Chemistry Reference
In-Depth Information
precursors in OAm, giving particles 15
-
25 nm in diameter.
17
The crystal
phase of the particles was di
cult to determine, due to peak broadening, but
was assigned as either a kesterite or stannite structure. The particles were
again used in photovoltaic devices with a device e
ciency of up to 0.74%.
Similarly,
18
Cu(CH
3
C(O)CH
2
C(O)CH
3
)
2
, Zn(O
2
CCH
3
)
2
, and Sn(O
2
CCH
3
)
4
were
combined in OAm, and heated to 150
C. In a separate
ask, sulfur was mixed
d
n
1
y
4
n
g
|
2
in OAm and sonicated, a
er which both precursor solutions were injected
into TOPO at 300
C for 45 minutes, giving faceted Cu
2
ZnSnS
4
particles
ca.
13 nm in diameter with a tetragonal crystalline core and an optical band
edge of 1.5 eV (
ca.
830 nm).
The composition of Cu
2
ZnSnS
4
has been tuned by altering the ratio of
precursors in the compound (Cu
2
Sn)
x
/3
Zn
1
x
S, where the material is essen-
tially tuned between ZnS (
x
0.75), keeping the
Cu : Sn ratio at 2 : 1 to maintain charge in the sphalerite phase.
19
The
resulting materials maintained the relatively small particle size of
ca.
3nm
regardless of composition, and the cubic sphalerite structure. The absorp-
tion band edge was easily tuned across the entire visible spectrum, with band
edges ranging from
ca.
350 nm for
x
¼
0) and Cu
2
ZnSnS
4
(
x
¼
0.75. The
material (Cu
2
Sn)
0.01
Zn
0.97
S was utilised in simple photovoltaic devices.
Likewise, spherical Cu
2
Zn
x
Sn
y
Se
1+
x
+2
y
nanoparticles of a tetragonal crystal-
line phase,
ca.
20 nm in diameter have been prepared, using amine
complexes of the metal chlorides and TOPSe as precursors in ODE at 295
C.
The identity of the material was con
¼
0to
ca.
1000 nm for
x
¼
rmed using a number of techniques,
including
Z
-contrast electron microscopy, as XRD alone was not su
cient to
.
identify the phase.
20
One of the simplest compounds with the most potential when one
consider the number of available crystalline phases is the binary copper
sul
de system, with numerous naturally occurring forms accessible, several
of which are semiconducting with applications in solar energy conversion.
21
Some of the copper(
I
) sul
des (Cu
2
x
S) are copper de
cient and hence give
rise to self-doped systems with su
cient free carrier densities to exhibit
valence band surface plasmons (when
x
> 0), notably in the near infrared
region (Figure 4.2).
22,23
Early studies into the organometallic-related
synthesis of copper sul
des used a solventless technique, using copper
octanoate and dodecanethiol as the starting materials for a single-source
precursor, which was heated at low temperatures to apparently yield Cu
2
S
nanorods,
24,25
although these were later found to be hexagonal discs which
self-assembled face to face.
26
Other single-source routes using solvents have been reported and are
described in Chapter 7. Typical binary routes which are based on high-
temperature solvent synthesis have also been reported, using precursors
such as Cu(CH
3
C(O)CH
2
C(O)CH
3
)
2
and elemental sulfur in OAm at 200
C for
1 hour, followed by a further hour at 230
C. The resulting hexagonal
b
-Cu
2
S
nanoparticle platelets with a hexagonal crystalline core were isolated using
solvent/non-solvent interactions, although spherical particles could also be
obtained using a synthesis temperature of 200
C for 2 hours.
27
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