Y.X. Wang

Reduction of the buckling strength of carbon nanotubes resulting from encapsulation of C60 fullerenes

The effect of encapsulation of C60 fullerenes upon the buckling strength of hosting carbon nanotubes (CNTs) has been investigated, using molecular dynamics (MD) simulations. The simulation results show that encapsulating C60 fullerenes into some CNTs with diameters larger than that of the (10, 10) CNT, in particular the (14, 14) and (18, 18) CNTs, significantly reduces the buckling strength, in contrast to the conventional wisdom that fillings increase the mechanical strength of hollow structures. The simulations have also confirmed the previous findings that filling a (10, 10) CNT with C60 increases the buckling strength. Our detailed analysis reveals that the interaction between the C60 fullerenes and the hosting (10, 10) CNT is cylindrically symmetric, while the presence of a zigzag array of C60 inside the (14, 14) CNT breaks the cylindrical symmetry and so does the presence of the three arrays of C60 inside the (18, 18) CNT. The induced asymmetries cause one peak for the C60@(14, 14) system and three peaks for the C60@(18, 18) system in the corresponding force distribution along the circumferential direction. The force concentration leads to observed reduction in the buckling strength. The reduction is more severe for C60@(14, 14) than for C60@(18, 18), because the force distribution of the former system is more asymmetric.

Stable configurations of C20 and C28 encapsulated in single wall carbon nanotubes

The stable configurations of small fullerenes (C20 and C28) encapsulated inside single wall carbon nanotubes (SWNTs) of different diameters were investigated by molecular dynamics simulations. The interactions between carbon atoms were described by a combination of the many-body Brenner potential with the Lennard-Jones (LJ) potential. We observed that the filling of small fullerenes into nanotubes with diameters larger than 10.85 A ((8, 8) SWNT) is an exothermic process. During the annealing process the fullerenes arrange themselves into complex phases, which may be one-(chain), two-(zigzag) or three-dimensional, depending on the tube diameter. This tube size dependence is very similar to that of C60, which has been experimentally observed. A comparison with the prediction of the hard-sphere model also finds a satisfactory level of consistency, indicating the dense packing nature of fullerene configurations in SWNTs.

Thermal effect on DWCNTs as rotational bearings.

We have investigated the rotational motion and dynamic friction in a molecular bearing composed of double-walled carbon nanotubes (DWCNTs) using molecular dynamics simulations. The main study was on thermal effects due to the rotational friction. The diameters of the bearings varied between 6xa0and 16xa0Åxa0for the inner shafts, and between 12xa0and 20xa0Åxa0for the outer sleeves. The rotation velocity varied from 0.05xa0rotationsxa0ps(-1) to 0.25xa0rotationsxa0ps(-1). The simulations show that the energy dissipation, and hence the temperature of the system, increases linearly with rotation time. The value of energy dissipation is around 0.59xa0meV/atom per rotation at ω = 0.05xa0rotationsxa0ps(-1) for a (15, 0)@(23, 0) bearing. Correspondingly, the average friction force is around 1.75 × 10(-5)xa0nN/atom. The dependence of the energy dissipation on the rotation velocity, the interwall distance, and the contact area of the DWCNT is also discussed. It was observed that the energy dissipation becomes lowest when the interwall distance of the DWCNT bearing reaches about 0.34xa0nm, the equilibrium distance of the Lennard-Jones (L-J) potential. This low energy dissipation suggests that the DWCNT can be a good candidate for a wearless rotational bearing, which supports the previous studies.

Predicting the stability of nanodevices

A simple model based on the statistics of single atoms is developed to predict the stability or lifetime of nanodevices without empirical parameters. Under certain conditions, the model produces the Arrhenius law and the Meyer-Neldel compensation rule. Compared with the classical molecular-dynamics simulations for predicting the stability of monatomic carbon chain at high temperature, the model is proved to be much more accurate than the transition state theory. Based on the ab initio calculation of the static potential, the model can give out a corrected lifetime of monatomic carbon and gold chains at higher temperature, and predict that the monatomic chains are very stable at room temperature.

Deposition of hydrocarbon molecules on diamond (001) surfaces: atomic scale modeling

The impact induced chemisorption of hydrocarbon molecules (CH3 and CH2) on H-terminated diamond (001)-(2x1) surface was investigated by molecular dynamics simulation using the many-body Brenner potential. The deposition dynamics of the CH3 radical at impact energies of 0.1-50 eV per molecule was studied and the energy threshold for chemisorption was calculated. The impact-induced decomposition of hydrogen atoms and the dimer opening mechanism on the surface was investigated. Furthermore, the probability for dimer opening event induced by chemisorption of CH, was simulated by randomly varying the impact position as well as the orientation of the molecule relative to the surface. Finally, the energetic hydrocarbons were modeled, slowing down one after the other to simulate the initial fabrication of diamond-like carbon (DLC) films. The structure characteristic in synthesized films with different hydrogen flux was studied. Our results indicate that CH3, CH2 and H are highly reactive and important species in diamond growth. Especially, the fraction of C-atoms in the film having sp(3) hybridization will be enhanced in the presence of H atoms, which is in good agreement with experimental observations

