1.7 Sensor Elements and Testing on the Nanoscale

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Figure 1.56: In a scanning tunneling microscope (STM), the tiny tunneling voltage between the atomically

sharp tip and the surface is measured and amplified. With a precise control over the distance between the

tip and the surface, this can be used to characterize the topography of the sample.

a tiny voltage between the two surfaces develops (the “tunneling voltage”). This voltage

can be amplified and measured. The STM keeps the tunneling voltage constant while

measuring the height of the tip very accurately and with that can scan the surface and

characterize the topography of the sample.

A similar method can be used for nonconductive surfaces: An atomic force micro-

scope (AFM) measures the repulsive force between two surfaces by measuring the move-

ment of the tip with a laser (Figure 1.57). The AFM keeps the repulsive force constant,

while measuring the height of the tip very accurately and with that can scan the surface

and characterize the topography of the sample. But since the measurement is based on

a force, AFM can also be used for force measurements on the nanoscale (Figure 1.58).

Depending on in which direction the force of the tip is applied, friction (via intermolec-

ular forces on the surface) or elasticity can be measured. The tip can also be used to

measure ligand binding by attaching a ligand to the tip. An example how these forces

are measured by the tip movement is shown in Figure 1.59.