The Use of Single-Source Precursors in Nanoparticle Synthesis - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 234

has been used at 65 C to prepare ultrathin PbS wires 64 and ultrasmall PbS
dots 65 ). Ethyl-, hexyl- and hexadecylxanthates were used to prepare CdS and
ZnS particles in HDA, whereas tertiary amines were used to synthesise CuS,
NiS and MnS due to the displacement of the xanthate from the metal by the
strongly binding primary amine. A one-pot reaction was used to prepare ZnS/
CdS particles, relying on the di ering decomposition temperatures of

the compounds. By slowly heating a mixture of the two complexes in HDA,
the zinc-containing precursor decomposed

d n 1 y 4 n g | 4

C), followed by the

 rst (70

170 C. It has been suggested
that these precursors may give better-quality particles than the dithiocarba-
mate analogues because of their lower decomposition temperature. In other
work, the thermolysis of the zinc and cadmium ethylxanthates in a range of
solvents resulted in the formation of a Zn x Cd x 1 S alloy particle system, the
composition (and hence the optical properties) of which was controllable
by varying the precursor ratio. Decreasing the zinc content also resulted in
a change of shape, from sphere, to rod, to multipod. 66 Zinc ethylxanthate
has also been used as a precursor for ZnS shells (deposited on preformed
CdSe particles). 67 In this case, the precursor was dissolved in tributyl
phosphate (TBP), added to the CdSe particles at 60 C and sonicated for ca. 10
minutes, whereupon the emission quantum yielded reached a maximum of
ca. 50%.
The solvents/capping agents used had a key role in lowering the decom-
position temperature of the precursors, which was explained with reference
to the Chugaev reaction, which has similarities to the decomposition of the
precursor (Figure 7.3). The introduction of the amine may have several
e

cadmium-containing precursor at 120 -

.

ects; activating the O - C - S 2 - Cd group by shi ing charge from the C - S bond



to the Cd S bond (Figure 7.3A); acting as a protonated intermediate aiding

the required proton transfer (Figure 7.3B) and coordinating directly to the
cadmium, weakening one Cd

-

- S bond, activating it as a leaving group and

hence forming one Cd S bond (Figure 7.3C).

Zinc ethylxanthate has also been used in the synthesis of ZnS rods and dots
where the stabilising solvent was used to control particle morphology and
crystalline phase. 68 Using the precursor in HDA and OA between 150 and
200 C, wurtzite (hexagonal crystalline core) rods were formed with tuneable
diameters and aspect ratios (20

-

4.5 nm in width)
which self-assembled into 2D lattices. Using HDA and TOP at 200 C resulted
in spherical particles with a zinc blende core. Using TOP alone resulted in
spherical particles with a wurtzite core. The growth of dots occurred even
with large monomer concentrations, indicating that anisotropic growth still
required the presence of ligands that bound to a speci

- 200 nm in length, 2.5 -

c crystal facet. Zinc
ethylxanthate has also been used to deposit a ZnS shell on CuInS 2 particles,
increasing the emission quantum yield from 8% to 60%, although
Zn(CO 2 (CH 2 ) 16 CH 3 ) 2 was also present during shell deposition. 69
Wurtzite rods of CdS up to 24 nm long and 5 nm wide have also been
grown using cadmium thiosemicarbazide [Cd(NH 2 CSNHNH 2 ) 2 Cl 2 ]as
a precursor in TOPO. 70



In this case, a precursor concentration consistent