title: Two-Dimensional Nanoparticle Supracrystals: A Model System for Two-Dimensional Melting.
abstract: In a Langmuir trough, successive compression cycles can drive a two-dimensional (2D) nanoparticle supracrystal (NPSC) closer to its equilibrium structure. Here, we show a series of equilibrated 2D NPSCs consisting of gold NPs of uniform size, varying solely in the length of their alkanethiol ligands. The ordering of the NPSC is governed by the ligand length, thus providing a model system to investigate the nature of 2D melting in a system of NPs. As the ligand length increases the supracrystal transitions from a crystalline to a liquid-like phase with evidence of a hexatic phase at an intermediate ligand length. The phase change is interpreted as an entropy-driven phenomenon associated with steric constraints between ligand shells. The density of topological defects scales with ligand length, suggesting an equivalence between ligand length and temperature in terms of melting behavior. On the basis of this equivalence, the experimental evidence indicates a two-stage 2D melting of NPSCs.
authors.forename: Jin Young
authors.initials: JY
authors.lastname: Kim
authors.forename: S Joon
authors.initials: SJ
authors.lastname: Kwon
authors.forename: Jae-Byum
authors.initials: JB
authors.lastname: Chang
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Ross
authors.forename: T Alan
authors.initials: TA
authors.lastname: Hatton
authors.affiliation: Institute of Materials, École Polytechnique Fédérale de Lausanne , MXG Station 12, 1015 Lausanne, Switzerland.
authors.forename: Francesco
authors.initials: F
authors.lastname: Stellacci
keywords: 2D melting; 2D nanoparticle supracrystals; equilibrated nanoparticle film; hexatic phase