| By using a feedback loop the tip
is vertically adjusted in such a way that the current always stays constant. As the
current is proportional to the local density of states, the tip follows a contour
of a constant density of states during scanning. A kind of a topographic image of
the surface is generated
by recording the vertical position of the tip. |
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In this mode the vertical position of the tip is not changed, equivalent to a slow or disabled feedback. The current as a function of lateral position represents the surface image. This mode is only appropriate for atomically flat surfaces as otherwise a tip crash would be inevitable. One of its advantages is that it can be used at high scanning frequencies (up to 10 kHz). In comparison, the scanning frequency in the constant current mode is about 1 image per second or even per several minutes. |
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Up to now homogoneous surfaces were considered. If there is an inhomogeneous
compound in the surface the work function will be inhomogeneous as well. This alters
the local barrier height. By using the two modes described above we would get
a virtual hole or adatom. But this can further be explored: Differentiation of (*) yields  . Thus the work function can directly be measured by variing the tip-sample distance, which can be done by modulating the current with the feedback turned on. |
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