Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films.
Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films.
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DOI：
10.1021/la036033k
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发表时间：
2004-03
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影响因子：
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通讯作者：
Zhijun Hu;Haiying Huang;Fajun Zhang;B. Du;T. He
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文献类型：
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作者：
Zhijun Hu;Haiying Huang;Fajun Zhang;B. Du;T. He
用透射电镜、广角X射线衍射、原子力显微镜和紫外吸收光谱研究了硅衬底上聚二正己基硅烷（PDHS）薄膜（厚度< 100 nm）的分子链和片晶取向。PDHS表现出膜厚依赖性的分子链和层状晶体取向。在PDHS的单层薄膜中，层状晶体优先以平贴取向生长，即，硅骨架沿着表面法线方向取向。相比之下，层状晶体的取向优先侧上的薄膜厚于约。13 nm，即，硅骨架平行于衬底表面取向。我们解释了分子链和层状晶体的不同取向分别是由于基底表面附近聚合物链的熵降低和平面层状晶体的晶体学（001）面的特殊性。由于σ电子沿硅骨架沿着离域的一维性质，硅骨架的取向对PDHS薄膜的紫外吸收有显著影响.当硅骨架垂直或平行于衬底表面时，紫外吸收率分别随入射方向与薄膜法线方向夹角的增大而增大或减小。
The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.