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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Charge transport in molecular electronic junctions:Compression of the molecular tunnel barrier in the strong coupling regime
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Charge transport in molecular electronic junctions:Compression of the molecular tunnel barrier in the strong coupling regime

机译：分子电子结中的电荷传输：强耦合状态下分子隧道势垒的压缩

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摘要

Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier.
机译：分子结本质上是电化学家熟悉的修饰电极，其中电解质被导电的“接触”所代替。通常假设分子结构的改变会改变系统能级，从而导致运输势垒的改变。在这里，我们显示了共价键合到碳上的七个不同芳族分子的电导率，尽管自由分子HOMO能量相差很大（> 2 eV范围），但在观察到的传输势垒中却有一个适度的范围（<0.5 eV）。通过考虑将分子结合到基底上的作用来解释这些结果。结合后，电子感应效应会调节系统的能级，从而导致隧穿势垒的压缩。用给体或撤离基团对分子的修饰可调节分子轨道能量和接触能级，从而产生流平效应，该效应将隧道势垒压缩到比预期小的范围内。尽管可以通过使用不同种类的分子（烷烃）来改变隧穿势垒的值，但是仅使用芳族结构会导致由于分子层和基板之间的部分电荷转移而导致的不同结构的隧穿势垒具有相似的平衡值。因此，该系统不遵守肖特基-莫特极限，并且分子层和底物之间的相互作用会影响能级排列。这些结果表明，必须考虑整个系统以确定各种电子因素的影响，这些因素会影响隧道的屏障。

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