S D Foulias

Barium and oxygen interaction on the Ni(110) surface at low coverages studied by soft x-ray photoemission spectroscopy: Ba negative binding energy shifts and their correlation with Auger electron spectroscopy shifts

In this work, barium and oxygen interaction on the Ni(110) surface is investigated, by means of soft x-ray photoemission spectroscopy (SXPS), mainly at submonolayer and monolayer coverages. The results show that oxygen interacts with barium and the substrate as well forming BaO and NiO respectively. The formation of both oxides is consistent with previous Auger electron spectroscopy (AES) results. The oxidation of barium on the surface induces negative binding energy shifts of the Ba low core atomic levels 4d, 5s and 5p. Both initial and final state effects are shown to be necessary in order to explain these peculiar energy shifts. This interpretation is based on correlating the core level binding energy shifts with previously recorded AES transition line shifts, also taking into account the changes of the work function of the surface. The analysis shows that the extra-atomic relaxation energy decreases on going from metallic Ba to BaO. This implies that the reduction of the free electron screening more than counteracts the increase in relaxation energy due to the polarizable O2− anions.

Development and characterization of Fe ultrathin films on the SrTiO3(100) surface

The adsorption of Fe on the SrTiO3(100) surface at room temperature has been studied in ultrahigh vacuum by means of Auger electron spectroscopy, low energy electron diffraction, electron energy loss spectroscopy, thermal desorption spectroscopy and work function measurements. The results show that iron probably grows in the mode of successive incomplete layers. For coverages 1.5 ML, a short range 1 × 1 order appears and the deposited Fe overlayer develops in body-centred cubic structure with Fe(100) � SrTiO3(100) and crystallographic orientation Fe[110 ]� SrTiO3[100]. The results of the electron spectroscopies do not indicate any iron oxidation at the metal‐oxide interface. Instead, an interaction between the Fe adatoms gradually leads to the metallization of the Fe overlayer. Thus, the Fe/SrTiO3(100) interface seems to be a rather abrupt metal‐oxide interface, which presents a good thermal stability for annealing up to ∼800 K. In conclusion, this adsorption system looks ideal for free-standing ultrathin Fe films and low-dimensional structures, useful for technological applications.

THE DEVELOPMENT OF NICKEL ULTRA-THIN FILMS AND THE INTERACTION WITH OXYGEN ON THE SrTiO3(100) SURFACE STUDIED BY SOFT X-RAYS PHOTOELECTRON SPECTROSCOPY

The electronic properties of very thin Ni films on the SrTiO3(100)-Fe doped surfaces and their interaction with oxygen have been studied by soft X-rays photoelectron spectroscopy measurements. Nickel starts to become metallic on the surface in the very early adsorption stages. Oxygen adsorption on the nickel covered SrTiO3(100) surface leads gradually to an almost complete oxidation of the nickel overlayer. The oxidation seems to take place through two different oxidation states, which according to the literature are due to the Ni2+ and Ni3+ species. The heating of the O/Ni/SrTiO3 system at 850 K, causes a partial reduction of the nickel overlayer.

Auger electron spectroscopy and work function characterization of oxygen adsorption on Ba-covered Ni(110)

The adsorption of oxygen on a Ba-covered Ni(110) surface has been investigated mainly by Auger electron spectroscopy (AES) and work function (WF) measurements. Low energy AES lines indicate that O interacts with Ba and Ni as well. Both Ba–O and Ni–O interactions take place simultaneously on the surface, progressively leading t oB aO and NiO formation. Ba enhances the oxidation of the substrate due to the higher sticking coefficient of O on the Ba/Ni surface. The oxygen interacts with the nickel substrate even at high adsorbate coverage, incorporating under the Ba layer. A part of the Ba adatoms remains non-oxidized even at a high O exposure.

ELECTRONIC PROPERTIES OF BARIUM ULTRATHIN LAYERS ON THE Ni(110) SURFACE

In this paper, we study the adsorption of Ba on the Ni(110) surface at room temperature. The investigation takes place mainly by soft X-ray photoelectron spectroscopy measurements. At low coverage (

Adsorption of oxygen on a nickel covered SrTiO3(100) surface studied by means of Auger electron spectroscopy and work function measurements

The interaction of oxygen with evaporated Ni films on an Fe-doped SrTiO3(100) substrate was investigated by means of LEED, AES and work function measurements (WF) at room temperature. The adsorption of oxygen takes place on the nickel overlayer firstly by chemisorption on nickel step sites, accompanied by a reduction of the WF, and secondly on terrace sites, followed by a WF increase. After the chemisorption phase, the oxidation of the nickel overlayer starts with NiO island formation followed by bulk NiO development, which is marked by a second WF reduction. The adsorption phases of oxygen correspond closely to those of oxygen on single crystals of nickel. This indicates that the character of the Ni predeposited layers on strontium titanate seems to be metallic.

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis-low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3-In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1-In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb-Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.

Development and Characterization of an Ultrathin Barium Oxide Film on a Surface Oxidized Ni(110) Substrate

In this work we develop an ultrathin barium oxide film by Ba adsorption on a pre‐oxidized Ni(110) surface, and characterize it by means of Auger electron spectroscopy, low energy electron diffraction and work function measurements. The results show that the Ba adatoms reduce the NiO surface oxide forming an amorphous and incomplete BaO layer. At higher coverages, the Ba adsorbate seems to approach the metallic state, with the BaO layer confined between NiO and the metallic Ba. The measured energy decreases of the atomic Ba(75 eV) and interatomic BaO(68 eV) low energy Auger transition lines are analyzed in terms of low core atomic levels, valence band and extra‐atomic relaxation energy changes.

The Low Energy Auger Electron Spectroscopy Lines as an Index of the Ba Overlayer Order on the Ni(110) Surface

We investigate the interaction of Ba with the Ni(110) surface at elevated temperatures by means of Auger electron spectroscopy and low energy electron diffraction. The results show that annealing of the substrate causes desorption and ordering of the initially amorphous overlayer, resulting in c and structures. It is observed that the induced ordering crucially affects the lineshape of the double Auger transition line Ba(73 eV)N45O23P1, establishing this line as an index of ordering of the Ba overlayer. The underlying physics of this effect is discussed.