Indentation and Self-Healing Mechanisms of a Self-Assembled Monolayer-A Combined Experimental and Modeling Study
Indentation and Self-Healing Mechanisms of a Self-Assembled Monolayer-A Combined Experimental and Modeling Study
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DOI：
10.1021/ja5048076
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发表时间：
2014-07-30
影响因子：
15
通讯作者：
Peukert, Wolfgang
中科院分区：
文献类型：
--
作者：
Meltzer, Christian;Paul, Jonas;Peukert, Wolfgang
结合原位振动和频产生（SFG）光谱和分子动力学（MD）模拟，使我们能够研究自组装十八烷基膦酸（ODPA）单分子膜的压痕α-Al 2 O 3（0001）的影响。应力引起的变化的振动签名的C-H伸缩振动SFG光谱和MD模拟的结果提供了明确的证据，在gauche缺陷密度的增加在单层作为一个响应压痕。应力相关分析表明，缺陷密度达到饱和在约155 MPa。应力释放后，MD模拟显示了几乎瞬时愈合的压力引起的缺陷与实验结果吻合良好。的接触区域的横向范围进行了研究与共定位SFG光谱和比较的压力梯度从赫兹接触理论的理论预测。SFG实验揭示了在接触中心附近的饱和之前，随着压力的增加，量规缺陷密度逐渐增加。此外，我们的MD模拟显示了ODPA域内的压力诱导效应的空间各向异性：在倾斜角增加的压力梯度的方向上倾斜的分子，而那些在相反侧形成gauche缺陷。
A combination of in situ vibrational sum-frequency generation (SFG) spectroscopy and molecular-dynamics (MD) simulations has allowed us to study the effects of indentation of self-assembled octadecylphosphonic acid (ODPA) monolayers on alpha-Al2O3(0001). Stress-induced changes in the vibrational signatures of C-H stretching vibrations in SFG spectra and the results of MD simulations provide clear evidence for an increase in gauche-defect density in the monolayer as a response to indentation. A stress-dependent analysis indicates that the defect density reaches saturation at approximately 155 MPa. After stress is released, the MD simulations show an almost instantaneous healing of pressure-induced defects in good agreement with experimental results. The lateral extent of the contact areas was studied with colocalized SFG spectroscopy and compared to theoretical predictions for pressure gradients from Hertzian contact theory. SFG experiments reveal a gradual increase in gauche-defect density with pressure before saturation close to the contact center. Furthermore, our MD simulations show a spatial anisotropy of pressure-induced effects within ODPA domains: molecules tilted in the direction of the pressure gradient increase in tilt angle while those on the opposite side form gauche-defects.