Angle-resolved Inverse Photoemission Spectroscopy Research Articles

Empty electron states in cobalt-intercalated graphene.

The dispersion of the electronic states of epitaxial graphene (Gr) depends significantly on the strength of the bonding with the underlying substrate. We report on empty electron states in cobalt-intercalated Gr grown on Ir(111), studied by angle-resolved inverse photoemission spectroscopy and x-ray absorption spectroscopy, complemented with density functional theory calculations. The weakly bonded Gr on Ir preserves the peculiar spectroscopic features of the Gr band structure, and the empty spectral densities are almost unperturbed. Upon intercalation of a Co layer, the electronic response of the interface changes, with an intermixing of the Gr π* bands and Co d states, which breaks the symmetry of π/σ states, and a downshift of the upper part of the Gr Dirac cone. Similarly, the image potential of Ir(111) is unaltered by the Gr layer, while a downward shift is induced upon Co intercalation, as unveiled by the image state energy dispersion mapped in a large region of the surface Brillouin zone.

Unoccupied electronic states of Ru(0001)

This report presents a combined theoretical and experimental description of the unoccupied electronic states of Ru(0001), along the Γ̅M̅ high symmetry direction of the Brillouin zone. A direct comparison between angle-resolved inverse photoemission spectroscopy and ab initio calculations of the 3-dimensional (3D) electronic structure of Ru(0001) have been used to determine the energy dispersion and the identification of different states and surface resonances. Both, measurements and calculations, complement previous reports regarding the electronic structure of Ru.

Conduction-band electronic structure of 1T-TaS2revealed by angle-resolved inverse-photoemission spectroscopy

We have investigated the conduction-band electronic structure of 1$T$-TaS${}_{2}$ in the incommensurate (IC) and commensurate (C) charge-density-wave (CDW) phases by means of angle-resolved inverse-photoemission spectroscopy (ARIPES) along the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)--M\phantom{\rule{0.28em}{0ex}}(L)$ direction. In the ARIPES spectra for the ICCDW phase, a continuous and dispersive Ta 5$d$ band is clearly observed near the Fermi level (${E}_{\mathrm{F}}$). For the CCDW phase, on the other hand, the Ta 5$d$ band splits up and down into two manifolds around the $M\phantom{\rule{0.28em}{0ex}}(L)$ point, reflecting formation of the $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure of the Ta plane. In addition, the CCDW-ARIPES spectrum around the $M\phantom{\rule{0.28em}{0ex}}(L)$ points exhibits a shoulder structure at 0.7 eV above ${E}_{\mathrm{F}}$, ascribed to the upper Hubbard band folded from the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)$ point. We have also observed an energy gap of $\ensuremath{\sim}$100 meV in the unoccupied density of states in the CCDW phase.

Nitrogen-doping processes of graphene by a versatile plasma-based method

We present controllable processes to fabricate nitrogen-doped graphene with different doping configurations, in particular pyridinic- or graphitic-nitrogen-rich graphene. Nitrogen-doping is realized by in situ post-growth doping of monolayer graphene on SiC(0001) using a tunable hybrid plasma source, which exposes the graphene to a stream of ions and/or to a neutral flow of thermalized species from a precursor nitrogen gas. Using angle-resolved inverse photoemission spectroscopy (ARIPES), we determine the doping level of the graphene through the analysis of the π∗ states dispersion. At a dose of about 1×1015ionscm−2 and an energy of ∼35eV, the Fermi level of the nitrogen-doped graphene is brought 0.4eV above its value for the pristine graphene monolayer on SiC(0001). This n-type doping is attributed to a 8.7 at.% graphitic-nitrogen as revealed by the core level spectroscopy (XPS). In contrast, exposure to a beam of thermalized nitrogen atoms mainly induces pyridinic-like nitrogen doping of graphene. It is thus possible to control the predominant bonding configuration of the doping nitrogen by choosing either a thermalized atom or a low-energy ion source.

Electronic structure of single crystal and highly oriented pyrolytic graphite from ARPES and KRIPES

We present a comparative study of the near fermi-level electronic structure of single crystal and highly oriented pyrolytic graphite (HOPG). Angle resolved photoelectron spectroscopy and angle resolved inverse photoelectron spectroscopy have been used to probe the occupied and unoccupied electronic states, respectively. The band dispersions showed by single crystal graphite along its Γ K and Γ M symmetry directions were found to be in agreement with calculated band structure of graphite. The π bands of single crystal graphite were found to have a splitting of ∼ 0.5 eV at the K-point. We also observe the presence of a quasiparticle peak below E F at the K point at low temperature which indicates a strong electron–phonon coupling in graphite. In HOPG, the M and K points like features were found to be present in the same radial direction due to the superposition of the Γ M and Γ K directions. Results from our angle resolved inverse photoemission spectroscopy present the dispersion of the conduction band states, particularly the lower π ⁎ band. We have also found the presence of some non-dispersive features in both the valence and the conduction bands.

