The well-ordered aluminum oxide film formed by oxidation of the NiAl(110) surface is the most intensely studied metal surface oxide, but its structure was previously unknown. We determined the structure by extensive ab initio modeling and scanning tunneling microscopy experiments. Because the topmost aluminum atoms are pyramidally and tetrahedrally coordinated, the surface is different from all Al2O3 bulk phases. The film is a wide-gap insulator, although the overall stoichiometry of the film is not Al2O3 but Al10O13. We propose that the same building blocks can be found on the surfaces of bulk oxides, such as the reduced corundum (0001) surface.

We have determined the structure of the ultrathin (sqrt[67] x sqrt[67])R12.2 degrees aluminum oxide on Ni3Al(111) by a combination of scanning tunneling microscopy and density functional theory. In addition to other local defects, the main structural feature of the unit cell is a 0.4-nm-diameter hole reaching down to the metal substrate. Understanding the structure and metal growth on this oxide allows us to use it as a template for growing highly regular arrays of nanoparticles.

A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.

The formation of one-layer surface oxides on Pd͑111͒ has been studied by scanning tunneling microscopy ͑STM͒ and density functional theory ͑DFT͒. Besides the Pd 5 O 4 structure determined previously, structural details of six different surface oxides on Pd͑111͒ will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of −0.95 to − 1.02 eV ͑570-605 K, 5 ϫ 10 −4 mbar O 2 ͒. Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is Pd 5 O 4 , Pd 9 O 8 , Pd 20 O 18 , Pd 23 O 21 , Pd 19 O 18 , Pd 8 O 8 , and Pd 32 O 32 . All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd͑111͒ substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than Pd 5 O 4 , we conclude that Pd 5 O 4 is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.

No abstract

No abstract

No abstract