Formation and regulation of unoccupied hybridized band with image potential states at perylene/graphite interface.

Occupied and unoccupied electronic structures of submonolayer perylene (C20H12) on a graphite surface have been investigated using two-photon photoemission (2PPE) spectroscopy for two phases at room and low temperatures. Low energy electron diffraction measurements indicated that the molecules are disordered at room temperature and form a well-ordered superstructure below 180 K. In 2PPE, a specific unoccupied peak (Lx) was observed at around room temperature (>180 K) but not at low temperature (<180 K). The temperature-dependence of the excitation probability was attributed to a contribution of a diffuse unoccupied state, which is characterized by the molecular orbital extending outside the perylene molecular framework. At around room temperature, perylene adopts a flat-lying molecular orientation so that the diffuse state can hybridize with a free-electron-like unoccupied surface state, image potential states (IPS). As a result, the hybridized Lx state can be excited from the occupied bulk band through the IPS-mediated process. In contrast, hybridization is not efficient in the low-temperature phase due to the standing molecular orientation, which decouples the molecule away from the image plane of the substrate. The size of molecular islands also affects hybridization between the diffuse states and IPS because the two states encounter each other at the edge part of molecular aggregates. The temperature-dependent 2PPE results indicate that the molecular orientation and island size of perylene are directly linked to the formation of hybridized states, and thus, the excitation probability at the interface can be regulated by the morphology on the surface.

The role of initial and final states in molecular spectroscopies.

Interpreting experimental spectra of thin films of organic semiconductors is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process. The discussion of spectroscopic data is frequently mired in confusion that originates in overlapping terminology with however distinct meaning in different spectroscopies. Here, we present a coherent framework that is capable of treating on equal footing most spectroscopies commonly used to investigate thin films of organic semiconductors. We develop a simple model for the expected energy level positions, as obtained by common spectroscopic techniques, and relate them to the energies of molecular states. Molecular charging energies in photoionization processes, as well as adsorption energies and the screening of molecular charges due to environmental polarization, are taken into account as the main causes for shifts of the measured spectroscopic features. We explain the relationship between these quantities, as well as with the transport gap, the optical gap and the exciton binding energy. Our considerations serve as a model for weakly interacting systems, e.g., various organic molecular crystals, where wave function hybridizations between adjacent molecules are negligible.

Hybridization of an unoccupied molecular orbital with an image potential state at a lead phthalocyanine/graphite interface.

The interaction of a molecular orbital with a surface state is important to understand the spatial distribution of the wave function at the molecule/substrate interface. In this study, we focus on hybridization of an unoccupied state of lead phthalocyanine (PbPc) with the image potential state (IPS) on a graphite surface. The hybridization modifies the energy-momentum dispersions of the IPS on PbPc films as observed by angle-resolved two-photon photoemission. On the PbPc 1 monolayer film, the IPS band forms a band gap and back-folding appears at the first Brillouin zone boundary due to the periodic potential by the adsorbate lattice. The modification of the dispersion is accompanied by the intensity enhancement of the IPS. We attributed the origin of the modified dispersion and intensity enhancement to a hybridization of the IPS with a molecule-derived unoccupied level. From the photon energy-dependent measurement on multilayer films, we have found the diffuse unoccupied molecular level in the vicinity of the IPS. The tail part of the IPS wave function in the substrate is enhanced by the hybridization with the unoccupied state, and thus strengthens the transition from the occupied substrate band to the hybridized IPS.

Direct visualization of diffuse unoccupied molecular orbitals at a rubrene/graphite interface.

Spectroscopic and nanoscale imaging investigations concerning the spatial extent of molecular orbitals at organic/substrate interfaces have been of intense interest to understand charge dynamics. Here, the spatial extent of unoccupied molecular orbitals of ultrathin rubrene [5,6,11,12-tetraphenyltetracene] films has been investigated with scanning tunneling microscopy and spectroscopy. Based on constant-current distance (z)-voltage (V) measurements, the unoccupied energy levels are elucidated and found to be consistent with previously reported macroscopic two-photon photoemission (2PPE) spectroscopy. In the diffuse unoccupied molecular orbitals reported with 2PPE (J. Phys. Chem. C, 2013, 117, 20098), nanoscale dz/dV spatial maps reveal that the local density of states of the orbitals extends over the rubrene molecules. Delocalization is also observed for the image potential states, which are inherently free-electron-like. This is in contrast to the localized nature of other unoccupied molecular orbitals. A nanoscale understanding of diffuse and delocalized molecular orbitals provides a fundamental insight into low-lying Rydberg states in polycyclic aromatic hydrocarbons.

