(-1) s(-1)) with exfoliated h-BN. By placing additional h-BN on a SiO2/Si substrate for a MoS2 (WSe2) field-effect transistor, the doping effect from gate oxide is minimized and furthermore the mobility is improved by four (150) times.
As an atomically thin material with low surface energy, graphene is an excellent candidate for reducing adhesion and friction when coated on various surfaces. Here, we demonstrate the superior ...adhesion and frictional characteristics of graphene films which were grown on Cu and Ni metal catalysts by chemical vapor deposition and transferred onto the SiO2/Si substrate. The graphene films effectively reduced the adhesion and friction forces, and multilayer graphene films that were a few nanometers thick had low coefficients of friction comparable to that of bulk graphite.
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We studied the surface-enhanced Raman scattering of an organic fluoropore (Rhodamine 6G, R6G) monolayer adsorbed onto graphene and two-dimensional (2D) molybedenium disulfides (MoS2) phototransistors ...and compared the results with the Raman scattering of R6G on 2D tungsten diselenides system (WSe2). The Raman enhancement factor of the R6G film adsorbed onto WSe2 was comparable to the corresponding value on graphene at 1365 cm–1 and was approximately twice this value at 615 cm–1. The amplitude of the charge transfer was estimated in situ by measuring the photocurrent produced in a hybrid system consisting of physisorbed R6G layer and the 2D materials. We found that the enhanced Raman scattering of R6G adsorbed onto the 2D materials was closely correlated with the charge transfer between the adsorbed molecules and the 2D materials. We also revealed that the intensity of Raman scattering generally decreased as the layer number of the 2D materials increased. For the R6G on the MoS2 nanosheet, a single layer system provided a maximum Raman enhancement factor, and this value decreased pseudolinearly with the number of layers. By contrast, the Raman enhancement factor of the R6G on WSe2 was greatest for both the mono- and bilayers, and it decreased dramatically as the number of layers increased. We provide qualitative theoretical explanations for these trends based on the electric field enhancement for the multile Fresnel phases and energy band diagrams of both systems.
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