Chemistry Reference
In-Depth Information
with dot growth was used, and it is possible that an amine decomposition
product of the precursor may have coordinated to a crystal facet and induced
the anisotropic growth. Using similar precursors, di
ering structures
were also grown; spherical particles of CdS
ca.
4 nm in diameter with
a wurtzite core were grown using cadmium
N
,
N
0
-bis(thiocarbamoyl)hydra-
zine (Cd(SCNHNH
2
)
2
Cl
2
) as a precursor under similar growth conditions
71
while thermolysis of a cadmium complex of dithiobiurea
Cd(NH
2
CSNHNHCSNH
2
)Cl
2
, again in TOPO, resulted in irregular particles
when grown at 150
C and large aggregates of
ca.
50 nm when grown at 240
C.
72
This wide variation in particle morphology when using subtly di
d
n
1
y
4
n
g
|
4
erent
precursors under the same growth condition highlights the role precursors
(and maybe decomposition products) play in de
ning nanostructure
composition.
An interesting reaction pathway is the heterocumulene metathesis route to
Cd(ESiMe
3
)
2
(E
S, Se) which can be used as an
in situ
single-source
precursor to CdSe or CdS.
73
The reaction between Cd[N(SiMe
3
)
2
]
2
and ECNR
(where E
¼
cyclohexyl) proceeds to give Cd(ESiMe
3
)
2
.
Attempts to isolate adducts of the two compounds using TOPO or dodeca-
nethiol as a Lewis base in a toluene solution resulted in the room-tempera-
ture preparation of TOPO-capped CdSe or thiol-capped CdS. Although few
details regarding the optical properties were given, the route is notable for
proceeding at a low temperature. Also, the precursor was relatively simple to
prepare, and the selenium source, cyclohexylisoselenocyanate, was easily
obtainable from the reaction between cyclohexylisonitrile and selenium
metal. This is in fact a key point to note, as in some cases the preparation of
a single-source precursor (such as diselenocarbamates) is o
¼
S, R
¼
t
Bu; E
¼
Se, R
¼
.
-
cult or dangerous than the preparation of the nanoparticles by simple binary
methods.
en more di
Likewise,
a
homoleptic
cadmium
selenolate,
[Cd(SeCH
2
CH
2
NMe
2
)
2
], has been shown to be an e
ective precursor for TOPO-
capped CdSe with a cubic crystal core,
74
but required the use of bis(2-dime-
thylaminoethyl)diselenide as a precursor. Although not di
cult to prepare, it
was still substantially harder than the simple binary routes to CdSe.
In the case of single-source precursors for cadmium selenide, the use of
cadmium diselenocarbamates is limited by the synthesis of carbon dis-
elenide, a toxic and evil-smelling compound that is di
cult to prepare. A
precursor system has been developed that avoids the use of such selenium
precursors. Cadmium imino-bis(diisopropylphosphine selenide), Cd
[N(SeP
i
Pr
2
)
2
]
2
, was prepared by deprotonating NH(SeP
i
Pr
2
)
2
in the presence
of cadmium chloride, yielding an air-stable complex.
75
Dissolution of Cd[N(SeP
i
Pr
2
)
2
]
2
in TOP, followed by injection into TOPO at
250
C and 30 minutes growth, resulted in CdSe nanoparticles
ca.
5nmin
diameter with a wurtzite structure. Similarly, diselenophosphinato-
complexes
76
of cadmium, [R
2
PSe
2
]
2
Cd (R
Ph,
i
Pr,
t
Bu), have been used as
precursors for CdSe QDs,
77
and the cobalt analogues have been used in the
synthesis of either CoSe
2
(when thermolysed in TOPO or HDA) or Co
2
P
(when thermolysed in TOPO and TOP). Extended thermolysis in TOPO/TOP
¼
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