Many properties of the thin films are different from the bulk value and in many cases, depend dramatically on the film thickness. In the metallic ultra-thin films epitaxially grown on the semiconductor substrate, the conduction electrons are confined by the vacuum and metal-semiconductor interface. When the film thickness is comparable to the electron Fermi wavelength, this confinement will produce discrete energy levels known as quantum well states (QWS), which dramatically modify the electronic structures of the thin film and this is called quantum size effect (QSE). QSE will have a profound effect on a lot of physical properties of the thin films. Among various systems exhibiting QSE, Pb/Si (111) is the most widely studied one and exhibits the richest phenomena in QSE. In this study, a home made low temperature Scanning Tunneling Microscopy/Spectroscopy (LT-STM/S) was used to study the superconductivities of the Pb thin films. Quantum oscillations of the superconductivity have been observed for the films down to 4 monolayer and the oscillation amplitude increases as the film gets thinner. To resolve the discrepancies between the superconductivities measured with ex-situ transport and in-situ STS. We also studied the influence of Au overlay on the Pb thin films with LT-STM/S, and found out the deposition of Au on Pb dramatically roughened the Pb films. Finally, we successfully grew large scale near perfect 2ML Pb films. There are two types of films which exhibit different Moiré patterns. LT-STS studies revealed there is big difference in the superconductivity Tc of these two films, both of which decreased dramatically from that of the 4ML film. / text

This dissertation describes experimental investigations of deposited atomic clusters and of films of metal particles produced by cluster deposition on graphite. It consists of three chapters presenting a review of the addressed subjects, the experimental techniques, and a summary of the results. The main body of results is described in full in afourth chapter as research papers. A systematic study of the impact of metal cluster trimers on highly oriented pyrolitic graphite (HOPG) is investigated. The creation of defects at the surface of graphite is found to be independento f the masso f the atoms in the two typeso f clusters considered The electron energy loss spectroscopy (EELS) of collective electronic excitations (plasmons) in a film of silver particles is presented A weak positive dispersion of the plasmon mode is measured and exhibits a higher ftequency of the mode with parallel momentum close to zero than in Ag surfaces. The adsorption of cinnamaldehydeis examined on nanostructured palladium surfaces. The EELS spectrum of cinnamaldehyde adsorbed on palladium is presented, as well as that of condensed layers of cinnamaldehyde on HOPG. Findings of this thesis involve different physical and chemical properties of the cluster surface system with a view in using cluster deposition for practical applications such as the investigation of model catalysts.

This thesis focuses on understanding the influence of defect sites in titanium dioxide that drive many types of thermal and photochemical reactions. Two of the most common defects in vacuum are titanium interstitials and oxygen vacancies. Molecular oxygen fills oxygen vacancies and creates oxygen adatoms. We broadly investigate reduction and oxidation reactions of oxygenates driven by titanium interstitials and oxygen adatoms. First, we focus on the thermal chemistry of oxygen adatoms with butyrophenone and find that it reacts with the adatoms to form a strongly bound complex. The large difference in mobility between complexed and uncomplexed butyrophenone, and the corrugated nature of the \(TiO_2(110)\) surface plane, allows a confined one-dimensional gas to persist, which is characterized by scanning tunneling microscopy (STM). Next, we focus on the reductive coupling of benzaldehyde to stilbene that is driven by titanium interstitials. The diolate intermediate of the reaction is identified by STM and the thermodynamic preference of molecular oxygen to interact with titanium interstitials is exploited to selectively reverse the benzaldehyde diolate intermediates. Additionally, we investigate the photo-oxidative coupling of methanol to methyl formate, the photo-oxidation of butyrophenone and the photo-stability of benzoate. Finally, we identify a water splitting mechanism on reduced titania that creates oxygen adatoms. We demonstrate that the photo-generated oxygen adatoms thermally react with titanium interstitials to make TiOx islands and drive the photo-oxidation of formaldehyde and butyrophenone. Methods used include temperature programmed reaction spectroscopy, STM, and density functional theory. / Chemistry and Chemical Biology

