In this work we investigate the vibrational and electronic properties of carbon nanoclusters. In the first part pure and modified adamantane as well as diamantane functionalized with amino, hydroxyl and thiol groups have been analyzed by Raman spectroscopy. The peaks in the spectra were assigned by supporting density functional theory computations. For functionalized diamantane two fingerprint regions for a fast determination of functionalization type and site were identified. With their help two different sites of the functional group can be distinguished. Functionalization in the 1-position leads to Raman activation and splitting of degenerate vibrational modes and hence to a more complex Raman spectrum and strong shifts of several modes. Small shifts of the same modes, on the other hand, indicate functionalization in the 4-position. We observe an anomaly in the mass dependence of the functional group, which we assign to the influence of the electronegativity on the vibrational frequency. Additionally, we can distinguish between the functional groups by the characteristic bands of the stretch vibration. Further the vibrational properties of two diamantane dimers have been analysed with respect to some vibrational modes playing a role in the following investigations. Polarization dependent Raman measurements on adamantylidene-diamantane have shown a clear crystalline orientation. In the second part we present resonance Raman measurements of crystalline trishomocubane and diamondoid dimers containing a C=C double bond. Raman spectra were recorded with excitation energies between 2.33 eV and 5.42 eV. The strongest enhancement is observed for the C=C stretch vibration and a bending mode involving the two carbon atoms of the C=C bond, corresponding to the B2g wagging mode of ethylene. This is associated with the localization of the π-HOMO and LUMO and the elongation of the C=C bond length and a pyramidalization of the two sp2-hybridized carbon atoms at the optical excitation. The observed Raman resonance energies of the trishomocubane and diamantane dimers are significantly lower than the HOMO-LUMO gaps of the corresponding unmodified diamondoids. In the last part we analyze the differences between crystalline and gas phase electronic properties of sp2-sp3-hybrid systems. Apparent differences of electronic properties experimentally observed by resonance Raman, absorption and photoluminescence measurements were investigated by first principle calculations. We found that in crystalline phase compared to the gasphase, the electronic structure, due to reorganization of the molecular orbitals, is changed and the energy gap must be lowered significantly. The reorganization of the particular molcular orbitals depends on the extension of the molecular orbitals, which in turn leads to a different weakening of the confinement effects, appearing in isolated molecules. Absorption and luminescence spectra show vibrational progressions, which fit well to the experimental Raman frequencies of the C=C stretch vibration. The length of the C=C bond could be estimated as having a value of 1.41 Angstrom.
|Commitee:||Maultzsch, Janina, Lange, Holger|
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
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