Preparation and characterization of lanthanum-based thin films on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Abstract

Silane coupling regent (3-mercaptopropyl trimethoxysilane (MPTS)) is prepared on a single-crystal silicon substrate to form two-dimensional self-assembled monolayer (SAM). The terminal thiol groups (–SH) in the film is oxidized to sulfonic acid groups (–SO3H) in situ to enhance the chemisorption property of the film. Lanthanum-based thin films are deposited on the oxidized MPTS-SAM by means of chemisorption with the –SO3H group. The surface energy, chemical composition, phase transformation and surface morphology of the films are analyzed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy. The results show that MPTS is a self-assembled layer on the substrate and that the terminal –SH groups of the self-assembled MPTS films are closer to the air/silane interface than the silicon atoms. The study also shows that the treatment of SAM with 30% nitric acid (HNO3) at 80 °C in sufficient duration can completely transform the terminal groups in the top-most layer into desirable sulfonate groups. Then the substrates coated with the sulfonated MPTS-SAM were immersed in the RE solution to form lanthanum-based thin films. The experimental results indicate that the lanthanum-based thin films are adsorbed on oxidized MPTS-SAM and that the lanthanum element with different states of oxidation exists in the thin films that are deposited on the surface of self-assembly monolayers. Also, shape analyses of the La3+ XPS peaks reveal that the rare-earth film can react with the substrate by chemical bonding and that some lanthanum molecules are adsorbed on the MPTS-SAM. The above results show that it is the –SO3H group that has good chemisorption trend.