Surface Segregation in Chromium-Doped Nanocrystalline Tin Dioxide Pigments

Surface properties play an important role in understanding and controlling nanocrystalline materials. The accumulation of dopants on the surface, caused by surface segregation, can therefore significantly affect nanomaterials properties at low doping levels, offering a way to intentionally control nanoparticles features. In this work, we studied the distribution of chromium ions in SnO2 nanoparticles prepared by a liquid precursor route at moderate temperatures (500 degrees C). The powders were characterized by infrared spectroscopy, X-ray diffraction, (scanning) transmission electron microscopy, Electron Energy Loss Spectroscopy, and Mossbauer spectroscopy. We showed that this synthesis method induces a limited solid solution of chromium into SnO2 and a segregation of chromium to the surface. The s-electron density and symmetry of Sn located on the surface were significantly affected by the doping, while Sn located in the bulk remained unchanged. Chromium ions located on the surface and in the bulk showed distinct oxidation states, giving rise to the intense violet color of the nanoparticles suitable for pigment application.

Persistent luminescence mechanisms: human imagination at work

The present status and future progress of the mechanisms of persistent luminescence are critically treated with the present knowledge. The advantages to be achieved by a further need as well as the pitfalls of the excessive use of imagination are shown. As usual, in the beginning of the present era of persistent luminescence since the mid 1990s, the imagination played a more important role than the sparse solid experimental data and the chemical common sense and knowledge was largely ignored. Since some five years, the mechanistic studies seem to have reached the maturity and - perhaps deceivingly - it seems that there are only details to be solved. However, the development of red emitting nanocrystalline materials poses a challenge also to the more fundamental studies and interpretation. The questions still luring in the darkness include the problems how the increased surface area affects the defect structure and how the "persistent energy transfer" really works. There is still some light to be thrown onto these matters starting with agreeing on the terminology: the term phosphorescence should be abandoned altogether. The long lifetime of persistent luminescence is due to trapping of excitation energy, not to the forbidden nature of the luminescent transition. However, the technically well-suited term "afterglow" should be retained for harmful, short persistent luminescence. (C) 2012 Optical Society of America