Molecular Order Induced Charge Transfer in a C₆₀-Topological Insulator Moiré Heterostructure

Abstract

We synthesized and spectroscopically investigated monolayer (ML) C₆₀ on the topological insulator (TI) Bi₄Te₃. This C₆₀/Bi₄Te₃ heterostructure is characterized by an excellent translational order in a novel (4 × 4) C₆₀ superstructure on a (9 × 9) cell of Bi₄Te₃. Angle-resolved photoemission spectroscopy (ARPES) of C₆₀/Bi₄Te₃ reveals that ML C₆₀ 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 C₆₀/Bi₄Te₃. At low temperatures, Raman spectroscopy and PL show a dramatic intensity increase of the C₆₀-related signal, suggesting a transition to a rotationally ordered state. Calculations explain the charge transfer by C₆₀ adsorption to Bi₄Te₃ surface defects. The temperature dependence of the charge transfer is attributed to the orientational order of C₆₀. The electron affinity of C₆₀ increases at low temperatures due to the freezing of the rotational motion.