Self-assembly and dynamics of oxide nanorods on NiAl(110)

Abstract

We observe the spontaneous formation of parallel oxide rods upon exposing a clean NiAl͑110͒ surface to oxygen at elevated temperatures ͑850-1350 K͒. By following the self-assembly of individual nanorods in real time with low-energy electron microscopy ͑LEEM͒, we are able to investigate the processes by which the rods lengthen along their axes and thicken normal to the surface of the substrate. At a fixed temperature and O 2 pressure, the rods lengthen along their axes at a constant rate. The exponential temperature dependence of this rate yields an activation energy for growth of 1.2± 0.1 eV. The rod growth rates do not change as their ends pass in close proximity ͑Ͻ40 nm͒ to each other, which suggests that they do not compete for diffusing flux in order to elongate. Both LEEM and scanning tunneling microscopy ͑STM͒ studies show that the rods can grow vertically in layer-by-layer fashion. The heights of the rods are extremely bias dependent in STM images, but occur in integer multiples of approximately 2-Å-thick oxygen-cation layers. As the rods elongate from one substrate terrace to the next, we commonly see sharp changes in their rates of elongation that result from their tendency to gain ͑lose͒ atomic layers as they descend ͑climb͒ substrate steps. Diffraction analysis and darkfield imaging with LEEM indicate that the rods are crystalline, with a lattice constant that is well matched to that of the substrate along their length. We discuss the factors that lead to the formation of these highly anisotropic structures.

Growth of thin alumina films on a vicinal NiAl surface

“…An oxide nature of these stripes would be compatible with their role as pinning lines for the triangular (110) planes. Oxide nano-rods of similar shape have previously been observed in LEEM studies performed during the initial oxidation of flat NiAl(110) surfaces [25]. Their one-dimensional growth has been traced back to an asymmetry in the lattice strain of this oxide phase (NiAl 2 O 4 as proposed by the authors), which favors elongation of the rod along the direction of smallest mismatch with the NiAl lattice, viz the [001] direction.…”

Al2O3scale development on iron aluminides

“…with doses of oxygen, rod-shaped oxides are formed along the substrate [001] direction, leaving unoxidized regions in between [24,25,26]. The preferential growth direction of these rods on the alloy surface is a result of strain minimization.…”

The formation of double-row oxide stripes during the initial oxidation of NiAl(100)

“…The structures of these oxides have been determined by electron energy loss spectroscopy, low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and low energy electron microscopy (LEEM). [11][12][13][14][15][16][17][18][19][20] Blum et al 12,13 showed that oxide stripes can form along h001i and h010i directions of the NiAl(100) surface and the stripes have a mean width of about 27 Å and a height of 3 Å or larger. Maurice et al 15 observed high resolution STM images on the oxides grown on NiAl(100) that are in agreement with the surface structure of the h-Al 2 O 3 phase.…”