The Preparation of II–VI Semiconductor Nanomaterials - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 22

TOPO and Hg(O 2 CCH 3 ) 2 . 145 The shape and size of the particles could be
controlled by varying the precursor : surfactant ratio. This report also sug-
gested why mercury chalcogenides could be prepared at room temperature as
opposed to the higher temperatures required for the analogous Pb, Cd and
Zn chalcogenides. It was suggested that the positive redox potential for Hg 2 2+
made reduction of the acetate precursor favourable, allowing the reaction to
proceed without heating. This also explained why Pb chalcogenides require
low reaction temperatures and why Zn chalcogenides required high
temperatures. Alloyed particles of HgSe x S 1 x with a cubic crystalline core
have also been prepared by a simple one-pot reaction: CH 3 Si

d n 1 y 4 n g | 1

SiCH 3 was
added to a frozen solution of TBP/Hg(O 2 CCH 3 ) 2 , whereupon a precipitate
formed. The precipitate decomposed upon warming to room temperature,
resulting in the alloyed particle formation, although few further details were
supplied. 146
Not all mercury chalcogenides can be prepared at room temperature,
highlighting that the reaction is not necessarily driven by the reactivity of
the cation precursor alone. 147 The reaction of Hg(O 2 CCH 3 ) 2 and TOPS did
not proceed at room temperature, due to the inert character of the chal-
cogen precursor. The TOPS was preheated to 150 C then injected, while
hot, into the TOPO/Hg 2+ at room temperature, which was then heated at
120 C for 15 minutes, forming HgS particles. The preheating was found to
be essential, while the typical hot injection into a metal precursor solution
resulted in reaction failure. The resulting cubic

- SeS -

d n 4 .

-HgS particles were ca. 3.9
nm in diameter, exhibited an excitonic absorption feature at ca. 910 nm
and broad, weak photoluminescence with a maximum at 1050 nm. The
synthesis follows a similar route to that of Xu et al. , who prepared HgS
nanomaterials using ODA as a capping agent, although no emission data
was reported. 148

b

1.6 Anisotropic Quantum Dots
The synthesis of QDs of di ering shapes 149,150 is a key element in the control

of the electronic structure of the materials, and numerous applications
utilise non-spherical particles. 151 The focusing of a particle size distribution
has been discussed, and the concentration of monomers has been identi ed

as a key factor; a lowmonomer concentration results in Ostwald ripening and
a defocusing of the size distribution, whereas an increased concentration
results in the smaller particles growing faster, producing a narrower size
range. CdSe of the wurtzite polytype is an inherently anisotropic material and
was the focus of the seminal work into the synthesis of anisotropic QD
structures, using dimethylcadmium and trialkylphosphine selenide as
precursors. Slow growth in TOPO resulted in spherical particles, whereas
a fast growth rate resulted in increased growth along the unique c -axis. A
massive increase in the monomer concentration (kinetic overdrive) also
resulted in the formation of elongated nanorods along the c -axis, due to the
di



ering growth rates of di ering crystal faces. 152