| Summary: | 博士 === 國立清華大學 === 化學系 === 96 === The theoretical studies on spectroscopic properties of the negatively charged diamond nitrogen-vacancy defect center, (NV)-, and related point defects presented in this report consists of three parts. The first part demonstrates the simulation of excitation and fluorescence spectra based on a symmetric double-well potential model constructed from a harmonic oscillator perturbed by a Gaussian-function barrier. Mathematical formulas were deduced to describe the vibronic transition involving the nitrogen-tunneling motion within this potential model, followed by numerical solutions to the Schroedinger equation that gave vibrational energy levels, wavefunctions, and Franck-Condon factors with consideration of overlap integrals. The model has been tested with the inversion vibrational mode of the ammonia molecule as well as the diamond (NV)- center, giving good agreements between experimental and simulated spectra.
The second part shows computational results of the diamond (NV)- center by ab initio and density functional theory calculations. Different-sized model clusters composed of 24 to 104 carbon atoms surrounding one nitrogen atom and one vacancy were constructed to imitate the local environment of the defect center. The structures were optimized using either HF or DFT algorithm, followed by TD-HF, TD-DFT, CIS, and CASSCF calculations to obtain excitation properties such as vertical excitation energies, one-photon and two-photon absorption cross sections corresponding to transitions from the ground to the first excited state. The dependences on computational methods and basis sets have been compared. While TD-DFT with the B3LYP functional and the 6-31G(d) basis set gave the most accurate prediction of the vertical excitation energy, TD-HF, CIS, and CASSCF provided suitable estimations on the absorption cross sections. The 6-31+G(d) basis set, however, always spoiled the results due to enhancements of electron density leakage from the defect center and interference from surface atoms. A large enough model is thus thought more important than a large basis set.
The last part concerns computational results of two other diamond defect centers related to nitrogen atoms and vacancy, the H3 (N-V-N) and the neutral (NV)0 centers. The excitation properties of the H3 center could be well reproduced analogously to the (NV)- case, giving satisfactory agreements to experimental values of vertical excitation energies by TD-DFT and one-photon absorption cross sections by TD-HF and CIS. For the (NV)0 center, on the other hand, TD-DFT could only characterize the first excited state while other states require CASSCF since they have the multiple-Slater-determinant character of electronic configurations. Again TD-DFT obtained agreement with the experimental vertical excitation energy; CASSCF overestimated it by size-limited models, but the very low one-photon absorption cross sections were predicted in consistent with experimental findings.
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