Reaction dynamics of molecular adsorptions onto silicon surface and their time-resolved vibrational spectra unveiled by ab-initio molecular dynamics and time-frequency analysis

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Title:	Reaction dynamics of molecular adsorptions onto silicon surface and their time-resolved vibrational spectra unveiled by ab-initio molecular dynamics and time-frequency analysis
Other Titles:	第一原理動態模擬矽表面分子吸附反應及其時間解析光譜
Authors:	李泳霆;Lee, Yung-Ting
Contributors:	淡江大學化學學系博士班林志興;Lin, Jyh-Shing
Keywords:	分子動態模擬;密度泛函理論;時頻分析;振動光譜;吸附;表面;Ab Initio Molecular Dynamics;Density Functional Theory;time frequency analysis;vibrational spectrum;adsorption;surface
Date:	2014
Issue Date:	2015-05-04 09:48:04 (UTC+8)
Abstract:	我們根據第一原理密度泛函理論(DFT)分子動態模擬搭配虛位勢與LCAO基底函數來模擬分子吸附到半導體表面系統的分子動態軌跡；並且，為了探討乙烯和甲醇吸附到矽(001)表面的化學反應，我們分別利用短時間傅利葉轉換和小波轉換來觀察分子的振動模式隨著反應進行的變化。分子動態模擬計算的結果顯示，乙烯吸附到Si(001)表面的非解離吸附反應路徑可分為間接吸附反應, 直接吸附反應,和repelling反應三種。首先，在間接吸附反應中，乙烯先利用π-bond與表面的buckled-down的矽原子作用，再透過π-bond的乙烯表面旋轉後在同一dimer上形成di-σ-bond的乙烯吸附結構。我們利用短時間傅立葉轉換(Short-time Fourier transform)分析隨著時間變化的C=C鍵振動模式，其時間解析光譜指出C=C雙鍵振動頻率在轉成di-σ-bond的吸附結構後變成C-C單鍵振動頻率，並與實驗光譜結果一致。另外，在直接吸附反應中，在48或150 K的模擬溫度下，乙烯可直接形成di-σ-bond的分子結構吸附在相同的dimer或兩個相鄰的dimer上。這可說明實驗上，在48及150 K溫度下，會有C-C單鍵振動頻率產生的原因。最後，repelling反應則是乙烯與表面產生無效碰撞，而導致分子離開表面。甲醇吸附到Si(001)表面的模擬解離吸附反應路徑可分為解離吸附反應和repelling反應二種，其中解離吸附反應為主要的反應路徑。在解離吸附反應中，甲醇先利用氧原子上的未共用電子對與表面buckled-down矽原子上的空懸鍵鍵結，再經由解離甲醇上的O-H鍵後形成吸附在表面上的methoxy與氫原子。同時，我們透過模擬紅外光譜，可以量測與實驗相符合的Si-H鍵的振動訊號。接著，我們使用小波轉換(Wavelet transform) 量測隨著反應進行的O-H鍵振動模式，透過時頻光譜的分析顯示當甲醇分子吸附到表面時，其O-H鍵振動頻率位移到3000~3400 cm-1的頻譜範圍。這主要是由於這個氫原子與表面上buckled-up矽原子的空懸鍵之間的吸引力會促使O-H鍵的拉伸，進而造成O-H鍵的斷裂後形成methoxy和氫原子吸附在表面上。 Density functional theory (DFT)-based molecular dynamic simulation in combination with localized basis sets and pseudopotentails is used to investigate the dynamic behaviors of molecules adsorption onto Si(001) surface. In particular, both short time Fourier transform (STFT) and wavelet transform (WT) are implemented to study evolution of molecular vibrational modes along the molecular adsorption process in order to unravel the elementary steps leading to ethylene adsorption and methanol adsorption onto the Si(001) surface. Based on the dynamic behaviors of ethylene adsorption onto Si(001) surface, three possible reaction pathways – the indirect adsorption, the direct adsorption, and the repelling reaction – have been found. Firstly, in the indirect adsorption, the ethylene (C2H4(ads)) forms the π-bonded C2H4(ads) with the buckled-down Si atom to adsorb on the Si(001) surface and then turns into the di-σ-bonded C2H4(ads). In addition, the time-resolved spectra constructed by STFT illustrates that the C=C stretching mode of the π-bonded C2H4(ads) shifts to the C-C stretching mode of di-σ-bonded C2H4(ads). Secondly, in the direct adsorption, the di-σ-bonded C2H4(ads) is formed directly with the Si intra-dimer or the Si inter-dimer. This reaction pathway leads to the C-C stretching mode and the C-H stretching mode of the di-σ-bonded C2H4(ads) appeared in the vibrational spectra at 48 K and 150 K, respectively. Finally, in the repelling reaction, the C2H4(g) first interacts with the Si dimer and then is repelled by Si atoms. Consequently, neither the π-bonded C2H4(ads) nor the di-σ-bonded C2H4(ads) is formed on the Si(001) surface. Regarding the dynamics behavior of methanol adsorption, the methanol (CH3OH(g)) firstly approaches the Si(001) surface to bond with the buckled-down Si atom within the temperature range between 100 K and 300 K, and then the O-H bond of CH3OH(ads) breaks within 10 ps only at 300 K due to the elongation of the O-H bond. Furthermore, the time-resolved vibrational spectrum constructed by WT illustrates that the O-H stretching mode of CH3OH(ads) shifts to below 3400 cm-1 when the H atom of the O-H bond is close to the buckled-up Si atom of the adjacent dimer. This analysis points out that the noticeable attraction force between the H atom of the O-H bond and the dangling bond at the buckled-up Si atom of the adjacent dimer prompts the O-H bond to break causing both CH3O and H species to adsorb on the buckled-down and buckled-up Si atoms, respectively.
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