22 research outputs found
Ab initio probing of the electronic band structure and Fermi surface of fluorine-doped WO3 as a novel low-TC superconductor
First-principles calculations were performed to investigate the electronic
structure and the Fermi surface of the newly discovered low-temperature
superconductor: fluorine-doped WO3. We find that F doping provides the
transition of the insulating tungsten trioxide into a metallic-like phase
WO3-xFx, where the near-Fermi states are formed mainly from W 5d with admixture
of O 2p orbitals. The cooperative effect of fluorine additives in WO3 consists
in change of electronic concentration as well as the lattice constant. At
probing their influence on the near-Fermi states separately, the dominant role
of the electronic factor for the transition of tungsten oxyfluoride into
superconducting state was established. The volume of the Fermi surface
gradually increases with the increase of the doping. In the sequence WO3
\rightarrow WO2.5F0.5 the effective atomic charges of W and O ions decrease,
but much less, than it is predicted within the idealized ionic model - owing to
presence of the covalent interactions W-O and W-F.Comment: 8 pages, 4 figure
Adsorption of simple molecules on metal oxides by ab initio calculations: application to the detection of gase
International audienc
Experimental and ab initio study of the O detection at the CuO (111) surface
International audienceCombining experiments and first-principles calculations, we present in this paper a detailed study of the O detection mechanism on the CuO (111) surface. The exchange-correlation functional is treated within both the LDA and the GGA including the spin polarization. In order to better take into account the on-site electronic interactions between electrons of Cu atoms a Hubbard term U has to be added in all calculations. We show that the O molecule is reduced to a O molecule with an enthalpy of reaction of −1.11 eV (−1.15 eV) within LDA+U (GGA+U). Along the reaction path, the O molecules are first physisorbed with a large adsorption energy of −1.83 eV (−1.03 eV) and a significant charge transfer from the surface to the molecule. The p-doping strengthening is compared to the electrical response of a CuO based sensor under O exposure
Ab initio study of oxygen point defects on tungsten trioxide surface
International audienceThe gas response of tungsten trioxide (WO 3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO 3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (-1.65 eV in LDA and-1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n–doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements
Mechanism of CO and O3 sensing on WO3 surfaces: First principle study
International audienc
Mechanism of NOx sensing on WO3 surface: First principle calculations
International audienceComputational study of NO 2 sensing on the WO 3 (001) surface is presented. Our ab initio calculations reveal a two-step process of NO 2 detection on the WO 3 surface. In a first step the NO 2 molecule is dissociated at an oxygen vacancy site, but a NO molecule remains adsorbed. In a second step NO is re-oxidized into NO 2 by O 2 of the surrounding air leading to the resistance increase which is experimentally observed. We also calculate the adsorption energy of NO on stoichiometric and non-stoichiometric WO 3 surfaces and propose a method for the NO detection
Correlation between rf-sputtering parameters and WO3 sensor response towards ozone
International audienc