Figure 5: Robust SCO-based single-molecule memristance.

(a) Demonstration of reversible switching by voltage pulses of +1.5 V inducing HS to LS switching and −1.2 V inducing LS to HS switching. Corresponding to the polarity of the voltage pulse, the tunnelling current for V=+0.1 V is clearly switched between two levels corresponding to the HS and LS states (150 pA and 130 pA, respectively). The tip was initially stabilized at +100 mV and 150 pA. (b) Demonstration of deterministic switching by the application of two consecutive pulses of the same polarity. The first positive pulse switches the molecules from the HS to the LS state. The second positive pulse does not alter the state, while a third negative pulse switches back the molecule to the HS state. The tip was initially stabilized at +100 mV and 100 pA. (c) Real-time current trace of an isolated HS state molecule on the CuN/Cu(100) surface. A voltage pulse of +1.5 V is applied for 3 s and the transition to the LS state is witnessed by a small decrease in current after a delay of 1.4 s (red arrow). (d) Observed switching rates for HS to LS states and vice versa by voltage pulses of ±1.5 V. The dotted lines indicate power law fits of the switching as function of the absolute tunnelling current. (e) Proposed model for the switching mechanism in both directions. While the switching from the LS to the HS state is a high order process (N≈8) involving heating by the tunnelling electrons, the transition from the HS to the LS state only requires one electron (N≈1) and involves an intermediate short-lived excited state.