Core Exciton Binding Energies Research Articles

Understanding 2p core-level excitons of late transition metals by analysis of mixed-valence copper in a metal-organic framework.

The L2,3-edge X-ray absorption spectra of late transition metals such as Cu, Ag, and Au exhibit absorption onsets lower in energy for higher oxidation states, which is at odds with the measured spectra of earlier transition metals. Time-dependent density functional theory calculations for Cu2+/Cu+ reveal a larger 2p core-exciton binding energy for Cu2+, overshadowing shifts in single-particle excitation energies with respect to Cu+. We explore this phenomenon in a Cu+ metal-organic framework with ∼12% Cu2+ defects and find that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.

Multitechnique Approach for Determining Energy Levels and Exciton Binding Energies of Molecules for Organic Electronics

The energies of the frontier orbitals and their electron–hole binding energy are key quantities in organic electronics, such as organic light-emitting diodes (OLEDs) and solar cells. A method is developed for obtaining these quantities from ultraviolet photoelectron spectroscopy (UPS) and optical absorption spectroscopy, combined with calculations of the ionization energy (IE) and the electron affinity (EA). This method provides a more efficient and accurate determination of the lowest unoccupied molecular orbital (LUMO) together with the exciton binding energy. The concept is extended to element-sensitive core-level spectroscopies, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), in order to obtain core exciton binding energies. Two prototypical organic molecules for OLED applications are used as examples, i.e., N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) and dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT:CN). These...

Electron-hole correlation effects in core-level spectroscopy probed by the resonant inelastic soft x-ray scattering map of C60

We have employed a unique spectroscopic approach, a resonant inelastic soft x-ray scattering (RIXS) map, to identify and separate electron-hole correlation effects in core-level spectroscopy. With this approach, we are able to derive a comprehensive picture of the electronic structure, separating ground state properties (such as the HOMO-LUMO separation) from excited state properties (such as the C 1s core-exciton binding energy of C(60)). In particular, our approach allows us to determine the difference between core- and valence exciton binding energies in C(60) [0.5 (±0.2) eV]. Furthermore, the RIXS map gives detailed insight into the symmetries of the intermediate and final states of the RIXS process.

Spectral properties of InN and its native oxide from first principles

AbstractElectronic single‐ and two‐particle excitations and their influence on electron, optical, and X‐ray spectra are studied by ab initio methods and discussed in the light of experiments for InN and its native oxide In2O3. The theoretical approaches are based on iterative solutions of the quasiparticle equation. The resulting gaps for InN and In2O3 are close to experimental data. The total density of states (DOS) explains the XPS measurements. There is excellent agreement between site‐ and orbital‐projected DOS and X‐ray absorption spectra. The rigid shift between theory and experiment is identified as core exciton binding energy. The inclusion of excitonic effects yields optical spectra whose lineshapes are in agreement with the measured peak structure. Our computational method possesses an accuracy that allows the prediction of binding energies EB in the meV range for Wannier–Mott excitons. For InN we predict $E_{\rm B} < 5\,{\rm meV}$. The parameter‐free methods give the energy gaps of the different In2O3 polymorphs and explain the so‐called indirect gap by interconduction band transitions. The band alignment using the branch‐point energy yields band discontinuities resulting in a type‐I heterostructure.

Theory of core exciton binding energies for excitons near interfaces

The major chemical trends in the binding energies of core excitons near interfaces are predicted for excitons in zincblende hosts.

Comparison of the size of excitonic effects in molecular π systems as measured by core and valence spectroscopies

In aromatic molecules characterised by extended π-systems valence and core excitons can have very similar binding energies. This is shown for a number of compounds, and in each case the core exciton binding energies are analysed in terms of the Coulomb screening and rehybridisation contributions. The latter semiquantitatively explain the binding energy differences between the valence and core excitons. This suggests that core and valence excitons have a similar spatial extent in such systems. In favourable cases core level techniques can thus be used to elucidate valence excited state properties.

