22 research outputs found
Ensemble modeling of very small ZnO nanoparticles
The detailed structural characterization of nanoparticles is a very important issue since it enables a precise understanding of their electronic, optical and magnetic properties. Here we introduce a new method for modeling the structure of very small particles by means of powder X-ray diffraction. Using thioglycerol-capped ZnO nanoparticles with a diameter of less than 3 nm as an example we demonstrate that our ensemble modeling method is superior to standard XRD methods like, e.g., Rietveld refinement. Besides fundamental properties (size, anisotropic shape and atomic structure) more sophisticated properties like imperfections in the lattice, a size distribution as well as strain and relaxation effects in the particles and-in particular-at their surface (surface relaxation effects) can be obtained. Ensemble properties, i.e., distributions of the particle size and other properties, can also be investigated which makes this method superior to imaging techniques like (high resolution) transmission electron microscopy or atomic force microscopy, in particular for very small nanoparticles. For the particles under study an excellent agreement of calculated and experimental X-ray diffraction patterns could be obtained with an ensemble of anisotropic polyhedral particles of three dominant sizes, wurtzite structure and a significant relaxation of Zn atoms close to the surface
Measurements of extremely low radioactivity levels in BOREXINO
The techniques researched, developed and applied towards the measurement of
radioisotope concentrations at ultra-low levels in the real-time solar neutrino
experiment BOREXINO at Gran Sasso are presented and illustrated with specific
results of widespread interest. We report the use of low-level germanium gamma
spectrometry, low-level miniaturized gas proportional counters and low
background scintillation detectors developed in solar neutrino research. Each
now sets records in its field. We additionally describe our techniques of
radiochemical ultra-pure, few atom manipulations and extractions. Forefront
measurements also result from the powerful combination of neutron activation
and low-level counting. Finally, with our techniques and commercially available
mass spectrometry and atomic absorption spectroscopy, new low-level detection
limits for isotopes of interest are obtained.Comment: 27 pages, 5 figures. Submitted to Astroparticle Physics (17 Sep
2001). Spokesperson of the Borexino Collaboration: G. Bellini. Corresponding
author: W. Hampe
Dramatic increase in charge carrier lifetime in a liquid crystalline perylene bisimide derivative upon bay substitution with chlorine
A perylene bisimide derivative bearing four chlorine substituents in the bay positions exhibits charge carrier mobility in the liquid crystalline phase similar to its parent compound, but the charge carrier lifetime is over 100 times greater than found in the non-chlorinated parent compound. This dramatic increase in lifetime is explained in terms of increased order in the modified material. This enhanced lifetime persists when the material is blended with hexabenzocoronene derivatives, and suggests the chlorinated derivative may find usefulness in device architectures where extended carrier lifetime is desirable