Yinfeng Li

Graphene microsheets enter cells through spontaneous membrane penetration at edge asperities and corner sites

Understanding and controlling the interaction of graphene-based materials with cell membranes is key to the development of graphene-enabled biomedical technologies and to the management of graphene health and safety issues. Very little is known about the fundamental behavior of cell membranes exposed to ultrathin 2D synthetic materials. Here we investigate the interactions of graphene and few-layer graphene (FLG) microsheets with three cell types and with model lipid bilayers by combining coarse-grained molecular dynamics (MD), all-atom MD, analytical modeling, confocal fluorescence imaging, and electron microscopic imaging. The imaging experiments show edge-first uptake and complete internalization for a range of FLG samples of 0.5- to 10-μm lateral dimension. In contrast, the simulations show large energy barriers relative to kBT for membrane penetration by model graphene or FLG microsheets of similar size. More detailed simulations resolve this paradox by showing that entry is initiated at corners or asperities that are abundant along the irregular edges of fabricated graphene materials. Local piercing by these sharp protrusions initiates membrane propagation along the extended graphene edge and thus avoids the high energy barrier calculated in simple idealized MD simulations. We propose that this mechanism allows cellular uptake of even large multilayer sheets of micrometer-scale lateral dimension, which is consistent with our multimodal bioimaging results for primary human keratinocytes, human lung epithelial cells, and murine macrophages.

An approximate continuum theory for interaction between dislocation and inhomogeneity of any shape and properties

An approximate continuum theory is developed to effectively handle the problem of interaction between dislocations and inhomogeneity of any shape and properties. The inhomogeneity is, based on the Eshelby equivalent inclusion theory, equivalent to a homogenous one with a transformation strain. The interaction force between dislocation and the inhomogeneity can then be evaluated from the work done by the dislocation stress field during the transformation. The proposed continuum theory is applicable to a variety of inhomogeneities, such as pore, gas bubble, shear band and plastically deformed zone. It can be reduced to the classical continuum theory for some special cases.

Surface hydrogenation regulated wrinkling and torque capability of hydrogenated graphene annulus under circular shearing

Wrinkles as intrinsic topological feature have been expected to affect the electrical and mechanical properties of atomically thin graphene. Molecular dynamics simulations are adopted to investigate the wrinkling characteristics in hydrogenated graphene annulus under circular shearing at the inner edge. The amplitude of wrinkles induced by in-plane rotation around the inner edge is sensitive to hydrogenation, and increases quadratically with hydrogen coverage. The effect of hydrogenation on mechanical properties is investigated by calculating the torque capability of annular graphene with varying hydrogen coverage and inner radius. Hydrogenation-enhanced wrinkles cause the aggregation of carbon atoms towards the inner edge and contribute to the critical torque strength of annulus. Based on detailed stress distribution contours, a shear-to-tension conversion mechanism is proposed for the contribution of wrinkles on torque capacity. As a result, the graphane annulus anomalously has similar torque capacity to pristine graphene annulus. The competition between hydrogenation caused bond strength deterioration and wrinkling induced local stress state conversion leads to a U-shaped evolution of torque strength relative to the increase of hydrogen coverage from 0 to 100%. Such hydrogenation tailored topological and mechanical characteristics provides an innovative mean to develop novel graphene-based devices.

Controllable far-infrared electromagnetic radiation from plasmas applied by dc or ac bias electric fields

It is shown theoretically and numerically that a dc/ac bias field applied over tenuous plasma can be converted efficiently into electromagnetic (EM) waves at the plasma frequency ωp, with the amplitude and polarization determined by the bias. The initial phase of the EM waves can be controlled by the triggering time of the bias and therefore circularly/elliptically polarized EM waves can be obtained by applying two bias fields perpendicular to each other. When the bias frequency is near ωp, the resonance appears and the EM waves are generated with the intensity enhanced considerably as compared with the dc-bias case. This approach provides a potential way to produce tunable far-infrared EM waves such as terahertz waves. Limits of this approach by available parameters of plasmas and bias are also discussed.

Boundary-dependent mechanical properties of graphene annular under in-plane circular shearing via atomistic simulations

Graphene annulus possesses special wrinkling phenomenon with wide range of potential applications. Using molecular dynamics simulation, this study concerns the effect of boundary on the mechanical properties of circular and elliptical graphene annuli under circular shearing at inner edge. Both the wrinkle characteristic and torque capacity of annular graphene can be effectively tuned by outer boundary radius and aspect ratio. For circular annulus with fixed inner radius, the critical angle of rotation can be increased by several times without sacrificing its torque capacity by increasing outer boundary radius. The wrinkle characteristic of graphene annulus with elliptical outer boundary differs markedly with that of circular annulus. Torque capacity anomalously decreases with the increase of aspect ratio, and a coupled effect of the boundary aspect ratio and the ratio of minor axis to inner radius on wrinkling are revealed. By studying the stress distribution and wrinkle characteristics, we find the decay of torque capacity is the result of circular stress concentration around the minor axis, while the nonuniform stress distribution is anomalously caused by the change of wrinkle profiles near the major axis. The specific mechanism of out-of-plane deformation on in-plane strength provides a straightforward means to develop novel graphene-based devices.

