We demonstrate heterogeneous chemistry between Li and anatase TiO2 nanoparticles under UHV. The reduction of TiO2 upon formation of lithium oxide proceeds via two different schemes: one that reduces Ti(4+) to Ti(3+) and one that reduces Ti(4+) directly to Ti(2+). The second scheme sets in only after a critical degree of reduction (i.e. Li amount) has been reached (Li/Ti = 0.28) and is associated with restructuring of the film. Two films with different morphologies were compared and the results demonstrate that the reaction between Li and larger TiO2 structures (30-50 nm) is kinetically restricted while such effects were significantly less prominent for small particles (10 nm).

The interaction between the metal organic precursor molecule titanium(IV) isopropoxide (TTIP) and three different surfaces has been studied: Si(111)-(7×7), SiOx/Si(111) and TiO2. These surfaces represent the different surface compositions encountered during TTIP mediated TiO2 chemical vapor deposition on Si(111). The surface chemistry of the titanium(IV) isopropoxide precursor and the film growth have been explored by core level photoelectron spectroscopy and x-ray absorption spectroscopy using synchrotron radiation. The resulting film morphology has been imaged with scanning tunneling microscopy. The growth rate depends on both surface temperature and surface composition. The behavior can be rationalized in terms of the surface stability of isopropoxy and isopropyl groups, confirming that growth at 573K is a reaction limited process.

We show that H/D exchange between H(2)O and D(2)O in ultrathin ice films adsorbed on Cu(100) does not occur through autoionization at temperatures below 140 K. The exchange is, however, facile if a proton deficiency is induced in the ice films by having small amounts of OH preadsorbed on the copper surface. The system was studied using surface infrared vibrational spectroscopy with the aid of density functional theory calculations.

Mössbauer measurements have been made on a series of stainless steel absorbers of different type and thickness. The analysis has been performed with different line shape approximations to illuminate the sensitivity to the method used, with focus on the recoilless fraction f.

The vibrational properties of the CO/Cu(100) surface adsorbate system have been explored by infrared spectroscopy and DFT cluster calculations. We show that all four fundamental, FT(x,y), FR(x,y), FT(z) and ν(C-O), vibrational modes are very well reproduced with respect to experiments by the present calculations and they are at the highest level reported to date. Our work demonstrates that it is essential to include both anharmonicity and cluster relaxation when modeling the CO/Cu(100) system. The absence and presence of binary modes: 2 ×ν(C-O) and FT(z) + ν(C-O) in our experimental data are discussed as well.

The adsorption of water on the anatase TiO2(001)-(4 x 1) surface is studied using synchrotron radiation-excited core level photoelectron spectroscopy. The coverage-dependent adsorption of water at low temperature is monitored and compared to the sequence obtained after heating of a water multilayer. Two adsorption phases of submonolayer coverage can be defined: Phase 1 consists only of dissociated water, observed as OH-groups. This phase is found at low coverage at low temperature (190 K) and is the only state of adsorbed water above similar to 230 K. The saturation coverage of phase 1 is consistent with dissociation on the 4-fold-coordinated Ti ridge atoms of the (4 x 1) surface reconstruction. Phase 2 is found at higher coverage, reached at lower temperature. It consists of a mixture of dissociated and molecular water with a ratio of 1:1 at 170 K. The molecular water is found to bond to the hydroxyl groups. The hydroxyl coverage of phase 2 is approximately 2 times that of phase 1. The results suggest that the OH and H2O species of phase 2 are confined to the ridges of the surface.

The properties of a sol-gel prepared Mn-modified TiO2 film have been studied with X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The chemical composition and oxidation state of the elements have been determined. The manganese content estimated by PES of the Mn-modified film is about 10% and both Mn2+ and Mn3+ are observed. Addition of Mn is found to modify the valence band edge. The Mn 3d states are found to extend about 1 eV into the TiO2 band gap region. It is demonstrated that lithium insertion into the sol-gel film can be performed in a stepwise fashion in situ under ultrahigh vacuum (UHV) conditions. Lithium is distributed evenly throughout the entire film and leads to reduction of Mn3+ to Mn2+ followed by reduction of Ti4+ to Ti3+. The XAS and PES measurements give fully consistent results regarding the amount of inserted lithium.

We present measurements of the Zr and Si core level photoelectron binding energies relative to the Fermi level and the vacuum level under a ZrO2 growth series on Si(100). It is shown that the Zr core level binding energy is most properly referenced to the local vacuum level already from the monolayer regime. This confirms the insulating properties of ZrO2. The Si core levels are referenced to the Fermi level and undergo shifts consistent with the disappearance of the mid-band-gap states originating from the (2×1) reconstruction on the clean Si(100) surface. The use of O 1s x-ray absorption spectroscopy (XAS) to determine the location of the conduction band edge of ZrO2 is discussed with the aid of ab initio calculations. It is demonstrated that the conduction band edge is located at the XAS peak position and that the position relative to the valence band can be determined by aligning the O 1s XAS spectrum to the O 1s photoelectron spectrum. The study thus establishes that photoelectron spectroscopy in conjunction with x-ray absorption spectroscopy forms a most powerful tool for studies of the band alignment at metal oxide–silicon interfaces.

The growth of ultrathin ZrO2 films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO2 (t-ZrO2), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO2 is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO2 is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO2 on Si(1 0 0) is discussed in terms of Zr–O valence electron matching to the number of dangling bonds per surface Si atom. The Zr–O hybridization within the ZrO2 unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C–O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.

