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Doping and Characterization of Nanocrystalline Materials

Thomas Wichert 1Zhiqun Q. Guan 

1. Universität des Saarlandes, Technische Physik, Saarbrücken, Germany


Synthesis and characterization of nanocrystalline materials is presented. The materials synthesised are ZnO and InP doped with shallow donors and acceptors, respectively, and Ni-Cu alloys. The characterization is performed with radioactive isotopes using the perturbed γγ angular correlation technique (PAC), thereby, yielding local information on an atomic scale. The characterization is supplemented by different techniques, such as XRD spectroscopy, TEM, UV-VIS, photoluminescence spectroscopy, and EXAFS. By using the radioactive probe atom 111In/111Cd, information about its local surrounding is obtained via the electric and magnetic hyperfine fields. The technique typically uses 1011 probe atoms and the signal amplitude does not depend on the Debye-Waller-factor, both making this technique well suited for the investigation of nanocrystalline materials. The information obtained in this way is discussed for the nanocrystalline materials ZnO, InP, and NiCu alloys.

ZnO nanoparticles synthesized by electrochemical deposition under oxidizing conditions (EDOC) are doped with radioactive 111In and stable In. The range of the relative concentrations of dopants is varied between 10-9 and 10-3. The as-prepared ZnO particles have a crystal size of 5 nm whereby the dopant atoms are incorporated into disturbed crystalline surroundings that are attributed to imperfect crystalline structures. It turns out that annealing at 473 K represents a good compromise for incorporating the dopant atoms into nanocrystals of high crystalline quality and, at the same time, avoiding a significant crystal growth.

InP nanoparticles with average particle size of 2.4 nm are synthesized via the reaction of dry InCl3 with P(Si(CH3)3)3 in trioctylphosphine (TOP) at 530 K. The InP crystals are doped with the acceptor Cd using the radioactive decay of the 111In/111Cd. After preparation no cubic lattice environments around the dopants are detected. Annealing the caped InP nanoparticles at 770 K results in a particle size of 4 nm and yields 10 % of the dopants to be incorporated into an undisturbed lattice environment, and at 870 K the corresponding values are 33 nm and 42 %, respectively. Supported by theoretical calculations, the results suggest the presence of distortions of the lattice structure in the InP nanocrystals, which decreases with increasing particle size.

In nanocrystalline Ni-Cu alloys, synthesized by pulsed electrodeposition (PED), the presence of microscopic inhomogeneity, e.g. caused by pure Ni precipitates, is controlled using the characteristic hyperfine field of nanocrystalline Ni. In this way, precipitates that are invisible by XRD are still detectable by PAC. By increasing the temperature of the electrolyte or lowering the current density of electrolysis, the formation of Ni precipitates is strongly suppressed but, at the same time, the grain size is increased. The addition of saccharin inhibits grain growth and improves the homogeneity of the Ni-Cu alloy on a nanocrystalline scale.

The financial support by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich (SFB) 277 is gratefully acknowledged.


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Presentation: Invited oral at E-MRS Fall Meeting 2006, Symposium C, by Thomas Wichert
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-12 16:44
Revised:   2006-05-24 11:21