Theory Of Photoemission Research Articles

Understanding the Unique Electronic Properties of Nano Structures Using Photoemission Theory.

Newly emerging experimental techniques such as nano-ARPES are expected to provide an opportunity to measure the electronic properties of nano-materials directly. However, the interpretation of the spectra is not simple because it must consider quantum mechanical effects related to the measurement process itself. Here, we demonstrate a novel approach that can overcome this problem by using an adequate simulation to corroborate the experimental results. Ab initio calculation on arbitrarily-shaped or chemically ornamented nano-structures is elaborately correlated to photoemission theory. This correlation can be directly exploited to interpret the experimental results. To test this method, a direct comparison was made between the calculation results and experimental results on highly-oriented pyrolytic graphite (HOPG). As a general extension, the unique electronic structures of nano-sized graphene oxide and features from the experimental result of black phosphorous (BP) are disclosed for the first time as supportive evidence of the usefulness of this method. This work pioneers an approach to intuitive and practical understanding of the electronic properties of nano-materials.

Nearly free electrons in a 5d delafossite oxide metal.

Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit-assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along k z . Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14m e. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below E F, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free-electron system in a 5d delafossite transition-metal oxide.

Theory of photoelectron spectroscopy for organic molecules and their crystals

In this short review we discuss recent progress in photoemission theory for organic molecules and their crystals. We discuss some important features in Keldysh Green's function theory for the photoemission. We briefly discuss many-body aspects in photoemission from core and extended levels. In particular phonon effects are investigated in more detail since organic solids are typically soft where electron–phonon interaction is important. Debye–Waller factor suppresses the interference effects of photoelectron waves which makes ARPES analyses useless, particularly in high energy region.

Comparative research on reflection-mode GaAs photocathode with graded AlxGa1−xAs buffer layer

The graded Al compositional AlxGa1−xAs buffer layer can not only form continuous internal electric field from buffer layer to active layer but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. By measuring the spectral response current (SRC) for two reflection-mode (r-mode) designed samples of graded and stationary Al compositional structure, we can find the special phenomenon that the graded structure had quite influence at the middle wavelength band from 550nm to 850nm, but not the short wavelength band from 400nm to 550nm, though the buffer layer can only absorb photon energy at the short wavelength band. Through the comparative research for designed samples through SPV before Cs–O activation and SRC after Cs–O activation, the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the photon absorption lengths are relative little at the short wavelength band and relative long at the middle wavelength band, so the optimizations of key parameters have little influence on photo-excited electrons at the short wavelength band which are mainly excited from the region in active layer near surface barriers. The optimizations of key parameter, mainly the back interface recombination velocity (Sv), can have quite impact on photo-excited electrons at the middle short wavelength band which are mainly excited from the internal active layer near the back interface. This comparative research can help to well study the photo-emission theory and structure design on graded Al compositional design for r-mode GaAs photocathodes in the future research.

Three-dimensional momentum-resolved electronic structure of1T−TiSe2:A combined soft-x-ray photoemission and density functional theory study

$1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ is a quasi-two-dimensional transition metal dichalcogenide, which exhibits a charge density wave transition at a critical temperature of $\ensuremath{\sim}200$ K as well as low-temperature superconductivity induced by pressure or intercalation. The electronic energy dispersion measured by soft x-ray angle-resolved photoemission is not only momentum resolved parallel to the surface but also perpendicular to it. Experiments are compared to density functional theory based band structure calculations using different exchange-correlation functionals. The results reveal the importance of including spin-orbit coupling for a good description of the experimental bands. Compared to calculations within the local density approximation, the use of the modified Becke-Johnson (mBJ) exchange functional leads to a band structure that does not need an artificial downwards shift of the valence band to fit the experiment. The mBJ functional thus clearly appears as the most adapted functional for the theoretical description of the $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ band structure within the DFT framework.

Spin texture with a twist in momentum space for Tl/Si(111)

The electronic structure and spin texture around the $\overline{\mathrm{M}}$ point of $\mathrm{Tl}/\mathrm{Si}(111)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ is investigated with spin- and angle-resolved inverse photoemission and density-functional theory. The $\mathrm{Tl}/\mathrm{Si}(111)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface exhibits a surface-derived unoccupied state, which extends over the complete surface Brillouin zone. It is known for its giant spin-orbit-induced splitting and unique symmetry- and spin-orbit-induced spin landscape: a rotating spin pattern in momentum space along $\overline{\mathrm{\ensuremath{\Gamma}}}\overline{\mathrm{K}}$ and pure in-plane spin polarization along $\overline{\mathrm{\ensuremath{\Gamma}}}\overline{\mathrm{M}}$. Our results demonstrate how the $\overline{\mathrm{M}}$ point is encircled by this state. Its spin texture shows a peculiar twist, which reflects the ${C}_{1h}$ symmetry of the $\overline{\mathrm{M}}$ point.

