arXiv subjects

Hisao Ishii

Publications and source records attributed to Hisao Ishii.

4 recordsLinked to original sources

arXiv2026

Determining the density of in-gap states in organic semiconductors: A pitfall of photoelectron yield spectroscopy

Accurate determination of low-density electronic states in the bandgap (in-gap states) is crucial for optimizing the performance of organic optoelectronic devices. Derivative photoelectron yield spectroscopy (PYS) is employed to estimate the density of states (DOS) of in-gap states. However, low-energy photons in PYS can generate excitons and anions in organic semiconductors, raising questions about whether derivative PYS spectra truly represent the DOS. We revealed that PYS signals originate from the single-quantum external photoelectron effect (SQEPE) of in-gap states, SQEPE of the singly occupied molecular orbital (SOMO) of anions, and the biphotonic electron emission (BEE) effect via exciton fusion. Because BEE signals mask the DOS contribution, derivative PYS misestimates the DOS of in-gap states. In contrast, constant final state yield spectroscopy (CFS-YS) reliably determines the DOS by separating these components. For a tris(8-hydroxyquinoline) aluminum (Alq3) film, CFS-YS revealed the DOS of in-gap and SOMO states over six orders of magnitude, clarifying why the Alq3 layer works effectively in organic light-emitting diodes. In the devices, BEE can act as carrier-generation and degradation processes, and CFS-YS can also probe it. We provide the practical guidelines of low-energy photon measurements for DOS determination, such as measurements of photon-flux dependency.

cond-mat.mtrl-sci↗

arXiv2025

Does Fermi Level Alignment Hold Across Organic Interfaces? -- An Investigation Using a Rotary Kelvin Probe

Understanding energy level alignment at organic interfaces is crucial for optimizing the performance of organic devices. Interface dipole and band bending significantly influence carrier recombination and generation mechanisms. A method of simulating energy level alignment at metal/organic and organic/organic interfaces by assuming a thermal equilibrium model has been proposed, but its validation against experimental methods is still limited. In this study, the work function change in the $α$-NPD/HAT-CN/Au interface was measured as a typical donor/acceptor system using a rotary Kelvin probe (RKP). Our findings demonstrate good agreement with simulations only at metal/organic interfaces which have "active" charge transfer. It is suggested that thermal equilibrium is not achieved simply by depositing the film under dark condition, and some treatment to supply carriers, such as exposure to UV light, is necessary for accurate evaluation. At the organic/organic interface, the the experimental results did not agree with thermal equilibrium model, highlighting the need to consider substrate-driven carrier supply and polarization effects when evaluating energy level alignment.

cond-mat.mtrl-sci↗

arXiv2025

Using optical tweezers to simultaneously trap, charge and measure the charge of a microparticle in air

Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a $\sim 1\,\mathrm{μm\;SiO_2}$ sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.

cond-mat.soft↗

arXiv2020

Off-resonant coherent electron transport over three nanometers in multi-heme protein bioelectronic junctions

Multi-heme cytochromes (MHC) are fascinating proteins used by bacterial organisms to shuttle electrons within and between their cells. When placed in a solid state electronic junction, they support temperature-independent currents over several nanometers that are three orders of magnitude higher compared to other redox proteins of comparable size. To gain microscopic insight into their astonishingly high conductivities, we present herein the first current-voltage calculations of its kind, for a MHC sandwiched between two Au(111) electrodes, complemented by photo-emission spectroscopy experiments. We find that conduction proceeds via off-resonant coherent tunneling mediated by a large number of protein valence-band orbitals that are strongly delocalized over heme and protein residues, effectively "gating" the current between the two electrodes. This picture is profoundly different from the dominant electron hopping mechanism supported by the same protein in aqueous solution. Our results imply that current output in MHC junctions could be even further increased in the resonant regime, e.g. by application of a gate voltage, making these proteins extremely interesting for next-generation bionanoelectronic devices.

physics.bio-ph↗