title: Energy landscape of self-assembled superlattices of PbSe nanocrystals.
abstract: Self-assembly of nanocrystals (NCs) into superlattices is an intriguing multiscale phenomenon that may lead to materials with novel collective properties, in addition to the unique properties of individual NCs compared with their bulk counterparts. By using different dispersion solvents, we synthesized three types of PbSe NC superlattices--body-centered cubic (bcc), body-centered tetragonal (bct), and face-centered cubic (fcc)--as confirmed by synchrotron small-angle X-ray scattering. Solution calorimetric measurements in hexane show that the enthalpy of formation of the superlattice from dispersed NCs is on the order of -2 kJ/mol. The calorimetric measurements reveal that the bcc superlattice is the energetically most stable polymorph, with the bct being 0.32 and the fcc 0.55 kJ/mol higher in enthalpy. This stability sequence is consistent with the decreased packing efficiency of PbSe NCs from bcc (17.2%) to bct (16.0%) and to fcc (15.2%). The small enthalpy differences among the three polymorphs confirm a closely spaced energy landscape and explain the ease of formation of different NC superlattices at slightly different synthesis conditions.
authors.affiliation: Earth and Environmental Sciences Division andMaterials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545;
authors.forename: Zewei
authors.initials: Z
authors.lastname: Quan
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616;
authors.forename: Di
authors.initials: D
authors.lastname: Wu
authors.affiliation: Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, NM 87545;
authors.forename: Jinlong
authors.initials: J
authors.lastname: Zhu
authors.affiliation: Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, 2628 BL, Delft, The Netherlands;Kavli Institute for Nanoscience, Delft University of Technology, 2628 CJ, Delft, The Netherlands; and.
authors.forename: Wiel H
authors.initials: WH
authors.lastname: Evers
authors.affiliation: Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545;
authors.forename: James M
authors.initials: JM
authors.lastname: Boncella
authors.affiliation: Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, 2628 BL, Delft, The Netherlands;
authors.forename: Laurens D A
authors.initials: LD
authors.lastname: Siebbeles
authors.affiliation: Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, NY 14853.
authors.forename: Zhongwu
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; anavrotsky@ucdavis.edu hxu@lanl.gov.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
authors.affiliation: Earth and Environmental Sciences Division and anavrotsky@ucdavis.edu hxu@lanl.gov.
authors.forename: Hongwu
authors.initials: H
authors.lastname: Xu
keywords: PbSe nanocrystal superlattices; ligand interaction; thermodynamics