In this study, photoemission spectroscopy (PES) was used to investigate the electronic properties of nanocrystalline titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium selenide (CdSe). Electrospray deposition technique enabled the preparation of thin films in vacuum from a dispersion prepared outside the vacuum chamber. This method also allowed the step-wise formation of interfaces and the monitoring of the evolution of the electronic structure with intermittent PES characterization. The work function of nanocrystalline TiO2 and ZnO was measured with ultraviolet photoemission spectroscopy (UPS) and low-intensity x-ray photoemission spectroscopy (LIXPS). Measurements on environmentally contaminated surfaces revealed an instantaneous and permanent work function decrease of 0.3-0.5 eV upon exposure to ultraviolet radiation during a UPS measurement. The work function reduction is likely to be related to the formation of a surface dipole caused by the photo-chemical hydroxylation of surface defects. This phenomenon was further investigated with regard to its influence on the electronic structure of the indium tin oxide (ITO)/TiO2 interface found in dye-sensitized solar cells. The experiments suggest that UV radiation can cause a small but significant change of the charge injection barriers at the interface. The determined band line-ups revealed electron injection barriers of ~0.3-0.5 eV, while UV radiation caused an increase of about 0.15 eV. This might have the potential to further impede electron transfer to the ITO electrode and affect the performance of solar cell device. Another type of photovoltaic cell using nanocrystalline material is a heterojunction bulk solar cell. Conversion efficiencies of such devices are currently only about 3% due to the inefficient charge separation at interfaces formed by blending organic and inorganic material. An approach to improve efficiencies in such devices is the use of covalently bonded conductive polymer/inorganic hybrid nanocrystals. In this study a prototypical model system was investigated with PES with the aim to develop a measurement protocol that allows the determination of electronic properties for such hybrid materials. The comparison of the relative core-level binding energies of the organics-functionalized CdSe nanocrystal compared to the ligand-free CdSe nanocrystal and the arylselenophosphate ligand material enabled the determination of the electronic structure at the interface. Core-level measurements support the hypothesis that the Se functionality of the organic ligand coordinates to the Cd sites on the nanopthesis surface.

The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.

In this study, we used UV-ozone Ar sputtering, X-ray photoelectron and ultra-violet photoelectron spectroscopies and sputtering based depositions of RuO2 and Se nano-layers on indium tin oxides (ITOs). We elucidated the effect of Ar sputtering on the composition and chemistry of Sn rich ITO surface. We demonstrated that while a combination of UV-ozone radiation and Ar sputtering removes most of the hydrocarbons responsible for degrading the work function of ITO, it also removes significant amount of the segregated SN at the ITO surface that's responsible for its reasonable work function of 4.7eV. We also demonstrated for the first time that sputtering cleaning ITO surface leads to the reduction of the charge state of Sn from Sn4+ to Sn2+ that adds to the degradation of the work function. For the nano-layers coverage of ITO studies, we evaluated both RuO2 and Se. For RuO2 coated ITO, XPS showed the formation of a Ru-Sn-O ternary oxide. The RuO2 nano-layer reduced the oxidation state of Sn in the Sn-rich surface of ITO from +4 to +2. The best work function obtained for this system is 4.98eV, raising the effective work function of ITO by more than 0.5 eV. For the Se coated ITO studies, a systematic study of the dependence of the effective work function on the thickness of Se overage and its chemistry at the Se/ITO interface was undertaken. XPS showed that Se reacts with Sn at the Sn-rich surface of ITO determined the presence of both negative and positive oxidation state of Se at the Se/ITO interface. The Se also reduced the oxidation state of Sn from Sn4+ to Sn2+ in the Sn-rich ITO surface. The highest effective work function obtained for this system is 5.06eV. A combination of RuO2/Se nanoscale coating of optimally cleaned ITO would be a good alternative for device applications that would provide work function tuning in addition to their potential ability to act as interface stabilizers and a barrier to reaction and inter-diffusion at ITO/active layers interfaces responsible for long term stability of devices and especially organic solar cells and organic light emitting diodes.

