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.PbSe nanocrystal superlattices | thermodynamics | ligand interaction S elf-assembly of colloidal nanocrystals (NCs) not only is a fascinating phenomenon, but also offers a promising route for design and fabrication of novel materials (1, 2). NCs possess unique size-and shape-dependent structures and properties that differ from those of their bulk counterparts (3). Analogous to formation of a crystal from constituent atoms/ions, highly uniform NCs can self-assemble into periodically ordered structures, referred to as superlattices or "artificial crystals." In addition to the unique properties of individual NCs, these NC superlattices manifest new collective behavior [such as electronic, plasmonic, magnetic, and catalytic properties (4)] through near-field coupling of neighboring NCs (5). In the past two decades, various NC superlattices were developed successfully via the controlled assembly of colloidal nanoparticles (2,(6)(7)(8)(9)(10)(11)(12)(13)(14), and, at the same time, extensive simulation efforts have been devoted to understanding this complicated process and to predict the assembly patterns (15-17). To achieve rational design of NC superlattices, however, it is essential to have a fundamental understanding of the mechanisms underlying their formation, including their energetics of formation and polymorphism, which form the focus of this study.Colloidal NCs usually are composed of a hard inorganic core and a soft shell of organic molecules. The organic shell helps stabilize the inorganic core, allows the formation of an almost monodisperse size distribution of NCs, and enables dispersion of NCs in an organic solvent. NC superlattices can be synthesized, starting with well-dispersed NCs in a solvent via slow and controlled solvent evaporation. We chose PbSe as a model system because of the relative ease of preparing NC building blocks. In addition, PbSe NC superlattic...

Hard convex lens-shaped particles: Densest-known packings and phase behavior

“…3(d). 3. bco-like crystal: This construction uses a Bain transformation 70,71 that views the fcc crystal of hard spheres as a special body-centered-orthorhombic (bco) crystal [ Fig. 4(a)].…”

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…1,2 Colloidal NCs stabilized by organic surfactants have been shown to pose excellent building blocks for the design of synthetic MCs with tailored structural properties which are conveniently obtained by self-assembly of NCs from solution on a solid or liquid substrate by exploiting ligand-ligand 3 interactions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Typically, the utilized ligands consist of wide-gap, bulky hydrocarbons which render the MCs insulating. [26][27][28][29][30][31][32][33] MCs obtained in this way exhibit average grain sizes of ~150 µm 2 , which enables a detailed characterization by electron and/or X-ray microscopy.…”

Nanoscale Study of Clausthalite-Bearing Symplectites in Cu-Au-(U) Ores: Implications for Ore Genesis

“…The latter is mostly observed around larger blebs or rods and is also associated with an increase in the offset in the orientation between clausthalite and the host. Self-assembly of nanocrystals into superlattices has been documented experimentally in PbSe, with three types of atom packing, one of which is tetragonal [51]. However, the superlattices given by Quan et al [51] are smaller than those observed here in the tetragonal chalcopyrite matrix.…”

“…Self-assembly of nanocrystals into superlattices has been documented experimentally in PbSe, with three types of atom packing, one of which is tetragonal [51]. However, the superlattices given by Quan et al [51] are smaller than those observed here in the tetragonal chalcopyrite matrix. None of the FFTs or SAEDs obtained from the mottled areas show the simple lattice of clausthalite (as, for example in the CcI study case) but instead show satellite reflections on SAEDs, representative of chalcopyrite on different zone axes.…”