Dimitrios Vlachos

Abstract: The morphology and growth mechanism of nanostructured metals on semiconducting substrates determine crucially the electronic and physicochemical properties of these adsorption systems. In some cases, these properties are affected by modification of the interfacial geometry, induced by the metal adsorbate on the semiconducting substrate. Thus, in this work we investigate indium growth on the Si(111)3 � 3 and Si(111)4 � 1 surfaces reconstructed by indium. The basic motivation of this study is to reveal how reconstruction of the silicon surface affects the growth mode and electronic properties of the indium overlayer. Therefore, the In/Si interface was mainly studied by Si 2p and In 4d photoemission spectra as well as by valence band measurements using synchrotron radiation. In addition, low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and electron energy loss spectroscopy were used to reveal the structure and adsorption states of the indium adsorbate on the reconstructed silicon substrates. The results indicate that the initial In-Si surface symmetry affects the growth mechanism of the indium overlayer. In particular, the Stransky-Krastanov mode holds for indium adsorption on the clean Si(111)7 � 7 and Si(111)3�3 In-reconstructed surface. On the other hand, indium develops on the Si(111)4 � 1 In surface according to the Volmer-Weber mechanism. The adsorbate approaches the metallic phase as the coverage approximates the monolayer irrespective of the substrate symmetry.

Abstract: Barium adsorption on the O(2 � 1)/Ni(110) surface has been studied by Auger electron spectroscopy and work function measurements in combination with photoemission measurements. The study was focused on the low coverage regime from submonolayer to double monolayer. The results show that during development of the first layer of Ba on the surface, a two-dimensional incomplete barium oxide layer, BaO, forms. This BaO layer is interspersed by Ba chemisorbed atoms reacting directly with Ni atoms. As the second layer of Ba is completed, the adsorbate approaches the metallic phase due to the Ba�Ba interaction. The low energy Auger transition lines of Ba (75 eV) and BaO (68 eV) shift towards lower energies as the Ba coverage increases. Previous photoemission measurements by synchrotron radiation are used to interpret these energy shifts, which are closely related to the barium oxidation process on the surface. The analysis shows the importance of the extra-atomic relaxation energy due to (1) the polarization of the O2� anions from BaO and (2) the screening from the electron density at the Fermi level of the barium overlayer and the nickel substrate.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: The interaction of oxygenwith evaporatedNi filmson an Fe-dopedSrTiO3(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 theWF, 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.

Abstract: The effects of specimen charging on X-ray absorption spectroscopy using total electron yield have been investigated using powder samples of zirconia stabilized by a range of oxides. The stabilized zirconia powder was mixed with graphite to minimize the charging but significant modifications of the intensities of features in the X-ray absorption near-edge fine structure (XANES) still occurred. The time dependence of the charging was measured experimentally using a time scan, and an algorithm was developed to use this measured time dependence to correct the effects of the charging. The algorithm assumes that the system approaches the equilibrium state by an exponential decay. The corrected XANES show improved agreement with the electron energy-loss near-edge fine structure obtained from the same samples.

Abstract: 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 (< 0.5ML), the Ba adatoms are in a partially ionic state, whereas at higher coverage, the barium overlayer becomes metallic. The nonmetal to metal transition is characterized by a new Ba 4d doublet appearing at higher binding energy. This more bound Ba 4d core state is attributed to initial state changes of the electrostatic potential at the atomic core region, due to changes in the hybridization of the Ba atoms from Ba 5d with Ni 3d, to Ba 5d, 6s and 6p states in the metallic phase. The latter states are more spatially extended than the Ba 5d ones, overlapping with the Ni 3d orbitals in the nonmetal and therefore lead to a reduced potential at the core electrons. A strong effect on the Ba 4d binding energy shifts, due to the surface dipole induced by the adsorbate itself, was observed.

Abstract: The adsorption of oxygen and potassium on the two-phase system: carbide-modified stepped- W(100) surface (CMT) in contact with the solid solution of carbon in bulk tungsten, was investigated by AES and WF measurements. The CMT surface shows metallic behavior judging from its interaction with K. The expected dissociative adsorption of oxygen appears to occur with 1 � θ kinetics, possibly via a molecularly chemisorbed state. The �dispersed phase � two-phase� model is clearly applicable when oxygen adsorbs on the K-pre-covered carbide. The initial sticking coefficient of oxygen increases drastically from the dispersed to the condensed phase, at least four-fold with respect to s0 on the clean carbide. It is proposed that this twophase carbon system can be advantageous compared with the bulk carbide since it can easily regenerate the surface if the latter is depleted from carbon.

