Sang Lingzi, Mudalige Anoma, Sigdel Ajaya K, Giordano Anthony J, Marder Seth R, Berry Joseph J, Pemberton Jeanne E
†Department of Chemistry and Biochemistry University of Arizona, 1306 E. University Boulevard, Tucson, Arizona 85721, United States.
‡Department of Physics and Astronomy, University of Denver, Denver, Colorado 80208, United States.
Langmuir. 2015 May 26;31(20):5603-13. doi: 10.1021/acs.langmuir.5b00129. Epub 2015 May 12.
Self-assembled monolayers (SAMs) of phosphonic acids (PAs) on transparent conductive oxide (TCO) surfaces can facilitate improvement in TCO/organic semiconductor interface properties. When ordered PA SAMs are formed on oxide substrates, interface dipole and electronic structure are affected by the functional group properties, orientation, and binding modes of the modifiers. Choosing octylphosphonic acid (OPA), F13-octylphosphonic acid (F13OPA), pentafluorophenyl phosphonic acid (F5PPA), benzyl phosphonic acid (BnPA), and pentafluorobenzyl phosphonic acid (F5BnPA) as a representative group of modifiers, we report polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) of binding and molecular orientation on indium-doped zinc oxide (IZO) substrates. Considerable variability in molecular orientation and binding type is observed with changes in PA functional group. OPA exhibits partially disordered alkyl chains but on average the chain axis is tilted ∼57° from the surface normal. F13OPA tilts 26° with mostly tridentate binding. The F5PPA ring is tilted 23° from the surface normal with a mixture of bidentate and tridentate binding; the BnPA ring tilts 31° from normal with a mixture of bidentate and tridentate binding, and the F5BnPA ring tilts 58° from normal with a majority of bidentate with some tridenate binding. These trends are consistent with what has been observed previously for the effects of fluorination on orientation of phosphonic acid modifiers. These results from PM-IRRAS are correlated with recent results on similar systems from near-edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. Overall, these results indicate that both surface binding geometry and intermolecular interactions play important roles in dictating the orientation of PA modifiers on TCO surfaces. This work also establishes PM-IRRAS as a routine method for SAM orientation determination on complex oxide substrates.
在透明导电氧化物（TCO）表面上，膦酸（PA）的自组装单分子层（SAMs）有助于改善TCO/有机半导体界面性能。当在氧化物衬底上形成有序的PA SAMs时，界面偶极子和电子结构会受到改性剂的官能团性质、取向和结合模式的影响。我们选择辛基膦酸（OPA）、F13-辛基膦酸（F13OPA）、五氟苯基膦酸（F5PPA）、苄基膦酸（BnPA）和五氟苄基膦酸（F5BnPA）作为一组代表性的改性剂，报告了在掺铟氧化锌（IZO）衬底上结合和分子取向的偏振调制红外反射吸收光谱（PM-IRRAS）。随着PA官能团的变化，观察到分子取向和结合类型有相当大的差异。OPA表现出部分无序的烷基链，但平均而言，链轴与表面法线倾斜约57°。F13OPA倾斜26°，主要为三齿结合。F5PPA环与表面法线倾斜23°，存在双齿和三齿结合的混合；BnPA环与法线倾斜31°，存在双齿和三齿结合的混合，F5BnPA环与法线倾斜58°，大部分为双齿结合，有一些三齿结合。这些趋势与先前观察到的氟化对膦酸改性剂取向的影响一致。PM-IRRAS的这些结果与最近来自近边X射线吸收精细结构（NEXAFS）和密度泛函理论（DFT）计算的类似系统的结果相关。总体而言，这些结果表明，表面结合几何结构和分子间相互作用在决定PA改性剂在TCO表面的取向上都起着重要作用。这项工作还将PM-IRRAS确立为一种用于在复杂氧化物衬底上确定SAM取向的常规方法。
