Chemistry Reference
In-Depth Information
ranging from 0.15 eV
25
0.32 eV.
56,124
The narrow bandgap and large
exciton make the materials ideal for applications in telecommunications, as
the emission can easily be tuned to the infrared region.
125
Aqueous-based methods of preparing HgTe QDs at room temperature
(using H
2
Te and Hg(ClO
4
)
2
as precursors and thiols as capping agents,
resulting in particles 3
to
6 nm in diameter) have proved extremely successful,
with particles exhibiting quantum yields of up to 50% and emission pro
-
d
n
1
y
4
n
g
|
1
les
in the 1050 nm region.
126
These particles were found to be extremely sensitive
to thermal e
ects and the environment, with emission shi
ing to
ca.
1200
nm upon standing, and up to 1500 nm upon re
uxing, with quantum yields
dropping below 1%.
127
Numerous studies have been carried out using this
system, including the synthesis of stable core/shell materials,
76
alloys,
128
investigations into growth conditions,
129
incorporation into emitting
devices
130
and photocurrent mechanisms associated with the capping
ligand.
131
The seminal investigations into solution routes to HgTe nanomaterials
using organometallic precursors were reported by Steigerwald, who used
complexes such as Hg(TeR)
2
.
132,133
Thermolysis of the majority of precursor
resulted in rapid uncontrolled growth of HgTe; however, the use of a speci
d
n
4
.
c
precursor, Hg(Te-
n
-Bu)
2
stopped the reaction proceeding to the bulk mate-
rial, giving nanoscopic materials. Few details of the optical properties were
recorded. The rapid growth of materials appears to be common to most
mercury chalcogenide species. Murray brie
y investigated organometallic
routes towards mercury chalcogenide nanoparticles capped with surfac-
tants.
17,21
The QDs were prepared using the precursors described by Stei-
gerwald (Ph
2
Hg and trioctylphosphine chalcogenides) using TOPO as
a surfactant at synthesis temperatures of
ca.
100
C, although the particles
produced were not described in any depth.
The use of metal ions stabilised by surfactants as described earlier by Peng
slows the e
ective mass transfer of reagent ions to the nanoparticle by either
hindering the reaction with a strongly coordinating ligand or by altering the
di
usion rate by the use of sterically demanding ligands.
134
Higginson re-
ported that in the case of HgE nanoparticles, the ligand coordination to the
metal ion is not su
cient to control particle growth and the stability
constant,
K
, of the ligand must be considered when preparing the mate-
rials.
134
Weakly binding ligands such as fatty acids and amines (log
K
18)
do not hinder particle growth and allow bulk material formation, while
strongly binding ligands such as polyamines and phosphines (log
K
6
-
z
z
17
30) result in the reductive
elimination of mercury metal at temperatures associated with nanoparticle
synthesis. Although this may not be a problem during nanoparticle prepa-
ration (previous results from Steigerwald have shown that eliminated
mercury o
-
30), phosphine oxide and thiols (log
K
17
-
z
en reacts with excess chalcogen), it may need to be considered
during any prolonged annealing step. From these results, a reverse micelle
route was used to prepare HgS nanoparticles with a
-HgS crystalline core, in
a similar manner to the early work on CdSe. An aqueous solution of mercury
b
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