Dissertations / Theses on the topic 'Nanoscale properties'

As the fast development of nanotechnology and further industrial applications, theoretical investigations upon nanoscale devices are in urgent need. Until now several formalisms have been well established in quantum transport of mesoscopic areas, including of tight-binding and first principle calculations. In this dissertation those methods were partly explored to explore transport and thermoelectric features in various models and actual devices. The density functional theory plus non-equilibrium Green’s function serves well in the probing process of transport properties like conductance in mesoscopic systems. Atoms’ positions were treated as the only input parameters and one computation package based on NEGF-DFT loop was utilized to get the numerical results, then the corresponding thermal quantities were analysed. The coherent transport exhibits an obvious character in transmission spectrum called transmission node, whose existence relies on the asymmetric structure of molecular junctions. In the main body of the thesis, firstly two types of model simulation were tested, and the following thermoelectric quantities showed that there’s one interesting signature in the thermopower performance, which was its temperature independence around transmission node. Through comparisons between different system parameters a rough regular pattern was obtained, that the degree of zero transmission and the energy difference around it influenced this temperature invariance feature at the same time. While these two properties were mainly determined by the natural structure of devices. Besides model simulations the ab initio investigations were also carried out. Although the actual device was not easily altered as ideal models, some similar behaviours in the transmission and thermal curves were still found out. The temperature insensitivity was considered to appear more often in a π electron dominated molecular structure rather than ones with σ electron interactions.
published_or_final_version
Physics
Master
Master of Philosophy

Nanodiamonds (ND) have been the subject of intense research in recent years, for they have unique physical properties normally associated with diamond, in addition to their rich surface chemistry and bio-compatibility. In this thesis, the electronic properties of intentionally boron-doped nanodiamond materials are studied. In chapter 5, the possibility of substitutional doping of NDs is investigated. The properties of boron-doped, detonation nanodiamonds (B-DND) are studied using electrical impedance measurements and spectral analysis, and are compared to un-doped detonation-NDs (DND). Activation energies from variable-temperature impedance spectroscopy are found to be lower in comparison to intrinsic NDs. Chapter 6 discusses the nucleation of high-pressure, high-temperature (HPHT) boron-NDs, as well as B-DNDs on silicon. By combining pH titration and ultra-sonication from solution, nucleation densities are measured using atomic force microscopy (AFM). It is found that for most samples, highly acidic solutions (pH~2) are ideal for high surface coverage. Chapter 7 describes the electrical properties and activation energies of boron-doped HPHT and detonation nanodiamonds. Thin films are aggregated on conductive silicon substrates, and are subjected to electrical impedance measurements in vacuum. Following vacuum annealing, electrical measurements showed activation energies comparable to highly boron-doped PE-CVD thin film diamond. Electrical conductivity and resistivity are also compared to literature. In chapter 8, aluminium-diamond Schottky-barrier diodes (SBD) are fabricated. HPHT nanodiamond films were used as both Ohmic contacts and as a source of dopant (boron), where aggregated nanodiamonds were subjected to PE-CVD film growth. Electrical (I-V) and capacitance-voltage (C- V) measurements are performed to study conduction mechanisms in fabricated devices. Resulting devices are found to have low carrier densities in the grown active layer (~1015 cm-3), which is desirable for SBDs. This is the first account of using doped-NDs as the source of low boron-doping in PE- CVD diamond films, paving the way for potentially economical nanoscale diamond electronic devices.

The idea of miniaturizing devices down to the nanoscale where quantum ffeffects become relevant demands a detailed understanding of the interplay between classical and quantum properties. Therefore, characterization of newly produced nanoscale materials is a very important part of the research in this fifield. Studying structural and magnetic properties of nano- and molecular magnets and the interplay between these properties reveals new interesting effects and suggests ways to control and optimize the respective material. The main task of this thesis is investigating the magnetic properties of molecular magnetic clusters and magnetic nanoparticles recently synthesized by several collaborating groups. This thesis contains two main parts focusing on each of these two topics. In the first part the fundamental studies on novel metal-organic molecular complexes is presented. Several newly synthesized magnetic complexes were investigated by means of different experimental techniques, in particular, by electron spin resonance spectroscopy. Chapter 1 in this part provides the theoretical background which is necessary for the interpretation of the effects observed in single molecular magnetic clusters. Chapter 2 introduces the experimental techniques applied in the studies. Chapter 3 contains the experimental results and their discussion. Firstly, the magnetic properties of two Ni-based complexes are presented. The complexes possess different ligand structures and arrangements of the Ni-ions in the metal cores. This difffference dramatically affffects the magnetic properties of the molecules such as the ground state and the magnetic anisotropy. Secondly, a detailed study of the Mn2Ni3 single molecular magnet is described. The complex has a bistable magnetic ground state with a high spin value of S = 7 and shows slow relaxation and quantum tunnelling of the magnetization. The third section concentrates on a Mn(III)-based single chain magnet showing ferromagnetic ordering of the Mn-spins and a strong magnetic anisotropy which leads to a hysteretic behavior of the magnetization. The last section describes a detailed study of the static and dynamic magnetic properties of three Mn-dimer molecular complexes by means of static magnetization, continuous wave and pulse electron spin resonance measurements. The results indicate a systematic dependence of the magnetic properties on the nearest ligands surrounding of the Mn ions. The second part of the thesis addresses magnetic properties of nano-scaled magnets such as carbon nanotubes fifilled with magnetic materials and carbon-coated magnetic nanoparticles. These studies are eventually aiming at the possible application of these particles as agents for magnetic hyperthermia. In this respect, their behavior in static and alternating magnetic fifields is investigated and discussed. Moreover, two possible hyperthermia applications of the studied magnetic nanoparticles are presented, which are the combination of a hyperthermia agents with an anticancer drug and the possibility to spatially localize the hyperthermia effffect by applying specially designed static magnetic fifields.

