Elsevier

Applied Surface Science

Volumes 144–145, April 1999, Pages 445-450
Applied Surface Science

Formation and evaluation of self-assembled monolayers derived from conjugated silylthiophene derivatives

https://doi.org/10.1016/S0169-4332(98)00839-3Get rights and content

Abstract

We synthesized soluble rod-like silylthiophene, silylbithiophene, and silylquaterthiophene derivatives with various functional groups. Their self-assembled monolayer films on silicon oxide were evaluated by atomic force microscopy, X-ray photoelectron spectroscopy, water contact angle measurements, and fourier transform infrared-reflection absorption spectroscopy. We found that the molecular density (molecules/area) of the resultant films increased with thiophene moiety. This is probably due to the effective π-stacking of aromatic rings between molecules.

Introduction

Highly ordered conjugated molecules on semiconductor are very important for fabricating molecular devices such as metal–oxide–semiconductor (MOS) structures. Self-assembling is one of the convenient techniques for the formation of homogeneous organic monolayers through the interaction between the head group of the molecule and the substrate. Oligothiophenes have been used as active materials in electronic devices such as field-effect transistors (FET) 1, 2, 3, 4, 5, 6and light-emitting diodes 7, 8, 9, 10, 11, 12. Especially, Garnier et al. reported that vacuum-evaporated quaterthiophene and quinquethiophene thin films showed a good conductivity and they made an FET structure using theses compounds [13]. For the formation of self-assembled monolayer (SAM) film of oligothiophenes, the solubility of the molecule is crucial. The conductivity increases with the number of the thiophene moiety, but the solubility into the solvents decreases. For fabricating conductive SAM films, we have to increase the solubility as well. The solubility of the corresponding compounds can be improved by introducing organosilyl group to the α-position of thiophene ring. We report herein the SAM formation of various substituted chlorodisilanyloligothiophene derivatives on silicon oxide/semiconductor Si and the evaluation of the resulting SAM films using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), water contact angle (WCA), and fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS).

Section snippets

Synthesis of silylthiophene derivatives

A typical synthetic method of the silylthiophene derivative ST 3 is shown in Scheme. 1. Phenylethynylbithiophene was prepared by the coupling reaction of the phenylethynylzinc chloride with 5-iodo-2,2′bithiophene in the presence of 1 mol% of Pd(PPh3)4 (Eqs. 1 and 2 in Scheme. 1). The resulting bithiophene derivative was lithiated by the hexane solution of n-butyllithium and added to the THF solution of 1,2-dichloro-1,1,2,2-tetramethyldisilane to afford the corresponding silylthiophene

Results and discussion

Unlike alkanethiol SAMs on Au, SAMs on silicon oxide do not give the molecular image by AFM nor STM so far. Therefore, we confirmed the formation of SAM films derived from silylthiophene derivatives ST 1-4 by RMS roughness, WCA, and XPS.

Conclusion

We could achieve the formation of SAMs on SiO2/Si(111) using the rod-like silylthiophene, silylbithiophene, and silylquaterthiophene derivatives with high solubilities toward organic solvents. The resulting SAM films were evaluated by AFM, XPS spectrum, and IR spectroscopy. XPS data for SAMs using ST 1-2 confirmed that the molecular density of the resulting film did not change so much between the end groups of n-hexyl and phenyl. Comparing the results of WCA and XPS for the films using ST 2-4,

Acknowledgements

The authors would like to thank I. Kojima (National Institute of Materials and Chemical Research) for his experimental assistance in the XPS measurements. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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    • 1

    Permanent address: Electrotechnical Laboratory, Tsukuba, Ibaraki, 305-8568, Japan.

    2

    Permanent address: Electrotechnical Laboratory, Tsukuba, Ibaraki, 305-8568, Japan.

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