Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Quantum-well states in bilayers of Ag and Au on W(1 1 0)

sp-Like quantum-well states (QWS) in thin monocrystalline bilayer films of Ag and Au on W(1 1 0) and of single Ag films were studied by angle-resolved photoemission. We find that the propagation of the electronic states in the bilayer films along [1 1 1] depends on the energy relative to the band edge of Au metal at the L point of the Brillouin zone. In particular, QWS with binding energies less than this band-edge energy (1.1 eV) are strongly confined to the Ag layer, while for higher binding energies the QWS extend across the whole bilayer film. This clearly demonstrates the weakness of the potential barrier at the Ag/Au interface in the context of QWS formation at energies where electronic states exist in both metals.     

The electronic structure of Ca-intercalated superconducting graphite CaC6

We have measured Ca-intercalated graphite superconductor CaC₆ (T_c = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E_F). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC₆, suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E_F. From a comparison of electronic structure of CaC₆ and other graphite intercalation compounds (GICs), we found the difference between CaC₆ and other superconducting GICs, which provides deeper understanding of the superconductivity of CaC₆.     

Electronic structure of 1 ML NTCDA/Ag(1 1 1) studied by photoemission spectroscopy

We performed high-resolution photoemission experiments at different photon energies to investigate the valence band structure of 1 ML of 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) on Ag(1 1 1). Besides the known occupied molecular orbitals HOMO and HOMO − 1 we observe a new state close to the Fermi level, which results from the interaction between NTCDA and the substrate, partially filling the lowest unoccupied orbital of the free molecule (LUMO) in the monolayer system. By tuning the photon energy through the carbon K-edge, a resonance like change of the spectral intensity at the HOMO and HOMO − 1 energies is clearly revealed, which we use for an assignment of the individual spectral features to a predominant localization either at the naphthalene core or the anhydride group.     

ARPES and STS investigation of noble metal Shockley states: Confinement in vicinal Au(1 1 1) surfaces and self-organized nanostructures

Spectroscopic effects associated with the superperiodic surface structure have been observed in Au(1 1 1) vicinal surfaces and nanostructured systems. In the vicinal Au(23 23 21) surface, high resolution angle resolved photoemission spectroscopy shows the opening of several gaps in the surface band structure, whereas scanning tunneling spectroscopy reveals the energy dependence of the electronic density. These combined spectroscopic data allow to determine the reconstruction potential by deducing their first Fourier components. We also demonstrate that due to the peculiar growth on this Au vicinal surface, we can obtain a self-assembled superlattice of triangular Ag islands. The high ordering of the nanostructures leads to homogenous electronic properties.     

Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL_2,3L_2,3 Auger transitions from SiO₂/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si⁺¹ and Si⁺⁴, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra.The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si₂O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å.The SiO₂ chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO₂ shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined.The relative final state relaxation energy ΔR(2p) is determined from the modified Auger parameter. This yields a value of ΔR(2p)=−1.7 eV and implies, for SiO₂/SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 Å thick.     

Theoretical study of time-resolved photoemission of graphite near Brillouin zone K and H points

The time-resolved photoemission spectra of graphite near the Brillouin zone K and H points are obtained by the many-body time-dependent approach, where the nonradiative relaxation process of the excited electron by the phonon release is considered. The relaxation of the spectral weight is highly suppressed near the H point compared to the K point. This implies that the effective electron-phonon coupling near the K point is stronger than that near the H point.     

Angle resolved photoemission study of the Ce/Pd(1 1 1) interface

X-ray and UV excitation angle resolved photoemission spectroscopy of ultra-thin films of cerium deposited on a Pd(1 1 1) single-crystal surface has been carried out. Both core level and valence band spectra show a formation of a surface alloy exhibiting d- and f-electron orbital hybridization. An azimuth and polar angle mapping of X-ray excited photoemission intensities results in a surface-geometrical structure information. Mapping of ultra-violet photoelectron intensities as a function of emission angles is used for a band mapping and an electronic structure determination. The d-, f-hybridization occurs even for 0.7 ML of Ce deposited at room temperature. The surface annealing at 260 °C enhances this behavior. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems is observed.     

Systematic studies on surface modifications by ARUPS on Shockley-type surface states

The quasi two-dimensional surface state on noble metal (1 1 1)-surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. Already one monolayer of physisorbed Xe on Au(1 1 1) is responsible for a characteristic shift of the Shockley state towards the Fermi level and the surface state experiences an increase in spin-orbit splitting of up to 35%. In contrast to the physisorption process of rare gases, a sub-monolayer coverage of an alkali metal, e.g., Na on Au(1 1 1), has the opposite effect on the Shockley state, i.e. an increase in binding energy, until it reaches the bottom of the L-gap and vanishes into the bulk states. Additionally, we studied the intermetallic system Ag/Au(1 1 1) which differs substantially from the other systems because of the similarity in the electronic structure between substrate and overlayer.     

Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p_3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

A core level and valence band photoemission study of the (1 1 1) and ( ) surfaces of 3C–SiC

A core level and valence band photoemission study of thick 3C–SiC(1 1 1) and 3C–SiC( ) epilayers grown by sublimation epitaxy is reported. The as introduced samples show threefold 1×1 low-energy electron diffraction patterns. For the Si face and reconstructed surfaces develop after in situ heating to 1100°C and 1300°C, respectively. For the C face a 3×3 reconstruction form after heating to 980°C. A semiconducting behavior is observed for the and 3×3 reconstructed surfaces while the reconstruction show a Fermi edge and thus a metallic-like behavior. The surface state on the surface is investigated and found to have Λ₁ symmetry and a total band width of 0.10 eV within the first surface Brillouin zone. For the Si 2p and C 1s core levels binding energies and surface shifted components are extracted and compared to earlier reported results for 6H– and 4H–SiC.     

Detection of Fe 3d electronic states in the valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy(L-MBE) and structurally characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM),all of which reveal the high quality of the film.No secondary phase was detected.Resonant photoemission spectroscopy(RPES) with photon energies around the Fe 2p-3d absorption edge is performed to detect the electronic structure in the valence band.A strong resonant effect at a photon energy of 710 eV is observed.Fe3+ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV～8 eV.There are no electronic states related to Fe near the Fermi level.Magnetic measurements reveal a typical superparamagnetic property at room temperature.The absence of electronic states related to Fe near the Fermi level and the high quality of the film,with few defects,provide little support to ferromagnetism.     

Low energy excitations in graphite: The role of dimensionality and lattice defects

In this paper, we present a high resolution angle resolved photoemission spectroscopy (ARPES) study of the electronic properties of graphite. We found that the nature of the low energy excitations in graphite is particularly sensitive to interlayer coupling as well as lattice disorder. As a consequence of the interlayer coupling, we observed for the first time the splitting of the π bands by ≈0.7 eV near the Brillouin zone corner K. At low binding energy, we observed signatures of massless Dirac fermions with linear dispersion (as in the case of graphene), coexisting with quasiparticles characterized by parabolic dispersion and finite effective mass. We also report the first ARPES signatures of electron-phonon interaction in graphite: a kink in the dispersion and a sudden increase in the scattering rate. Moreover, the lattice disorder strongly affects the low energy excitations, giving rise to new localized states near the Fermi level. These results provide new insights on the unusual nature of the electronic and transport properties of graphite.     

Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces: I. Fe on TiO2(1 1 0)

The electronic states of the Fe overlayers on TiO₂(1 1 0) surfaces have been investigated using normal-emission and resonant photoelectron spectroscopy with synchrotron radiation. It was found that Fe grows in a Stranski-Krastanov mode. At low coverages, Fe deposition on TiO₂(1 1 0) is supposed to create surface Ti³⁺(3d¹) ions leading to the same in-gap emission as that is produced by surface oxygen vacancies of TiO₂. At high coverages, Fe-induced in-gap emission is evolved into a bulk Fe spectrum. However, at the beginning, a Fermi edge is not observed, indicating that the small Fe clusters of non-metallic nature are formed. A sharp Fermi edge is formed at higher coverages, indicating that the cluster becomes metallic as the size increases.     

The observation of energy transitions from lower valence band states in CuGaSe2

Electroreflectance data has been obtained with CuGaSe₂ single crystals which can be associated with transitions from a band of hybridized p-d states low in the valence band. The electrically p-type crystals were produced by a zone-growth technique and compensation doping was employed in order to produce resistivity values ～2Ωcm.