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.
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Jung et al. (1995) studied this question.
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