The influence of crystal structure on the Zn 3d states in Zn 1−xMn xS

High resolution reflectance spectroscopy experiments were performed to study the Zn 3d states of Zn 1- x Mn x S over the whole concentration range (0≤ x≤0.56). For zincblende-type samples ( x<0.05), we observe the excitation of three core level excitons reflecting the threefold split Zn 3d states at the Γ-point. The splitting of the Γ 5 state due to spin-orbit interaction is Δ so=251 meV. For the core exciton binding energy, a value of 517 meV is determined. For wurtzite-type samples ( x>0.05), the core exciton derived structures are broadened, reflecting the tenfold splitting of the 3d states under C 6v symmetry.

Conduction-band states in GaSb(110) and GaP(110) at the Brillouin-zone center.

Angle-resolved constant-initial-state spectroscopy (ARCIS) from the valence-band maximum (VBM) as initial state was used to determine conduction-band energies at the \ensuremath{\Gamma} point up to 66 eV above the VBM for GaSb and GaP. With the help of nonlocal-empirical-pseudopotential (EPM) calculations, structure in the ARCIS spectra up to 17 eV can be assigned to particular interband transitions. The experimental results are discussed in light of our previous work on the conduction-band states of GaAs, InP, and InAs. Above 40 eV there appears to be no correspondence between conduction-band states as calculated in the EPM framework and structure in the spectra. Autoionizing resonances due to surface core excitons have been observed in both materials. From their energies a surface core-exciton binding energy of 0.6 eV for GaSb and 1.4 eV for GaP has been determined.

Al L2,3 core excitons in AlxGa1-xAs studied by soft-x-ray reflection and emission.

The Al ${\mathit{L}}_{2,3}$ soft-x-ray reflection and emission spectra of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As have been measured for x between 0.17 and 1.0. The valence-band maximum was monitored via the emission spectra using a fit based on band-structure parameters. The dominant features in the reflection spectra are produced by the ${\mathit{L}}_{2,3}$ core excitons. Since both the reflection spectra and emission spectra were obtained using the same soft-x-ray spectrometer, the energies of the excitons above the valence-band maximum are accurately obtained. The known values of the optical band gap, as a function of x, are then used to determine the core exciton binding energy. The core excitons are found to follow and lie above the conduction-band minimum by 0.3 eV as the Al composition changes. Our results are compared to various core exciton theories and to previous experiments.

Dynamical Effects on the Binding Energies of Core Excitons in Covalent and Polar Semiconductors

AbstractThe binding energies of core excitons in covalent and polar semiconductors are obtained by the polaron model taking into account the dynamical screening by the valence electrons. The binding energies of core excitons thus obtained are comparable to the available experimental values.

Electronic structure of cationic core excitons in II–VI semiconductors

The electronic structure of cationic core excitons in ZnS, ZnSe, ZnTe, and CdTe was analyzed by means of self-consistent-field multiple scattering Xα molecular cluster calculations. The exciton level introduced in the gap by the core hole is found to be weakly localized with a charge distribution similar to that of an isocoric single donor level. Core exciton binding and formation energies and also core ionization energies were calculated. The binding energies of the core excitons in ZnS are found to be almost independent of the depth of the core level involved. This behavior seems to be a general property of core excitons in semiconductors. Trends for core exciton binding energies, core level shifts and charge distributions are analyzed in the ZnX (X = S, Se, Te) series and also in CdTe.

Electronic properties of the precrystallization regime of germanium: A photoemission study.

Ultraviolet photoemission spectroscopy with synchrotron radiation has been used to measure the electronic properties of germanium films grown under different conditions. The substrates were germanium (111) single crystals and oxidized tungsten. Measurements were performed on amorphous germanium and on hydrogenated amorphous germanium films annealed at different temperatures and on films grown on substrates kept at temperatures higher than room temperature. The evolution of the electronic properties from amorphous to crystal-like behavior indicates the existence of a precrystallization regime where germanium clusters---mainly (111) oriented---are embedded in an amorphous matrix. Cluster size depends on the substrate temperature. We also observed a large variation of the photoemission gap corresponding to a different degree of ordering of the grown films. A comparison of photoemission and optical gaps shows that the core-exciton binding energy increases with an increasing degree of disorder.

Binding energy and spectral width of Si 2p core excitons in SixGe1-x alloys.

