Tip-sample interaction effects

In order to achieve atomic resolution, even with p and d tip states, the tip-sample distance should be very short. At such short distances, the tip-sample interactions arc strong. There are two kinds of interaction effects 

A series of first-principles calculations of the combined system, that is, the tip and the sample, has been carried out by many authors, for example, Ciraci, Baratoff, and Batra (1990, 1990a). The three-dimensional shape of the potential barrier as well as the force between the tip and the sample are calculated. Three systems have been studied graphite-carbon, graphite-aluminum, and aluminum-aluminum. All those studies reached the same conclusion The top of the potential barrier between the tip and the sample is either very close to or lower than the Fermi level within the normal tip-sample distances of STM. 

The effect of atomic forces in STM was observed repeatedly by direct experiments. Before the invention of the STM, Teague (1978) already 

To summarize, the existence and role of force in STM is now a well-established scientific fact. At a relatively large absolute distance, for example, 5 A, the force between these two parties is attractive. (By absolute distance we mean the distance between the nucleus of the apex atom of the tip and the top-layer nuclei of the sample surface.) At very short absolute distances, for example, 1.5 A, the force between these two parts is repulsive. Between these two extremes, there is a well-defined position where the net force between the tip and the sample is zero. It is the equilibrium distance. On the absolute distance scale, the equilibrium distance is about 2-2.5 A. Therefore, the tip-sample distance of normal STM operation is 3-7 A on the absolute distance scale. In this range, the attractive atomic force dominates, and the distortion of wavefunctions cannot be disregarded. Therefore, any serious attempt to understand the imaging mechanism of STM should consider the effect of atomic forces and the wavefunction distortions. 

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Ebenstein, Y., Nahum, E., and Banin, U., Tapping mode atomic force microscopy for nanoparticle sizing Tip-sample interaction effects, Nano Lett., 2, 945, 2002. 

Ciraci, S., Baratoff, A., and Batra, I. P. (1990). Tip-sample interaction effects in scanning-tunneling and atomic-force microscopy. Phys. Rev. B 41, 2763-2775. 

Most users purchase AFM cantilevers with their attached tips from commercial vendors, who manufacture the tips with a variety of microlithography techniques. A close inspection of any AFM tip reveals that it is rounded off. Therefore, AFM microscopists generally evaluate tips by determining their end radius. In combination with tip-sample interaction effects, this end radius generally limits the resolution of AFM. As such, the development of sharper tips is currently a major concern. 

The atomic resolution in STM can be understood in terms of tip electronic states and tip-sample interactions. We will discuss the effect of tip electronic states in this section, and the tip-sample interactions in the next section. 

The one exception in which phase contrast is not due to the dissipation arises when the tip jumps between attraction phases (>90°) and repulsion phases (<90°). Since sine is a symmetric function about 90°, the phase changes symmetric even if there are no losses in the tip-sample interaction. The relative contribution of the repulsive and attractive forces can be estimated experimentally from the frequency-sweep curves in Fig. lib by measuring the effective quality factor as Qe=co0/Ao)1/2, where Ago1/2 is the half-width of the amplitude curve. The relative contribution of the attractive forces was shown to increase with increasing the set-point ratio rsp=As/Af. Eventually, this may lead to the inversion of the phase contrast when the overall force becomes attractive [110,112]. The effect of the attractive forces becomes especially prominent for dull tips due to the larger contact area [147]. 

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. 

For PtRh(lOO) an alloying effect decreases the LDOS at the Fermi edge for the Pt atoms and therefore the Pt atoms appear darker (lower) than the Rh atoms [34]. Chemical contrast by tip-sample interaction (see next section) will also lead to Rh atoms appearing bright and may therefore enhance this effect. 

Figure 19.4 Squeeze effect on cantilever oscillation amplitude and sensitivity to tip-sample interaction. The amplitude-distance curves were obtained using a cantilever with a 1.5 pm long, bird-bill-shaped tip (a) or a cantilever having an additional 1.0 pm long needle-shaped tip (b),...

C. (2011) Polarization effects in non-contact atomic force microscopy a key to model the tip-sample interaction above charged adatoms. Phys. Rev. B, 83, 035411. 

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