title: Entropy-Driven Pt3Co Nanocube Assembles and Thermally Mediated Electrical Conductivity with Anisotropic Variation of the Rhombohedral Superlattice.
abstract: Understanding the shape-dependent superlattices and resultant anisotropies of both structure and property allows for rational design of materials processing and engineering to fabricate transformative materials with useful properties for applications. This work shows the structural evolution from square lattice of two-dimensional (2D) thin film to rhombic lattice of large three-dimensional (3D) assembles of Pt3Co nanocubes (NCs). Synchrotron-based X-ray supercrystallography determines the superlattice of large 3D supercrystal into an obtuse rhombohedral (Rh) symmetry, which holds a long-range coherence of both NC translation and atomic crystallographic orientation. The Rh superlattice has a trigonal cell angle of 104°, and the constitute NCs orient their atomic Pt3Co(111) planes to the superlattice Rh[111] direction. The temperature-dependent in situ small and wide-angle X-ray scattering (SAXS/WAXS) measurements reveal a thermally induced superlattice contraction of supercrystal, which maintains translational ordering but slightly develops orientational disordering. The observed increases of both the packing density and the rotation magnitude of NCs indicate a rational compromise between configurational and rotational entropies of NCs. The resultant minimization of the total free energy is responsible for the formation and stability of the obtuse Rh superlattice. The temperature-dependent in situ measurements of SAXS and electrical resistance reveal that, in conjunction with the thermally induced sharp contraction of superlattice at 160 °C, the supercrystal becomes measurable of electrical resistance, which was followed by a temperature-dependent linear increase. Upon rapid annealing from 250 °C, the supercrystal remains almost constant in both structure and electrical resistance. The heating-enabled electrical conductivity of the supercrystal at high temperature implies the formation of a NC-interconnected architecture. The experiments and overall analysis provide solid evidence and essential information for the use of shape-dependent structural anisotropies of supercrystal to create nanobased novel architecture with desired properties.
authors.affiliation: State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum , Qingdao 266580, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Department of Physics and Astronomy, University of Nevada , Las Vegas, Nevada 89154, United States.|Center for High Pressure Science and Technology and Advanced Research , Beijing 100094, China.
authors.forename: Jinlong
authors.initials: J
authors.lastname: Zhu
authors.affiliation: Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University , Ithaca, New York 14850, United States.
authors.forename: Ruipeng
authors.initials: R
authors.lastname: Li
authors.affiliation: Department of Chemistry, State University of New York at Binghamton , Binghamton, New York 13902, United States.
authors.forename: Jiye
authors.initials: J
authors.lastname: Fang
authors.affiliation: Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University , Ithaca, New York 14850, United States.
authors.forename: Zhongwu
authors.identifier: 0000-0001-9742-5213
authors.initials: Z
authors.lastname: Wang
keywords: 2D square-to-rhombic transformation; 3D rhombohedral superlattice; Pt3Co nanocube; anisotropic structure; heating-induced 1D electrical conductivity; nanocube assembly