Chemistry Reference
In-Depth Information
The discrete cadmium complexes could be prepared from either Me
2
Cd or
CdO (but not CdCl
2
, in contrast to the work reported by Lazell) and used in
the synthesis of high-quality nanoparticles, suggesting the metal alkyl was
not necessary. Therefore, a one-pot method of preparing particles was
developed, based on the high-temperature dissolution of CdO in TOP with
a known
such as tetradecylphosphonic acid (TDPA). Injection of
the chalcogen precursor resulted in the formation of cadmium chalcogenide
particles that followed the growth kinetics suggested earlier. A notable
feature of this method is the slow nucleation due to the stability of the
cadmium
'
impurity
'
d
n
1
y
4
n
g
|
1
phosphonic acid complex, which has important advantages; the
injection temperature need not be 350
C to obtain a monodispersed sample
and the reaction is very reproducible and controllable,
55,56
even allowing the
growth of bimodal and trimodal size distributions through kinetic pertur-
bation.
57
The slow nucleation has allowed studies into the crystallisation
process, which have revealed that when using cadmium stearate as
a precursor and a growth temperature of 200
C, the initial particles formed
are amorphous and required 10 minutes of annealing to undergo the
required reconstruction and crystallisation.
58
Work by Stucky has revealed that CdCl
2
,CdI
2
, and Cd(CO
2
CH
3
)
2
could also
be used as precursors.
59
Peng further showed that cadmium acetate was the
most useful precursor when used with weakly anionic fatty acids, and that
a number of solvents such as phosphines, phosphine oxides and amines
could be used, while the use of thiols was found to hinder nucleation due to
the stability of cadmium
-
d
n
4
.
thiolate complexes. Particles between 4 and 25 nm
in diameter could be prepared, although the larger particles showed a wide
size distribution. Controlling the rate of growth was achieved by varying the
chain length of the acid, with shorter chains resulting in faster growth. As
grown, the highest quantum yields observed were 20
-
cant
increase on emission reported from particles prepared by metal alkyl. This
route is notable for producing particle sizes not normally accessible by the
Me
2
Cd route.
60
Kumar also reported the preparation of CdTe nanoparticles of
varying morphology by the use of cadmium stearate, Cd(CO
2
(CH
2
)
16
CH
3
)
2
,as
a precursor.
61
The particles prepared exhibited a hexagonal crystalline core,
as opposed to the cubic materials reported by Talapin.
Detailed work by Liu
et al.
has uncovered the reaction mechanism behind
the formation of CdSe QDs using CdO as a precursor in a non-coordinating
solvent (discussed later).
62
As suggested, the reaction between CdO and
a phosphonic acid (present as an impurity in the capping agent or a speci
-
30%, a signi
c
ligand) resulted in the formation of phosphonic acid complex and the
generation of water. The addition of TOPSe resulted in the formation of an
TOPSe
-
Cd(X)
2
intermediate (where X was various groups), which weakened
the P
Se bond was cleaved by the nucleophilic attack of
either the carboxylic acid or phosphonic acid, leaving CdSe, a phosphine
oxide and an acid anhydride. Complementary work by Steckel
63
con
Se bond. The P
]
]
rmed
this mechanism and also suggested another competing mechanism based on
experimental observations, in which the surfactant (TOP) reduced the metal
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