Z.Y. Pan

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.

MECHANICS OF NANOTUBES FILLED WITH C60, C36 AND C20

The mechanical properties of single-walled nanotubes (SWNTs) filled with small fullerenes (C20, C36 and C60) were investigated using molecular dynamics (MD) simulation. The interaction between carbon atoms was described by a combination of the many-body Brenner potential with a two-body pair potential. We found that below the critical value of the strain, the stress of SWNT increases linearly with the strain and the Youngs modulus of certain SWNT with different filling densities is almost the same for small strain. It was also observed that the buckling force, which corresponds to the critical strain, becomes higher as the filling density of SWNT is increased in general. However, in the case of SWNT of larger radius filled with smaller fullerenes, the dependence of the buckling force on the filling density is expected to be different, which was attributive to the long-distance attractive interaction between atoms of fullerene and those of SWNT.

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

Energy dependence of C60–graphite surface collisions

The resilience of C60 fullerene with graphite (0001) surfaces has been investigated by means of molecular dynamics simulations with empirical model potentials. The initial energy of C60 ranges from 30 to 300 eV. It is shown that when the impact energy is above 60 eV, the rebounding energy of C60 is nearly independent of the impact energy. The scattering is highly inelastic and the internal excitation energy of the scattered molecule increases with the incident energy. These results are consistent with experiment. Furthermore, the simulations provide insight into the microscopic aspects of the scattering. The rebounding processes at different energies are found to exhibit similar dynamic behavior and the molecular center-of-mass motion can be regarded as moving in a quadratic harmonic potential. All of these results support a schematic picture to describe nonreactive C60-surface collisions.

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.

The role of energetic atoms in the deposition of Au/Au(001) thin films : a computer simulation study

The behavior of Au/Au (100) thin film growth with energetic deposition has been investigated by kinetic Monte Carlo simulations with the description of the deposition process of energetic atoms based on molecular dynamics simulation results. We present the simulation results on the morphology, islands distribution, Bragg intensity and roughness of homoepitaxial Au (100)-films growth with energetic deposition at various substrate temperatures. We found the energetic atoms can promote the nucleation and island growth in the early stages of film growth and thus enhance the smoothness of the film surface at the temperatures of film growth in three-dimensional mode and in quasi-two-dimensional mode. The atomistic mechanism that promotes the nucleation and island growth and enhances the smoothness of the film surface is discussed.

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