順反式有機分子在金屬覆蓋的鍺(111)單晶表面上的吸附及熱脫附反應

The Adsorption and Thermal Desorption of Cis-Trans Organic Molecules on Metal Covered Ge(111)-(√3×√3)R30° Surfaces

簡廷達(Tyng-Dar Chien)

指導教授 ： 蘇昭瑾

國立臺北科技大學/工程學院/有機高分子研究所/碩士(2005年)

摘要

本論文分兩部分，第一部份研究偶氮苯分子在Ag/Ge(111)-(√3×√3)R30°表面上的吸附及熱脫附反應，在超高真空(UHV)系統中，結合歐傑電子能譜儀(AES)、低能量電子繞射儀(LEED)、程式控溫熱脫附法(TPD)等表面分析技術，鑑定Ag單層覆蓋結構、表面週期性、偶氮苯吸附量之變化及該吸附分子與Ag/Ge(111)-√3表面間的作用力。氮苯分子的表面吸附溫度為100 K，由程式控溫熱脫附法追蹤偶氮苯的TPD圖譜發現低曝氣量時(<0.6 L)只有單一脫附峰出現，最大脫附速率溫度(peak temperature, Tp)為250 K。隨著覆蓋度的增加(至12 L)，脫附溫度亦往高溫位移，並維持單一脫附峰行式，即使再高曝氣量(至30 L)也只有一個脫附峰出現。推論在加熱過程中由於偶氮苯分子間的作用力比較強，多層(multilayer)吸附物由於分子間作用力影響不直接從多層脫附，反而趨向聚集(aggregate)成3D cluster，因為3D cluster的分子較穩定且兩者脫附溫度可能接近，它寧願先聚集成3D cluster，再從3D cluster脫離表面，我們對其TPD數據做脫附動力學零級(zeroth-order)配套模擬，得到脫附活化能Ea=18.84 kcal/mol、前指數因子ν= 5.3×10 14 s-1。
第二部份是研究順式二苯乙烯(CSB)在Au/Ge(111)-(√3×√3)R30°表面上的吸附及脫附情形。我們也嘗試利用LEED觀察二苯乙烯吸附在(√3×√3)R30°表面的規則排列，並且成功看到(2×1)的LEED繞射圖譜，另外我們也架設一組氦氖雷射來測定表面上吸附物的光學反射率變化(optical reflectivity change)，在金屬覆蓋的鍺(111)單晶表面上，確定Au覆蓋量不同會造成反射的變化。
在程溫脫附法追蹤CSB有兩個脫附峰出現在243及274 K、分別代表多層吸附物(multilayer)及單層吸附物(monolayer)，在脫附的過程中沒有其他熱反應產物被偵測到。比較CSB在Au/Ge(111)-√3或Ag/Ge(111)-√3表面上熱脫附之TPD圖譜，發現CSB在兩者表面的脫附溫度差異不大，表示CSB在表面結構吸附位置和作用力差不多。

關鍵字

超高真空 ； 低能電子繞射 ； 歐傑電子能譜 ； 程序控溫反應/脫附 ； 鍺(111)單晶 ； 有機分子 ； 吸附 ； 自組裝 ； 光異構化反應

並列摘要

This thesis is composed of two major parts. In the first part, the adsorption and thermal desorption of azobenzene on Ag/Ge(111)-(√3×√3)R30° (Ag/Ge(111)-√3) has been studied using combinative techniques of temperature programmed desorption (TPD) for azobenzene desorption kinetics, Auger electron spectroscopy (AES) for Ag coverage determination, and low energy electron diffraction (LEED) for surface structure characterization under ultra-high vacuum (UHV) condition. The Ag/Ge(111)-√3 surface was prepared by thermal deposition of one monolayer of Ag onto Ge(111) single crystal surface at 100 K followed by annealing at 700 K for 1 min. The resultant Ag/Ge(111) surface showed a sharp (√3×√3)R30° LEED pattern. For all the azobenzene exposures (0.6 L to 30 L) which have been studied, the TPD traces show only molecular desorption with single desorption peak at peak temperature (Tp)=250 K. The Tp shifts constantly from 250 to 267 K with increase of azobenzene exposure from 0.6 to 30 L. The leading edge shows excellent match for all the acquired TPD curves. Those results strongly suggest the zeroth-order kinetics for azobenzene desorption from Ag/Ge(111)-√3. Such behavior can be explained by the thermal-induced pre-aggregation of azobenzene molecules to form 3D cluster on Ag/Ge(111)-√3 before desorption. The zeroth-order desorption kinetics was also confirmed by the leading-edge analyses of TPD results. The plausible explanation in terms of molecular-molecular interaction and molecular-substrate interaction is discussed in this thesis.
In the second part of this thesis, the adsorption and thermal desorption of cis-stilbene (CSB) on Au/Ge(111)-(√3×√3)R30° (Au/Ge(111)-√3) surface has been studied using the same approach. One monolayer of CSB deposited on Au/Ge(111)-√3 at 220 K followed by cooling to 100 K leads to the formation of a well ordered (2×1) structure as observed by LEED. The TPD spectra for CSB thermal desorption from Au/Ge(111)-√3 exhibit showed two well-resolved peaks at Tp = 243 and 274 K corresponding to multilayer and monolayer desorption, respectively. Only molecular desorption was detected during the heating process. Interestingly that the Tp for both multilayer and monolayer desorption peaks continually increase upon increase of CSB exposure and show unsaturated feature even up to 30 L. Similar behavior was also found for CSB desorption on the Ag/Ge(111)-√3 surface. Comparison between these two systems and possible explanation for the desorption mechanism is discussed in this thesis.

並列關鍵字

UHV ； LEED ； AES ； TPR/D ； Ge(111) ； organic molecule ； adsorption ； self-assembly ； photoisomerization

參考文獻

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