Scanning Tunneling Microscopy and Spectroscopy of Metal-Organic Interfaces for Electronics and Spintronic Applications

This dissertation aims to develop an understanding of metal-organic interfaces which are crucial for determining electronic and spintronic device characteristics. Model systems investigated involve a well-known organic semiconductor tris (8-hydroxyquinolinato) aluminum (Alq3) adsorbed on several metal substrates (Cu(110), Au(111), Cr(001)) which have distinct surface properties and reactivities. The electronic and magnetic properties at Alq3/metal interfaces have been studied in detail with scanning tunneling microscopy (STM) and spectroscopy (STS). The observations can provide insights for interpreting the performance of organic devices and provide guidance for future organic electronic and spintronic applications.;Molecular resolved STM imaging of Alq3 on both Cu(110) and Cr(001) surfaces reveals strong structural disorder in the sub-monolayer thickness region which suggests strong metal-molecule interactions at the interface. While the film exhibits an amorphous structure, the molecules at sub-monolayer coverages tend to form random quasi-linear structures. This linear tendency is driven by anisotropic intermolecular interactions. The strong interfacial structural disorder does not lead to remarkably strong electronic disorder at the interface. The local density of states measurement by constant-current STS shows the spatial variation of lowest unoccupied molecular orbitals (LUMO)-derived levels for both Alq3/Cu(110) and Alq3/Cr(001) systems. The observed electronic disorder is significantly less than the strong electronic disorder inferred from device measurements. The weak distribution in energies of electronic states is attributed to several closely spaced unoccupied states known for the isolated molecule. The discrepancy between our observation and device characteristics suggests that electronic disorder in devices has a significant contribution from polycrystalline electrodes.;Spin-polarized scanning tunneling microscopy (SP-STM) was developed to study spin dependent properties of the Alq3 molecule at metal surfaces. We observed a strong spin depolarization effect of Alq3 molecules on Cr(001) in a low bias region. Furthermore, we observed spin polarized electronic structure effects near the LUMO-derived states. Near the LUMO energies, spin up molecules always coexist with spin down molecules. The phenomenon reveals that Alq3 can have surface induced spin-polarization near LUMO-derived states. We argue that the directions of local magnetic moments are controlled by molecule chirality. This observation highlights the importance of metal-organic interfaces in determining the characteristics of spintronic devices. It shows that molecular geometric shape can alter magnetic properties indirectly by controlling metal-molecule interactions.;Lastly, by comparing iron phthalocyanine (FePc) growth on Au(111) and Cr(001), we observe that FePc molecules completely dissociate on Cr(001) surfaces. By creating a less reactive iron buffer layer on the chromium surface, it is possible reduce the likelihood of decomposition. The high likelihood of chemical reactions occurring at 3d metal-organic interfaces should be taken into serious consideration in manufacturing real electronic or spintronic devices.