This dissertation investigates four diverse molecular systems using ultrahigh vacuum low-temperature scanning tunneling microscope (UHV-LT-STM) manipulation and spectroscopic techniques to understand the fundamental properties of future nanotechnology applications.
The first system demonstrated an extensive and unusual redistribution of spin density of cobalt porphyrin (TBrPP-Co) molecules adsorbed on a Cu(111) surface. Here the Kondo temperature increased dramatically on the porphyrin macrocycle as compared to the cobalt atom location. This effect was confirmed by monitoring the Kondo temperature with differential conductance (dI/dV) tunneling spectroscopy and spectroscopic mapping. In particular, it was found that an interfacial charge-transfer process between the molecule and the surface led to a spin delocalization in the TBrPPCo molecule, and resulted in a higher Kondo resonance on the lobe than the center of the molecule.
The next two aspects of the research focused on a molecular level investigation of two different charge-transfer (CT) systems, 6T/F4TCNQ and TCNQ/ Mn(C5(CH3)5)2 on Au (111) surface. In the former system, the formation of new hybrid molecular orbitals in the 6T/F4TCNQ complex in a lateral configuration was evident by STM imaging at different bias voltages and differential conductance spectra. The CT led to the shift of HOMO and LUMO orbitals in the complex with respect to the pure molecular orbitals. A voltage dependent resonance-tunneling scheme was used to demonstrate a molecular switch made of 6T/ F4TCNQ CT complexes. In the latter system, charge-transfer process in a vertical stacking configuration was investigated. The comparative tunneling spectroscopy between the Mn(C5(CH3)5)2/Au(111) and Mn(C5(CH3)5)2/TCNQ/Au(111)assemblies clearly revealed the shift of frontier molecular orbitals. This explained detailed charge transfer mechanism induced by the deposition of a single Mn(C5(CH3)5)2 molecule on TCNQ layer.
The last part of the dissertation focused on 4Fc3SEt double-decker molecular rotors on Au(111) surface. A conformational change of the rotator part of the molecule was realized by inelastic tunneling electron excitation using the STM-tip and a single tunneling electron energy transfer caused the conformational changes of the molecule. Furthermore, Kondo resonance and vibrational features of the molecular rotors were also identified from scanning tunneling spectroscopy measurements.