M. Kamaratos

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.

Interactions between Se, Cs and Si upon Se adsorption on Cs/Si(111)-7×7 surfaces

This report involves the room and elevated temperature ultrahigh-vacuum study of Se adsorbed on caesiated Si(111)-7×7 surfaces using Auger electron spectroscopy, low-energy electron diffraction, work-function measurements and thermal desorption spectroscopy. Adsorption of Se on Cs/Si(111)-7×7 surfaces initially occurs on the uncaesiated portions of the Si substrate and subsequently on the Cs adatoms. Heating the Se/Cs/Si(111)-7×7 surface causes Se to diffuse below the Cs adlayer to form strong dipoles with the alkali metal adatoms. The presence of Se increases the energy of binding of Cs to the substrate, and CsxSeySiz compound formation was observed with a binding energy of 2.8 eV/atom. The Cs-Se bonds most probably break at 1050 K, and two binding states of Cs and Se were observed after heating to temperatures >1050 K. Their calculated binding energies are 3.0 and 3.2 eV/atom. The coadsorption of Cs and Se induces a high degree of surface disorder, while desorption most probably causes surface etching. Finally, the presence of Cs on the Si(111)-7×7 surface greatly suppresses the formation of SiSe2, detected when Se is adsorbed on clean Si(111)-7×7 surfaces.

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.

K and S coadsorption on Ni(100) surfaces

In this contribution we report a study of K and S coadsorption on Ni(100) surfaces by means of Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and WF (work-function) measurements in UHV. Adsorption of elemental S on clean Ni(100) increases the binding energy, and the maximum coverage of the subsequently deposited K. The presence of the S underlayer maintains the ionicity of K adatoms close to the ionic state up to the completion of the first layer, at ΘK = 0.67 ML. The latter coverage is, thus, greater than the estimated 0.38 ML of K on clean Ni(100). The K overlayer on S-covered Ni(100) weakens the S-Ni bond, and forms a compound with S. The presence of S on Ni(100) lowers the WF minimum, Φmin, of K on Ni(100) to a value of ~1 eV, which is attributed to K-S compound formation. Deposited S atoms on K-covered Ni(100) are submerged under the K layer up to 0.5 ML and subsequently they are adsorbed on top of the K layer.

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.

Adsorption of Br2 on Na-intercalated layered compounds: Bromine-induced deintercalation

This work refers to a study of the behavior of Br2 adsorbed on Na-intercalated 1T-TaSe2(0001) and TiSe2(0001) surfaces at 100 K and during subsequent warming up to 300 K. The investigation was performed in UHV with LEED and photoemission with synchrotron radiation measurements. At 100 K, the bromine forms molecular multilayers on Na/TaSe2. The Br2 layer, close to the substrate, interacts weakly with Na near the surface, forming . At 300 K, part of the Br2 overlayer desorbs while the remainder on both 1T-TaSe2 and Tise2 surfaces interacts strongly with Na, leading to deintercalation of Na to the surface in the tendency to form NaBr. The formation of NaBr overlayers causes the transition of the incommensurate surface structure of 1T-TaSe2(0001) to 1×1. ESD from the Br/Na/1T-TaSe2 surface causes the restoration of the .

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.

Na and Cl2 interaction on 1T and 2H-TaSe2 (0001) surfaces

In this paper we study the interaction ofCl2and Na on 1T–TaSe2and 2H–TaSe2(0001)surfaces in the temperature range of 100–300 K. The experiments are performed in UHV with the use of LEED and SXPS by synchrotron radiation measurements. Deposition of Na onCl2-covered 1T–TaSe2at 100 K forms initially a

Potassium adsorption on MoS2(0001) at low temperature

{rm Na}mbox{-}{rm Cl}_2^-