Page 72 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

58 Principles and Methods

Given the analogous structure of Fe(O)(OH) (lepidocrocite) to boehmite,
it is not surprising that the iron analog of alumoxane nanoparticles
(i.e., ferroxanes) is readily prepared. First prepared by Rose et al. [90],
ferroxanes have been extensively characterized, and have shown iden-
tical structural features to alumoxanes and undergo similar exchange
reactions [91].

Semiconductor Nanoparticles
(Quantum Dots and Quantum Rods)

The synthesis of semiconductors as nanoscale particles yields materials
with properties of absorbance and fluorescence that differ considerably
from those of the larger, bulk-scale material. Highly specific bands of
absorbance or fluorescence arise from the quantum confinement of elec-
trons that are excited in these materials when exposed to light. These
materials are therefore of great interest in applications ranging from
medical imaging to tagging and sensing.
Solution processes
The most studied nonoxide semiconductors are cadmium chalcogenides
(CdE, with E sulfide, selenide, and telluride). CdE nanocrystals were
probably the first material used to demonstrate quantum-size effects cor-
responding to a change in the electronic structure with size—that is, the
increase of the band gap energy with the decrease in size of particles [4–6,
92, 93]. These semiconductor nanocrystals are commonly synthesized by
thermal decomposition of an organometallic precursor dissolved in an
anhydrous solvent containing a source of chalcogenide and a stabilizing
material (polymer or capping ligand). Stabilizing molecules bound to the
surface of particles control their growth and prevent particle aggregation.
Although cadmium chalcogenides are the most studied semiconduct-
ing nanoparticles, the methodology for the formation of semiconducting
nanoparticles was first demonstrated independently for InP and GaAs
[94, 95]. In both cases it was demonstrated that the reaction of the metal
halide with the trimethylsilyl–derived phosphine or arsine resulted in
the formation of the appropriate pnictide and Me 3 SiCl:

InCl P(SiMe ) → InP 3 Me SiCl
3
3
3 3
Although these initial studies were performed as solid-state reactions, car-
rying them out in high boiling solutions led to the formation of the appro-
priate nanoparticle materials. The most widely used development of the
7
Barron/Wells synthetic method was by Bawendi and colleagues in which
an alkyl derivative was used in place of the halide. Dimethylcadmium
Cd(CH 3 ) 2 is used as a cadmium source and bis(trimethylsilyl)sulfide,
(Me 3 Si) 2 S, trioctylphosphine selenide or telluride (TOPSe, TOPTe) serve as