Tip-surface interactions

If the tip is contaminated, its apex is most likely attached to a hydrogen molecule or H atoms. As a consequence, the conductance of this tip should be much lower than that of a clean tungsten tip. Since this conductance change has not been reported, it can be concluded that the reduced 0—0 distance is not the effect of a contaminated tip. Surface-tip interactions are evaluated by calculating the interaction between the reacted surface and a tungsten cluster at low distance. Here, the calculations indicate that there is no substantial relaxation due to interactions between the two leads. Consequently, the only possibility left is that the electronic surface structure somehow changes the appearance of the oxygen positions. 

That is, an AFM probe responds to the average force between the sample surface and a group of tip atoms that are in close proximity to the surface. In order to image individual atoms by SPM, the surface-tip interactions must be limited to the nearest atom(s) on the tip periphery. Hence, an AFM image will not show individual atoms, but rather an average surface, with its ultimate resolution dependent on the sharpness of the tip structure. In contrast, STM is capable of atomic resolution since the tunneling current passes only through the tip atom that is nearest the sample surface. Named after Bmnauer, Emmett, and Teller. 

Perez, R., Stich, 1., Payne, M.C., and Terakura, K. (1998) Surface-tip interactions in noncontact atomic-force microscopy on reactive surfaces ... 

Figure 7.13 Left interaction potential and force between an atom at the apex of the tip and an atom in the surface. Tip-surface interactions can be described by a summation of these potentials over all combinations of atoms from the tip and the surface. Right interaction potential between the tip, approximated as a sphere, and a plane surface, valid in the non-contact mode of force microscopy. To stress the long-range character of the non-contact potential, the Lennard-Jones interaction potential between two atoms has been included as well (dotted line).

Fig.2. Interaction forces acting in vacuum between a two atoms (f r 7) and b macroscopic particles (e.g., for surface-sphere interaction, F D 2). The tip position at D=0 corresponds to the tip-sample contact, while the range at D<0 corresponds to the sample indentation...

Fig. 5. a Schematic representation of the SFM set-up using the optical beam deflection method, b When the tip interacts with the sample surface, the cantilever exhibits deflection perpendicular to the surface as well as torsion parallel to the surface plane. The normal force Fn and the lateral force FL corresponds to the force components which cause the deflection and torsion, respectively. Both responses are monitored simultaneously by the laser beam which is focused on the back side of the cantilever and reflected into a four-quadrant position sensitive detector (PSD)... 

Fig. 7.16 (a) I interaction potential and force between an atom at the apex of the tip and an atom in the surface. Tip-surface interactions can be described by a summation of these potentials over all combinations of atoms from the tip and the surface. 

Methods for directly observing supermolecules have recently been developed. Scanning probe microscopy (SPM) has become a particularly useful method in the field of nanotechnology. The general concept of SPM is summarized in Fig. 5.3. A very fine tip is used in this method. As the tip approaches the sample surface, various interactions occur between the tip and the surface which result in various kinds of forces (such as atomic forces). These forces are felt by the tip and converted into electrical signals. 

The simple textbook solution of a harmonic damped oscillator becomes complex when the vibrating tip interacts with the surface of a sample, e.g., in tapping mode AFM. Although the different imaging modes and the experimental observables may vary, the underlying physics is similar. Amplitude and frequency modulated AFMs are most commonly used, labeled by AM-AFM (amplitude modulation or tapping) and FM-AFM. For a detailed review of this topic several reviews are available, e.g., [15]. 

The AFM probe tip interacts via various mechanisms with the sample surface, including simple quasi-static and dynamic normal and lateral forces. These (and other) interactions depend on local physical properties of the sample, on the scale of the probed area. Tribological, viscoelastic, and microhardness mechanical properties can be probed under (more or less) controlled environmental conditions. 

As time has passed since the invention of these instruments, however, the collective expertise of the research community involved in proximal probe studies has increased enormously. Steady progress has been made in learning how to produce more uniform tips of consistent shape and character. Our knowledge of the basic phenomena involved in STM and AFM tip interactions with sample surfaces has become more comprehensive, as have our skills in interpreting the results obtained by these instruments. 

The heart of force detection in an AFM apparatus is a scanning cantilever microfabricated with a tip for force sensing (Figure 6.29). Forces generated by surface-to-tip interactions, Fst> lead to vertical tip displacements, Az, that are related to each other in a quasi-static way and to a hrst approximation by a form of the Hooke s law equation ... 

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