Molecular order induced charge transfer in a C$_{60}$-topological insulator moir\'e heterostructure

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.