Nanoclusters (NC) are an intermediate state of matter between molecules and solids. This makes them a very promising system for both fundamental science and technologies. Due to their unique properties they are of a high interest for many applications like catalysis, biomedical or photovoltaic. All applications require understanding and control of NC properties such as size and morphology. A deep understanding of the growth process has already been achieved for Inert Gas Aggregation (IGA) that has been intensively studied in the literature [1].
A novel approach to NC synthesis, employing pulsed highly-ionized plasma has been suggested in [2]. The main benefit of this method is a high growth rate of 470 nm/s. On the other hand, there is little fundamental understanding of the process of charged NC growth from ions. Molecular dynamics is a powerful tool for simulations of NC growth, but it does not allow to accounting for charges, present in the plasma. We have developed a model that considers charges indirectly [3]. We have considered a negatively charged cluster interacting with the environment of positive ions. This model has allowed us to solve the problem of interaction of charged particles analytically. The obtained solution gives the angular distribution of velocities of ions near the cluster surface, see Fig. 1. Thus the atoms positions and velocities can be generated near the surface of the cluster without considering the charges but with angular distribution that accounts for the influence of charges on the dynamics.
Even though the distribution for particles with lowest energies, 0.03 eV looks similar to the one with 1 eV, most of those particles will still have normal incidence upon the collisions with the cluster surface. Indeed, they move so slow that the interactions with the NC inside the cutoff radius are sufficient to change angles 0. Thus, conventional assumptions made in studies of NC growth in IGA should not be affected by our results.
We simulated the growth process of Cu NC with the embedded-atom method (EAM) potential. We used a cluster consisting of 147 atoms with icosahedral morphology [1] as an initial growth seed. In every simulation an incoming particle was randomly generated on a sphere with radius 13 Å. This radius was chosen to be slightly larger than the cutoff radius of the potential. Particles were generated with velocities corresponding to 1 eV directed with angles. In order to understand the difference in kinetic processes during IGA and the growth in plasma from ions a detailed characterization of heat transfer and surface diffusion was studied.
We consider a high-energy particle as thermalized when its energy has decreased 3 times. Simulations have shown that in the IGA process the angle of incoming particles have not affected the cooling rate at all, whereas in plasma growth the nonlinear dependence has been found [3]. Besides it was shown that in the IGA process the angle of incoming particles does not influence the diffusion length, whereas in plasma growth the quadratic dependence has been found.
For the simulations of the growth process we used 147 atoms cluster as a growth seed, new atoms were randomly generated every 10 ps slightly above the cut off radius of the potential with velocity corresponding to 1eV. We considered two different cases. In the first case incoming atoms had incidence angles corresponding to the angular distribution shown in Fig. 1. In the second case incoming particles always had normal incidences. The final clusters consisted of 447 atoms in both cases. The simulations were performed in NVT-ensemble and the temperature was controlled by Nosé-Hoover thermostat at 300 K.