<p>In the present work, the structural, electronic properties, thermodynamic stability and adatom surface movements of oxygen and sulfur adsorption on the Fe surface were studied based on the ab initio method.</p><p>Firstly, the oxygen adsorbed on the iron (100) surface is investigated at the three adsorption sites top, bridge and hollow sites, respectively. Adsorption energy, work function and surface geometries were calculated, the hollow site was found to be the most stable adsorption site, Which is in agreement with the experiments. In addition, the difference charge density of the different adsorption systems was calculated to analyze the interaction and bonding properties between Fe and O. It can be found out that the charge redistribution was related to the geometry relaxation.</p><p>Secondly, the sulfur coverage is considered from a quarter of one monolayer (1ML) to a full monolayer. Our calculated results indicate that the most likely site for S adsorption is the hollow site on Fe (100). We find that the work function and its change Df increased with S coverage, in very good agreement with experiments. Due to a recent discussion regarding the influence of charge transfer on Df, we show that the increase in Df can be explained by the increasing surface dipole moment as a function of S coverage. In addition, the Fe-S bonding was analyzed. Finally, the thermodynamic stabilities of the different structures were evaluated as a function the sulfur chemical potential.</p><p>Finally, a two dimensional (2D) gas model was proposed to simulate the surface active elements, oxygen and sulfur atoms, movement on the Fe (100) surface. The average velocity of oxygen and sulfur atoms was found out to be related to the vibration frequencies and energy barrier in the final expression developed. The calculated results were based on the density function and thermodynamics & statistical physics theories. In addition, this 2D gas model can be used to simulate and give an atomic view of the complex interfacial phenomena in the steelmaking refining process.</p>

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In dieser Arbeit wurde die Austrittsarbeit von Graphen, einer vielversprechenden Elektrodenmaterial für (opto)- elektronische Bauteile, durch die Adsorption von luftbeständigen konjugierten organischen Molekülen (KOMs), welche als Akzeptoren und Donatoren fungieren, modifiziert. Die Eigenschaften der Valenz- und Rumpfniveaus sowie die Austrittsarbeitsmodifikation der vakuumverdampften KOMs wurden mit Photoelektronenspektroskopie (PES) untersucht, während die Orientierung der KOMs mit Röntgen-Nahkanten-Absorptions-Spektroskopie (NEXAFS) aufgeklärt wurde. Die Austrittsarbeit von Graphen auf Quartz (G/Qu) lässt sich auf maximal 5.7 eV und minimal 3 eV anpassen, welches aus einem Ladungstransfer direkt an der Grenzfläche resultiert, der keine Ausbildung von kovalenten Bindungen zwischen der molekularen Monolage und dem Graphen beinhaltet. Zudem, für den starken molekularen Akzeptor Hexaazatriphenylen-Hexacarbonitril (HATCN) verläuft die Austrittsarbeitserhöhung über eine Orientierungsänderung der Moleküle im Monolagenbereich. Für alle anderen auf G/Qu abgeschiedenen Akzeptoren (Donatoren) wurde beobachtet, dass der Ladungstransfer eine positive (negative) Oberflächen-ladungsdotierung der Graphen-Schicht bewirkt, welches in einer Austrittsarbeitserhöhung (-erniedrigung) resultiert. Letztere ließ sich jeweils in zwei Beiträge zerlegen: (a) Verschiebung des Vakuumniveaus durch einen Grenzflächendipol an der KOM/Graphen-Grenzfläche und (b) Verschiebung des Fermi-Niveaus durch Oberflächenladungstransferdotierung der Graphen-Schicht. Weiterhin wurde der molekulare Akzeptor Hexafluoro-tetracyano napththoquinodimethan (F6TCNNQ) sowohl auf G/Qu als auch auf Graphen auf Kupfer abgeschieden, wobei sich herausstellte, dass der Ladungstransfer im ersteren Fall vom Graphen stammt, und im letzteren von der Kupferunterlage. Die Ergebnisse werden von Dichtefunktionaltheorieberechnungen gestützt und tragen erheblich zum Verständnis von Graphen/KOM-Grenzflächen bei. / In this thesis, the work function of graphene, a promising electrode for (opto)electronic devices was modified by adsorption of air-stable conjugated organic molecules (COMs) that act as strong molecular acceptors or donors. The valence and core level properties, together with the work function modification of the vacuum-deposited COMs on graphene were investigated with photoelectron spectroscopy (PES), while the orientation of COMs was studied with near edge X-ray fine structure spectroscopy (NEXAFS). The work function of graphene-on-quartz (G/Qu) is modified up to 5.7 eV and down to 3 eV as a result of charge transfer (CT) occurring right at the interface, which does not invoke covalent bond formation between the molecular monolayer and the graphene. In addition to the CT, in the case of the molecular acceptor hexaazatriphenylene-hexacarbonitrile (HATCN), the work function increase proceeded via a density-dependent re-orientation of the molecule in the monolayer regime. For all the other tested molecular acceptors (donors) deposited on graphene-on-quartz, the CT was observed to induce positive (negative) surface CT doping of the graphene layer, leading to a work function increase (decrease) and was disentangled into two contributions: (a) shift of the Vacuum level due to the formation of an interface dipole at the COM/graphene interface and (b) shift of the Fermi level of the graphene due to the surface CT doping. Additionally, the molecular acceptor hexafluoro-tetracyanonapththoquinodimethane (F6TCNNQ) was deposited on both G/Qu and graphene-on-copper, where the CT was found to originate from graphene and copper support respectively. The findings were supported by density functional theory calculations and significantly add to a fundamental understanding of graphene/COM interfaces.

