Molecular dynamics simulations of cluster growth in nanostructured materials
Résumé
Cluster growth on nanostructured materials is of interest in many fields among them catalysis, nanoelectronics and thermal transport in micro and nanodevices. Recent experiments displayed interesting behavior of nanoclusters embedding carbon nanofibers and nanotubes. Especially, plasma sputtered platinum clusters are suitable as electrodes catalysts for low temperature fuel cells. Plasma sputtered Ni, Ti and W layers have interesting properties as photothermal transducers for thermal characterization of embedded nanotubes by using a UV pulsed photothermal method. In this method, metal clusters are able to absorb a high amount of UV photons leading to a rapid surface temperature rise, and then to a high IR radiation (especially for Ti). Molecular dynamics simulations are carried out for understanding the early stages of such nanosized films especially the cluster growth steps along the carbon nanofibers. These molecular dynamics simulation are aimed to understand growth phenomena of fuel cell electrodes and photothermal transducers. For this study, amorphous model carbon nanofibers of 40 nm in height and 20 nm in diameter are built. Due to periodic boundary conditions, this represents periodic array of nanofibers. The metal deposition is simulated by injecting atom each two ps. This is a suitable time delay allowing thermal relaxation of deposited atoms. Metal interaction potential is chosen as the so-called analytical tight binding - second moment approximation while metal-carbon interaction are modeled by Lennard-Jones potential as available in the literature. Relevant properties as sticking coefficients, concentration profiles along the nanofibers, cluster size distribution are calculated and compared against the properties of the impinging atoms (energy and angular distribution), which is a function of plasma deposition parameters (as pressure).