Identification of metals in scanning tunneling microscopy via image states

An oscillatory reversal of the contrast between Cu and Mo is observed with scanning tunneling microscopy (STM), using sample bias voltages of +5 V and higher. It is attributed to tunneling via a series of discrete states that are induced by a combination of the image potential and the applied field. They are offset in energy due to the different work functions of Cu and Mo. This effect provides a generally applicable mechanism for elemental contrast in STM. © 1995 The American Physical Society.