No abstract

Differences between the properties of PbS nanocrystals prepared using a large excess of PbCl2 and those prepared using other Pb sources have led to speculation that a thin PbCl x shell may be present on the surface of PbCl2-derived PbS nanocrystals. However, characterization of this chloride layer has proven to be challenging because of the poor contrast provided by electron and X-ray scattering probes. Here, we show that small-angle neutron scattering (SANS) provides the compositional sensitivity needed to unambiguously quantify the PbCl x shell that is present on the surface of PbCl2-derived PbS nanocrystals. Using a charge-balanced structural model, the scattering contribution of a ∼0.3 nm thick surface PbCl x layer is parsed separately from ligand carboxylate head groups and PbS in the core. Global fitting of the SANS data across a solvent deuteration series enables unique determination of the spatial distribution of each material. These results are corroborated by quantitative nuclear magnetic resonance (NMR) and energy-dispersive X-ray spectroscopy (EDS). This work resolves a discrepancy in reported sizing curves for PbS nanocrystals prepared by different syntheses and further demonstrates the power of SANS in resolving molecular structure in soft and hybrid nanomaterials.

Significant experimental and theoretical work has been devoted to understanding why colloidal nanocrystals (NCs) self-assemble into such a diverse array of structures. Previous research has focused on factors such as nanocrystal charging, the ratio of ligand length to core radius, core faceting, and ligand coverage among many controllable parameters. Here, we demonstrate that the presence of unbound/free ligand in colloidal suspension plays a pivotal role in determining NC superlattice (SL) structure and orientation. We investigated the structure of PbS NC SLs with grazing-incidence small-angle X-ray scattering (GISAXS) while using nuclear magnetic resonance (NMR) to quantify the bound and unbound ligand populations. Through a series of controlled additions of unbound oleic acid to solutions of identically sized oleate-capped NCs with different bound ligand coverages, we mapped the continuous evolution of the final SL structure from body-centered cubic (BCC) to face-centered cubic (FCC) through a series of body-centered tetragonal (BCT) intermediate phases. Strikingly, this phase transformation pathway is identical to the uniaxial contraction observed when evaporating solvent, suggesting that unbound ligand and solvent occupy a similar space within the SL unit cell. Molecular dynamics simulations of single NCs confirm that unbound ligand readily swells the bound ligand shell over all exposed NC facetseven without explicit rebinding to the NC surfaceand we establish limitations on the range of tunability via this approach based on Flory–Rehner gel-swelling theory. Furthermore, we explain the effect of high free ligand fraction on the early time dynamics of spin coating concentrated colloidal dispersions, which can disrupt the formation of long-range SL order. The controlled addition of unbound ligand represents a novel mechanism for directing superlattice structure and highlights the experimental importance of fully characterizing bound and unbound ligand populations.

University (R. B.)7 AtlantaBears are extremely popular among the zoo-going public, yet while many zoo exhibits have undergone dramatic design changes in recent years, most bears continue to be housed in moated grottos constructed largely of gunite. In these traditional exhibits they frequently demonstrate stereotypic locomotor patterns and are often encouraged by the public to beg. Thus, the manner in which most captive bears are exhibited does not facilitate conservation education. It is possible, however, to provide bears with opportunities to demonstrate species-typical feeding and foraging behaviors, even in standard exhibits. Subjects were four individuals of three bear species. Feeding enrichment was provided to one bear per week during three mornings during the summers of 1989 and 1990. Overall, animals were more active, less passive and less often engaged in abnormal behaviors during sessions with enrichment. Effects showed individual variation and were more profound during the second year of the study, when a greater variety of enrichment items was presented. These results suggest that simple and inexpensive methods of enrichment may have a significant, positive influence on the behavior of captive bears. 0 1992 Wiley-Liss, Inc.

In semiconductors, increasing mobility with decreasing temperature is a signature of charge carrier transport through delocalized bands. Here, we show that this behavior can also occur in nanocrystal solids due to temperature-dependent structural transformations. Using a combination of broadband infrared transient absorption spectroscopy and numerical modeling, we investigate the temperature-dependent charge transport properties of well-ordered PbS quantum dot (QD) solids. Contrary to expectations, we observe that the QD-to-QD charge tunneling rate increases with decreasing temperature, while simultaneously exhibiting thermally activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence small-angle X-ray scattering, we show that this trend is driven by a temperature-dependent reduction in nearest-neighbor separation that is quantitatively consistent with the measured tunneling rate.