D. Vlachos

Photoelectron spectroscopy of UHV in situ intercalated Li/TiSe2: Experimental proof of the rigid band model

Abstract Li was adsorbed at room temperature onto UHV cleaved TiSe 2 (0001) van der Waals surfaces and investigated by soft X-ray photoelectron spectroscopy. The adsorbed Li is readily intercalated into the substrate without interface decomposition. The electronic structure of the intercalated phase is analyzed from the valence band spectra giving clear evidence that the intercalation follows the rigid band model. Shifts in the Fermi level and related changes in the work function are analyzed in terms of the electronic contribution of the free energy of intercalation.

Ba deposition on Si (1 0 0)2 × 1

The Ba deposition on Si(1 0 0)2 × 1 at room temperature has been investigated by LEED, AES, TDS, EELS and work function measurements. Ba grows, though disordered, layer by layer with a constant sticking coefficient. Barium on Si(1 0 0)2 × 1 gives rise to two adsorption states which exhibit relatively high binding energy for Θ ≤ 2 ML, a strong Ba-Si ionic interaction and relatively low binding energy for Θ ≤ 2 ML where the Ba overlayer has a metallic character. Heating of the Ba-Si interface at T < 700°C promotes BaSi interaction, probably with a tendency to form silicide compounds. Higher temperatures cause the appearance of the 2 × 4, 2 × 1 and 2 × 3 diffraction patterns.

A synchrotron radiation study of the interaction of Na with WSe2 and TaSe2: oxygen-induced deintercalation

The interaction of O with UHV cleaved TaSe 2 and WSe 2 , previously intercalated in situ with Na, was studied by synchrotron radiation in the region 30-80 eV. The observed strong fluctuations of the Na 2p emissions relative to the metal 4f ones are explained in terms of deintercalation of Na, in the tendency to form Na x O y surface oxides, along with clustering as oxides are forming. The deintercalation is proposed to occur in two stages : a fast one attributed to the lowering of the near surface potential due to the O-Na interaction and a slow one due to volume diffusion.

The behavior of K on the basal plane of MoS2

Abstract The adsorption of K on MoS 2 (0001) has been investigated by LEED, AES, TDS and WF measurements. The results suggest that the initial sticking coefficient of K on MoS 2 is less than 1 (~ 0.7). From the known flux and sticking coefficient, the K coverage could be determined at any time. At low coverage, K forms strongly ionized isolated adatoms as on metals and other semiconductors. However, with increasing coverage, K atoms form 2D clusters which with more K adsorption coalesce and grow to 3D clusters. The growth of K to 3D clusters contrasts the uniform deposition exhibited on metals and other semiconductors.

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.

Ba deposition on Ni(110)

The deposition of Ba on Ni(110) at room temperature was studied by AES, TDS, LEED, as well as SEECC and work function measurements. Ba was found to form a first layer consisting of a mixture of random c(2 × 2) domains and disordered adatoms. The first break in the AES uptake curve is estimated to be ∼ 3/4 ML. A second, more weakly bound disordered layer follows. Heating to ∼ 200°C turns ∼ 0.35 ML to a subsurface phase which is stable up to 800°C. The system becomes mobile at ∼ 500°C opening up two kinetic channels: (a) desorption of some of the second layer atoms at ∼ 500°C, (b) conversion of the rest of these atoms to long-range c(2 × 2). Finally, desorption from the c(2 × 2) phase occurs at ∼ 780°C. The binding energy of the first layer atoms is ∼ 2.9 eV at all coverages contrary to alkali metals deposition where it decreases with coverage, whereas the initial dipole moment is ∼ 13 D, i.e. similar to the alkali metals

Potassium adsorption on MoS2(0001) at low temperature

The authors report on the properties of potassium on MoS2(0001) at 100 K as deduced from LEED, AES, EELS and TDS measurements. Potassium grows in a layer-by-layer mode on the surface of MoS2. The intercalation of K into MoS2, which occurs at RT is negligible at LT. At low coverages K is deposited in ionic form, whereas for theta K>0.5 there is clear evidence of metallization of the K overlayer on MoS2.