*2.2.1. Tip fabrication and related issues*

A sharp metallic tip is a central part of the TERS setup and its quality (size and shape) defines the spatial resolution of TERS and, to some extent, the magnitude of the enhanced field. Owing to its importance, several papers have discussed various tip fabrication methods [32–34]. Tips are usually made of gold or silver as these metals have plasmon resonances in the visible region of the electromagnetic spectrum. Electrochemical etching is a common method to prepare the tips. In a typical procedure, Au tips are prepared by direct current electrochemical etching in a 50:50 (v:v) mixture of concentrated HCl and ethanol. Au wire (a tip after etching) serves as the anode and a gold or platinum ring acts as the cathode. The end of the Au wire is submerged 1–2 mm into the solution at the center of the gold ring. A voltage in the range of 1.7–2.5 V is applied between the cathode and the anode. The etching reaction proceeds until the electrochemical current drops to zero. Typically tips with a tip-apex size in the range of 20–50 nm are obtained in this way.

#### *2.2.2. Optical geometries*

TERS requires integration of STM/AFM with the optical components used in Raman spectroscopy. Various optical geometries have been adapted in the past 15 years to work with a variety of different samples [34]. They include a bottom-, side-, top- and a parabolic mirror illumination depicted in **Figure 1**. Each geometry has its own advantages and disadvantages, which are summarized hereafter.

The bottom illumination geometry (**Figure 1a**), in which the incident laser light is fed through the microscope objective placed at the bottom and the rear side of the sample, is not practiced among STM-based TERS users as it requires transparent samples. This geometry is commonly used in AFM-based TERS, in which an inverted optical microscope platform is used. The advantage of this geometry is a very high collection efficiency of scattered light as oilimmersion objectives with a high numerical aperture (NA) can be used. Recently, the Deckert group has modified the bottom illumination geometry to work with opaque samples by using

**Figure 1.** Four common optical geometries used in TERS (a) bottom illumination, (b) side illumination,(c) top illumination, (d) parabolic mirror illumination.

a dichroic mirror and passing the incident light through the transparent side of the opaque sample [35]. The bottom illumination is well suited for biological samples.

Side illumination (**Figure 1b**) is the most commonly used setup in TERS built on STM. In this geometry, the incoming laser light as well as the outgoing scatter signal is collected by a long working distance microscope objective placed at an angle in the range of 45–70° relative to the tip axis that maximizes the electric light component along the tip axis. The advantage of this geometry is an easy integration with commercial STM setups. It can be used both for transparent and nontransparent samples. However, only conductive samples can be studied. The disadvantage of the side illumination geometry is relatively low collection efficiency in comparison to the bottom light illumination geometry. This is because objective lenses with a low numerical aperture are employed due to space restriction around the STM tip. Another disadvantage of the side illumination geometry is an asymmetric illumination of the tip, which leads to larger spot size and higher far-field background. In addition to tip resonances, so-called gap mode resonances can be excited in the side illumination geometry [36, 37]. They play a big role in the enhancement of the signal when the distance of the tip to the surface is below 2 nm [38]. Excitation of the gap-mode resonances improves sensitivity and resolution of the TERS technique. The side illumination geometry is widely used in the field of surface science.

Top illumination (**Figure 1c**) is the second most commonly used geometry in the STM-TERS community. This geometry makes focusing of the incident beam on the STM tip easier. The tip is placed at an angle to the surface with the microscope objective placed normal to the surface. Although some of the scattered light is shadowed by the tip, this geometry offers still higher excitation and collection efficiency than the side-illumination geometry does. Another advantage is a reduction in the far-field background. Both opaque and transparent samples can be studied. This geometry is ideally suited for investigating nanoscale phenomena on silicon or graphene samples.

A geometry utilizing a parabolic mirror (**Figure 1d**) was first demonstrated by researchers working with an STM unit operating in UHV conditions [30]. It was developed in order to increase the collection efficiency by allowing collection of light from all directions. In this geometry, a hole is made in a parabolic mirror to accommodate the STM tip. STM-TERS setups with a parabolic mirror work in a reflection mode, allowing both opaque and transparent samples to be studied. Compared to the side and top illumination geometries, the parabolic mirror geometry offers high collection efficiency. Two major disadvantages of this geometry are: it is difficult to integrate into commercial STM units and optical alignment is difficult. A small angular mismatch results in defocusing of the laser spot and loss of the signal. Thus, this geometry has not been widely used.
