Two different types of amorphous carbon films were deposited on Si substrates, with film hardness of 22 GPa and 40 GPa, by pulsed laser evaporation of graphite targets. The x-ray photoemission spectra (XPS) of the C 1s core level in these films shown two components at 284.3±0.1 eV and 285.2±0.1 eV, which were identified with the ${\mathit{sp}}^2$ and ${\mathit{sp}}^3$ hybrids forms of carbon. The ${\mathit{sp}}^3$/${\mathit{sp}}^2$ concentration ratio deduced from the area of the components had a value of 2/5 for the harder amorphous carbon film and 1/4 for the softer. Upon annealing the harder film at different temperatures, the ${\mathit{sp}}^3$/${\mathit{sp}}^2$ ratio remained nearly constant up to about 900 K and then decreased until reaching a value of zero above 1100 K. The C 1s core level shifted 0.3±0.1 eV toward lower binding energy in the films for annealing temperatures above 900 K. This shift was correlated with an increase in the asymmetry of the C 1s XPS spectra and of the density of states at the Fermi level, as observed by ultraviolet photoemission spectroscopy. There was no detectable π plasmon in the harder films below 900 K, despite the presence of ${\mathit{sp}}^2$ atoms and π bonds at those temperatures. © 1996 The American Physical Society.

We have determined the effective attenuation length of photoelectrons over the range of kinetic energies from 4 to 6 keV in Co, Cu, Ge, and ${\mathrm{Gd}}_2{\mathrm{O}}_3$. The intensity of the substrate (Si) and overlayer core level peaks was measured as a function of the thickness of the wedge-shaped overlayers. Experimental values vary between 45–50 Å at 4 keV and 60–65 Å at 6 keV in Co, Cu, and Ge. Smaller values (30 Å to 50 Å, respectively) are found in ${\mathrm{Gd}}_2{\mathrm{O}}_3$. Our results confirm that, for different classes of materials, high energy photoemission spectroscopy has the necessary depth sensitivity to go beyond surface analysis, yielding important information on the electronic properties of the bulk and of buried layers and interfaces.

The instrument VOLPE (volume photoemission from solids) is an experimental setup dedicated to high energy photoemission (PE) experiments. The instrument is equipped with an electrostatic hemispherical spectrometer especially designed to analyze high energy electrons (up to 10 keV) with high resolving power. In order to attain an energy resolution of a few tens of millielectron volts, we designed and constructed a dedicated input lens system, high stability power supplies, and a low dark-count detector and readout electronics. The system has been tested and is now operational on the ID16 beamline at European Synchrotron Radiation Facility, where an optical layout has been developed to perform high energy, high resolution PE experiments. First results show an overall energy resolution (electron + photon) of $71\pm 7\mathrm{meV}$ at 5934 eV. The effective attenuation length of the photoelectrons is estimated to be $5\pm 0.5\mathrm{nm}$ at a kinetic energy of 5 keV.

We exploited complementary synchrotron radiation spectroscopies to study the Yb $4f$ electronic configuration in three representative intermediate-valence materials: $\mathrm{Yb}{\mathrm{Al}}_3$, $\mathrm{Yb}\mathrm{In}{\mathrm{Cu}}_4$, and $\mathrm{Yb}{\mathrm{Cu}}_2{\mathrm{Si}}_2$. High-resolution x-ray absorption (PFY-XAS), resonant inelastic x-ray scattering (RIXS), and hard-x-ray photoemission (HAXPES) data all show characteristic temperature-dependent changes of the Yb valence. For each material, the increments measured from low $\left(20\mathrm{K}\right)$ to high $\left(300\mathrm{K}\right)$ temperature by the different probes are quite similar. The estimated RIXS and XAS valences are consistently higher than the HAXPES values. We briefly discuss the possible origin of this discrepancy.

We present hard x-ray photoemission measurements from GaAs samples with a 10-Å-thick layer of AlAs buried at different depths. The intensity trend versus kinetic energy of the Al $1s$ signal allows extraction of the x-ray attenuation length, which we find to reach $\sim 100\mathrm{\AA }$ at a kinetic energy of 6 keV. On one sample exposed to air for several days we obtain qualitative information on the oxidation at different depth scales by exploiting the energy dependence of the attenuation length. This suggests the strong potential of hard x-ray photoemission in the nondestructive characterization of diluted materials on a depth scale interesting to modern nanotechnologies.

Friction characteristics of graphene deposited on different substrates have been studied by atomic force microscopy (AFM). In particular, we compared mechanically exfoliated graphene transferred over Si/SiO ₂ with respect to monolayer (ML) graphene grown in our laboratory by low temperature chemical vapor deposition on Ni(111) single crystal. Friction force measurements by AFM have been carried out as function of load under different environment conditions, namely vacuum (10 ⁻⁵ Torr), nitrogen and air. The typical decrease of friction force with increasing number of layers has been observed on graphene over Si/SiO ₂ in all environment including vacuum. Continuum mechanical approximation has been used to analyze the friction versus load curves of ML graphene on Ni(111). Analysis shows that Derjaguin–Mueller–Toporov model is in good agreement with our experimental data indicating that overall behavior of the interface graphene–Ni(111) is relatively rigid respect to out of plane deformations. This result is consistent with the structural characteristics of the interface since graphene grows in registry with Ni(111) surface with covalent bonding character. Finally, the shear strength and the work of adhesion of the two systems with respect to AFM tip in vacuum have been compared. The result of this procedure indicates that shear strength and work of adhesion measured on graphene–Si/SiO ₂ interface are always greater than those on graphene–Ni(111) interface.