Дисертації з теми "Nanoscaled films"

Dünne Filme aus konjugierten Molekülen werden vermehrt in der organischen Optoelektronik, Bio-Sensorik und Oberflächenmodifikationen eingesetzt. Jedoch steckt das nanoskopische Verständnis von elementaren Prozessen bzgl. des molekularen Wachstums, der Film-Stabilität und thermisch-mechanischer Eigenschaften noch in den Kinderschuhen. Im ersten Teil dieser Arbeit nutzen wir Echtzeit in situ spekulare und diffuse Röntgenstreuung in Kombination mit Kinetik-Monte-Carlo Simulationen, um die Nukleation und das Multilagen-Wachstum von C60 zu studieren. Wir quantifizieren einen konsistenten Satz von Energieparametern, die die Oberflächenprozesse während des Wachstums beschreiben: eine effektive Ehrlich-Schwoebel Barriere von EES = 110 meV, eine Oberflächendiffusions-Barriere von ED = 540 meV und die Bindungsenergie von EB = 130 meV. Durch die Analyse der Teilchendynamiken finden wir, dass die laterale Diffusion ähnlich derer von Kolloiden ist, jedoch weist die Stufenkanten-Diffusion eine atom-ähnlichen Schwoebel-Barriere auf. Außerdem haben wir für die erste Monolage ein thermisch-aktiviertes Dewetting nach dem Wachstum von C60 auf Mica mit einer effektiven Aktivierungsbarriere von (0.33 ± 0.14) eV für die Aufwärts-Diffusion beobachtet. Im zweiten Teil der Arbeit untersuchen wir die thermomechanischen Eigenschaften der supra-molekularen Anordnung von dem organischen Halbleiter PTCDI-C8. Temperaturabhängige GIXD-Experimente decken einen außergewöhnlich großen positiven und negativen thermischen Expansionskoeffizienten der Kristallstruktur auf. Die Moleküle vollführen kooperative rotierende Bewegungen als Reaktion auf die Temperaturänderung, die zu dieser anomalen thermischen Expansion führen. Unsere Beschreibung der Bewegungen einzelner adsorbierter Moleküle während des Wachstums und der kooperativen Bewegungen einzelner Moleküle in supra-molekularen Ensembles auf der molekularen Skala wird die weitere Arbeit auf dem Weg zu funktionalen molekularen dünnen Filmen beleben.
Thin films of conjugated molecules are increasingly used in organic optoelectronics, biosensing and surface modification. However, nanoscopic understanding of elementary processes regarding the molecular film growth, the stability of these films and regarding the thermal and mechanical properties of supra-molecular assemblies are in its infancy. In the first part of this thesis we use real-time in situ specular and diffuse X-ray scattering in combination with kinetic Monte Carlo simulations to study C60 nucleation and multilayer growth. We quantify a consistent set of energy parameters, which describe the surface processes during growth, yielding an effective Ehrlich-Schwoebel barrier of EES = 110 meV, a surface diffusion barrier of ED = 540 meV and a binding energy of EB = 130 meV. Analysing the particle-resolved dynamics, we find that the lateral diffusion is similar to colloids, but step-edge crossing is characterized by an atom-like Schwoebel barrier. Furthermore, a thermally-activated post-growth dewetting for C60 on mica has been observed for the first monolayer with an effective activation barrier for upward interlayer transport of (0.33 ± 0.14) eV. In the second part we investigate the thermomechanical properties of the supra-molecular assembly of the organic semiconductor PTCDI-C8. Temperature-dependent Grazing Incidence X-ray Diffraction (GIXD) experiments reveal extraordinary large positive and, surprisingly, negative thermal expansion coefficients of the thin film crystal structure. The molecules perform temperature-controlled cooperative rotational motions leading to the change of the molecular crystal structure at different temperatures. We hope that our molecular scale picture of the movement of single ad-molecules during growth and the cooperative motions of single molecules in supra-molecular ensembles will stimulate further work towards the optimized, rational design of functional molecular thin films and nanomaterials.

As optoelectronic components become nano-dimensional, controlling the coupling between light and matter at the nanoscale has become a major technological challenge, as well as the subject of theoretical studies. The aim of this work is threefold. First, to assess the suitability of ferroelectric thin films - Barium Strontium Titanate (BST), and Strontium Barium Niobate (SBN), as active media for plasmonic devices. Second, to find suitable thin film electrodes for such devices, by exploring and optimizing the plasmonic behaviour of already known conductive materials, conductive oxides (Strontium Ruthenate - SRO), and transitional metal nitrides (Titanium nitride - TiN). Third, to optimize the deposition process of metallic (Silver – Ag) films, so as to improve their smoothness, and thus their suitability for plasmonic applications and lithography in general. SBN ceramic targets were sintered. SBN and BST films were deposited by PLD and ellipsometry and normal incidence reflectometry were used to examine their optical tunability. Ellipsometry was further used to measure the effects of the residual strain of the BST thin films on their optical properties. BST and SBN films were found to exhibit a birefringence under bias along the direction of growth. A residual strain variation along the films’ direction of growth was inferred from an observed non-linear change in the refractive index of BST films along that same direction. SRO and TiN films were fabricated using PLD and reactive magnetron sputtering, respectively. The effects of the deposition pressure upon structure, charge carrier concentration and mobility, and optical properties were studied using X-ray diffraction (XRD), Hall-effect measurements, and ellipsometry. The optical properties of SRO were explained based on electron concentration and structure. SRO was confirmed as a promising plasmonic material, for applications in the near infrared range and at elevated temperatures. The influence of the deposition temperature upon the optical properties of TiN films was shown. Films grown at high temperature (800oC) had quasi-metallic optical properties, while films grown at room temperature exhibited well defined plasmon bandwidth, between two distinct Epsilon-near-zero (ENZ) frequencies, which has been linked to the uniform oxidation of the samples). Finally, Ag thin films were deposited using magnetron sputtering, in an Ar/He atmosphere. The effect of the sputtering gas ratio on the films structure, morphology and reflectivity was studied using XRD, Atomic force microscopy (AFM) and visual-range normal incidence reflectometry. The addition of Helium to the sputtering atmosphere was found to reduce the roughness of Ag films and improve their reflectivity, due to the Penning effect present in the Ar-He plasma. The work undertaken has, by developing new plasmonic materials (SRO, oxidized TiN), and expanding the knowledge of the behaviour (BST) and fabrication (Ag) here has paved the way for the development of active plasmonic devices.

Thesis (Ph. D.)--Ohio State University, 2001.
Advisor: Bharat Bhushan, Dept. of Mechanical Engineering. Includes bibliographical references (leaves 145-152). Available online vai OhioLINK's ETD center.

Thesis (M.S.)--West Virginia University, 2001.
Title from document title page. Document formatted into pages; contains vii, 44 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 35-37).

