Graduate School of Energy, Environment, Water, and Sustainability, Korea Advanced Institute of Science and Technology , 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea.
The Institute of Scientific and Industrial Research, Osaka University , 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
J Am Chem Soc. 2017 Jun 21;139(24):8286-8294. doi: 10.1021/jacs.7b03393. Epub 2017 Jun 7.
Molecule-electrode contact atomic structures are a critical factor that characterizes molecular devices, but their precise understanding and control still remain elusive. Based on combined first-principles calculations and single-molecule break junction experiments, we herein establish that the conductance of alkanedithiolate junctions can both increase and decrease with mechanical stretching, and the specific trend is determined by the S-Au linkage coordination number (CN) or the molecule-electrode contact atomic structure. Specifically, we find that the mechanical pulling results in the conductance increase for the junctions based on S-Au CN two and CN three contacts, while the conductance is minimally affected by stretching for junctions with the CN one contact and decreases upon the formation of Au monatomic chains. Detailed analysis unravels the mechanisms involving the competition between the stretching-induced upshift of the highest occupied molecular orbital-related states toward the Fermi level of electrodes and the deterioration of molecule-electrode electronic couplings in different contact CN cases. Moreover, we experimentally find a higher chance to observe the conductance enhancement mode under a faster elongation speed, which is explained by ab initio molecular dynamics simulations that reveal an important role of thermal fluctuations in aiding deformations of contacts into low-coordination configurations that include monatomic Au chains. Pointing out the insufficiency in previous notions of associating peak values in conductance histograms with specific contact atomic structures, this work resolves the controversy on the origins of ubiquitous multiple conductance peaks in S-Au-based single-molecule junctions.
分子-电极接触原子结构是分子器件的关键特征，但对其精确的理解和控制仍然难以实现。基于第一性原理计算和单分子断键实验的结合，我们在此建立了烷二硫键结的电导可以随机械拉伸而增加和减少，具体趋势取决于 S-Au 键合配位数 (CN) 或分子-电极接触原子结构。具体来说，我们发现机械拉伸导致基于 S-Au CN 二和 CN 三接触的结的电导增加，而对于具有 CN 一接触的结，电导受拉伸的影响最小，并且在形成 Au 单原子链时会降低。详细的分析揭示了在不同接触 CN 情况下，拉伸诱导的最高占据分子轨道相关态向电极费米能级上移与分子-电极电子耦合恶化之间的竞争机制。此外，我们实验上发现更快的拉伸速度下更容易观察到电导增强模式，这可以通过从头分子动力学模拟来解释，该模拟揭示了热涨落在帮助接触变形为包括单原子 Au 链在内的低配位数构型方面的重要作用。指出了将电导图谱中的峰值与特定接触原子结构相关联的先前概念的不足，本工作解决了 S-Au 基单分子结中普遍存在的多个电导峰起源的争议。
