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B. Supracrystals of Silver Nanoparticles (5,6) Figure 6A shows a TEM image of aggregates formed from 5-nm silver particles coated with dodecanethiol on an amorphous carbon substrate. High-resolution TEM (Fig. 6B) shows that the silver nanoparticles appear to be arranged in either cubic or hexagonal structures. The transition from one phase to the other is abrupt and analogous to polycrystalline atomic lattices, wherein each nanocrystal domain has a different orientation. In a perfect hexagonal crystal, patterns of fourfold symmetry should not be observed. However, if one considers an FCC lattice, fourfold symmetry may be observed at certain tilt angles (we confirmed this by actually changing the tilt angle in the TEM studies). Thus, the “pseudohexagonal� structure seen in Fig. 12B can be attributed to the stacking of the {110} plane of an FCC lattice, and the features with fourfold symmetry arise from stacking of the {011} planes. The cell parameter of the aggregate can be determined from either the fourfold symmetry or pseudohexagonal patterns in the TEM. In the former case, the cell parameter is 9  1 nm. From the pseudohexagonal pattern, we find that the lattice constants (a,b,c) are not equivalent as they should be in a perfect hexagonal lattice. We find that a  b  9 nm and c  6.6 nm. This discrepancy may be attributed to a tetragonal distortion of the FCC structure, which was recently predicted from molecular dynamics simulations of alkylthiolate-coated gold nanoparticles (34). STM images of 3D aggregates of silver nanoparticles on an Au(111) substrate largely confirm the interpretation of the TEM images. For example, in Fig. 6C, the fourfold symmetry arising from the stacking of {011} planes is clearly visible. The line trace (see inset) from the dark (low) to bright (high) regions of the image indicates clearly the stacking of monolayers of silver nanocrytals. Several such line scans were obtained, and the distance between two layers is consistently found to be in the 2- to 2.8-nm range. This distance is smaller than the total diameter of one coated nanocrystal (6.1 nm  4.3 nm  1.8 nm) and thus indicates that the particles of one layer sit in the center of the triangle formed by particles in an adjacent layer. The change in height of the line scan as it crosses nearly three layers of nanocrystals is consistent with interdigitation and zigzag conformation of the alkyl chains. Employing a similar deposition procedure, but with slower solvent evaporation, well-defined FCC supracrystals of silver nanoparticles can be obtained (64).

C. Supraaggregates of Ferrite Nanocrystals (65) By depositing a 5-mL solution containing cobalt ferrite nanocrystals (0.0093 mass percent) on an amorphous carbon-coated TEM grid and allowing the solution to evaporate, an open mesh of randomly aggregated particles is obtained

Nanoparticles  

Chemistry of metal nanoparticles

Nanoparticles  

Chemistry of metal nanoparticles

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