Theory of valence-band and core-level photoemission from plutonium dioxide

The correlated-band theory implemented as a combination of the local-density approximation with the dynamical mean-field theory is applied to PuO2. An insulating electronic structure, consistent with the experimental valence-band photoemission spectra, is obtained. The calculations yield a nonmagnetic ground state that is characterized by a noninteger filling of the plutonium 5f shell. The noninteger filling as well as the satellites appearing in the 4f core-level photoemission spectra originate in a sizable hybridization of the 5f shell with the 2p states of oxygen.

Channeling Vibrational Energy To Probe the Electronic Density of States in Metal Clusters.

We investigate the electronic density of states (DOS) of isolated neutral cobalt clusters by probing the temperature-modulated population of electronic states through UV photoionization. The temperature is controlled via resonant excitation of lattice vibrations using the free-electron laser FELICE, after which the vibrational and electronic systems equilibrate through the electron-phonon coupling, redistributing the population of electronic states. The data are analyzed by surface photoemission theory, modified to incorporate the realistic DOS.

Thermionic and Photo-Excited Electron Emission for Energy-Conversion Processes

This article describes advances in thermionic and photoemission materials and applications dating back to the work on thermionic emission by Guthrie in 1873 and the photoelectric effect by Hertz in 1887. Thermionic emission has been employed for electron beam generation from Edison’s work with the light bulb to modern day technologies such as scanning and transmission electron microscopy. The photoelectric effect has been utilized in common devices such as cameras and photocopiers while photovoltaic cells continue to be widely successful and further researched. Limitations in device efficiency and materials have thus far restricted large-scale energy generation sources based on thermionic and photoemission. However, recent advances in the fabrication of nanoscale emitters suggest promising routes for improving both thermionic and photo-enhanced electron emission along with newly developed research concepts, e.g., photonically enhanced thermionic emission. However, the abundance of new emitter materials and reduced dimensions of some nanoscale emitters increases the complexity of electron emission theory and engender new questions related to the dimensionality of the emitter. This work presents derivations of basic two and three-dimensional thermionic and photoemission theory along with comparisons to experimentally acquired data. The resulting theory can be applied to many different material types regardless of composition, bulk and surface structure.

Angle-resolved synchrotron photoemission and density functional theory on the iridium modified Si(1 1 1) surface

The physical and electronic properties of the Ir modified Si(1 1 1) surface have been investigated with the help of angle resolved photoemission spectroscopy and density functional theory. The surface consists of Ir-ring clusters that form a reconstruction. A comparison between the measured and calculated band structure of the system reveals that the dispersions of the projected bulk states and the states originating from domains are heavily modified due to Umklapp scattering from the surface Brillouin zone. Density of states calculations show that Ir-ring clusters contribute to the states in the vicinity of the Fermi level.

CuPc/Au(1 1 0): Determination of the azimuthal alignment by a combination of angle-resolved photoemission and density functional theory

Here we report on a combined experimental and theoretical study on the structural and electronic properties of a monolayer of Copper-Phthalocyanine (CuPc) on the Au(110) surface. Low-energy electron diffraction reveals a commensurate overlayer unit cell containing one adsorbate species. The azimuthal alignment of the CuPc molecule is revealed by comparing experimental constant binding energy (kxky)-maps using angle-resolved photoelectron spectroscopy with theoretical momentum maps of the free molecule's highest occupied molecular orbital (HOMO). This structural information is confirmed by total energy calculations within the framework of van-der-Waals corrected density functional theory. The electronic structure is further analyzed by computing the molecule-projected density of states, using both a semi-local and a hybrid exchange-correlation functional. In agreement with experiment, the HOMO is located about 1.2eV below the Fermi-level, while there is no significant charge transfer into the molecule and the CuPc LUMO remains unoccupied on the Au(110) surface.

Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional.

Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett.2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1–0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found—a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

Exceptional behavior of d-like surface resonances on W(110): the one-step model in its density matrix formulation

Spin–orbit-induced spin splitting of surface states has attracted great interest in recent years because of the high potential for technological applications associated with this phenomenon. This Rashba physics is found in a variety of systems ranging from simple metals like Ag or Au to the so-called topological insulators which are of special interest in spintronics. A very special and unique case is found at the W(110) surface. In this metal d-like surface resonances exhibit energy dispersions and spin-polarization structures which are reminiscent of topological surface states. In our theoretical study, we present a complete analysis of the surface electronic structure of W(110) and show that the atypical linear-shaped dispersion behavior is triggered by the amount of charge transfer from the bulk into the first few vacuum layers. Furthermore, we compare our theoretical spectra with experimental photoemission data on W(110) and demonstrate that our state-of-the-art photoemission theory is able to deal with these peculiar surface features in a quantitative way. Our analysis is based on a generalization of the relativistic one-step model of photoemission, recently extended by us to study photoelectron spectroscopy at high photon energies. This theoretical approach was realized in the full spin-density matrix formulation for the photocurrent, which allows for an unrestricted calculation of the spin-polarization vector of the photoelectron. As an additional result we predict very peculiar behavior of these surface features showing up even at soft and hard x-ray energies. This observation is very surprising, unprecedented for ordinary surface features on simple metal surfaces.

Formation of an Organic/Metal Interface State from a Shockley Resonance.

