Observation of slowly decreasing molecular oscillations in ultrathin liquid films using X-ray reflectivity

Abstract

We studied similar to 0.5 mu m and 30-80 angstrom thick films of a normal dielectric liquid, tetrakis(2-ethylhexoxy) silane (TEHOS), at temperature range 228-286 K, deposited onto silicon ( 111) substrate with native oxide using X-ray reflectivity. TEHOS is spherical with size similar to 10 angstrom, non-polar, non-reactive, and non-entangling; TEHOS has been reported to show interfacial layering at room temperature and surface layering at 0.23 T-c (T-c approximate to 950 K). For. lms similar to 0.5 mu m thick, the reflectivity data did not change significantly as a function of temperature; for. lms 30-80 angstrom thick, the re. ectivity data did change. The data could be fitted with an electron density model composed of a minimum necessary number of Gaussians and a uniform density layer with error-function broadened interfaces. When the film thickness is 60-80 angstrom below 246 K, we found that the interface and the surface layering coexist but do not overlap. When the film thickness is 30-40 angstrom below 277 K, they overlap and the electron density pro. le shows slowly decreasing molecular oscillations at the air-liquid interface.