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Nanostrukturen und Grenzflächen Vortrag: Mi., 18:40–19:00 M-V24
Structure determination of nanoparticles using the pair distribution function
Reinhard Neder 1 , Vladimir Korsunkiy 1
1 Institut für Mineralogie, Am Hubland 97074 Würzburg
PDFs of interatomic distances G(R) are obtained by Fourier transformation of powder
diffraction pattern [1]. The PDF peak positions correspond to interatomic distances
and the peak intensity is proportional to the number of corresponding distances. This
allows the modelling of the real atomic mutual arrangement in extremely small (1-
3 nm) nanoparticles [1,2], whose structure cannot be investigated with the standard
methods like Rietveld fitting of powder diffraction intensities. By the use of high intensity
high energy synchrotron radiation (λ=0.1 ˚A, E=100-120keV), one can reliably
obtain highly resolved PDFs. This is due to the possibility to precisely measure the
very weak scattering up to a very high diffraction vector qmax=4πsinΘ/λ=30 ˚A −1 .
Our measurements were carried out at the beam line BW5 at HASYLAB DESY in
Hamburg [1]. The PDFs of both, polycrystalline and nanocrystalline ZnSe show sharp
peaks, characteristic of interatomic distances in a crystal. The PDF of crystalline ZnSe
does show these up to very large R, while in the PDF of nanocrystalline ZnSe, equivalent
peaks have already disappeared at R>30 ˚A due to the finite nanoparticle size. This
allows to estimate the particle size as being ca. 30 ˚A. The modelling of the PDF shows
that the nanoparticles have the zincblende structure but with several defects. Small
(D=18 ˚A) CdS nanoparticles have a quasi-amorphous structure and the two types of S
atoms the internal inorganic and the external belonging to the organic ligands define
two quite distinct CdSCd angles (109 and 100 ◦ ) between the Cd-S (2.52 ˚A) bonds. In
the RDF, the two peaks produced by distances between heavy Cd atoms (at 3.85 and
4.11 ˚A) correspond to these different bond angles. In the PDF of composite nanoparticles
consisting of a CdSe core and a ZnS shell, interatomic distances characteristic for
both components were observed directly. In the diffraction pattern, however, only very
broad and overlapping peaks are observed due to the small particle size and structural
defects.
[1] V.I. Korsounski, R.B. Neder, at all, J.Appl.Cryst. 36 (2003) 1389.
[2] V.I. Korsunskiy, , R.B. Neder, J.Appl.Cryst. 38 (2005) 1020.