自組裝長鏈醋酸分子薄膜於θ-氧化鋁上的成長行為; The Growth of Self-Assembled Docosanoic Acid Monolayers on θ-Al2O3 / NiAl(100)

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题名:	自組裝長鏈醋酸分子薄膜於θ-氧化鋁上的成長行為;The Growth of Self-Assembled Docosanoic Acid Monolayers on θ-Al2O3 / NiAl(100)
作者:	李建緯;Chien-wei Lee
贡献者:	物理研究所
关键词:	反射式高能電子繞射;超高真空;氧化鋁;原子力顯微鏡;自組裝分子膜;Al2O3;AFM;SAMs;RHEED;UHV
日期:	2011-06-16
上传时间:	2012-01-05 14:41:01 (UTC+8)
摘要:	我們使用原子力顯微鏡 (Atomic Force Microscopy, AFM) 觀察 docosanoic acid (CH3(CH2)20COOH) 自組裝分子薄膜 (Self-Assembled Monolayers, SAMs) 在θ-Al2O3/NiAl(100) 表面上的成長樣貌及穩定性。在薄膜成長初期，吸附在樣品表面的分子首先會聚核成分子島嶼增加樣品表面的分子島密度。隨著吸附的持續進行，這些分子島的高度會由一開始的 1 – 2 奈米增加為 2 – 3 奈米。這告訴我們，較密堆積的分子島於表面形成，因此由原子力探針所導致的形變或傾斜便不再明顯。最後，個別的分子島開始彼此癒合，降低樣品表面的分子島密度。藉由40 – 70 ℃ 的溫度將樣品作退火處理，以及把樣品放置在溶液中測試此分子薄膜的穩定性。經過退火處理後，表面的雙層分子結構島嶼 (bilayer islands) 的密度會減少；但是單層分子結構的島嶼不會。這是因為在不同介面的吸附力不同所導致：第一層的分子是利用化學鍵和氧化層鍵結;雙層分子中的上層分子及吸附在單層分子島上的雙層分子島是藉由物理吸附。我們也發現原本散亂分布在表面的單層分子島，在經過退火後，會出現沿著氧化條紋排列的現象。這是因為熱能促進了分子在表面的移動能力。在持續搖動的酒精溶液內，大部分的單層薄膜結構脫附但單層分子島只受到些微的影響，這代表著此兩種結構顯示截然不同的穩定性。然而在水溶液裡面，吸附在樣品表面的單層分子薄膜並不穩定，它們會在水溶液中持續的脫附。這脫附的行為，是由於分子尾端的厭水程度與前端之於氧化層親和力競爭後的結果。論文的最後，我們觀察到一個特殊的現象：表面吸附雙層分子島的樣品，經由退火處理後，會程現出由單層分子島所排列而成的封閉線圈圖案。這些線的高度主要分佈在 2 – 3 奈米間，平均寬度則在 50 – 90 奈米區間 (約為2 – 3 顆發生癒合前的分子島直徑)。 The formation of docosanoic acid (CH3(CH2)20COOH) self-assembled monolayers (SAMs) and their stability were investigated by atomic force microscopy (AFM). The growth begins with the formation of single-layer islands by exposing the sample to solution of docosanoic acid. In an initial growth stage, new islands form through nucleation and the island density increases. As adsorption continues, the heights of nucleated islands (single-layer islands) increase from 1 – 2 nm to 2 – 3 nm. This observation suggests that the densely packed islands form, so the deformation or tilting induced by AFM tip is not apparently. Finally, the individual islands coalesce decreasing the island density. We annealed the sample to the temperature ranging from 40 – 70 ℃ for varied times and placed the sample in solution to measure the stability of the docosanoic acid SAMs. In the annealing experiments, the density of bilayer islands decreased dramatically after the annealing, but the single-layer islands do not desorb. This difference is attributed to the different bonding strength at the interface: the first layer molecules are chemically bonded to the oxide; the upper layer in the bilayer islands and the bilayer islands adsorbed on the single islands are physically bonded to the lower molecular layer. The single-layer islands were randomly distributed on the surface at the beginning, and became aligned along the oxide strips after the annealing - the annealing facilitates the diffusion of the islands on the surface. In the agitated alcohol solution most single-layer films on the surface desorb but the single-layer islands are less affected, indicating disparate stabilities for these two structures. In addition, the docosanoic acid SAMs are not stable in water; they desorb continuously in water. We attribute this desorption kinetics to the competition of the hydrophobic interactions of tails and affinities between head groups and surface. Finally, we demonstrate an atypical behavior of this molecule: after annealing the bilayer islands, they formed great closed-loop lines consisting of single-layer islands. The height of these lines ranges primarily from 2 – 3 nm, and the average width is in the range of 50 – 90 nm, 2 – 3 times the diameter of a single-layer island prior to coalescence.
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