Oxidation of uranium nanoparticles produced via pulsed laser ablation
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Oxidation of uranium nanoparticles produced via pulsed laser ablation Thomas W. Trelenberg, Stephen C. Glade, James G. Tobin, Thomas E. Felter, and Alex V. Hamza Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory Livermore, CA 94550, U.S.A. ABSTRACT An experimental apparatus designed for the synthesis, via pulsed laser deposition, and analysis of metallic nanoparticles and thin films of plutonium and other actinides was tested on depleted uranium samples. Five nanosecond pulses from a Nd:YAG laser produced films of ~1600 Å thickness that were deposited showing an angular distribution typical of thermal ablation. The films remained contiguous for many months in vacuum but blistered due to induced tensile stresses several days after exposure to air. The films were allowed to oxidize from the residual water vapor within the chamber (2x10-10 Torr base pressure). The oxidation was monitored by in-situ analysis techniques including x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and scanning tunneling microscopy (STM) and followed Langmuir kinetics. INTRODUCTION Surface science studies of actinide electronic structure should ideally be carried out on clean, single-crystalline samples, however, producing these large samples has proven difficult. Bandmapping requires that the orientation of the crystalline facet under measurement be known. At the same time, the probe used must not extend beyond this facet or to neighboring crystallites. While efforts to grow larger, single-crystal actinide samples have met with some success, isolation of these crystals for study via traditional techniques can still be difficult. Another approach is to study thin films grown on suitable substrates. Crystalline films may be produced in this way; however, the observed structure may have substrate influences present and care must be taken to ensure that these influences are taken into account. Our approach was to attempt to isolate and analyze laser ablated nanocrystallites using microprobe techniques. A film would be ablated onto a substrate, verified clean with photoemission (XPS an UPS), crystallite location and orientation determined using standard STM, and finally probing electronic structure using STS (scanning tunneling spectroscopy). To this end, we have constructed an experimental apparatus designed for the synthesis and analysis of metallic nanoparticles and thin films of plutonium and other actinides [1, 2]. Pulsed laser ablation was chosen as the method for producing our samples, as the production of both highpurity nanoscale material [3] and actinide films [4] (through careful annealing) have been demonstrated. Laser ablated nanocrystallites naturally occur with a range of sizes, so further processing, other than choosing a particle size via an STM scan, would not be required. One of the chief challenges we faced was to deposit actinide material that contained no oxide. The high reactivity of the actinide elements makes this a challenging task. O
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