Theoretical Studies On The Rectification Of Arylethynylene Single-molecular Diodes

Over recent years,the size of the electronic device is shrinking to the nano size due to the rapid development of electronic processing technology,which forces the researchers to design molecular devices with different functions based on the quantum theory rather than classic theory.Theoretical and experimental researchers have paid great attention on the molecular diodes or rectifiers as it is one of the most important segments of the electronic circuits.In general,the rectifying properties of molecular device can be attributed to the asymmetry of the electrode-molecule contact or the intrinsic asymmetry of the molecule itself.In recent years,a series of arylethynylene molecule has been synthesized and studied.It is found that the arylethynylene molecule has good conducting properties and can be tuned by modulating its length and conjugation bridge.In addition,arylethynylene synthesized through self-assembly monolayer(SAM)experiment exhibits obvious rectifying phenomenon.Up to now,there have been several studies on the arylethynylene molecule,but several problems still remain to be solved for the arylethynylene molecule: the rectifying properties still needs to be analyzed and demonstrated through theoretical calculation,the rectifying performance need to be tuned by introducing asymmetric in the molecular backbone and so on.By using the nonequilibrium Green's function(NEGF)method combined with density functional theory(DFT),we investigate the conjugation type effects on the rectifying properties of arylethynylene diodes and rectifying regulation by introducing asymmetric side groups and asymmetric anchoring groups in the arylethynylene molecule.Finally,the rectifying mechanism is explained from theoretical analysis method.The research contents and results are as follows: 1.Bridge conjugation type effects on the rectifying properties of D-Bridge-A arylethynylene diodes with asymmetric side group substitutionBy using the first-principles method,conjugation effects on the rectifying properties of the three D–B–A arylethynylene diodes are theoretically investigated,which are denoted as linearly conjugated anthracene(sAC),cross-conjugated 9,10-anthraquinone(sAQ),to broken-conjugated 9,10-dihydroanthracene(sAH).Then we focus on studying the conjugation type effects on the rectifying properties of arylethynylene diodes and explain the rectifying mechanism from theoretical analysis method.The results show that rectifying performance of the junctions correlates negatively with conjugation of the molecules,which reveals a much more pronounced rectification behavior for that with broken-conjugated dihydroanthracene core(sAH)than the counterpart with linearly-conjugated anthracene core(sAC)or cross-conjugated anthraquinone core(sAQ).It is demonstrated that the conjugation breaking induced localization of molecular orbital and asymmetric evolution of its spatial distribution under positive and negative biases play the key role for the rectification enhancement in sAH.2.Bridge conjugation type effects on the rectifying properties of D-Bridge-A arylethynylene diodes with asymmetric anchoring groupBy using the nonequilibrium Green's function(NEGF)method combined with density functional theory(DFT),we investigate the bridge conjugation type effects and asymmetric thiol-cyano regulation ability on the rectifying properties of the three arylethynylene diodes AC?AQ and AH.The rectification performance is improved and the rectifying direction is inverted when the central bridge is changed from cross conjugated AQ to linearly-conjugated AC.Moreover,when the conjugation of the bridge fragment is broken for AH,the rectification performance is further enhanced remarkably.Analysis of the molecular eigenstates as well as the transmission spectra reveals that the asymmetric shift of frontier orbital under bias are responsible for the invertion of the rectification direction,and great enhancement in rectification ratio for AH results from asymmetric evolution of strongly localized frontier molecular orbitals under positive and negative bias voltages.The thesis is divided into five chapters as follows.The first chapter is the introduction part,which includes the brief introduction of the molecular electronics and the basic classification of the molecular diodes,as well as the latest progress of the molecular rectifiers and arylethynylene molecule device.In second chapter,we present the adiabatic approximation,Hartree-Fock method,then we focus on the density functional theory,which is one of the most successful methods in studying the electronic structures,in the end,we introduce the non-equilibrium Green's function method and the self-consistent solution process of the electron transport properties by combining these two methods.In the next third chapter,we investigate the rectifying properties of three cyano side group substituted arylethynylene thiolate molecules.Based on the study of the third chapter,the rectifying properties of arylethynylene molecules with asymmetric anchoring groups are studied.The last chapter gives a brief conclusion and outlook for our work.