•Polarization-dependent differential photoelectric cross sections in valence photoemission.•Bulk electronic structure from hard X-ray angle-resolved photoemission.•Depth-resolved photoemission using ...standing-wave and total reflection excitation.•Standing-wave ambient-pressure photoemission as a probe of solid–liquid interfaces.•Molecular dissociation dynamics from photoelectron holography with free-electron laser excitation.
We discuss several recent developments in photoemission, with comments on their perspectives for the future. These include an adequate allowance for differential cross section effects in core- and valence-angular distributions, as well as more accurate one-step modeling of angle-resolved photoemission (ARPES); the use of higher photon energies from the soft- to hard- X-ray regime to permit probing bulk electronic structure and buried layers and interfaces; extending ARPES into the soft- and hard- X-ray regimes; tailoring the X-ray wave field through X-ray optical effects including standing waves, total reflection, and tuning through resonances; using standing-wave excitation to provide much enhanced depth sensitivity in studying solid/gas and solid/liquid interfaces; and applying photoelectron holography to time-resolved studies of molecular reactions and dissociation. Specific application examples include a magnetic semiconductor, multilayer structures of complex metal oxides, a thin water solution on a metal oxide surface, and a halo-substituted benzene molecule.
•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.
We report that recent direct experimental observation of multiple highly dispersive C60 valence bands has allowed for a detailed analysis of the unusual photoemission traits of these features through ...photon energy- and polarization-dependent measurements. Previously obscured dispersions and strong photoemission traits are now revealed by specific light polarizations. The observed intensity effects prove the locking in place of the C60 molecules at low temperatures and the existence of an orientational order imposed by the substrate chosen. Most importantly, photon energy- and polarization-dependent effects are shown to be intimately linked with the orbital character of the C60 band manifolds which allow for a more precise determination of the orbital character within the third highest occupied molecular orbital (HOMO-2). In conclusion, our observations and analysis provide important considerations for the connection between molecular and crystalline C60 electronic structure, past and future band structure studies, and for increasingly popular C60 electronic device applications, especially those making use of heterostructures.
Recent direct experimental observation of multiple highly dispersive C60 valence bands has allowed for a detailed analysis of the unusual photoemission traits of these features through photon energy- ...and polarization-dependent measurements. Previously obscured dispersions and strong photoemission traits are now revealed by specific light polarizations. The observed intensity effects prove the locking in place of the C60 molecules at low temperatures and the existence of an orientational order imposed by the substrate chosen. Most importantly, photon energy- and polarization-dependent effects are shown to be intimately linked with the orbital character of the C60 band manifolds which allow for a more precise determination of the orbital character within the third highest occupied molecular orbital (HOMO–2). Our observations and analysis provide important considerations for the connection between molecular and crystalline C60 electronic structure, past and future band structure studies, and for increasingly popular C60 electronic device applications, especially those making use of heterostructures.
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.
This review reports on experimental and theoretical results on the inelastic decay of optically excited volume electrons in different types of metals, including simple metals (Al), noble metals (Au, ...Ag, Cu), transition metals (Ta, Mo, Rh, Co, Fe, Ni) and rare earth metals (Gd, Tb, Yb, La). The comparison of the different materials and material classes provides particular insight into the relevance of the localization and delocalization of electronic states for inelastic carrier scattering processes. The discussion of the data illustrates furthermore the capabilities and limitations of the time-resolved two-photon photoemission technique as well as current theoretical approaches in analyzing and determining inelastic lifetimes of excited electrons.
We demonstrate the ultrafast generation of electrons from tailored metallic nanoparticles and unravel the role of plasmonic field enhancement in this process by comparing resonant and off-resonant ...particles, as well as different particle geometries. We find that electrons become strongly accelerated within the evanescent fields of the plasmonic nanoparticles and escape along straight trajectories with orientations governed by the particle geometry. These results establish plasmonic nanoparticles as versatile ultrafast, nanoscopic sources of electrons.
We use subcycle time-resolved photoemission microscopy to unambiguously distinguish optically triggered electron emission (photoemission) from effects caused purely by the plasmonic field (termed ...“plasmoemission”). We find from time-resolved imaging that nonlinear plasmoemission is dominated by the transverse plasmon field component by utilizing a transient standing wave from two counter-propagating plasmon pulses of opposite transverse spin. From plasmonic foci on flat metal surfaces, we observe highly nonlinear plasmoemission up to the fifth power of intensity and quantized energy transfer, which reflects the quantum-mechanical nature of surface plasmons. Our work constitutes the basis for novel plasmonic devices such as nanometer-confined ultrafast electron sources as well as applications in time-resolved electron microscopy.
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