Studies of Nanoclusters’ Collision Depositions on a Crystalline Surface and Graphene, and Negative Restitution Coefficient of Nanoclusters

In this thesis, we show the results of the molecular dynamics (MD) simulations of the collisions between nanoscale objects such as (i) nanocluster depositions on a crystalline surface, (ii) nanocluster depositions on a free-standing graphene, and (iii) oblique impacts of two nanoclusters. The aim of this thesis is to analyze the mechanics of nanoscale objects and to explain the results of the molecular dynamics simulations in terms of the continuum mechanics. At first, depositions of amorphous Lennard-Jones nanocluster on a crystalline surface are numerically investigated. From the results of the MD simulation, we found that the deposited nanocluster exhibits a transition from multilayered adsorption to monolayered adsorption at a critical incident speed. Employing the energy conservation law, we estimate the incident speed at which evaporation occurs during the impact. We demonstrate the estimated value well agrees with the results of the MD simulation. We also explain that the scaled adsorption parameter η ≡ Nadh/Ncls, where Nadh and Ncls are the number of atoms adsorbed on the substrate and the cluster size, respectively, is proportional to the square of the incident speed. The boundary shape of the adsorbed nanocluster depends strongly on the incident speed, and some unstable modes grow during the impact. We also analyze the wettability between different Lennard-Jones atoms, and temperature dependence of a deposited nanoclusters. At second, the motion of a free-standing graphene induced by nanocluster impact is investigated. The graphene is bended by the incident nanocluster and a transverse deflection wave is observed. We find that the time evolution of the deflection is semi-quantitatively described by the elastic beam theory. The time evolution of the temperature profile of graphene is also measured, and we demonstrate that the analysis based on the least dissipation principle well explains the result in the early stage of impact. At third, the oblique impacts of nanoclusters are studied by the MD simulation and theoretically. In the MD simulations, we explore two models – Lennard-Jones clusters and particles with covalently bonded atoms. In contrast to the case of macroscopic bodies, the standard definition of the normal restitution coefficient yields negative values for oblique collisions of nanoclusters. We explain this effect and propose a proper definition of the restitution coefficient which is always positive. We develop a theory of an oblique impact based on continuum model of particles. A good agreement between the macroscopic theory and simulations leads to the conclusion that macroscopic concepts of elasticity, bulk viscosity and surface tension remain valid for nanoparticles of a few hundred atoms.