Mechanical and electrical properties of nanostructured ‘plastic metals’ Bartosz A. Grzybowski a,b, * , Christopher E. Wilmer a , Marcin Fiałkowski a a Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd., IL 60208, United States b Department of Chemistry, Northwestern University, 2145 Sheridan Rd., IL 60208, United States article info Article history: Available online 22 June 2009 Keywords: Electrical and electronic properties Mechanical properties abstract Deformable, supraspherical (SS) aggregates of metal nanoparticles (NPs) connected by long-chain dithiol ligands self-assemble into nanostructured materials of macroscopic dimensions. These materials are both malleable/moldable at room temperature as well as electrically conductive. Upon gentle heating, they harden into polycrystalline metals. The paper discusses qualitative models that rationalize the mechan- ical and electrical properties of these unusual ‘plastic metals’. Ó 2009 Published by Elsevier B.V. 1. Introduction Assemblies of nanoscopic components have recently attracted considerable scientific attention both for their unique fundamental properties [1–3] as well as potential technological applications in optoelectronics [4], high-density data storage [5], catalysis [6], and biological sensing [7], to name just a few. Although various techniques have been developed for the preparation of such assemblies at the microscale (e.g., nanoparticle superlattices [8,9] three-dimensional crystals [1,10,11]), up-scaling these procedures to freestanding, macroscopic materials [12] has proven challeng- ing. We have recently described a procedure that overcomes these limitations by a two-step, hierarchical self-assembly process (SS) [13,14]. In our method (Fig. 1), individual metal nanoparticles coated with appropriate self-assembled monolayers first self- assemble into deformable spherical aggregates (‘supraspheres,’ SS) and then ‘glue’ together like pieces of Play-doh Ò (in Europe, plasticine) into macroscopically-sized materials. These materials have several remarkable properties (Fig. 2). On one hand, they are plastic and moldable into arbitrary shapes; on the other, they are electrically conductive and – despite their metal/insulator nat- ure – show linear current–voltage (I–V) characteristics without any thresholds for conductance. In addition, owing to the metastability of the supraspherical building blocks, they can be hardened at low temperatures – indeed, as low as 40 °C – and structurally evolved into polycrystalline metals. The low-temperature processability of these ‘plastic metals’ is in sharp contrast to – and in some sense, quite ‘opposite’ of – the traditional metals and alloys that become malleable only at very high temperatures and solidify upon cool- ing. The present paper revisits two important aspects of the plastic metals. First, we investigate the mechanical properties of the ‘sticky’ SS building blocks that give rise to the plasticity of the assembled materials. Second, we attempt to rationalize the electri- cal conductivity of these materials and its relationship to the underlying nanoscale structure. While the models we discuss re- flect the trends observed in experiments, they do so only qualita- tively and should therefore be construed only as starting points for more through studies of the unusual properties of plastic metals. 2. Self-assembly of the supraspheres and processing of ‘plastic metals’ In a typical synthetic procedure, 2 mM solutions of metal (Au, Ag, Pt, or Pd) nanoparticles 5 nm in diameter and with dispersity r  10%; were stabilized in toluene by DDA (dodecylamine; 35 mM) capping agent and DDAB (didodecyldimethylammonium bromide; 10 mM) surfactant. To such solutions, different amounts (up to 2.4 mM) of cross-linking dithiol (DT) ligands were added. The dithiols could be either based on all-carbon alkyl chains [14] or could incorporate other chemical functionalities [13] (the exact composition was not crucial to the properties of the assembling supraspheres). When the dithiol was rapidly added to the NP solu- tion mixture (Fig. 1(A) and (B)), it displaced portion of the loosely bound surfactants and simultaneously cross-linked the NPs. The cross-linking continued until all NPs in solution were aggregated into spherical, internally disordered aggregates (SS), each com- posed of hundreds to thousands of nanoparticles (Fig. 1(B)). The fi- nal sizes of the supraspheres depended on the concentrations of the DT cross-linkers and/or of the NPs used. By varying these parame- ters [13,14], it was possible to prepare SS of diameters ranging from 80 to 320 nm. Importantly, irrespective of dimensions, the supra- spheres were able to deform and stick to one another upon contact (Fig. 1(C)). This property was central to the formation of extended 0022-3093/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2009.05.035 * Corresponding author. Address: Department of Chemical and Biological Engi- neering, Northwestern University, 2145 Sheridan Rd., IL 60208, United States. E-mail address: grzybor@northwestern.edu (B.A. Grzybowski). Journal of Non-Crystalline Solids 355 (2009) 1313–1317 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol