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End-Graft Technique for Single Polymer Science

Kazuaki Furukawa*, a, Keisuke Ebataa, Nobuo Matsumotob, Michiya Fujikia, Masao Moritaa, Keiichi Torimitsua

aNTT Basic Research Laboratories,
3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198 Japan

bShonan Institute of Technology
Tsujido-Nishikaigan, Fujisawa, Kanagawa 251-8511 Japan

This is an abstract for a presentation given at the
Eighth Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is available on the web.

 

The development of molecular scale devices has recently become a very important area of research. Work in this field includes the development of molecular wire, namely a nanoscale lead for connecting functional molecules and macroscale electrodes, as well as the design and synthesis of signal-processing molecules. The investigation of the electronic properties of a single conducting/semiconducting polymer chain is of great interest because of its potential application to molecular wire. Polysilane, a silicon-catenated polymer with semiconducting properties based on the s-conjugation, is one of the target materials. For instance, photoconductivity, hole drift mobility and electroluminescence have been observed using polysilane thin films. The observation of these properties at the single polysilane level is desirable not only in terms of defining possible molecular wire applications but also for clarifying the intrinsic properties of the polysilanes.

For the purposes of single polymer science, it is necessary to pick out a single molecule from a mole amount of the material. This is not such an easy task. We developed the 'end-graft' technique as an effective way to fix single polysilane molecules in isolation on a substrate surface. [1] The end-graft technique utilizes a one-to-one chemical reaction between a reactive anchor built on a substrate surface and an end-functionalized polysilane molecule. We found primary alkylbromide and end-lithiated polysilane to be a good combination. The building of the reactive anchor is surface dependent: the silane coupling reaction between Br(CH2)11Si(CH3)2Cl and a surface -OH bond is effective on an SiO2 substrate and the photo-induced insertion reaction between Br(CH2)9CH2=CH3 and a surface Si-H bond is effective on Si(111). [4] We can control the end-graft polysilane density by diluting the density of the reactive anchor on the surface with non-reactive anchors. There are also several ways of preparing end-lithiated polysilane: the living anionic polymerization of masked disilene using n-C4H9Li as an initiator or the scission reaction of the polysilane chain by lithium reagent such as CH3Li. [2]

We can approach the following fascinating problems in polymer science by using our end-graft polysilane samples: (i) The direct observation of single polymer structures and possible supramolecular structures by atomic force microscopy. [3,4] (ii) The chain conformations of single polysilane in good/poor/intermediate solvent conditions by electronic absorption spectroscopy (rather than neutron scattering and reflection etc.). (iii) The dynamics of end-grafted polysilane that is different from that of polysilane in dilute solution (ie. isolated and free from intermolecular interaction) or polysilane thin solid film (ie. with unavoidable intermolecular interaction). (iv) Surface selective grafting on a micro-patterned SiO2/SiH substrate. We also expect that our end-graft technique will lead us into the area of single polysilane electronics and polysilane-based molecular device fabrication.

References:
[1] K. Ebata, K. Furukawa, N. Matsumoto, J. Am. Chem. Soc. 120, 7367-8 (1998).
[2] K. Ebata, K. Furukawa, N. Matsumoto, M. Fujiki, Polym. Prepr. 40(2), 157-8 (1999).
[3] K. Furukawa, K. Ebata, N. Matsumoto, Appl. Phys. Lett. 75, 781-3 (1999).
[4] K. Furukawa, K. Ebata, M. Fujiki, Adv. Mater. (in press).


*Corresponding Address:
Kazuaki Furukawa
NTT Basic Research Laboratories
3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198 Japan
Email: furukawa@will.brl.ntt.co.jp
Web: http://www.brl.ntt.co.jp/people/furukawa/top.html



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