Temperature-dependent X-ray Photoemission Research Articles

Nonuniform Debye Temperatures in Quasi-One-Dimensional Transition-Metal Trichalcogenides

The Debye–Waller factor plots for S 2p core-level, based on temperature-dependent X-ray photoemission measurements on quasi-one-dimensional (1D) chains of TiS3, suggest effective Debye temperatures...

The lattice stiffening transition in UO2 single crystals

The effective Debye temperatures () of the surface region of UO2 single crystals, prepared by the hydrothermal synthesis technique, were obtained from temperature-dependent x-ray photoemission in the temperature range of 300 K–623 K. A lattice stiffening transition, characterized by different regions of different effective Debye temperature, 500 ± 59 K below 475 K and 165 ± 21 K above 475 K is identified. A comparison of the temperature dependence of the effective UO2 Debye temperature, with the changes in the lattice expansion coefficient for UO2, support strong lattice-phonon interaction arising from the Jahn–Teller distortion.

Thermally Activated Interaction of Co Growth with ZnO(101̅0) Surface

The nucleation and sintering characteristics of Co on ZnO(1010) during annealing were investigated by scanning tunneling microscopy (STM). The extent of coverage and annealing temperature were varied to modify the size of the Co nanoclusters. Temperature-dependent X-ray photoemission spectra of the Co chemical state reveal that metallic Co dominates the annealing procedure on ZnO(1010) and is more thermally stable than Co on both ZnO(0001) and ZnO(0001). The effects of annealing time and the distribution of Co adatoms were elucidated by STM. The results have potential implications for the thermal stability of Co/ZnO(1010) in real catalytic processes.

Competitive Bonding of Amino and Hydroxyl Groups in Ethanolamine on Si(100)2×1: Temperature-Dependent X-Ray Photoemission and Thermal Desorption Studies of Nanochemistry of a Double-Chelating Agent

Competitive Bonding of Amino and Hydroxyl Groups in Ethanolamine on Si(100)2×1: Temperature-Dependent X-Ray Photoemission and Thermal Desorption Studies of Nanochemistry of a Double-Chelating Agent

Relative reactivities of amino and ethenyl groups in allylamine on Si(100)2 × 1: Temperature-dependent X-ray photoemission and thermal desorption studies of a common linker molecule

The room-temperature adsorption and thermal evolution of allylamine on Si(100)2 × 1 have been investigated by using temperature-dependent X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The presence of a broad N 1 s feature at 398.9 eV, attributed to a N―Si bond, indicates N―H dissociative adsorption. On the other hand, the presence of C 1 s features at 284.6 eV and 286.2 eV, corresponding to C═C and C―N, respectively, and the absence of the Si―C feature expected at 283.2 eV shows that [2 + 2] C═C cycloaddition does not occur at room temperature. These XPS data are consistent with the unidentate staggered and eclipsed allylamine conformer adstructures arising from N―H dissociation and not [2 + 2] C═C cycloaddition. The apparent conversion of the N 1 s feature for Si―N(H)―C ▪ at 398.9 eV to that for Si―N(H) at 397.7 eV and the total depletion of C 1 s feature for C―N at 286.2 eV near 740 K indicates cleavage of the C―N bond, leaving behind a Si―N(H) radical. Furthermore, the C═C C 1 s feature at 284.6 eV undergoes steep intensity reduction between 740 K and 825 K, above which a new C 1 s feature at 283.2 eV corresponding to SiC is found to emerge. These spectral changes suggest total dissociation of the ethenyl fragment and the formation of SiC. Moreover, while the total N 1 s intensity undergoes a minor reduction (24%) upon annealing up to 1090 K, a considerable reduction (43%) is found in the overall C 1 s intensity. This observation is consistent with our TDS data, which shows the desorption of C-containing molecules including propene and ethylene at 580 K and of acetylene at 700 K. The lack of N-containing desorbates suggests that the dissociated N species are likely bonded to multiple surface Si atoms or diffused into the bulk. Interestingly, both the staggered and eclipsed N―H dissociative adstructures are found to have a less negative adsorption energy than the [N, C, C] tridentate or the [2 + 2] C═C cycloaddition adstructures by our DFT calculations, which suggests that the observed formation of N―H dissociative adstructures is kinetically favored on the Si(100)2 × 1 surface.

Evidence for incipient O dimerization and absence of Cu+3 in temperature-dependent XPS of highT c superconducting oxides

From temperature-dependent X-ray photoemission (XPS) spectra between 300 and 10 K of Sr doped La2CuO4 and “123” compounds (YBa2Cu3O7−δ) we deduce the absence of the Cu3d8(Cu3+) configuration in the ground state of these Mott insulators. The metallic character of these samples and involving those Cu atoms which are relevant for the superconducting properties arises from α(d9p6)+β(d10p5) valence fluctuations, while the superconducting transition is indicated to be closely related to an incipient oxygen dimerization which is best detected upon cooling far belowTc.

Dynamics of polyimide curing and degradation: An i n s i t u x-ray photoemission study

Dynamic time- and temperature-dependent x-ray photoemission studies of polyimide (PI) show the evolving valence bands and core level spectra, starting from partially polymerized PI and continuing through PI degradation at 500 °C. For temperatures below∼200 °C, we see little change, but curing above 300 °C is accompanied by changes in core level and valence-band positions relative to the Fermi level. These changes are associated with the removal of defect induced states in the band gap of the polymer. Comparison of the valence-band spectrum of the fully cured PI to recent electronic structure calculations shows good agreement. PI degradation occurs at temperatures between 450 and 500 °C, where there is a decrease in carbonyl and pyromellitic dianhydride benzene ring carbon 1s emission due to loss of CO or CO2, and an increase in carbon atom density. Nitrogen 1s spectra exhibit the growth of new low binding energy components which indicate that the feature previously assigned to an isoimide structure is a metastable reaction intermediate of the degradation process. The oxygen 1s features are primarily affected in the carbonyl oxygen peak, indicating ether oxygen bonds are more stable with respect to high temperatures.

Valence fluctuations and oxygen dimerization in high-temperature superconductors from X-ray photoemission spectroscopy

The high-temperature superconducting Cu oxides — which are intrinsically Mott insulators — are strongly correlated metals because of valence fluctuations involving Cud9−d10 configurations. Temperature-dependent X-ray photoemission spectroscopy of theO 1s and Cu 2p levels in the range 10–300 K provides evidence for O-dimerization, with corresponding increase of the Cud10O 2p5 “well-screened” XPS-final state belowT c . This supports the suggestion of dynamical mixed-valence and double-exchange antiferromagnetic interaction as an important mechanism for the origin of highT c superconductivity.