Theoretical Study On Electronic Transport Properties Of Oligophenyleneethynylene Molecular Junctions

With the minimization of traditional electronic devices, molecular electronic devices haveattracted more and more attention. In addition to molecular device’s intrinsic characteristics,external conditions also affect the structure and the electronic transport property of the moleculardevice, such as pressure, temperature, electric field, optical field and pH value in solutions.Changing the pH value of the solution has become one of the effective methods in chemistry tocontrol the charge transport properties of molecular devices. The available results have shownthat the side groups of molecular wires play a crucial role in determining the electron transportproperties. In particular, amino/carboxyl groups are special ones because they may beprotonated/deprotonated when acid/alkali is applied, which have been tested by a lot ofqualitative experimental works. Nevertheless, to the best of our knowledge, there is no enoughstudy on the transport properties in molecular with amino/carboxyl groups at the first-principleslevel. Thus, a systematical theoretical study is highly needed.In this paper, using nonequilibrium Green’s function method combined with densityfunctional theory, we theoretically investigated the electronic transport properties inoligophenyleneethynylene (OPE) molecular junctions with amino and carboxylic groups underalkaline and acid solutions. The numerical results show that comparing with neutral state, theconductance of the molecular junction is double improved and the rectifying direction is invertedwhen carboxylic group is deprotonated in the alkaline environment. On the other hand, therectification ratio is enhanced trebly and the rectifying direction is inverted similarly when aminogroup is protonated in the acid environment. Further analysis shows that protonation ordeprotonation change the energy level of frontier molecular orbital and the spatial distribution ofthe wave function, then influence the electronic transport properties as a bias voltage is applied.This theoretical work presents a chemically controllable method on manipulating conductanceand rectification of molecular junctions.