Theoretical Research Of The Performances And Mechanisms Of Field-controlled Single-molecule Devices

With the continuous development and innovation of semiconductor manufacturing technology,the limitations of silicon-based semiconductor materials become more and more apparent.As semiconductor electronic devices shrink to nanometer level,the quantum effects of the devices become prominent.The electron tunneling effects between neighboring devices are inevitable,which brings significant challenges to the manufacturing,packaging and normal operation of chips.Single-molecule devices,which use individual molecules as functional units connected between two electrodes,are promised to overcome the difficulties and limitations encountered in further miniaturizing traditional semiconductor devices,meanwhile achieving superior electrical performance.Field-effect molecular devices,as crucial components of molecular integrated circuits,possess many potential advantages,such as low power consumption,high sensitivity,small size,rapid response,etc.To elucidate the mechanisms of field-controlled molecular devices（FCMD）,researchers have conducted extensive experimental and theoretical studies,and designed diverse types of FCMDs.However,current understandings on the mechanisms of FCMDsare still limited.Therefore,further investigations on the performance and mechanisms of FCMD are very necessary.In this thesis,the modulation mechanisms of gate field and optical field on the transport properties of molecular devices are investigated based on density functional theory and non-equilibrium Green’s function methods.For the molecular junctions under the control of gate electrode,the effects of molecular orientation,electrode shape and distance,and electrode symmetry on the electronics transport properties are investigated.The differences between the electronic transport properties of azobenzene isomers caused by light are further studied.The investigations of this thesis encompasses the following 5 parts:1.The effect of gate voltage on the rectification properties of DPE-2F moleculeGate-modulation is achieved by introducing a third terminal electrode（gate electrode）between the source and drain electrodes,which generates a gate field acting on the molecule to adjust the alignment of molecular frontier orbitals,and further effectively modulates the electronic transport properties of the molecule.Due to its asymmetric non-planar structure,DPE-2F molecules are generally believed to exhibit good rectification characteristics,with rectification properties can vary with gate voltage.Therefore,we systematically investigated the effects of gate voltage and bias voltage on the molecular orbitals and energy levels of DPE-2F molecules and consequently on their electronic transport properties.The results show that under positive gate voltage,the lowest unoccupied molecular orbital（LUMO）dominates electron transport,with its wave function more delocalized and extending throughout the entire molecule.Under bias voltages of-1 V and-2 V,the LUMO becomes localized and located at the right side of the molecule compared with that of the molecular junction at 0 V gate voltage.Thus,positive gate voltage significantly alters the spatial distribution of LUMO and further enhances the rectification performance of the molecule.Within the range of bias voltages from-2 V to 2 V under negative gate voltage,the highest occupied molecular orbital（HOMO）becomes highly delocalized relative to its initial localized state,with its wave function extending throughout the entire molecule.The influence of positive and negative biases on molecular orbitals under negative gate voltage is similar,thus the magetive gate voltage having little effect on the rectification of DPE-2F molecules.2.Influence of gate voltage on the electron transport properties of S-（CH₂）₃-Fc-（CH₂）₉-S molecular junctionThe studies show that molecules with a ferrocene core connected to electrodes via different lengths of saturated carbon chains exhibit remarkably superior rectification characteristics.Thus,the electric transport properties of the molecule S-（CH₂）₃-Fc-（CH₂）₉-S（abbreviated as C3Fc C9）under the combined influence of gate voltage and bias voltage are investigated.The numerical results reveal that under the controls of gate and bias voltage,the current and conductance maps of C3Fc C9 molecular junctions exhibit regular and intriguing resonance transport areas and Coulomb blockaderegions.The regular variation characteristics of the maps entirely stemming from the linear movement of molecular energy levels induced by gate and bias voltages.Further,by selecting the variation intervals of gate and bias voltage from the current maps,we predicted multiple logic gate functions and signal frequency doubling functions for ferrocene-hased single-molecule junctions,which are successfully validated experimentally.According to the calculations,the energy differences between two effective conductive molecular orbitals in twelve three-terminal devices with different structures are stably around 110 me V,indicating that the electrode interface structures have little impact on the stability of the energy levels of the two effective conductive molecular orbitals,which demonstrates that the stability of two adjacent conductive molecular orbitals is an intrinsic characteristic of ferrocene-based molecular devices.Due to the competing effects of molecule-gate coupling and molecule-electrode coupling,some molecular devices exhibit a negative correlation between gate voltage control rate and bias voltage control rate,which results in bias voltage control rates of three-terminal devices typically being lower than those of two-terminal molecular devices without gates.3.Influence of electrode distance on the electron transport properties of S-（CH₂）₃-Fc-（CH₂）₉-S molecular junctionDue to the highly flexible nature of the S-（CH₂）₃-Fc-（CH₂）₉-S molecule,there may be significant differences in the electrode distances in experimentally constructed C3Fc C9molecular junctions.Varying electrode distances can influence on the connections between the molecule and the electrodes as well as the molecular geometry,which thereby further modulates the electronic transport properties of the molecule.Therefore,we computed the structures and electronic transport properties of C3Fc C9 molecular junctions under the control of gate voltage at