This
paper describes the fundamental mechanism for the formation of a 3-dimensional
porous template during the anodization of Al with less than 1 atom
% Cu percentages. It is known that the presence of Cu impurities in
an Al film introduces horizontal pores interconnecting the vertically
aligned porous structure of the anodized aluminum oxide (AAO) template.
We show that the formation of these horizontal pores is accompanied
by current density oscillations when the anodization is performed
at a constant voltage. The frequency of these oscillations is directly
related to the horizontal interpore distance. We propose a mechanism
that links the current density oscillations to the Cu accumulation
at the metal/oxide interface through the cyclic change in anode potential.
The distance between the horizontal pores is found independent of
the current density, temperature, and electrolyte concentration. Instead,
it was found that the spacing between the vertical pores, and thus,
the anodization voltage determines the spacing between the horizontal
pores. A model based on the plastic flow of the alumina barrier layer
was suggested to link the spacing between the horizontal and the vertical
pores. These results provide important insights in the formation of
3D AAO templates. In addition, we show the fabrication of rigid 3D
metal nanomeshes by electrochemical deposition into these 3D porous
templates.
In this paper, we show the electrochemical deposition of a subnanometer film of nickel (Ni) on top of titanium nitride (TiN). We exploit the concept of cluster growth inhibition to enhance the nucleation of new nuclei on the TiN substrate. By deliberately using an unbuffered electrolyte solution, the degree of nucleation is enhanced as growth is inhibited more strongly. This results in a very high particle density and therefore an ultralow coalescence thickness. To prevent the termination of Ni deposition that typically occurs in unbuffered solutions, the concentration of Ni(2+) in solution was increased. We have verified with RBS and ICP-MS that the deposition of Ni on the surface in this case did not terminate. Furthermore, annealing experiments were used to visualize the closed nature of the Ni film. The closure of the deposited film was also confirmed by TOF-SIMS measurements and occurs when the film thickness is still in the subnanometer regime. The ultrathin Ni film was found to be an excellent catalyst for carbon nanotube growth on conductive substrates and can also be applied as a seed layer for bulk deposition of a smooth Ni film on TiN.
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