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 C 60 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/C 60 and is suppressed near 1 e/C 60 . 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/C 60 and larger cations favoring band-like transport, higher overall conductances at all charge densities up to 1.7 e/C 60, and weaker suppression at 1 e/C 60 . Displacement current measurements on C 60 EDLTs with small cations show clear evidence of sub-band filling at 1 e/C 60, which correlates very well with the minimum in the C 60 sheet conductance. Overall, the data suggest a significant Mott-Hubbard-like energy gap opens up in the surface density of states for C 60 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 C 60 EDLTs, but it can be manipulated by choice of electrolyte.