Single-molecule Junctions:Conductance Properties And Molecular Raman Images

With the development of science and technology,nanotechnology that aims at manipulating a single atom and molecule has flourished.Benefited by the precise nanofabrication,one can connect a single molecule with macroscopic electrodes to form the single molecule junction.Single-molecule junctions exhibit many interesting conductance properties,dawning the exploration of multifunctional molecular devices in molecular electronics.Many theoretical and experimental works have been devoted to unravel the relationship between molecular device conductance and its electronic and geometric structures.However,some mechanisms that affect the conductance properties of single-molecule devices are still debatable.Single-molecule junctions have also provided a platform for optical single molecule Raman images in recent years.Using the properties of near-field light,the Raman imaging technique for the molecule inside single-molecule junctions has achieved sub-nanometer spatial resolution,which breaks the spatial resolution diffraction limit of traditional spectroscopy techniques.As an emerging technology,it is urgent to explore potential applications of the molecule Raman imaging.Herein,the present thesis mainly includes researches on two aspects:one is to specifically investigate the effect of the molecular aromaticity as well as the adsorbed molecule of electrodes on the conductivity of single-molecule junctions through the density functional theory combined with the nonequilibrium Green's function method,the other is to theoretically predict that the emerging Raman imaging technique can manipulate excited states and characterize structures of the molecule inside single-molecule junctions based on the ab initio theory of light-matter interaction at the nanoscale.The main contents and results of this thesis are as follows:?1?Effect of molecular aromaticity on the conductance properties of single-molecule junctionsIn 2014,the negative relationship between conductance and molecular aromaticity?NRCA rule?was proposed based on the experimental conductance measurement of three five-membered ring molecules with different degrees of aromaticity.By modeling the possible experimental structures of single-molecule junctions,we calculated the corresponding electrical transport properties,which is in good agreement with experimental results.Further analyses show that the aromaticity affects the energy position of the molecular orbital relative to the Fermi level of the electrode,which is responsible for the observed NRCA rule in experiments.However,it is well known that the molecular terminal group as well as the molecule-electrode contact configuration also plays a key role for the molecular orbital energy position relative to the Fermi level of the electrode.Therefore,we further studied the robustness of the NRCA rule.By changing the terminal group of the experimental molecule into an isocyano-group or modulating the contact between the methylene-terminal experimental molecule and electrodes into a parallel style,we find that the experimental NRCA rule is no longer applicable.Therefore,the NRCA rule is not universal.?2?Effect of molecular adsorption on the conductance properties of boron-doped graphene nanoribbon molecule junctionsSingle-molecule sensors based on boron-doped graphene nanoribbon electrodes have attracted attention due to their high sensitivity and rapid detection.The theoretical study of graphene nanoribbons has mainly focused on the changes in the electronic structure of graphene induced by adsorbed molecules.But the subsequent detecting of variations in electronic states is rare.By connecting an azulene-like molecule with two graphene nanoribbon electrodes,the two-probe junction is theoretically proposed,in which the current can be detected as practical signals for gas molecule adsorption.Calculated results show that the gas molecules CO,NO and NO₂ are adsorbed on the boron sites of doped graphene nanoribbon electrodes.The variations in current induced by different molecular adsorptions is distinguished,achieving the requirement of monitoring different molecules.It finds that the adsorbed molecules cause the energy band change near the Fermi level of the boron-doped graphene electrode,which influences the conductance properties of the corresponding single-molecule junction.?3?Characterization of the configuration for a single azobenzene by Raman imagingWith highly localized plasmon fields,molecule Raman imaging has reached atomic resolution,which provides an optical means of exploring the configuration of individual molecules.Our calculations for various isomers of azobenzene and its derivatives show that the resonance Raman imaging for the first excited state can unambiguously identify different molecular adsorption structures.Therefore,Raman imaging technology is expected to directly characterize the geometric changes of switching molecules,playing an important role in single molecule device research.Calculated results of other excited states show that the resonance Raman image can reflect electronic transitions?e.g.n??