Reentrant Adhesion Behavior in Nanocluster Deposition

We simulate the collision of atomic clusters with a weakly attractive surface using molecular dynamics in a regime between soft landing and fragmentation, where the cluster undergoes large deformation but remains intact. As a function of incident kinetic energy, we find a transition from adhesion to reflection at low kinetic energies. We also identify a second adhesive regime at intermediate kinetic energies, where strong deformation of the cluster leads to an increase in contact area and adhesive energy.

Figure 1

The evolution of the velocity and radius of gyration of the 147-atom cluster is shown over the duration of the collision illustrated in a series of snapshots. The cluster has an initial velocity of $1.0(\epsilon /m{)}^{1/2}$. In this case, the cluster is captured by the surface, and the velocity oscillates about zero after the collision.

Figure 2

The changes in cluster potential energy (both the cluster internal energy $\Delta E^c$ and cluster total potential energy $\Delta E^{\mathrm{pot}}$) and the thermal kinetic energy ($E_{\mathrm{thermal}}^K$) are shown during the collision in Fig. 1.

Figure 3

The change in the cluster potential energy per atom ($\Delta E^c$), the change in total potential energy per atom ($\Delta E^{\mathrm{pot}}$), the thermal kinetic energy per atom $E_{\mathrm{thermal}}^K$, and the center of mass velocity are shown as a function of time in a collision that results in reflection. The cluster is a 147-atom icosahedron, and its initial velocity is $1.0(\epsilon /m{)}^{1/2}$.

Figure 4

The probability of sticking as a function of incident velocity for each cluster size. At each velocity, we conducted 50 trial simulations with randomly selected cluster orientations. Note the minimum and maximum in adhesion probability (at 1 and 2, respectively).

Figure 5

The coefficient of restitution as a function of initial velocity $v_0$ for $c=0.35$ for all three cluster sizes. Also shown (on the right-hand axis) is the relative deformation of the 147-atom cluster as a function of velocity which is well-fitted by a quadratic in $v_0$.

Figure 6

The adhesion energy $E^a$ at the peak reflected velocity and the corresponding Weber number as a function of $v_0$ for the 147-atom cluster with $c=0.35$. The fit to the $E^a$ points is a linear function of $v_0^{0.50}$ and is shown by the solid line. Note the maximum (at 1) and minimum (at 2) in the Weber number which correspond, respectively, to the minimum and maximum in the adhesion probability in Fig. 4.