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among the incident beam and small-angle diffraction beams through
these supercrystalline superparticles.
46
The interplanar spacing
and angles obtained from the TEM images are consistent with the
corresponding SAED patterns. These patterns show sharp spot
arrays (Fig. 13.11b,d,f ), which are single-crystal-like ED patterns,
demonstrating the three-dimensional (3D) perfection of the
superlattice. Taken together, these results show that the colloidal
superparticles have a “single supercrystal” structure where all of the
5.8 nm Fe
nanocrystal building blocks occupy the lattice points in
the fcc superlattice.
O
3
4
1
Similar to those single-crystalline nanocrystals made of atoms,
these single-supercrystalline superparticles also exhibit stacking
faults. TEM images indeed show that a few of these particles did have
clear stacking faults along the (111)
planes of the fcc superlattice
(Fig. 13.11g). In addition, the fcc superlattice structure was further
confirmed by small-angle X-ray diffraction (XRD). The XRD spectrum
of a sample with larger supercrystalline superparticles (560 nm in
diameter) exhibits six distinguishable peaks (Fig. 13.11h). These
peaks are located at the positions corresponding to the Bragg
reflections from planes specified by the Miller indices as (111), (200),
(311), (400), (333), and (444) of the fcc superlattice, respectively.
The lattice constant determined from this XRD spectrum is 11.9
SL
±
0.3 nm, which is in excellent agreement with the value of 11.7
±
0.2
nm from the TEM measurements.
The development of CIS synthesis is important for three
major fundamental reasons. First, the synthesis approach can be
generalized for making supercrystalline colloidal superparticles
from nonpolar-solvent-dispersible nanocrystals with other sizes
and chemical compositions such as metals, metal oxides, and
semiconductors (Fig. 13.12a
20
Second, the properties of these
supercrystalline colloidal superparticles can be easily modified via
“doping” with organic molecules such as dye sensitizers. For example,
rhodamine-6G-doped supercrystalline colloidal superparticles
(made of 5.4 nm gold nanocrystals) exhibit strong surface-enhanced
Raman scattering (Fig. 13.12d), owing to the electromagnetic field
c).
20
enhancement from gold superparticles.
Third, because of their
excellent stability in polar solvents, these colloidal superparticles
can be further assembled into more complex and hierarchically
ordered materials in which new properties may occur.
15
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