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The picture shows the construction of an STM. In the midst lies the sample. Below are the piezos with whom you can move the sample in x, y and z direction. Just beyond is the tip which moves over the surface. This tip is attached to a cantilever. A voltage lies between the tip and the sample so there is a current whose strength is dependend on the distance between the tip and the sample. The current is measured by a control unit. If the current gets stronger, which means the distance shortens or the material gets higher, the voltage on the z piezo is adjusted until the actual voltage between tip and sample equals the original volatge. Through this procedure the distance between the tip and the sample is kept constant to ensure that the tip does not collide with the sample.
To get a voltage between the tip and the sample the latter has to be conductive. Therefore the STM can only scan conductive materials.
In this case the sample is moved by the piezos. But it is also possible to move the tip by fixing the piezos on the tip.
To get rid of external disturbances (thermal, mechanical, accoustic vibrations) the sample and the microscope should build one closed system.
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