Theoretical Studies On The Rectification Of Diblock Co-oligomer Molecular Junctions

With the minimization of traditional electronic devices, much attention has beendevoted to the study of molecular devices. As a basic functional unit of molectronics, themolecular diode has taken an important place since it was first proposed by Aviram andRatner in1974. However, the original idea to use a donor-σ-acceptor (D-σ-A) molecule forrectification has been challenged in the further chemical synthesis and theoreticalverification. As an alternative way, a series of diblock co-oligomers consisting of anelectron-rich moiety (D) and an electron-deficient one (A) similar to p-n junctions has beenexperimentally synthesized and investigated in recent years. A clear and repeatable currentrectification has been observed in these diblock co-oligomers, which has been proved toarise from the built-in asymmetry in the molecules. A lot of experiments and theoreticalresearches have been carried out so far. However, there are still many issues to be resolvedfor D-A diblock co-oligomer molecular diodes: the mechanisms of rectification need to befurther discussed, some experimental observations need to be understood, and there is someroom and necessity to improve the rectification and so on.Using the fully self-consistent nonequilibrium Green’s function method combined withdensity functional theory, the effects of molecule/electrode contact configuration,protonation, anchoring group, molecular length, and stretching and compressing themolecular junction on the rectification of D-A diblock co-oligomer diodes have beensystematically investigated respectively. The main contents and corresponding results of theresearches in this thesis are as follows.First, effects of molecule/electrode contact configuration on the rectification of D-Adiblock co-oligomer diodes have been theoretically investigated. As is known, the alignmentof molecular energy levels with respect to energy bands of electrodes would be affected bythe molecule/electrode contact configuration. So study on the effects of molecule/electrode contact configuration on the transport properties of molecular junctions is an interesting andimportant issue. The molecule/electrode contact configuration effects on rectification of aconjugated dipyrimidinyl-diphenyl diblock co-oligomer sandwiched between two goldelectrodes have been theoretically investigated using the fully self-consistentnonequilibrium Green’s function method combined with density functional theory. We havefound that the rectification and the mechanisms of electron transport indipyrimidinyl-diphenyl diblock co-oligomer molecular junctions vary, depending on thedetails of molecule/electrode contact configuration. Compared with the plane contactconfiguration, the triangular contact configuration contributes to a much larger rectificationratio. However, an inversion of the rectification takes place for the tetrahedral case in asmall bias region while the negative differential resistance is observed as the bias gets larger.The further analysis suggests that electrons are the main charge carriers in the molecularjunction with triangular contact configurations between the molecule and electrodes and it isholes for the tetrahedral case. When the triangular contact configuration is adopted, bothelectrons and holes are responsible for the conductivity of the molecular junction.Second, the protonation effects on rectification of a dipyrimidinyl-diphenyl diblockco-oligomer sandwiched between two gold electrodes have been theoretically studied usingthe density functional theory based fully self-consistent nonequilibrium Green’s functionmethod. Morales et al. have reported that the rectification direction in the dipyrimidinyl-diphenyl diode molecule is controlled by the PH of the surrounding solution. That is, aninversion of the rectification has been observed when the dipyrimidinyl-diphenyl diodemolecule is placed in perchloric acid (HClO4) solution. Moreover, it is reversible byneutralization of the HClO4solution. They attributed the rectification inversion to theprotonation of the nitrogen atoms on the dipyrimidinyl block. So far, this phenomenon hasnot been verified by theoretical work. Based on the developed numerical treatment ofprotonation, all possible configurations of protonation on the dipyrimidinyl block of thedipyrimidinyl-diphenyl molecule have been considered. The numerical results suggest thatthe electron transport is largely enhanced by protonation of the dipyrimidinyl-diphenylmolecule. And the rectification is tuned by the number and locations of protons residing in the molecular junction. Furthermore, it is observed that protonation in the outer pyrimidinylis favorable for enhancing the rectification ratio, while protonation in the inner pyrimidinylplays a dominative role in inverting the rectifying direction. The numerical results explainthe experimental findings of inversion of the rectifying effect in diblock molecular diodescaused by protonation.Third, the effects of anchoring groups on the rectification of D-A diblock co-oligomerdiode molecules have been theoretically investigated using the density functional theorybased fully self-consistent nonequilibrium Green’s function method. As parts thatconnecting the molecule to electrodes in molecular junctions, anchoring groups affect thecouplings between the molecule and the electrodes. So, anchoring groups are expected toinfluence the transport properties of molecular junctions. In addition, different molecularanchoring groups would create an asymmetry in molecular junctions leading to asymmetriccurrent-voltage curves. Therefore, asymmetric anchoring groups in the molecule