Thesis (Ph. D.) -- University of Maryland, College Park, 2005.
Thesis research directed by: Material Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Modification of planar graphitic carbon surfaces by the attachment of molecular films has been investigated in this work. Molecular layers have been grafted to glassy carbon (GC) and pyrolyzed photoresist film (PPF) by employing a range of techniques, which involved electrochemically and photochemically assisted procedures. Modification methods involve the electrochemical reduction of aryldiazonium salt, electrochemical oxidation of arylcarboxylate and photolysis of alkene, alkyne and azide on carbon surfaces. For these methods, it is proposed that reactive species are generated by the procedures, which leads to the grafting of modifiers to the carbon surfaces. A selection of molecular species was grafted to GC and PPF by these method containing different terminal R-functional groups that include —COOH, -NO₂, -NH₂, and —NCS. The grafted R-functional groups permit for further chemical reactions on the surface. Electrochemically and photochemically grafted films were examined with a combination of water contact angle measurements, cyclic voltammetry, X-ray electron spectroscopy XPS, optical microscopy, scanning electron microscopy SEM and atomic force microscopy AFM. Film properties such as surface concentration, film thickness, wettability, chemical composition and reactivity were characterized by the above mentioned techniques. Films electrochemically prepared from aryldiazonium salts and arylcarboxylates, under the conditions applied in this work, formed loosely packed multilayers with typical film thicknesses of les than 5 nm. Photochemically grafted films prepared from alkenes and azides, in general, formed loosely packed monolayers with film thicknesses of less than 2 nm. Loosely packed multilayers were also prepared from alkene and alkyne by photochemical procedures. ii Chemical reactions on grafted films were demonstrated and analyzed by a combination of the above mentioned characterization techniques. In particular, the reduction of nitrophenyl (NP)films, amine-coupling reactions, photoactivation of grafted films with oxalyl chloride and electrostatic assembly of anionic gold nanoparticles were investigated. Selected chemical reactions permitted identification and evaluation of the grafted layers, and demonstrated the ability to control the immobilization of chemical species. Microscale chemical patterning of two different types of modifiers on carbon surfaces was demonstrated using photolithographical techniques that utilized photochemical reactions with azides. Patterns of line-arrays with line widths of hundreds of micrometers to 10 µm were formed.

A wide range of polymer film products are available with varying degrees of permeability to a variety of gases, from oxygen to water to aromatic species. The barrier properties are determined primarily by the choice of the matrix polymer, and fine tailoring can be introduced through the addition of filler materials such as inorganic platelets and polymer blend materials. It was identified that hollow and porous addition of such particles appears little studied. Gel particles were prepared by free radical emulsion polymerisation of styrene and DVB in presence of hydrophobic solvents. Porous particles were prepared by the surfactant free emulsion copolymerisation of VBC and DVB, followed by Friedel- Crafts alkylation to yield hypercrosslinked microporous nanoparticles. Porous and gel particles were added to polymer films and the oxygen permeability and mechanical properties of these films determined. It was found that the addition of hypercrosslinked microporous nanoparticles increased the oxygen permeability of the films up to 〜50 % when added at 40 wt% loading. Further to these studies, hollow nanoparticles were prepared by the emulsion phase ATRP of DVB when initiated by mPEG-6-PS inisurf in the presence of a hydrophobic solvent.

L’assemblage couche-par-couche permet de fabriquer des films multimatériaux aux propriétés variées présentant une structure stratifiée. Ce travail décrit les études structurelles des films multicouches de polyélectrolytes à l’aide de mesures de diffusion de neutrons.L’effet du vieillissement des films a été déterminé par réflectométrie des neutrons. Nous avons observé un léger tassement des films après 5 ans et une forte expansion après 15 ans.Nous avons aussi montré que le substrat et l’air en surface ont une influence sur la structure des couches proches des extrémités des films, conduisant à une structure inhomogène perpendiculairement à la surface.Nous avons finalement étudié la conformation des chaînes de polyélectrolytes dans les films multicouches ; nous avons déterminé que les chaînes de PSS dans des films préparés par trempage ont une conformation en pelotes aplaties, contrairement aux chaînes de polyélectrolytes en solution qui présentent une conformation sphérique
The Layer-by-Layer assembly allows the build-up of multimaterial films with various properties showing a stratified structure. This work describes the structural strudies of multilayer films of polyelectrolytes with neutron scattering measurements.Ageing effect on films was determined by neutron reflectometry. We observed a slight shrink of the films after 5 years and a strong expansion after 15 years.We also showed that the proximity of the substrate and the air at the surface have an influence on the structure of the layers at the extremities of the films, leading to an inhomogeneous structure perpendicularly to the surface.Finally, we studied the conformation of polyelectrolyte chains in the multilayer films ; we determined that PSS chains in dipped films have a flattened coil conformation, whereas the polyelectrolyte chains in solution have a spherical conformation

Nanoscale polymer films have numerous potential applications such as protective coatings, flexible electronics, energy harvesting devices, and drug delivery systems. For realization of these potential applications, the mechanical properties of these materials and the underlying physics need to be understood. This dissertation focuses on understanding the responses of nanoscale films to mechanical deformations. In this regard, an elastic instability was exploited to locally bend and impart a local tensile stress in a nanoscale polystyrene film, and directly measure the resulting residual stress caused by the bending. Our results indicate that the onset of permanent deformation for thin polystyrene films is an order of magnitude smaller than what has been reported for the bulk value. In addition, not only is the onset of failure strain found to be small but also it increases with increased confinement. Using similar processing techniques, the yield strain of a more complex material - poly(styrene-b-divinylpyridine) - was studied. Similar to the polystyrene films, failure in polystyrene-b-poly(2-vinylpyridine) is also initiated at extremely low strain and is influenced by thin film confinement effects. In addition, we have demonstrated that internal nanostructure of self-assembled polystyreneb- poly(2-vinylpyridine) affects the onset of failure strain. Having introduced an idealized heterogeneity to a sample through ultraviolet/ozone treatment, we have created samples ranging from continuous thin films to sets of isolated plates. We demonstrated that, when subjected to mechanical deformation, the unbounded plates form isotropic undulations that persist even beyond high strain. In contrast, isolated plates undergo non-isotropic undulations in the range of high strains. The non-isotropic undulation shape has been described through a simple numerical modeling subjected to controlled boundary conditions. The agreement between experiment and numerical modeling is remarkable. Finally, through an integrated experimental methods and theoretical modeling, the response of discrete colloidal layers to mechanical deformations have been exploited. The buckling profiles measured experimentally demonstrate a great insight that the continuum model may not be able to predict the response of discrete systems. Theoretically, a granular model was constructed and structural stability analysis was investigated to predict the experimental observations. The overall agreement of the experiment and the modeling was good.
ND EPSCoR

