Chemistry Reference
In-Depth Information
CHAPTER 3
d
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y
4
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7
The Preparation of IV
-
VI
Semiconductor Nanomaterials
3.1 PbE (E
S, Se, Te)
Bulk lead chalcogenides are narrow-bandgap semiconductors with face-
centred cubic structures, lending themselves to several potential applica-
tions, notably in thermoelectrics due to the large
¼
gure of merit (
ZT
)
associated with the materials, and photovoltaics due to the ideal position of
the quantum-con
ned bandgap.
1
Bulk PbSe exhibits a bandgap of 0.26 eV
and an excitonic diameter of 92 nm.
2
PbTe has a bandgap of 0.25 eV and will
reportedly exhibit quantum con
ects below 152 nm diameter
3
(which is the longitudinal Bohr radius; the transverse Bohr radius is
reportedly 12.9 nm), whereas PbS has a bandgap of 0.37 eV and an excitonic
diameter of 40 nm.
4
Upon quantum con
nement e
.
of the
bandgap results in materials with optical properties in the infrared region,
making these particles the materials of choice for most infrared-dependent
applications,
5
with numerous reports of PbE (E
nement, the blue shi
¼
S, Se, Te)-based photode-
tectors,
6
-
9
ect transistors.
16
Quantum dots (QDs) of lead chalcogenides have even been used as biological
labels,
17
where phase transfer of PbS particles using simple thiolated ligands
was successful and maintained emission quantum yields of up to 26%.
18
PbSe particles have also been capped with SiO
2
and used in cellular imaging
without any obvious toxicity issues.
19,20
Unlike the CdSe family of nano-
particles, which have analogous organic compounds with similar optical
properties, there are currently no organic equivalents for strongly lumines-
cent QD infrared emitters such as PbSe.
The most common method of preparing QDs is the hot injection method
where, for example, a metal salt is dissolved in solution with a capping agent
solar cells,
10
-
12
LEDS,
13,14
lasers,
15
and
eld-e
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