Novel self-assembled films of La-based phosphonate 3-aminopropyltriethoxysilane

Silane coupling reagent (3-aminopropyltriethoxysilane (APTES)) was prepared on single-crystal silicon substrates to form two-dimensional self-assembled monolayer (SAM). The terminal –NH₂ groups in the film were in situ phosphorylated to –PO(OH)₂ group to endow the film with good chemisorption ability. Then La-based thin films were deposited on phosphorylated APTES-SAM in order to make good use of the chemisorption ability of –PO(OH)₂ groups. The thickness of the film was determined with ellipsometer, while phase transformation and surface morphology, surface energy, phase composition were analyzed by means of atomic force microscope (AFM), contact angle measurements and X-ray photoelectron spectroscopy (XPS). The results indicated that the terminal –NH₂ groups could be completely transformed into desirable –PO(OH)₂ groups after phosphorylation of APTES-SAM. Detailed XPS analysis of the La³⁺ peaks revealed that lanthanum element existed in the films in different states. As a result, conclusion could be made that lanthanum reacted with –PO(OH)₂ groups on the surface of the substrate by chemical bond which would improve the bonding strength between the film and silicon substrate. Since the La-based thin films were well adhered to the silicon substrate, it might find promising application in the surface-modification of single-crystal Si and SiC in microelectromechanical systems (MEMS).