*?for a single molecule.As a result,the electronic structure of all excited states of the molecular system in a specific energy range could be characterized by Raman imaging.?4?Selective manipulation of molecular excited states by Raman imagingThe highly localized plasmon field inside a single-molecule junction is a prerequisite for molecule Raman imaging.Herein,we theoretically investigated the effect of spatially confined plasmon on the electronic transition of molecules.Calculated results for the donor/conjugated-bridge/acceptor molecular system show that various long-range charge transfer states that are previously forbidden can be selectively activated,which is attributed to the modulation of the position and distribution of the local light field.We find that the spatial distribution of the field can thus be used as an extra means that can affect the selection rule and intensity of the optical transition by breakdown of the system symmetry.As a result,it is possible to selectively manipulate arbitrary excited state of molecules.In addition,we find that resonance Raman images of the donor/conjugated-bridge/acceptor system for different excited states can unambiguously identify molecular donor/acceptor moieties and thus provide a powerful means of characterization for the rational design of functionalized materials.?5?Detection of inner hydrogen/deuterium transfer in single porphyrin by Raman imagingInner hydrogen transfer between two trans isomers in porphyrins is important in many biological systems as well as molecular nanotechnology.It is now widely accepted that hydrogen transfer between trans isomers via an intermediate cis isomer.However,the involved cis has not been directly detected owing to its short lifetime and the extremely weak intensities of corresponding hydrogen vibrations.The direct detection of hydrogen atoms of porphyrins in real space is needed.Our calculations show that the highly localized plasmon field can enhance the vibrational signal of inner hydrogen in porphyrins.Corresponding pre-resonance Raman image can provide position information of inner hydrogen atoms.As a result,Raman images effectively distinguish trans and cis isomers,making it possible to directly capture cis porphyrin.When a single hydrogen in the porphyrin ring is replaced by deuterium,Raman images for specific vibration modes can distinguish the position of hydrogen/deuterium,highlighting the chemical characterization of molecule Raman imaging.In addition,pre-resonant/non-resonant Raman imaging calculations requires the summation of all vibrational and electronical excited states,which is computationally expensive.Taking advantage of the linear response theory,the summation of all vibrational states can be calculated by the numerical differentiation.This greatly improves the efficiency of simulating pre-resonance Raman images.This thesis includes eight chapters as follows.The first chapter is the review section,which briefly introduces the specific research of single-molecule devices as well as the origin and development of the molecule Raman imaging technology.In the second chapter,we describe the method of electronic transport that based on the density functional theory and nonequilibrium function method.Meanwhile,the ab initio theory of light-matter interaction at the nanoscale is also introduced for calculating the molecule Raman images.Some calculated results obtained by using the above method are presented from the third chapter to the seventh chapter.In chapter three,the relationship between molecular aromaticity and its conductance was investigated.It finds that the negative correlation between molecular aromaticity and its conductance does not always hold.The terminal group and the molecule-electrode contact configuration can change this relationship.In the next chapter,we studied the effect of molecular adsorption on the conductance of boron-doped graphene nanoribbon molecular devices and theoretically proposed a single-molecule sensor that can distinguish different gas molecules.In chapter five,the application of characterizing single molecular configurations by resonance Raman imaging technology was comprehensively investigated.In the following chapter,taking a donor/conjugated-bridge/acceptor system as an example,we evaluated the effect of localized light field on the molecular optical transitions.Calculated results show that the localized field can selectively manipulate molecular arbitrary excited state and the resonant Raman images for the corresponding excited state can unambiguously identify the molecular donor and acceptor moieties.In chapter seven,we further developed an algorithm for efficiently calculating the molecule non-resonant Raman image.And calculated Raman images of porphyrins for specific vibrational modes predict that the Raman imaging technology can detect inner hydrogen atoms in a single porphyrin in real space.The last chapter summarizes the whole work in this thesis and presents a prospect on the future research.