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.

Co films were grown on top of topological insulator Bi 2 Se 3 (111) substrate and studied using Magneto-Optic Kerr Effect (MOKE) and X-ray Magnetic Circular Dichroism (XMCD). Both MOKE and XMCD results show that there exists a ~1.2nm thick magnetic dead layer of the Co film, i.e., ~1.2nm Co film at the Co/Bi 2 Se 3 (111) interface losses its ferromagnetic order. The Co ferromagnetic order can be completely resumed by inserting a 3 nm Ag layer between the Co and Bi 2 Se 3 , indicating that the magnetic dead layer is due to the Co/Bi 2 Se 3 interfacial mixing which is supported by the Co L edge spectroscopy result.