Self-Assembled Monolayers Directly Attached to Silicon Substrates Formed from 1-Hexadecene by Thermal, Ultraviolet, and Visible Light Activation Methods

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Using 1-hexadecene as a precursor, self-assembled monolayers (SAMs) were fabricated on hydrogen-terminated silicon (111) [H-Si(111)] surfaces without forming an interfacial oxide layer on the basis of thermal (180 °C, 2 h), ultraviolet (UV; 500 mW cm-2, 10 h), and visible-light activation (330 mW cm-2, 16 h) processes. As characterized by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and ellipsometry, the hexadecyl SAM fabricated by the visible-light process had a highly-ordered molecular arrangement and a closely-packed methyl-terminated surface similar to the SAMs prepared by the thermal and UV activation processes. The photo-irradiation wavelength dependence of the visible-light activation process was further studied at irradiation wavelengths of 400, 550, and 700 nm. The SAM formation reaction was certainly promoted at all the wavelengths, even at 700 nm. However, oxidation of the Si surface became apparent due to the slow rate in SAM growth, and thus the monolayer coverage of SAM at 700 nm became smaller. The reaction rate became faster with the decreasing wavelength for activation, probably due to the increase in the light adsorption coefficient of Si. The excitation of Si, namely, the generation of hole/electron pairs at the Si substrate, is assumed to be the rate-controlling step of the visible-light activation process.