Surface termination of cleaved Bi2Se3 investigated by low energy ion scattering.

It has been widely assumed that Se terminates the surface of the topological insulator, bismuth selenide. Although some Se is initially at the surface after cleaving at 80 K, low energy ion scattering reveals a complete Bi termination at room temperature. Density functional theory shows that a Bi bilayer atop the bulk-terminated structure is energetically favorable. It is thus proposed that a thermally activated process induces a spontaneous termination change after cleaving. This has profound implications on the electrical transport and long-term stability of such materials and devices.