氧化还原分子导电结中的不可逆性和滞后现象
Irreversibility and hysteresis in redox molecular conduction junctions
摘要：
在这项工作中，我们提出并讨论了氧化还原分子结的理论模型，占最近观察到的非线性电荷传输现象，如滞后和滞后负微分电阻（NDR）。此类模型的一个定义特征是涉及至少两个传导通道--一个决定桥的电荷状态之间转变的慢通道和一个主导其传导的快通道。使用马库斯理论的异质电子转移（ET）在金属分子界面，我们确定和描述不同的制度的非线性传导通过氧化还原分子桥，其中转移电荷可以高度本地化周围的氧化还原部分。这种定位及其通过周围介质的极化和/或构象变化的稳定化可以导致电流响应动力学与电压扫描的时间尺度的解耦（即，电流不动态地跟随电压），因此导致该响应中的记忆（热力学不可逆性）的出现，其通过电流-电压循环中的滞后来表现。在标准伏安法中，这种不可逆性导致电流峰值沿着正向和反向电压扫描的相对移位。指出了这些现象的共同根源，并给出了阈值电压扫描速率的表达式。此外，该理论被扩展到（a）分析不同的方式，通过这种现象表现在单扫描周期和系综平均这样的周期，和（b）检查量子效应在快速传输通道。
英文摘要：
In this work we present and discuss theoretical models of redox molecular junctions that account for recent observations of nonlinear charge transport phenomena, such as hysteresis and hysteretic negative differential resistance (NDR). A defining feature in such models is the involvement of at least two conduction channels - a slow channel that determines transitions between charge states of the bridge and a fast channel that dominates its conduction. Using Marcus theory of heterogeneous electron transfer (ET) at metal-molecule interfaces we identify and describe different regimes of nonlinear conduction through redox molecular bridges, where the transferring charge can be highly localized around the redox moiety. This localization and its stabilization by polarization of the surrounding medium and/or conformational changes can lead to decoupling of the current response dynamics from the timescale of the voltage sweep (that is, the current does not adiabatically follow the voltage), hence to the appearance of memory (thermodynamic irreversibility) in this response that is manifested by hysteresis in current-voltage cycles. In standard voltammetry such irreversibility leads to relative shift of the current peaks along the forward and backward voltage sweeps. The common origin of these behaviors is pointed out and expressions of the threshold voltage sweep rates are provided. In addition, the theory is extended (a) to analyze the different ways by which such phenomena are manifested in single sweep cycles and in ensemble averages of such cycles, and (b) to examine quantum effects in the fast transport channel.
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