Can electrons tunnel in reverse, from tip to surface?
Yes, but there is a net tunneling from surface to tip. This is because the tip has a small attractive voltage on it (positive voltage to attract an electron, negative potential energy). This was represented in the diagram by the tip potential well shifted a bit downward with respect to the metal surface potential well. Electrons in the metal surface can tunnel through the gap to an energy state in the tip’s potential well more easily than vice versa (the metal surface electrons see a shorter barrier than the tip ones). So there is a net tunneling in the direction of the tip, and a net current.
