160 research outputs found
Electronic excitation spectrum of doped organic thin films investigated using electron energy-loss spectroscopy
The electronic excitation spectra of undoped, and potassium as well as
calcium doped phenantrene-type hydrocarbons have been investigated using
electron energy-loss spectroscopy (EELS) in transmission. In the undoped
materials, the lowest energy excitations are excitons with a relatively high
binding energy. These excitons also are rather localized as revealed by their
vanishing dispersion. Upon doping, new low energy excitation features appear in
the former gaps of the materials under investigation. In Kpicene and
Kchrysene they are characterized by a negative dispersion while in
Capicene they are dispersionless
Absence of photoemission from the Fermi level in potassium intercalated picene and coronene films: structure, polaron or correlation physics?
The electronic structure of potassium intercalated picene and coronene films
has been studied using photoemission spectroscopy. Picene has additionally been
intercalated using sodium. Upon alkali metal addition core level as well as
valence band photoemission data signal a filling of previously unoccupied
states of the two molecular materials due to charge transfer from potassium. In
contrast to the observation of superconductivity in K_xpicene and K_xcoronene
(x ~ 3), none of the films studied shows emission from the Fermi level, i.e. we
find no indication for a metallic ground state. Several reasons for this
observation are discussed.Comment: 15 pages, 6 figure
Mapping of the energetically lowest exciton in bulk -HfS
By combining electron energy-loss spectroscopy and state-of-the-art
computational methods, we were able to provide an extensive picture of the
excitonic processes in -HfS. The results differ significantly from the
properties of the more scrutinized group VI semiconducting transition metal
dichalcogenides such as MoS and WSe. The measurements revealed a
parabolic exciton dispersion for finite momentum parallel to the
K direction which allowed the determination of the effective exciton
mass. The dispersion decreases monotonically for momentum exchanges parallel to
the M high symmetry line. To gain further insight into the excitation
mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The
results matched the experimental loss spectra closely, thereby confirming the
excitonic nature of the observed transitions, and produced the
momentumdependent binding energies. The simulations also demonstrated that the
excitonic transitions for || M occur exactly along that
particular high symmetry line. For || K on the other hand,
the excitations traverse the Brillouin zone crossing various high symmetry
lines. A particular interesting aspect of our findings was that the calculation
of the electron probability density revealed that the exciton assumes a
six-pointed star-like shape along the real space crystal planes indicating a
mixed Frenkel-Wannier character.Comment: 12 pages, 10 figure
Electron Energy-Loss Spectroscopy: A versatile tool for the investigations of plasmonic excitations
The inelastic scattering of electrons is one route to study the vibrational
and electronic properties of materials. Such experiments, also called electron
energy-loss spectroscopy, are particularly useful for the investigation of the
collective excitations in metals, the charge carrier plasmons. These plasmons
are characterized by a specific dispersion (energy-momentum relationship),
which contains information on the sometimes complex nature of the conduction
electrons in topical materials. In this review we highlight the improvements of
the electron energy-loss spectrometer in the last years, summarize current
possibilities with this technique, and give examples where the investigation of
the plasmon dispersion allows insight into the interplay of the conduction
electrons with other degrees of freedom
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Charge transfer characteristics of F6TCNNQ–gold interface
The metal–organic interface between polycrystalline gold and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) was investigated by photoelectron spectroscopy with the focus on the charge transfer characteristics from the metal to the molecule. The valence levels, as well as the core levels of the heterojunction, indicate a full electron transfer and a change in the chemical environment. The changes are observed in the first F6TCNNQ layers, whereas for further film growth, only neutral F6TCNNQ molecules could be detected. New occupied states below the Fermi level were observed in the valence levels, indicating a lowest unoccupied molecular orbital (LUMO) occupation due to the charge transfer. A fitting of the spectra reveals the presence of a neutral and a charged F6TCNNQ molecules, but no further species were present
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