Thin film growth from the vapor phase has for a long time intrigued researchers endeavouring to unravel and understand atomistic surface processes that govern film formation. Their motivation has not been purely scientific, but also driven by numerous applications where this understanding is paramount to knowledge-based design of novel film materials with tailored properties. Within the above framework, this thesis investigates growth of metal films on weakly bonding substrates, a combination of great relevance for applications concerning e.g., catalysis, graphene metallization and architectural glazing. When metal vapor condenses on weakly bonding substrates three dimensional islands nucleate, grow and coalesce prior to forming a continuous film. The combined effect of these initial growth stages on film formation and morphology evolution is studied using pulsed vapor fluxes for the model system Ag/SiO2. It is shown that the competition between island growth and coalescence completion determines structure evolution. The effect of the initial growth stages on film formation is also examined for the tilted columnar microstructure obtained when vapor arrives at an angle that deviates from the substrate surface normal. This is done using two metals with distinctly different nucleation behaviour, and the findings suggest that the column tilt angle is set by nucleation conditions in conjunction with shadowing of the vapor flux by adjacent islands. Vapor arriving at an angle can in addition result in films that exhibit preferred crystallographic orientations, both out-of-plane and in-plane. Their emergence is commonly described by an evolutionary growth model, which for some materials predict a double in-plane alignment that has not been observed experimentally. Here, an experiment is designed to replicate the model’s growth conditions, confirming the existence of double in-plane alignment. New and added film functionalities can further be unlocked by alloying. Properties are then largely set by chemistry and atomic arrangement, where the latter can be affected by thermodynamics, kinetics and vapor flux modulation. Their combined effect on atomic arrangement is here unravelled by presenting a research methodology that encompasses high resolution vapor flux modulation, nanoscale structure v vi probes and growth simulations. The methodology is deployed to study the immiscible Ag-Cu and miscible Ag-Au model systems, for which it is shown that capping of Cu by Ag atoms via near surface diffusion processes and rough morphology of the Ag-Au growth front are the decisive structure forming processes in each respective system. The results generated in this thesis are of relevance for tuning structure of metal films grown on weakly bonding substrates. They also indicate that improved growth models are required to accurately describe structure evolution and emergence of a preferred in-plane orientation in films where vapor arrives at an angle that deviates from the substrate surface normal. In addition, this thesis presents a methodology that can be used to identify and understand structure forming processes in multicomponent films, which may enable tailoring of atomic arrangement and related properties in technologically relevant material systems.

Дисертаційна робота присвячена визначенню закономірностей формування фазового складу, структури і магнітних властивостей в нанорозмірних плівках Fe50Pt50-Au та багатошарових композиціях [Pt/Fe]n (n = 1, 4, 8) на підкладках SiO2(100 нм)/Si(001) та Al2O3 (1010) при термічних відпалах. Встановлено, що контролюючи рівень механічних напружень та їх знак у шарі Fe50Pt50 зміненням товщини, розташування, кількості додаткових шарів Au, швидкості нагріву та атмосфери при відпалі (вакуум, азот, водень), можна керувати процесами упорядкування та формуванням фазового складу, структури та магнітними властивостями в плівкових композиціях. Застосування водневої термообробки прискорює процеси упорядкування в плівках Fe50Pt50/Au/Fe50Pt50 порівняно з відпалом у вакуумі за рахунок створення додаткових стискаючих напружень при втіленні атомів водню у пустоти кристалічної гратки фази L10-FePt. При цьому вісь легкого намагнічування c у зернах фази L10-FePt розташовується у площині плівки. Швидкий термічний відпал плівкових композицій [Pt/Fe]n (де n=4, 8) на підкладках SiO2(100 нм)/Si(001) в атмосфері азоту призводить до орієнтованого росту зерен фази L10-FePt з віссю легкого намагнічування c, розташованою в напрямку [001], перпендикулярному площині плівки.
The work is devoted to definition of the phase composition formation regularities, structure and magnetic properties in nanoscale Fe50Pt50-Au films and multilayered [Pt/Fe]n (n = 1, 4, 8) compositions on SiO2(100 nm)/Si(001) and Al2O3 (1010) substrates at thermal annealings. It is established that by supervising of mechanical stresses level and their sign in Fe50Pt50 layer by change of a thickness, location, quantity of additional Au layers and annealing conditions (temperature, duration, speed of heating and atmosphere  vacuum, nitrogen, hydrogen) one can operate by ordering processes and phase compound formation, structure and magnetic properties of film compositions The variations in residual stresses/strains level and sign in the FePt layer of as-deposited films influense the change in the ordered L10-FePt phase formation temperature, structure and the coercivity in the film compositions. Increasing the level of compressive stresses in the Fe50Pt50 layer causes a decrease in the ordering temperature and improvement of the magnetic properties. It is established that oriented grain growth with c-axis of easy magnetization in the [001] direction perpendicular to the film plane at annealing in vacuum occurs in films with a smaller thickness of the intermediate Au(7.5 nm) layer due to the higher level of compressive strains in the deposited films. Increasing the thickness of the Au layer to 15 nm and reducing the level of compressive deformations contributes to the growth of FePt grains with the c-axis of easy magnetization in the plane of the film. The same orientation can be achieved by increasing the thickness of the intermediate Au layer to 30 nm. 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Thus the c-axis of easy magnetization in L10-FePt phasephasephase phase grains is located in the film plane. Hydrogen treatment allows to obtain higher values of coercivity (27.3 kOe) in Fe50Pt50/Au/Fe50Pt50 film compositions at a lower annealing temperature of 700 °C than at annealing in vacuum (900 °C), due to the intensive penetration of hydrogen atoms into the film. It was determined that due to the action of the compressive stress during the diffusion of gold along the grain boundaries and the increase in the number of interfaces in films with an intermediate Au(7.5 nm) layer, the ordered L10-FePt phase formation temperature the can be reduced compared to the other Au layer location. In the films with various Au layer location (top, intermediate, under-) separated from the substrate, the same tendency of the A1→ L10 phase transformation temperature changing as in the films on the substrate is remained: the ordering temperature is lower in film with intermediate Au(7.5 nm) layer then in Au/FeAu/FeAu/FeAu/Fe 50 Pt 50 and and and and Fe 50 Pt 50 /Au filmsfilmsfilmsfilmsfilms. In this work it is also shown that the increase in the number of interfaces in [Pt/Fe]n film compositions, where n = 1, 4, 8, while maintaining the total film thickness, promotes the activation in diffusion processes and the formation of the disordered phase A1-FePt in the composition [Pt/Fe]4 and partially ordered regions with tetragonal distortions in the [Pt/Fe]8 composition already during deposition. Rapid thermal annealing of [Pt/Fe]n film compositions (where n = 4, 8) on SiO2(100 nm)/Si(001) substrates in nitrogen atmosphere leads to the oriented growth of L10-FePt phase grains with a c-axis of easy magnetization, located in [001] direction, perpendicular to film plane. The recommendations for controlling the stress state, the reduction of the temperature of the ordered L10-FePt phase formation, the obtaining of c-axis of easy magnetization oriented perpendicular or parallel to the film plane in the film based on FePt, application of which by thermal activated method will allow to increase the magnetic recording density and storage information were developed
Диссертационная работа посвящена определению закономерностей формирования фазового состава, структуры и магнитных свойств в наноразмерных пленках Fe50Pt50-Au и многослойных композициях [Pt/Fe]n (n = 1, 4, 8) на подложках SiO2(100 нм)/Si(001) и Al2O3 при термических отжигах. Установлено, что контролируя уровень механических напряжений и их знак в слое Fe50Pt50 изменением толщины, расположения, количества дополнительных слоев Au, скорости нагрева и атмосферы при отжиге можно управлять процесами упорядочения и формированием фазового состава, структуры и магнитными свойствами в пленочных композициях. Применение водородной термообработки ускоряет процессы упорядочения в пленках Fe50Pt50/Au/Fe50Pt50, по сравнению с отжигом в вакууме, за счет создания дополнительных сжимающих напряжений при внедрении атомов водовода в пустоты кристаллической решетки фазы L10-FePt. При этом ось легкого намагничиваня c в зернах фази L10-FePt располагается в плоскости пленки. Быстрый термический отжиг пленочных композиций [Pt/Fe]n (где n=4, 8) на подложках SiO2(100 нм)/Si(001) в атмосфере азота приводит к ориентированному росту зерен фазы L10-FePt с осью легкого намагничивания c, расположенной в направлении [001], перпендикулярном плоскости пленки.

