Visualizing Molecular Orientational Ordering and Electronic Structure in Cs_nC₆₀ thin films

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here a cryogenic scanning tunneling microscopy and first-principles calculation are combined to investigate the sub-molecular structural and electronic properties of expanded fullerene C₆₀^n- films on inert graphene with various cesium (Cs) doping. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C₆₀^n- anions, yet the tunneling conductance spectra are robustly characteristic of energy gaps, hallmarks of the Jahn-Teller distortions and electron correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of n = 1, 2, 3 and 4, apart from which it moves toward the band edges with concomitant electronic states in the gap. Our findings establish the universality of Jahn-Teller instability, and clarify the relationship among the doping, structural and electronic structures in Cs_nC₆₀ fullerides.