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Nanostrukturen und Grenzflächen Poster: Do., 13:00–15:30 D-P302
Structure Determination of ZnO and CdSe/ZnS Core-Shell particles
Franziska Niederdraenk 1 , Pawel Luczak 1 , Reinhard Neder 2 , Christine
Barglik-Chory 3 , Sofia Dembski 4 , Christina Graf 4 , Eckart Rühl 4 , Eberhard
Umbach 1
1 Experimentelle Physik II, Universität Würzburg – 2 Mineralogisches Institut, Universität
Würzburg – 3 Institut für Physikalische Chemie, Universität Würzburg –
4 Lehrstuhl für Silicatchemie, Universität Würzburg
During the last 15 years, semiconductor nanoparticles have attracted more and more
attention in both, fundamental and applied research. Some applications, e.g. in bioimaging
or quantum electronics, are already under way. Yet, the basic knowledge is
still far from complete, in particular for nanoparticles with diameters well below 5 nm.
The size range between 1 and 5 nm is also of fundamental interest since it represents
the transition regime between solid state, cluster, and molecular physics.
One important aspect is the determination of precise geometric parameters like size,
shape, crystallinity, ” lattice“ constants, and structural defects. In principle, powder diffraction
methods can provide detailed structural information. However, the diffraction
peaks become very broad for particle diameters fairly below 5 nm, and the analysis
of the data is crucial. Commonly used approaches for a grain size evaluation like the
Rietveld refinement or the Scherrer equation are based on periodic structures, an assumption
which is often not sustainable for nanoparticles with diameters below 5 nm.
Other more intricate parameters like relaxation effects or stacking faults cannot be
considered at all.
Here we use an alternative, more direct way for the data analysis. We model the entire
particle and calculate the powder diffraction pattern using the Debye formula. In this
way, all important parameters like size, shape, stacking faults, strain, etc. are intrinsically
included in the calculated data. Furthermore, parameter distributions, e.g., the
size distribution, can now be taken into account, which are essential since they are
likely to occur in wet-chemically synthesized particles.
We report on powder diffraction data with high signal-to-noise quality and low background
measured at beamline BW2 at HASYLAB. The measured data are fitted using
an evolutionary algorithm which includes the Debye method described above. Our
results show that a stacking fault probability needs to be considered in most cases.
This means that the particles are not necessarily single crystalline but consist of both
zincblende- and wurtzite-like stacked layers. Distributions or probabilities are implemented
by averaging the diffraction patterns of several different particles with different
sizes, stacking sequences, or other parameters. Since the atomic model is highly flexible,
the method can easily be expanded to core-shell particles by adding an outer layer
consisting of different atoms with different lattice parameters.
In our contribution we present several examples demonstrating the capability of our
new data analysis method. Diffraction data for ZnO and CdSe/ZnS core-shell particles
will be shown.