Influence of van der Waals interactions on morphology and dynamics in ultrathin liquid films at silicon oxide interfaces

摘要

Single molecule tracer diffusion studies of evaporating (thinning) ultrathintetrakis-2-ethyl-hexoxysilane (TEHOS) films on silicon with 100 nm thermaloxide reveal a considerable slowdown of the molecular mobility within less than4 nm above the substrate (corresponding to a few molecular TEHOS layers). Thisis related to restricted mobility and structure formation of the liquid in thisregion, in agreement with information obtained from a long-time ellipsometricstudy of thinning TEHOS films on silicon substrates with 100 nm thermal or 2 nmnative oxide. Both show evidence for the formation of up to four layers.Additionally, on thermal oxide, a lateral flow of the liquid is observed, whilethe film on the native oxide forms an almost flat surface and shows negligibleflow. Thus, on the 2 nm native oxide the liquid mobility is even morerestricted in close vicinity to the substrate as compared to the 100 nm thermaloxide. In addition, we found a significantly smaller initial film thickness incase of the native oxide under similar dipcoating conditions. We ascribe thesedifferences to van der Waals interactions with the underlying silicon in caseof the native oxide, whereas the thermal oxide suffices to shield thoseinteractions.