The emergence of an organic/metal interface state is studied for the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(100) by means of time- and angle-resolved two-photon photoemission and density functional theory. The interface state is located in the projected band gap of Ag(100), 2.3 eV above the Fermi level, and has an electron lifetime in the 10 fs range. The interface state has no direct counterpart on the clean Ag(100) surface, which only exhibits a broad resonance at ≈1 eV lower energy. Our results thus show that organic overlayers are able to shift such a resonance substantially and even qualitatively change its character to a distinct electronic state at the interface.

Exploring the XPS limit in soft and hard x-ray angle-resolved photoemission using a temperature-dependent one-step theory

We present a theory of temperature-dependent photoemission which accurately describes phonon effects in soft and hard x-ray angle-resolved photoemission. Our approach is based on a fully relativistic one-step theory of photoemission that quantitatively reproduces the effects of phonon-assisted transitions beyond the usual $\mathbf{k}$-conserving dipole selection rules which lead to the so-called XPS limit in the hard x-ray and/or high temperature regime. Vibrational atomic displacements have been included using the coherent potential approximation in analogy to the treatment of disordered alloys. The applicability of this alloy analogy model is demonstrated by direct comparison to experimental soft x-ray data from W(110) showing very satisfying agreement.

Momentum-resolved electronic structure at a buried interface from soft X-ray standing-wave angle-resolved photoemission

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for the study of electronic structure, but it lacks a direct ability to study buried interfaces between two materials. We address this limitation by combining ARPES with soft X-ray standing-wave (SW) excitation (SWARPES), in which the SW profile is scanned through the depth of the sample. We have studied the buried interface in a prototypical magnetic tunnel junction La0.7Sr0.3MnO3/SrTiO3. Depth-and momentum-resolved maps of Mn 3d eg and t2g states from the central, bulk-like and interface-like regions of La0.7Sr0.3MnO3 exhibit distinctly different behavior consistent with a change in the Mn bonding at the interface. We compare the experimental results to state-of-the-art density-functional and one-step photoemission theory, with encouraging agreement that suggests wide future applications of this technique.

Anisotropic Eliashberg function and electron-phonon coupling in doped graphene

We investigate, with high-resolution angle-resolved photoemission spectroscopy, the spectral function of potassium-doped quasi-free-standing graphene on Au. Angle-dependent x-ray photoemission and density functional theory calculations demonstrate that potassium intercalates into the graphene/Au interface, leading to an upshift of the K-derived electronic band above the Fermi level. This empty band is what makes this system perfectly suited to disentangle the contributions to electron-phonon coupling coming from the $\ensuremath{\pi}$ band and K-derived bands. From a self-energy analysis we find an anisotropic electron-phonon coupling strength $\ensuremath{\lambda}$ of 0.1 (0.2) for the $K\ensuremath{\Gamma}$ ($KM$) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in $KM$ does an additional low-energy part appear.

Photonically excited electron emission from modified graphitic nanopetal arrays

Efficient electron emission for energy conversion requires a low work function and a stable emitter material. The work function of graphene-based carbon materials can decrease significantly by intercalation with alkali metals, thus increasing their emission current. In this work, electron emission from potassium-intercalated carbon nanosheet extensions grown on electrode graphite is investigated. These petal-like structures, composed of 5–25 layers of graphene, are synthesized using microwave plasma chemical vapor deposition. Samples are intercalated with potassium, and a hemispherical energy analyzer is used to measure the emission intensity caused by both thermal and photonic excitation. The emission from the potassium-intercalated structures is found to consistently decrease the work function by 2.4 to 2.8 eV relative to non-intercalated samples. High emission intensity induced by photonic excitation from a solar simulator, with a narrow electron energy distribution relative to established theory, suggests that electron scattering decreases emitted electron energy as compared to surface photoemission. A modified photoemission theory is applied to account for electron scattering, and the sample work function and mean number of scattering events are used as parameters to fit theory to experimental data. The thermal stability of the intercalated nanopetals is investigated, and after an initial heating and cooling cycle, the samples are stable at low temperatures.

Photoemission and density functional theory study of Ge(100): Clean surface and Yb-induced (2×4) reconstruction

Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2×4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).

X-ray Photoemission and Density Functional Theory Study of the Interaction of Water Vapor with the Fe3O4(001) Surface at Near-Ambient Conditions

The interaction of water with the Fe3O4(001) surface was investigated in a combined ambient pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. The uptake of molecular water and hydroxyl species on the (001) surface of a natural magnetite sample at near-ambient conditions was quantified using O 1s spectra taken in the p(H2O) range from 10–9 to 2 Torr. At low p(H2O) (≤10–4–10–5 Torr) and room temperature, we found that water does not adsorb dissociatively on the surface, except on defect sites. In contrast, progressive dissociation into surface hydroxyl species was observed between 10–4 and 10–2 Torr p(H2O). The onset of hydroxylation coincides with the increasing presence of molecular water species on the surface, which demonstrates the key role played by cooperative interactions between adsorbed water molecules, leading to dissociation and surface hydroxylation. The measured O 1s chemical shifts of hydroxyl and molecular water species from both isotherm and isobar d...