Title: Multi-Photon Spectroscopic Studies of Molecule/Metal Interfaces and Graphene
Description: This dissertation presents multi-photon spectroscopic studies on molecule/metal interfaces and graphene. Two different aspects of these ultrathin molecular or atomic materials were investigated: (1) the electronic structure of molecule/metal interfaces and (2) nonlinear optical properties of graphene. For the case (1), two-photon photoemission (TPPE) using a femotosecond laser was employed to investigate occupied and unoccupied electronic states of molecule/metal interfaces. Here we selected two specific examples of interfaces, benzenethiols on Cu(111) and hexa-cata-hexabenzocoronene (HBC) on Cu(111), which are important model systems for an organic / electrode interface of organic semiconductor devices. Although the same copper substrate was used for all the experiments, the nature of interfaces was strongly affected by the interaction between molecular adlayers and metal substrate. Our TPPE measurements on two benzenthiol species, thiophenol and p-fluorothiophenol, on Cu(111) focus on the role of adsorbates in shifting surface polarization and effecting surface electron confinement.

As the coverage of each molecule increases, their photoemission-measured work functions exhibit nearly identical behavior up to 0.4-0.5 ML, at which point their behavior diverges; this behavior can be fit to an interfacial bond model for the surface dipole. In addition, our results show the emergence of an interfacial electronic state 0.1-0.2 eV below the Fermi level. This electronic state is attributed to quantum-mechanical-confinement shifting of the Cu(111) surface state by the molecular adsorbates. Another TPPE study of ours was carried out on an organic semiconductor, HBC, deposited on Cu(111). An increase of HBC coverage continuously shifts the vacuum level of the Cu substrate until a coverage of 2 ML is reached. In the same range of coverage, the Shockley state and the image potential states are quenched while new unoccupied states develop. The momentum- and polarization-resolved photoemission spectra reveal that the new states are originated from a Cu image state. Electron localization is discussed with respect to the structural evolution of HBC.

For the case (2), nonlinear optical scanning microscopy was designed to study third-harmonic generation (THG) from micron- scale graphene crystals on glass substrate. The polarization-, thickness-, and orientation- dependence of THG signals from the graphene were measured and compared to theoretical prediction using the nonlinear optical slab model of Bloembergen and Pershan. The results revealed in-plane isotropy and out-of-plane anisotropy of the THG signals and sub-quadratic dependence of the layer number. Due to the strong THG signal, background-free imaging of graphene crystal was carried out. This result implies the potential application of THG for imaging graphene on arbitrary substrates.