Question

why you need a conducting sample to study in STM?

Answer 1

Because this kind of microscopes uses tunneling current. The needle of the microscope has a very small potential with regard to the sample. The distance between the sharp end of the needle and the sample is 0.2 to 0.7 nm. At such small distances electrons tunnel to the sample. If we want that tunneling current to flow, we need a conducting sample. That current varies with distance according to exponential law, so if we measure the current, we receive data about the surface if the sample.