标题： 一系列高传输分子结的电导饱和现象 显示英文标题

标题： Conductance saturation in a series of highly transmitting molecular junctions

摘要： 理解金属-分子界面处的电子输运特性对于控制多种基于分子的器件至关重要，例如有机发光二极管、纳米尺度有机自旋阀和单分子开关。揭示通过金属-分子界面的电子输运机制的主要实验方法之一是研究分子结中电导率随分子长度的变化。以前的研究集中在由隧穿或跳跃主导的输运上，两者都处于低电导率。然而，尽管在高效信息传输、电荷注入和复合过程中具有巨大潜力，分子结中的电导率上限尚未被探索。在这里，我们研究了高传输金属-分子-金属界面的电导特性，使用了一系列基于逐渐增长长度的寡聚苯的单分子结。我们发现电导率在某个上限处趋于饱和，此时电导率与分子长度无关。此外，我们表明这个上限可以通过金属-分子界面的轨道杂化特性进行调控。使用两种原型系统，其中分子分别通过银或铂电极接触，我们揭示了电导率饱和的两种不同原因。在银基分子结的情况下，电导率饱和归因于能级对齐和能级展宽之间的竞争，而在铂基结的情况下，饱和归因于类似带状的输运。结果通过一个直观的模型进行了解释，并得到了从头算输运计算的支持。我们的发现阐明了在高传输极限下限制电导率的机制，为设计高导电性的金属-分子界面提供了指导原则。 显示英文摘要

摘要： Understanding the properties of electronic transport across metal-molecule interfaces is of central importance for controlling a large variety of molecular-based devices such as organic light emitting diodes, nanoscale organic spin-valves and single-molecule switches. One of the primary experimental methods to reveal the mechanisms behind electronic transport through metal-molecule interfaces is the study of conductance as a function of molecule length in molecular junctions. Previous studies focused on transport governed either by tunneling or hopping, both at low conductance. However, the upper limit of conductance across molecular junctions has not been explored, despite the great potential for efficient information transfer, charge injection and recombination processes. Here, we study the conductance properties of highly transmitting metal-molecule-metal interfaces, using a series of single-molecule junctions based on oligoacenes with increasing length. We find that the conductance saturates at an upper limit where it is independent of molecule length. Furthermore, we show that this upper limit can be controlled by the character of the orbital hybridization at the metal-molecule interface. Using two prototype systems, in which the molecules are contacted by either Ag or Pt electrodes, we reveal two different origins for the saturation of conductance. In the case of Ag-based molecular junctions, the conductance saturation is ascribed to a competition between energy level alignment and level broadening, while in the case of Pt-based junctions, the saturation is attributed to a band-like transport. The results are explained by an intuitive model, backed by ab-initio transport calculations. Our findings shed light on the mechanisms that constrain the conductance at the high transmission limit, providing guiding principles for the design of highly conductive metal-molecule interfaces.