This thesis focuses on the use of surface modifiers to tune the properties of both metals and metal oxides. Particular attention is given to examine the modification of transparent conducting oxides (TCOs) including indium tin oxide and zinc oxide both through the use of phosphonic acids as well as organic and metal-organic dopants. In this thesis a variety of known and new phosphonic acids are synthesized. A subset of these molecules are then used to probe the relationship between the ability of a phosphonic acid to tune the work function of ITO and how that interrelates with the coverage and molecular orientation of the modifier on the surface. Experimental techniques including XPS, UPS, and NEXAFS are coupled with theoretical DFT calculations in order to more closely examine this relationship. Literature surrounding the modification of zinc oxide with phosphonic acids is not as prevalent as that found for the modification of ITO. Thus, effort is placed on attempting to determine optimal modification conditions for phosphonic acids on zinc oxide. As zinc oxide is already a low work function metal oxide, modifiers were synthesized in an attempt to further decrease the work function of this substrate in an effort to minimize the barrier to carrier collection/injection. Etching of the substrate by phosphonic acids is also examined. In a related technique, n- and p-dopants are used to modify the surfaces of ITO, zinc oxide, and gold and it was found that the work function can be drastically altered, to approximately 3.3 – 3.6 eV for all three of the substrates examined. Surface reactions are straightforward to conduct typically taking only 60 s to achieve this change in work function.