The demand for electrical energy is steadily increasing. Highly efficient organic solar cells based on mixed, strongly absorbing organic molecules convert sunlight into electricity and, thus, have the potential to contribute to the worlds energy production. The continuous development of new materials during the last decades lead to a swift increase of power conversion efficiencies (PCE) of organic solar cells, recently reaching 12%. Despite these breakthroughs, the usage of highly complex organic molecules blended together to form a self-organised absorber layer results in complicated morphologies that are poorly understood. However, the morphology has a tremendous impact on the photon-to-electron conversion, affecting all processes ranging from light absorption to charge carrier extraction. This dissertation studies the role of phase-separation of the self-organised thin film blend layers utilized in organic solar cells. On the molecular scale, we manipulate the phase-separation, using different molecule combinations ranging from the well-known ZnPc:C 60 blend layers to highly efficient oligothiophene:C60 blend layers. On the macroscopic scale, we shape the morphology by depositing the aforementioned blend layers on differently heated substrates (in-vacuo substrate temperature, Tsub). To characterise the manufactured blend layers, we utilize high resolution microscopy techniques such as photoconductive atomic force microscopy, different electron microscopic techniques, X-ray microscopy etc., and various established and newly developed computational simulations to rationalise the experimental findings. This multi-technique, multi-scale approach fulfils the demands of several scientific articles to analyse a wide range of length scales to understand the underlying optoelectronic processes. Varying the mixing ratio of a ZnPc:C60 blend layer from 2:1 to 6:1 at fixed in vacuo substrate temperature results in a continuous increase of surface roughness, decrease of short-circuit current, and decrease of crystallinity. Additionally performed density functional theory calculations and 3D drift-diffusion simulations explain the observed crystalline ZnPc nanorod formation by the presence of C60 in the bulk volume and the in turn lowered recombination at crystalline ZnPc nanorods. Moving to oligothiophene:C60 blend layers used in highly efficient organic solar cells deposited at elevated substrate temperatures, we find an increase of phase-separation, surface roughness, decrease of oligothiophene-C60 contacts, and reduced disorder upon increasing Tsub from RT (PCE=4.5%) to 80 °C (PCE=6.8%). At Tsub =140 °C, we observe the formation of micrometer-sized aggregates on the surface resulting in inhomogeneous light absorption and charge carrier extraction, which in turn massively lowers the power conversion efficiency to 1.9%. Subtly changing the molecular structure of the oligothiophene molecule by attaching two additional methyl side chains affects the thin film growth, which is also dependent on the substrate type. In conclusion, the utilized highly sensitive characterisation methods are suitable to study the impact of the morphology on the device performance of all kinds of organic electronic devices, as we demonstrate for organic blend layers. At the prototypical ZnPc:C60 blend, we discovered a way to grow ZnPc nanorods from the blend layer. These nanorods are highly crystalline and facilitate a lowered charge carrier recombination which is highly desirable in organic solar cells. The obtained results at oligothiophene: C60 blends clearly demonstrate the universality of the multi-technique approach for an in-depth understanding of the fragile interplay between phase-separation and phase-connectivity in efficient organic solar cells. Overall, we can conclude that both molecular structure and external processing parameters affect the morphology in manifold ways and, thus, need to be considered already at the synthesis of new materials.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references (leaves 46-48).
Vascular grafts are prosthetic tubes that serve as artificial replacements for damaged blood vessels. Poly(ethylene-terephthalate), PET, has been successfully used in large diameter grafts; however, small caliber grafts are still a major challenge in biomaterials. Due to surface forces, blood plasma proteins adsorb to the graft, resulting in inflammation, infection, thrombus formation, and ultimately, vessel reclosure. The object of this project was to characterize and analyze the nanoscale surface properties of three different commercial vascular grafts, woven collagen-coated, knitted collagen- coated, and knitted heparin-bonded, all PET-based. The study was performed in order to ascertain differences in biocompatibility due to surface coating and morphology. Scanning Electron Microscopy, Atomic Force Microscopy and High Resolution Force Spectroscopy techniques were used to characterize the surface of the samples as well as to measure the forces between these surfaces and blood plasma proteins. The results will serve as a basis for the understanding of the nanoscale interactions between the biomaterial and blood plasma proteins. Such interactions are brought about by the different surface topologies and components, therefore a thorough understanding of surface properties will act as a building block for further changes in small caliber vascular grafts in order to enhance their biocompatibility.
by Celia Edith Macias.
S.B.

