The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 113

CHAPTER 5

d n 1 y 4 n g | 3

The Synthesis of Core/Shell
Quantum Dots

5.1 Early Studies on Core/Shell Materials
Surface passivation is a key element in a robust nanoparticle system. As
the number of surface atoms is high (50% of particles are at the surface in
a 3 nm diameter spherical particle) the preparation of a surface

with as few defects, dangling bonds or charge carrier trapping sites as
possible is desirable. Early studies relied on polymers and surfactants to
passivate the surface, although these o

' clean '

 en desorbed leaving unprotected

particles.
A further inorganic layer is ideal for numerous reasons; once deposited, it
is unlikely to desorb and may provide a more consistent, complete surface
passivation. Surfactant cone angles no longer matter, the particles are more
resistant to processing, and, by a judicious choice of material, quantum
dot (QD) heterojunctions can be formed where the band energy o

.

sets of
the two materials can result in materials with novel and interesting optical
properties. 1,2
The two main factors to consider when choosing a shell material are the
band energy levels and lattice mismatch. The lattice constants need to be
di erent enough to avoid alloy formation, yet close enough to allow even

shell growth. It is worth noting that although some core/shell systems have
large lattice mismatches ( ca. 15%), homogenous shells can form easily. The
dynamics of shell growth on a small particle are signi

 cantly di erent from

multilayer growth in thin  lm, and structures with large mismatches that

would not form in at systems may form in nanoparticle systems because of

the short facets and reconstructed surfaces.