Chemistry Reference
In-Depth Information
(CH
3
)
3
CSSC(CH
3
)
3
into a solution of In(CH
3
C(O)CH
2
C(O)CH
3
)
3
and OAm.
39
A
er growth at 180
C for 7 hours, nanorods (with a cubic crystalline core)
of varying lengths were obtained, although a relatively monodispersed
average width of 11.5 nm
1.3 nm was observed. The rods were also found
to have a remarkably square edge. The rods had broad, weak emission at
ca.
445 nm attributed to either trap states or structural defects, slightly Stokes
shi
d
n
1
y
4
n
g
|
2
ed from the absorption edge with an excitonic feature at
ca.
350 nm. It
is worth noting that elemental sulfur could not be used as a precursor in
this reaction, although anisotropic In
2
S
3
nanotubes were reported elsewhere
using InCl
3
, elemental sulfur and OAm, followed by heating at 50
Cfor
30 minutes, and further heating at 220
C for up to 5 hours.
40
During the
reaction, it was found that 13 nm hexagonal nanoplates of
b
-In
2
S
3
formed
a
er 30 minutes. The particles increased in size during prolonged heating,
stacking face on, and then converting to nanotubes upon further heating. It
was suggested that the crystalline phase of the particles drove the conver-
sion, as full coordination could be achieved by reactions between similar
surfaces, reducing the energy of the system as dictated by dangling bonds.
The OAm was also suggested to be essential, as it bound to the (111) facet
of the crystals, allowing the growth
via
oriented attachment.
Other layered semiconductors, such as GaSe, have been prepared by
analogous chemistry.
41,42
GaSe has a direct bandgap of 2.11 eV (although the
di
erence between the direct and indirect transition is only 25 meV), and is
composed of weakly bound Se
Se sheets. The material is unusual as it
display two types of exciton; a 2D exciton associated with the indirect tran-
sition, and a 3D exciton, with a Bohr radius of 3.1 nm, associated with the
direct transition. The injection of a solution of (CH
3
)
3
Ga in TOP into TOPO,
TOP and TOPSe at 278
C, followed by growth at
ca.
270
C, resulted in the
formation of GaSe particles
ca.
4 nm in diameter. A
-
Ga
-
Ga
-
.
er observation of an
excitonic shoulder in the absorption spectrum, the reaction was cooled to
room temperature to stop the reaction, a process that took in total
ca.
2 hours. The particles were isolated by methanol/toluene solvent/non-solvent
interactions, although interestingly, if the system was extremely water-free,
the particles were retained by methanol. Trace amounts of water resulted in
the particles remaining in the non-polar solvent. The TOPO on the resulting
GaSe particles could be exchanged for a long-chain amine, which reduced
agglomeration. This synthesis is extremely interesting as previous attempts
to prepare gallium-containing QDs have proved di
cult; this is attributed to
the tight binding of gallium precursors to ligands, as described in Chapter 2.
The synthesis of monodispersed GaSe particles therefore needed the high
temperature to proceed: any higher, and the particles became polydispersed;
any lower, and the reaction proceeded too slowly. Analysis of the selected
area di
raction patterns obtained from the particles suggested a single tet-
ralayer structure. The emission of the particles was found to be relatively
strong, with quantum yields of up to 10%. In a similar manner to InSe
particles, the wavelength of the emission was tuneable from
ca.
425 nm to
ca.
525 nm by altering the excitation wavelength.
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