Sub-Band Filling, Mott-like Transitions, and Ion Size Effects in C₆₀ Single Crystal Electric Double Layer Transistors

Electric double layer transistors (EDLTs) based on C60 single crystals and ionic liquid gates display pronounced peaks in sheet conductance versus gate-induced charge. Sheet conductance is maximized at electron densities near 0.5 e/C60 and is suppressed near 1 e/C60. The conductance suppression depends markedly on the choice of ionic liquid cation, with small cations favoring activated transport and essentially a complete shutdown of conductance at ∼1 e/C60 and larger cations favoring band-like transport, higher overall conductances at all charge densities up to 1.7 e/C60, and weaker suppression at 1 e/C60. Displacement current measurements on C60 EDLTs with small cations show clear evidence of sub-band filling at 1 e/C60, which correlates very well with the minimum in the C60 sheet conductance. Overall, the data suggest a significant Mott-Hubbard-like energy gap opens up in the surface density of states for C60 crystals gated with small cations. The causes of this energy gap may include both electron–electron repulsion and electron-cation attraction at the crystal/ionic liquid interface. The energy gap suppresses the insulator-to-metal transition in C60 EDLTs, but it can be manipulated by choice of electrolyte.