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| Copyright by Gary Larson: The Far Side |
A voltage is impressed between the tip and the sample (1mV to 3V). If the distance is enough short (<10 Å), a current (tunneling current, pA to nA) flows. The current exists without any contact between the two materials. With decreasing distance or increasing voltage the wave functions of the electrons overlap increasingly. So the probability for an electron to jump (to tunnel) to the other material rises.
The tunneling current is very sensible to changes in the distance between the tip and the sample. Calculations prove that the tunneling current increases by a factor of 10 while the distance is reduced by 1 Å.
Thus we learn that with an STM we examine the density of electrons rather than the atomic surface. STM images depend sharply on the nature of the surface and on the strength and the leading sign of the tunneling current.
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Tip