aqueous precipitations but it was at first surprising to discover that the luminescence of such particles depended markedly on the surface coat. Particles treated
with hydroxide were much more efficient than as-prepared particles. The importance of the surface in such materials reflects the relative importance of the surface
sites for a typical nanodimension particle around 4 nm
3 e.g. a few percent of the
atoms are found on the surface. Defects tend to anneal to the surface and hence
the cores of such small particles are often ‘relatively’ defect free.
These observations and one further criterion start to define quantum dot systems. The second major possibility is the coating of the central particle with a
second material e.g. CdS or ZnS on top of CdSe. When a wider band gap material
is coated onto the outside of a narrow band gap material the confinement on
the ‘core’ is enhanced, leading to enhanced optical properties, especially photoluminescence efficiencies.
The above enables us to define the nature of an isolated quantum dot. It will
depend on
. The nature of the central materials, the core.
. The nature of any subsequent coating ‘shell’ layer.
. The nature of the final coat on the material, often an organic capping layer.
A typical such material is shown in Figure 2.1. In high quality materials the core of
the dot will be a single crystal, the shell epitaxial or close to and the final coat will
pacify defects and also confer solubility and or functionality for binding to a substrate or target molecule. However, what is shown in both these schematics and
images is a single quantum dot. Each dot is a mesoscopic entity with individual
properties, the properties of an ensemble of dots will additionally be determined
by the particle size distribution and any differences in morphology within the ensemble. The best quality dots are themselves single crystals, as nicely illustrated by
the PbS sample [11] shown in Figure 2.2.
There are as yet no standards defining the properties of such samples; these will
clearly need to emerge as these materials become more generally utilised. Such
standards will owe more to the definitions of materials and/or polymer science
than to the world of the molecule and will include definitions probably statically
derived from:
. Size, including aspect ratio.
. The distribution of particle sizes.
. The nature of the core.
. The nature of the shell.
. The nature of the final coat.
The above describe the physical composition of the particle; other properties may
define its use, typically quantum efficiency in an optical material and other measures as appropriate, such as coercivity in a magnetic material.
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
14
with hydroxide were much more efficient than as-prepared particles. The importance of the surface in such materials reflects the relative importance of the surface
sites for a typical nanodimension particle around 4 nm
3 e.g. a few percent of the
atoms are found on the surface. Defects tend to anneal to the surface and hence
the cores of such small particles are often ‘relatively’ defect free.
These observations and one further criterion start to define quantum dot systems. The second major possibility is the coating of the central particle with a
second material e.g. CdS or ZnS on top of CdSe. When a wider band gap material
is coated onto the outside of a narrow band gap material the confinement on
the ‘core’ is enhanced, leading to enhanced optical properties, especially photoluminescence efficiencies.
The above enables us to define the nature of an isolated quantum dot. It will
depend on
. The nature of the central materials, the core.
. The nature of any subsequent coating ‘shell’ layer.
. The nature of the final coat on the material, often an organic capping layer.
A typical such material is shown in Figure 2.1. In high quality materials the core of
the dot will be a single crystal, the shell epitaxial or close to and the final coat will
pacify defects and also confer solubility and or functionality for binding to a substrate or target molecule. However, what is shown in both these schematics and
images is a single quantum dot. Each dot is a mesoscopic entity with individual
properties, the properties of an ensemble of dots will additionally be determined
by the particle size distribution and any differences in morphology within the ensemble. The best quality dots are themselves single crystals, as nicely illustrated by
the PbS sample [11] shown in Figure 2.2.
There are as yet no standards defining the properties of such samples; these will
clearly need to emerge as these materials become more generally utilised. Such
standards will owe more to the definitions of materials and/or polymer science
than to the world of the molecule and will include definitions probably statically
derived from:
. Size, including aspect ratio.
. The distribution of particle sizes.
. The nature of the core.
. The nature of the shell.
. The nature of the final coat.
The above describe the physical composition of the particle; other properties may
define its use, typically quantum efficiency in an optical material and other measures as appropriate, such as coercivity in a magnetic material.
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
14