When an electronic system is confined in one or more dimensions to a length scale comparable to the de Broglie wavelength, quantum confinement occurs. In metallic quantum thin films grown on semiconductor substrates, such confinement occurs between the vacuum-solid and the solid-solid interfaces, which results in the formation of distinctive quantum well states (QWS). Due to this confinement, many physical phenomena occurring in the thin metal system are totally different from the bulk system, which makes the study of quantum thin films interesting and important. In this thesis, quantum thin film studies, mainly based on the Pb/Si(111) system, were performed utilizing low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) with a focus on three main aspects: phase relationship, surface reactivity, and coherent coupling. The Pb/Si(111) system is chosen due to its unique phase matching between the Fermi wavelength and the lattice spacing along [111], leading to a bi-layer quantum oscillation in many physical properties, including the surface energy and the work function. Surprisingly, STM/STS measurement revealed that quantum oscillations of work function and surface energy have identical phase, in contrast to a theoretically predicted 1/4 wavelength phase shift in the phase relationship. Here, a new solution to this puzzle is provided. Furthermore, it is found out that the oxidation rate of Pb/Si(111) system is greatly enhanced in the presence of atomic scale catalyst -- Cs substitutional atoms, while the reactivity to CO is saturated after the initial enhanced nucleation. Finally, by inserting thin Ag layers in between Pb/Si(111) system, the coherent coupling of double quantum wells (a Pb quantum well and a Ag quantum well) are probed, where combined QWS features are observed by STS measurement. The growth mechanism of these heterostructures -- Pb/Ag/Si(111) -- is also investigated. / text

In this thesis, I present a comprehensive study of the interfacial electronic structure and thin film growth of two types of dipolar organic semiconductors on noble metals by employing a surface science approach, which underlines the critical role of surface electronic states in determining the interfacial electronic structure and self-assembly of organic semiconductors. I show that the electronic structure at organic/metal interfaces is complex and depends on important factors such as molecular adsorption configuration, surface/molecule coupling strength, reactivity of the substrate, molecular electrostatics, and local film structure. I demonstrate the fundamental capability of the image potential states and resonances in probing the local film environment, especially in systems consisting of inhomogeneous film structure. I also show that the presence of adsorbates on a surface allows one to investigate quantum mechanical interference effects otherwise not accessible on the bare surface. The dipolar organic semiconductors studied here are vanadyl naphthalocyanine (VONc) and chloroboron-subphthalocyanine (ClB-SubPc). The single crystals of gold and copper with hexagonal surface symmetry (111) were used to investigate the interfacial properties of VONc and ClB-SubPc, respectively. The fundamental understanding of self-assembly of large π-conjugated organic semiconductors on metals is a crucial step in controlling fabrication of supramolecular structures. Here, I provide a first step in this direction with a detailed and quantitative analysis of molecular nearest-neighbor distances that unravels the fundamental intermolecular interactions of organic semiconductors on transition metal surfaces. I additionally investigated the interfacial electronic structure of these organic semiconductors to examine the relation between molecular adsorption orientation and charge transfer across the interface.

Since 1982, the Si (111)-(7x7) surface has been extensively studied both theoretically and experimentally with the modern powerful tools of STM and Scanning Tunneling Spectroscopy (STS). In this work, a simple atomic orbital model for the Si (111)-(7x7) surface is developed to simulate the experimental results of STM and STS. Based on Tersoff-Hamann’s theory for the tunneling current, simulations of clean Si (111)-(7x7) constant-current images are presented. The direct, real-space simulated topographic images of the surface are compared to experimental results qualitatively and quantitatively. The simulation of spectroscopic imaging and normalized conductance spectra are also included. The adsorption of atomic hydrogen atoms onto the Si (111)-(7x7) surface is also simulated. / xiv, 146 leaves ; 29 cm.

