Lead sulfide (PbS)
based quantum dots have been chosen to study quantum confinement. PbS
QD have almost unique characteristics that tend to amplify the quantum
confinement effect making it an excellent candidate for quantum dot experiments.
However, these characteristics are responsible for the poor fluorescence
efficiency of PbS quantum dots. Many groups have tried to improve the
fluorescence efficiency of PbS quantum dots using surface state passivation
with organic molecules. These approaches have met with little success
and in general are not appropriate in PbS quantum dots, due to strong
quantum confinement of both electrons and holes. Therefore, a more appropriate
technique of epitaxial overgrowth with another semiconductor was considered.
Fluorescence activated PbS quantum dots exhibiting
both epitaxial (red) and polycrystaline growth
Sulfide (CdS) is a compatible wide band-gap semiconductor, having
a similar lattice structure to PbS while presenting a small lattice
mismatch of less than 2%. Epitaxial overgrowth of CdS around the PbS
quantum dots was conducted in solution using wet chemical methods.
Immediately following epitaxial overgrowth, fluorescence can be observed
in the colloidal solutions under illumination with a UV lamp. This
fluorescence was further increased by an activation step, which involves
coating the nanocrystallites with cadmium hydroxide.
The chemical synthesis
was found to produce a wide range of quantum dots with visible fluorescence
ranging from deep red to almost white. It is believed that the difference
in fluorescence properties relates to the surface quality and morphology
of the PbS quantum dots.The
chemical conditions that control the formation of these different types
of quantum dot are presently under investigation.
transmission electron microscopy (HRTEM) has been used to characterise
the structure and shape of the quantum dots. Initial results indicate
that the PbS quantum dots grow to a size of approximately 2 nm. However
it was also found that epitaxial over-coating with CdS can result
in a polycrystalline structure, rather than a uniform over-coating.
Control of the epitaxial growth mode, between uniform over-coating
and polycrystalline growth, is presently under investigation.
HRTEM (inverted) of CdS/PbS composite quantum
dot showing polycrystalline growth.