Feb 7, 2013
Patterned cathodes offer fine control over nanorod size and position
The electrochemical growth of gold nanorods in anodized aluminium templates offers a low-cost method for creating an optical metamaterial with applications in bio-sensing, nanophotonics and surface enhanced Raman spectroscopy (SERS). In recent experiments, a cathode under layer is pre-patterned before the growth of gold nanorods. This results in the localized growth of nanorods at specific sites on the substrate. By shrinking the size of the pre-patterned cathode bridge to less than 200 nm, the scientists can control both rod height and location.
Reporting their results in the journal Nanotechnology, researchers at Queens University Belfast have demonstrated greater control over the growth dynamics of nanorods electrodeposited in anodized aluminum (AAO) templates. This builds on previous work, which showed how to grow bulk nanorod arrays in thin-film templates using electrochemical deposition.
The new technique uses a focused ion beam to etch specific shapes into the gold cathode under layer. These patterned regions define where electrochemical growth of gold nanorods is allowed. This localization process can also be used to study the growth dynamics involved.
By examining nanorod growth along cathode bridges of various widths, the scientists observed that, as the cathode bridges narrow, the nanorods decrease in height compared with heights of structures grown in bulk.
These growth dynamics were generalized by creating chains of narrow cathode bridges to larger cathode pads, to create islands of isolated growth. The creation of the nanorod islands results from the significant increase in the resistivity along the narrow cathode bridges compared with the cathode pads.
The FIB method is not the only approach for structuring the cathode under layer. This can also be accomplished using electron beam lithography or other photolithographic techniques. However, the FIB approach is particularly favourable for high-resolution prototyping experiments, such as the ones described in the work.
Plasmonic wave guiding
Bulk samples of gold nanorods have already found applications as biosensors and for SERS studies. Thanks to the ability to vary spatial position and rod height all within a specific region, new applications related to plasmonic wave guiding should become possible. One way this could be achieved is by tailoring the nanorod islands to cluster in specific shapes.
More information can be found in the journal Nanotechnology.
About the author
Joshua F Einsle is a research fellow at Queens University Belfast. Currently, he is investigating plasmonic antennas for improved magneto-optical Kerr microscopy in the Centre for Nanostructured Media. His research interests include nanofabrication techniques, electron microscopy, plasmonics as well as nanomagnetic systems.