The Use of Thermally Decomposable Ligands for Conductive Films of Semiconductor Nanocrystals
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The Use of Thermally Decomposable Ligands for Conductive Films of Semiconductor Nanocrystals Andrew W. Wills1, Moon Sung Kang2, Ankur Khare2, Wayne Gladfelter1and David Norris2 1 Department of Chemistry and 2Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455-0132 ABSTRACT Poor conductivity is a bottleneck hindering the production of nanocrystal-based devices. In most nanocrystal syntheses, ligands with long alkyl chains are used to prepare monodisperse, crystalline particles. When these nanocrystals are incorporated into devices as films, the bulky ligands form an insulating layer that prevents charge transfer between particles. While annealing or post-deposition chemical treatments can be used to strip surface ligands, each of these approaches has disadvantages. Here we demonstrate the use of a novel family of ligands comprised of primary alkyl dithiocarbamates to stabilize PbSe/CdSe core-shell nanocrystals. Primary dithiocarbamates, which can bind to cadmium and lead, are known to decompose to the corresponding sulfides when heated under mild conditions. In our scheme, PbSe/CdSe core-shell nanocrystals are first synthesized with standard ligands. These ligands are then exchanged to short chain dithiocarbamates in solution. When a film is cast and annealed at low temperature, the dithiocarbamates are removed. Electron microscopy reveals that the particles move closer together, and, along with x-ray diffraction, shows that the nanocrystals remain quantum confined. Transport measurements show a 10,000-fold increase in conductivity after annealing. INTRODUCTION Semiconductor nanocrystals have attracted substantial interest due to their unique optical, electronic, and magnetic properties. These properties arise from quantum confinement, in which a carrier is isolated inside a nanocrystal that is smaller than the carrier’s natural length scale.1 In wet chemical syntheses, long chain surfactants are commonly employed to prepare uniform, crystalline particles of appropriate size. These surfactants stabilize the colloidal dispersion and passivate surface trap states but also insulate the nanocrystal from its environment. To exploit the properties of these nanocrystals in devices, the insulating ligands must be removed. The principal means of stripping ligands from semiconductor nanocrystals to date has been to deposit a film of nanocrystals and then treat the film with nucleophilic chemicals such as hydrazine,2 ethanedithiol,3 methylamine,4 or sodium hydroxide.5 Alternatively, nanocrystals have been exchanged with weakly binding ligands in solution that can be removed by annealing at elevated temperatures under vacuum.6 All of these methods are effective at increasing the conductivity of nanocrystal solids, but they also place limits on processing conditions. For example, the chemicals used in post-deposition treatment are too harsh to be used with organic semiconductors and lead to cracking in the nanocrystal film. Dispersions of
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