Wen-Jay Lee

A molecular dynamics study of the tensile behaviour of ultrathin gold nanowires

A molecular dynamics simulation of ultrathin gold nanowires for tensile behaviour has been carried out. Ultrathin nanowires, unlike the bulk FCC structure, have a multi-shell helical structure. This work compares the mechanical properties of the 7-1 multi-shell helical structure with those of the FCC structure. The results reveal that the temperature and the strain rate influence the yielding stress. One atom chain was detected before the nanowire broke when the temperature was near absolute zero. At room temperature, severe interaction between atoms caused successive yielding and a cluster of three atoms in cross-section was formed before the nanowire broke.

Modeling of polyethylene and poly (L-lactide) polymer blends and diblock copolymer: chain length and volume fraction effects on structural arrangement.

Dissipative particle dynamics (DPD), a mesoscopic simulation approach, has been used to investigate the chain length effect on the structural property of the immiscible polyethylene (PE)/poly(L-lactide) (PLLA) polymer in a polymer blend and in a system with their diblock copolymer. In this work, the interaction parameter in DPD simulation, related to the Flory-Huggins interaction parameter chi, is estimated by the calculation of mixing energy for each pair of components in molecular dynamics simulation. The immiscibility property of PE and PLLA polymers induces the phase separation and exhibits different architectures at different volume fractions. In order to observe the structural property, the radius of gyration is used to observe the detailed arrangement of the polymer chains. It shows that the structure arrangement of a polymer chain is dependent on the phase structure and has a significantly different structural arrangement character for the very short chains in the homopolymer and copolymers. The chain length effect on the degree of stretching or extension of polymers has also been observed. As the chain length increases, the chain exhibits more stretching behavior at lamellae, perforated lamellae, and cylindrical configurations, whereas the chain exhibits a similar degree of stretching or extension at the cluster configuration.

Hydrogen-bond structure at the interfaces between water/poly(methyl methacrylate), water/poly(methacrylic acid), and water/poly(2-aminoethylmethacrylamide).

The molecular dynamics approach was employed to study the structural characteristics at the interface of water/poly(methyl methacrylate) (PMMA), water/poly(methacrylic acid) (PMAA), and poly(2-aminoethylmethacrylamide) (PAEMA). It is found that the water on the PMAA surface shows a significant increase in the density at the interface, with a greater number of water molecules permeating into the bulk of the substrate region. The structure of hydrogen bonds of water and the radial distribution function for given polar atoms in the polymer substrate are calculated. We found that a network structure of hydrogen bonding between water and the polar atom of the polymer forms at the interface. PMAA exhibits a more hydrophilic property than PMMA and PAEMA because it generates a shell-like structure of water molecules around its functional group. Finally, the hydrogen bond numbers of PMMA, PMAA, and PAEMA are also analyzed. The results detail the hydrogen bond structure of each specific atom and find that, in all three cases, the carboxyl oxygen attracts the greatest number of water molecules compared with other atoms.

Structure-dependent mechanical properties of ultrathin zinc oxide nanowires.

Mechanical properties of ultrathin zinc oxide (ZnO) nanowires of about 0.7-1.1 nm width and in the unbuckled wurtzite (WZ) phase have been carried out by molecular dynamics simulation. As the width of the nanowire decreases, Youngs modulus, stress-strain behavior, and yielding stress all increase. In addition, the yielding strength and Youngs modulus of Type III are much lower than the other two types, because Type I and II have prominent edges on the cross-section of the nanowire. Due to the flexibility of the Zn-O bond, the phase transformation from an unbuckled WZ phase to a buckled WZ is observed under the tensile process, and this behavior is reversible. Moreover, one- and two-atom-wide chains can be observed before the ZnO nanowires rupture. These results indicate that the ultrathin nanowire possesses very high malleability.

Structure and dynamics of water surrounding the poly(methacrylic acid): A molecular dynamics study

All-atom molecular dynamics simulations are used to study a single chain of poly(methacrylic acid) in aqueous solutions at various degrees of charge density. Through a combination of analysis on the radial distribution functions of water and snapshots of the equilibrated structure, we observe that local arrangements of water molecules, surrounding the functional groups of COO− and COOH in the chain, behave differently and correlated well to the resulting chain conformation behavior. In general, due to strong attractive interactions between water and charged COO− via the formation of hydrogen bonds, water molecules tend to form shell-like layers around the COO− groups. Furthermore, water molecules often act as a bridging agent between two neighboring COO− groups. These bridged water molecules are observed to stabilize the rodlike chain conformation that the highly charged chain reveals, as they significantly limit torsional and bending degrees of the backbone monomers. In addition, they display different d...

A molecular dynamics study of nanoindentation on a methyl methacrylate ultrathin film on a Au(111) substrate: interface and thickness effects.

