Abstract
The single bilayer of Bi (111) has been theoretically predicted as a prototype 2D topological insulator.1 However, a bilayer film of good quality is difficult to be grown because Bi films less than 4 atomic layers in thickness prefer energetically the black-phosphorous structure instead of rhombohedral structure on substrates such as Si (001),3 Si (111),2 and HOPG.4 Therefore, it is essential to find a new method to fabricate a Bi bilayer. In this research, we expose atomic deuterium (D) on newly cleaved Bi2Se3 surfaces. Scanning tunneling microscopy (STM) and synchrotron-radiation X-ray photoelectron spectroscopy (XPS) are employed to study the resulting surface chemical compositions, morphology, and atomic structures. Core-level spectra from XPS measurement can reveal the surface chemistry and STM can image the surfaces in real space and with atomic resolution. After atomic D exposures, XPS shows a rapid reduction of the selenium 3d core-level intensities, suggesting strongly that Se atoms are removed via the reaction of 2D+Se→D2Se(g). In the meantime, a new component arises on the lower binding energy side of the original Bi 5d component. The binding energy shift of the two components suggests that the new component is originated from a Bi bilayer. With increasing D exposures, the new component gradually gets more intensity than the original Bi 5d component. The original one almost vanishes at large exposure. Corresponding STM images show the areas of negative islands with three-fold symmetry increases with D exposure. The final surface exhibits a hexagonal superstructure. The negative islands are attributed to the Se atoms removed from the surface while the hexagonal superstructure results from the lattice mismatch between Bi2Se3 and the topmost Bi bilayer. In conclusion, we provide a simple new way to form a Bi bilayer. The bilayer film is reasonably smooth and ordered and can be utilized for further investigation of its electronic properties.