金电极与吡啶末端连接界面结构的力学变化过程理论研究

Theoretical study on mechanical evolution process of interface between gold electrode and pyridyl anchor group

利用基于第一性原理的绝热拉伸模拟方法计算了4,4’-二吡啶分子与不同构型金电极之间的作用过程,研究了分子在外力作用下逐渐远离金电极过程中分子与电极间界面结构特有的演化过程以及体系能量与作用力的变化特征.结果显示,分子在远离锥形电极过程中很容易出现近于垂直地连接到锥形电极第二层金原子上的特有连接构型,同时由于吡啶末端的排斥作用,电极尖端的金原子偏向一侧.分子从第二层金原子上断开并连接到尖端金原子上需要1.3—1.5 nN的拉力作用,明显大于分子从尖端电极上断开所需要的0.8—1.0 nN的作用力,从而揭示了实验中二吡啶分子结在形成过程中作用力与界面构型变化之间的对应关系.4,4’-二吡啶分子与平面金电极的作用较弱,只需要不到0.5 nN的作用力就可断开,而当分子连接到吸附在平面电极表面的孤立金原子上时,可以承受约1.7 nN的作用力.以上研究表明基于第一性原理的绝热拉伸模拟方法不仅可以揭示分子与电极之间的界面结构演化过程,而且通过作用力的计算可以很好地识别实验中分子与电极间的特有界面结构.

Pyridyl-ended molecular junctions show high and low breaking forces successively in formation process and at the same time exhibit intriguing conductance switching behaviors.To understand the forming process of pyridyl-ended molecular junctions,the interaction between 4,4’-bipyridine molecule and gold electrode is studied by the ab initio-based adiabatic simulation method.The processes that the molecule moves away from electrode tip with different contact configurations are simulated,and the molecule-electrode interface evolutions,energy of the molecule-electrode system and the force between molecules and electrode are calculated in the simulations.The numerical results show that during the molecule moving away from the pyramid-shaped electrode,the pyridyl is easy to vertically adsorb on the second gold layer of the electrode tip.In this contact configurations,the tip Au atom deviates from the original position due to the lateral pushing force of the pyridyl.It needs about 1.3-1.5 nN stretching force for the pyridyl breaking from the second gold layer and switching to the tip Au atom,which is evidently larger than the force of 0.8-1.0 nN for the molecule breaking from the tip Au atom.This result is well consistent with the experimental observations,which thus reveals the relationship between the interface structures and the stretching force in the formation process of bipyridyl molecular junction in the experiment.The interaction between 4,4’-bipyridine molecule and plane-shaped gold electrode is very weak.It needs no more than 0.5 nN for the molecule breaking from the plane-shaped gold electrode.However,when the molecule adsorbs on the single Au atom which is adsorbed on the surface of planeshaped electrode,the molecule can sustain 1.7 nN stretching force.Our study shows that the ab initio-based adiabatic stretching simulation method can not only reveal the geometric evolution process of molecule-electrode systems,but also identify the specific contact configurations between molecule and electrode.