The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.
The
valence band spectra of three cyano-ionic liquids based on
1-ethyl-3-methylimidazolium (Im2,1
+) paired with thiocyanate (SCN–), dicyanamide (N(CN)2
–), and tricyanomethanide
(C(CN)3
–) have been measured using ultraviolet
and X-ray photoemission spectroscopy. Experimental spectra are compared
to their corresponding density of states, weighted by photoemission
cross sections, calculated for clusters of ions pairs of increasing
size. Thus, this study bridges single ion approaches to 3D periodical
DFT studies and enables the exploration of the different aspects of
electronic structure establishment in ILs. Even for a relatively small
cluster size, the relative energy of cation and anions states shifts
by an amount that corresponds closely to that expected from the Madelung
energy of a bulk IL, and the photoemission cross section-weighted
DOS spectra are in good agreement with the measured valence bands.
Trends in the relative energy and ionic character of the frontier
orbitals across this series of cyano-ILs are discussed.
The geometric and electronic structures of a monolayer of rubrene and of a fluorinated rubrene derivative (FM-rubrene), adsorbed on a Ag(100) surface, were determined using scanning tunneling microscopy (STM), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPS) to study the influence of fluorinefunctionalization on self-assembly, molecular energy levels, and energy-level alignment. STM images at room temperature reveal that the molecules form different molecular assemblies at the monolayer coverage and have the tetracene backbones nearly parallel to the surface, with strongly splayed phenyls. While the assemblies are highly ordered, the molecular orientation and intermolecular spacings differ from those of the respective molecular crystals and thus do not act as templates for the epitaxial growth of ordered multilayer molecular films. UPS and IPS measurements indicate that the frontier orbitals of an adsorbed FM-rubrene molecular layer are shifted downward with respect to the Ag(100) Fermi level by 0.2 eV as compared to those of a rubrene monolayer. Moreover, the intrinsic molecular dipole of FM-rubrene contained in the first layer leads to an interface dipole of 0.2 eV, further shifting added molecular layers to higher energies. Comparison of the adsorption of rubrene to that of FM-rubrene on Ag(100) provides valuable insights into the nature of the molecule surface and intermolecular interactions that drive self-assembly and energy-level alignment, as well as their effects on the potential growth of ordered molecular multilayers.
Experiments are presented in which strong optical absorption at the surface of small metal particles is been observed. This was accomplished by the following method. Na clusters with rough surfaces were prepared on a transparent substrate. They provide the possibility of localized electronic excitations in the surface region in addition to the well-known plasmon-polaritons in the bulk. The particles were irradiated with pulsed laser light and their temperature rise was derived from measurements of the kinetic energy distribution of thermally desorbed Na dimers. From the temperature increase, the heat generated in the particles during decay of both surface and bulk excitations was calculated and taken as a measure of the total absorption cross-section. Separation of the bulk and surface contributions was achieved by repeating the experiments after removal of the surface roughness by annealing. We find that the surface contributes as much as 32% to the total optical absorption at 2 = 532 nm for Na clusters of a mean radius R of 21 mm.
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