Molecular Order Induced Charge Transfer in a C 60 -Topological Insulator Moiré Heterostructure.

We synthesized and spectroscopically investigated monolayer (ML) C ₆₀ on the topological insulator (TI) Bi ₄ Te ₃ . This C ₆₀ /Bi ₄ Te ₃ heterostructure is characterized by an excellent translational order in a novel (4 × 4) C ₆₀ superstructure on a (9 × 9) cell of Bi ₄ Te ₃ . Angle-resolved photoemission spectroscopy (ARPES) of C ₆₀ /Bi ₄ Te ₃ reveals that ML C ₆₀ 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 ₆₀ /Bi ₄ Te ₃ . At low temperatures, Raman spectroscopy and PL show a dramatic intensity increase of the C ₆₀ -related signal, suggesting a transition to a rotationally ordered state. Calculations explain the charge transfer by C ₆₀ adsorption to Bi ₄ Te ₃ surface defects. The temperature dependence of the charge transfer is attributed to the orientational order of C ₆₀ . The electron affinity of C ₆₀ increases at low temperatures due to the freezing of the rotational motion.