The molecular dynamics simulation model of nanoindentation is proposed in order to study the mechanical and structural deformation properties of an ultrathin MMA (methyl methacrylate) film on a Au(111) surface. First, the significant differences in the structural arrangement of MMA thin films with different thicknesses are observed. Two layers are apparent in the thinnest MMA thin film next to the Au(111) surface, while three layer structures are apparent in the thicker film. Second, this study examines the indentation tip that penetrates the MMA thin film into the Au(111) substrate in order to understand the influence of the interface on the properties and deformation behavior in both the thin film and substrate. The result shows that the indentation force is influenced both by the layer structure and by the thickness of the MMA film. The thinnest case exhibits different deformation behavior from that of the thicker cases. In addition, the deformation of MMA molecules becomes significant at the interface between the MMA film and the Au(111) surface with the increase of film thickness, and detailed deformation behavior of the Au surface for different thicknesses of MMA film is reported in this paper. Finally, both the rigid and the active models for the indentation tip are utilized in the simulation to examine the interaction differences between the tip and the film and the deformation mechanism.

Modeling of the polyethylene and poly(L-lactide) triblock copolymer: A dissipative particle dynamics study

Dissipative particle dynamics (DPD), a mesoscopic simulation approach, has been used to investigate the effect of the arrangement of the microstructure and the effect of the volume fraction on the structural properties of the immiscible polyethylene (PE)/poly(L-lactide) (PLLA) polymer in the triblock copolymer system. In this work, the interaction parameter in DPD simulation, related to the Flory-Huggins interaction parameter chi, is estimated by the calculation of mixing energy for each pair of components in molecular dynamics simulation. The immiscibility property of PE and PLLA polymers induces phase separation and exhibits different architectures at different volume fractions. In order to obtain the structural property, the radius of gyration and the end-to-end distance are used to observe the detailed arrangement of the triblock copolymer. The results show first that the relative volume fractions of PE and PLLA directly affect the bridge and loop fraction, and, second, that whether or not the arrangement is symmetrical or asymmetrical affects the equilibrium structure in the triblock copolymer system. Moreover, as the chain length of the component chains within the PE-PLLA-PE triblock copolymer increase, those component chains become softer; therefore, this directly affects the bridge and loop fraction.

Adsorption mechanism of water molecules surrounding Au nanoparticles of different sizes

Molecular dynamic simulation is used to investigate the adsorption mechanism of water molecules surrounding Au nanoparticles with different sizes. Our results show that the adsorption mechanism of the water molecules in the first water shell will be influenced by the size of the Au nanoparticle. For the larger Au nanoparticles, the hydrogen bonding of water molecules adsorbed on the surface of the Au nanoparticles are arranged in a two-dimensional structure, while those adsorbed on the edge of the surface of the Au nanoparticles are arranged in a three-dimensional structure. However, in the case of the smallest Au nanoparticle, the hydrogen bonding of the water molecules on the first adsorbed layer are arranged only in a three-dimensional structure. The arrangement of the water molecules in the first water shell can be determined by orientation order parameter. The water molecules that adsorb on the larger Au nanoparticles tend to arrange in an irregular arrangement, while those adsorbed on the smallest Au nanoparticle tend to arrange a regular arrangement. Interestingly, the water molecules adsorbed on the smallest nanoparticle are arranged in a bulklike structure in the first shell.

Dynamical behaviour of 7-1 gold nanowire under different axial tensile strains

The phonon spectrum has in recent years attracted much interest in investigating electrical transfer in nanowires. In this study, molecular dynamics is employed to calculate the phonon property of thinness for 7-1 gold nanowire at room temperature. This study finds that the phonon density distribution of 7-1 nanowire is different from both that of bulk face-centred cubic (fcc) structure and that of nanoparticles. We also observed that the high frequency of the phonon spectrum in the bulk fcc structure has a higher intensity than the low frequency of the phonon spectrum. This phenomenon, however, does not appear in the 7-1 nanowire. The strain effect of 7-1 nanowire on the phonon property has also been studied; moreover, the phonon spectra of core–core and shell–shell structures of nanowire exhibit two different modes. The relationship between the phonon profile of these core and shell structures is analysed in this study.

Penetration and Adsorption of a Water Droplet Causing Local Deformation of the Poly(methyl methacrylate) Surface

A molecular dynamics approach has been employed to study the wetting behavior of poly(methyl methacrylate) (PMMA). The flexible and rigid models of PMMA substrate are compared. The results verify that the rigid model is not suitable to act as the PMMA substrate in simulation because it prevents the diffusion of PMMA molecules, which affects the penetration behavior of water molecules. The contact angle of different sizes of water droplets on the PMMA surface and local root-mean-square roughness have been calculated. The local root-mean-square roughness increases with decreasing size of water droplet. As the number of water molecules in the droplet falls below 1000, the contact angle of the water droplet and the permeability of the PMMA substrate are affected significantly because of the lower adsorption energy between the water molecules and the diffusion of the outmost PMMA molecules on the surface. When the droplet is composed of more than 1000 water molecules, the contact angle shows an agreement with experimental results. Finally, the hydrogen-bonded number as a function of distance along the Z direction and the distribution of the hydrogen-bonded number and adjacent number in different regions are discussed, with results showing that the hydrogen-bonded number is in inverse proportion to the number of water molecules of the droplet in the region below the dividing surface.