Chemistry Reference
In-Depth Information
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Figure 5.1
Band edge alignments and mismatches for the diering classes of
core/shell materials. Grey boxes represent the valence band, white
boxes the conduction band.
The energy level mismatch is the key parameter. Four types of core/shell
semiconductor particles can be described, depending on the alignment of
the energy levels (Figure 5.1). It must be noted that the values taken for these
materials are usually based on bulk systems, and hence do not take into
account the widening bandgap in a semiconductor nanoparticle; whether
a shell of semiconductor a few monolayers thick can have a band structure at
all is debatable. The bulk energy diagrams are, however, a good indication of
what might be expected and some key publications describe relevant energy
levels and mismatches.
3
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9
The most common are termed
type I
, where the energy levels of the core are
narrower than those of the shell. By choosing a semiconducting shell with
a wider bandgap than that of the core, photoluminescence should be
enhanced (although not shi
.
ed), because the charge carriers are restricted
from the shell by the energetic potential barriers, forcing charge recombi-
nation in the core species. The shell protects the emitting core from envi-
ronmental factors by passivating surface defects and hindering oxidation.
The other structures are classi
ed as
type II
, where the energy levels are
staggered;
type III
, where the energy levels are completely misaligned or
broken; and
inverse
,e
ectively the opposite of type I structures.
For type II structures, electron and holes are spatially separated between
the core and the shell, and emission is from the recombination at an e
ec-
tively smaller indirect bandgap, making the emission red-shi
ed. As a result
of these dynamics, type II particles have long excited-state lifetimes, which
may
nd applications in photovoltaics. There are, to date, no examples of
type III core/shell nanoparticles prepared by organometallic precursors in
solution. Inverse structures can exhibit either type I or type II behaviour,
depending on core size and shell thickness.
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