different electrode distances.The calculations reveal that the electrode distance interval for stable logic computing is[D₀-1.2（?）,D₀+1.2（?）],where D₀ represents the equilibrium electrode distance.When the electrode distance changes in this interval,the energy and spatial distribution of molecular orbitals contributing significantly to transport show minimal variations,which indicates that the contributions of the resonant transport orbitals are hardly affected.Beyond a deviation of 1.2（?）in electrode distance,molecular deformation induces the substitution of effective conductive molecular orbitals,which further results in larger energy differences.Under electrode bias,the energies of molecular orbitals effectively contributing to the electronic transport linearly vary with the chemical potentials of the two electrodes.The variations in energy levels induced by bias are 0.35 e V/V and 0.45 e V/V with and without gate field effects,respectively.As the electrode distance increases from the equilibrium distance to 3.8（?）,the modulation rate of orbitals by gate voltage decreases from 0.50 e V/V to 0.11 e V/V,while the modulation rate by bias voltage increases from 0.34 e V/V to 0.48 e V/V.The reduction in electrode distance does not affect the modulation rate of orbitals by gate voltage.4.Influence of electrode symmetry on the electron transport properties of S-（CH₂）₆-Fc-（CH₂）₆-S molecular junctionIn single-molecule junctions,electrode symmetry directly influences on the coupling strength between the molecule and the electrodes,the distribution of molecular orbitals and energy levels,as well as the polarization characteristics within the molecule.Deeply understanding of the influences of electrode symmetries on gate-modulated single-molecule junctions is very necessary.Thererfore,we calculated the transport properties of S-（CH₂）₆-Fc-（CH₂）₆-S（abbreviated as C6Fc C6）field-effect molecular junctions with symmetric and asymmetric electrode configurations,and comparedwith C3Fc C9 molecular junctions.The stabilities of the logic compution functions for the two molecular junctions are further analyzed.The numerical results show that symmetric molecules and electrodes lead to a greater sensitivity of the junction to changes in electrode distance.Increasing electrode distance causes the molecular orbitals which have evident contribution to the electronic transport moving away from the Fermi level,and further results in a larger energy gap between the two main conductive molecular orbitals.Compressing the electrode distance increases the number of delocalized resonant-transport orbitals.The symmetric C6Fc C6 molecule connected to asymmetric electrodes also induces delocalized molecular orbitals moving away from the Fermi level,but it is less sensitive to the electrode distance changes.Although asymmetric electrodes enhance the stability of logic compution functions,C6Fc C6 molecular junctions are less stable compared to C3Fc C9 molecular junctions.5.Light-induced isomerization causes changes in the electronic transport properties of azobenzene-based molecular junctionExperimental results show that there is one conductance peak for the4,4’-diamineazobenzene molecular junction without light radiating,but the conductance peaks become two when the 4,4’-diamineazobenzene molecular junction were continuously irradiated with 365 nm UV light for different periods.Additionally,the two peaks shift towards higher conductance values with the increase of the irradiation time.To explain these results,we investigated the electronic transport properties of monomeric and dimeric4,4’-diamineazobenzene molecules.The results indicate that cis-azobenzene molecules can connect with trans-azobenzene molecules in different dimeric configurations between two electrodes,which leads to a transition from semi-parallel conduction to series conduction during electrode stretching.Therefore,the low conductance results from the series connection of two azobenzene molecules,while the high conductance is attributed to the formation of semi-parallel structures by adsorption of cis-azobenzene molecules onto trans-azobenzene molecules.If the end group is a pyridine group,the electrode stretchingresults in high and low conductance plateaus due to pyridine adsorption at the tip and the second-layer electrode,respectively.These results explain well the observed conductance changes of azobenzene molecules in the experiment and provide theoretical support for the design of molecular switches using azobenzene-pyridine molecules.The thesis consists of the following sections:Chapter 1 provides an overview of the background and progress in molecular electronics,and introduces the development of molecular electronics and factors influencing the electronic transport properties of molecular junctions.The progresses of studies on the gate field and light-induced molecular isomerization effects on the electronic transport properties of molecular junctions are further focused on.Chapter 2 introduces the theoretical methods used in our calculations which includes density functional theory and non-equilibrium Green’s function method.Chapters 3 to 7 present the results and discussion of the main work of this thesis.Chapter 3 investigates the effect of gate voltage on the rectification properties of DPE-2F molecular junctions,systematically studying and analyzing the influence of gate voltage and bias voltage on the molecular orbitals and energy levels of DPE-2F molecules and their electronic transport properties.Chapter 4 discusses the effects of gate voltage and bias voltage on the electronic transport properties of C3Fc C9molecular junctions,proposing a specific scheme for designing C3Fc C9 molecular junctions as single-molecule logic gates.Chapter 5 systematically discusses the influence of electrode distance on the electronic transport properties and logic compution functions of C3Fc C9field-effect molecular junctions.Chapter 6 investigates the effect of electrode symmetry on the electronic transport properties of C6Fc C6 field-controlled molecular junctions.Chapter 7 studies the changes in the electronic transport properties of azobenzene molecular junctions induced by photoisomerization,and explains the underlying mechanisms through theoretical calculations.Chapter 8 provides a summary of the entire work and prospects for the future study on field-effect molecular junctions.