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Title: Structural Investigations of Surfaces and Orientation-SpecificPhenomena in Nanocrystals and Their Assemblies

Thesis/Dissertation ·
DOI:· OSTI ID:901229
  1. Univ. of California, Berkeley, CA (United States)

Studies of colloidal nanocrystals and their assemblies are presented. Two of these studies concern the atomic-level structural characterization of the surfaces, interfaces, and interiors present in II-VI semiconductor nanorods. The third study investigates the crystallographic arrangement of cobalt nanocrystals in self-assembled aggregates. Crystallographically-aligned assemblies of colloidal CdSe nanorods are examined with linearly-polarized Se-EXAFS spectroscopy, which probes bonding along different directions in the nanorod. This orientation-specific probe is used, because it is expected that the presence of specific surfaces in a nanorod might cause bond relaxations specific to different crystallographic directions. Se-Se distances are found to be contracted along the long axis of the nanorod, while Cd-Se distances display no angular dependence, which is different from the bulk. Ab-initio density functional theory calculations upon CdSe nanowires indicate that relaxations on the rod surfaces cause these changes. ZnS/CdS-CdSe core-shell nanorods are studied with Se, Zn, Cd, and S X-ray absorption spectroscopy (XAS). It is hypothesized that there are two major factors influencing the core and shell structures of the nanorods: the large surface area-to-volume ratio, and epitaxial strain. The presence of the surface may induce bond rearrangements or relaxations to minimize surface energy; epitaxial strain might cause the core and shell lattices to contract or expand to minimize strain energy. A marked contraction of Zn-S bonds is observed in the core-shell nanorods, indicating that surface relaxations may dominate the structure of the nanorod (strain might otherwise drive the Zn-S lattice to accommodate the larger CdS or CdSe lattices via bond expansion). EXAFS and X-ray diffraction (XRD) indicate that Cd-Se bond relaxations might be anisotropic, an expected phenomenon for a rod-shaped nanocrystal. Ordered self-assembled aggregates of cobalt nanocrystals are examined with transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). SAED patterns from multilayered assemblies show that the nanocrystals have preferred crystallographic orientations. It is proposed that the nanocrystals are organized in a vortex-like or ''loop-closing'' arrangement, possibly due to magnetism. SAED and dark-field imaging used to investigate this hypothesis are presented, along with the data analysis. The effects of magnetism and nanocrystal faceting are discussed.

Research Organization:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR)
DOE Contract Number:
Report Number(s):
LBNL-60295; R&D Project: 506101; BnR: KC0203010; TRN: US200713%%202
Country of Publication:
United States