Thermal Behavior of Self-assembled Stearic Acid Monolayers on Sapphire Surface

A stearic acid self-assembled monolayer was fabricated by solution deposition on single-crystal C-plane (0001) aluminum oxide (sapphire). Thermal stability was investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy(XPS), and contact angle(CA) measurements. The AFM results revealed that the surface coverage remains unchanged in air until the temperature reached ∼80°C, then decreases with further increasing of temperature and finally becomes zero at 220°C. In addition, varies of specific transition (phase, surface coverage rate and height difference) were found in the temperature range between 130oC and 140oC. Below the temperature range the desorption species are physically adsorbed stearic acid molecules, and above this temperature range, the chemisorbed stearic acid molecules would desorb and the left stearic acid SAM would melt. The complex bonding type is thought to play an important role in the thermal desorption.