Combining sonicated cold development and pulsed electrodeposition for high aspect ratio sub-10 nm gap gold dimers for sensing applications in the visible spectrum

Strong interactions between localized surface plasmons and nanoscale objects have led to the development of highly sensitive biochemical sensing in planar metallic nanostructures with sensing performance mainly dependent on the interaction volume and the local electric field. However, the sensitivity and the interaction volume of these planar structures have been limited by the achievable aspect ratios based on the standard lift-off process. We propose a new technique which involves cold sonicated development and pulsed electrodeposition to overcome this limitation, and demonstrate robust gold square dimers with sub-10 nm gaps and a gap aspect ratio of similar to 8. We show that smooth gold surfaces can be achieved by growing the gold film directly on a transparent ITO substrate without a gold seed layer, and demonstrate a significant improvement in Q factors and resonance contrast in electrodeposited dimers compared to dimers fabricated by physical vapor deposition. We demonstrate that the electrodeposited dimers exhibit near 50% higher bulk refractive index sensitivity than their planar counterparts. The technique may be used to grow a variety of metals of arbitrary geometries and spatial arrangements.