Tunneling conditions

Fig. 2. The fluctuating difference between the proton potential at the product side relative to that at the reactant side (the difference between the two wells in a double-well proton potential). Whenever this difference is close to zero, tunneling conditions are favourable.

Figure 8.5 STM image of Ni (110) exposed to CO at 1 x 10-6 mbar (a) raw data (60 x 60 A) (b) and (c) unit cell averaged (30 x 30 A) at two different tunnelling conditions the unit cell is indicated. (Reproduced from Ref. 19).

Figure 8.6 STM image of Ni (11 l)-c(4 x 2) CO structure with (a) (4 x 2) (white) and c(4 x 2) (black) unit cells shown with corresponding corrugation line scan (0.2 A full scale) (b) similar to (a) under different tunnelling conditions and corresponding line scan (0.3 A full scale). (Reproduced from Ref. 20).

The control parameter in an STM, the current in the tunneling junction, is always due to the same physical process. An electron in one lead of the junction has a nonvanishing probability to pass the potential barrier between the two sides and to tunnel into the other lead. However, this process is highly influenced by (i) the distance between the leads, (ii) the chemical composition of the surface and tip, (iii) the electronic structure of both the systems, (iv) the chemical interactions between the surface and the tip atoms, (v) the electrostatic interactions of the sample and tip. The main problem, from a theoretical point of view, is that the order of importance of all these effects depends generally on the distance and therefore on the tunneling conditions [5-8]. 

Figure 4.7b shows a close-up of Figure 4.7a, a 300 pA constant It contour, which has a corrugation of approximately 100 pm and is located approximately 300 pm from the 02c surface atoms. These values disagree quantitatively with experimental STM results at the same tunneling conditions on two accounts. First, a set point of It= 300 pA is not a particularly large value for constant current STM imaging, and... 

Fig. 1.18. Four STM images of 4Hb-TaS2 at 4.2 K. These images were taken during a period of about 2 h on the same area of the surface under identical tunneling conditions (7=2.2 nA, V=25 mV). These images demonstrate the role of tip electronic states on the STM images. (Reproduced from Coleman et al., 1988, with permission.)...

It is often observed that the actual appearance of the atom-resolved images varies from time to time, and the details are not reproducible. Figure 1.18 shows four STM images of the same area on the same surface, under identical tunneling conditions, within a 2 h period. The difference is due to different electronic states at the tip apex. 

Furthermore, in the case of Au(110)-3X1, the topographic images are approximately independent of the tip-sample distance, or independent of tunneling conditions. Actually, here q< Q.25 A, k 1 A" . Thus,... 

The picture begins to come somewhat into focus. Starting off with some basic mechanics of electrons, one was able to define the quantum mechanical condition for the tunneling of electrons from a metallic donor to electron acceptors through an electron-energy barrier. The tunneling condition could be expressed in terms of an energy barrier for ion movement, e.g., the movement of protons toward the metal in the reaction ... 

Fig. 6. Experimental results of oxygen adsorbed on Ru(0001) with half a monolayer coverage. While the clean surface has hexagonal symmetry (a), the oxygen covered surface shows a 2 X 2 superstructure (b). The shape of the ensuing features depends on the tunneling conditions (c) it changes from circular to triangular as surface and tip move closer together.

Fig. 6. Comparison between tunneling conditions on a semiconductor in vacuum (top) and in the electrolytic environment (bottom). In vacuum empty states (a) and occupied states (b) are imaged with a negative and positive tip respectively. In the liquid the position of band edges is fixed with respect to the tip Fermi level a cathodic bias stabilizes the tip above the n-type electrodes and occupied states are imaged in (c). Under depletion the tip comes into contact (d). Arrows refer to the direction of tunneling electrons.

Tip-induced effects are expected to be less important and even to disappear at highly doped materials and also when majority carriers are accumulated at the surface. It should be noted that images are experimentally featureless when extreme tunneling conditions such as those described at the beginning of this section are used. 

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