Numerical investigation of the stability of Ag-Cu nanorods and nanowires Francesco Delogu, 1, * Elisabetta Arca, 2 Gabriele Mulas, 2 Giuseppe Manai, 3 and Igor Shvets 3 1 Dipartimento di Ingegneria Chimica e Materiali, Università degli Studi di Cagliari, Piazza d’Armi, 09123 Cagliari, Italy 2 Dipartimento di Chimica, Università degli Studi di Sassari, via Vienna 2, 07100 Sassari, Italy 3 School of Physics, Trinity College Dublin, Dublin 2, Ireland Received 22 February 2008; revised manuscript received 9 June 2008; published 8 July 2008 Molecular dynamics simulations have been employed to investigate thermally-induced phase separation processes in nanometer-sized Ag 50 Cu 50 rods and wires. In the absence of concentration gradients, the mecha- nism underlying the system decomposition consists of two stages. Roughly below 260 K, the thermal response is governed by the displacement of individual surface atoms. Above such temperature, phase separation pro- ceeds via cooperative rearrangements involving also bulklike atomic species. The result is the formation of systems with an Ag-rich phase segregated at the surface. Significantly different thermal responses are obtained in the presence of concentration gradients perpendicular or parallel to the wire axis. First, the phase separation process is favored and takes place at lower temperatures. Second, an almost complete decomposition of the system in Ag- and Cu-rich domains is obtained and not the surface segregation of the Ag-rich phase. The decomposition is also accompanied by a considerable distortion of the originally regular nanowire shape. DOI: 10.1103/PhysRevB.78.024103 PACS numbers: 64.70.Nd, 81.07.Bc, 82.60.Qr I. INTRODUCTION Nanometer-sized systems NSs are currently the focus of intense scrutiny due to the impressive suite of novel physical and chemical properties exhibited in contrast to bulk counterparts. 1–3 All such properties can be in principle connected with two fundamental aspects characteristic of NSs. 1–3 First, the fraction of atoms with unsaturated coordi- nation shells is no longer negligible and the rate of surface- involving processes such as self-assembly and catalysis is correspondingly enhanced. 1–3 Second, the number of atoms in NSs is far from the thermodynamic limit. 1–4 The con- straints imposed by classical thermodynamics can corre- spondingly significantly relax, allowing the occurrence of processes unusual for bulk phases. 1–4 The two above-mentioned factors govern to a various ex- tent the stability of NSs and their capability of withstanding external perturbations. For example, melting points and la- tent heats of transition have been shown to critically depend on the system size, 1–3,5 decreasing as it decreases. 5 Deforma- tion mechanisms and mechanical properties of NSs such as particles, wires, and tubes are affected as well by size effects. 1–3,6–9 Also the thermodynamic stability of phases is seriously undermined by the dynamics of surface atoms, which are characterized by a significantly larger mobility than bulk atoms. Precisely, this latter general observation takes a particular importance in the case of metastable phases formed by immiscible elements. The equilibrium phase diagram for immiscible systems in bulk form indicates a very small terminal mutual solubility. The latter is limited to very small atomic percentages as a consequence of the positive enthalpy of mixing. 10 In spite of this, massive crystalline alloys of immiscible elements can still be synthesized by imposing suitable kinetic constraints. 11–13 High cooling rates, 11,12 for example, or the codeformation of metals 13,14 permit one to bypass the ten- dency to decomposition originating from the positive en- thalpy of mixing 5 and keep the elements together in a crys- talline lattice. Even though the obtained bulk alloys are expected to be metastable according to equilibrium thermo- dynamics, they are stable at relatively low temperatures on long time scales. The aforementioned form of metastability is achieved by the very low mobility of atomic species that prevents, for example, an appreciable growth of local compositional fluc- tuations. In a bulk system the diffusion paths remain there- fore long enough to make decomposition a quite improbable event at relatively low temperatures. 11 In contrast, NSs are characterized by small volumes and the surface atoms pos- sess relatively high mobility. 1–3 It follows that local compo- sitional fluctuations can be unexpectedly amplified, thus originating a structural instability for NSs formed by immis- cible elements. The present work aims at investigating pre- cisely this possibility in the case of Ag-Cu NSs by exploit- ing, in particular, molecular dynamics MD methods to analyze their thermal behavior. Ag-Cu represents a classical immiscible system with positive enthalpy of mixing and metastable solubility over the whole compositional range. 5,11,13 Due to the qualitative nature of this study, it should, however, be regarded as a model immiscible system. Attention has been focused first on the role of spontaneous compositional fluctuations. To such an end, the thermal be- havior of a small nanometer-sized rod consisting of an equal number of Ag and Cu atoms randomly distributed on the crystalline lattice was characterized. Once the role of spon- taneous fluctuations was addressed, attention was shifted to a nanometer-sized wire characterized by concentration gradi- ents along its main axis. It is worth noting that all the NSs dealt with can be in principle prepared via refined solution methods 9–20 and the questions analyzed in this work could be amenable to experimental investigation. II. MOLECULAR DYNAMICS SIMULATIONS Reproducing to a satisfactory extent the basic features of metallic systems requires the use of a many-body treatment PHYSICAL REVIEW B 78, 024103 2008 1098-0121/2008/782/02410313 ©2008 The American Physical Society 024103-1