Earth and Environmental Sciences Division andMaterials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545;
Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616;
Proc Natl Acad Sci U S A. 2014 Jun 24;111(25):9054-7. doi: 10.1073/pnas.1408835111. Epub 2014 Jun 9.
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.
纳米晶体（NCs）自组装成超晶格是一种有趣的多尺度现象，除了与体相比，单个 NCs 具有独特的性质外，它还可能导致具有新型集体性质的材料。通过使用不同的分散溶剂，我们通过同步加速器小角 X 射线散射证实合成了三种类型的 PbSe NC 超晶格——体心立方（bcc）、体心四方（bct）和面心立方（fcc）。在正己烷中的溶液量热测量表明，由分散的 NCs 形成超晶格的形成焓约为-2 kJ/mol。量热测量表明，bcc 超晶格是能量上最稳定的多晶型物，bct 的焓高 0.32 kJ/mol，fcc 的焓高 0.55 kJ/mol。这种稳定性顺序与 PbSe NCs 的堆积效率从 bcc（17.2%）降低到 bct（16.0%）和 fcc（15.2%）相一致。三种多晶型物之间的小焓差证实了能量景观的紧密间隔，并解释了在略有不同的合成条件下，不同 NC 超晶格的形成的容易程度。