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The studies presented here address the characterization of the electronic structure of various self-assembled monolayers (SAMs) of peptide nucleic acid (PNA) and tetraphenylporphyrin (TPP) SAMs and arrays, formed on gold substrates. PNA is a promising alternative to DNA for bio-sensing applications, as well as for strategies for self-assembly based on nucleic acid hybridization. In recent years charge transfer through PNA molecules was a focus of research due to possible applications in self-assembled molecular circuits and molecular tools. In light of this research it is interesting to investigate the electronic structure of PNA interfaces to gold, a potential electrode material. TPP is, due to its electronic structure, an organic p-type molecular semiconductor. Such a material can provide an alternative to standard micro- and optoelectronic devices and in recent years more attention was paid to semiconducting polymers and organic compounds offering these low-cost and flexible alternatives. Therefore, it is of high importance to investigate the prospect of using modified TPP molecules for the formation of interconnected molecular networks on metallic surfaces. All investigated monolayers were formed from solution in a nitrogen atmosphere inside a homemade glove box. This process allowed for PNA SAM and TPP SAM and array formation on clean Au substrates without the exposure to the ambient atmosphere. Ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements on the resulting PNA SAMs and TPP SAMs and arrays, which were performed in a to the glove box attached vacuum chamber containing a photoemission spectrometer, revealed the hole injection barriers at the interfaces and the interface dipoles. In addition to the UPS and XPS measurements on PNA, electronic structure calculations based on molecular dynamics sampling of the PNA structure were obtained, yielding the HOMO-LUMO gap and the electronic density of states for PNA. Combined with the UPS data, the theoretical calculations enabled estimation of the charge injection barriers for the PNA SAMs at the interface, as well as the assignment of individual UP-spectral features to specific molecular orbitals. The orbital line-up at the interface between the Au substrate and the PNA indicated a significant interface dipole resulting in the alignment of the Au Fermi level near the center of the PNA HOMO-LUMO gap. This alignment causes large charge injection barriers for both holes and electrons, and thus impedes charge transfer from Au into the PNA SAM. The study of PNA molecules with ferrocene termini showed that this hole injection barrier is shifted to lower energies at the PNA/ferrocene interface. This shift was explained with a molecular orbital reconfiguration through the presence of the ferrocene terminus. The further investigation of the dependence of the electronic structure of PNA SAMs, based on their orientation, showed that incomplete films containing flat lying molecules can have a significant impact on the charge injection barriers. The close proximity of the nucleobases to the Au surface offers new ways for charge transfer between the substrate and the PNA molecule through its nitrogen sites, leading to a lowering of the hole injection barrier at the interface. The TPP arrays were formed by depositing AgNO3 on the Au substrate prior to TPP incubation using the electrospray technique. The interaction of AgNO3 with the TPP promoted the formation of an interconnected thin film forming a network on the Au substrate. The line-up at the Au/TPP interface without AgNO3 exposure showed an interface dipole formation with injection barriers that would potentially obstruct charge injection into the molecule. However, the addition of AgNO3 to the process resulted in the formation of fine structures, and lead to a lower hole injection barrier due to an induced dipole, which would ultimately improve charge transfer between the substrate and the thin film. A separate thiolated TPP derivative was used to form SAMs on a gold substrate. The SAM exhibited an even lower injection barrier than the mentioned TPP thin film with AgNO3 exposure, leading to the conclusion that a mix of both TPP derivatives could potentially lead to a SAM with long range interconnectivity and a low hole injection barrier towards the substrate.

L'introduction du SiGe dans les pMOS (Bulk et FDSOI) exige un bon contrôle de la tension de seuil (VT). Ceci nécessite une extraction précise des paramètres électriques ainsi qu'une compréhension des effets électriques du Ge dans de tels dispositifs. Dans cette thèse, nous avons d'abord proposé des méthodes pour une identification précise des paramètres électriques du « gate stack »: VT, la tension de bande plate (VFB) et l'épaisseur équivalente d'oxyde (EOT). Ces méthodes ont été validées avec des simulations Poisson-Schrödinger (PS) et appliquées avec succès aux mesures. Dans un second temps, nous avons étudié les effets électriques du Ge sur les paramètres du « gate stack » des pMOS. La comparaison des caractérisations électriques (C-V) avec les simulations PS a montré un décalage supplémentaire du travail de sortie effectif qui croit avec le Ge. Des caractérisations STEM, EELS et SIMS ont prouvé que ce décalage est due à la présence de dipôles à l'interface SiGe/oxyde. / Maintaining good threshold voltage (VT) centering is a paramount challenge for CMOS technology. The SiGe introduction in bulk and FDSOI pFETs requires VT control for such devices. To this end, we have to extract accurately electrical parameters and to understand Ge integration effects in SiGe based pFETs. In this thesis, first, we have proposed extraction methods to determine VT, flat band voltage (VFB) and equivalent oxide thickness (EOT) parameters in bulk and FDSOI transistors. The extraction methods have been validated via Poisson-Schrodinger (PS) simulations and successfully applied to measurements. Second, we have highlighted and explained electric effects of Ge on pMOS gate stack parameters. Electrical characterizations compared with PS simulations have evidenced an additional effective work function increase, induced by Ge, related to interfacial dipoles. STEM, EELS and SIMS characterizations have demonstrated that dipoles are located at SiGe/IL interface.