A calculation is presented to explain the anomalous experimental behavior of the Si 2p core-exciton binding energy and linewidth in ${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys. The observed minimum in the line width near x\ensuremath{\simeq}0.15 can be explained as being the result of a competition between intrinsic broadening due to screening and extrinsic alloy broadening. For pure Si, the binding energy is estimated to be 0.15\ifmmode\pm\else\textpm\fi{}0.05 eV and the width is shown to be smaller that that observed at x\ensuremath{\simeq}0.15.

Core excitons in cuprous halides

The binding energies of core-excitons in cuprous halides are calculated with help of the electron polaron model in which the valence electrons are considered as quantum oscillators. We advocate that when dealing with core-excitons the hole polaron should be considered as localized due to its infinite mass. The binding energies thus calculated are of right magnitude.

Core excitons in the electronic polaron model

The electronic polaron model of the exciton is used to study the dielectric response of a medium to the excitation of a «core» level, by adopting the method of direct solution of the Eulerian functional variational equations. The dynamical response of the electronic polarization affects the electron-hole attraction and the exciton binding energy, in a way which depends on the basic parameters of the crystal (dielectric constant, effective masses, lattice parameter) through the core exciton radius and the polaron radius. When the former is much larger than the latter, static dielectric screening results. When the exciton radius is comparable to the polaron radius, the screening is reduced and the binding energy is increased. Core exciton binding energies are computed in a number of substances using the effective-mass approximation. Space dispersion of the dielectric function coupled to intervalley interaction may, however, contribute in some cases to reducing the excitonic radius and bringing about an instability to a deep state that would invalidate the effective-mass approximation.

Luminescence excitation spectra of RbI and RbBr between 40 and 130 eV

Using synchrotron radiation, the intrinsic luminescence excitation spectra of RbI and RbBr were measured between 40 and 130 eV, at low temperatures, and the impurity luminescence at room temperature. Some surprising features correlated with competing decay routes after photoexcitation, which increase in complexity as the exciting energies enlarge. Luminescence can be used to determine core-edge thresholds and core-exciton binding energies.

Optical determination of exciton sizes in semiconductors

We have studied the gradual disappearance of Ga3d core-exciton-related features in energy derivative reflectance spectra of GaAs crystals ion-implanted to increasing fluences. Our data yield radii rL = 23 Å and rx = 28 Å for the L and X excitons. In addition, a lineshape fit for unimplanted GaAs yields core exciton binding energies EL = 0.172 eV and Ex = 0.129 eV. These radii and binding energies are not related through an effective mass approximation description of the core exciton.

Core excitons in semiconductors with a decaying core hole

It is shown that deviations from static electronic screening, in addition to contributing an increase of the core-exciton binding energy EB, produce also a reduction of the core-exciton spectral width Γ as compared with the corresponding spectral width γ of the core hole alone. The occurence of this effect is interpreted as a fingerprint of dynamical screening since it cannot be obtained within the framework of a static theory. Numerical estimates for the Si 2p transition give about 70 meV for the difference γ-Γ, in qualitative agreement with experimental data.

The Ga core exciton at unrelaxed GaAs (110)

Core excitonic effects at an unrelaxed GaAs (110) surface are studied using a localized orbital approach. A realistic Hamiltonian for the conduction bands is obtained with inclusion of 5s orbitals and third nearest neighbor interactions. The binding energies of core excitons both in the bulk and at the surface are calculated. The results are in good agreement with experimental measurements, with a contact potential of ∠l.0 eV. In addition it is shown that the binding energy has a maximum value and the limiting case can be thought of as an ideal vacancy.

Band Structure and Optical Properties of In1-xGaxAsyP1-y

Dielectric function spectra in the visible-near uv and energy derivative reflectance spectra in the vacuum uv spectral ranges are reported for In1-xGaxAsyP1-y at compositions between y=0 and 1 lattice-matched to InP. Analysis of these spectra yields bowing parameters for the valence-conduction band critical points E1, E1+Δ1, and E'0, estimates of core exciton binding energies, and the ordering of the lower conduction band minima at L and X.