We study the dynamics of solid islands deposited on nanopillars using kinetic Monte Carlo simulations. The islands are initially placed on the top of the pillars, in the so-called Cassie-Baxter state. For high pillars, the dynamics is divided into two phases. The first phase corresponds to the deterministic and irreversible impalement of the island. The dynamics of this phase is governed by surface diffusion. Once the island has collapsed, a second phase is observed where the island exhibits Brownian motion along the pillars, characterized by a diffusion constant D i and a kinetic coefficient K i accounting for the interaction of the island with the top of the pillars. The random walk stops when the island reaches the bottom of the substrate, where it sticks irreversibly. When the island wettability is small, the island diffusion constant D i is controlled by adatom diffusion, and scales as the inverse of the number of atoms in the island. In contrast, for large wettabilities, we observe that D i oscillates as the island size is increased. The minimum of the oscillations corresponds to nucleation-limited dynamics, where D i is independent of the island size. We also determine the time for partial irreversible collapse on shorter pillars, leading to the so-called Wenzel state. Finally, we discuss the orders of magnitude of the typical duration of these processes.

Solids and liquids are both known to exhibit Cassie-Baxter states, where a drop or a solid nanoparticle is maintained on top of pillars due to wetting forces. We point out that due to elastic strain, solid nanocrystals exhibit a behavior different from that of liquids. First, the equilibrium Cassie-Baxter state on a single pillar exhibits a spontaneous symmetry breaking due to elastic effects. The second consequence of elasticity is the existence of stable partially impaled states, resulting from a compromise between wetting forces which favor impalement and elastic strain which resists impalement. Based on kinetic Monte Carlo simulations which include elastic strain, we discuss these effects and we propose a global phase diagram for the stability of nanocrystals on nanopillars.

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Some of these parameters are defined by the installation used: this is the case in particular of the incident beam. At ALTO we use an electron beam of 50 MeV energy and 10 μA intensity to induce photofission [1].