Question 4: During scanning tunnelling microscope (STM) operation, a student obserred a tunnelling current of 10nA for flat and smooth surface. However, when the probe tip is scanned across the surface of a specimen, he observed different tunnelling currents 4, 5 , 14 and 20nA respectively. Identify surface feature associated with these tunnelling currents (concave or convex feature). Justify your answer. (8)