Lanqing Xu

Strain engineering of thermal conductivity in graphene sheets and nanoribbons: a demonstration of magic flexibility

Graphene is an outstanding material with ultrahigh thermal conductivity. Its thermal transfer properties under various strains are studied by reverse nonequilibrium molecular dynamics. Based on the unique two-dimensional structure of graphene, the distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties are demonstrated under strains. For example, the corrugation under uniaxial compression and helical structure under light torsion, as well as tube-like structure under strong torsion, exhibit enormously different thermal conductivity. The important robustness of thermal conductivity is found in the corrugated and helical configurations of graphene nanoribbons. Nevertheless, thermal conductivity of graphene is very sensitive to tensile strain. The relationship among phonon frequency, strain and thermal conductivity are analyzed. A similar trend line of phonon frequency dependence of thermal conductivity is observed for armchair graphene nanoribbons and zigzag graphene nanoribbons. The unique thermal properties of graphene nanoribbons under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.

Mechanical properties of grafold: a demonstration of strengthened graphene

Morphological patterns and structural features play crucial roles in the physical properties of functional materials. In this paper, the mechanical properties of grafold, an architecture of folded graphene nanoribbon, are investigated via molecular dynamics simulations and intriguing features are discovered. In contrast to graphene, grafold is found to develop large deformations upon both tensile and compressive loading along the longitudinal direction. The tensile deformation is plastic, whereas the compressive deformation is elastic and reversible within the strain range investigated. The calculated Youngs modulus, tensile strength, and fracture strain are comparable to those of graphene, while the compressive strength and strain are much higher than those of graphene. The length, width, and folding number of grafold have distinctive impacts on the mechanical performance. These unique behaviors render grafold a promising material for advanced mechanical applications.

Graphene-nanotube 3D networks: intriguing thermal and mechanical properties

Carbon-based nanomaterials have drawn strong interest for potential applications due to their extraordinary stability and unique mechanical, electrical and thermal properties. For the minimization of microelectronics/micromechanics circuits, bridging the low dimensional microscopic structure and mesoscopic modeling is indispensable. Graphene and carbon nanotubes are suggested as ideal ‘building blocks’ for the bottom-up strategy, and recently the integration of both materials has stimulated research interests. In this work we investigated the thermal and mechanical performance in the pillared-graphene – constructed by combining graphene sheets and carbon nanotubes to create a three-dimensional nano network. Reverse non-equilibrium molecular dynamics simulations were carried out to analyze the thermal transport behavior. The obtained thermal conductivities are found to be possibly isotropic in two specific directions or highly anisotropic for certain structure configurations. In the mechanical performance analysis, tensile deformations are loaded along graphene plane and along tube axis. The elongation responses and stress-strain relations are observed to be nearly linear, and the calculated strength, fracture strain and Youngs moduli are lower than the pristine graphene or carbon nanotubes. The alterations in the thermal and mechanical performances are ascribed to the bond conversion on the junctions.

Mechanical properties of highly defective graphene: from brittle rupture to ductile fracture

Defects are generally believed to deteriorate the superlative performance of graphene-based devices but may also be useful when carefully engineered to tailor the local properties and achieve new functionalities. Central to most defect-associated applications is the defect coverage and arrangement. In this work, we investigate, by molecular dynamics simulations, the mechanical properties and fracture dynamics of graphene sheets with randomly distributed vacancies or Stone-Wales defects under tensile deformations over a wide defect coverage range. With defects presented, an sp-sp(2) bonding network and an sp-sp(2)-sp(3) bonding network are observed in vacancy-defected and Stone-Wales-defected graphene, respectively. The ultimate strength degrades gradually with increasing defect coverage and saturates in the high-ratio regime, whereas the fracture strain presents an unusual descending-saturating-improving trend. In the dense vacancy defect situation, the fracture becomes more plastic and super-ductility is observed. Further fracture dynamics analysis reveals that the crack trapping by sp-sp(2) and sp-sp(2)-sp(3) rings and the crack-tip blunting account for the ductile fracture, whereas geometric rearrangement on the entire sheet for vacancy defects and geometric rearrangement on the specific defect sites for Stone-Wales defects account for their distinctive rules of the evolution of the fracture strain.

