Biomedical Engineering Reference
In-Depth Information
largest. These semiconductor nanocrystals are commonly synthesized by thermal
decomposition of an organometallic precursor dissolved in an anhydrous solvent
containing the 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.
In the case of CdSe, dimethylcadmium Cd(CH
3
)
2
is used as a cadmium source
and bis(trimethylsilyl)sulfide, (Me
3
Si)
2
S, trioctylphosphine selenide (TOPSe),
or trioctylphosphine telluride (TOPTe) serve as sources of selenide in trioctyl-
phosphine oxide (TOPO) used as solvent and capping molecule. The mixture is
heated at 230-260
C over a few hours while modulating the temperature in
response to changes in the size distribution, as estimated from the absorption
spectra of aliquots removed at regular intervals. The particles, capped with TOP/
TOPO molecules, were non-aggregated and easily dispersible in organic solvents to
form optically clear dispersions. When similar syntheses are performed in the
presence of surfactant, strongly anisotropic nanoparticles are obtained, e.g., rod-
shaped CdSe nanoparticles can be obtained.
Because Cd(CH
3
)
2
is extremely toxic, pyrophoric, and explosive at elevated
temperatures, other Cd sources have been used. CdO appears to be an interesting
precursor. CdO powder dissolves in TOPO and HPA (hypophosphorous acid) or
TDPA (tetradecylphosphonic acid) at about 300
C giving a colorless homogeneous
solution. By introducing selenium or tellurium dissolved in TOP, nanocrystals grow
to the desired size.
Nanorods of CdSe or CdTe can also be produced by using a greater initial
concentration of cadmium as compared to reactions for nanoparticles. This
approach has been successfully applied for synthesis of numerous other metal
chalcogenides including ZnS, ZnSe, and Zn
1-x
Cd
x
S. Similar procedures enable
the formation of MnS, PdS, NiS, and Cu
2
S nanoparticles, nanorods, and nanodisks
[
65
,
67
,
69
-
73
]. An alternative route by which passivation of CdSe QDs is achieved
employs one of the nucleobases, cytosine, that makes the CdSe QDs more biocom-
patible because of its versatility. Cytosine molecules alone do not contribute to
fluorescence yield [
74
,
75
] as the changes in the electronic and optical properties of
cytosine-capped CdSe QDs are insignificant; rather, cytosine molecules only act as
a capping agent for reducing the surface defect density of QDs. The QD was
represented by a 32-atom cluster and capping was performed with either four or
eight cytosine molecules. The zero of the energy is aligned to the Fermi energy, as
shown in Fig.
18a,b
. Figure
19
a shows the optical absorption and Fig.
19
b shows the
photoluminescence spectra of CdSe and CdSe QDs capped with different
concentrations of cytosine molecules.
6.4.2 Oxide Semiconductors
Oxide semiconductor nanoparticles such as ZnO were surface-modified by Dutta
et al. using a different approach [
76
]. They described the design and fabrication of
