The Effect Of Anchoring Groups And Molecular Length On Electrical Transport Properties Of Molecular Devices

With the size of electronic components down to the nanometer scale step by step,nanoelectronics is emerged to explore electronic properties of nanomaterials,in which classical theory is no longer suitable and quantum theory needs to describe behavior of electrons.As an important branch of nanoelectronics,molecular electronics is to construct functional molecular devices by using single molecules,supramolecules or molecular clusters and to explore the charge transport properties by using experimental or theoretical methods.Up to now,some functional molecular devices,such as molecular diodes,molecular switches,molecular sensors,have been fabricated and measured in the laboratory.However,problems such as high preparation cost,poor reproducibility and integration seriously hinder application of molecular devices.Thus,how to understand the working theory of molecular devices by means of theoretical methods and to develop the fabrication and measurement technology become an important research content of molecular electronics.In this thesis,we investigated the effects of anchoring groups and molecular length on the transport properties of the biphenyl molecular rectifier.The specific research contents and results are as follows:(1)The influence of anchoring groups on the charge transport of trans-stilbene based molecular junctions.Effects of the anchoring groups on transport properties of trans-stilbene based molecular junctions are investigated by using nonequilibrium Green's function method combined with density functional theory.Numerical calculation indicates that when we use methylene as an anchoring group,the current is an order of magnitude higher than that by using isocyano as an anchoring group,and twice over than that by using thiol as an anchoring group.What is more,as we choose isocyano as an anchoring group,negative differential resistance is observed.When asymmetric anchoring groups are used for the trans-stilbene junctions,a slight rectifying phenomenon is observed.As the carbon-carbon double bond that links the two benzene of trans-stilbene is turned into carbon-carbon single bond,the rectifying performance becomes obvious.(2)The effects of molecular length on rectification of biphenyl molecules.By applying nonequilibrium Green's function method in combination with density functional theory,we study the molecular length effect on rectification of biphenyl molecules.We choose biphenyl molecules as our prototypes.The numerical results show that increase of the molecular length leads to high enhancement of the rectification ratio of the molecular junctions,which is caused by the asymmetric shift of the molecular orbital energy related to the positive and negative bias.In addition,there are two mechanisms that affect the direction of rectification in the biphenyl molecule,and the two mechanisms compete for determining the direction of the rectification.The rectifying direction of molecular devices depends on the dominant mechanism.When the molecular length increases,the dominant mechanism of the two mechanisms is converted from one to the other,resulting in inversion of the rectifying direction of the molecular junctions.This thesis consists of five chapters.The first chapter is the introduction,which briefly introduces the development of molecular electronics.In the second chapter,we briefly introduce the theory of density functional theory and the non-equilibrium Green's function.At the same time,the nonequilibrium Green's function method and the self-consistent iterative method based on the density functional theory are introduced.The chapters three and four are numerical results and discussions on basis of the above method.The influence of anchoring groups on the charge transport is investigated in chapter three and the length effect on the molecule rectifier is studied in chapter four.The fifth chapter summarizes the work of this thesis and puts forward expectation in future.