title: Energetic and entropic contributions to self-assembly of binary nanocrystal superlattices: temperature as the structure-directing factor.
abstract: We studied the effect of temperature on self-assembly of monodisperse colloidal nanocrystals into single-component and binary superlattices. Temperature, which serves as a weighting factor for the internal energy (U) and entropy (S) contributions to the Helmholtz free energy F = U - TS, allows tailoring relative weights of the interparticle interactions and free-volume entropy during the formation of nanocrystal superlattices. Temperature also provides a convenient tool for directing self-assembly of nanocrystals toward desired superlattice structures. We found that temperature strongly affects the structures of binary superlattices self-assembled from the mixtures of CdSe + PbS nanocrystals and PbSe + Pd nanocrystals. In the former case, small Hamaker constants for CdSe and PbS nanocrystals led to a relatively simple phase diagram, including only high-density NaZn(13)-, AlB(2)-, and NaCl-type binary superlattices. In contrast, binary superlattices self-assembled at different temperatures from PbSe and Pd nanocrystals showed a number of low-density complex phases stabilized by strong local van der Waals interactions between Pd nanocrystals. The structural diversity of nanoparticle superlattices is shown to be a result of the cooperative effect of the entropy-driven crystallization and the interparticle interactions. Both DeltaU and TDeltaS terms associated with the superlattice formation should be of the same order of magnitude, with |DeltaU| < |TDeltaS| for the assembly of CdSe and PbS nanocrystals and |DeltaU| > |TDeltaS| for the PbSe and Pd nanocrystals.
authors.affiliation: Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Maryna I
authors.initials: MI
authors.lastname: Bodnarchuk
authors.forename: Maksym V
authors.initials: MV
authors.lastname: Kovalenko
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Heiss
authors.forename: Dmitri V
authors.initials: DV
authors.lastname: Talapin
mesh_terms: D019187:Cadmium Compounds / Q000737:chemistry; D003102:Colloids / Q000737:chemistry; D007854:Lead / Q000737:chemistry; D046911:Macromolecular Substances / Q000737:chemistry; D015394:Molecular Structure; D049329:Nanostructures / Q000737:chemistry*; D018036:Selenium Compounds / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D019187:Cadmium Compounds; D003102:Colloids; D046911:Macromolecular Substances; D018036:Selenium Compounds; D013440:Sulfides; C088065:lead selenide; C018391:lead sulfide; D007854:Lead; C058667:cadmium selenide