Investigation of the Thermal Stability and Orthogonal Functionalization of Organophosphonate Self-Assembled Monolayers on the Si Substrate

Abstract

In this study, we investigated the thermal stabilities of butylphosphonic acid (BPA) and aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) on a Si substrate. The thermal desorption, thermal cleavage, and surface morphology of the BPA and APTES SAM film on the Si substrate were studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) upon thermal treatment from 150 °C to 500 °C. XPS analyses shows that the thermal desorption of the APTES monolayer occurs above 250 °C. However, AFM analyses revealed the formation of aggregated cluster in the APTES SAM film upon thermal treatment at 150°C for 1 h. As the annealing temperature increased from 150 °C–400 °C, the size of the APTES aggregates increased from 10 to 30 nm with a concurrent increase in surface roughness. In contrast, BPA SAM on Si first underwent bond cleavage at 150°C at the C2–C3 of the hydrocarbon chain, leaving a smooth film of ethylphosphonate SAM anchored onto the Si surface. At 350 °C, the P–O–Si bonding detached from the Si substrate, and the remaining organophosphonate SAM completely desorbed from the Si surface. A simple orthogonal functionalization method using thiolate SAM as a sacrificial layer was also demonstrated to overcome the spontaneous reaction of the organophosphonate film on the metal substrate.

Keywords: monolayer, thermal stability, morphology, surface roughness, organophosphorus compounds