Micro-orientation control of silicon polymer thin films on graphite surfaces modified by heteroatom doping

Significance 

Recently, organic electronic devices have gained considerable interests due to their wide range of applications creditable to the huge variety of constituent organic molecules. Such diverse applicability can be attributed to versatility derived from their lightness, ease of enlargement and softness. Unfortunately, the miniaturization of orientation structure remains a challenge for achieving the large-scale integration and functional improvement of organic electronic devices. Scholars have attempted to control the orientation of organic thin films utilizing various technique best suited for formation of homogeneous films with lying or standing orientations on a substrate. Unluckily, such attempts have proven futile since the orientations are difficult to miniaturize due to the influence of homogeneous external fields. Directed self-assembly is a promising technique proposed for achieving both orientation control and micro patterning for block co-polymer thin films, however, at the expense of several drawbacks. With time, it has been observed that if a carbon atom of graphene is substituted for another heteroatom, the absorption of small molecules depends strongly on the dopant atom. Inspired by this, we proposed that heteroatom doping may be an efficient means of controlling the adsorption properties of polymer molecules.

Atomic Energy Agency researchers in Japan, proposed a study on the orientation change of polydimethylsilane (PDMS) thin films deposited on highly oriented pyrolytic graphite (HOPG) substrates with and without surface modification by a N2+ ion beam using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. They hoped to clarify the orientations of the PDMS films on the non-irradiated, N2+-irradiated and Ar+-irradiated substrates respectively. They aimed at developing a novel micro-orientation control technique for various organic thin films. Their research work is now published in the journal, Applied Surface Science.

The researchers commenced their empirical work by preparing HOPG samples and performing ion beam irradiation using cold-cathode ion gun. The research team then employed silicon K-edge NEXAFS so as to clarify the electronic structures and orientations of PDMS thin films because of element specificity due to silicon 1s core excitation and polarization dependence of X-rays. They also studied the orientation of a PDMS thin film on the highly oriented pyrolytic graphite substrate irradiated by Ar+ ions to compare the effect of the radiation damage induced by the ion beam.

Using first-principles calculations, Iwao Shimoyama and colleagues were able to interpret the NEXAFS spectra theoretically. Consequently, they clarified that PDMS films have lying, standing, and random orientations on the non-irradiated, N2+-irradiated, and Ar+-irradiated substrates, respectively. Furthermore, the research team noted that the photoemission electron microscopy indicated that the orientation of a PDMS film could be controlled with microstructures on the order of micrometers by separating irradiated and non-irradiated areas on the graphite surface.

Herein, the silicon K-edge NEXAFS spectra of PDMS thin films deposited on non-irradiated, Ar+-irradiated and N2+-irradiated highly oriented pyrolytic graphite substrates have been presented. The results of the three peaks observed in the NEXAFS spectra have been interpreted using first principles method. Clarification of the PDMS films orientations on the non-irradiated, Ar+-irradiated, and N2+-irradiated substrates have also been presented. It has been seen that, from the photoelectron emission microscopy measurements, micro-orientation control of polydimethylsiloxane on the order of micrometer is possible. Therefore, the technique: surface modification by heteroatom doping, can be developed in to a novel micro-orientation control technique for various organic thin films.

Reference

Iwao Shimoyama, Yuji Baba, Norie Hirao. Micro-orientation control of silicon polymer thin films on graphite surfaces modified by heteroatom doping. Applied Surface Science volume 405 (2017) pages 255–266.

 

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