The behavior of polymeric systems confined into thin films is a situation that has numerous practical consequences. One particular application in which the properties of thin polymer films is becoming crucially important is in the design, formulation, and processing of photoresists for semiconductor microlithography. As devices continue to be scaled down into the nano-regime, the microelectronics industry will ultimately rely upon a molecular understanding of materials for process development. The majority of these devices are now confined in planar geometries; thus, thin films have played an ever-increasing role in manufacturing of modern electronic devices. This movement towards thinner resist films creates larger surface to volume ratios, and hence thin films can exhibit thermodynamic, structural, and dynamic properties that are different from those of the bulk material. It is thus extremely important to understand the properties of polymers when confined in such geometries for various applications including resists for lithographic patterning. In present work, the influence of a variety of factors including film thickness, molecular weight, and substrate interactions on the polymer thin film physical properties such as the glass transition temperature, coefficient of thermal expansion, dissolution rate, and diffusion coefficient was studied in detail using a combination of experimental characterization and molecular modeling simulation techniques.

In the framework of the two-band (Dirac) model, we analyze the electronic structure of nanoscale systems, based on narrow-gap semiconductors of Pb,_xSnx (Se, S) type. Themain attention is paid to the influence of properties of the surface, encoded in appropriate boundary conditions, on the size-quantized spectrum. From this point of view we consider two types of systems: spherical (quantum dots) and quasi one-dimensional (films).It is shown that the spectrum of the spherical quantum dot consists not only of usual size-quantized states, located above the gap edge, but also surface modes residing inside the gap. Such states manifest themselves in the far infrared part of the absorption spectrum, the measurement of which allows one to extract information about the dot surface.Next, we consider a film with the energy gap modulated in the <111> (growth) direction. It is shown that the spectrum of the infinite crystal possesses a supersymmetrical structure. The film boundaries, generally speaking, destroy the supersymmetry, i.e. size-quantized subbands turn out to be spin-split. However, there exists a class of boundary conditions that do not lift spin degeneracy. Physically, in this case there is no band mismatch at interfaces. Our central statement, therefore, consists of the following: even when the inversion symmetry is destroyed by the bulk inhomogeneity, the spin-splitting of the spectrum is a purely surface effect. This is illustrated on a simple example, when the energy gap varies linearly over the film width.Finally, we investigate the role of boundary conditions in the problem of scattering of spinor waves by a quantum dot. It is shown that the existence of surface states greatly modifies the scattering data; in particular, outgoing waves may turn out to be fully polarized.
Department of Physics and Astronomy

This thesis presents surface science studies, investigating several aspects of titanium dioxide at the atomic scale. The greater part of this work is devoted to the preparation by chemical vapor deposition (CVD) of titanium(IV) tetraisopropoxide (TTIP) of ultrathin TiO2 or TiOx films on Au(111). Four ordered structures were growth and characterized. It was also demonstrated how the morphology of the film (wetting film vs island) can be tailored. The acquired knowledge about the CVD process was exploited to load nano porous gold with titania, enhancing its catalytic activity. The reactivity towards water adsorption of the titania structures on Au(111) was also investigated. Finally, part of this work concerned the studying of the behavior of water on the stoichiometric rutile TiO2(110) surface, combining the experiments with density-functional theory (DFT) calculations and (kinetic) Monte Carlo simulations. The main experimental techniques used in this work are low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES).

Ionically self-assembled monolayer (ISAM) films are typically an assemblage of oppositely charged polymers built layer by layer through Coulombic attraction utilizing an environmentally friendly process to form ordered structures that are uniform, molecularly smooth and physically robust. ISAM films have been shown to be capable of the noncentrosymmetric order requisite for a second-order nonlinear optical response with excellent temporal and thermal stability. However, such films fabricated with a nonlinear optical (NLO) polyanion result in significant cancellation of the chromophore orientations. This cancellation occurs by two mechanisms: competitive orientation due to the ionic bonding of the polymer chromophore with the subsequent polycation layer, and random orientation of the chromophores within the bulk of each polyanion layer. A reduction in film thickness accompanied by an increase in net polar ordering is one possible avenue to obtain the second-order susceptibility chi(2) necessary for practical application in electro-optic devices. In this thesis, we discuss the structural characteristics of ISAM films and explore a novel approach to obtain the desired characteristics for nonlinear optical response. This approach involves a hybrid covalent / ionic self-assembly technique which affords improved net dipole alignment and concentration of monomer chromophores in the film. This technique yields a substantial increase in chi(2) due to the preferential chromophore orientation being locked in place by a covalent bond to the preceding polycation layer. The films fabricated in this manner yield a chi(2) that substantially exceeds that of any known polymer-polymer ISAM film. This covalent-hybrid ionically self-assembled multilayer (CHISAM) technique is demonstrated to result in films suitable for electro-optic devices, with measured electro-optic coefficient (14 pm/V) comparable to that of the inorganic crystal lithium niobate (30 pm/V). Thermal and temporal stability are important properties of electro-optic device implementation, and are demonstrated for CHISAM films. CHISAM films have remained stable at room temperature for more than 420 days, and suffered no loss of chi(2) when held at 80 C for 36 hours, followed by 150 C for 24 hours. Studies are also presented that demonstrate the ability to produce ISAM chi(2) films that are nearly one micron thick, and exhibit no evidence of a thickness limitation to the polar order. Analytical considerations for second-order NLO characterization of thick films are addressed in detail. The effect of absorption of the second harmonic wavelength and resonant enhancement of chi(2) are investigated, and it is demonstrated that accurate determination of chi(2) may be made for thick films and for films that absorb the second harmonic. The temporal and thermal stability of a variety of ISAM and CHISAM NLO films are examined in detail. In some cases, a decrease in the NLO response is observed at elevated temperature that is completely restored upon cooling. Studies are presented that suggest this effect is a result of thermally induced trans-to-cis isomerization of azo linkages in the NLO chromophores.
Ph. D.

