First-Principles Studies On Switching Characteristics Of Dimethyldihydropyrene Single-molecule Devices

With the rapid development of miniaturization of microelectronic devices,traditional silicon-based electronic technology has been unable to meet the demand for device miniaturization due to high manufacturing cost and the limitation of physical laws.Therefore,single-molecule devices constructed by using single molecules as basic elements,which have the advantages of smaller size,lower energy consumption,and faster response speed,are proposed to further promote the continuously miniaturizing electronic devices.In 1974,Aviram and Ratner put forward the conception of the molecular rectifier and validated its feasibility theoretically.This was the first concrete single-molecule device,which marked the birth of a rapidly emerging field named molecular electronics.In recent decades,a large number of single-molecule devices have been designed and fabricated theoretically and experimentally,such as molecular spin filters,molecular diodes,molecular field effect transistors,molecular wires,molecular sensors,and molecular switches.Among these functional molecular devices,the molecular switch has attracted particular attention due to its extensive application in molecular circuits.A functioning molecular switch requires two or more stable states with distinct conductivity,which can be reversibly shifted subject to external stimuli,such as electric field,environmental temperature,ambient chemicals,and light,among which light radiation has been used by researchers as a common trigger because of its advantages of ease of implementation,precise spatial and temporal control and low toxicity.Recently,experimental and theoretical works have been devoted to the switching characteristics of the photosensitive dimethyldihydropyrene molecule,which is found to have a significant ON/OFF ratio.However,some factors influencing the performance of dimethyldihydropyrene molecule switch still need to be explored.In this thesis,by using the nonequilibrium Green’s function method(NEGF)in conjugation with the density functional theory(DFT),the effect of linkage site on the switching characteristics of dimethyldihydropyrene single-molecule junction was firstly investigated.On this basis,the effect of the length of carbon atomic chain(CAC)on the charge transport properties and switching performance of dimethyldihydropyrene molecular device have been studied when gold or zigzag graphene nanoribbon were used as electrodes and CACs of different lengths were used as bridges between the core dimethyldihydropyrene molecule and the electrodes.The contents and conclusions of this thesis are summarized as follows.1.Effect of the linkage site between molecule and electrodes on the switching performance of dimethyldihydropyrene molecular devices.Designing molecular switches with high stability and performance is still a great challenge in the field of molecular electronics.In fact,many factors affect the charge transport properties of molecular devices,among which the linkage site between the core molecule and electrodes is a key factor that is often studied.Using the fully self-consistent DFT based NEGF method,we investigated the effect of different linkage sites of the core dimethyldihydropyrene(DHP)/cyclophanediene(CPD)molecule to electrodes on its switching performance.The numerical results show that the fully conjugated DHP is more conductive than the less conjugated CPD,which is due to the difference in the distance between their highest occupied molecular orbitals(HOMO)and the Fermi level(E_F).At the same time,it was found that the different linkage sites of the core DHP/CPD molecule to electrodes have a significant effect on the ON/OFF ratio.Further analysis shows that the transmission pathway of the electron is closely related to both the conjugation characteristics of the core molecule and the linkage site of the core molecule to electrodes.Therefore,the transmission pathway of the electron can be largely modified by changing the linkage site of the core molecule to electrodes.2.The study on the switching characteristics of dimethyldihydropyrene molecular devices when CACs connect the molecule to gold electrodes.Linear CACs can be regarded as a class of graphene derivatives or extremely narrow graphene nanoribbons.They are ideal one-dimensional systems,which have attracted more and more attention due to their unique electronic properties.Theoretical and experimental studies show that the molecular devices based on CACs often exhibit many interesting physical phenomena.Using the fully self-consistent DFT based NEGF method,we used CACs as the connection bridge between the core DHP/CPD molecule and the gold electrodes to explore whether it could improve the switching performance.It is found that the current values and ON/OFF ratio of DHP/CPD molecular devices show an evident odd-even effect with the change of the number of carbon atoms in CACs,and the odd-even effect of ON/OFF ratio is opposite to that for the current values.The maximum ON/OFF ratio for DHP/CPD molecular devices with an even number of carbon atoms in CACs is about 150,while the maximum ON/OFF ratio of molecular devices with an odd number of carbon atoms in CACs is only 19.Further analysis shows that bondings between the carbon atoms in the CACs are closely dependent on the number of carbon atoms,which results in very different arrangements of the molecular frontier orbital relative to the E_F of the electrodes.In addition,the conducting channels around E_F for CPD single-molecule junctions depend on the odevity of the number of carbon atoms in CACs,which results in an order of magnitude variation in the switching performance of the DHP/CPD single-molecule junction.3.The study on the switching characteristics of dimethyldihydropyrene molecular devices using graphene nanoribbons as electrodes.So far,controlling the interface between electrode and molecule is still one of the most challenging problems in preparing reliable molecular devices.The core molecule is usually connected to metal electrodes through the thiolate anchoring group,but the main disadvantage of this method is that the surface morphology of the metal electrode is complex and unstable as well as the sulfur atom in the thiolate group being susceptible to oxidation.The two-dimensional graphene has become the most ideal material for replacing metal as electrode due to its novel properties such as ultrahigh electron mobility,easily tunable electronic structure,and stable chemical properties.Therefore,we investigated the switching characteristics of DHP/CPD molecular devices when zigzag graphene nanoribbons were used as the electrode.In this study,one-dimensional CAC is still used to connect the DHP/CPD molecule to graphene nanoribbon electrodes.It is found that the ON/OFF ratio can reach to 10~2 and even 10~3 when the CACs with an odd number of carbon atoms,and no larger than 57 when the CACs with an even number of carbon atoms.And a spin filtering effect higher than 80%or even 100%can be obtained for each molecular device.Further analysis reveals that the transmission eigenstates of DHP molecular devices are more delocalized than those of CPD molecular devices.And the spatial distributions of spin up transmission eigenstates depend on the odevity of the number of carbon atoms in CACs for CPD molecular devices,which leads to an odd-even effect on the switching performance of DHP/CPD molecular devices.For each DHP or CPD molecular device,the distributions of spin-up transmission eigenstates are delocalized.On the contrary,the spin-down transmission eigenstates of DHP and CPD molecular devices are localized on the left electrode,which leads to the high spin filtering effect in the molecular devices.This thesis consists of six chapters as follows.The first chapter briefly introduces the development of molecular electronics and the research progress of molecular switches.Chapter two involves the density functional theory,which is a widely used method for electronic structures of multi-electron systems,and the nonequilibrium Green’s function method that can be used to obtain the charge transport properties of molecular devices.In addition,the self-consistently iterative process of combining the nonequilibrium Green’s function method with the density functional theory for two-probe systems has also been described.Some concrete works by using the above-mentioned method and corresponding result discussion are presented in chapter three to chapter five.Effect of different linkage sites of the core dimethyldihydropyrene molecule to the electrodes on the switching performance of the molecular device has been systematically studied in chapter three.In the next chapter,the switching characteristics of the dimethyldihydropyrene molecular device have been studied when CACs of different lengths are used as the bridge between the core molecule and the gold electrodes.In chapter five,the gold electrodes of the molecular devices are replaced by zigzag graphene nanoribbons and effects of CACs with different lengths on the switching characteristics and spin transport properties of the dimethyldihydropyrene molecular device have been studied.Finally,a summary of the thesis and a prospect on the development of molecular devices in the future are given in chapter six.