Dissociation of trichloroethylene (TCE) molecules on the Si(111)-7x7 and the Si(100)-2x1 surfaces was studied using STM. Though molecular adsorption may also be observed on the Si(111)-7x7 surface, dissociation is the dominant process. From the STM images acquired, products of dissociation were identified, namely chlorine atoms and dichlorovinyl groups. Dissociation of chlorine from the TCE molecule was confirmed by studying not just appearance in STM images but also from studies of tip-induced diffusion. Different binding configurations were proposed for the vinyl group on the Si (111)-7x7 and the Si(100)-2x1 surfaces. Site preference for each product of dissociation is reported on the Si(111)-7x7 surface. Dissociation of molecules such as ammonia, dimethylamine and methyl chloride on the Si(111)-7x7 and Si(100)-2x1 surfaces is reviewed. The field emission process is explained in detail. The usefulness of making field emission measurements is in evaluating the sharpness of STM tips. / xviii, 175 leaves : ill. (some col.) ; 29 cm

This research work is aimed at understanding the electronic properties of Bi(110) nanostructures. This study chiefly uses Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS) and Non Contact Atomic Force Microscope (NCAFM) to investigate the geometric and electronic structure of Bi(110) islands on highly oriented pyrolytic graphite (HOPG) substrate. STM measurements are the primary focus of the thesis which involves imaging the bismuth islands and study of its atomic structure. STM images of the Bi(110) islands reveal a ‘wedding cake’ profile of the bismuth islands that show paired layers on top of a base. I(V) (Current vs voltage) data was acquired via STS techniques and its first derivative was compared to DFT calculations. The comparison implied the presence of a dead wetting layer which was present only underneath the bismuth islands. We observed bilayer damped oscillations in the surface energy that were responsible for the stability of paired layers in Bi(110) islands. Interesting Moiré pattern arising out of misorientation between the substrate and the overlayer are also observed in STM images on some bismuth islands. Bright features pertaining to enhanced LDOS (local density of states) were observed on the perimeter of the bismuth islands and stripes in the STM images and STS dI/dV maps which appear at energies around the Fermi level. The bright features which we termed as ‘bright beaches (BB)’ are also observed on grain boundaries and defects that suggest that they are related to termination of the chain of bismuth atoms. The Bi(110) islands and stripes were observed to form preferred widths with a well defined periodicity. This peculiar phenomenon was attributed to a lateral quantum size effect (QSE) that results from a Fermi wave vector with appropriate shifts in Fermi energy. The widths of the islands prefer to adjust themselves at the nodes of this in-plane Fermi wavelength. NaCl deposited on a HOPG substrate forms cross shaped islands which were used as spacers to limit the interaction between the bismuth films and the underlying HOPG substrate. The NaCl islands are transparent to the flow of tunneling current and allow STS measurements. The LDOS of Bi/HOPG was very similar to the LDOS of Bi deposited on NaCl/HOPG which suggests that the wetting layer underneath the bismuth islands plays an important role in decoupling the film from the underlying substrate.

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The thesis presents fundamental studies of phthalocyanines (Pc), a group of organic macro-cycle molecules, similar to systems found in nature. The use of phthalocyanine molecular films in devices with a variety of possible technological applications has been the reason of the many studies during the last decades. Core and valence photoelectron spectroscopies (PES), X-ray absorption spectroscopy (XAS) and scanning tunneling microscopy (STM) techniques are used to study Pc molecules in gas phase and adsorbed on gold Au(111) and silicon Si(100)-2x1 substrates. Density function theory (DFT) is used to obtain further insights in the electronic structure of the phthalocyanines.We aim to obtain a deeper understanding of the molecule-molecule and molecule-substrate interactions, a fundamental requirement for improving devices based on organic materials. Gas phase PES experiments and DFT calculations performed on different Pcs show the influence of the metal atom on the valence band spectra, where FePc and H2Pc HOMO is formed from C2p states, whereas MnPc has mainly Mn3d character.PES and STM studies of H2Pc and LuPc2 on Au(111) show a formation of a monolayer for H2Pc and a bi-layer of LuPc2 at lower thicknesses. XAS studies show a parallel to surface orientation of both H2Pc and LuPc2 with a change in tilt angle of the molecules with increasing thickness.A comparison of LuPc2 adsorbed on pristine and passivated Si surfaces show a bulk-like LuPc2 character on passivated Si, whereas a surface induced difference in adsorption geometry is evidenced on pristine Si and two energetically different adsorption geometries are proposed.