時間相關函數與第一原理分子動態模擬於表面系統的理論研究

The theoretical studies of time-correlation functions with the first principle molecular dynamics simulations on surfaces

周文祺(Wen-Chi Chou)

指導教授 ： 林志興

淡江大學/理學院/化學學系博士班/博士(2007年)

https://doi.org/10.6846/TKU.2007.00051

摘要

在這個博士研究論文中，我們的主要工作是開發一套IR光譜的模擬與解析工具，這套工具根據第一原理密度泛函理論(Density Functional Theory, DFT)分子動態模擬搭配虛位勢(pseudopotential)與LCAO基底函數來計算分子或半導體表面系統的分子動態軌跡與電荷分布動態，進而透過我們開發的電偶極矩自動相關函數(dipole moment auto-correlation function)與其傅立葉分析(Fourier analysis)成功地模擬分子與半導體Si(100)表面的紅外線振動光譜，最後搭配我們所設計的光譜解析技術來研究分子在半導體表面的振動動態與反應動態。這項技術首先成功地計算氣態有機分子的IR光譜，我們提出(I)結構座標自動相關函數(Structural Coordinate Autocorrelation Function, SCAF)，(II)部分電偶極矩自動相關函數解析(Partial Dipole-moment Autocorrelation Function, PaDAF)與(III)非等向性(Anisotropy) IR光譜解析方案使得模擬的IR光譜得以被解析成相關的振動模式。在成功確認光譜模擬工具與分子動態模擬的可行性之後，這套工具被應用到Si(100)表面、monohydride飽和的Si(100)表面、styrene吸附於Si(100)以及styrene一維分子線的IR光譜模擬上，光譜模擬結果與實驗間在定性上有相當好的一致性。此外，我們也分別針對(1)styrene在Si(100)表面上的[2+2] cycloaddition與(2)styrene在Si(100)表面上形成一維分子線的self-directed growth兩種重要表面反應進行在數個皮秒(picosecond)尺度下的反應動態與振動動態的研究，透過振動光譜與部分態密度(PDOS)的動態變化清楚地捕捉到吸附分子與表面的動態作用過程。此外，我們也計算了H-abstraction的反應能障、過渡態結構與PDOS變化，結果顯示styrene分子線的一維性質是導因於dimer row方向上具有兩種較低的H-abstraction反應能障途徑。最後我們也進行了電場對H-Si(100)系統的IR光譜影響以及電場於styrene-Si(100)-H系統所造成的電荷動態研究。結果顯示不同方向的垂直表面電場將導致H-Si振動頻率的紅位移與藍位移，另一方面我們也觀察到styrene-Si(100)-H系統在電場開啟後250飛秒(femtosecond)下的電荷重新分布動態與所伴隨的結構動態。

關鍵字

相關函數 ； 分子動態模擬 ； 密度泛函理論 ； 紅外線光譜 ； 振動動態 ； 反應動態 ； 態密度 ； 自由基傳遞 ； 分子線 ； 聲子光譜

並列摘要

We successfully developed a series of IR spectrum simulation and corresponding spectrum analysis methods for studying the vibrational dynamics and the reaction dynamics of both molecular and surface systems. The time-correlation function theories were thoroughly studied to build 1) the Fourier trnsformed dipole-moment AutoCorrelation Function(dpACF) for IR spectrum simulation, 2) the Partial Dipole-moment AutoCorrelatoin Function(PaDAF) and the Structural Coordinate AutoCorrelation Function(SCAF) schemes for spectrum analysis, and 3) the Fourier transformed anisotropic dpACF for the IR spectrum simulations on surface systems. By utilizing Density Functional Theory in connection with accurate LCAO basis sets and norm-conserving pseudopotentials, efficient Born-Oppenheimer molecular dynamics(BOMD) simulations would thereby be performed to study the reaction dynamics of adsorbed molecules on surfaes at microscopic level. First of all, our IR spectrum simulation methods were validated by calculating the IR spectra of a series of organic molecules. Based on the success of IR spectrum simulations on gas-phase molecules, furthermore, we employed this IR spectrum simulation and analysis scheme to Si(100) clean surface, monohydride saturated Si(100) surface, i.e. H-Si(100)-2x1 surface, adsorbed styrenes on Si(100) surfaces, and adsorbed one dimensional(1-D) styrene molecular wire on Si(100) surface. Spectrum simulation results for these surface systems are also qualitatively consistent with experimental measurements. In addition, we also successfully studied the reaction dynamics, Partial Density of States(PDOS) evolutions, and the vibrational dynamics within several picoseconds for two kinds of surface reactions of adsorbed styrene on Si(100) surface, say, (1) the [2+2] cycloaddition of styrene on Si(100), and (2) the self-directed growth of 1-D styrene molecular wire on Si(100) via the H-abstraction reaction. On the other hand, we also calculated the transition state structures and their corresponding PDOS variations, however, our results show that the one dimensionality of styrene molecular wire is due to two possible lower energy barrier pathways of H-abstraction along dimer row direction. Finally, we also studied the effects on both IR spectrum of H-Si(100) and the charge dynamics of styrene-Si(100)-H under external electric field. The results show that the H-Si vibrational frequency could be red-shifted or blue-shifted under the presence of external electric field along surface normal, depending on the direction of the electric field. Charge redistribution dynamics of styrene-Si(100)-H accompanying the structure relaxation within 250 femtoseconds were also observed after turning on the external electric field.

並列關鍵字

correlation function ； molecular dynamics ； Density Functional Theory ； IR spectrum ； vibrational dynamics ； reaction dynamics ； Density of States ； Si(100) ； styrene ； radical propagation ； self-directed growth ； molecular wire ； phonon spectrum ； H-abstraction ； SIESTA

參考文獻

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