For an electronic device well-designed interfaces are critical for the performance. Studies of interfaces down to an atomic level are thus highly motivated both from a fundamental and technological point of view. In this thesis, a surface science approach has been employed to study the formation of interfaces in systems relevant for transistor and solar cell applications. Surface science methodology entails ultra high vacuum environment, single crystalline surfaces, submonolayer control of deposited material, surface sensitive spectroscopy and atomic resolution microscopy.The primary experimental method for characterization is electron spectroscopy. This is a family of very powerful experimental techniques capable of giving information on the atomic level. Additionally, studies have been performed using scanning tunnelling microscopy. Combined these two methods can provide an atomic level characterisation of the geometric and electronic properties of the surface.The emphasis of this work is placed on ultra thin TiO2 and ZrO2 films grown on silicon substrates by means of ultra-high vacuum metal-organic chemical vapour deposition. ZrO2 has also been grown on SiC and FeCrAl. Deposition has been performed with different process parameters. The interface region of each film has been characterised. The band alignment, a most important issue with regard to the development of new transistor devices, for the ZrO2/Si(100) system has been explored. Decomposition pathways of the metal organic precursors have been studied in detail. Changing process parameters is shown to alter both the precursor decomposition pathway and the nature of the interface region, thus opening the possibility to tailor the material function.The titanium dioxide films grown in situ have shown to be excellent models of nanostructured electrode materials. In this spirit, interfaces of model systems for the solid-state dye-sensitized solar cell have been studied. Links between device performance and interface structure have been elucidated.

The band alignment of ultrathin ZrO2 films of different thickness formed on Si(100) have been monitored with synchrotron radiation photoelectron spectroscopy and x-ray absorption spectroscopy. The films were deposited sequentially by way of metal-organic chemical-vapor deposition in ultrahigh vacuum. A significant decrease in the conduction band offset is found for increasing film thickness. It is accompanied by a corresponding increase of the valence band offset. The variations originate in the formation of an interfacial layer characterized by a lower degree of Zr-O interaction than in bulk ZrO2 but with no clear evidence for partially occupied Zr4d dangling bonds.

The chemistry Of HfI4 adsorbed on the Si(100)-(2x1) surface has been studied by core level photoelectron spectroscopy in ultra-high vacuum. Two stable surface intermediates are identified: HfI3 and HfI2, both of which remain upon heating to 690 K. The dissociation of HfI4 is accompanied by the formation of SiI. In addition, HfI4 is Observed up to 300 K. Complete desorption of iodine occurs in the temperature regime 690-780 K. Deposition of HfI4 at 870 K results in a layer consisting of metallic Hf, whereas deposition at 1120 K results in the formation of Hf silicide. The results indicate that the metallic Hf formed at 870 K is in the form of particles. Oxidation of this film by O-2 at low pressure does not result in complete Hf oxidation. This suggests that complete oxidation of Hf is a critical step when using HfI4 as precursor in atomic layer deposition. (c) 2006 Elsevier B.V. All rights reserved.

Zirconium Dioxide Formation on Silicon Surfaces by Metal-Organic Chemical Vapor Deposition in UHV

The catalytic cracking of ethanol on clean and oxygen precovered Rh(111) surfaces has been studied by a combination of synchrotron based high-resolution core level spectroscopy and surface infrared spectroscopy. On the clean surface the decomposition of ethanol occurs without any formation of atomic oxygen. The presence of oxygen on the surface changes the reactivity significantly and acetate (CH(3)COO) was identified as an important surface intermediate in the ethanol oxidation. (c) 2006 Elsevier B.V. All rights reserved.

Metal-organic chemical vapour deposition growth of titanium oxide on moderately pre-oxidised Si(1 1 1) using the titanium(IV) isopropoxide precursor has been studied for two different growth modes, reaction-limited growth at 300 degrees C and flux-limited growth at 500 degrees C. The interfacial properties have been characterized by monitoring synchrotron radiation excited Si 2p photoemission spectra. The cross-linking from oxidised Si to bulk Si after TTIP exposure has been found to be very similar to that of SiOx/Si(1 1 1). However, the results show that the additional oxidation of Si most probably causes a corrugation of the SiOx/Si interface. Those conclusions are valid for both growth modes. A model is introduced in which the amorphous interface region is described as (TiO2)(x)(SiO2)(y) where x and y changes linearly and continuously over the interface. The model quantifies how (TiO2)(x)(SiO2)(y) mixing changes the relative intensities of the signals from silicon oxide and silicon. The method can be generalised and used for the analyses of other metal-oxides on silicon. (c) 2005 Elsevier B.V. All rights reserved.

Metal-organic chemical vapor deposition of TiO2 on silicon substrates using titanium (IV)isopropoxide : Surface chemistry and interfacial properties

The anharmonic properties of a surface intermediate, methoxy, adsorbed on Cu(100) are investigated by surface infrared overtone spectroscopy and density-functional-theory electronic structure calculations. The anharmonicity is measured in the zero-coverage limit, and it is observed that the anharmonicity is increased upon adsorption as compared with the free methanol. By combining experiments with calculations we demonstrate that modifications of the anharmonicity of the methoxy species is indeed induced by adsorption onto the copper surface and not by the formation of the methoxy species.