Abstract: The Ni ultrathin film development on the SrTiO3(1 0 0) surface doped by Fe acceptors (0.14 wt%) has been studied by AES, LEED, TDS, EELS and relative WF measurements. Heating of the clean STO surface above 1100 K causes TiO desorption. The adsorbate grows in the simultaneous multilayers (SM) mode, approaching a rather metallic character at high coverages. The nickel adatoms in the metal�oxide interface, interact with the outmost surface oxygen atoms resulting in a two-dimensional NiO compound. The formation of this surface oxide is: (i) time dependent, (ii) temperature independent for T > 300 K and (iii) promoted by the presence of surface defects. A significant amount of Ni (~1.4 ML) remains on the surface, strongly chemisorbed, after high temperature annealing.

Abstract: The electronic properties of very thin Ni flms 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.

Abstract: The adsorption of oxygen on aBa-coveredNi(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 to BaO 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.

Abstract: The electron energy-loss near-edge structure (ELNES) at the O K edge has been studied in yttria-stabilized zirconia (YSZ). The electronic structure of YSZ for compositions between 3 and 15 mol% Y2O3 has been computed using a pseudopotential-based technique to calculate the local relaxations near the O vacancies. The results showed phase transition from the tetragonal to cubic YSZ at 10 mol% of Y2O3, reproducing experimental observations. Using the relaxed defect geometry, calculation of the ELNES was carried out using the full-potential linear muffin-tin orbital method. The results show very good agreement with the experimental O K-edge signal, demonstrating the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

Abstract: Liwas deposited at low temperature (80 K) onto cleaved van derWaals surfaces of the layered compounds MSe2 (M � Ti, Zr and Hf). The adsorption systems were investigated by means of low-energy electron diffraction, work-function measurements and soft-x-ray photoelectron spectroscopy using a synchrotron radiation source. The results suggest that at low coverages, Li is uniformly distributed near the surface, leading via a decomposition reaction to the formation of Li2Se and M0. At high adsorbate concentration, some of the Li intercalates into the substrate in the interlayer region. The intercalation process seems to depend on the temperature and the lattice parameter of the substrate.

Abstract: Atomistic configurations of yttria-stabilized zirconia between 3 and 10 mol% Y2O3 were relaxed using the pseudopotential technique. The results showed a phase transition to the cubic (c) (ZrO2)1002x(Y2O3)x at x~10 mol%. The electron-energy-loss near-edge spectra, calculated using the linear muffin-tin orbital method and relaxed defect geometry, agree with experiment. In the displacive limit of the double-well potential model, the vibration modes, corresponding to a soft phonon of c-ZrO2, were calculated for each composition of yttria-stabilized zirconia. The effect of anharmonicity yields the fine structure in the spectral density which is associated with stabilization at x<10 mol%. In studying the phonon dynamics, we use the displacement probability density which quantifies accurately the transition temperature above which the c phase is stabilized.

Abstract: The electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia (YSZ) materials. The positions of the peaks in the near-edge structure are identical in both techniques. Differences observed in the intensities of the features are attributed to the effect of specimen charging in the XANES experiments. Analysis of near-edge structure reveals that both the crystallographic phase and the metal fraction of yttrium present can be determined directly from the oxygen K-edge data opening up opportunities for characterization of interfacial phenomena in YSZ materials with sub-nanometre resolution using ELNES.

Abstract: The electron energy-loss near-edge structure ~ELNES! at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia (YSZ) materials. The electronic structure of the three polymorphs of pure ZrO2 and of the doped YSZ structure close to the 33 mol%Y2O3 composition have been calculated using a full-potential linear muffin-tin orbital method (NFP-LMTO) as well as a pseudopotential based technique. Calculations of the ELNES dipole transition matrix elements in the framework of the NFP-LMTO scheme and inclusion of core hole screening within Slater�s transition state theory enable the ELNES to be computed. Good agreement between the experimental and calculated ELNES is obtained for pure monoclinic ZrO2. The agreement is less good with the ideal tetragonal and cubic structures. This is because the inclusion of defects is essential in the calculation of the YSZ ELNES. If the model used contains ordered defects such as vacancies and metal Y planes, agreement between the calculated and experimental O K-edges is significantly improved. The calculations show how the five different O environments of Zr2Y2O7 are connected with the features observed in the experimental spectra and demonstrate clearly the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

Abstract: A comparative study on negative ion formation in the scattering of a proton beam from both a clean and one monolayer of barium-covered Ag(111) surface is presented. The angular and energy dependence of the backscattered negative hydrogen ions as a function of incoming and azimuthal angles has been determined for a beam energy of 750 eV. The backscattered negative particles emerge from the surface as well as from deeper layers of the crystal. The angular dependence of the outgoing particles shows a very rich structure, which is explained by shadowing and blocking of the incoming and outgoing particles. In addition, the angular dependence of the outgoing neutral particles is determined. The essential features appear the same, but distinct differences can be observed. These are due to changes in the probability for negative ion formation as a function of outgoing angle. The energy distributions of the outgoing particles suggest a large penetration depth along the crystal channels. We have performed classical trajectory calculations that simulate the angular distributions of the backscattered particles very well. These calculations also show considerable penetration of particles into the bulk of the crystal and complicated zigzag trajectories through the bulk before leaving the crystal. The (electronic) stopping inside the Ag solid is at least one or two orders of magnitude smaller (<0.3 eV/� at E=700 eV) than the values found in the literature. Comparing the Ag(111) data and the data of Ag(111) covered by one monolayer barium, we conclude that the barium atoms occupy lattice positions of the crystal. The overlayer must contain vacancies to accommodate the large size mismatch between the barium atoms and those of the substrate.

