Immobilization of Alkyl Chain Molecules on Oxide Surface Using Phosphonic Acid as an Anchor

• Published in: E-journal of surface science and nanotechnology Vol. 10; pp. 367 - 373
• Main Authors: Narita, Ayumi, Baba, Yuji, Sekiguchi, Tetsuhiro, Shimoyama, Iwao, Hirao, Norie, Yaita, Tsuyoshi
• Format: Journal Article
• Language: English
• Published: The Japan Society of Vacuum and Surface Science 2012
• Subjects: Aluminum oxide; Near edge extended X-ray absorption fine structure (NEXAFS); Phosphorus; X-ray photoelectron spectroscopy
• ISSN: 1348-0391; 1348-0391
• DOI: 10.1380/ejssnt.2012.367

For the purpose of formation of stable and uniform organic films on oxide surface, the alkyl chain molecules were immobilized on oxide surfaces using phosphoric acid as an anchor. The molecule investigated was decyl-phosphonic acid (DPA), which is composed of C10-alkane chain and one of the ends is terminated by phosphonic acid (PO3H2). DPA films are formed on sapphire surfaces by solution methods; immersing the substrate in DPA ethanol solution. The chemical-states of the interface between DPA molecules and the sapphire were investigated by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) using synchrotron soft X-rays. As to the P K-edge NEXAFS spectra of the DPA film on the sapphire C-plane, the resonance energy of the main peak was the same as that of the molecular DPA. This result indicates that the DPA molecules keep the molecular state. XPS spectra measured under total reflection condition shows that the phosphonic acid group in DPA molecules is located at the lower side, while the alkyl chain is oriented at the upper side on the surface. In order to confirm the thermal stability of the DPA film, the film was heated at 250°C. The peak intensity of the P K-edge NEXAFS spectra decreased, while the intensities of P 1s and C 1s in XPS spectra increased by heating. This discrepancy was explained by the difference of the detection depths between NEXAFS and XPS. Based on the simulation using a simple model, it was found that the DPA molecules form island structures at room temperature, while they become the uniform film after heating. As a result, it was concluded that the uniform and thermally stable films of alkyl chain molecules was formed on the sapphire surface, and phosphonic acid is an excellent anchor that combines alkyl molecules on oxide surface. [DOI: 10.1380/ejssnt.2012.367]