The electrostatic gating effects on molecular transistors are investigated using the density functional theory (DFT) combined with the nonequilibrium Greens function (NEGF) method. When molecular energy levels are away from the Fermi energy they can be linearly shifted by the gate voltage, which is consistent with recent experimental observations [Nature 462, 1039 (2009)]. However, when they move near to the Fermi energy (turn-on process), the shifts become extremely small and almost independent of the gate voltage. The fact that the conductance may be beyond the gate control in the ON state will challenge the implementation of molecular transistors.
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