Institute of Physics, Chemnitz University of Technology, Chemnitz, Germany.
Langmuir. 2013 Mar 19;29(11):3583-93. doi: 10.1021/la3043796. Epub 2013 Mar 6.
Single molecule tracer diffusion studies of evaporating (thinning) ultrathin tetrakis-2-ethyl-hexoxysilane (TEHOS) films on silicon with 100 nm thermal oxide reveal a considerable slowdown of the molecular mobility within less than 4 nm above the substrate (corresponding to a few molecular TEHOS layers). This is related to restricted mobility and structure formation of the liquid in this region, in agreement with information obtained from a long-time ellipsometric study of thinning TEHOS films on silicon substrates with 100 nm thermal or 2 nm native oxide. Both show evidence for the formation of up to four layers. Additionally, on thermal oxide, a lateral flow of the liquid is observed, while the film on the native oxide forms an almost flat surface and shows negligible flow. Thus, on the 2 nm native oxide the liquid mobility is even more restricted in close vicinity to the substrate as compared to the 100 nm thermal oxide. In addition, we found a significantly smaller initial film thickness in case of the native oxide under similar dipcoating conditions. We ascribe these differences to van der Waals interactions with the underlying silicon in case of the native oxide, whereas the thermal oxide suffices to shield those interactions.
利用单分子示踪扩散研究，我们发现蒸发（变薄）过程中，在距硅基底不到 4nm（对应于几个分子的 TEHOS 层）的范围内，热氧化层厚度为 100nm 的硅基底上的超薄四乙氧基硅烷（TEHOS）薄膜的分子迁移率显著降低。这与从热氧化层厚度为 100nm 或 2nm 的自然氧化层硅基底上的 TEHOS 薄膜减薄的长时间椭圆光度法研究获得的信息一致，这表明在该区域内液体的迁移率受到限制，结构形成受限。两种情况都表明形成了多达四层的结构。此外，在热氧化层上观察到液体的横向流动，而在自然氧化层上形成的薄膜几乎是平坦的，流动可以忽略不计。因此，与热氧化层相比，在距离基底更近的范围内，2nm 自然氧化层中液体的迁移率受到的限制更大。此外，我们发现，在类似的浸涂条件下，自然氧化层的初始薄膜厚度明显更小。我们将这些差异归因于自然氧化层与下面的硅之间的范德华相互作用，而热氧化层足以屏蔽这些相互作用。