Electronic excitation and relaxation dynamics of the LUMO-derived level in rubrene thin films on graphite.

Time resolved two-photon photoemission (TR-2PPE) spectroscopy has been performed for rubrene films on highly oriented pyrolytic graphite. When a second layer is formed on the first monolayer (ML), 2PPE intensity from the lowest unoccupied molecular orbital (LUMO)-derived level shows a clear resonance at a pump photon energy of 4.1 eV. In contrast, the resonance is very weak for sub-ML films. Substrate-molecule interaction blurs the intramolecular resonant transition for sub-ML films. The lifetime of electrons in the LUMO-derived level increases exponentially with increasing film thickness, for thickness up to 3 ML. The lifetime increase becomes more moderate for further increase in the film thickness. This change in the slope of the increase in lifetime suggests a transition in the relaxation mechanism, from electron tunneling to intramolecular relaxation medicated by the substrate. When ultraviolet photons of 4.45 eV are used to pump electrons to the LUMO-derived level, the decay profiles for films thicker than 1 ML deviate from a simple exponential decay. Such deviation is not significantly observed for sub-ML films. When visible photons of 2.97 eV are used for pumping, the decay profiles are well reproduced by a simple exponential decay, irrespective of the film thickness. The deviation from simple exponential decay is attributed to the relaxation of holes produced at deep occupied levels to the highest occupied molecular orbital-derived level.

The complex polymorphism and thermodynamic behavior of a seemingly simple system: naphthalene on Cu(111).

Naphthalene, C10H8, is a polycyclic aromatic hydrocarbon (PAH) consisting of two fused benzene rings. From previous studies, it is known to form three different commensurate structures in thin epitaxial films on Cu(111), depending on the preparation conditions. One of these structures even exhibits a chiral motif of molecular rotations within the unit cell. In an attempt to elucidate this polymorphism, we performed in situ low-energy electron diffraction (LEED) as a function of temperature and surface coverage, revealing an unexpected and extraordinarily complex structural and thermodynamic behavior. We present experimental evidence for a phase transition from a two-dimensional gas to a highly ordered molecular solid via an intermediate metastable phase with moderate order (extending over a few lattice constants only) which undergoes a reversible orientational shift upon temperature variation. At monolayer coverage and above, we find that two different point-on-line (POL) coincident epitaxial relations constitute the dominant structures. This is remarkable because, so far, POL structures of naphthalene on Cu(111) and other substrates have either not been recognized or not obtained under the respective experimental conditions. Our results are corroborated by the analysis of characteristic moiré patterns observed in scanning tunneling microscopy (STM), indicative of a noncommensurate epitaxial registry.

Dispersive Electronic States of the π-Orbitals Stacking in Single Molecular Lines on the Si(001)-(2×1)-H Surface.

One-dimensional (1D) molecular assemblies have been considered as one of the potential candidates for miniaturized electronic circuits in organic electronics. Here, we present the quantitative experimental measurements of the dispersive electronic feature of 1D benzophenone molecular assemblies on the Si(001)-(2×1)-H. The well-aligned molecular lines and their certain electronic state dispersion were observed by scanning tunneling microscopy (STM) and angle-resolved ultraviolet photoemission spectroscopy (ARUPS), respectively. Density functional theory (DFT) calculations reproduced not only the experimental STM image but also the dispersive features that originated from the stacking phenyl π-orbitals in the molecular assembly. We obtained the effective mass of 2.0me for the hole carrier along the dispersive electronic state, which was comparable to those of the single-crystal molecules widely used in organic electronic applications. These results ensure the one-dimensionally delocalized electronic states in the molecular lines, which is requisitely demanded for a charge-transport wire.

Dispersions of image potential states on surfaces of clean graphite and lead phthalocyanine film.

Dispersions of image potential states on a graphite surface (denoted IPS1) and on 1 monolayer (ML) film (denoted IPS2) of lead phthalocyanine (PbPc) are investigated by the micro-spot angle-resolved two-photon photoemission (micro-AR-2PPE) spectroscopy. On the graphite surface, whole dispersions of the two members of IPS1 (n = 1 and 2) are observed. The n = 1 IPS1 peak is weakly visible at energy higher than the vacuum level. The effective mass of an electron in the n = 1 IPS1 becomes slightly light at the high momentum region, suggesting the interaction between the IPS1 and the unoccupied σ-band of graphite. On the PbPc film, the IPS2 band forms a band gap and back-folds at the boundary of the Brillouin zone. A 1-dimensional Kronig-Penny model is used to reproduce the effective mass and the shift of binding energy.