Satoshi Kera

Phenacenes, in which benzene rings are fused in a zigzag manner, are chemically robust π-conjugated hydrocarbons of interest for organic electronic applications and superconductivity. However, quantitative electronic parameters of phenacene thin films remain limited compared with those of acenes. Here, we determine the ionization energies (IEs), electron affinities (EAs), bandgaps, exciton binding energies, and polarization energies of orientation-controlled thin films of picene, [6]-phenacene, and [7]-phenacene using ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy. The energy parameters depend only weakly on molecular size. In contrast, both IE and EA exhibit large orientation-dependent shifts of approximately 1 eV: lying films show IEs of 6.28–6.42 eV and EAs of 2.20–2.39 eV, whereas standing films show IEs of 5.54–5.74 eV and EAs of 1.46–1.73 eV. Nevertheless, the bandgaps of approximately 4 eV and exciton binding energies of approximately 1 eV remain nearly independent of molecular size and orientation. Analysis of polarization energies shows that the induction term is nearly orientation independent, whereas the electrostatic term strongly depends on orientation and originates primarily from molecular quadrupole moments. These results identify molecular orientation as a key factor governing frontier energy levels in phenacene thin films.

Recent measurements using high-resolution, angle-resolved photoelectron spectroscopy have revealed intrinsic electron–phonon interactions of molecular crystals, such as organic semiconductors. The observed spectra shows that the rubrene single crystals have a clear intra-band gap in the VB structure, which is beyond the framework of conventional density functional theory. The electron–phonon interaction associated with the coupling between electrons and intramolecular vibrational phonons has been considered as the physical origin of this band modulation, whereas the theoretical analysis has been limited to the one-dimensional lattice model. Therefore, we present a theoretical calculation of the electronic-band structure considering the electron–phonon interaction using the first-principles material parameters of the rubrene single crystals. We conduct quantitative comparisons of our results of band modulation with experimental ones.

We report the development and initial applications of the multi-functional research and development beamlines BL-11A and BL-11B at the Photon Factory, which are designed to enable simultaneous use of hard and soft x-rays. The two beamlines share a common bending-magnet source and deliver independently monochromatized hard- and soft-x-ray beams that can be focused onto the same sample position in a downstream experimental hutch, providing a versatile platform for two-beam experiments in which hard and soft x-rays can serve as pump and/or probe beams in various combinations. As proof-of-concept demonstrations, we performed probe–probe measurements of water electrolysis on a cobalt electrocatalyst and pump–probe measurements of x-ray-induced dissociation of a caged compound. In the probe–probe scheme, hard- and soft-x-ray absorption spectroscopy was carried out simultaneously to track correlated changes in the catalyst oxidation state and oxygen-related intermediates during the oxygen evolution reaction. In the pump–probe scheme, chemical changes in 2-iodosobenzoic acid induced by hard-x-ray excitation at the iodine L3 edge were monitored using soft-x-ray absorption spectroscopy at the oxygen K edge.

Alkali-metal incorporation is widely employed to improve the performance and stability of organic halide perovskites, yet the underlying doping mechanisms and diffusion pathways remain poorly understood. Here, we use ultraviolet photoemission spectroscopy to probe the evolution of the electronic structure of CH3NH3PbX3 (X = I, Br, and Cl) single crystals upon potassium and cesium deposition. We observe two distinct regimes governed by halide chemistry: facile bulk diffusion in CH3NH3PbI3 and CH3NH3PbBr3, in contrast to surface-confined accumulation in CH3NH3PbCl3, arising from kinetically hindered alkali migration in the chloride perovskite. Temperature-dependent measurements of the doped crystals further detect opposite valence band shifts for CH3NH3PbBr3 relative to CH3NH3PbI3 and CH3NH3PbCl3, highlighting the role of surface termination and charge redistribution. Notably, alkali metal deposition induces substantial energy level shifts while leaving band dispersions largely unchanged, indicating a predominantly interfacial doping mechanism. These results establish a direct link among halide composition, diffusion behavior, and electronic structure evolution, providing microscopic insight into alkali-metal interactions in perovskites.

