THERMAL-STABILITY OF SELF-ASSEMBLED MONOLAYERS FROM ALKYLCHLOROSILANES

Contact angle measurements and scanning force microscopy (SFM) have be en used to study the morphology of self-assembled monolayers of octade cyltrichlorosilane (OTS) and undecyltrichlorosilane (UTS) on silicon o xide surfaces as a function of annealing temperature. Characterization of the monolayers before annealing indicates that the as-formed monol ayers cover the substrate fully and have thicknesses of 2.9 and 1.5 nm , respectively, with the average surface having a roughness of approxi mately 0.3 nm. Water contact angle results and SFM roughness analysis showed that UTS monolayers annealed for 2 h at temperatures over 125 d egrees C exhibited permanent changes in monolayer structure. Experimen tal data indicated a higher transition temperature for permanent struc tural changes for the OTS monolayers. Water contact angle measurements indicated a transition centered at 125 degrees C, hexadecane contact angle measurements indicated a transition around 130 degrees C, and SF M indicated a transition centered at 155 degrees C for OTS monolayers. We have observed that contact angle measurements can be sensitive to surface structure on an angstrom length scale. These results observed after 2 h are not equilibrium results, as evidenced by the further dev elopment in the monolayer after an anneal for 5 h. Thermodynamic consi derations support the earlier (lower temperature) changes observed for UTS than for OTS monolayers. The observed transition is likely the re sult of hydrolysis of the organic molecules by water molecules existin g at the monolayer-substrate interface.