Negative differential resistance of oligo(phenylene ethynylene) self-assembled monolayer systems: The electric-field-induced conformational change mechanism

We investigate here a possible mechanism for the room temperature negative differential resistance (NDR) in the Au/AN-OPE/RS/Hg self-assembled monolayer (SAM) system, where AN-OPE = 2′-amino,5′-nitro-oligo(phenylene ethynylene) and RS is a C₁₄ alkyl thiolate. Kiehl and co-workers showed that this molecular system leads to NDR with hysteresis and sweep-rate-dependent position and amplitude in the NDR peak. To investigate a molecular basis for this interesting behavior, we combine first-principles quantum mechanics (QM) and mesoscale lattice Monte Carlo methods to simulate the switching as a function of voltage and voltage rate, leading to results consistent with experimental observations. This simulation shows how the structural changes at the microscopic level lead to the NDR and sweep-rate-dependent macroscopic I-V curve observed experimentally, suggesting a microscopic model that might aid in designing improved NDR systems.