title: Manipulating the dynamics of self-propelled gallium droplets by gold nanoparticles and nanoscale surface morphology.
abstract: Using in situ surface-sensitive electron microscopy performed in real time, we show that the dynamics of micron-sized Ga droplets on GaP(111) can be manipulated locally using Au nanoparticles. Detailed measurements of structure and dynamics of the surface from microns to atomic scale are done using both surface electron and scanning probe microscopies. Imaging is done simultaneously on areas with and without Au particles and on samples spanning an order of magnitude in particle coverages. Based on this, we establish the equations of motion that can generally describe the Ga droplet dynamics, taking into account three general features: the affinity of Ga droplets to cover steps and rough structures on the surface, the evaporation-driven transition of the surface nanoscale morphology from rough to flat, and the enhanced evaporation due to Ga droplets and Au nanoparticles. Separately, these features can induce either self-propelled random motion or directional motion, but in combination, the self-propelled motion acts to increase the directional motion even if the directional force is 100 times weaker than the random force. We then find that the Au particles initiate a faster native oxide desorption and speed up the rough to flat surface transition in their vicinity. This changes the balance of forces on the Ga droplets near the Au particles, effectively deflecting the droplets from these areas. The model is experimentally verified for the present materials system, but due to its very general assumptions, it could also be relevant for the many other materials systems that display self-propelled random motion.
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Zakharov
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Erik
authors.initials: E
authors.lastname: Mårsell
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Emelie
authors.initials: E
authors.lastname: Hilner
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Rainer
authors.initials: R
authors.lastname: Timm
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Jesper N
authors.initials: JN
authors.lastname: Andersen
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Edvin
authors.initials: E
authors.lastname: Lundgren
authors.affiliation: †Department of Physics and the Nanometer Structure Consortium (nmC@LU) and ‡MAX IV Laboratory, Lund University, P.O. Box 118, 221 00 Lund, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Mikkelsen
keywords: III−V semiconductor; SPELEEM; SPM; controlled self-propelled motion; droplets