Molecular Order Induced Charge Transfer in a C-Topological Insulator Moiré Heterostructure.

We synthesized and spectroscopically investigated monolayer (ML) C on the topological insulator (TI) BiTe. This C/BiTe heterostructure is characterized by an excellent translational order in a novel (4 × 4) C superstructure on a (9 × 9) cell of BiTe. Angle-resolved photoemission spectroscopy (ARPES) of C/BiTe reveals that ML C accepts electrons from the TI at room temperature, but no charge transfer occurs at low temperatures. This temperature-dependent doping is further investigated by Raman spectroscopy, photoluminescence (PL), and calculations of C/BiTe. At low temperatures, Raman spectroscopy and PL show a dramatic intensity increase of the C-related signal, suggesting a transition to a rotationally ordered state. Calculations explain the charge transfer by C adsorption to BiTe surface defects. The temperature dependence of the charge transfer is attributed to the orientational order of C. The electron affinity of C increases at low temperatures due to the freezing of the rotational motion.