Page 76 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

62 Principles and Methods

quantum dots and quantum rods with controllability of their size,
monodispersity, and aspect ratio. This approach has been successfully
applied for synthesis of numerous other metal chalcogenides, including
ZnS, ZnSe, and Zn 1 x Cd S [101]. CdS nanorods have also been obtained
x
from Cd(S CNCH CH ) , an air-stable compound, thermally decomposed
2
2
3 2
in hexadecylamine HDA at around 300 C [102]. Various shapes of CdS
nanocrystals are obtained in changing the growth temperature. Rods are
formed at elevated temperatures (300 C) and armed rods (bipods, tripods)
are obtained as the growth temperature is decreased to 180 C. Around
120 C, tetrapods of four armed rods are dominant.
A similar procedure, using Mn(S CNCH CH ) , enables formation of
2
2
3 2
MnS nanocrystals with various shapes including cubes, spheres,
monowires, and branched wires (bi-, tri- and tetrapods) [103]. Nanorods
of diluted magnetic semiconductors, Cd 1 x Mn S, have also been
x
obtained by this procedure. Various shapes of PbS nanocrystals have
similarly been produced from Pb(S CNCH CH ) [104]. NiS nanocrys-
3 2
2
2
tals, elongated along the 110 direction, were prepared by solventless
thermal decomposition of a mixture of nickel alkylthiolate and octade-
canoate. Similarly, Cu S nanorods or nanodisks are obtained by
2
solventless thermal decomposition of a copper alkylthiolate precursor
[105]. Finally, a very interesting design of nano-objects with advanced
shapes results from oriented attachment of nanoparticles. PbSe
nanowires of 3.5 to 18 nm in diameter and 10 to 30 mm in length
(Figure 3.17) are obtained from the reaction between lead oleate with
TOPSe at 250 C in solution in diphenylether in the presence of TDPA
[106]. In the presence of hot (250 C) hexadecylamine in diphenylether,
lead oleate and TOPSe form PbSe nanorings resulting very likely from
a similarly oriented attachment of nanoparticles (Figure 3.24).

Nanoparticles from the vapor phase
The chemical vapor deposition (CVD) of semiconductors from molecu-
lar precursors has been extensively studied. One class of precursors is
the so-called single source precursors, those in which all of the desired
elements are in the same molecule. The use of single-source precursors
allows for the structure of films grown by CVD to be controlled by the
structure of the precursor molecule employed [107–109]. Such a process
requires the precursor structure to remain intact during deposition
[110]. However, vapor phase molecular cleavage can alter film mor-
phology as well as influence phase formation. It has been observed
that a major consequence of precursor decomposition in the vapor phase
is cluster formation, leading to a rough surface morphology [111]. While
particulate growth during CVD is often an undesirable component of
the film deposition process, it is possible to prepare highly uniform