IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 40 (2007) 3767–3770 doi:10.1088/0022-3727/40/12/031 Influence of crystallinity on the Rayleigh instability of gold nanowires ∗ S Karim 1,4,5 , M E Toimil-Molares 2 , W Ensinger 1,3 , A G Balogh 3 , T W Cornelius 2 , E U Khan 4 , and R Neumann 2 1 Department of Chemistry, Philipps University, Marburg, Germany 2 Gesellschaft f ¨ ur Schwerionenforschung (GSI), Darmstadt, Germany 3 Institute of Material Science, Darmstadt University of Technology, Darmstadt, Germany 4 Department of Physics, CIIT, Islamabad, Pakistan E-mail: s.karim@gsi.de Received 26 February 2007, in final form 26 April 2007 Published 4 June 2007 Online at stacks.iop.org/JPhysD/40/3767 Abstract The influence of the crystalline structure of nanowires on their thermal instability has been systematically investigated. Both poly- and single-crystalline (SC) cylindrical nanowires with diameters 87 and 132 nm transform into chains of spheres during annealing at 600–700 ◦ C. SC nanowires oriented along the 110 direction are found to be more stable, i.e. longer annealing times are needed for their complete transformation into sphere chains. Sphere size and spacing between adjacent spheres formed after decay are controlled by the crystallinity of the wires and both are larger in the case of SC nanowires. 1. Introduction Metallic nanowires are regarded as key components of future nanoscale devices. Among other important factors, the thermal stability of nanowires is crucial for a reliable performance of nanowire-based modules. Recent experimental and theoretical results demonstrate that the so-called Rayleigh instability causes the breakup of a cylindrical nanowire into a linear row of nanospheres in particular at elevated temperatures where atomic movements by diffusion become significant [1– 5]. This poses a serious obstacle to the sustained reliability of components based on nanowires. Due to their small diameter, nanowires are far more vulnerable to fragmentation by Rayleigh instability, and this spurred tremendous interest in this process. An extensive stability analysis by Nichols and Mullins explains many aspects of Rayleigh instability that induces morphological changes in cylindrical solid rods by mass transport occurring either by surface or volume diffusion [6]. Their analysis resulted in two main conclusions: (1) the rod morphology is unstable against any perturbation having a wavelength larger than the rod circumference, i.e. λ min = 2πR o with R o being the initial unperturbed radius of the rod. (2) The perturbation growth rate reaches the maximum value at ∗ This paper is dedicated to Professor Dr H Fueß on the occasion of his 65th birthday. 5 Author to whom any correspondence should be addressed. a specific wavelength, λ max . This wavelength is a function of the mass transport mechanism. Nichols and Mullins derived λ max /λ min = √ 2, 1.43, and 2.06 for surface diffusion, and internal- and external-volume diffusion, respectively. Thus, if a cylinder breaks up, the final average spacing between the resulting spheres is defined by λ max . If this decay occurs with constant volume, the diameter (d sp ) of a sphere can be calculated assuming that the volume between two consecutive radii minima transforms into a sphere. For the case of surface diffusion, d sp is given by 3.78R o . Nichols and Mullins assumed in their analysis an isotropic surface energy of the initial rod. This assumption has been demonstrated to be inadequate in many cases, for example, concerning the instability of cylindrical voids in sapphire and for Cr filaments [7, 8]. As the surface energy of the crystalline solids is generally anisotropic, varying with crystallographic orientation, analyses in [3, 9, 10] suggest a substantial influence of the anisotropy on the Rayleigh instability. Cahn was the first to improve the model by taking into account the variation of surface energy with orientation [9]. Recently, Gurski and McFadden investigated theoretically the role of anisotropic surface energy on the Rayleigh instability in nanowires and found that the anisotropy can either promote or suppress the instability [3, 5]. Experimental work on the morphological evolution (depending on crystalline orientation) of cylindrical cavities of controlled geometries and orientation in sapphire also supports these theories [7]. 0022-3727/07/123767+04$30.00 © 2007 IOP Publishing Ltd Printed in the UK 3767