Chemistry Reference
In-Depth Information
Initial reports on the use of dithio- and diselenocarbamates as precursors
described the thermolysis of the air-stable, symmetrical compounds
[Cd(S
2
CNEt
2
)
2
]
2
and [Cd(Se
2
CNEt
2
)
2
]
2
in 4-ethylpyridine.
15
Although the
dithiocarbamate decomposed cleanly to produce quantum dots (QDs) of CdS
with a hexagonal crystal core, the diselenocarbamate formed both nano-
particulate CdSe and micrometre-sized selenium particles, both of which
exhibited hexagonal crystal cores. Absorption band edges consistent with
nanosized CdS and CdSe were observed, although no emission data was
presented in this case. The formation of impurities from the simple dis-
elenocarbamate presented a signi
d
n
1
y
4
n
g
|
4
cant problem, which was overcome by
using a related diselenocarbamate, [MeCd(Se
2
CNEt
2
)
2
]
2
, prepared by the
comproportionation
reaction
16
between Me
2
Cd
and
the
simple
cadmium(diethyldiselenocarbamate).
Simple dissolution of [MeCd(Se
2
CNEt
2
)
2
]
2
in trioctylphosphine (TOP) fol-
lowed by injection into TOPO at 200
250
C resulted in clean decomposition
yielding CdSe nanoparticles
ca.
5 nm in diameter which displayed band edge
emission comparable with CdSe particles prepared by binary organometallic-
based methods.
17
This can be thought of as the
-
rst single-source route to
high-quality semiconductor nanoparticles. The neopentyl analogue
([(CH
3
)
3
CH
2
Cd(Se
2
CNEt
2
)
2
]
2
) was also prepared and found to produce
nanoparticles of a similar quality. The thermolysis of the thio-analogues
[RCd(S
2
CNEt
2
)
2
]
2
(R
Me, (CH
3
)
3
CH
2
) in TOPO produced nanoparticles of
CdS which were again of comparable quality to materials produced by the
binary route, as were nanoparticles obtained from the simple dithiocarba-
mate [Cd(S
2
CNEt
2
)
2
]
2
. In all cases, particles were
ca.
4
¼
.
-
5 nm in diameter,
exhibiting blue-shi
ed absorption edges and near band edge emission.
18
Despite the clean decomposition of [Me/(CH
3
)
3
CH
2
Cd(Se
2
CNEt
2
)
2
]
2
, the
alkylated precursors are air sensitive, unlike the simple dithio/selenocarba-
mates which exhibit excellent stability when stored in ambient conditions;
ideally, a precursor should be air stable and decompose cleanly to the desired
product.
In-depth investigations into the decomposition of symmetrical and
asymmetrical dithio/selenocarbamates showed an intimate link between
decomposition pathway and the constituent alkyl groups.
19,20
Thermolysis of
symmetrical diethyldiselenocarbamates proceeded by the elimination of
EtSe
2
Et, which decomposed further to elemental selenium as determined by
gas chromatography-mass spectrometry (GC-MS). Ethane elimination on
particle-bound diethyldiselenocarbamate groups was also identi
ed as
a potential decomposition pathway that resulted in a selenium-rich product
(Figure 7.1a).
Mass spectrometry on the asymmetric diselenocarbamate thermolysis by-
products identi
ve-membered heterocycle, 5-butyl-3-methyl-2,3-dihy-
dro-1,3-selenazole, which was formed by the elimination of a hydrogen atom
in the
b
position to the nitrogen (Figure 7.1b), e
ed a
ectively removing the excess
selenium and giving a clean product. No evidence of EtSe
2
Se was found in the
decomposition by-products of asymmetrical diselenocarbamates.
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