Sketch of the etching experimental setup. The zoomed area shows the details of the tip immersion zone, highlighting the geometry of the electrodes and the meniscus formation at the air-liquid interface.
Time evolution of the etching process. (a) Meniscus formation at the air-liquid interface upon wire immersion in solution. (b) The meniscus lowers rapidly during the etching at 10 VDC as consequence of the diameter reduction of the wire in bulk. (c) The meniscus drops slowly during the etching at 2.4 VDC. The process ends with the detachment of immersed wire leaving a sharp gold tip at the end of the second tapered zone.
(a)–(d) SEM snapshots of the gold wire during the etching at 10 VDC and (e) after the conclusion of the etching at 2.4 VDC. The horizontal white line indicates the meniscus contact point at T = 0. The oblique line is guide-to-the-eye of the meniscus lowering at increasing times T 1. (f) Picture of the wire portion detached at the end of the two-step process. (g) Increase of the taper length (black diamonds) as a function of the etching time. Black squares are experimental values, while the red line is the linear fit. (h) Temporal evolution of the wire diameter at the meniscus contact point (red circles) and in bulk (black squares). Continuous lines represent their respective linear fits (red at meniscus and black in bulk).
Plots of the electrical current as a function of etching time during (a) the etching at high voltage (10 VDC, black symbols) and (b) the etching at low voltage (2.4 VDC). At high voltage we observe a linear current drop of −3 mA/s ((a), red line). On the contrary, (b) the electrical current during the low voltage etching shows spikes with intensity of several tens of mA superimposed to a continuously decreasing baseline. The end of the etching process is characterized by a sudden drop of the current to zero.
(a) SEM images of a pilum-shaped tip with subsequent zooms ((b) and (c)) on the apical part showing a radius of curvature r ∼ 25 nm (c).
(a) SEM image of a rough tip and (b) zoom on its apex showing a blunt apex with r ∼ 250 nm. (c) Occasionally, rough tips can exhibit extremely sharp edges with r in the 50 nm range. (d) SEM image of a tip covered with crystallite impurities.
(a) Current curves measured during the etching at low voltage of several tips, evidencing that tips with smooth surfaces (red curves) are characterized by much shorter etching times compared to tips with a rough surface (black curves). (b) Correlation plot between T 2 and r measured by SEM. Tips with a smooth surface (red circles) have r < 50 nm and preferentially are found whenever the etching time is lower than 250–300 s. Conversely, tips with a rough surface (black circles) and r > 50 nm require longer etching times (> 300 s). (c) Statistical distribution of r for tips produced with our method before (violet) and after (yellow) the application of the T 2 < 250 s screening criterion. (d) Cumulative percentage of tips having r smaller than each given amount before (violet) and after (yellow) the application of the screening criterion.
(a)–(d) SEM pictures of four different tips produced with T 2 < 250 s, typically showing (a) a smooth surface and (d) r value as low as 15 nm.
TERS (red line) and Raman (blue line) spectra of silicon acquired with a P polarized excitation and S polarized detection, with the tip in contact with the surface (red spectrum) and withdrawn by a few microns from the surface (blue spectrum).
(a) Percentage of tips with r smaller than 75 nm. (b) Percentage of tips showing SERS effect and r < 75 nm. (c) Percentage of tips with r < 75 nm that presents an SERS effect on different parts of the tip (apex and shaft).
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