The creation and characterization of high-quality crystalline monolayer transition metal dichalcogenides (TMDCs) are crucial for the development of future electronic device applications. Traditional methods face challenges in achieving the necessary spatial resolution in electronic structure analysis, especially when dealing with identical sample work functions and chemical environments, e.g., those in antiparallel twin domains. Here, we employed momentum-selected (or dark-field) photoelectron emission microscopy (PEEM) via a momentum microscope (PMM) apparatus to successfully characterize domains of one monolayer WS ₂ on Au(111) that was not possible with conventional PEEM. The spatial mapping of the selected band structure contribution of a specific domain revealed the existence of many twin domains with a size of around 5–50 μm forming a continuous film of over 500 μm × 500 μm. Simultaneously, i.e., within a single instrument, we verified the one monolayer thickness of the grown film. Our work highlights the potential of PMM as a versatile instrument for surface characterization, paving the way for advancements in materials science and nanotechnology.

We investigated the influence of van der Waals interactions on the electronic structure at the interface between a pentacene monolayer and a graphite surface. Upon cooling below 130 K, the pentacene molecules form a densely packed monolayer characterized by newly developed dispersive bands. These bands, observed using low-energy angle-resolved photoelectron spectroscopy with photon energy varying from 7.2 to 8.5 eV, exhibit constant final-state characteristics that overlap with nondispersive molecular-orbital states. This overlap results in variations in photoemission intensity—both enhancement and suppressions—indicative of Fano resonance. Such resonance arises from the interaction between a discrete molecular state and a continuum state. The Fano profile analysis of spectral fine features reveals the wave-function connection by the broader spread in unoccupied states at the physisorbed interface.

Two-dimensional crystallization of chiral molecules on achiral crystal surfaces typically exhibits mirror-imaged growth, defined by the substrate's mirror plane and the lattices of each enantiomer. While various commensurate molecular overlayers have shown such growth, the possibility of achieving substrate-defined enantiospecific structures in non-commensurate chiral molecular overlayers remains elusive. Here, enantiopure thiadiazole-[9]helicene on Au(111) is shown to form overlayers without commensurability, exhibiting substrate-defined mirror-imaged growth. This study experimentally demonstrates that the rotational orientation locking without two-dimensional interface potential minima can support mirror-imaged growth, suggesting that it can serve as benchmark for enantiospecific growth in a broader range of chiral molecular systems.

Despite the rapid progresses in the field of organic semiconductors, aided by the development of high-mobility organic materials, their high carrier mobilities are often unipolar, being sufficiently high only for either electrons or holes. Yet, the basic mechanisms underlying such significant mobility asymmetry largely remains elusive. We perform angle-resolved photoelectron spectroscopy to reveal the occupied band structures and the many-body interactions for low-mobility hole carriers in a typical n-type semiconductor perylene diimide derivative. The band dispersion exhibits strong renormalization to the calculated non-interacting electronic structure. The analysis including many-body interactions elucidate that the significant mass enhancement can be understood in terms of strong charge–phonon coupling, leading to an important mechanism of polaron band transport of low intrinsic carrier mobility in organic semiconductors.

The soft X-ray photoelectron momentum microscopy (PMM) experimental station at the UVSOR Synchrotron Facility has been recently upgraded by additionally guiding vacuum ultraviolet (VUV) light in a normal-incidence configuration. PMM offers a very powerful tool for comprehensive electronic structure analyses in real and momentum spaces. In this work, a VUV beam with variable polarization in the normal-incidence geometry was obtained at the same sample position as the soft X-ray beam from BL6U by branching the VUV beamline BL7U. The valence electronic structure of the Au(111) surface was measured using horizontal and vertical linearly polarized (s-polarized) light excitations from BL7U in addition to horizontal linearly polarized (p-polarized) light excitations from BL6U. Such highly symmetric photoemission geometry with normal incidence offers direct access to atomic orbital information via photon polarization-dependent transition-matrix-element analysis.