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- Song, Ce, 1980-
(författare)
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Theoretical Study of Vibronic Spectra of Molecule Systems Generated by Photo- and Electronic Excitations
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Spectra represent fingerprints of molecules, which contain unique information about their properties. Through analyzing the spectral data, one can reveal the molecules' energy level alignments, identify their species and geometric structures, and explore relevant chemical processes and microscopic mechanisms. Currently, spectroscopy is one of the main means for human beings to enter the mysterious world of molecules and hear their stories. However, interpreting molecular spectra is not a straightforward process, because the occurrence of spectra involves complex interactions between molecules and external stimuli. Theoretical simulations based on quantum chemistry play an indispensable role in this regard, which makes developing and applying related computational software become very important.This thesis focuses on the theoretical simulations of two types of molecular spectra, namely the vibrationally resolved optical spectra and the inelastic electron tunneling spectra (IETS). The former involves the transitions of electrons between a molecule's ground state and its excited states with the involvement of molecular vibrations, and the latter comes from the excitations of a molecule's vibrational states within its electronic ground state by inelastic tunneling electrons across a molecular junction. By performing time-dependent density functional theory calculations as well as applying the DynaVib code, I have systematically investigated the optical absorption properties of two types of functional molecules, i.e., naphthalenediimide cyclophane (NDIC) derivatives and fused porphyrin derivatives, which have been proposed as building blocks for future single-molecule optoelectronic devices. Based on the Franck-Condon (FC) principle, the simulations well explain the energy shifts induced by chemical substitutions in the first intense absorption bands of the three NDIC derivatives, and nicely reproduce the vibrational features of their first two bands. Furthermore, by using three different exchange-correlation functionals (i.e., the conventional functional B3LYP and two long-range corrected functionals CAM-B3LYP and wB97XD), it is found that long-range corrections are very important for the description of the spectral features owing to the strong charge transfer in the related excited states. By taking into account both the FC and the non-FC Herzberg-Teller (HT) contributions, the experimentally measured electroluminescence spectrum of a single fused 5,15-(diphenyl)-10,20-(dibromo)porphyrin (fused-H2P) molecule is nicely reproduced by the simulations. It is found that the FC contribution also dominates the emission of the molecule, while the HT terms mainly contribute to the low-energy tail of the spectrum. The vibrational fine structures as observed in the experiments are unambiguously assigned based on the simulation results. In terms of the development of computational software, I have developed a Windows version for the QCME package - an efficient package to perform first principles calculations of electron transport through molecules such as simulating the IETS. The implementation has been achieved by using the C# language and the Windows Presentation Foundation (WPF) user interface framework. The Windows version of QCME exhibits compatibility, stability, scalability, and strong operability. It has a beautiful interface, is easy to learn and to use, and has improved human-computer interactions. Such an approach of the implementation can be also extended to other quantum chemistry packages.
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| 2. |
- Xie, Zhen
(författare)
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High Resolution Tip-Enhanced Raman Images of Single Molecules from First Principles Simulations
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the precise control of spatially confined plasmon (SCP), tip-enhanced Raman spectroscopy (TERS) has achieved sub-nanometer resolution, leading to the chemical and physical characterization of the single molecule by optical Raman images. In the high resolution TERS measurements, the SCP spatial distribution generates the position-dependent Raman images. The position dependence challenges the conventional response theory, because the assumption of interactions between the molecule and the uniform electromagnetic field does not hold anymore. Moreover, as an emerging technology, potential applications of high resolution TERS are required to be fully explored. In this thesis, the developed theory for modeling high resolution Raman images is presented. By taking a series of typical molecular systems as examples, we theoretically predict some fine applications of single-molecule TERS.The first part of the thesis introduces the development of Raman spectroscopy and images. To achieve the final target of single molecule characterization, high spatial resolution single-molecule TERS is established and improved. As a nondestructive measuring tool, Raman imaging technology offers the means to study single molecules with unprecedented spatial resolution.The high resolution Raman images theory with detailed derivations is given in the second part of the thesis. The key factor is to take the inhomogeneous spatial distribution of SCP field into account, when we construct the interaction Hamiltonian between the localized light field and the molecule. This makes the numerical simulations of Raman images feasible.Other parts of the thesis give some theoretical predictions for potential applications of the emerging Raman imaging technology. Specifically, resonance Raman images can visualize the geometric changes of a single molecule switch and the intramolecular structure in real space. Since the localized plasmonic field can affect the electron transition, the excited quantum states can thus be effectively manipulated. This breaks down the intrinsic spatial selection rule imposed in conventional spectra. In addition, an effective linear response algorithm is used to simulate nonresonance Raman images. The unique superiority of spatial vibration resolution from non-resonance cases provides rich information about the single molecule. By constructing images from different vibrational modes, the spatial chemical distribution within a single molecule can be visualized. All these findings will facilitate fine applications of the emerging TERS technology in the coming years.
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| 3. |
- Cheng, Xiao
(författare)
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Multiplet computation methods for core level X-ray spectroscopy of transition metal and rare earth elements
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the development of new generation synchrotron facilities, the performances of various X-ray spectroscopies have become more advanced. In order to interpret the X-ray spectrum experiments of various novel materials related to transition metal and rare earth elements, new advanced theoretical methods are required. The present thesis incorporates four modus operandi based on the classic multiplet theory to study the core level X-ray spectroscopy of transition metal and rare earth element. The four approaches consist of new methods developed from classic multiplet approach to high level first-principles method assisted multiplet calculation. Some methods are selected from previous researches and some are invented by original researches. These methods are integrated together to form a complete set of multiplet computational methods. This set of multiplet computational methods can perform calculations on various X-ray spectroscopies such as XAS, XPS, XES and RIXS related to the core-level electron. These wide range of spectroscopic methods coupled to different multiplet theory approaches serve as efficient tools to understand the electronic structure of metal sites and their unique contribution to the physical/chemical properties of the materials.The thesis creatively improves the classic multiplet theory on several aspects: (1) the relation between crystal field parameters and local structure factors; (2) the difficulty of processing point group symmetry branching chain in low symmetric structure; (3) the first-principles calculation of semi-empirical parameters. Four modus operandi are presented in this thesis: the first is the classic multiplet theory consisting of the multiplet effect, crystal field effect and charge transfer effect via several semi-empirical parameters as description for these effects. The second level multiplet theoretical approach analyze the crystal field potential matrix in various symmetries according to the point group symmetry branching rules. Then the crystal field effect parameters used in classic multiplet theory are linked analytically to the specific structural factors such as bond length and angles. This approach is a good tool to study the structural distortion from higher to lower order symmetry with analysis of X-ray spectral feature changes in experiment. The third modus operandi adopts large cluster model consisting of point charges at equivalent atoms position to simulate the crystal field effect on the center metal site. This approach handles low order symmetric crystal field with long range effect in multiplet calculation in an easier way than the classic multiplet theory. The fourth modus operandi initially studies the system of interest in first-principles calculation for the electronic wavefunctions. Then the electronic wavefunctions are used to derive the maximally localized Wannier functions at metal/ligand sites. The analysis of these Wannier functions provide a lot of semi-empirical parameters required in the classic multiplet calculation approach in a first-principles way. This modus operandi has substantially resolved the problem of finding the best set of semi-empirical parameters to fit the calculated X-ray spectrum with experimental data.In order to study the core electrons of the light elements (such as C/N/O) around center metal ions, a theoretical calculation method used to study the core electrons' vibrationally-resolved X-ray spectroscopy is also introduced as a complementary research and applied to C1s core ionized XPS calculation as an example.
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