Dmitri Schebarchov

Capillary absorption of metal nanodroplets by single-wall carbon nanotubes.

We present a simple model that demonstrates the possibility of capillary absorption of nonwetting liquid nanoparticles by carbon nanotubes (CNTs) assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius there is a critical size below which a metal droplet will be absorbed. The model also explains recent observations of capillary absorption of nonwetting Cu nanodroplets by carbon nanotubes. This finding has implications for our understanding of the growth of CNTs from metal catalyst particles and suggests new methods for fabricating composite metal-CNT materials.

Grand and Semigrand Canonical Basin-Hopping.

We introduce grand and semigrand canonical global optimization approaches using basin-hopping with an acceptance criterion based on the local contribution of each potential energy minimum to the (semi)grand potential. The method is tested using local harmonic vibrational densities of states for atomic clusters as a function of temperature and chemical potential. The predicted global minima switch from dissociated states to clusters for larger values of the chemical potential and lower temperatures, in agreement with the predictions of a model fitted to heat capacity data for selected clusters. Semigrand canonical optimization allows us to identify particularly stable compositions in multicomponent nanoalloys as a function of increasing temperature, whereas the grand canonical potential can produce a useful survey of favorable structures as a byproduct of the global optimization search.

Molecular dynamics simulations of nanoparticles

Nanoparticles are becoming increasingly important in many areas of nanotechnology. Here we use classical molecular dynamics simulations to investigate the competition between surface and volumetric effects in metal nanoparticles. In particular, we review work on the melting of isolated nanoparticles, solid-solid transitions in nanoparticles and the deposition of nanoparticles on substrates. In all these examples the delicate balance between surface and volumetric effects can lead to a complex dependence of behaviour on size, from non-monoticity to more exotic phenomena that have no counterpart in bulk materials. In melting, we find that the nature of the wetting of the solid by the melt is important in determining both the melting temperature and the nature of the melting transition. Furthermore, we find that the preference of the melt to wet certain facets can induce solid-solid transitions in partially melted particles. Finally, we observe a re-entrant adhesion transition in nanoparticle deposition as the collision switches from elastic to plastic and the particle begins to spread on the surface. These examples provide an interesting insight into nanoparticle physics.

Comment on “Dynamic Catalyst Restructuring during Carbon Nanotube Growth”

’ In a recent study, Moseler et al. use environmental transmission electronmicroscopy to observe the restructuring of solid Ni catalyst during the growth of carbon nanotubes. They also simulate the evolution of the catalyst shape during growth using molecular dynamics and derive a continuum model to describe this process. The continuum model, which assumes that the catalyst restructuring is capillary-driven and surface-diffusion-mediated, is found to give quantitative agreement with the experimentally measured time scales. However, in this letter, we would like to correct the assertion by Moseler et al. that ref 2 and ref 3 (ref 32 and ref 33 in the article) contain experimental evidence that supports this continuum model. This is not the case. Rather, these works contain derivations of continuum models that are mathematically equivalent to that derived by Moseler et al. Experimental evidence which supports such models is available elsewhere (e.g., ref 4). In fact, the theory of capillary penetration was extended to droplets of finite size by Marmur, whose model we generalized and tested in the context of carbon nanotubes andmetal nanoparticles via molecular dynamics simulations. The sole distinction between the model of Moseler et al. and that in refs 2, 3, 5, and 6 lies in the mass-transport mechanism: refs 2, 3, 5, and 6 consider a fully developed Newtonian fluid flow, while Moseler et al. model a steady surface diffusion current. For constant transport coefficients (i.e., spatially independent fluid viscosity μ and surface diffusion coefficient Ds), the resultant equations of motion for the tail length (eq 2 in ref 1 and eq 2 in ref 5) have exactly the same form. An analytic solution to this particular ordinary differential equation is known, and as we will show below, it is easily generalized to deal with diffusion coefficients Ds(z) that vary in a piecewisemanner along the tube axis z (as is the case in eqs 3 and 4 of ref 1). Following ref 5, we first use volume conservation to eliminate the tail length L(t) from eq 3 of ref 1 and then integrate the resultant differential equation for the outer head radius R(t). This yields