Abstract: In this work, we study the coadsorption of barium (Ba) and hydrogen (H). on Ni(100) surfaces using mainly thermal desorption spectroscopy (TDS) in correlation with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and work function(WF) measurements. Two different processes have been used for the coadsorption experiments: (1) Ba deposition on hydrogenated Ni(110) and (2) H adsorption on Ba-covered Ni(110). In both cases, H gives two different TD energy states, b1and b2. The first state is the same with that of H adsorption on clean Ni(110), pointing to a direct H�Ni bonding, while the second, which is attributed to a H�Ba interaction, shifts to higher temperature as Ba-coverage increases up to Ba coverage 0.9 ML. This shift indicates a gradually developing attractive H�Ba interaction leading to BaH2 formation. The formation of BaH2 does not depend on the sequence of Ba and H deposition. For Ba coveraage 0.3 ML, H adsorption on Ba/Ni(110) does not induce any WF change, as H2 does on alkali-covered surfaces. This means that the Ba/Ni(110) surface might be a stable negative H ion source.

Abstract: The interaction of O with UHV cleaved TaSe2 and WSe2, 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 NaxOy 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.

Abstract: An experimental study of Ba and H adsorption on Si(100)2x1 by Auger electron spectroscopy, thermal desorption spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy, and work function measurements has been made. Measurements of hydrogen adsorption on a clean silicon surface have been made mainly for reference purposes. H on Si forms two different states, known as monohydride state Si(100)2x1:H and dihydride state Si(100)1x1: : 2H. Preadsorption of H made the surface order more stable without changing the sticking coefficient of Ba on the Si surface. The results supported the double-layer (DL) model for the first Ba layer on the monohydrided Si surface. Ba adatoms up to coverage 1 ML on the dihydride phase were relaxed at symmetric and equivalent sites following the (1x1) symmetry of the restored Si surface. TDS measurements showed that during Ba adsorption on the monohydride phase some of the H atoms were removed from their initial adsorption sites, and a new H energy state was formed at 425 C which was attributed to the weakening of the Si�H bond in the presence of Ba adatoms. When Ba deposition took place on the dihydride phase, two new H states were successively developed. The first state at 220 C was attributed to BaH2 formation, and the subsequent one to a complex Ba�H�Si compound near 680 C. The presence of hydrogen caused a considerable delay of barium overlayer metallization, in contrast to the early metallization of alkali on hydrogenated surfaces.

Abstract: The deposition of Ba on Ni(ll0) 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.

Abstract: Effects of the surface structure on H- scattering from clean and Ba covered Ag(111) are reported. The collision energy in this study is around 750 eV. The H-atoms are scattered from the surface as well as from deeper layers of the crystal. This is clearly seen in the angular distributions of the back scattered ions. The Ba atoms sit at lattice positions of the Ag(111) substrate. The adsorbate layer contains vacancies to accommodate the large Ba atoms on the substrate.

Abstract: The Ba deposition on Si(1 00)2 x 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 x 1 gives rise to two adsorption states which exhibit relatively high binding energy for coverages < 2 ML, a strong Ba-Si ionic interaction and relatively low binding energy for coverages > 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 x 1 and 2 x 3 diffraction patterns.

Abstract: Li was adsorbed at room temperature onto UHV cleaved TiSes (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.

Abstract: In this work we report on the properlies of potasium on MoS2(0001) at 100 K as deduced from LEED, AES EELS and TDS measuremenls. 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 coverages > 0.5 there is clear widence of metallization of the K overlayer on MoS2.

Abstract: The coadsorption of K and O2 on MoS2(0001) at RT, has been investigated by LEED, AES, TDS and WF measurements. The sticking coefficient and the maximum amount of oxygen on the surface increase with K coverage. For coverage > 0.5 the amount of desorbed oxygen increases almost linearly with the coverage. The oxygen interacts strongly with K and forms K-O complexes on the surface of MoS2. The TD spectra show K2O2, K2O and KO2. The interaction of the adsorbed oxygen with K causes the diffusion of intercalated K to the surface (deintercalation). The deintercalation of K to the surface helps estimating the coverage of K on MoS2.

Abstract: The adsorption of K on MoS2(0001) has been investigated by LEED, AES, TDS and WF measurements. The results suggest that the initial sticking coefficient of K on MoS, 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.