La ferroelectricitat es una propiedat functional que trova aplicacions en dispositius microelectronics com les memories no volatils. En aquesta tesi se han estudit les propiedats ferroelectriques de capes primes nanométriques y supexarxes combinant técniques de caracterizació macroscópiques y nanoscópiques. En primer lloc, se ha estudiat la configuració de dominis ferroelectrics de BaTiO3 en supexarxes de diferents periods (gruix de les capes) utilitzant microscopia electronica en transmissió (STEM). Se ha observat una fort dependencia amb el periodi, con una distorsió polar decreixent per a periodis mayors, d'acord amb la polarizació macroscópica mesurada. Se han observat rotacions de la polarizació en las capes de BaTiO3 de la supexarxa de periodi mes grand (amb capes de BaTiO3 de 10 celles unitat de gruix), contrastant amb els dominis amb nomes una orientació (fora del pla) en les supexarxes de periodi mes petit. En una segona part se han estudiat capts fints de Hf0.5Zr0.5O2 crescudes epitaxialment sobre substrats de perovskita.
La ferroelectricidad es una propiedad funcional que encuentra aplicaciones en dispositivos microelectrónicos tales como las memorias no volátiles. En esta tesis se han estudiado las propiedades ferroeléctricas de capas finas nanométricas y superredes combinando técnicas de caracterización macroscópicas y nanoscópicas. En el primer bloque, se ha estudiado la configuración de dominios ferroeléctricos de BaTiO3 en una serie de superredes de diferentes periodos (grosor de las capas) mediante microscopía electrónica de barrido en transmisión (STEM). Se ha observado una fuerte dependencia con el periodo, con una distorsión polar decreciente para periodos mayores, de acuerdo con la polarización macroscópica medida. Se han observado rotaciones de la polarización en las capas de BaTiO3 de la superred de mayor periodo (con capas de BaTiO3 de 10 celdas unidad de grosor), contrastando con los dominios con una única orientación (fuera del plano) en las superredes de menor periodo. En un segundo bloque se han estudiado capas finas de Hf0.5Zr0.5O2 crecidas epitaxialmente sobre substratos de perovskita. Las capas están generalmente compuestas de una mezcla de granos cristalinos de estructuras ortorrómbica (ferroeléctrica) y monoclínica (no polar), obteniendo más o menos de una fase u otra en función de distintas condiciones. También se ha estudiado el rol del electrodo inferior en la estabilización de las fases y la ferroelectricidad de las capas. Para estabilizar la fase ortorrómbica, se ha de depositar previamente una capa de manganita (i.e. La0.7Sr0.3MnO3) sobre los sustratos; en caso contrario, la cantidad de fase ortorrómbica obtenida, así como la ferroelectricidad de las capas es muy baja. Por otro lado, el efecto del stress epitaxial también es evaluado depositando capas de Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 sobre una serie de substratos de distinto parámetro de red, obteniendo capas puras de fase ortorrómbica con mayor ferroelectricidad en aquellas capas depositadas sobre substratos que incrementan el parámetro de red en el plano del La0.7Sr0.3MnO3. Por otra parte, se ha explorado y descrito la epitaxia de Hf0.5Zr0.5O2 sobre La0.7Sr0.3MnO3 y la intercara de Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 con resolución átomica mediante STEM.
Ferroelectricity is a functional property that can be exploited in microelectronics devices such as non-volatile memories. In this thesis, the ferroelectric properties of nanometric thin films and superlattices have been studied by combining macroscopic and nanoscale characterization techniques. Firstly, the configuration of ferroelectric domains of BaTiO3 has been studied in a set of BaTiO3/SrTiO3 superlattices of different periods by Scanning Transmission Electron Microscopy (STEM). A strong dependence of the superlattice period (layers’ thickness) has been observed, with a decreasing polar distortion for longer periods, in agreement with the macroscopic polarization. Polarization rotations have been observed inside the BaTiO3 layers of the superlattice with longest period (10 unit-cell thick BaTiO3 layers), contrasting with the single oriented (out-of-plane oriented) domains in superlattices of shorter period. Secondly, Hf0.5Zr0.5O2 thin films grown epitaxially on perovskite substrates have been studied. The films generally show a mixture of orthorhombic (ferroelectric) and monoclinic (non-polar) epitaxial crystallites, with more of one or the other phase depending on different conditions. The role of the bottom electrode on the stabilization of phases and the ferroelectricity of the films has also been studied. In order to stabilize the orthorhombic phase, the substrates require to be buffered with a manganite (i.e. La0.7Sr0.3MnO3); otherwise, the amount of the orthorhombic phase is very small, and so is the ferroelectricity of the films. Besides, the effect of the epitaxial stress has been explored by growing Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 films on a set of substrates with different lattice parameters, achieving pure orthorhombic films with enhanced ferroelectricity on films grown on those substrates that impose a tensile strain to the La0.7Sr0.3MnO3 electrode. Furthermore, the epitaxy of Hf0.5Zr0.5O2 on La0.7Sr0.3MnO3 as well as the Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 interface have been explored and described with atomic resolution by means of STEM.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

Thin films of organic semiconducting materials, such as copper phthalocyanine (CuPc) and C60, can be used in photovoltaic devices. The interface between these materials is the site of exciton dissociation, and thus a key region of interest in their study. The processes that occur within these films and at interfaces are governed by the local morphology and structure. Studying these films and interfaces at high spatial resolution has previously been challenging given their soft nature and scale. Using electron transparent cross-sections prepared with a focussed ion beam (FIB), high resolution transmission electron microscopy (HRTEM) has been used to probe the local crystallography of three archetypical organic photovoltaic device structures grown on silicon and indium tin oxide (ITO). In HRTEM images lattice fringes of unprecedented clarity are observed, validating the optimised FIB method. HRTEM examination of device structure cross-sections on silicon reveals lattice fringes throughout pure films of CuPc and C60. The structure of the CuPc thin film can be correlated with bulk characterisation methods however, the observation of stacking faults demonstrates film non-uniformity. Lattice fringes in C60 films show an orientation preference with respect to the interface, which allows conclusions to be made about C60 when grown on molecular films. Mixed films show no lattice fringes. Structures grown on ITO are more complex than those on silicon, which is attributed the relatively rougher growth surface. Due to this rougher surface, the morphological changes occurring result in reduced crystallinity, a conclusion supported by bulk characterisation methods. The cross-sectional methodology has been extended to thicker films, revealing the presence of structural deviations that lie parallel to the surface. Scanning transmission electron microscopy, in combination with energy dispersive X-ray spectroscopy, high resolution quantitative compositional mapping reveals the morphology of the interface for the structures studied. This been correlated with the morphology of single CuPc film surfaces, with the conclusion that morphology of the CuPc surface remains unchanged after C60 film growth.

Thesis (M.S. in Physics)--Naval Postgraduate School, June 2010.
Thesis Advisor(s): Karunasiri, Gamani ; Grbovic, Dragoslav. "June 2010." Description based on title screen as viewed on July 15, 2010. Author(s) subject terms: Bi-material Pixel, Terahertz Imaging, Deformation, Multi-layer stack, Terahertz Absorption, COMSOL finite element modeling, Fresnel's Equations, Dielectric Bragg reflector, FTIR Spectroscopy, maximizing the THz absorption. Includes bibliographical references (p. 51-52). Also available in print.

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xviii, 190 p.; also includes graphics. Includes bibliographical references (p. 181-190).

Thesis (Ph. D.)--University of California, San Diego, 2008.
Title from first page of PDF file (viewed September 4, 2008). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 77-82).

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.

