分子结拉伸与界面识别:破解4,4′-二吡啶分子结拉伸过程中高低电导之谜

Molecular junction stretching and interface recognition:Decode the mystery of high/low conductance switching in stretching process of 4,4′-bipyridine molecular junction

4,4′-二吡啶分子结在拉伸过程中呈现出独特的高低电导现象,是分子电子学近十几年研究中的未解之谜.根据实验测量过程以及所采用的技术手段,发展了基于第一性原理计算的分子结绝热拉伸模拟方法,对4,4′-二吡啶分子结的拉伸过程进行了模拟计算.并利用一维透射结合三维修正近似(OTCTCA)方法计算了拉伸过程中体系电导的变化,成功破解了4,4′-二吡啶分子结在拉伸过程中的高低电导之谜.结果显示,在4,4′-二吡啶分子结的拉伸过程中,分子末端的氮原子很容易吸附到探针电极的第二层金原子上,并且导致分子对尖端金原子产生特有的侧向推动作用,将探针尖端金原子推向一侧,从而在拉伸过程中出现高电导平台.进一步拉伸分子结,分子上端氮原子移动并吸附到探针尖端金原子上,同时尖端金原子重新回到原来的晶格位置上.体系电导也因此降低大约5—8倍,形成低电导平台.根据计算结果,4,4′-二吡啶分子结双电导平台的出现同时表明基底电极很容易存在表面金原子,且只有分子吸附到表面金原子上才会出现高低电导现象.可见,利用分子结拉伸过程中测量到的电导曲线并借助理论计算可以有效识别分子结界面结构.另外,对4,4′-二吡啶分子结高低电导现象物理过程和内在物理机制的破译,为更好利用含吡啶末端分子构建分子开关、分子存储器、分子传感器等功能分子器件提供了重要技术信息与依据.

The high/low conductance switching in stretching process of 4,4′-bipyridine molecular junction is a distinctive phenomenon in molecular electronics,which is still a mystery and has been unsolved for more than one decade.Based on the techniques and processes of experimental measurement,the ab initio-based adiabatic molecule-junction-stretch simulation(AMJSS)method is developed,by which the stretching processes of 4,4′-bipyridine molecular junctions are calculated.The conductance traces of the molecular systems in the stretching processes are studied and the mystery of high/low conductance switching in the stretching processes of 4,4′-bipyridine molecular junction is decoded by using the one-dimensional transmission combined with the threedimensional correction approximation(OTCTCA)method.The numerical results show that,in the stretching process of 4,4′-bipyridine molecular junction,the upper terminal nitrogen atom in the pyridine ring is easy to vertically adsorb on the second gold layer of the probe electrode.At the same time,the molecule produces unique lateral-pushing force to push the tip atoms of the probe electrode aside.Thus,the high conductance plateau arises.With the molecular junction further stretched,the upper terminal nitrogen atom of the molecule shifts from the second gold layer to the tip gold atom of the probe electrode with the tip gold atom moving back to the original lattice position.Consequently,the conductance value decreases by about 5–8 times,and the low conductance plateau is presented.According to our calculations,the phenomenon of high/low conductance switching in the stretching process of 4,4′-bipyridine molecular junction also indicates that,single surface gold atom often lies on the surface of substrate electrode.Moreover,the phenomenon of high/low conductance switching can only be found when the molecule is adsorbed on the surface gold atom of the substrate electrode.Thus,using conductance traces measured in the stretching processes of molecular junction and with the help of theoretical calculations,the interface structures of molecular junctions can be recognized efficiently.Our study not only decodes the physical process and intrinsic mechanism of the high/low conductance switching phenomenon of 4,4′-bipyridine molecular junction,but also provides significant technique information for using pyridine-based molecule to construct functional molecular devices,such as molecular switch,molecule memory,molecular sensor,etc.