The nucleation and growth of uranium on the basal plane of graphite studied by scanning tunneling microscopy
For the first time, nanometer scale uranium clusters were created on the basal plane of highly oriented pyrolytic graphite by laser ablation under ultra-high vacuum conditions. The physical and chemical properties of these clusters were investigated by scanning tunneling microscopy (STM) as well as standard surface science techniques. Auger electron and X-ray photoelectron spectroscopies found the uranium deposit to be free of contamination and showed that no carbide had formed with the underlying graphite. Clusters with sizes ranging from 42 [Angstrom][sup 2] to 630 [Angstrom][sup 2] were observed upon initial room temperature deposition. Surface diffusion of uranium was observed after annealing the substrate above 800 K, as evidenced by the decreased number density and the increased size of the clusters. Preferential depletion of clusters on terraces near step edges as a result of annealing was observed. The activation energy for diffusion deduced from these measurements was found to be 15 Kcal/mole. Novel formation of ordered uranium thin films was observed for coverages greater than two monolayers after annealing above 900 K. These ordered films displayed islands with hexagonally faceted edges rising in uniform step heights characteristic of the unit cell of the P-phase of uranium. In addition, atomic resolution STM images of these ordered films indicated the formation of the [beta]-phase of uranium. The chemical properties of these surfaces were investigated and it was shown that these uranium films had a reduced oxidation rate in air as compared to bulk metal and that STM imaging in air induced a polarity-dependent enhancement of the oxidation rate.
- Research Organization:
- Lawrence Livermore National Lab., CA (United States)
- Sponsoring Organization:
- USDOE; USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 6774924
- Report Number(s):
- UCRL-LR-112417; ON: DE93006279
- Resource Relation:
- Other Information: Thesis (Ph.D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
URANIUM
CRYSTAL GROWTH
NUCLEATION
AUGER ELECTRON SPECTROSCOPY
CRYSTAL LATTICES
DIFFUSION
GRAPHITE
SCANNING ELECTRON MICROSCOPY
THIN FILMS
X-RAY SPECTROSCOPY
ACTINIDES
CARBON
CRYSTAL STRUCTURE
ELECTRON MICROSCOPY
ELECTRON SPECTROSCOPY
ELEMENTAL MINERALS
ELEMENTS
FILMS
METALS
MICROSCOPY
MINERALS
NONMETALS
SPECTROSCOPY
360102* - Metals & Alloys- Structure & Phase Studies
665100 - Nuclear Techniques in Condensed Matter Physics - (1992-)