Surface electronic structures of Ba overlayers on W(100), W(110), and W(111)

The total energy distributions (TEDs) in field emission (FE) and photofield emission (PFE) and the work functions have been measured at room temperature for Ba adsorbed on W(100), W(110), and W(111) in the range of coverage from 0 to 1 ML (monolayer). We observe two initial state peaks and three final state peaks on $\mathrm{W}(100)∕\mathrm{Ba}$, six initial state peaks and one final state peak on $\mathrm{W}(110)∕\mathrm{Ba}$, and two initial state peaks and two final state peaks on $\mathrm{W}(111)∕\mathrm{Ba}$. We extend the full-potential linear augmented plane wave method for the electronic structures of periodic lattices to calculate the emission current in FE and surface PFE at a metal-adsorbate-vacuum interface. Our calculations account for the energies of all of the initial state features observed experimentally in FE and surface PFE from clean W(100), in PFE and angle-resolved inverse photoemission spectroscopy from $\mathrm{W}(100)∕\mathrm{Ba}$ at 1 ML, and for all of the peaks observed in FE, surface PFE, and photoemission from $\mathrm{W}(110)∕\mathrm{Ba}$ at 0.6 ML and from $\mathrm{W}(111)∕\mathrm{Ba}$ at 1 ML. The $d{z}^{2}$-like surface states of the Swanson hump [L. W. Swanson and L. C. Crouser, Phys. Rev. Lett. 16, 389 (1966)] of clean W(100) hybridize with $s$-like states of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ overlayer and are shifted by $\ensuremath{-}1.60\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to yield a prominent peak in PFE. An isolated Ba $c(2\ifmmode\times\else\texttimes\fi{}2)$ layer is found to be weakly metallic; the metallicity is greatly enhanced when it is adsorbed on a W(100) substrate. The TEDs in PFE from the atomically less dense overlayer $\mathrm{W}(110)∕\mathrm{Ba}$ $(2\ifmmode\times\else\texttimes\fi{}2)$ are dominated by substrate-overlayer interactions, while those from the atomically denser overlayer $\mathrm{W}(111)∕\mathrm{Ba}$ $(1\ifmmode\times\else\texttimes\fi{}1)$ are dominated by interactions within the overlayer. Our results yield evidence that above $1∕3$ ML Ba coverage on W(111), which corresponds to a commensurate $({3}^{1∕2}\ifmmode\times\else\texttimes\fi{}{3}^{1∕2})R30\ifmmode^\circ\else\textdegree\fi{}$ overlayer, the interstitial sites fill in randomly to form a commensurate $(1\ifmmode\times\else\texttimes\fi{}1)$ overlayer at 1 ML coverage.

Temperature dependent electronic correlation effects in GdN

We investigate temperature dependent electronic correlation effects in the conductionbands of gadolinium nitride (GdN) based on the combination of many-body analysis of themulti-band Kondo lattice model and the first principles TB-LMTO band-structurecalculations. The physical properties like the quasi-particle density of states (Q-DOS),spectral density (SD) and quasi-particle band structure (Q-BS) are calculated anddiscussed. The results can be compared with spin and angle resolved inverse photoemissionspectroscopy (ARIPS) of the conduction bands of GdN. A redshift of 0.34 eV of the lowerband edge () is obtained and found to be in close agreement with earlier theoretical prediction andexperimental values reported in the literature.

Correlated surface bands of the prototypical interface Sn/Si(1 1 1)-α-[formula omitted

Using angle-resolved inverse photoemission spectroscopy (KRIPES), we have investigated the unoccupied electronic structure of the model interface Sn/Si(111)-α-3 at room temperature. In addition to a “metallic” surface state crossing the Fermi level (EF) near the K′ point, we unambiguously assign a second feature of our KRIPES spectra, located around 1.5eV above EF, to a second surface state U2′. We will experimentally show that U2′ is an intrinsic feature of the α-3 reconstruction which cannot be associated with defects. The existence of these two surface states is not compatible with the ideal T4 model which would show either a single, half-occupied metallic band crossing EF, or an insulating phase if strong correlation effects, important for these narrow surface bands, are considered. Rather, both U1′ and U2′ receive a natural explanation, once many-body effects are introduced, in the framework of a dynamical fluctuations model, where two kinds of Sn adatoms sites reminiscent of a low-temperature 3×3 phase do persist at room temperature. Correlated surface bands incorporating many-body effects in a non restricted way provide a complete description of the experimental surface bands and their dispersions.

