Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films
Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films
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DOI：
10.1021/la048069y
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发表时间：
2005-03-15
期刊：
影响因子：
3.9
通讯作者：
Moeller, M
中科院分区：
文献类型：
--
作者：
Mourran, A;Tartsch, B;Moeller, M
二十烷基全氟十四烷，F（CF 2）（14）（CH 2）（20）H，在云母，氧化硅，或水的单分子膜上的扫描力显微镜显示自发组织，定义良好的纳米级带状和螺旋或环形超结构。形成的是带状物还是纳米螺旋取决于浇铸分子单层膜的溶剂。当使用烃或全氟化碳溶剂时观察到条带，萘烷或全氟萘烷。然而，当化合物从非选择性的六氟二甲苯沉积时，分子组装成确定尺寸的螺旋。当暴露于十氢萘或全氟十氢萘蒸气时，螺旋/环形转变为带状，并且当暴露于六氟二甲苯蒸气时，带状转变为环形。这些变化可以在现场观察到。扫描力显微镜产生了相同的高度和宽度的带形成的螺旋和平行的扁平带。X射线反射率产生3.61 +/-0.05nm的高度，同样对于两种形态相同。然而，延伸的F（CF 2）（14）（CH 2）（20）H分子的长度，即，4.65 nm，超过从X射线反射计获得的层厚度。然而，这与其中氟化链垂直于表面层取向并且其中烷基链段在两个链段之间以122度角倾斜的布置一致。在由倾斜限定的平面内，该角度允许烷基链段的密集堆积，以补偿氟碳链段的较大横截面。倾斜平面定义了单层结构可以保持有序的“易”方向。在垂直于该轴的平面中，烷基链的长程有序致密堆积是不可能的。原则上，不可压缩的堆积可以解释带状物的有限而规则的宽度以及螺旋中的阶梯式转弯。
Scanning force microscopy on monomolecular films of eicosylperfluorotetradecane, F(CF2)(14)(CH2)(20)H, on mica, silicon oxide, or water revealed spontaneous organization to well-defined nanoscopic ribbon and spiral or toroidal superstructures. Whether ribbons or nanospirals were formed depended on the solvent from which the molecular monofilm was cast. Ribbons were observed when a hydrocarbon or a perfluorocarbon solvent was used, e.g., decalin or perfluorodecalin. When the compound, however, was deposited from nonselective hexafluoroxylene, the molecules assembled into spirals of defined size. The spirals/toroids transformed to ribbons when exposed either to decalin or perfluorodecalin vapor, and the ribbons transformed to toroids when exposed to hexafluoroxylene vapor. These changes could be observed in situ. Scanning force microscopy yielded an identical height and width for the bands forming the spirals and for the parallel flat ribbons. X-ray reflectivity yielded a height of 3.61 +/- 0.05 nm, again identical for both morphologies. Yet, the length of the extended F(CF2)(14)(CH2)(20)H molecule, i.e., 4.65 nm, exceeds the layer thickness obtained from X-ray reflectometry. It is, however, consistent with an arrangement where the fluorinated chains are oriented normal to the surface layer and where the alkyl segments are tilted with a 122 degrees angle between the two segments. Within the plane defined by the tilt, this angle allows a dense packing of the alkyl segments compensating for the larger cross-section of the fluorocarbon segment. The tilt plane defines an "easy" direction along which the monolayer structure can preserve order. In the plane perpendicular to this axis, long-range ordered dense packing of the alkyl chains is not possible. Incommensurable packing can in principle explain the finite and regular width of the ribbons and the stepwise turn in the spirals.