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  • Title: Energetic and entropic contributions to self-assembly of binary nanocrystal superlattices: temperature as the structure-directing factor.
    Author: Bodnarchuk MI, Kovalenko MV, Heiss W, Talapin DV.
    Journal: J Am Chem Soc; 2010 Sep 01; 132(34):11967-77. PubMed ID: 20701285.
    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.
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