How does a Tunneling electron microscopy work?
A small conducting tip is bought towards the surface of the material, when it is very close (a few nanometres away) a current is formed between the sharp tip and the surface of the material. Electrons appear to jump across from the surface to the tip forming a current. This is due to quantum mechanics and the shrodinger equation (if you are not familiar see wikipedia or physics.edu) where the wavefunctions of the tip and surface overlap and make it possible for electron to jump across (or tunnel) through this potential barrier (the gap) and form a current. Current constant mode allows the z height axis to be adjusted which forms an image of the surface on a computer screen due to tip tomography.
