Selective Hydrogen Etching Leads to 2D Bi(111) Bilayers on Bi₂Se₃: Large Rashba Splitting in Topological Insulator Heterostructure

Ultrathin bilayers (BLs) of bismuth have been predicated to be a two-dimensional (2D) topological insulator. Here we report on a new route to manufacture the high-quality Bi bilayers from a 3D topological insulator, a top-down approach to prepare large-area and well-ordered Bi(111) BL with deliberate hydrogen etching on epitaxial Bi₂Se₃ films. With scanning tunneling microscopy (STM) and X-ray photoelectron spectra (XPS) <i>in situ</i>, we confirm that the removal of Se from the top of a quintuple layer (QL) is the key factor, leading to a uniform formation of Bi(111) BL in the van der Waals gap between the first and second QL of Bi₂Se₃. The angle resolved photoemission spectroscopy (ARPES) <i>in situ</i> and complementary density functional theory (DFT) calculations show a giant Rashba splitting with a coupling constant of 4.5 eV Å in the Bi(111) BL on Bi₂Se₃. Moreover, the thickness of Bi BLs can be tuned by the amount of hydrogen exposure. Our ARPES and DFT study indicated that the Bi hole-like bands increase with increasing the Bi BL thickness. The selective hydrogen etching is a promising route to produce a uniform ultrathin 2D topological insulator (TI) that is useful for fundamental investigations and applications in spintronics and valleytronics