willprovide an alternative for modulation of the rectification in molecular junctions. The effectsof asymmetric isocyanide-thiol end groups on the rectification of dipyrimidinyl-diphenyldiode molecule have been theoretically investigated. By introducing the isocyanide grouponto one of the two terminals of dipyrimidinyl-diphenyl diodes, the frontier molecularorbitals, especially the spatial distributions of wavefunctions, are changed significantly,which further lead to entirely different energy evolutions under bias voltages. The rectifyingdirection or rectification ratio of single dipyrimidinyl-diphenyl diode is modulated bysubstituting an isocyanide group for a thiol group. That is, the rectifying direction ofdipyrimidinyl-diphenyl diode molecules is inversed when the thiol group connecting to thephenyl ring is replaced by an isocyanide group. And the rectification ratio is highlyimproved when the thiol group at the pyrimidinyl side is substituted by an isocyanide group.Our theoretical results are well consistent with the experimental observations from Hihath etal. and the numerical values from Nakamura et al. However, the values differ muchcompared with the experimental results given by Lee et al. This disagreement is explainedby asymmetric molecule/electrode contacts.Next, we have discussed the effects of molecular length on the rectification of D-A diblock co-oligomer molecules. The HOMO-LUMO gap would decrease along with theincreasing of diblock co-oligomer length. The variation of HOMO-LUMO gap wouldprobably modify the resonance properties of electron transport and the onset bias ofmolecular junctions. We have chosen pyrimidinyl-phenyl diblock co-oligomer molecules asour prototypes and investigated the molecular length effects on the rectification using thedensity functional theory based nonequilibrium Green’s function method. The results revealtwo competitive mechanisms in determining the rectifying direction, asymmetric shift ofmolecular energy levels and spatial asymmetry of molecular wave functions upon thereversal of bias voltage. It is demonstrated that the dominated mechanism of rectificationmay be converted from the former to the latter as molecular length increases, which inducesan inversion of the rectification direction. And our first-principles calculation confirms ourprevious results of the Su-Schrieffer-Heeger (SSH) model.At last, the effects of stretching and compressing the molecular junction on therectification of D-A diblock co-oligomer diodes have been explored. As is known,mechanically controllable break junction is a frequently-used technique in molecularelectronics to form single molecule junctions and investigate the transport properties. Withthis technique, one can precisely control the distance between two electrodes. And then, thecoupling between the target molecule and electrodes can be modulated. Stimulated by this,we have theoretically explored the effects of stretching and compressing on the rectificationof pyrimidinyl-phenyl diblock co-oligomer molecular junctions. Our calculations reveal thatthe rectification direction may be controlled by stretching and compressing the molecularjunction. Stretching and compressing drive LUMO of pyrimidinyl-phenyl molecule to be atdifferent locations relative to the Fermi level and then the rectifying direction would beinverted. Our calculations propose a mechanically controllable manipulation of therectification in pyrimidinyl-phenyl diblock co-oligomer diodes, which is deserved to beverified in the future mechanically controllable break junction experiments.This thesis consists of eight chapters as follows. The first chapter includes anintroduction of molectronics and the development of molecular diodes. In chapter two, wehave briefly introduced the density functional theory, a commonly used tool to study the electronic structure, and the nonequilibrium Green’s function method, which is usually usedto solve the electron transport properties in nanoscale. In addition, we also present thecombination of density functional theory and nonequilibrium Green’s function toself-consistently get the electron transport properties at an ab-initio level. Some calculationsand the corresponding results obtained by using the above method are presented in chapterthree to chapter seven. The effects of molecule/electrode contact configurations on therectification of single dipyrimidinyl-diphenyl molecular junctions have been elaboratelystudied in chapter three. In the next chapter, we have systematically investigated theprotonation effects on the rectification of dipyrimidinyl-diphenyl molecule. And theinversion of rectifying direction for dipyrimidinyl-diphenyl diblock molecule in HClO4solution observed in experiment can be successfully explained. In chapter five, by analyzingthe influences of asymmetric isocyanide-thiol anchoring groups on the moleuclar orbitals indipyrimidinyl-diphenyl diblock diodes, we not only give an explanation to the experimentalobservation but also design a molecular diode with a high rectification ratio. In thefollowing chapter, the molecular length effects on the rectification have been theoreticallyexplored, the relationship between the rectifying direction of pyrimidinyl-phenyl diblockmolecular diode and the molecular length has been declared. A proposal of mechanicallycontrollable manipulation of the rectification in pyrimidinyl-phenyl diblock co-oligomerdiodes has been delivered in chapter seven, which is deserved to be verified in the futuremechanically controllable break junction experiments. The last chapter draws a conclusionfor the whole work of the thesis and gives a prospect on the development of molecularrectifying devices in the future.