Mechanical Properties of Graphene Nanobuds: A Molecular Dynamics Study

Abstract: Carbon-based hybrid nanostructures are believed to entail certain advantages of their parent low-dimensional materials and would serve as building blocks to bridge the nanoscale geometry to the large-scale application requirements. Fullerene, carbon nanotubes and graphene are suggested as ideal ‘building blocks’ for this kind of bottom-up strategy. In this work a series of hybrid gra-phene/fullerene architectures, termed graphene nanobuds, are constructed by attaching or fusing C 60 molecules on a defect graphene sheet and the mechanical properties are investigated through molecular dynamics simulations. The elastic moduli are observed to degrade by a certain amount but are still rather high. The obtained Young’s moduli are of several hundred GPas and the tensile strengths are above 50 GPa. Furthermore, the intriguing feature of the nearly linear stress-strain response could attract intense follow-up investigations and could be utilized in various application branches such as position sensing.

A molecular dynamics investigation of the mechanical properties of graphene nanochains

In this paper, we investigate, by molecular dynamics simulations, the mechanical properties of a new carbon nanostructure, termed a graphene nanochain, constructed by sewing up pristine or twisted graphene nanoribbons (GNRs) and interlocking the obtained nanorings. The obtained tensile strength of defect-free nanochain is a little lower than that of pristine GNRs and the fracture point is earlier than that of the GNRs. The effects of length, width and twist angle of the constituent GNRs on the mechanical performance are analyzed. Furthermore, defect effect is investigated and in some high defect coverage cases, an interesting mechanical strengthening-like behavior is observed. This structure supports the concept of long-cable manufacturing and advanced material design can be achieved by integration of nanochain with other nanocomposites. The technology used to construct the nanochain is experimentally feasible, inspired by the recent demonstrations of atomically precise fabrications of GNRs with complex structures [Phys. Rev. Lett., 2009, 102, 205501; Nano Lett., 2010, 10, 4328; Nature, 2010, 466, 470].

Defect-activated self-assembly of multilayered graphene paper: a mechanically robust architecture with high strength

In this work molecular dynamics simulations are carried out to investigate the defect-mediated self-assembly of graphene paper from several layers of graphene sheets with vacancy defects. Tensile and shear deformations are applied to the obtained architectures to investigate both the in-plane and the out-of-plane mechanical properties. The effect of incipient defect coverage is analyzed and super-ductility is observed in the high defect density situation. While the stiffness and strength decrease with the increasing of incipient defect coverage under in-plane deformations, they increase under out-of-plane deformations, which can be attributed to the enhanced defect-induced interlayer cross-linking. Effects of crack-like flaws are also investigated to demonstrate the robustness of this structure. Our results demonstrate that defects, which are sometimes unavoidable and undesirable, can be engineered in a favorable way to provide a new approach for graphene-based self-assembly of vertically aligned architectures with mechanical robustness and high strength.

Target recognition in a turbid medium with backscattered polarization patterns

Optical systems have been extensively studied for target recognition in a turbid medium because of its promising applications in many fields such as driver assistant system. Several recent studies have demonstrated that relevant information of the turbid medium including the hidden object in the medium can be derived by analyzing the polarization state of diffusely backscattered light of the sample. In this paper Stokes/Mueller formula was introduced to investigate polarized light transportation in a turbid medium such as atmosphere; Mie scattering theory was applied to calculate the scattering property of polarized light; Monte Carlo method was used to compute backscattered polarization patterns from a turbid medium containing hidden object. Results showed that the backscattered polarization patterns are strongly influenced by optical parameters of the medium. The two-dimensional distribution of degree of polarization (DOP) calculated from backscattered Mueller matrix can well discriminate different objects within limited distance. For applications in driver assistant system, the effective recognition distance in a foggy weather was also calculated; and results could be several times of visibility distance.