Spectroscopic and microscopic investigations of organic ultrathin films: Correlation between geometrical structures and unoccupied electronic states

In this review, we summarize recent progress in experimental approaches to the investigation of the unoccupied electronic structures of organic ultrathin films, based on a combination of spectroscopic and microscopic techniques. At the molecule/substrate interface, electronic structures are greatly affected by the geometrical structures of adsorbed molecules. In addition, a delicate balance between substrate-molecule and intermolecular interactions plays an important role in the formation of complex polymorphism. In this context, we have clarified the correlation between geometric and electronic structures using a combination of two-photon photoemission (2PPE) spectroscopy, low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Organic ultrathin films of metal phthalocyanines and polycyclic aromatic hydrocarbons (naphthalene, rubrene and perylene) on graphite substrates were examined as model systems. Depending on the substrate temperature and coverage, unique morphologies, including well-ordered films, a metastable phase and a two-dimensional gas-like phase, were determined at the molecular level. The data show that variations in molecular orientation have a significant impact on the occupied/unoccupied electronic structures. In addition to static information regarding electronic states, ultrafast electron excitation and relaxation dynamics can be tracked in real time on the femtosecond scale by time-resolved 2PPE spectroscopy. The excited electron dynamics of rubrene films are discussed herein, taking into account structural information, in the presence and absence of an overlap of the wave function with the substrate. Spatial resolution at the molecular level is also obtainable via STM-based local spectroscopy and mapping, which have been utilized to elucidate the spatial extent of unoccupied orbitals in real space. Visible photon emissions from the unoccupied states of perylene monolayer films were observed using 2PPE, representing a characteristic deexcitation process from electronically excited states, depending on the surface structure. These spectroscopic and molecular level microscopic investigations provide fundamental insights into the electronic properties of organic/substrate interfaces.