J-Aggregates Granting Giant Second-Order NLO Responses in Self-Assembled Hybrid Inorganic–Organic Materials
✍ Scribed by E. Cariati; R. Ugo; F. Cariati; D. Roberto; N. Masciocchi; S. Galli; A. Sironi
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 501 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0935-9648
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✦ Synopsis
formed only at the central part of each nanowire because of the extremely high energy density. The corresponding size of the SiC core for PS1 (11.8 nm AE) and PS5 (12.4 nm AE) can also be estimated from the conversion ratio. This suggests that the b-SiC core size, r, depends only on LET, and the thickness of the crosslinked polymer coating, dr, can be independently controlled by the molecular weight of the polymer.
The variation of dr in PS1±PS5 is discussed based on the theoretical aspects on the physical interaction of ion beams with matter. The following formulae already exist for the values of the coaxial energy in an ion track: [12]
where r c and r p are the deposited energy density at core and penumbra area, respectively; r c and r p are the radii of core and penumbra area; and e is an exponential factor. The equations give a value of deposited energy density at a boundary of a nanowire by using its radii. The density is estimated to be 1.2±1.6 eV/nm 3 for all the ion beams in PS5. The small discrepancy in the density indicates that the radial distribution of deposited energy influences the size of a nanowire and its surface structure. The density (~1 g/cm 3 ) and molecular weight of PS5 give the volume of a molecule roughly as 10 2 nm 3 . The deposited energy per molecule is ~150 eV, indicating that the ion beams form only a few crosslinking points at the boundary because the G-value of crosslinking (number of reactions per absorbed 100 eV) has previously been reported as 0.5±1. Thus simple crosslinking of polymer molecules occurs at the nanowire surface. It suggests the structure of a nanowire having a b-SiC core lapped by crosslinked polysilanes. Ion beam irradiation is a very unique and powerful technique to produce nanosized cylindrical structures as displayed in the present study. It not only produces isolated nanowires on the Si surface but also controls their size and length. The size controllability is also supported by theoretical models of energy distribution in a single ion track. We believe that the present technique has the potential to be developed for producing nanostructured materials other than SiC nanowires in the future.
Experimental
PMPS was synthesized by reaction of methylphenyldichlorosilane with sodium in refluxing toluene or n-undecane (Kipping method). The reaction was carried out under an atmosphere of predried argon with or without 12-crown-4 ether. The PMPS was fractionated by separatory precipitation, leading to PS1 (M n = 4.6±2.2 10 5 ), PS2 (M n = 1.5±1.1 10 5 ), PS3 (M n = 2.6±2.1 10 4 ), PS4 (M n = 1.0 10 4 ±9.0 10 3 ), and PS5 (M n = 5.0±4.0 10 3 ) with a small dispersion less than 1.5. PMPS was spin-coated on Si substrate that had been treated with NaOH solution in ethanol/water = 1:1 over a period of 5 min. These films were irradiated in a vacuum chamber (<1 10 ±6 hPa) with the ion beams listed in Table 1 from a cyclotron accelerator at the Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Laboratory. Ion beams from a Van de Graaff accelerator were also used at the