La morphologie de films minces métalliques polycristallins élaborés par condensation d’une phase vapeur sur des substrats à faible interaction (SFI) possède un caractère 3D intrinsèque. De plus, la nature hors équilibre de la croissance du film depuis une phase vapeur conduit souvent à la génération de contraintes mécaniques, ce qui peut compromettre davantage la fiabilité et la fonctionnalité des dispositifs optoélectroniques. Les objectifs de cette thèse sont liés à la croissance de films métalliques sur SFI et visent à : (i) contribuer à une meilleure compréhension des processus à l'échelle atomique qui contrôlent l'évolution morphologique des films ; (ii) élucider les processus dynamiques qui régissent la génération et l'évolution des contraintes en cours de croissance ; et (iii) développer des méthodologies pour manipuler et contrôler la morphologie des films à l'échelle nanométrique. L’originalité de l’approche mise en œuvre consiste à suivre la croissance des films in situ et en temps réel par couplage de plusieurs diagnostics, complété par des analyses microstructurales ex situ. Les grandeurs mesurées sont confrontées à des modèles optiques et des simulations atomistiques.L’ensemble des résultats obtenus dans cette thèse fournissent les bases pour : (i) déterminer les coefficients de diffusion sur une large gamme de systèmes films/SFI; (ii) concevoir des stratégies non invasives pour les contacts multifonctionnels dans les dispositifs optoélectroniques;(iii) apporter des éléments de compréhension à l’origine du développement de contrainte, qui permettent de prédire et contrôler le niveau de contrainte intrinsèque à la croissance de films minces polycristallins
Vapor-based growth of thin metal films with controlled morphology on weakly-interacting substrates (WIS), including oxides and van der Waals materials, is essential for the fabrication of multifunctional metal contacts in a wide array of optoelectronic devices. Achieving this entails a great challenge, since weak film/substrate interactions yield a pronounced and uncontrolled 3D morphology. Moreover, the far-from-equilibrium nature of vapor-based film growth often leads to generation of mechanical stress, which may further compromise device reliability and functionality. The objectives of this thesis are related to metal film growth on WIS and seek to : (i) contribute to the understanding of atomic-scale processes that control film morphological evolution; (ii) elucidate the dynamic competition between nanoscale processes that govern film stress generation and evolution; and (iii) develop methodologies for manipulating and controlling nanoscale film morphology between 2D and 3D.Investigations focus on magnetron sputter-deposited Ag and Cu films on SiO2and amorphous carbon (a-C) substrates. Research is conducted by strategically combining of in situand real-time film growth monitoring, ex situchemical and (micro)-structural analysis, optical modelling, and deterministic growth simulations.The overall results of the thesis provide the foundation to: (i) determine diffusion rates over a wide range of WIS film/substrates systems; (ii) design non-invasive strategies for multifunctional contacts in optoelectronic devices; (iii) complete important missing pieces in the fundamental understanding of stress, which can be used to expand theoretical descriptions for predicting and tuning stress magnitude

This dissertation demonstrated that the manipulation of substances at the molecular or nanometer level can lead to the discovery and development of new materials with interesting properties and important applications. Chapter 1 describes the development of a nanoscale molecular thin film material for corrosion protection. By using a self-assembled monolayer film with a thickness of only about 1 nanometer as a linkage, a covalent bonding was achieved between a polyurethane top coating and an aluminum alloy substrate. This covalent bonding between polymer top coating and the aluminum alloy substrate significantly improved the corrosion resistance of the substrate. Chapter 2 and Chapter 3 describe the development of a gold nanoparticle-polymer composite material in different forms with a number of applications. Gold nanoparticles are among one of the most extensively studied nanomaterials. When the size of gold is shrunk to the nanometer scale, many interesting and new physical properties start to appear from gold nanoparticles. The optical properties of gold nanoparticles, particularly the surface plasmon resonance absorption, have been investigated in this dissertation for the development of multifunctional nanocomposite materials. Chapter 2 presents the preparation of a gold nanoparticle/poly(methyl methacrylate) (PMMA) nanocomposite film and the application of such films for microstructure fabrication using a direct laser writing technique. Gold nanoparticles are excellent photon-thermal energy converters due to their large absorption cross section at the surface plasmon resonance region. Upon laser irradiation of the nanocomposite film, the thermal energy converted from the absorbed photon energy by gold nanopaticles induced a complete decomposition of PMMA, leading to the formation of various microstructures on the nanocomposite films. Chapter 3 reports the further development of a nanoparticle/polymer composite nanofiber material fabricated through an electrospinning process. The matrix of the nanofiber is made of two polyelectrolytes, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Three methods were developed to incorporate gold nanoparticles into the polymer matrix. The composite nanofiber materials developed in this study demonstrate multifunctional properties, including good electrical conductivity, photothermal response, and surface-enhanced IR absorption. This material may be used for many important applications including catalysis, chemical and biological sensors, and scaffold materials for tissue engineering. In Chapter 4, another most important nanomaterial, carbon naotubes (CNTs), were introduced as fillers to prepare polymer nanocomposites. A dispersion method for multi-walled carbon nanotubes (MWCNTs) using a conjugated conducting polymer, poly(3-hexylthiophene) (P3HT) as the third component and trifluoroacetic acid (TFA) as a co-solvent was developed. Due to the excellent dispersion of carbon nanotubes in PMMA and enhanced conductivity of the nanocomposites by the conjugated conducting polymers, the prepared composite materials has an extremely low percolation threshold of less than 0.006 wt% of MWCNT content. The potential use of MWCNT/conducting polymer composites for energy storage applications such as suppercapacitors was further investigated by Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and charging-discharging cycles. Compared to pure carbon nanotubes, the nanocomposite materials have significantly improved properties and are promising for supercapacitor applications.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Materials Science & Engr PhD

Existing as much more than just a gemstone or refractory material, boron-doped diamond is a semiconductor with enormous potential as an electronic material. Diamond will form the basis of future high-power electronic devices, radiation-hard electronics and radiation detectors. It is also a highly effective electron emitter and a unique, biocompatible material for biomedical devices. Still, many questions remain surrounding the versatile, easy to grow polycrystalline form of boron-doped diamond. What is the surface atomic structure of these films after growth? How uniform is the boron-induced conductivity? How and why does the work function vary across the films? All of these properties affect how diamond electronic devices can be designed, fabricated and used. In this thesis we investigate nanoscale variation in properties across the surfaces of a number of differently grown boron-doped diamond films under ultra-high vacuum and evaluate the potential impact of the changes in these properties to surface electronic applications of diamond. Kelvin probe force microscopy results demonstrate significant variation in work function across the diamond surface, with a step change in work function of 0.8 eV measured between hydrogen and oxygen surface terminations, and variations across a single diamond surface are calculated to correspond to a 310% change in dopant concentration . The effect of this dopant variation is demonstrated by conductive atomic force microscopy studies in which entire crystallites exhibit insulating behaviour, with significant variation in conduction also observed across the surface. Finally, scanning tunneling microscopy studies of the diamond surface demonstrate that nanoscale roughness on large microcrystals is caused by the existence of a layer of nanocrystalline diamond at the surface. These nanocrystals persist throughout the growth process and exhibit many different surface reconstructions. The implications of all of these discoveries are discussed, with possibilities and suggestions for further work given also.

The first thermal cycling experiments of ionic self-assembled multilayer (ISAM) films have been reported examining their survivability through repeated thermal cycles from -20° C to 120° C in ambient atmospheric conditions. The films were constructed from alternating layers of Nile Blue A and gold nanoparticles which provided a strong absorbance in the optical wavelength range. No degradation of the optical characteristics of the ISAM films was observed [1]. Techniques for measuring the capacitance and resistivity of various ISAM films have also been developed allowing for a more complete electrical characterization of ISAM films. Capacitance measurements enabled a calculation of the dielectric function and breakdown field strength of the ISAM films. The capacitance measurement technique was verified by measuring the dielectric function of a spin-coated thin film PMMA, which has a well characterized dielectric function [2]. Surface-enhanced Raman spectroscopy (SERS) has been studied as a possible detection method for malignant melanoma revealing spectral differences in blood sera from healthy horses and horses with malignant melanoma. A SERS microscope system was constructed with the capability of resolving the Raman signal from biologically important molecules such as beta-carotene and blood sera. The resulting Raman signals from sera collected from horses with malignant melanoma were found to have additional peaks not found in the Raman signals obtained from sera collected from healthy horses. A systematic analysis of the combination of absorbance and fluorescence signals of blood sera collected from populations of healthy dogs and dogs with cancer has resulted in a rapid and cost-effective method for monitoring protein concentrations that could possibly be used as part of a cancer screening process. This method was developed using the absorbance and fluorescence signals from known serum proteins, the combinations of which were used to match the absorbance and fluorescence signals of blood sera allowing for an accurate determination of protein concentrations in blood sera [3]. Finally, a novel method for measuring the melting point of DNA in solution using capacitance measurements is presented. This method allows for the determination of the melting temperature as well as the melting entropy and melting enthalpy of DNA strands. Two different short strands of DNA, 5'-CAAAATAGACGCTTACGCAACGAAAAC-3' along with its complement and 5'-GGAAGAGACGGAGGA-3' along with its complement were used to validate the technique as the characteristics of these strands could be modeled using theoretical methods. This experimental technique allows for the precise determination of the melting characteristics of DNA strands and can be used to evaluate the usefulness of theoretical models in calculating the melting point for particular strands of DNA. Additionally, a micro-fluidic device has been proposed that will allow for a rapid and cost-effective determination of the melting characteristics of DNA [4].
Ph. D.

