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Now showing items 17 - 26 of 26

  • Formation and coalescence of linear chains in growth of nanostructured sp-sp(2) amorphous carbon films

    Ma, Tian-Bao   Hu, Yuan-Zhong   Wang, Hui  

    Formation and coalescence of linear chains are studied in low-energy atom-by-atom deposition of nano-structured sp-sp(2) amorphous carbon films. At the energy of 1 eV, the adsorption of incident atoms on the top of the surface atoms and on the top of existing chain-tips prevails, which explains the initial nucleation and the elongation of sp linear chain structures. During the whole growth process, the film experiences a transition from a sp-dominated linear chain structure to a sp(2)-dominated network structure. This transition happens when linear chains get instable beyond the critical length limit, and coalesce into the three-dimensional network structure. (C) 2008 Elsevier B.V. All rights reserved.
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  • Growth of ultrathin diamond-like carbon films by C(60) cluster assembly: Molecular dynamics simulations

    Ma, Tian-Bao   Hu, Yuan-Zhong   Wang, Hui  

    The deposition of C(60) clusters to produce cluster-assembled ultrathin diamond-like carbon (DLC) films is investigated using the molecular dynamics (MD) simulations. The deposition dynamics, especially the fragmentation process of the C(60) cluster are studied. The trajectory and instantaneous velocity of each atom in the cluster reveal a close relationship between cluster deposition and atom-by-atom deposition of DLC film. The atomistic structures, sp(3) fractions. and radial distribution functions of DLC films are quantitatively studied. For relatively low energies (E<20 eV/atom), the C(60) structural features preserve partially. The cluster-assembled films show big cavities, non-uniform structures, and rough surfaces. For relatively high energies (E>20 eV/atom), the films turn out to be structurally amorphous, densely-packed and sp(3) dominated. A marked discrepancy is observed between the sp(3) fraction of cluster and atom-by-atom deposited DLC films. The kinetic energy dissipation and the oblique incidences of freed atoms are the main reason for this discrepancy. (C) 2008 Elsevier B.V. All rights reserved.
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  • Shear-induced lamellar ordering and interfacial sliding in amorphous carbon films: A superlow friction regime

    Ma, Tian-Bao   Hu, Yuan-Zhong   Xu, Liang   Wang, Lin-Feng   Wang, Hui  

    A shear-induced phase transition from disorder to lamellar ordering in amorphous carbon films are investigated by molecular dynamics simulations. Formation of well-separated graphene-like interfacial layers is observed with large interlayer distances, diminishing and ultimately vanishing interlayer bonds, which provides a near-frictionless sliding plane. The steady-state velocity accommodation mode after the running-in stage is interfacial sliding between the graphene-like layers, which explains the experimentally observed graphitization and formation of carbon-rich transfer layers. A superlow friction or superlubricity regime with friction coefficient of approximately 0.01 originates from the extremely large repulsive and low shear interactions across the sliding interface. (C) 2011 Elsevier B. V. All rights reserved.
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  • Tribochemical Mechanism of Amorphous Silica Asperities in Aqueous Environment: A Reactive Molecular Dynamics Study

    Yue, Da-Chuan   Ma, Tian-Bao   Hu, Yuan-Zhong   Yeon, Jejoon   van Duin, Adri C. T.   Wang, Hui   Luo, Jianbin  

    Reactive molecular dynamics (ReaxFF) simulations are used to explore the atomic-level tribochemical mechanism of amorphous silica (a-SiO2) in a nanoscale, single-asperity contact in an aqueous environment. These sliding simulations are performed in both a phosphoric acid solution and in pure water under different normal pressures. The results show that tribochemical processes have profound consequences on tribological performance. Water molecules could help avoid direct adhesive interaction between a-SiO2 surfaces in pure water under low normal load. However, formation and rupture of interfacial siloxane bonds are obviously observed under higher normal load. In phosphoric acid solution, polymerization of phosphoric acid molecules occurs, yielding oligomers under lower load, and tribochemical reactions between the molecules and the sliding surfaces could enhance wear under higher load. The bridging oxygen atoms in silica play an important role in the formation of interfacial covalent bonds, and hydrogen is found to have a weakening effect on these bonds, resulting in the rupture during shear-related loading. This work sheds light on tribochemical reactions as a mechanism for lubrication and wear in water-based or other tribological systems.
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  • Achieving a superlubricating ohmic sliding electrical contact via a 2D heterointerface:a computational investigation

