Study Of Charge Transport Through Intermolecular π-π Interaction With Mechanically Controllable Break Junction Technique

π-π interaction is a kind of non-covalently weak interaction,widely existed in aromatic ring systems,and it has important applications in the fields of chemistry,biology and materials science.Since the mechanism of π-π interaction is relatively complicated and the experimental support is not sufficient,it is difficult for us to fully and thoroughly understand the nature and cause of π-π interaction.Studies show that the substituent groups on the aromatic ring directly affect the π-π interaction between aromatic rings,affecting the stability of π system.In addition,the π-π interaction between aromatic rings can be tuned by changing the nature,number,position and the like of the substituents.With the development of single-molecule measurement techniques,Scientists have been able to observe the state of matter units and study their properties from the perspective of individual molecules or even atoms.Mechanically controllable break junction technique,as one of single-molecule electrical measurement techniques,not only enables precise construction of nano-spaces,but also has higher mechanical stability.In this thesis,mechanically controllable break junction technique is used to construct a single π-π unit,of which the charge transport properties are investigated.We hope to understand the nature of π-π weak interaction from a new perspective,and provide experimental support and theoretical guidance for the development of its applications.Specifically,the work of this thesis is to construct a single dimer molecular junction through π-π interaction between two benzenethiol molecules with single sulfhydryl as anchor group by using mechanically controllable break junction technique,and to investigate the effect of position and number of the substituted methyls on charge transport properties of the π-π molecular junction.It has been found that benzenethiol molecules can form π-π molecular junctions of three different types(face to face,edge to face and point to face configuration)with two different conductance signals.The high conductance is from the face to face and edge to face configurations,and the low conduction is from the point to face configuration.The conductance order of these three different n-n configurations is:face to face ≈ edge to face>point to face.When the benzene ring is substituted by one methyl group(2-methyl benzenethiol,3-methyl benzenethiol,4-methyl benzenethiol),the substitution position of the methyl group directly affects the π-π configuration of the molecular junction,making different charge transport properties.When the benzene ring is substituted by two methyl groups(2,4-dimethyl benzenethiol,2,5-dimethyl benzenethiol,3,5-dimethyl benzenethiol),methyl group substituted at the ortho position of the sulfhydryl group makes the molecular junction shows a higher conductance.According to the analysis,the increase of the conductance is due to the significant enhancement of charge transport capacity of the face to face configuration in π-π molecular junction.In addition,the polarity of the solvent environment has no effect on the charge transport properties of the π-π dimer molecular junctions in this system.