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er injection and during the growth stage, the reaction could be moni-
tored by removing aliquots and recording the emergence of a band edge
via
absorption spectroscopy, and the growth could also hence be tuned by
altering the reaction temperature. A
A
er the particles had grown to the
required size, the reagents were then le
to cool to
ca.
60
C, followed by
addition of a polar solvent (termed a non-solvent) such as methanol, which
induced precipitation. The precipitate, collected as a waxy powder by
centrifugation, was dispersed in non-polar solvents producing an optically
clear solution. Addition of small amounts of non-solvent increased the
average polarity of the solvent and resulted in the precipitation of the larger
particles. This size-selective precipitation resulted in a solution of nano-
particles with an extremely narrow size distribution.
The materials prepared were monodispersed (<5% standard deviation),
1.2
d
n
1
y
4
n
g
|
1
11.5 nm in diameter and slightly prolate with aspect ratios up to 1.3. The
particles were also crystalline, capped with a monolayer of surfactant mole-
cules (TOPO and TOP) and displayed excellent optical properties. Figure 1.1
shows typical absorption spectra from CdE (E
-
d
n
4
.
¼
S, Se, Te) nanoparticles
prepared by this method, with the
rst excitonic transitions clearly visible.
Figure 1.1
Absorption spectra for CdS, CdSe and CdTe nanoparticle prepared
by organometallic chemistry. Reprinted with permission from C. B.
Murray, D. J. Norris and M. G. Bawendi,
J. Am. Chem. Soc.
, 1993, 115,
8706. Copyright 1993 American Chemical Society.
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