Impact induced chemisorption of C-20 isomers on diamond (001)-(2 x 1) surface

The adsorption of low-energy C20 isomers on diamond (0 0 1)–(2×1) surface was investigated by molecular dynamics simulation using the Brenner potential. The energy dependence of chemisorption characteristic was studied. We found that there existed an energy threshold for chemisorption of C20 to occur. Between 10 and 20 eV, the C20 fullerene has high probability of chemisorption and the adsorbed cage retains its original structure, which supports the experimental observations of memory effects. However, the structures of the adsorbed bowl and ring C20 were different from their original ones. In this case, the local order in cluster-assembled films would be different from the free clusters.

Simulations of C28 chemisorption on diamond (001)-(2×1) surface: The comparison between cluster–cluster interaction and cluster–surface interaction

In this article, the dynamic behavior of C28 chemisorption on diamond (001)-(2×1) surface was investigated by molecular dynamics simulation. The many-body Brenner potential was employed to describe the interaction between carbon atoms. With the incident energy ranging from 25 to 40 eV, the single C28 was found to have more than 50% of the probability to be chemisorbed on a diamond surface and to form two C–C bonds with one dimer of the surface. Then the chemisorption of two C28 clusters was simulated at the above energy range. The cluster–cluster interaction was found to hinder the next incident cluster to be chemisorbed. Besides, the juxtaposition configuration of two C28 on the surface was observed when their impact points were along the same dimer row. For multicluster impacting, when two or three clusters formed a nucleation site, the forthcoming cluster was easily to be adsorbed close to it. The growth of the C28 cluster assembled film is typically a three dimensional island mode. Our study also showed that within the energy range the C28 clusters retained their cage structure after chemisorption. This is in agreement with experimental results.In this article, the dynamic behavior of C28 chemisorption on diamond (001)-(2×1) surface was investigated by molecular dynamics simulation. The many-body Brenner potential was employed to describe the interaction between carbon atoms. With the incident energy ranging from 25 to 40 eV, the single C28 was found to have more than 50% of the probability to be chemisorbed on a diamond surface and to form two C–C bonds with one dimer of the surface. Then the chemisorption of two C28 clusters was simulated at the above energy range. The cluster–cluster interaction was found to hinder the next incident cluster to be chemisorbed. Besides, the juxtaposition configuration of two C28 on the surface was observed when their impact points were along the same dimer row. For multicluster impacting, when two or three clusters formed a nucleation site, the forthcoming cluster was easily to be adsorbed close to it. The growth of the C28 cluster assembled film is typically a three dimensional island mode. Our study also show...

Energy dependence of methyl-radical adsorption on diamond (001)-(2 × 1) surface

The deposition of hyperthermal CH3 on diamond (001)-(2×1) surface at room temperature has been studied by means of molecular dynamics simulation using the many-body hydrocarbon potential. The energy threshold effect has been observed. That is, with fixed collision geometry, chemisorption can occur only when the incident energy of CH3 is above a critical value (Eth). Increasing the incident energy, dissociation of hydrogen atoms from the incident molecule was observed. The chemisorption probability of CH3 as a function of its incident energy was calculated and compared with that of C2H2. We found that below 10 eV, the chemisorption probability of C2H2 is much lower than that of CH3 on the same surface. The interesting thing is that it is even lower than that of CH3 on a hydrogen covered surface at the same impact energy. It indicates that the reactive CH3 molecule is the more important species than C2H2 in diamond synthesis at low energy, which is in good agreement with the experimental observation.

Structural and dynamical properties of Al clusters adsorbed on Ni surface

The impact-induced deposition of All:, clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis-Sinclair potentials. The incident kinetic energy (E-m) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However. the original icosahedral structure was transformed to the fee-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 1 1 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying

Molecular dynamics simulation of structural characteristics in metal cluster deposition on surfaces

The deposition of small metal clusters (Cu, Au and Al) on f.c.c. metals (Cu, Au and Ni) has been studied by molecular dynamics simulation using Finnis–Sinclair (FS) potential. The impact energy varied from 0.01 to 10 eV/atom. First, the deposition of single cluster was simulated. We observed that, even at much lower energy, a small cluster with (Ih) icosahedral symmetry was reconstructed to match the substrate structure (f.c.c.) after deposition. Next, clusters were modeled to drop, one after the other, on the surface. The nanostructure was found by soft landing of Au clusters on Cu with increasing coverage, where interfacial energy dominates. While at relatively higher deposition energy (a few eV), the ordered f.c.c.-like structure was observed in the first adlayer of the film formed by Al clusters depositing on Ni substrate. This characteristic is mainly attributive to the ballistic collision. Our results indicate that the surface morphology synthesized by cluster deposition could be controlled by experimental parameters, which will be helpful for controlled design of nanostructure.