| dc.description.abstract | The growth and structures of aluminum oxides on NiAl(100) have been investigated by RHEED(reflection high energy diffraction) ,complemented by LEED (low energy electron diffraction) ,AES (Auger electron
spectroscopy) and STM (scanning tunneling microscopy).
Crystalline θ-Al2O3 phase grows through gas-phase oxidation on the NiAl(100) substrate with its a and b-axes parallel to [0-10] and [001] direction of the substrate,respectively,forming a (2 × 1) unit cell.Whilst,three-dimensional nano-sized NiAl(100) protrusions and Al2O3,NiAl (011)clusters were found to co-exit at the surface,evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns,particularly,the NiAl (011) clusters develop with their (011) plane parallel to the NiAl(100) surface,and [100] axis parallel to the [0-10] direction of the substrate.STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxide during annealing.On the other hand,smoother (2×1) oxide islands with thickness close to a complete monolayer of θ-Al2O3 can be formed on NiAl(100) by electro-oxidation,in contrast with the large crystalline films formed by gas-oxidation.
Moreover,the Co nanoclusters grown from vapour deposition on thin film Al2O3/NiAl(100) have been studied by reflection high energy electron diffraction (RHEED),scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES).The results show that the annealed Co nanoclusters,with mean diameters of 2.4 ,3.4 ,5.8 nm and heights of 0.7 ,1.5 ,1.5 nm respectively,are highly crystalline.Structural analysis based on the RHEED patterns indicate that these Co clusters,although with different sizes,have an fcc phase and grow with their (001) facet parallel to the NiAl(100).It is optimal growth for the Co clusters,as the Co fcc
(001) facet is to match better with the rectangular oxygen mesh of the oxide surface even though there is an appreciable lattice mismatch.The lattice constant of the Co clusters is consequently expanded by 4-5 ％ in comparison with that of fcc Co bulk,in order to match better with the oxide surface and minimize the interface strain.The preferential formation of the Co fcc phase is also driven by such significant interfacial interaction and perhaps a relatively smaller surface free energy of the Co-fcc clusters. | en_US |