Switchable silver clusters : crystals for novel electronics

Metallic nanoclusters form a family of novel materials with atomically defined hybrid organic/inorganic structures. Among these materials, clusters containing 44 silver atoms have emerged as one of the most interesting, because of their exceptional electronic and optical properties. Recently synthetic progresses made possible their crystallisation. Crystals of Ag44 clusters form what is defined as a superlattice, a periodic three dimensional structure, whose properties are expected to depend on the collective interaction of its constituents. In this thesis, clusters with a molecular formula Ag((SC6H4F)30)4- have been crystallized in the presence of four tetraphenyl phosponium [(C6H5)4P+] counterions and the crystals have been fully characterized. The response of these crystals to an external electric field was then tested in a two- and four- terminal configuration, and showed conductivity switching at high voltages between a less and a more conductive phase. Single crystal X-ray diffraction measurements showed that each of the conductive phase was characterized by a different crystal structure, suggesting an electric-field induced phase transition. The change in properties is attributed to the collective displacement of the tetraphenyl phosphonium counterions, which causes a change in orientation of 3 of the 30 fluorophenyl thiols that coat the cluster. Overall all these local changes lead to a global redistribution of the charges in the crystal. To the best of my knowledge, this work presents the first experimental observation of the phenomenon due to a collective behavior in a crystal of nanoclusters as well as in a crystal with more than 1000 atoms in the unit cell.