The use of nanoscale, nanowires in microfluidic devices and sensors require the tip size to be of the order of molecules to be detected, while integrating them to micro and macro-scale connectors. The work presented here creates multiscale wires through galvanic displacement and subsequent etching techniques. Nanosized, rod-like structures were generated from electrodeposited Fe-Ni-Co nanowires, followed by displacement of a part of the Fe-Ni-Co by Cu. A second Fe-Ni-Co layer was then deposited onto the Cu layer.  Once the wires are released, a part of the Cu was selectively etched.

Experimental

Then Fe-Ni-Co nanowires were fabricated with a pulsed current electrodeposition method, 2 s on/2 s off¹, in a polycarbonate membrane having pores of ~100 nm, from a boric acid-sulfate-sulfamate electrolyte. Displacement was carried out in a copper citrate solution and the etching step was completed in a citrate-boric acid electrolyte. The pH of the copper displacement solution was a variable. A commercial gold displacement solution was used to coat the resulting nanowire. A rotating cylinder electrode cell design was also used to characterize the deposition and displacement reactions. The resulting structure was characterized with Field Emission Scanning Electron Microscope (FESEM) and High-resolution transmission electron microscopy (HRTEM).

Results and Discussion

FESEM and TEM results indicate that rod-like regions occurred between two supporting Fe-Ni-Co regions in the nanowires (Figure 1), with the diameter of the rods on the order of 10 nm. Inner-rods was formed at low pH (pH=0, 1) however gaps were generated at high pH (pH=4). A complexation model predicted different Cu(II) species that govern the displacement reaction at low and high pH. SEM-EDS analysis show that there is indeed Fe and Ni remaining in the rod-like region. The results suggest that the copper ions are penetrating into the template and around the Fe-Ni-Co nanowires. A thin (~5 nm) Au layer was successfully coated on the rod-like nanowires.  

Conclusion

A new multistep method was developed to create nanowires with smaller, inner rod regions via electrodeposition, galvanic displacement and etching. The displacement reaction was affected by pH attributed to the type of complex Cu-citrate species. The resulting structure was then uniformly coated with gold by displacement.

Acknowledgement

The authors acknowledge and thank Roche Diagnostics for support of this project.

Reference

1. H. Kim, S.  A. Soper, E. J. Podlaha, ECS Trans. 53, 9 (2013).