    Song, Aisheng   Gao, Lei   Zhang, Jie   Liu, Xiao   Hu, Yuan-Zhong   Ma, Tian-Bao   Zheng, Quanshui   Luo, Jianbin  

    Simultaneously achieving low friction and fine electrical conductance of sliding electrical contacts is a crucial factor but a great challenge for designing high-performance microscale and nanoscale functional devices. Through atomistic simulations, we propose an effective design strategy to obtain both low friction and high conductivity in sliding electrical contacts. By constructing graphene(Gr)/MoS2 two-dimensional (2D) heterojunctions between sliding Cu surfaces, superlubricity can be achieved with a remarkably lowered sliding energy barrier as compared to that of the homogeneous MoS2 lubricated Cu contact. Moreover, by introducing vacancy defects into MoS2 and substituting Cu with active metal Ti, the Schottky and tunneling barriers can be substantially suppressed without losing the superlubricious properties of the tribointerface. Consequently, a high conductivity ohmic contact with low sliding friction could be realized in our proposed Ti-MoS1.5-Gr-Ti system, which provides a potential strategy for tackling the well-known dilemma for high performance sliding electrical contacts.
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  • Three-Dimensional Graphene Networks with Abundant Sharp Edge Sites for Efficient Electrocatalytic Hydrogen Evolution

    Wang, Huaping   Li, Xu-Bing   Gao, Lei   Wu, Hao-Lin   Yang, Jie   Cai, Le   Ma, Tian-Bao   Tung, Chen-Ho   Wu, Li-Zhu   Yu, Gui  

    To achieve sustainable production of hydrogen (H-2) through water splitting, establishing efficient and earth-abundant electrocatalysts is of great necessity. Morphology engineering of graphene is now shown to modulate the electronic structure of carbon skeleton and in turn endow it with excellent ability of proton reduction. Three-dimensional (3D) graphene networks with a high density of sharp edge sites are synthesized. Electrocatalytic measurements indicate that the obtained 3D graphene networks can electrocatalyze H-2 evolution with an extremely low onset potential of about 18 mV in 0.5 m H2SO4 solution, together with good stability. A combination of control experiments and density functional theory (DFT) investigations indicates that the exceptional H-2 evolution performance is attributed to the abundant sharp edge sites of the advanced frameworks, which are responsible for promoting the adsorption and reduction of protons.
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  • Molecular dynamics simulations on atomic friction between self-assembled monolayers: Commensurate and incommensurate sliding

    Hu, Yuan-zhong   Zhang, Tao   Ma, Tian-bao   Wang, Hui  

    Atomic friction between self-assembled monolayers (SAMs) on Au(111) has been studied through molecular dynamics simulations, with emphasis on the comparison of the performances of commensurate and incommensurate SAMs in relative sliding. Results show that the shear stress on commensurate SAMs exhibits a clean periodic pattern, manifesting the atomic stick-slip friction, while random fluctuations and a much lower average value of the shear stress are observed for incommensurate sliding. The different frictional behavior can be traced to the difference in molecule movements, especially in the collective nature of the motion. In commensurate sliding, all molecules move synchronously in each period and phase, but they swing randomly and independently for incommensurate monolayers. Simulations provide clear evidence that under the same working conditions friction forces on commensurate SAMs are always higher than those in incommensurate cases. The results also show a linear dependence of shear stress on normal pressure and logarithmic dependence on sliding velocity. (c) 2006 Elsevier B.V. All rights reserved.
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  • Moire superlattice-level stick-slip instability originated from geometrically corrugated graphene on a strongly interacting substrate

