Electronic and vibrational properties of fullerenes and metallofullerenes studied by STM and STS

This thesis presents the results of a low temperature scanning tunneling microscopy (STM) study on fullerenes and endohedrally doped fullerenes. The measurements have been performed for three different molecules: C60, Ce@C82 and Ce2@C80, adsorbed on metal surfaces. The investigated molecules belong to different classes of the fullerene family and exhibit substantial differences in structural and electronic properties. The study has been focused mainly on the metallofullerenes, special attention has been paid to the features arising due to the presence of the encapsulated atoms. The electronic and vibrational structure of the fullerenes have been studied by scanning tunneling spectroscopy (STS) and inelastic electron tunneling spectroscopy (STM-IETS). In particular, the influence of the chemisorption on the molecular properties has been explored. The STM images resolve the internal structure of the molecules and give insight into the bonding configuration of the fullerenes. The preferential orientation of C60 on Cu(111) has been established by considering the symmetry of the molecular orbitals. For Ce@C82 on Cu(111) no favoured adsorption geometry has been found. The different bonding configurations observed for Ce@C82 could be identified based on the density functional theory (DFT) calculations. The investigation of the electronic properties by tunneling spectroscopy indicates that the electronic structure of the molecules is influenced by the interaction with the substrate, in particular the charge donation from Cu(111). In the STS spectrum of C60 an additional feature has been identified, originating from the partial filling of the LUMO orbitals of the molecule. For Ce@C82 a strong dependence of the dI/dV spectra on the molecular orientation has been observed. Modifications in the electronic structure have been found for different bonding configurations of Ce2@C80. The observed changes in the electronic spectra of the metallofullerenes are related to the presence of highly localized metal-cage hybridized orbitals. As indicated by DFT calculations, such hybrid states dominate the density of states (DOS) of the endohedral fullerenes. The inelastic tunneling spectroscopy study reveals that only few vibrational modes are active in the STM-IETS spectra of the fullerenes. Two internal cage phonons have been identified if case of C60. In the vibrational spectra of Ce@C82 only pure C82 modes have been resolved and no signature of the dynamics of encapsulated cerium has been found. However, in case of Ce2@C80, apart from the features related to the cage phonons also a low frequency mode has been observed. The theoretical calculations of the vibrational structure of the molecule indicate that the feature corresponds most probably to the movement of Ce atoms. Interesting conclusions follow from the spectroscopic measurements on Ce2@C80 molecules. The results reveal an unusually high increase in the differential conductance of this molecule at low bias voltages. The experiments indicate that the effect may be related to the excitation of vibrational modes, that strongly modify the tunneling current. Finally also the electron transport properties of the fullerenes have been investigated. The controlled contact formation to the C60 and Ce2@C80 molecules is demonstrated. The conductance measurements of single-molecule STM junctions indicate the main differences in the transport properties of the hollow and endohedrally doped fullerenes. In particular it has been found that the encapsulation of metal ions hinders the conduction process along the carbon cage which results in the reduced conductivity of the doped fullerenes compared to the hollow species