Discrepancies in reported values of exciton binding energy (Eb) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, Eb is defined as the difference between the ...transport gap (Et) and the optical gap (Eopt). Here, the Eb values of PBnDT‐TAZ polymer variants are determined using two commonly employed methods: a combination of ultraviolet photoemission spectroscopy and low‐energy inverse photoemission spectroscopy (UPS‐LEIPS) and solid‐state cyclic voltammetry (CV). Eb values obtained by UPS‐LEIPS show low dispersion and no clear correlation with the polymer structure and thedielectric properties. In contrast, CV reveals a larger dispersion (200 meV‐1 eV) and an apparent qualitative Eb‐molecular structure correlation, as the lowest Eb values are observed for oligo‐ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of Eb. Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of Et is discussed. The differences in intrinsic and extrinsic factors highlight the context‐dependent nature of measurement when drawing global conclusions. Notably, the observed Eb trend derived from CV is not intrinsic to the pure materials but likely linked to electrolyte swelling and associated changes in dielectric environment, suggesting that high‐efficiency single‐material organic photovoltaics with low Eb may be possible via high dielectric materials.
Investigating a Practical Definition of the Transport Gap. Top: Single molecule regime. Middle: Solid state with relaxation and disorder effects, and Bottom: Mobility edge model in solid state capture the charge transport character in device application.
•We present a scheme to generate high intensity XUV pulses from HHG with variable time-bandwidth product.•Shorter-wavelength driven high-harmonic XUV trARPES provides higher photon flux and increased ...energy resolution.•High-quality high-harmonic XUV trARPES data with sub 150meV energy and sub 30fs time resolution is presented.
Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to <150meV while preserving excellent time resolution of about 30fs.
A chemical approach to controlling the work function of few‐layer graphene is investigated. Graphene films are synthesized on Cu foil by chemical vapor deposition. Six metal chlorides, AuCl3, IrCl3, ...MoCl3, OsCl3, PdCl2, and RhCl3, are used as dopants. The sheet resistance of the doped graphene decreases from 1100 Ω/sq to ≈500–700 Ω/sq and its transmittance at 550 nm also decreases from 96.7% to 93% for 20 mM AuCl3 due to the formation of metal particles. The sheet resistance and transmittance are reduced with increasing metal chloride concentration. The G peak in the Raman spectra is shifted to a higher wavenumber after metal chloride doping, which indicates a charge transfer from graphene to metal ions. The intensity ratio of ICC/IC−C increases with doping, indicating an electron transfer from graphene sheets to metal ions. Ultraviolet photoemission spectroscopy data shows that the work function of graphene increases from 4.2 eV to 5.0, 4.9, 4.8, 4.68, 5.0, and 5.14 eV for the graphene with 20 mM AuCl3, IrCl3, MoCl3, OsCl3, PdCl2, and RhCl3, respectively. It is considered that spontaneous charge transfer occurs from the specific energy level of graphene to the metal ions, thus increasing the work function.
A chemical approach to controlling the work function of graphene is investigated. The work function of graphene increases from 4.2 eV to 5.0, 4.9, 4.8, 4.68, 5.0, and 5.14 eV for the graphene with 20 mM AuCl3, IrCl3, MoCl3, OsCl3, PdCl2, and RhCl3, respectively. Spontaneous charge transfer occurs from graphene to the metal ions, increasing the work function.
Surface functionalization with dipolar molecules is known to tune the electronic band alignment in semiconductor films and colloidal quantum dots. Yet, the influence of surface modification on ...plasmonic nanocrystals and their properties remains little explored. Here, we functionalize tin-doped indium oxide nanocrystals (ITO NCs) via ligand exchange with a series of cinnamic acids with different electron-withdrawing and -donating dipolar characters. Consistent with previous reports on semiconductors, we find that withdrawing (donating) ligands increase (decrease) the work function caused by an electrostatic potential shift across the molecular layer. Quantitative analyses of the plasmonic extinction spectra reveal that varying the ligand molecular dipole affects the near-surface depletion layer, with an anticorrelated trend between the electron concentration and electronic volume fraction, factors that are positively correlated in as-synthesized NCs. Electronic structure engineering through surface modification provides access to distinctive combinations of plasmonic properties that could enable optoelectronic applications, sensing, and hot electron-driven processes.
Carbon surfaces incorporating densely packed spikes with nanometer sharpness have shown exceptional electrocatalytic properties attributed to the enhanced electric field from the topography of the ...spikes. In this article, the composition of nanospike surfaces is identified by x-ray photoelectron spectroscopy (XPS) and their absolute work function determined using ultraviolet photoemission spectroscopy (UPS). Low temperature annealing of as-grown samples above 275 °C is required to produce a clean surface which has a 4.13 eV work function, a half volt lower than that of flat graphite. Treatments that contaminate the spiked surface, including exposure to air, oxidation at elevated temperature, or immersion in water or hydrocarbons, all increase the work function. Processing that blunts the spikes, including exposure to an oxygen plasma, argon sputtering, or high temperature annealing (800 °C) result in a work function close to that of flat graphite. An unusual double onset in the UPS secondary electron intensity is observed on as-grown nanospike samples and is reproduced by absorbing hydrocarbons on the surface. This double onset appears to be unique to carbon substrates and may originate in inelastic scattering of photoelectrons.
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The electronic structure at the α-NPD/MoO
3/Au interfaces has been investigated with ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission ...spectroscopy (IPES). It was found that the MoO
3 layer contains some number of oxygen vacancies prior to any treatment and gap states are induced by the partial filling of the unoccupied 4d orbitals of molybdenum atoms neighboring oxygen vacancies. The α-NPD thickness dependence of XPS spectra for the α-NPD/MoO
3 system clearly showed that molybdenum atoms at the surface of the MoO
3 film were reduced by α-NPD deposition through the charge-transfer interaction between the adsorbed α-NPD and the molybdenum atoms. This reduction at the α-NPD/MoO
3 interface formed a large interface dipole layer up to −1.79
eV. The deduced energy-level diagram for the α-NPD/MoO
3/Au interfaces describes the energy-level matching that explains well the significant reduction in the hole-injection barrier due to the MoO
3 buffer layer.
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