Structure and dynamics of nanoconfined glass-forming oligomers and diblock coplymers (BPCs) are investigated by a combination of infrared transition moment orientational analysis (IR-TMOA), positron annihilation lifetime spectroscopy (PALS), grazing incidence small angle X-ray scattering (GISAXS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and broadband dielectric spectroscopy (BDS). The oligomers probed are the van der Waals type, tris(2-ethyhexyl)phosphate (TEHP) and the self-associating molecules of 2-ethyl-1-hexanol (2E1H). Symmetric and asymmetric poly(styrene-b-1,4-isoprene) P(S-b-I) are studied for the case of BCPs. The samples are confined either in one-dimensional (1D) in form of thin films or in 2D (nanopores) geometrical constraints. The molecular order of TEHP in nanopores as studied by IR-TMOA shows that about 7% of the molecules are preferentially oriented perpendicular to the long axis of the pores due to their interaction with the pore walls. PALS results reveal that 2E1H confined in nanopores exhibit larger free volume with respect to the bulk. In thin films (1D), P(S-b-I) having volume fraction of isoprene blocks f(PI)= 0.55 exhibits randomly oriented lamellae and their thicknesses are directly proportional to the film thickness d(film). For f(PI) = 0.73, perpendicular cylinders with respect to the substrate are observed for d(film)>50 nm but they lie along the substrate plane when d(film) < 50 nm. In AAO pores (2D) with average pore diameter d(pore) of 150 nm, straight nanorods are formed which change to helical structures in 18 nm pores. Molecular dynamics of 2E1H and TEHP constrained in nanopores (2D), is influenced by the interplay between confinement and surface effects. Confinement effects show up as an increase in the structural relaxation rate with decreasing pore sizes at the vicinity of the glass transition temperature. This is attributed to the reduced packing density of the molecules in pores as quantified by PALS results for 2E1H. Whereas the orientation and morphologies of the domains in P(S-b-I) and the chain dynamics of isoprene chains are influenced by the finite--size and dimensionality of confinement, the segmental motion, related to the dynamic glass transition (DGT) of both styrene and isoprene blocks remains unaffected-in its relaxation time-within experimental accuracy. Effects of nanoscale confinement on the molecular dynamics therefore depend on a number of factors: the type of molecules (polymers, low molecular liquids), interfacial interactions and the dimensionality of the constraining geometries.

Organic and inorganic-metal oxide thin film transistors can be solution-processed, providing large area, low cost and low processing temperature leading to strong industrial and research interest. The nanoscale properties of each material system have been investigated using scanning probe techniques. Pentacene is a model small molecule organic semiconductor but it is an air sensitive material and insoluble. However, its derivative 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) is amenable to solution processing. The motivation to study TIPSPEN is not to chase performance figures only but to investigate how the solution deposited film topography can be controlled by varying the solvent composition and the insulating polymer binder concentration. We report the effect of anisole / decane binary solvent mixture and the subsequent addition of low percentage weight of (atactic amorphous polystyrene) aPS on the surface morphology of TIPS-PEN thin films. It was found that addition of up to 20 wt% decane has little impact on micro-scale crystal morphology but has a significant influence on the growth mode, mean grain size and terrace roughness. The effect of the TIPS-PEN / aPS blend ratio up to 20 wt% of aPS in drop-cast thin film was similarly investigated on untreated SiO2 and on poly(4-vinylphenol) ( PVP) interlayers. It is found that addition of aPS has a vital effect on macroscopic crystal properties such as surface coverage, unity of orientation, long range order, and average field effect mobility. It also changes the surface morphology and layer ordering on the nano-scale. Mobility values obtained from PVP treated surface were higher than those from SiO2 surface by five times, which was consistent with preferred TIPS-PEN crystal growth on hydrophobic surface PVP of low surface energy. Indium oxide (In2O3) is an n-type (electron transport), optical transparent with wide bandgap values between 3.5 and 4 eV, and high performance semiconductor. Spin coating was used to prepare indium oxide thin film transistors (TFT) of two different thicknesses of the device channel. As the thickness of the In2O3 active layer is increased, the device mobility increases, the threshold voltage is shifted in the negative direction, off-drain current is increased, and on-drain current is less pronounced. A systematic study was carried out to investigate the transistor stability under bias stress. Applying a positive gate bias stress to indium oxide transparent TFTs was found to induce a parallel shifts of threshold voltage in positive direction without changing the device mobility or the subthreshold gate voltage swing. Scanning Kelvin Probe Microscopy (SKPM) was used to study the surface potential distribution of operating devices under a range of drain and gate biases. The potential profiles showed evidence of metal contact diffusion into the channel region which appeared as a flatting of the profile close to the drain electrode. The experimental data was confirmed and complemented with simulation results for contact material diffusion into the In2O3 channel. A new method of performing Electrostatic Force Microscopy (EFM) was developed and applied to the In2O3 TFT. The tip bias was modulated and the resulting phase difference between the cantilever response and the applied bias was recorded. The measurements were affected by a time delay, likely to be caused by capacitive charging of the semiconductor beneath the tip. The effects of the time delay were corrected by the use of an inverse filter in the analysis software developed. This new version of the EFM-phase method was applied to the operating biased In2O3 TFTs and the results compared with those obtained with SKPM.

