Chemistry Reference
In-Depth Information
appear counter-intuitive as larger particles have less surface defects. This
unusual size dependence has been investigated using X-ray photoemission
spectroscopy, which revealed a complex range of lead and selenium envi-
ronments, essentially a core PbSe system, with a shell of Pb
1
x
Se and a
nal
capping layer of selenium ions bound to trioctylphosphine (TOP), essentially
a layer of TOPSe. Interestingly, two environments for TOP were observed,
suggesting a second layer of TOP above the TOPSe layer. The smaller particles
were found to have a thinner Pb
1
x
Se layer, which was suggested as a source
of defects.
29
In contrast, investigations by NMR have reported that the
passivation is almost entirely due to oleic acid coordinating to a uniquely
lead surface, with just 0
d
n
1
y
4
n
g
|
7
-
5% of the surface ligands being TOP.
30
Although
di
erent, these results are clearly dependent upon reaction conditions. Other
noteworthy investigation revealed important optical data, such as the
extinction coe
cient and the oscillator strength of QD PbSe.
31
QDs of PbSe have been shown to be extremely air sensitive, spontaneously
oxidising in ambient conditions to give PbO, PbSeO
3
and SeO
2
on the particle
surface.
32
The resulting oxidation reduces the particle size signi
cantly, blue-
shi
ing and broadening the optical properties. The emission quantum yield
initially quenches in minutes, attributed to oxygen on the particle surface
enhancing non-radiative carrier trapping, although this reverses in a matter
of hours to enhance the emission, o
en beyond the original quantum yield,
attributable to the lowering of the rate of non-radiative recombination due to
the widening bandgap in oxidised samples. This obviously impacts on the
durability of devices fabricated with such materials.
33
-
35
Similar results were
observed by Stouwdam
et al.
36
and Dai
et al.
,
37
who also observed complete
oxidation and dissolution of PbSe over a 12 day period under ultraviolet
excitation.
Despite the inherent air sensitivity, highly luminescent magic-sized
clusters (MSCs) of PbSe have been grown in ambient conditions using very
simple, related chemistry.
38
Heating PbO with oleic acid, followed by
cooling to 20
C produced the lead precursor in a similar manner to the hot
injection method. The precursor was then dissolved in octadecene (ODE),
to which was added TOPSe in ambient conditions, and le
.
stirring for
a few hours while the solution darkened notably. The resulting PbSe
particles exhibited reproducible,
c electronic transitions
consistent with magic cluster formation of various sizes. The clusters
exhibited absorption excitonic peaks between
ca.
625 and 880 nm, with
broad emission between
ca.
800 and 1000 nm and quantum yields of up to
90% despite being prepared in the presence of oxygen. Even when phase-
transferred to water, the particles maintain relatively high quantum yields
(up to 30%) over a matter of weeks. The inorganic core of the particles was
suggested to be less than 2 nm, with PEGylated samples displaying
hydrodynamic diameters of
ca.
2.4 nm, extremely small when taking into
account that a signi
very
speci
cant portion of the size could be attributed to the
capping agent. This makes PbSe magic clusters potentially relevant to
biological imaging applications.
Search WWH ::
Custom Search