Chain-Length Dependence of Optical Properties for an Alkanethiol Monolayer on an Ultrathin Gold Film Revealed via Reflected Light Microscopy
2021
Hattori, Yoshiaki | Takahashi, Hayato | Ikematsu, Naoki | Kitamura, Masatoshi
In this work, a monolayer of n-alkanethiol formed on an approximately 8 nm thick Au film, deposited onto a Si substrate with a thermally grown SiO₂ layer, was characterized using reflected light microscope with a narrow band-pass filter. The semitransparent, thin Au film, which is electrically conductive, provides interference in the films, and the optical properties of the monolayer depend on the alkanethiol length. When the surface of a Au film is photographed with a digital camera attached to the microscope, the color changes slightly with formation of a monolayer. The difference depends on the wavelength of a light source. Observations with narrow band-pass filters of 490 and 520 nm were found to provide a large difference, which is useful to characterize a monolayer. The difference increases with the length of the alkyl chains in the monolayer. Thus, the presented approach enables the fact that the number of the alkyl-chain units is identified through observations at an appropriate wavelength. This simple technique, which uses a general commercial microscope, can thus be used to confirm the presence of a monolayer with a molecular-order thickness.
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