Small-angle X-ray-scattering study of silver-nanocrystal disorder-order phase tr

2019-11-01 15:40:03

phase potential packing nanocrystal lang

责任者: Korgel, B.A.;Fitzmaurice, D. 单位: Dept. of Chem., Univ. Coll. Dublin, Ireland 来源出处: Physical Review B (Condensed Matter)(Phys. Rev. B, Condens. Matter (USA)),1999/06/01,59(22):14191-201 摘要: A conceptually unique approach was developed to study the interparticle interactions between organized alkanethiol-capped silver nanocrystals. Dense nanocrystal fluids were formed by evaporating the solvent from a “size-polydisperse” (σ~±12%) nanocrystal dispersion on a substrate. The sample polydispersity prevented the disorder-order phase transition (i.e., superlattice formation) from occurring. Small-angle X-ray scattering was then used to measure the static structure factors S(q), of these disordered nanocrystal films as a function of the ratio ⟨L⟩/R between the capping ligand chain length to the core nanocrystal radius. The pair-distribution and direct correlation functions were then calculated from Fourier transformations of S(q). This enabled the use of the hypernetted chain approximation to calculate the pair interparticle potential u(r). The 6-12 Lennard-Jones potential provided reasonable fits to all experimentally determined values of u(r), indicating the predominance of relatively short-range repulsion between nanocrystals. Monodisperse dodecanethiol- and octanethiol-capped silver nanocrystals were then condensed into ordered arrays. Face-centered-cubic (fcc) packing was favored for ⟨L⟩/R<0.60, and body-centered-cubic (bcc) packing was favored when ⟨L⟩/R>0.60. Lower-symmetry body-centered-tetragonal packing was observed for octanethiol-capped silver nanocrystals with ⟨L⟩/R>0.66. A simple model employing the experimentally determined values for u(r), predicts that the fcc→bcc superlattice phase transition occurs when ⟨L⟩/R≳0.65 关键词: disperse systems;Lennard-Jones potential;metallic superlattices;metallic thin films;nanostructured materials;order-disorder transformations;quantum dots;self-assembly;silver;X-ray scattering;small-angle X-ray-scattering;Ag;SAXS;disorder-order phase transitions;interparticle interactions;polydispersity;static structure factors;capping ligand chain length;core nanocrystal radius;pair-distribution functions;direct correlation functions;Fourier transformations;hypernetted chain approximation;pair interparticle potential;6-12 Lennard-Jones potential;short-range repulsion;fcc packing;bcc packing;fcc→bcc superlattice phase transition;alkanethiol-capped nanocrystal films