Thin films and nanowires of Co-Pt have been prepared by means of electrodeposition. Composition, structure, microstructure and magnetic properties have been intensively studied using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry and correlated to the deposition parameters such as electrolyte composition, deposition current and/or potential. Co rich Co-Pt films have been deposited at various current densities. A nearly constant composition of Co70Pt30 was achieved for current densities between 18 and 32 mA/cm². Detailed texture measurements confirmed an increasing fraction of the hexagonal phase with its c-axis aligned perpendicular to the film plane with increasing current density. Accordingly, magnetic properties are strongly affected by the magnetocrystalline anisotropy of the hexagonal phase that competes with the shape anisotropy of the thin film geometry. Co-Pt nanowires have been prepared within alumina templates at different deposition potentials between -0.6 and -0.9VSCE changing the composition from nearly pure Pt to Co. The composition Co80Pt20 was observed at a deposition potential of -0.7VSCE. Co-Pt nanowires are nanocrystalline in the as-deposited state. Magnetic measurements reveal changing fcc and hcp phase fractions within the wires as the effective anisotropy significantly differs from the expected shape anisotropy for nanowires with high aspect ratio. This change in effective anisotropy is attributed to the preferential alignment of the c-axis of hcp Co-Pt phase perpendicular to the nanowires axis. A promising alternative with much smaller feature sizes is the diblock copolymer template. Electrodeposition of Co and Co-Pt into these templates has been carried out. Inhomogeneities in the template thickness as well as a certain substrate roughness have been identified to be the reasons for inhomogeneous template filling. Thus magnetic properties are dominated by large deposits found on top of the template. Additionally, rolled-up tubes of several nm thick Au/Co/Au films have been characterized magnetically. Temperature dependent measurements show an exchange bias behaviour that is explained in terms of induced stresses during cooling. Changes of magnetic properties in the investigated samples are finally discussed in terms of competing effects of different magnetic anisotropies in various geometries
Co-Pt Dünnschichten und Nanodrähte wurden mittels elektrochemischer Abscheidung hergestellt. Zusammensetzung, Struktur, Mikrostruktur und magnetische Eigenschaften wurden intensiv mit Röntgenbeugung, Rasterelektronenmikroskopie und Magnetometrie untersucht und mit den Depositionsparametern wie Elektrolytzusammensetzung, Abscheidestrom und/oder-potential korreliert. Co reiche Co-Pt-Filme wurden mit verschiedenen Stromdichten hergestellt. Eine nahezu konstante Zusammensetzung im Bereich Co70Pt30 wurde für Stromdichten zwischen 18 und 32 mA/cm² erreicht. Detaillierte Texturmessungen bestätigen einen zunehmenden Anteil an hexagonaler Phase mit senkrecht zur Filmebene ausgerichteter c-Achse mit zunehmender Stromdichte. Dementsprechend werden die magnetischen Eigenschaften stark von der magnetokristallinen Anisotropie der hexagonalen Phase beeinflusst, die mit der Formanisotropie der Dünnschicht-Geometrie konkurriert. Co-Pt-Nanodrähte wurden in nanoporöse Aluminiumoxidmembranen bei verschiedenen Potentialen zwischen -0,6 und -0.9 VSCE abgeschieden, wobei sich die Zusammensetzung von nahezu reinem Pt zu Co verändert. Die Zusammensetzung Co80Pt20 wurde bei einem Abscheidepotential von -0.7 VSCE erhalten. Die so hergestellten Co-Pt Nanodrähte sind nanokistallin. Magnetische Messungen weisen jedoch auf veränderte Phasenanteile der fcc und hcp Phase innerhalb der Drähte hin, da die effektive Anisotropie erheblich von der für Nanodrähte mit hohem Aspektverhältnis erwarteten Formanisotropie abweicht. Diese Änderung der effektiven Anisotropie ist auf die bevorzugte Ausrichtung der hexagonalen c-Achse des Co-Pt senkrecht zur Drahtachse zurückzuführen. Vielversprechende Template mit deutlich kleineren Dimensionen sind Diblockcopolymertemplate. Es wurden Versuche zur Abscheidung von Co und Co-Pt in diese Template durchgeführt. Als Gründe für die inhomogene Templatfüllung wurden Inhomogenitäten in der Schichtdicke sowie eine gewisse Rauhigkeit der Substrate identifiziert. Aufgrund der ungleichmäßigen Fülleg werden die magnetischen Eigenschaften durch große, halbkugelförmige Abscheidunge auf der Oberfläche des Templates bestimmt. Darüber hinaus wurden aus wenige nm dicken Au/Co/Au Filmen hergestellte Mikroröhren magnetisch charakterisiert. Temperaturabhängige Messungen zeigen ein Exchange Bias Verhalten, das durch beim Abkühlen induzierte Spannungen erklärt wird. Unterschiede im magnetischen Verhalten der untersuchten Proben werden abschließend im Hinblick auf die verschiedenen konkurrierenden magnetischen Anisotropien in verschiedenen Geometrien diskutiert

Cerium oxide (ceria)-based materials in the nanoscale regime are of significant fundamental and technological interest. Nanoceria in pure and doped forms has current and potential use in solid oxide fuel cells, catalysis, UV- screening, chemical mechanical planarization, oxygen sensors, and bio-medical applications. The characteristic feature of Ce to switch between the +3 and + 4 oxidation states renders oxygen buffering capability to ceria. The ease of this transformation was expected to be enhanced in the nanoceria. In most the practical scenarios, it is necessary to have a stable suspension of ceria nanoparticles (CNPs) over longer periods of time. However, the existing literature is confined to short term studies pertaining to synthesis and property evaluation. Having understood the need for a comprehensive understanding of the CNP suspensions, this dissertation is primarily aimed at understanding the behavior of CNPs in various chemical and physical environments. We have synthesized CNPs in the absence of any surfactants at room temperature and studied the aging characteristics. After gaining some understanding about the behavior of this functional oxide, the synthesis environment and aging temperature were varied, and their affects were carefully analyzed using various materials analysis techniques such as high resolution transmission electron microscopy (HRTEM), UV-Visible spectroscopy (UV-Vis), and X-ray photoelectron spectroscopy (XPS). When the CNPs were aged at room temperature in as-synthesized condition, they were observed to spontaneously assemble and evolve as fractal superoctahedral structures. The reasons for this unique polycrystalline morphology were attributed to the symmetry driven assembly of the individual truncated octahedral and octahedral seed of the ceria. HRTEM and Fast Fourier Transform (FFT) analyses were used to explain the agglomeration behavior and evolution of the octahedral morphology. Some of the observations were supported by molecular dynamic simulations. Poly (ethylene glycol) (PEG) and ethylene glycol (EG) were used to control the kinetics of this morphology evolution. The ability to control the agglomeration of CNPs in these media stems from the lower dielectric constant and an increased viscosity of the EG and PEG based solvents. CNPs when synthesized and aged in frozen conditions, i.e. in ice, were found to form one dimensional, high aspect ratio structures. A careful analysis has provided some evidence that the CNPs use the porous channels in ice as a template and undergo oriented attachment to form nanorods. When the aging treatment was done near freezing temperature in solution, the nanorods were not observed, confirming the role of channels in ice. When synthesized in aqueous media such as DI water, PEG and EG; CNPs were observed to exhibit a reversible oxidation state switching between +3 and +4. Band gap values were computed from the optical absorption data. The changes in the band gap values observed were attributed to the changes in the oxidation state of CNPs as opposed to the quantum confinement effects, as expected in other nanoparticle systems. The work presented in this dissertation demonstrates, with evidence, that in order to obtain a comprehensive understanding of the properties of nanoscale materials it is of paramount importance to monitor their behavior over relatively longer periods of time under various ambient environments. While the solution based techniques offer a versatility and low cost route to study the fundamental properties of nanomaterials, they suffer some inherent problems such as precursor contamination and uncontrolled chemical reactions. Especially when analyzing the behavior of ceria-based materials for applications like solid oxide fuel cells, a great control in the density and crystalline quality are desired. In order to achieve this, as a first step pure ceria thin films were synthesized using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The ceria films were analyzed using various in situ and ex situ techniques to study the crystal structure, growth mode and epitaxial quality of the films. It was observed that the epitaxial orientation of the ceria films could be tuned by varying the deposition rate. When the films were grown at low deposition rate (< 8 Å/min) ceria films with epitaxial (200) orientation were observed where as the films grown at high deposition rates (up to 30 Å/min) showed (111) orientation. Theoretical simulations were used to confirm some of the experimental facts observed in both nanoparticles and thin films.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science & Engr PhD