A controlled and reproducible tip-to-sample gap is a crucial requirement in TERS experiments. For the regulation of the tip above the sample surface, three SPM (Scanning Probe Microscope) methods, AFM, STM, and Normal/Shear-Force Microscopy, have mainly been employed.
Cantilever-Based Atomic Force Microscopy (AFM):
The Atomic Force Microscope relies on measuring the bending forces of a beam that holds the actual tip. Most commonly, this is achieved by detecting the changed reflection of a laser beam induced by forces once the cantilever senses a tipsample interaction (contact mode), or a change in resonance frequency of an induced tip oscillation due to the proximity of the sample (semi-contact mode, a.k.a. intermittent mode, a.k.a. tapping mode).
In all cases, these forces are related to typical atomic or molecular interactions, ranging from van der Waals forces to repulsive electrostatic interactions.
The main advantage of the cantilever-based AFM feedback modes is that almost no special sample requirements exist. The system works on any surface with a roughness up to several microns, and a lot of additional information related to the tip-to-sample interactions can be probed (topography, phase imaging, conductivity, friction, contact potential and many more).
In TERS, AFM contact mode has been used successfully; however, special care must be taken to avoid damage of the metal (coated) tip. The tapping mode feedback is more appropriate for sticky biological samples, such as lipids, proteins, etc. However, if the tip oscillation amplitudes are big, the tip is only for a fraction of the time in the actual nearfield region. Consequently, the challenge is to perform the TERS experiments at the lowest possible amplitudes, and a well-known tip-sample distance, while maintaining a stable feedback. As a technical answer, a specific imaging mode, the so-called βSpec-stackβ mode, has been developed on the SmartSPM so that the tip is alternately going into contact for the TERS acquisition and moving in tapping mode to the next pixel to avoid any damage of the tipβs vicinity.
Scanning-Tunneling Microscopy (STM):
In STM, a conductive tip is kept in electron tunneling distance from the surface. For tunneling, both tip and sample surface need to be conductive; thus, mostly metals are used as substrates as well as for the tips. The tunneling distance between the tip and sample is usually considered to be in the region of 1 nm or less.
The disadvantage of the STM approach is the limitation to either conductive samples or molecular monolayers and other very thin samples. On the other hand, the main advantages of the STM feedback are highest spatial resolution, best tip-distance control, and easy tip preparation (most often by electrochemical etching). Also, by changing tip bias, the STM can investigate further parameters that are specifically of interest in electrochemistry.
Shear Force and Normal Force Microscopy:
In a Shear Force microscope, a metal tip (generally the same TERS tips as the one used in STM) is glued to the prong of a quartz tuning fork. Upon excitation, this system (tip and fork) vibrates at its resonance frequency using the natural piezoelectricity of quartz. When this oscillating system is brought in close proximity of the surface by damping of the free oscillation, similar to tapping mode AFM, a detuning of the resonance is used as a feedback signal. Two different approaches can be used: (i) Shear Force mode when the oscillation direction of the tuning fork is parallel to the sample surface (similar interaction than the mode used with fiber-based SNOM) and (ii) Normal Force mode when this oscillation direction is normal to the sample surface (leading thus to a typical atomic interaction like in AFM regulation). The main advantage is easy tip preparation, drawbacks are the intrinsically lower lateral resolution and the very low reproducibility in gluing the tip onto the prong.