Chemistry Reference
In-Depth Information
Steigerwald, who described the precursors as
.
31
The general
suitability of trialkylphosphine selenides as precursors was originally ques-
tioned because the P
'
masked atoms
'
chalcogen bond is stronger than in the analogous
telluride compunds,
32
which resulted in phosphine selenides requiring
further heating to yield selenium in the synthesis of, for example, NiSe.
33
Although TOPTe is also routinely used in the synthesis, of for example, CdTe,
it is found to be relatively unreactive when compared to TOPSe. This made
synthesis of materials such as ZnTe di
-
d
n
1
y
4
n
g
|
6
cult, unless the TOPTe was reduced
by superhydride giving an Te
2
intermediate.
34
TOP, when heated to rela-
tively high temperatures (>300
C) decomposes to yield phosphorus and can
therefore be used as a phosphorus precursor,
35,36
as outlined in Chapter 2.
TOP is generally thought to coordinate to the surface of CdSe QDs
via
the
selenium sites, supplying a more complete surface passivation.
19
Modelling
has also con
rmed that TOP preferentially binds to selenium-terminated
sites such as the selenium-terminated (0001
), whereas most other ligands
0) surface.
37,38
The role of TOP in the electronic
structure of CdSe nanocrystals is not clear; Kalyuzhny
et al.
attributed the
deep trap emission observed in CdSe particles to the complex of TOP with the
surface selenium sites, leading to a lower energy chemical state,
39
although
Jasieniak showed that deep trap emission occurs in both TOP-passivated and
TOP-free nanoparticles and that the lower energy states are surface related.
40
The presence of TOP on selenium-rich surfaces has also been shown to be the
source of enhanced photoluminescence,
41
and a major contributing factor to
the hexagonal crystal character of CdSe particles.
42
A related ligand, tributyl
phosphate (TBP), has been shown to remove surface selenium adatoms while
passivating selenium dangling bonds.
43
TOPSe is also microwave-absorbing,
allowing for the synthesis of QDs under microwave irradiation.
44
TOP also
plays a pivotal role in the formation mechanism of QDs, as discussed in
Chapter 1.
Few other phosphines are used routinely, other than TBP as discussed in
Chapter 1, which appeared to make little di
prefer the non-polar (112
.
erence when used as a replace-
ment for TOP,
45,46
although 1,3,5-triaza-7-phosphaadamantane (PTA) has
been used as the reaction solvent for the synthesis of ruthenium and plat-
inum nanoparticles using organometallic precursors.
47
The resulting parti-
cles were water-soluble because the PTA coordinated to the particle surface.
6.3 Amines
TOPO is not an ideal solvent, however; heating it above 300
C is known to
induce decomposition,
48
the product of which is unidenti
ed but is known to
luminesce (albeit weakly). This decomposition product emission can in some
cases be mistaken for semiconductor emission, especially where the parent
semiconductor is expected to exhibit a low-emission quantum yield.
49
Long-
chain amines are found to be more suitable surfactants for II
VI-based
semiconducting systems. Nanoparticles of CdSe prepared using long-chain
primary amines are generally found to have emission quantum yields of 60%
-
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