Bi(111) thin film with insulating interior but metallic surfaces

The electrical conductance of epitaxial Bi thin films grown on BaF(2)(111) by molecular beam epitaxy has been systematically investigated as a function of both film thickness (4-540 nm) and temperature (5-300 K). Unlike bulk Bi as a prototypical semimetal, the Bi thin films up to 90 nm are found to be insulating in the interior but metallic on the surface. This finding not only has unambiguously resolved the long-standing controversy about the existence of the semimetal-semiconductor transition in Bi thin films but also provided a straightforward interpretation for the perplexing temperature dependence of the resistivity of Bi thin films, which in turn might have some potential applications in spintronics.