Mesoscopic structures in two-dimensional ferroelectric polymers

Unique insight into the nature of ferroelectricity is emerging from the study of the first two-dimensional ferroelectric films, which are made by Langmuir-Blodgett deposition of vinylidene fluoride copolymers. These films are the first truly two-dimensional ferroelectrics, with thickness-independent bulk ferroelectric properties and a separate surface phase transition. The films offer a unique system for the study the relationship between ferroelectric properties and mesoscopic structure. The structure at all length scales was probed using complementary techniques: X-ray and neutron diffraction, scanning tunneling microscopy, atomic force microscopy, electric field microscopy, low-energy electron diffraction, angle-resolved inverse photoemission spectroscopy, infrared ellipsometry, electron energy-loss spectroscopy, scanning electron microscopy, optical polarization microscopy. We also introduce new results from dynamic pyroelectric scanning microscopy.

Solid-state graphitization mechanisms of silicon carbide 6H–SiC polar faces

Due to the higher vapour pressure of silicon, silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The comparison of graphite formation on the silicon and carbon terminations of 6H–SiC reveals significant differences in the graphitization mechanisms involved. The conduction-band structure of these interfaces has been determined by angle-resolved inverse photoemission spectroscopy (KRIPES). Although the graphite layers grown on the C face are essentially polycrystalline, a small fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 30° with respect to the basis vectors of the SiC lattice as in the case of the Si face. This in-plane disorder is in contrast with the growth of graphite on the Si face that takes place on a “passivated” adatom-terminated surface, leading to single-crystalline, heteroepitaxial graphite growth. The observation of unshifted π* states indicates a very small interaction of the first graphite monolayer with the Si face. In contrast, KRIPES reveals that the first graphite layer is strongly bound to the C face. A rehybridization of the graphite π* states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.

High-temperature graphitization of the 6H-SiC (0001) face

Silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The gradual graphitization of the 6H-SiC (0001̄) face (carbon termination) has been studied by angle-resolved inverse photoemission spectroscopy (KRIPES). The initial growth of graphite occurs on a 2×2 reconstruction which is best observed by LEED after annealing at 1050°C. Graphite begins to grow at temperatures as low as 1080°C, above which a hexagonal array of tangentially elongated diffraction spots appears superposed with the 2×2 spots of the substrate. The observation by KRIPES of a fingerprint of the three-dimensional character of graphite reveals the growth of several layers as low as 1150°C. Above 1200°C, the elongated spots join to form closed diffraction rings whose brightest parts define a hexagonal lattice corresponding to graphite. The graphite layers grown on the C face are thus essentially azimuthally disordered, although a significant fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 30° with respect to the basis vectors of the SiC lattice. KRIPES reveals that the first graphite layer is strongly bound to the C face since the π∗ states are lacking at low carbon coverage. This strong interaction explains why graphite is not free to orient at the surface, which is best viewed as a mosaic of small crystalline domains with azimuthal disorder, similar to the case of HOPG. A rehybridization of the graphite π∗ states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.

Compact grating spectrometer for inverse photoemission spectroscopy

We report on a normal incidence grating spectrometer for angle-resolved inverse photoemission spectroscopy covering a photon energy range from 10 to 40 eV. Following diffraction from a spherical grating with 201.4 mm radius and 3600 lines/mm the photons are recorded by a position sensitive detector. The compact mounting of the grating and detector on a single 150 mm CF flange is established by displacing detector and sample from the Rowland circle. The performance of the spectrometer concerning energy calibration, spectral efficiency, optical resolution, and count rates was experimentally investigated and found to match the design goals well. Operated together with a low energy electron gun using a low temperature BaO cathode a typical total energy resolution of 360 meV at hν=15 eV is obtained in inverse photoemission spectroscopy. It is shown that due to its compactness the spectrometer unit can be effectively combined with other components for surface analysis.

Unoccupied surface states of Cr(001) detected by K-resolved inverse photoemission

The energies of unoccupied surface states of Cr(001) have been measured using angle-resolved inverse photoemission spectroscopy. Spectra of the clean surface were recorded varying the momentum parallel to the surface along the \\ ̄ gG- \\ ̄ gD-X̄ and \\ ̄ gG- \\ ̄ gS-M̄ high-symmetry directions. The surface states were distinguished from the bulk d bands by contaminating the surface and comparing the difference between the clean and contaminated spectra. This showed surface states near E F and 1.7 eV above it. In the spectra of the clean surface, the surface states could be distinguished from the bulk d bands at some angles of incidence and appeared to disperse very little. The observed surface states are identified with the unoccupied majority and minority spin surface states calculated by Klebanoff et al. [1]. The energies of the bulk bands were in agreement with the ASA-LMTO calculation of Skriver [2].

