N.-T.H. Kim-Ngan

Positive and negative ion emissions induced by low-energy ion single scattering from a single crystalline Fe3O4(001) surface

Abstract We investigated the energy spectra of positive and negative ions emitted from the (0xa00xa01) surface of a single crystalline magnetite, Fe3O4, under low-energy He+, Ne+ and Ar+ ion bombardments (4–8 keV). Under He+ ion bombardments, two distinct peaks in the LEIS spectra were observed at the energies where the single binary elastic scattering respectively from oxygen (He+–O) and from iron (He+–Fe) ions are expected. A large enhancement of the He+–O scattering peak was observed at 90 K. Under Ne+ ion bombardments, two observed peaks were assigned to the O+-recoil peak and to the Ne+–Fe scattering peak. Under Ar+ ion bombardments, the Fe-recoil peak and the Ar+–Fe scattering peak were revealed. The intensity and the characteristic of the Fe-recoil peak are found to depend strongly on the azimuthal angle. In all cases the large peak-width and the very high background indicated a large contribution of ion trajectories from the quasi-single scattering and multiple scattering processes. A very large O−1-recoil peak was observed under the Ne+ and Ar+ ion bombardments. The LEIS results suggest that the octahedral-Fe ions and a large amount of negative-charge oxygen ions were present on the surface and that the neutralization related to the change in the electronic state of magnetite played a dominant role in a final yield of ions.

Low energy ion scattering from the single crystal magnetite and from magnetite covered by condensed xenon at the Verwey transition

Abstract Energy spectra of He+, Ne+, Ar+ and Kr+ ions, scattered from a cleavage surface of a single crystal of magnetite, have been measured in small-angle geometry and in the temperature range of 85–300xa0K. The large energy losses of scattered ions and a very deep minimum around 120xa0K in the temperature dependence curve of ion yield ( R+(T) ) have been observed for all bombarding ions. The large energy losses were well explained in the frame of the zigzag collisions of incoming ions with Fe ions forming the surface semi-channels. The minimum in the R+(T) curve corresponding to the Verwey phase transition of magnetite can be explained by a change of the neutralization probability of incoming and outgoing ions, by a change of the target material transparency and by a change in the number of the trajectories containing ionizing collisions. The trajectory effect is visible especially for Ne+ and Ar+ ion interactions. Large suppression of the phase transition effect on ion scattering was observed on the magnetite surface covered by layers of condensed xenon.

Hydrogen storage in Ti?TiO2 multilayers

Multilayered thin films of Ti‐TiO2 system have been investigated, focusing on all of the important parameters in both photocatalysis and H storage. Numerous Ti‐TiO2 thin films with a single-, bi- and tri-layered structure have been deposited on different substrates by means of dc pulsed magnetron sputtering from a metallic Ti target in an inert Ar or reactive Ar+O2 atmosphere. The film chemical composition, depth profile, layer thickness and structure were determined by combined analysis of x-ray diffraction, x-ray reflectometry, Rutherford backscattering and optical reflectivity spectra. The results show that the Ti films deposited on Si(111) exhibit a strong preferred orientation with the (00.1) plane parallel to the substrate, while a columnar structure was developed for TiO2 films. H charging at 1bar and at 300 C revealed that, in the case of the tri-layered structure of Ti/TiO2/Ti/Si(111), H diffused through the TiO2 layer without any accumulation in it. Pd acts as a catalyst for gathering H in Ti layers and up to 50% of H is stored in the topmost and bottom Ti layers. The preferential orientation in the Ti films was found to be destroyed upon hydrogenation at 100bar. The hydride TiHx phase (x < 0.66) was formed under such a high H pressure.

Characterization of U-based thin films: the UFe2+x case

We have characterized UFe2+x films prepared by sputter deposition onto fused silica (SiO2) and Si(111) substrates with the film thickness ranging from 75 nm to 900 nm. The X-ray diffraction results showed an amorphous character of the deposited material. Some of the films showed in addition a pattern of highly textured cubic Laves phase. Rutherford Backscattering Spectroscopy with 2 MeV He+ ions has been used to determine the composition, thickness and concentration depth profile of the films. A large ageing affect was observed within 1 month after that the films were exposed to air. Magnetic measurements revealed TC increasing with relative Fe concentration and reaching approx. 450 K in UFe3.0.

Control of interface properties, layer quality and interdiffusion in selected nanoscaled heterostructures

Applications of ion beam techniques (IBA and IBMM) on nanoscaled thin films of selected materials (e.g. magnetite Fe3O4, uranium nitride UN) are presented. 1u2009MeV Ar+ and Kr+ ion beam with different ion fluences in the range of 1015–1017u2009ionsu2009cm−2 have been used to modify the film interfaces. Selected heterostructure interfaces were treated by subsequent thermal annealing. By analyzing the results obtained for the films in three different states (as-deposition, after annealing and after ion irradiation), the effects of ion beam mixing and interdiffusion were determined.