Methods for Controlling the TERS Probe Position 44 

The enhancement of the electric field in TERS is confined in close proximity of the tip apex. The dimension of the confined area ranges from few to tens of nanometers. However, it has been theoretically1,2 and experimentally3-5 evidenced that the tip- enhanced signal decays quite rapidly as the tip-sample distance increases. Owing to its near-field nature, the intensity of the Raman signals decrease rapidly by extension of the tip-surface distance.6-9 Consequently, it is essential to precisely control the position of the

tip with respect to the sample surface. To fulfill this purpose, the precise control over the tip position (feedback mechanism) provided by scanning probe microscopy (SPM), is ideal. Atomic force microscopy (AFM),6,8 scanning tunneling microscopy (STM)7,9and sheer force (SF)10,11 feedback controls are the three SPM methods that have mainly been utilized in TERS. The feedback system, along with a piezoelectric sample scanner, enables the scanning of the tip and its positioning on a particular region of the sample with sub-nanometer resolution.

3.1.1.1 AFM Feedback

In AFM, the reflection of a laser beam from the cantilever backside is used as the feedback parameter. The reflection of this laser allows one to monitor the tip-sample distance and to maintain a constant interaction using a feedback loop. The feedback loop is equipped by a piezoelectric tube that controls the tip position in xyz directions as illustrated in Figure 3.1a.

Figure 3.1 Illustration of different SPM feedback systems (a) AFM feedback with tip oscillating vertically with respect to the sample surface (b) STM feedback with tip oscillating laterally with respect to the sample surface (c) shear force feedback with tuning fork mounted vertically and the tip moving laterally with respect to the surface (d) feedback mechanism with tuning fork mounted horizontally and the tip oscillating vertically with respect to the sample surface

The tip-sample interaction is enabled through various AFM modes such as contact and non-contact (intermittent contact or tapping mode. Both contact and non-contact modes are applicable for TERS measurements. The advantage of contact over non-contact mode is the constant distance between the tip and the sample which results in a consistent enhancement over the surface. On the contrary, in tapping mode the tip oscillates and its distance from the surface changes constantly.12 However, upon each Raman acquisition the tip oscillates several times during which the tip-sample distance ranges from a few to tens of nanometers. As a result, the Raman signal is an average of contact and non- contact signals. In general, lower interaction between the tip and the sample is expected in non-contact mode, which is an advantage specially when working with delicate samples and/or probes that are coated with a thin layer of noble metal. The time during which the tip is in closer proximity (near-field) of the sample can be controlled by optimizing the oscillation amplitude. In addition, near-field contributions can be

separately detected by synchronizing the laser with the time duration when tip is in near- field of the sample and using a shutter to collect the signal only when the tip is in sample’s close proximity.13 In addition, by using fast detectors such as avalanche photodiodes (APD) and a lock-in amplifier, optimization of the signal/noise ratio becomes possible through discriminating the far-field from the near-field contribution.14 There is however one important limitation involved with AFM based TERS setups arising from the use of an internal laser diode located inside the AFM head and with wavelength usually in the near-IR range. This light source used to maintain the feedback between the surface and the tip restricts the choice of Raman excitation wavelength to UV-visible due to the possible interference of the AFM laser with the detector of the spectrometer. Nevertheless, the addition of filters can be useful to eliminate parasitic wavelength of the AFM laser.

3.1.1.2 STM Feedback

In STM, the tip-sample distance is controlled through keeping a constant tunneling current between a conductive tip and a conductive sample, as illustrated in Figure 3.1b. The tip-sample separation in STM setups is in the sub-nanometer range, which is

generally smaller compared to AFM feedback.15 Metal tips obtained by electrochemical etching of gold or silver wires are generally employed. The fact that the substrate has to be conductive restricts the choices of TERS optical configuration and the samples. Nevertheless, the need for a conductive sample could be resolved by utilizing conductive substrates which are typically metallic or carbon thin plates. Top or side illuminations configuration is generally combined with STM feedback to be compatible with opaque substrates.14 TERS optical configurations including top and side illuminations will be discussed in detail in section 3.1.2 of this Chapter.

3.1.1.3

Shear Force Feedback

In shear force feedback systems, etched TERS metallic tip is mounted to a tuning fork which usually oscillates horizontally with respect to the sample surface as illustrated in

Figure 3.1c. In such cases the feedback system holds relatively constant distance

between the tip and the sample which resembles contact AFM modes. This configuration is mostly applicable for samples with low roughness and horizontal features. The tuning fork is usually made from quartz crystals and resonates at a frequency in the range of 30- 100 kHz.10 The oscillation occurs in the plane parallel to the sample surface and the feedback is maintained by monitoring the change of the current induced to the tuning fork by the tip-sample shear force interactions. The tuning fork itself is attached to a

piezoelectric tube as a part of the feedback loop.

It is also possible that the tuning fork is mounted horizontally with respect to the sample surface (Figure 3.1d). Instead of shear forces, tip-sample interactions will be mainly induced by Van der waals and electrostatic interactions in this setup.

An advantage of using a tuning fork scheme compared to standard contact AFM comes from the fact that the choice of Raman excitation laser is not limited to UV-visible due to the absence of feedback laser diode. In shear force setups however, the tip has to be glued to the tuning fork where the quality of the gluing could result in a less stable and