Spin resolved empty energy bands in Fe/Ag(1 0 0) films

We present a room temperature study of the empty electronic states in thin Fe films expitaxially grown on Ag(1 0 0). The spin resolved band structure of the magnetic film is determined vs film thickness in the range from 1 to 100 bcc Fe monolayers by means of spin- and angle-resolved Inverse Photoemission Spectroscopy. Measurements on films thicker than 5 monolayers point out the presence of two peaks in the spectra exhibiting opposite polarization and related to majority and minority empty bands split-off by the exchange interaction. No inplane spin polarization is observed in thinner films.

Angle-resolved inverse photoemission study of Bi 2Sr 2CaCu 2O 8: Existence of two CuO 2 bands across the Fermi level

Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2Sr 2CaCu 2O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.

The unoccupied electronic structure of 1T-VSe2

By use of angle-resolved inverse photoemission spectroscopy (ARIPES) in the isochromate mode using 9.88 eV photon energy the authors have studied the unoccupied electronic structure of 1T-VSe2 along the principal symmetry directions Gamma M, Gamma M' and Gamma K. The data are found to be in good agreement with the recent ARIPES study of Law et al (1990) employing 20 eV photon energy. The experimental bands are reasonably well described by band-structure calculations but viewed together with the 20 eV data appear to be less dispersive perpendicular to the surface than theoretically predicted. A structure at 5 eV above EF for normal incidence can be identified as an image potential state.

Angle-resolved inverse photoemission spectroscopy with longitudinally spin-polarized electrons

We have built and tested a spin-polarized electron source based on photoemission from negative electron affinity GaAs(100). It provides 5–10 μA of longitudinally spin-polarized electrons in the energy range 5–20 eV. Following the design of Erdman and Zipf, the electron optics are simple and consist of only five elements. Their voltages are all proportional to each other and are linearly driven by one ramping voltage. The spin-polarized electrons are used for angle-resolved inverse photoemission measurements from magnetized sample surfaces. Our first results obtained from Ni(110) mounted on a horseshoe magnet agree well with the literature. From these measurements the degree of spin polarization is determined to be 35%±5%. We also checked the spin polarization using an absorbed-current spin detector in combination with a 90° cylindrical deflector.

Ultrathin magnetic Fe films on W(001): A spin-polarized inverse photoemission study of Fe electronic structure evolution (abstract)

By using spin-dependent electron spectroscopic techniques, one can explore the relevant problems in 3d metallic ferromagnetism, including temperature effects. Current surface science methods of ultrathin-film preparation and characterization make it possible to study interesting two dimensional magnetic systems. In the present work, ultrathin epitaxially grown Fe films on W(001) have been investigated by spin- and angle-resolved inverse photoemission spectroscopy. The evolution of the electronic structure has been studied for thickness from one monolayer to ‘‘bulklike’’ Fe thin films. In-plane remanent magnetization is observed. In the thick bulklike sample, transitions to majority and minority states near the H point of bcc Fe were identified, in contrast with data from bulk Fe(001).1 The thinner Fe films showed a trend of ‘‘collapsing’’ d bands at H′25. The results suggested a reduced Curie temperature for monolayers thick films. Information obtained for examining the correlation length in the itinerant ferromagnetic systems does not support the local-band picture at finite temperatures.2

Conduction-band structure and charge-density waves in 1T-TaS2.

Angle-resolved inverse photoemission spectroscopy (ARIPES) has been utilized to study the conduction-band structure of 1T-${\mathrm{TaS}}_{2}$ in the presence of charge-density waves (CDW's). In room-temperature spectra taken along the principal symmetry directions \ensuremath{\Gamma}M, \ensuremath{\Gamma}M', and \ensuremath{\Gamma}K we can clearly identify Ta 5d--derived ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ manifolds, which reveal a substructure of almost dispersionless peaks attributable to CDW effects. The intensity modulation of the substructure appears to reflect the band dispersion of the undistorted 1T phase. Distinct differences in intensity are observed between the \ensuremath{\Gamma}M and \ensuremath{\Gamma}M' spectra as a consequence of the threefold symmetry of the unit cell. In temperature-dependent ARIPES between 120 and 400 K we do not, within the experimental accuracy, observe changes of the electronic structure other than a linear shift due to the electron-lattice interaction. In particular, there is no evidence of an opening of a localization gap in the unoccupied band structure as has been observed in the occupied states. From combined photoemission and inverse photoemission results the Fermi level is rather found to be pinned close to the bottom of the lowest unoccupied band.

Destabilization of the Si(111) pi -bonded chain structure upon Cs adsorption.

Contrary to previous knowledge, we have found an ordered $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ surface reconstruction on the cleaved Si(111) surface after room-temperature deposition of one monolayer of Cs. The preparation and characterization of this surface, and the dispersion of the lowest unoccupied surface-state band as measured with angle-resolved inverse photoemission spectroscopy are presented. The surface is observed at the saturation of the work function and found to be semiconducting.