Computer simulations of the effect of atomic structure and coordination on the stabilities and melting behaviour of copper surfaces and nano-particles Thomas D. Daff a,b, * , Iman Saadoune b , Isabelle Lisiecki c , Nora H. de Leeuw b a School of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK b Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK c Laboratoire LM2N, UMR CNRS 7070, Universite Pierre et Marie Curie, Bat F, 4 Place Jussieu 75005 Paris, France article info Article history: Received 25 August 2008 Accepted for publication 13 November 2008 Available online 6 December 2008 Keywords: Surface melting Surface structure, morphology, roughness, and topography Nano-particles Surface defects Copper Computer simulations Molecular dynamics abstract We have studied the structures and stabilities of copper nano-particles and the melting properties of cop- per surfaces using interatomic potential-based molecular dynamics simulations, where the (1 1 1) surface has been shown to be the most stable in terms of surface energy and melting behaviour. Low energy shapes of nano-particles are influenced by the surfaces present and therefore have a higher proportion of (1 1 1) surface. The effect of surface structure on stability becomes less marked as the size of the nano-particle is increased. Melting is observed to occur below the bulk melting temperature in all the surfaces investigated, at increasingly lower temperatures from the (1 1 1), (1 0 0), (1 1 0) down to the (2 1 0) surface, confirming their order of decreasing stability. The melting processes of defective close- packed copper surfaces were also simulated. Steps, kinks, and facets were all shown to accelerate the melting of the surfaces. The melting is shown to initiate at the site of the defect and the results demon- strate that it is the low-coordinated atoms, at the step edge or kink, that are more mobile at lower tem- peratures. These features facilitate surface melting even further below the melting temperature than was observed for the perfect surfaces. Furthermore, facets of (1 0 0) surface were shown to be unstable even at moderate temperatures on the close-packed surface. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction In the rapidly expanding field of nanotechnology, applications for nano-particles are widespread, ranging from catalysis, electron- ics and high density magnetic recording media devices to biologi- cal identification and labelling [1–3]. The range and diversity of their uses is determined by their unique properties that differ markedly from those of both isolated atoms and the bulk phase, and which are dependent on their size and structure [4–7]. By understanding the mechanism of their formation we are able to tune their properties to specific applications. As a result, much re- search is currently directed towards synthesising and characteris- ing nano-particles, aiming to achieve better size and shape control [8,5]. This remains a big challenge since tuning the size and shape of nano-particles depends on the synthesis media and reaction conditions, where several parameters such as temperature, electro- static and sterical confinements, that influence their shape and size, are dependent on each other [9–13,4]. Hence singling out the effect of each of these parameters on the eventual shape and size of nano-particles is difficult to achieve experimentally, which makes computational research an attractive alternative or comple- mentary technique. In order to understand the mechanisms that influence the formation of different nanoclusters, the study of sur- face properties of these materials is crucial [14]. In this paper we focus on the study of copper surface properties and their effect on the stabilities of a range of copper nano-parti- cles. It has been reported that the control of copper nanocrystal size is achieved [15], although there are still open questions regarding the tuning of the shape and size of these nanocrystals, which requires a better understanding of copper surface properties at the atomic level. Several computational studies have been per- formed to study the melting behaviour of perfect copper surfaces [16,17], but with little reference to defective copper surfaces. The processes observed in the melting of perfect surfaces dem- onstrate the formation of adatoms and their behaviour at high temperatures [16,18]. The influence of the adatoms and surface features is shown as a result of melting processes [19–21] and, if we are to understand the metal surfaces’ behaviour in experimen- tal conditions, we must account for imperfections in the surface structures. The effect of defects on surface properties is found to differ from their effects on the bulk, but detailed studies are lacking [22]. Atomic Force Microscppy (AFM) and Scanning Tunneling Microscopy (STM) studies of metal surfaces have indicated the importance of steps, kinks and facets in the determination of the Equilbirum Crystal Shape (ECS) and the derivation of surface 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.11.031 * Corresponding author. E-mail address: t.daff@mail.cryst.bbk.ac.uk (T.D. Daff). Surface Science 603 (2009) 445–454 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc