X-ray Diffraction Photoemission Research Articles

Periodic Modulation of Graphene by a 2D-FeO/Ir(111) Moiré Interlayer

Ultrathin films of iron oxide form a two-dimensional (2D) FeO layer on Ir(111). Because of difference in lattice constant between 2D-FeO and Ir(111), a moire superstructure is formed. The 2D-FeO/Ir(111) structure is examined by soft X-ray photoelectron spectroscopy, X-ray photoemission diffraction, scanning tunneling microscopy, and low-energy electron diffraction. A 2D-FeO layer may also be grown by iron intercalation and subsequent oxidation underneath a graphene layer on Ir(111). Thus, the graphene can be decoupled from the metal by the 2D-FeO layer. Changes in the graphene C 1s binding energy can be mainly explained by shifts in the Fermi level of graphene as a consequence of interface band alignment for weak interactions between graphene and the substrate. A shift of C 1s to lower binding energy, for graphene supported on FeO/Ir(111), is a consequence of the dipole moment in the 2D-FeO layer normal to the Ir(111) surface. Broadening of the C 1s peak is consistent with a locally varying 2D-FeO dipole ...

Photoelectron spectroscopic studies of ultra-thin CuPc layers on a Si(111)-(√3 × √3)R30°-B surface

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)-(√3 × √3)R30°-B surface is investigated at room temperature under ultra-high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results shed light on the growth mechanism of CuPc on this substrate. At one monolayer coverage the growth mode was characterized by the formation of crystalline 3D nanoislands. The molecular packing deduced from this study appears very close to the one of the bulk CuPc α phase. The 3D islands are formed by molecules aligned in a standing manner. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate. However there is charge transfer from molecules to the substrate. During the growth, the work function (WF) was found to decrease from 4.50eV for the clean substrate to 3.70eV for the highest coverage (30 monolayers). Within a thickness of two monolayers deposition, an interface dipole of 0.50eV was found. A substrate band bending of 0.25eV was deduced over all the range of exposure. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.30eV. The changes in the work function, in the Fermi level position and in the onset of the molecular HOMO state have been used to determine the energy level alignment at the interface.

Adsorption study of copper phthalocyanine on Si(111)(√3 × √3)R30°Ag surface

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)(√3×√3)R30°Ag surface is studied at room temperature under ultra high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results indicate that after one monolayer deposition the molecular layer is highly ordered with a flat lying adsorption configuration. The corresponding pattern reveals the coexistence of three symmetrically equivalent orientations of molecules with respect to the substrate. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate; however there is evidence of Fermi level movement. During the growth, the work function was found to decrease from 4.90eV for the clean substrate to 4.35eV for the highest coverage (60 monolayers). Within a thickness of two monolayer deposition an interface dipole of 0.35eV and a band bending of 0.2eV have been found. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.55eV. The changes in the work function, in the Fermi level position and in the HOMO state have been used to determine the energy level alignment at the interface.

Fe nanoparticles on ZnSe: Reversible temperature dependence of the surface barrier potential

The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of $\ensuremath{\sim}$1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.

Excitation of phonons in medium-energy electron diffraction.

The ``elastic'' backscattering of electrons from crystalline surfaces presents two regimes: a low-energy regime, in which the characteristic low-energy electron diffraction (LEED) pattern is observed, and a medium-energy regime, in which the diffraction pattern is similar to those observed in x-ray photoemission diffraction (XPD) and Auger electron diffraction (AED) experiments. We present a model for the electron scattering which, including the vibrational degrees of freedom of the crystal, contains both regimes and explains the passage from one regime to the other. Our model is based on a separation of the electron and atomic motions (adiabatic approximation) and on a cluster-type formulation of the multiple scattering of the electron. The inelastic scattering events (excitation and/or absorption of phonons) are treated as coherent processes and no break of the phase relation between the incident and the exit paths of the electron is assumed. The LEED and the medium-energy electron diffraction regimes appear naturally in this model as the limit cases of completely elastic scattering and of inelastic scattering with excitation and/or absorption of multiple phonons. Intensity patterns calculated with this model are in very good agreement with recent experiments of electron scattering on Cu(001) at low and medium energies. We show that there is a correspondence between the type of intensity pattern and the mean number of phonons excited and/or absorbed during the scattering: a LEED-like pattern is observed when this mean number is less than 2, LEED-like and XPD/AED-like features coexist when this number is 3--4, and a XPD/AED-like pattern is observed when this number is greater than 5--6. \textcopyright{} 1996 The American Physical Society.

High-angular resolution x-ray photoelectron diffraction measurements of Si(111)

The surface crystallography of a single Si(111) 7×7 crystal has been investigated, for the first time, using high-angular resolution (±1°) X-ray photoemission diffraction measurements. Polar scans of the Si2p core-level intensity have been measured along the principal azimuthal directions of the Si(111) substrate. Observation of prominent forward scattering features along with fine structure characteristic of a given azimuth, allow us to identify unambigously the orientation of the substrate and more specially to distinguish the opposite [1 2 1] and [ 1 2 1 ] directions. The forward scattering peaks correspond to the [111], [011] and [11 1 ] ([111] and [100]) low-index directions along the [1 2 1] ([ 1 2 1 ]) azimuth and to the [111] and [ 1 31] crystallographic axis along the [ 1 10] azimuth. The maximum polar intensity anistropy, measured as (Imax-Imin)/Imax, is ∼51% for the peak located at normal emission. Improvement of angular resolution results in a strong enhancement of the profile contrast, and much fine structure which cannot be explained in the simple forward focusing picture. Such data can be used to test the capabilities of single and multiple scattering simulations.

Epitaxy of fcc and bcc Co, Ni, and Cu studied by X-ray photoelectron diffraction

The structures of epitaxial transition-metal films with thicknesses of only a few monolayers were determined by X-ray photoemission diffraction (XPD) techniques. Ni ultrathin films were studied on two Cu single-crystal surfaces, and Cu/Ni multilayers (or “sandwich” structures) were also grown. On Cu(100), the initial Ni epitaxy is found to result in expansion of the Ni lattice constant to achieve lateral coherence with the substrate, along with a reduction in the Ni interplanar spacing perpendicular to the substrate, approximately maintaining a fixed Ni-Ni nearest-neighbor distance. This structure is a body-centered tetragonal distortion of the normal fcc(100) lattice. The behavior on Cu(111) is different, in that the epitaxial Ni films retain the c/a axis relationship of bulk Ni(111). Films of Co, Ni, and Cu, which are normally fcc or hcp crystals at room temperature, were grown on single-crystal Fe(100) substrates to induce bcc ordering. Both Co and Ni grow as bcc(100) films on iron, but the Cu films are disordered. This is in contrast to epitaxy of Cu on strained layer Fe(100) films, which do support the growth of bcc Cu(100). Modelling of ultrathin film XPD using multiple-scattering calculations was used for bond-length determinations with high accuracy, and for a determination of growth modes.

X-ray photoemission diffraction as a novel technique for antiphase domain detection in the epitaxial growth of polar compounds on nonpolar substrates

We show that x-ray photoemission diffraction can be used as a means of detection of antiphase domains in the epitaxial growth of polar compounds on nonpolar substrates. An example is given in the case of the GaAs growth on a patterned (001) Si substrate, 4° off towards [110], where antiphase domains have been observed by scanning transmission electron microscopy.