Near-field optical taper antennas fabricated with a highly replicable ac electrochemical etching method

Abstract

This paper describes a novel chemical etching method to fabricate high quality near-field optical antennas-tapered metallic tips-from gold wire in a reproducible way for optically probing a specimen on the nanoscale. A new type of an electrochemical cell is introduced and different dc and ac etching regimes are studied in detail. The formation and dynamics of a meniscus around a gold wire immersed in an electrolyte when supplying a square wave voltage are considered. We show that in situ etching current kinetics allows one to improve a yield of tips with a well-defined geometry up to 95% by filtering these on the basis of a cutoff current and a power spectrum of etching current fluctuations. As a quantitative measure for estimating the yield we introduce a probability to find tips with curvature radii falling in the range of interest. Testing the tips for a plasmonic effect is implemented with tip-enhanced Raman spectroscopy and sub-wavelength imaging of a thin fullerene film.

Electrochemical design of plasmonic nanoantennas for tip-enhanced optical spectroscopy and imaging performance

“…− [11,22] which block an access for Cl − anions to WE in the vicinity of an etchant meniscus. This value corresponds to an electrode potential of 1.68 V for gold at which a leading corrosion mechanism Au ↔ Au + + e − takes place [21]. Importantly, electrochemical etching is generally non-steady, since convex areas on the gold surface dissolve strongly in comparison with concave ones and, in addition, the gold under stress corrodes faster than that in a normal state.…”

“…The latter for different gold tips ranges statistically from 2 ms to 20 ms and it means that a time sampling when etching is estimated to be ~100 μs. Such a time is enough to meet a current cutoff event in an optimal manner [21,24]. The centre of gravity of the histogram is long-term shifted when the handling parameter η increases and vice versa.…”

“…Previously, the reproducibility of gold tips etched with the method had been determined as a probability to find tips with curvature radii ρ falling in the range of interest [21]. However, this approach may distort the yield of the well-shaped tips produced with the dc-voltage etching method because of non-uniform distribution.…”

“…Electrochemical etching is widely used as a low-cost, simple and reliable fabrication tool for designing plasmonic optical conical nanoantennas with a high yield of >90% [21,22]. A drawback of this method is related to optimization of a large number of parameters such as the chemical nature of etching electrolyte, electrode arrangement, an applied voltage shape, etching timing, a cutoff current and others.…”

Electrochemical fabrication of silver tips for tip‐enhanced Raman spectroscopy assisted by a machine vision system

“…[16][17][18][19][20][21][22][23] There are already quite some attempts to fabricate Ag tips for TERS by the EC etching, because silver has a much more efficient LSPR than gold. The developed EC etching methods to fabricate silver tips can be classified into two types: single step and two step methods.…”