Chemistry Reference
In-Depth Information
There are also instances where metal or metal oxide shells can be depos-
ited on a semiconducting core, giving bifunctional core/shell materials, and
in these cases the optics are dependent on the particular system.
10
For
example, the deposition of gold on to a CdSe surface resulted in the
quenching of the semiconductor emission. However, deposition of gold on to
a CdTe surface resulted in enhanced emission.
11
In these cases, the
absorption of the metal must be taken into account, and in the case of gold,
silver and copper (which are free-electron metals), the surface plasmon
resonance of the nanosized metal must be considered. Not all semi-
conductor/metal or metal oxide particles form core/shell particles, and these
will be discussed later.
Initial work into what can be described as the earliest core/shell systems
was reported in 1984, where CdS islands were grown on ZnS.
12
This was
followed by numerous notable investigations, including CdS/Cd(OH)
2
,
13
and
the CdS/HgS series,
14
which also included the
d
n
1
y
4
n
g
|
3
rst multilayer structure CdS/
HgS/CdS,
15
-
19
which could be considered the most studied and technically
advanced core/shell system before the advent of organometallic chemical
routes. The genesis of organometallic core/shell systems is covered in
Chapter 1, with CdSe/ZnS
20
and CdSe/ZnSe
21
systems being prepared by
a pseudo-organometallic route, using a mixture of organometallic and
inorganic chemistry.
5.2 Organometallic Routes to Core/Shell
Nanoparticles
The synthesis of CdSe nanoparticles capped with trioctylphosphine oxide
(TOPO) initiated numerous studies into size quantisation e
.
ects; the rela-
tive ease of preparation, the manipulation of the high-quality material ob-
tained and the superior optical properties make QDs prepared by the
organometallic route an ideal method of studying low-dimensional systems.
The particles, capped with TOPO, were crystalline, monodispersed and
exhibited band edge luminescence. However, the surfactant monolayer that
passivates surface defects, ensuring band edge emission while protecting
the particles from agglomeration, is not ideal as the surfactant does not
completely cap the surface and o
en desorbs. Gradual oxidation of TOPO-
capped CdSe particles over
ca.
24 hours was found to result in the formation
of SeO
2
, which desorbed o
the particle surface over a 96 hour period.
22
The
resulting surface vacancies provided non-radiative recombination pathways
for the charge carriers and induced deep trap emission from the particle,
giving optical properties similar to poorly passivated particles. It is common
to see nanoparticles capped with just organic surfactants referred to as
'
particles prepared with
a monolayers of surfactant eventually display deep trap emission, usually
manifest as a low-energy shoulder or feature in emission spectra. Di
naked
'
or
'
bare
'
dots. The majority of
'
bare
'
erent
capping agents result in di
ering degrees of protection, with amine-capped
Search WWH ::
Custom Search