Ordering of the Nanoscale Step Morphology As a Mechanism for Droplet Self-Propulsion Emelie Hilner, † Alexei A. Zakharov, ‡ Karina Schulte, ‡ Peter Kratzer, § Jesper N. Andersen, † Edvin Lundgren, † and Anders Mikkelsen* ,† Department of Physics, MAX-lab, Lund UniVersity, Box 118, 22100 Lund, Sweden, and Fachbereich Physik, UniVersita ¨t Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany Received April 14, 2009; Revised Manuscript Received May 29, 2009 ABSTRACT We establish a new mechanism for self-propelled motion of droplets, in which ordering of the nanoscale step morphology by sublimation beneath the droplets themselves acts to drive them perpendicular and up the surface steps. The mechanism is demonstrated and explored for Ga droplets on GaP(111)B, using several experimental techniques allowing studies of the structure and dynamics from micrometers to the atomic scale. We argue that the simple assumptions underlying the propulsion mechanism make it relevant for a wide variety of materials systems. Droplet dynamics on solid surfaces is a fascinating topic that involves a complex interplay between processes from the single atom level through the nanoscale up to mesoscopic length scales. Droplet motion was traditionally found to be a consequence of externally induced gradients in temperature, gravity, chemistry, or morphology. 1-5 However, self- propelled droplet motion has more recently been observed in a number of very different material systems ranging from organic liquids to metal alloys and has been intensely studied. 6-9 Droplets are found in many technologies such as self-cleaning mechanisms with tailored morphology (for example the lotus effect). 10 Self-propelled droplets are also very useful as means of mass transport especially in nanoscale systems, and they are used in droplet epitaxy and as seeds for the growth of nanowires. 11-14 In binary compounds, droplet formation and motion are frequently observed as one species condensed on the surface. 6,7,15-22 A very recent study by Tersoff et al. 9 found that Ga droplets on GaAs perform a spontaneous motion across the surface. Droplet dynamics was followed live using mirror electron microscopy, a very important tool for live surface studies at elevated temperatures; however, due to the resolution limit of this electron microscopy mode it reveals little about the nanoscale and atomic scale structure on the surface. The motion was attributed to the noncongruent evaporation, but no direct evidence on the atomic scale or nanoscale structures induced by the droplets was given. To study how dynamical behavior of micrometer size objects depend on atomic/nanoscale structure, we used an experimental combination of video rate spectroscopic photoemission and low energy electron microscopy (SPELEEM) 23,24 and scanning tunneling microscopy (STM) resolving individual atoms, enabling live imaging, and spanning many orders of mag- nitude in image size. As a result, we can present a thorough experimental investigation from micrometers to the atomic scale of Ga droplets on the binary semiconductor system GaP(111)B. The GaP(111)B surface has been extensively used for growth of III-V nanowires and nanotrees. 25 As in the work by Tersoff et al., 9 we observe spontaneous droplet motion (or self-propelled droplets); however, using several low energy electron microscopy modes at high resolution and STM we show that the motion is related to ordering of the nanoscale surface step structure, and propose an alterna- tive model for droplet propulsion based on the ordering and sublimation of surface steps. This mechanism turns out to be potentially useful both as a propulsion mechanism and as a pathway toward steering the droplet by tailoring the surface step structure. The few general assumptions of the mechanism make it relevant for many different material systems as discussed below. While only mentioned in passing, similar behavior as observed in the present materials system has presumably been observed in the eutectic Au-Si and Cu-Si systems, 21 also important for nanowire growth like the Ga droplets. Metal on metal thin film dewetting to form three-dimensional 3D islands (droplets) have also in some cases been explained by processes that involve stepped surfaces; interestingly in this case island motion perpendicular to the steps was also observed. 26 * To whom correspondence should be addressed. E-mail: anders.mikkelsen@ sljus.lu.se. † Department of Physics, Lund University. ‡ MAX-lab, Lund University. § Universita ¨t Duisburg-Essen. NANO LETTERS 2009 Vol. 9, No. 7 2710-2714 10.1021/nl9011886 CCC: $40.75  2009 American Chemical Society Published on Web 06/09/2009 Downloaded by BIBSAM CONSORTIA SWEDEN on August 7, 2009 Published on June 9, 2009 on http://pubs.acs.org | doi: 10.1021/nl9011886