From High Density Nanoparticle Deposits to Nanometer Thin Continuous Films By Growth-Inhibited Nucleation Enhancement
For the application of dense CNT forests, we were interested in fabrication of densely packed Ni, Co and Fe nanoparticles. We have shown that the density of the ECD catalyst particles is directly translated to the CNT density; the highest density of 2x1011cm-2 from an alkaline NiNO3-NH4Cl bath . Based on the know-how from copper, several typical growth inhibiting additives (e.g. saccharine, 2-butyne-1,4-diol, PEG) were tried to increase the Ni and Co nanoparticle density. However, with no avail, as the adsorption and inhibition was typically too slow. In copper deposition, the additives already form complexes with the cuprous intermediate formed during copper deposition. As such the adsorption is very fast, whereas for Ni and Co, the additives need to adsorb via a physisorption which happens too late to affect the nucleation stage. Recently we found that the Ni particle density is strongly affected by the boric acid concentration . The formation of growth inhibiting hydroxides gives the desired effect for growth-inhibited nucleation. However, the local pH cannot rise to fast as otherwise precipitation from solution already occurs in the depletion layer. Therefore a small concentration of boric acid was still needed at 5mM NiCl2 concentration to allow nickel growth to proceed long enough. At the best trade-off conditions a density of 4x1011 cm-2 Ni nanoparticles on TiN was obtained. By tuning the NiCl2concentration, the growth-inhibited nucleation process could be extended to higher Ni island densities resulting in nanometer thin nickel films ; similar to what was achieved for copper. In this paper, we will go in on the mechanism of growth-inhibited growth for the fabrication of high density magnetic metals such as Ni, Co and Fe.
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