Phase Transitions in Octanethiol-Capped Ag, Au and CdS Nanocluster Assemblies
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Phase Transitions in Octanethiol-Capped Ag, Au and CdS Nanocluster Assemblies A.V. Ellis§, K. Vijayamohanan§†, C. Ryu‡, and G. Ramanath§∗ § Materials Science and Engineering Department, and ‡Chemistry Department, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. †National Chemical Laboratories, Pune, India ABSTRACT We describe phase transitions in assemblies of octanethiol (OT)-capped nanoclusters of Ag and Au and CdS of sizes ranging from 2 to 5 nm, created by a new variant of the Brust synthesis method, without the use of phase transfer agents. We probed the stability of these assemblies by a combination of UV-Visible spectrometry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and in situ polarized microscopy (PLM). Our results show that Ag nanoclusters form a crystalline assembly at ambient temperatures. Upon heating, these assemblies undergo two reversible phase transitions corresponding to melting of (i) a phase comprised of excess thiols that are not linked to the nanoclusters, and (ii) the nanocluster assembly, at ~60 and 124 °C, respectively. In contrast, Au nanocluster assemblies are softer, waxy solids, and show sub-zero melting transitions. Both these assemblies show no observable mass loss up to ~180 °C. CdS nanocluster assemblies are also waxy solids, but show a nonreversible melting transition at 137 °C, with simultaneous mass loss due to OT desorption. From our results the thermal stability of the nanoclusters was OT-Ag > OT-Au > OT-CdS. INTRODUCTION Creating ordered assemblies of protected organic-inorganic hybrid assemblies, with selective sizes, is of great technological interest, and is critical in reaping the benefits of these properties in potential applications such as microelectronics, sensors, catalysis, nonlinear optics and single electron devices [1-3]. For example, assembly of superstructures of semiconductor nanoclusters (quantum dots) comprising of arrays of alternating small and large sized clusters adjacent to each other could give rise to resonant tunneling effects that could be harnessed to make nanodevices for optical applications [4]. From a fundamental viewpoint, it is of interest to understand the thermal and chemical stability of different nanocluster assemblies. Nanoclusters that are isolated from each other, e.g., by capping agents, tend to coalesce slower than their uncapped counterparts, and are hence more stable. Typically these types of “protected” nanoclusters can be prepared by different methods, including hydrolysis and reduction of precursors, thermal decomposition, photolysis and nanosphere lithography [5, 6]. Commonly studied monolayer-protected nanoclusters involve the attachment of a thiol to the nanocluster surface, be it single metals [7], alloys [8] or semiconductors [1]. Of particular importance is maximizing the self-assembly of the thiol protecting layer to produce strong binding asymmetry and a high degree of conformal packing. This effectively exploits chainlength dependent lateral van der Waals forces, thus preventi
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