Vibrational Identification Of Molecular Configurations At Metallic Interfaces

Understanding and controlling the molecular structures at the interface are critical to the heterogeneous catalysis, electrochemistry, corrosion protection and trace-level detection. Metallic junctions provide special interfaces for molecules. Because of the generation of the plasmon between two electrodes, the electromagnetic field interacting with the molecule inside the junction is enhanced to a very large extent. It is a property that would be helpful for the photochemical reaction and single molecule detection.By now, many experimental techniques have been developed to detect the inter-facial structure. However, it is still a challenge to identify the structure at the single molecular level. Surface enhanced Raman spectroscopy (SERS) and inelastic electron tunnelling scattering (IETS) are two important approaches that can be applied in-situ to detect the molecular configuration at the interface. However, it is difficult to directly infer the details of the interfacial structure based solely on experimental observations. A good combination of the theoretical simulations and experimental observations be-comes necessary.In the past decades, along with the fast increase of computer capability and the development of theoretical methods, the first principles calculations are approaching the level of "chemical accuracy". In addition, it is also possible to model large systems that include metallic surfaces. In this thesis, I will introduce how to model interfacial structure, to calculate properties, and to determine accurate molecular configurations at the interface and to compare with experimental observations.In the first Chapter, I will first give a brief introduction on the experimental tech-niques for detecting interfacial structures, including the photoelectron spectroscopy, the scanning probe microscope and the vibrational spectroscopy. Then I introduce the the-oretical methods to model the interface with both molecular dynamics and quantum mechanics. Moreover, I give an introduction on the cluster and periodic boundary con-dition models. The advantage and dis-advantage of them are also mentioned.In the second Chapter, I give a brief introduction on the theoretical background and the calculation methods employed in current studies. Firstly, I introduce the basis of the quantum chemistry:the Hartree-Fock theory and density functional theory (DFT); Secondly, I present the basic theory of Raman spectroscopy:the mechanism both from the point view of classic electromagnetic theory and the quantum mechanics. Then I briefly introduce two enhancement mechanics of SERS:namely physical and chemical enhancements. Thirdly, I explain how to calculate the electron scattering through the junction in both elastic and inelastic manners. Finally, I show how to explorer reaction pathway and locate the transition state.In the third Chapter, we used the cluster model (tetrahedral Au20) to study trans-1,2-bis (4-pyridyl) ethylene/gold (BPE/Au) system. We consider both junction and complex forms. The peak at 1581 cm-1 is predicted to be more intense in the junction forms than that in the complex forms. Time dependent density functional theory (TDDFT) calcu-lations show that the relative intensity is mainly controlled by the excitation energy derivative respect to the normal modes, and the total intensity is governed by the ex-citation energy. Compared with the SERS spectra, IES spectra can provide additional construction information of the junctions with different contact configuration. Among six molecular junctions we studied here, different vibrational modes are dominant in the IET spectra, illustrating the contact configuration of junctions could be reflected by the IET spectra.In the forth Chapter, we proposed a quasi-analytical method that allows to effec-tively evaluate Raman tensors in periodic systems based on density functional perturba-tion theory and finite-difference approach. Using this method we studied Raman spec-tra of 4,4’-bipyridine (4,4’-bpy) in various conditions. The calculated Raman spectra of isolated 4,4’-bpy as well as its adsorption on flat gold surfaces nicely reproduce their experimental counterparts. The same method has also been applied to a more compli-cate system:4,4’-bpy sandwiched inside gold nano junctions. By comparing with the in-situ experimental spectra (three types of Raman spectra appear), four interfacial con-figurations are identified, which are further verified by the good agreement between the simulated charge transfer properties and the experimental measurements. These results indicate that the proposed quasi-analytical method can provide accurate interpretation for the experimentally measured surface enhanced Raman spectra and unambiguously determine the structures of the molecules on metal surfaces.In the fifth Chapter, we designed a kind of photoswitchable molecular devices based on dimethyldihydropyrene (DHP) and cyclophanediene (CPD) isomers. The re-action pathways for the forth-and back-isomerization between DHP and CPD have been explored. We found that along the ground state, the calculated barrier for the back-isomerization from CPD to DHP is as high as 23.2 kcal/mol, indicating the instability of CPD in room temperature. Substituting the methyl in the central part with hydro-gen atom or cyan group can significantly enhance the thermal stability of the molecular switch. A desirable substitution (with hydrogen) that gives lager ON/OFF ratio and higher thermal stability is proposed for these isomeric systems. Finally, we found that the electrode distance has huge impact on the electron transport properties, as well as the switching performance, of these junctions, which nicely explains some puzzling experimental observations.