Strength nature of two-dimensional woven nanofabrics under biaxial tension:

Woven nanostructures have been acknowledged as a platform for solar cells, supercapacitors, and sensors, making them especially of interest in the fields of materials sciences, nanotechnology, and renewable energy. By employing molecular dynamics simulations, the mechanical properties of two-dimensional woven nanofabrics under biaxial tension are evaluated. Two-dimensional woven nanostructures composed of graphene and graphyne nanoribbons are examined. Dynamic failure process of both graphene woven nanofabric and graphyne woven nanofabric with the same woven unit cell initiates at the edge of interlaced ribbons accompanied by the formation of cracks near the crossover location of yarns. Further stress analysis reveals that such failure mode is attributed to the compression between two overlaced ribbons and consequently their deformation under biaxial tension, which is sensitive to the lattice structure of nanoribbon as well as the density of yarns in fabric. Systemic comparisons between nanofabrics with different yarn width and interval show that the strength of nanofabric can be effectively controlled by tuning the space interval between nanoribbons. For nanofabrics with fixed large gap spacing, the strength of fabric does not change with the ribbon width, while the strength of nanofabric with small gap spacing decreases anomalously with the increase in yarn density. Such fabric strength dependency on gap spacing is the result of the stress concentration caused by the interlace compression. The outcomes of simulation suggest that the compacted arrangement of yarns in carbon woven nanofabric structures should be avoided to achieve high strength performance.

Harvest of ocean energy by triboelectric generator technology

Water wave energy widely distributed in the globe is one of the most promising renewable energy sources. However, it has not been effectively exploited by current energy harvesting technologies which primarily rely on electromagnetic generator (EMG). EMGs have various limitations, especially when operating in environment with irregular and/or low frequencies ( < 5 Hz) wave motions. Triboelectric nanogenerators (TENGs) exhibit obvious advantages over EMG in harvesting energy from low-frequency water wave motions. The networking of TENGs has been regarded as a potential method towards large-scale blue energy harvesting. In this review, recent progress of the TENG technology for blue energy harvesting is presented, including a comparison between TENG and EMG in physics and engineering design, and the fundamental mechanism of nanogenerator based on Maxwells displacement currents is systematically introduced. The review of hydrodynamic TENG, liquid-solid contact electrification TENG, hybrid (dual-modes) TENG, fully enclosed TENG, and TENG network for blue energy harvesting is discussed. The TENG networks are expected to harvest large-scale blue energy from the ocean, which will be a feasible approach for realizing the blue energy dream. Moreover, the energy harvested by TENG from various sources, such as human motion and vibration, is not only new energy, but more importantly, energy using for the new era-the era of internet of things.At the request of the authors, this article is being retracted effective 15 May 2019.Water wave energy widely distributed in the globe is one of the most promising renewable energy sources. However, it has not been effectively exploited by current energy harvesting technologies which primarily rely on electromagnetic generator (EMG). EMGs have various limitations, especially when operating in environment with irregular and/or low frequencies ( < 5 Hz) wave motions. Triboelectric nanogenerators (TENGs) exhibit obvious advantages over EMG in harvesting energy from low-frequency water wave motions. The networking of TENGs has been regarded as a potential method towards large-scale blue energy harvesting. In this review, recent progress of the TENG technology for blue energy harvesting is presented, including a comparison between TENG and EMG in physics and engineering design, and the fundamental mechanism of nanogenerator based on Maxwells displacement currents is systematically introduced. The review of hydrodynamic TENG, liquid-solid contact electrification TENG, hybrid (dual-modes) ...

Tunable thermal conductivities of graphene and graphyne under in-plane torsion

Using the non-equilibrium molecular dynamics method, the thermal properties of two dimensional nanomaterials are investigated by considering graphene and graphyne nanosheets with circular boundaries. The thermal transport efficiency of graphene and graphyne under heat flux from the inner boundary to outer boundary is revealed to be tunable by applying in-plane torsion at the inner boundary, and the tunable range of thermal conductivity for graphyne could be up to 37% (15% for graphene). With the increase of rotation angle, the thermal conductivities of both graphene and graphyne are found to increase at small rotation angles and then decrease after the occurrence of wrinkle deformation. The maximum thermal conductivity appears at the onset of wrinkling which depends on the lattice structure and stiffness of the nanosheets. By systematically investigating the morphological characteristics and the phonon spectra under different torsion angles, the tunable thermal conductivities of both graphene and graphyne are found to be controlled by three factors including surface smoothness, stress concentration and lattice instability. The increase of thermal conductivity with small torsion angles is caused by the suppressed surface fluctuation which decreases the phonon scattering, while the wrinkling and lattice instability occurring under large torsion angles accounts for the deterioration of thermal conductivity. Since the fluctuation of graphyne is efficiently compressed at smaller torsion angles compared to graphene, the maximum thermal conductivity of graphyne appears earlier than graphene. Such correlation between out-of-plane deformation and in-plane thermal conductivity provides new insights into the thermal management of two dimensional nanomaterials.

Castigliano’s Second Theorem for Deformation Determination of a Cracked Body

AbstractIn this paper, a continuum theory capable of describing the deformation of a cracked body based on Castigliano’s second theorem is presented. The additional deformation due to the crack presence is described by the concept of stress intensity factor (SIF) of linear elastic fracture mechanics. Both the crack opening distance and the body deformation can be easily computed for any arbitrary loading on any boundary. As a demonstration, the proposed theory is applied to cases of edge-cracked infinite plane and edge-cracked beam, and the results show high accuracy when compared to those predicted by the numerical finite-element method.