Tip-Enhanced Raman Scattering Imaging of Two-Dimensional Tungsten Disulfide with Optimized Tip Fabrication Process

Abstract

We successfully achieve the tip-enhanced nano Raman scattering images of a tungsten disulfide monolayer with optimizing a fabrication method of gold nanotip by controlling the concentration of etchant in an electrochemical etching process. By applying a square-wave voltage supplied from an arbitrary waveform generator to a gold wire, which is immersed in a hydrochloric acid solution diluted with ethanol at various ratios, we find that both the conical angle and radius of curvature of the tip apex can be varied by changing the ratio of hydrochloric acid and ethanol. We also suggest a model to explain the origin of these variations in the tip shape. From the systematic study, we find an optimal condition for achieving the yield of ~60% with the radius of ~34 nm and the cone angle of ~35°. Using representative tips fabricated under the optimal etching condition, we demonstrate the tip-enhanced Raman scattering experiment of tungsten disulfide monolayer grown by a chemical vapor deposition method with a spatial resolution of ~40 nm and a Raman enhancement factor of ~4,760.

Fabrication of highly uniform nanoprobe via the automated process for tip-enhanced Raman spectroscopy

“…The Au etching reaction mainly occurs at the meniscus of the etchant around the Au wire and is expressed as follows [21][22][23][24][25][26][27][28][29]:…”

“…The contrast value can be determined by obtaining the intensity ratio of both the 2LA(M) peaks depicted in Figure 5b, and the value of the TERS enhancement factor can be calculated using the obtained contrast value, radius of the laser spot, and measured R tip . The defined equations for both the contrast and enhancement factor are represented as follows [7,22,34]:…”

“…Thus far, various methods, such as mechanical shaping [14][15][16], ion beam milling [17,18], chemical polishing [19], template stripping [20], and electrochemical etching [21][22][23], have been developed to fabricate Au nanotips. Particularly, most research groups have been using electrochemical etching because it offers several advantages as follows: low fabrication cost [18,21], reliable tip quality [24][25][26], and simple manufacturing procedure [22,23]. However, they typically used a strong acid, such as HCl or H 2 SO 4 , as an etchant to fabricate the tips.…”

Spectroscopic Imaging at the Nanoscale: Technologies and Recent Applications

“…Ag and Au) tips, which are typically used in STM-TERS, is electrochemical etching, which electrochemically dissolves a metal wire in the etching electrolyte. 23–27 By adjusting the etching parameters, the size and shape of the etched tip can be controlled. 24 Recent studies have also shown the etchant composition and concentration affects the shape of the resulting tips.…”

“…24 Recent studies have also shown the etchant composition and concentration affects the shape of the resulting tips. 26, 27 However, it is still difficult to precisely tune the tip size and shape by electrochemical etching alone. Over-etching is a common problem, particularly in the case of silver tips.…”