Formation of Colloidal Semiconductor Nanocrystals

The present work describes different techniques to control some ma
jor parameters of colloidal nanocrystals. The individual techniques
rely on the manipulation of the nucleation event. The sensitive
control of the nanocrystals’ size and shape is discussed.
Furthermore the formation of hybrid nanocrystals composed of
different materials is presented. The synthesis technique for the
production of the different samples involves organic solvents and
surfactants and reactions at elevated temperatures. The presence of
magic size clusters offers a possibility to control the size of the
nanocrystals even at very small dimensions. The clusters produced
comprise ca. 100 atoms. In the case of CdSe, nanocrystals of this
size emit a blue fluorescence and therefore extend the routinely
accessible spectrum for this material over the whole visible range.
Samples fluorescing in the spectral range from green to red are
produced with standard recipes. In this work a reaction scheme for
magic size clusters is presented and a theoretical model to explain
the particular behaviour of their growth dynamics is discussed. The
samples are investigated by optical spectroscopy, transmission
electron microscopy, X-ray diffraction and elemental analysis.
Shape controlled nanocrystals might be of interest for a variety of
applications. The size dependent properties of nanocrystals are
dominated by their smallest dimension. Therefore anisotropically
shaped nanocrystals exhibit similar optical and electronic
properties as spherical nanocrystals with a compatible diameter.
This makes nanorods and nanowires an appealing object for
electronics. Another possible application for these materials is to
incorporate them into synthetic materials to influence their
mechanical stability. Here, a method to form branched nanocrystals
is discussed. It turned out that the presence of small impurities
in the reaction vessel triggers the formation of branching points.
Furthermore this synthesis technique offers some insights into the
architecture of the branching point. The branching point is
analysed by high resolution transmission electron microscopy and
proves for the occurrence of a multiple twinned structure are
strengthened by simulation of the observed patterns. Incorporation
of a second material into a nanocrystal adds different
functionality to the entire ob ject. Ideally both materials
contribute with their own functionality and they are not affected
by the presence of the other material. Two different techniques to
generate nanocrystals of this type are presented. The first relies
on a seeded growth approach in which the nucleation of the second
material is allowed only on defined sites of the seeds. Anisotropic
nanorods show a reactivity that varies for the individual facets.
Using such nanorods as seeds dumbbell structures are formed. The
second technique uses the tips of pre-formed nano-dumbbells as
sacrificial domains. The material on the tips is replaced by gold.
In any of the processes a different aspect of the nucleation event
or the earliest stage of the growth is of relevance. In the growth
of the magic size clusters the nucleation event itself is slowed
down to a pace at which the experimenter can follow any step. The
occurrence of branching can be traced down to the emergence of
defects in the crystalline structure in the earliest stage of the
growth. Hybrid materials are formed by a seeded-growth mechanism.
Pre-formed nanocrystals provide the nucleation sites for the second
material.