In STM the tunnelling current is given by the equation. (a) Explain the significance of each of the terms in this equation and explain how the tunnelling current is used to provide information at an atomic resolution. In particular, explain why the tunnelling current is very sensitive on the sample to probe distance and why this is significant. (b) Estimate the change in the tunnelling current if the distance d is varied from d1 = 0.6 nm to d2 = 0.2 nm. Let U + E = 3.2 x 10-19 J.