Study On Quantum Interference Mechanism And Design Of Molecular Electronic Device

With the development of miniaturization of electronic devices,the requirements for low power consumption and high performance in device design are constantly increasing.On the one hand,the size reduction means that the probability of direct tunneling of electrons is increased,inducing leakage current in the device.On the other hand,the highdensity integration also brings about the high heating phenomenon of the device,affecting the performance of the device.Organic molecular materials are regarded as an important alternative to traditional silicon materials in the future due to their smaller structure,lower heat generation and structural diversity.For molecular devices,their rich electronic properties such as switching effect,negative differential conductance,spin filtering and magnetoresistance etc,have attracted the majority interests of researchers.In addition,the quantum interference,as an important phenomenon in single-molecule electronics,has been discovered in many experiments and theoretical studies about molecular junctions.The constructive and destructive quantum interference can greatly enhance and suppress the conductance of molecule junction,respectively.Therefore,an in-depth understanding of the quantum interference mechanism of electrons in molecular devices is of great significance for the interpretation of experimental phenomena,and beneficial to the design and application of molecular devices in the future.In this dissertation,we focus on investigating the organic molecular devices by using Density Functional Theory combined with Non-equilibrium Green's Function,studying the effect of the stacking way between weakly coupled molecules,the length of magnetic molecular junctions,the symmetry of magnetic molecular junctions and the connection type between molecule and electrodes on the electronic transport properties.We have also studied the influence of quantum interference mechanism and the impact of the inelastic process caused by electro-phonon coupling to the transport properties of the device.Specifically,we have done the following researches:First,we studied the electron transport phenomenon of weakly coupled molecular junctions based on diphenylacetylene molecules with different stacking types.The results show that the transport properties of the device can be affected by changing the stacking method between molecules.We found that the molecular junctions of the para and meta stacking types can raise the destructive and constructive quantum interference phenomena,respectively.The quantum interference in both junctions can realize the off state and on state of conductance which is opposite to the phenomenon in the in-plane strong coupling molecular junction where no stacking occurs.Furthermore,considering the influence of electron-phonon coupling,we proposed a molecular switch based on quantum interference,and by introducing a gate model,we achieved a 5-fold molecular switch ratio enhanced by quantum interference at a specific gate potential of-0.6V.More importantly,we also observed negative differential conductance in the meta-molecular junction caused by in-plane vibration excitation at higher gate potential,which means that the electron-phonon coupling can be enhanced under near resonance conditions.This work can provide ideas for the use of quantum interference to design molecular switches.Secondly,we further extended the quantum interference to the magnetic systems,and studied the spin transport properties of chromium porphyrin molecular junctions with different molecular lengths.It was found that the destructive quantum interference phenomenon that changes with the length of the molecular junction can alternately change in the two spin channels,and can produce a length-dependent spin filtering behavior.Moreover,by further considering the electron-phonon coupling and temperature effects,we found that when the chromium porphyrin molecules are monomers and dimers,the electrons can often maintain better electron coherence characteristics.When the molecular length is further increased to trimer,the vibration induced decoherence phenomenon will become stronger.Finally,we compared the transport properties of chromium-doped and undoped of porphyrin monomer molecular junctions,and found that magnetic atom of chromium atoms can greatly enhance the spin quantum interference phenomenon,and produce nearly 100% spin filtration efficiency under low bias voltage.This work provides ideas for spin device design.Then,we further considered the influence of the symmetry of the magnetic molecular junction,and studied the spin transport properties of the Co-Salophene dimer molecular junction under different magnetic states.The results show that the spin dependent quantum interference phenomenon in the ferromagnetic state molecular junction can greatly improve the spin injection efficiency of spin devices.We also found that symmetric molecular junction has higher tunneling magnetoresistance ratio,which is about the same as that of asymmetric molecular junctions 2 times.By further considering the electron-phonon coupling and temperature effects,we found that symmetric molecular junctions have stronger ability to resistant the molecular vibrations and temperature effects.This work provides new ideas for the design of spin devices,and also provides a certain reference for the design of molecular integrated circuits in the future.Finally,we studied the effects of different contact sites and electron-phonon coupling to the electronic transport properties of tetraphenylbenzene molecular junction.The results show that the quantum interference can be achieved by changing the contact point between the molecule and the electrode.In addition,we have also built a molecular junction with four anchor points,and compared the conductance with the terphenyl molecular junction.We found that the quantum interference and electron-phonon coupling can indeed break the Kirchhoff's law of conductance.This work provides theoretical evidence for the quantum interference in molecular devices to break Kirchhoff's law.At the same time,it also provides ideas for constructing intramolecular multi-channel integrated circuits.