Fluorescence from Coated Oxide Nanoparticles
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Fluorescence from Coated Oxide Nanoparticles D. Vollath, I. Lamparth, D. V. Szabó Forschungszentrum Karlsruhe, Institut fuer Materialforschung III P.O.Box 3640, D-76021 Karlsruhe, Germany ABSTRACT In many cases, coated nanoparticles behave like isolated ones. Using the microwave plasma process, it is possible to produce oxide nanoparticles with ceramic or polymer coating. Coating the particles has the additional advantage that by proper selection of the coating it is possible to suspend the particles in distilled water without using any colloid stabilizer. From quantum dots made of sulfides or selenides, it is well known from literature that fluorescence depends strongly on the coating of the kernels. In the case of CdSe, the kernels are coated with CdS. Within this study, similar phenomena are found with coated oxide nanoparticles having sizes of ca. 6 nm exhibiting a very narrow particle size distribution. The coating consists of a second ceramic phase or a polymer one, each one influencing fluorescence differently. Obviously, the type of coating is a tool to modify fluorescence. This behavior is demonstrated on kernels made of Al2O3, ZrO2, HfO2, ZnO etc. PMMA, PTFE, or Al2O3 were used as coating material. In most cases, the fluorescence spectra showed broad bands. In some cases, such as ZnO, additionally, a sharp emission line in the UV appears. It is interesting to note that coatings made of fluorine containing polymer materials did not lead to fluorescence intensities comparable with PMMA coatings. The observed spectra are equivalent whether the powder is in aqueous suspensions or dry on a quartz glass carrier. The experimental results in this study indicate that the combination of non-fluorescent oxide core with a non-fluorescent polymer coating may lead to a nanocomposite with strong fluorescence. This is a phenomenon not described in literature until now. INTRODUCTION Many applications of nanoparticles use fluorescence properties, e.g. in medical, biological, or pharmaceutical area. [1-7] Usually, fluorescence is a property of organic molecules or single isolated nanoparticles. Therefore, in most cases, fluorescence is determined in suspensions. Best fluorescence efficiency is obtained with semiconductor nanoparticles and semiconductor quantum dots based on sulfides, selenides, or tellurides. [8, 9] The best-described fluorescent nanoparticles are GaN [10, 11], doped ZnS [12], or doped CdSe. [13] All of these particles have the disadvantage of being as well poisonous as carcinogenic. Additionally, these particles show a very limited thermodynamic stability against oxidation. Therefore, research in the direction of fluorescent oxide nanoparticles is under way. [14-16] In most cases, fluorescence is obtained by doping with small amounts of rare earth ions. [17] In many cases, electron excitation of the fluorescent material led to the best results. The Karlsruhe Microwave Plasma Process is capable to synthesize ceramic nanoparticles coated with a second ceramic layer [18,19] or a polymer. [20] In situ coating o
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