Tip-Sample Distance Control

The principle behind SFM is that the lateral or shear force between an oscillating probe tip and the sample increases as the distance decreases. The probe is usually mounted in a support such that several millimeters of the aperture end of the optical fiber extends beyond the clamping point. The probe thus forms a cantilever having one fixed and one free end. It is driven transversely at a so-called tip resonance , which indicates that the resonance is due to the cantilever rather than the support structure of the microscope, with an amphtude 5nm. Shear forces between the probe tip and sample surface damp the oscillation. The amplitude is measured and fed back to the sample height position so as to maintain constant oscillation amplitude and presumably constant tip-sample distance. The amplitude was measured, originally, with optical deflection methods. Recently, a number of electrical measurement schemes have been demonstrated that may prove to have a number of advantages in speed, sensitivity or ease-of-use [12]. In near-field single molecule experiments the bandwidth of the feedback is not an issue as scan rate is limited by 

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Karrai, K., and R. D. Grober. 1995. Piezoelectric tip-sample distance control for near-field optical microscopes. Appl. Phys. Lett. 66 1842-1844. 

While the experimental details involved in implementing NSOM can be found elsewhere [7,8], it is instructive to briefly discuss the two main obstacles that must be overcome in order to conduct NSOM measurements. These revolve around aperture formation and implementing a feedback system for tip-sample distance control. For the former, as in all scanning probe techniques, the quality of the measurements is in large part dictated by the quality of the probe. For the latter, as the schematic in Fig. 1 suggests, high resolution requires that the NSOM probe be maintained within nanometers of the sample surface. 

So far, two types of metallic tips have been typically reported. One is a metal-coated cantilever and the other is etched metallic wire, each of which is used in a suitable combination with the feedback scheme of tip-sample distance control (see Sect. 4.3). The metal-coated cantilever is fabricated conveniently from a commercially available silicon (Si) or silicon nitride (SisN4) cantilever of AFM by depositing thin metallic film onto the probe surface (Fig. 16.5a) [31-33]. Metal with >99.999% purity is thermally evaporated under vacuum condition and deposited onto the probe surface slowly with a rate of less than 1 A/s in order not to damage the tip apex. The deposition thickness is typically several tens of nanometers. A small diameter of several tens of nanometers at the tip is easily obtained. 

The reported tip-sample distance control methods and the typical values of the resultant controlled distance in TERS experiments are summarized in Table 16.1. 

Kramer, A., T. Hartmann, S. M. Stadler and R. Guckenberger (1995). "An optical tip-sample distance control for a scanning near-field optical microscope." Ultramicroscopy 61(1 -4, Select Papers from the 3rd International Conference on Near-Field Optics and Related Techniques, 1995) 191-195. 

If the time consumption is acceptable and the image drift is negligible, a scan line can be scanned twice to separate topography and electrical properties. In this case, a first scan in contact or better in a dynamic mode without an electrical excitation is performed. The tip is lifted and for the following second scan the z-piezo is controlled in a way that the tip follows the same topography as for the first scan (constant tip-sample distance or interleave scan). During this second line scan, one of the above-mentioned measurements of electrical properties can be performed [396]. 

The main differences between the different SPM techniques lie in the type of interaction that is used to control the tip-sample distance. Although the SPM offspring are remarkably numerous [3,4], here we will focus only on STM and AFM, as they remain the most widely used and the best suited for high resolution imaging of surface structures. 

In TERS microscopy and spectroscopy, the tip enhancement due to the SPP resonance plays the most essential role both for signal sensitivity and spatial resolution. However, the tip-enhancement effect is not the only one affecting Raman spectra. There coexist other interaction mechanisms between a metal tip and sample molecules, chemical interactions similar to SERS [120-122], and, in addition, mechanical interactions (see Sect. 5.4.1). The latter two interactions show up only when sample molecules are in a close vicinity of a tip. In the TERS system using a ccaitact mode AFM, an experimentally observed TERS spectrum is a complex combination of the contributions of these three interactions, which makes it difficult to interpret experimental TERS spectra. Therefore, elucidation and discrimination of the tip-sample interactions are of scientific and practical importance. This can be realized by measuring a tip-sample distance dependence of TERS, since those three interaction mechanisms have different dependencies on the tip-sample distance. The active control of the distance between the tip and sample is a unique feamre only possible in TERS not in SERS. Two system configurations, time-gated detection and timegated illumination, are described below. 

This mode is often simply called the non-contact mode. This mode can provide true atomic resolution and image quality comparable to an STM. The cantilever is excited by the piezoactuator to oscillate at or near its resonant frequency. The vibrating tip is brought near a sample surface, but not touching the surface, to sense the weak attractive force between tip and sample instead of strong repulsive force in the contact mode. As illustrated in Figure 5.13, the interactions between the tip and the sample shift the oscillation frequency. The frequency shift signals are used to control the tip-sample distance. The interaction forces detected in the non-contact mode provide excellent vertical resolution. However, this mode cannot be operated in a liquid environment. Even in air, liquid attached on the tip can cause failure in operation. 

As mentioned before, AFM can measure surface forces using two different operation modes, DC and AC. In the DC mode, one measures the deflection (AZ) of a cantilever as a function of the tip-sample distance (D) that is varied usually hy a piezoelectric transducer (Fig. 3). The tip-sample force is given by Hooke s law in terms of AZ, F = kAZ, where k is the spring constant of the cantilever. This force clearly depends on the tip-sample distance, D, which is given by D = Z — AZ, in the absence of sample deformation. Z in the expression is the displacement of the piezo and the one that can be controlled in the experiment. When the tip is far away from the sample (large D), the force is zero. When the tip approaches the sample, it experiences various forces, electrostatic, van der Waals, double-layer, solvation forces, and so on. This is the regime of interest in... 

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