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Carbon Materials for Advanced Technologies Part 4

Tham khảo tài liệu 'carbon materials for advanced technologies part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 85 plished through a saturation of the transmitted light intensity with increasing incident intensity. Outstanding performance for C6Q relative to presently used optical limiting materials has been observed at 5320Ả for 8 ns pulses using solutions of C60 in toluene and in chloroform CII3CI 77 The proposed mechanism for the optical limiting is that C60 is more absorptive for molecules in the triplet excited state than for the ground state see 2.4 . In this process the initial absorption of a photon takes an electron from singlet So state to an excited singlet state. This is followed by a rapid inter-system crossing from the singlet to a metastable triplet state from which dipole-allowed transitions to the higher-lying triplet states can occur. Because of the higher excitation cross section for electrons in the metastable triplet state relative to those in the ground state an increase in the population of the metastable triplet state promotes further stronger absorption of photons 77 . Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole PVK and other selected polymers such as poly paraphcnylene-vinylene PPV and phenylmethylpolysilane PMPS doped with a mixture of C60 and C70 have been reported to exhibit exceptionally good photoconductive properties 206 207 208 which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes e.g. 3 by weight lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes thereby promoting the conduction of mobile holes in the polymer 209 Fullerene-doped polymers also have significant potential for use in applications such as photo-diodes photo-voltaic devices and as photo-refractive materials. Fullerenes have been shown to benefit the synthesis of Sic and diamond. Gruen and coworkers 210 have demonstrated that by fragmentation of individual CG0 molecules diamond .