Molecular order induced charge transfer in a C${}_{60}$-topological insulator moiré heterostructure

Abstract

We synthesize and spectroscopically investigate monolayer C${}_{60}$ on the topological insulator (TI) Bi${}_4$Te${}_3$. This C${}_{60}$/Bi${}_4$Te${}_3$ heterostructure is characterized by excellent translational order in a novel (4 x 4) C${}_{60}$ superstructure on a (9 x 9) unit of Bi${}_4$Te${}_3$. We measure the full two-dimensional energy band structure of C${}_{60}$/Bi${}_4$Te${}_3$ using angle-resolved photoemission spectroscopy (ARPES). We find that C${}_{60}$ accepts electrons from the TI at room temperature but no charge transfer occurs at low temperatures. We unravel this peculiar behaviour by Raman spectroscopy of C${}_{60}$/Bi${}_4$Te${}_3$ and density functional theory (DFT) calculations of the electronegativity of C${}_{60}$. Both methods are sensitive to orientational order of C${}_{60}$. At low temperatures, Raman spectroscopy shows a dramatic intensity increase of the C${}_{60}$ Raman signal, evidencing a transition to a rotationally ordered state. DFT reveals that the orientational order of C${}_{60}$ at low temperatures has a higher electron affinity than at high temperatures. These results neatly explain the temperature-dependent charge transfer observed in ARPES. Our conclusions are supported by the appearance of a strong photoluminescence from C${}_{60}$/Bi${}_4$Te${}_3$ at low temperatures.