The Surface and Interface Chemistry of Bi2(Te,Se)3 Topological Insulators

Abstract

The effects of ambient exposure on Bi 2(Te,Se)3 topological insulators (TIs) were studied, and surface modification methods were developed to improve device integration of TI materials. X-ray photoelectron spectroscopy (XPS) studies of air exposed Bi2Se3, Bi2Te3, and Bi2Te2Se showed that surface oxide does not grow deeper than the first atomic layer after at least one month of air exposure; only about 80% of the surface Te could be oxidized under ambient conditions. Scanning photoelectron microscopy (SPEM) showed that air oxidation of Bi2Te2Se is most rapid at step edge defects, which addresses a controversy regarding the surface reactivity of Bi2(Te,Se)3 materials.

Surface modification methods were developed that did not disrupt the sensitive TI surface states. Titanium dioxide (TiO2) grown from Ti(OtBu)4 onto Bi2Te2Se surfaces followed a nucleation-and-growth mechanism to create uniform thin films, which was confirmed by atomic force microscopy (AFM), XPS, and SPEM measurements. These films served as adhesion layers for growth of self-assembled monolayers of phosphonates (SAMPs). Growth of SAMPs with systematically varied dipole moments was able to tune the surface work function of the SAMP/TiO2/Bi2Te2Se system as measured by ultraviolet photoelectron spectroscopy (UPS) and vibrating Kelvin probe measurements. Shubnikov-de Haas (SdH) quantum oscillation measurements proved that the growth of TiO2 and SAMP layers did not create surface electronic defects or disrupt the TI states.

Bi2Te2Se surfaces were chemically activated by reaction with a strong, outer-sphere oxidizing agent, [Fe(III)(1,10-phenanthroline)3](PF6)3 . Reaction of this [Fe(phen)3]+3 compound with Bi2Te2Se in the presence of water rapidly formed a uniform surface oxide coating, in which nearly all of the surface Te was oxidized. SdH oscillation measurements proved that this surface treatment preserved TI surface states; most importantly, this method caused a profound shift in the Fermi energy (EF) relative to the TI Dirac point, which is the first known demonstration of a surface chemical treatment that results in such a change. This oxide layer was reactive towards phosphonic acids, while the pristine Te-terminated surface was not. Thus, SAMPs can be grown directly on Bi2Te2Se oxide surfaces without the use of an adhesion interlayer.