Tip-Enhanced Raman Spectroscopy
221
a gold wire is immersed in an etchant solution, which is gradually etched through the
electrochemical reaction induced by an applied voltage. The electrochemical reaction sharpens the gold wire, and finally an extremely sharp tip in conical shape is
obtained as illustrated in Fig. 7a. Although gold is the most common material, silver
and aluminum can also be used as a plasmonic material [43–45]. Usually the electrochemically etched tips are long and have smooth surface. They do not have any
nano-sized plasmonic resonant structure that can be considered as an optical antenna.
That is why they are often used in the gap-mode geometry to confine the light field.
Since the STM-based TERS uses a conductive substrate, the gap-mode configuration
is easily adopted. In the gap-mode regime, light is confined at the gap between the
tip and the substrate. Compared with normal plasmon resonance mode with a single
plasmonic structure, the gap-mode offers strong light confinements, however, with
weak dependence on the incident wavelength because it does not have a plasmon
resonance that one finds in the case of an antenna. The negative aspect of this is that
one cannot selectively control the enhancement of one particular wavelength, but the
positive aspect is that such system works readily with a large range of wavelengths.
Moreover, the strong confinement contributes to strong signal enhancement as well
as high spatial resolution. Recently, a single molecular resolution was reported in
TERS [16], which was also achieved in the gap-mode configuration in STM-based
TERS, as we will discuss later in this chapter. Although the gap-mode has several
advantages, it can instantly disappear with increasing gap distance, similarly with
the tunneling current. It thus requires a special care for thick samples.
Another popular approach to control tips in TERS is AFM, as introduced in Fig. 3
[26, 32, 33]. Since AFM operates through weak forces such as van der Waals force
between the tip and the sample, the sample does not have to be conductive. Therefore, either conductive or insulating, any types of samples can be measured with
AFM-based TERS. Furthermore, AFM works not only in vacuum or in air but also
in the liquid environment, which is highly compatible to biological samples [46–
48]. Recently, most of the commercially available AFM systems can work in liquid
environment. Therefore, AFM-based TERS could be more versatile compared with
STM-based TERS. A cantilever tip is usually utilized in AFM-based TERS as shown
in Fig. 3, where the optical lever detection technique is used in most AFM to
monitor applied forces between the tip and samples by detecting the deflection of
the cantilever.
The most common method of tip fabrication is to simply deposit plasmonic material on a cantilever tip via thermal evaporation [26, 33, 34]. A variety of AFM
cantilever tips are commercially available. By evaporating silver or gold on the
cantilever tip, granular plasmonic structures are formed on the tip body (Fig. 7b).
Figure 7c shows a scanning electron microscopy (SEM) image of a cantilever tip
after the thermal evaporation of silver. A metallic nanoparticle is often attached at
the apex of the tip that facilitates LSPR so that near-field light can be efficiently
excited at the apex. Since AFM-based TERS relies on the LSPR at the tip apex,
it does not necessarily need the gap-mode configuration to confine the light, and
hence the substrate does not have to be conductive. However, one can of course use
a conductive substrate in AFM-based TERS if the gap-mode is preferred.
221
a gold wire is immersed in an etchant solution, which is gradually etched through the
electrochemical reaction induced by an applied voltage. The electrochemical reaction sharpens the gold wire, and finally an extremely sharp tip in conical shape is
obtained as illustrated in Fig. 7a. Although gold is the most common material, silver
and aluminum can also be used as a plasmonic material [43–45]. Usually the electrochemically etched tips are long and have smooth surface. They do not have any
nano-sized plasmonic resonant structure that can be considered as an optical antenna.
That is why they are often used in the gap-mode geometry to confine the light field.
Since the STM-based TERS uses a conductive substrate, the gap-mode configuration
is easily adopted. In the gap-mode regime, light is confined at the gap between the
tip and the substrate. Compared with normal plasmon resonance mode with a single
plasmonic structure, the gap-mode offers strong light confinements, however, with
weak dependence on the incident wavelength because it does not have a plasmon
resonance that one finds in the case of an antenna. The negative aspect of this is that
one cannot selectively control the enhancement of one particular wavelength, but the
positive aspect is that such system works readily with a large range of wavelengths.
Moreover, the strong confinement contributes to strong signal enhancement as well
as high spatial resolution. Recently, a single molecular resolution was reported in
TERS [16], which was also achieved in the gap-mode configuration in STM-based
TERS, as we will discuss later in this chapter. Although the gap-mode has several
advantages, it can instantly disappear with increasing gap distance, similarly with
the tunneling current. It thus requires a special care for thick samples.
Another popular approach to control tips in TERS is AFM, as introduced in Fig. 3
[26, 32, 33]. Since AFM operates through weak forces such as van der Waals force
between the tip and the sample, the sample does not have to be conductive. Therefore, either conductive or insulating, any types of samples can be measured with
AFM-based TERS. Furthermore, AFM works not only in vacuum or in air but also
in the liquid environment, which is highly compatible to biological samples [46–
48]. Recently, most of the commercially available AFM systems can work in liquid
environment. Therefore, AFM-based TERS could be more versatile compared with
STM-based TERS. A cantilever tip is usually utilized in AFM-based TERS as shown
in Fig. 3, where the optical lever detection technique is used in most AFM to
monitor applied forces between the tip and samples by detecting the deflection of
the cantilever.
The most common method of tip fabrication is to simply deposit plasmonic material on a cantilever tip via thermal evaporation [26, 33, 34]. A variety of AFM
cantilever tips are commercially available. By evaporating silver or gold on the
cantilever tip, granular plasmonic structures are formed on the tip body (Fig. 7b).
Figure 7c shows a scanning electron microscopy (SEM) image of a cantilever tip
after the thermal evaporation of silver. A metallic nanoparticle is often attached at
the apex of the tip that facilitates LSPR so that near-field light can be efficiently
excited at the apex. Since AFM-based TERS relies on the LSPR at the tip apex,
it does not necessarily need the gap-mode configuration to confine the light, and
hence the substrate does not have to be conductive. However, one can of course use
a conductive substrate in AFM-based TERS if the gap-mode is preferred.