    Shi, Ruoyu   Gao, Lei   Lu, Hongliang   Li, Qunyang   Ma, Tian-Bao   Guo, Hui   Du, Shixuan   Feng, Xi-Qiao   Zhang, Shuai   Liu, Yanmin   Cheng, Peng   Hu, Yuan-Zhong   Gao, Hong-Jun   Luo, Jianbin  

    Two dimensional (2D) materials often exhibit novel properties due to various coupling effects with their supporting substrates. Here, using friction force microscopy (FFM), we report an unusual moire superlattice-level stick-slip instability on monolayer graphene epitaxially grown on Ru(0001) substrate. Instead of smooth friction modulation, a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the moire superlattice structure, which is robust against high external loads and leads to an additional channel of energy dissipation. In contrast, the long-range stick-slip instability reduces to smooth friction modulation on graphene/Ir(111) substrate. The moire superlattice-level slip instability could be attributed to the large sliding energy barrier, which arises from the morphological corrugation of graphene on Ru(0001) surface as indicated by density functional theory (DFT) calculations. The locally steep humps acting as obstacles opposing the tip sliding, originates from the strong interfacial electronic interaction between graphene and Ru(0 0 0 1). This study opens an avenue for modulating friction by tuning the interfacial atomic interaction between 2D materials and their substrates.
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  • Atomic simulations of effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface

    Zhu, Peng-Zhe   Hu, Yuan-Zhong   Ma, Tian-Bao   Li, Rui   Wang, Hui  

    Understanding the effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface is critically needed for the design and development of nanoscale ultra-low friction devices. This study uses molecular dynamics simulations to explore the sliding friction behaviors of an incommensurate interface consisting of a diamond slider and a silver substrate. The instantaneous relative lattice constant is proposed to quantitatively describe the commensurability of contacting surfaces in the sliding process. It is found that when the contact size is large, the slider exhibits ultra-low friction force. While for small contact size, superlubricity behavior breaks down,which is due to the transition of incommensurate-commensurate interfacial configuration in the local contact region. It is also found that when the interfacial interaction strength is reduced below a critical value, the obvious stick-slip motion observed for the small slider with large interfacial interaction strength disappears and superlubricity behavior occurs, which results from the incommensurate interfacial configuration in the contact region maintained during the sliding process. These results provide a first demonstration that the instantaneous incommensurate-commensurate transition in the local contact region can result in the breakdown of superlubricity in a realistic three-dimensional sliding system. The obtained results not only may guide the design of nanoscale ultra-low friction devices, but also provide some insights into the origins of friction at macroscopic interfaces which usually consists of many small nanoscale contacts.
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  • Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films

    Chen, Yi-Nan   Ma, Tian-Bao   Chen, Zhe   Hu, Yuan-Zhong   Wang, Hui  

    Tribological behaviors of hydrogenated amorphous carbon (a-C:H) films Under single, asperity contact are investigated by molecular dynamics (MD) Simulations. Hydrogen concentration and normal load are found to play essential roles in the frictional behavior of a-C:H films. With low hydrogen concentration, the a-C:H film shows high adhesion 150 studied hydrogen concentrations, which is greatly enhanced with increasing normal load. At high normal loads, formation of nanocrystalline,graphene-like lamellar structures is observed locally, usually beneath the instantaneous contact area, acting as a lubricating agent, demonstrating local graphitization due to combined effects of the compression and shear process. With high hydrogen concentration, the friction shows a linear increase along with the normal load and higher load bearing capability. Hydrogen atoms accumulate on the film surface during the sliding process, reducing friction significantly. Hydrogen passivation becomes more obvious with higher hydrogen content especially at low normal loads, where a hydrogen-rich monolayer is found to attach onto the asperity, functioning as a lubricating transfer layer. This work shows the combined effects of structural transformation and surface passivation on the frictional behaviors of a-C:H films and may shed light on the well-known but riot well-understood superlubricity mechanism of a-C:H films.
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