Study Of Temperature Dependence Of C70 Molecules By SERS

Fullerene is the third form of pure carbon, and the other two well-known are diamond and graphite. Since fullerenes C60 and C70 were discovered in the soot of carbon, in particular appreciable amount of fullerenes was obtained by Kr?tschmer et al., a lot of theoretical and experimental work has been done. Fullerene is a special material, which has a close hollow carbon-cage structure. C70 is the second stable and abundant molecule next to C60 in the fullerenes family. Consisting of 25 hexagons and 12 pentagons, the molecule C70 has a rugby-shaped structure. This thesis for the first time reports the temperature dependence of C70 molecules adsorbed on silver film by the surface enhanced Raman scattering (SERS). Both SERS spectra of C70 molecules on the surface of silver mirror at different temperatures and the normal Raman spectrum of C70 powder at the room temperature were measured. More complete and accurate vibration modes of C70 were expected, considering the high sensitivity of the SERS active substrate (silver mirror) and the high purity of the C70 sample (better than 95%) used in our experiment. The He-Ne laser with the wavelength 632.8nm was used as the excitation line. Judged by the UV-VIS spectrum of C70, a preresonant or weak resonant effect existed in the Raman spectra in our measurement. SERS spectrum was very similar with the normal Raman spectrum at the room temperature 298K. This means there was only weak interaction between C70 molecules and the silver substrate. Therefore, the remarkable enhancement of the signal in SERS spectra dominantly originated from the physical mechanism. In addition, several new spectral features appeared in the SERS spectra. According to the theoretical computation, some of these new features were assigned to the predicted vibrationmodes. The results of cryogenic investigation showed the intensities of most features in SERS increased greatly at the low temperature, but not accompanied with the shifts of center positions. This kind of increment of intensity originated from the change of resonant condition in the low temperature. According to the experiment reported by V. K. Dolganov, the electronic gap can shift to the low energies under hydrostatic pressure. Then the temperature influence of electronic gap can be estimated. Considering there is a wide absorption band at about 472nm with a wavelength shoulder tailing to about 670nm, the Raman preresonant or weak resonant effect may be enhanced at the low temperature. This induced the great enhancement of the Raman signal at the low temperature and the reversible temperature dependent change in the measurement. Furthermore, there were some interesting phenomena in our experiment, such as the "abnormal" change of intensity in the range of 273K to 298K and the prominent "double peak" phenomenon at the lowest temperature 93K.Finally, in the fourth chapter, we primarily discussed the SERS effect of C70 molecules adsorbed on the surface of copper. The difference of relative intensity between the SERS spectra of C70 adsorbed on the silver surface and the copper surface originated from the different adsorption state of the C70 molecules on them. This investigation is valuable for the selection of the active substrate for C70 molecules in SERS measurement and the research of the adsorption state of C70 molecules on the surfaces of metals...