III-nitrides are wide-band gap materials that have applications in both electronics and optoelectronic devices. Because to their inherent strong polarization properties, thermal stability and higher breakdown voltage in Al(Ga,In)N/GaN heterostructures, they have emerged as strong candidates for high power high frequency transistors. Nonetheless, the use of (Al,In)GaN/GaN in solid state lighting has already proved its success by the commercialization of light-emitting diodes and lasers in blue to UV-range. However, devices based on these heterostructures suffer problems associated to structural defects. This thesis primarily focuses on the nanoscale electrical characterization and the identification of these defects, their physical origin and their effect on the electrical and optical properties of the material. Since, these defects are nano-sized, the thesis deals with the understanding of the results obtained by nano and micro-characterization techniques such as atomic force microscopy(AFM), current-AFM, scanning kelvin probe microscopy (SKPM), electron beam induced current (EBIC) and scanning tunneling microscopy (STM). This allowed us to probe individual defects (dislocations and cracks) and unveil their electrical properties. Taking further advantage of these techniques,conduction mechanism in two-dimensional electron gas heterostructures was well understood and modeled. Secondarily, origin of photoluminescence was deeply investigated. Radiative transition related to confined electrons and photoexcited holes in 2DEG heterostructures was identified and many body effects in nitrides under strong optical excitations were comprehended.

When we stretch and contract a rubber band a hundred times, we expect the rubber band to fail. Yet our heart stretches and contracts the same amount every two minutes, and does not fail. Why is that? What causes the significantly higher elasticity of certain molecules and the rigidity of others? Equally importantly, can we use this information to design materials for precise mechanical tasks? It is the aim of this dissertation to illuminate key aspects of the answer to these questions, while detailing the work that remains to be done. In this dissertation, particular emphasis is placed on the nanoscale properties of elastomeric proteins. By better understanding the fundamental characteristics of these proteins at the nanoscale, we can better design synthetic rubbers to provide the same desired mechanical properties.
Ph. D.

Thesis (Ph. D.)--University of Cincinnati, 2005.
Title from electronic thesis title page (viewed Apr. 11, 2006). Includes abstract. Keywords: Al-Cu nanoparticle; Al nanoparticle; Al-Al2O3 composite; 2024Al-Al2O3 composite; nanocomposite; nanoparticle; phase transformation; precipitate; plasma ablation; inert gas condensation; exploding wire; consolidation; sinter; cold roll; hot roll; aging; thermal-soaking; mechanical properties; strengthening mechanism. Includes bibliographical references.

Nanoscale properties of conjugated polymers by Scanning Probe Microscopy Atomic force microscopy (AFM) and electrostatic force microscopy (EFM) are explored and developed to study the surface potential distribution for a range of applications, including semiconductor laser devices, the electrical conductivity of aligned DNA molecules. The main focus of the thesis is the application of these techniques to investigate the nanoscale structures and electrical properties of conjugated polymers, including poly-(3-exylthiophene)s (P3ATs), polyfluorene (PFO), and poly-(3,4,-ethylenedioxythiophene) (PEDOT). EFM is a SPM technique, used to measure electrostatic force in non-contact mode. Two modes of EFM, scanning Kelvin probe microscopy (KPM or SKPM) and EFM/phase, are explored. Analytical calculations of tip-surface capacitances and their gradients are presented, aiming at quantifying the measurement. Based on the calculation results, the origin of the measurement resolution in EFM/phase and SKPM is explained, and a procedure is developed to convert the phase shift to the local surface potential. Thus, EFM/phase can also be used to measure the surface potential with higher resolution than SKPM. The self-assembled/aggregation structures of the polymers, as varied by molecular weight, solution preparation and substrates used, are investigated by AFM. The self-assembled structure, usually in the form of a network, obeys certain laws in its formation. The surface potential distributions and charge transport properties in polymer films and network structures are investigated with both EFM modes. The electrical properties of Au on poly-(3-hexylthiophene) (P3HT) and P3HT on Au contacts are investigated. The electrochemical reaction of conjugated polymers, and electropolymerisation of 3,4-ethylenedioxythiophene (EDOT) are carried out on micro electrodes, and studied by AFM. The EDOT electropolymerization is shown to grow polymer nano-wires or a uniform polymer film, depending on conditions the electropolymerization process.

Barium strontium titanate thin film varactors have been widely investigated for the purpose of creating tunable front-ends for RF and microwave systems. There is an abundance of literature observing the capacitance-voltage behavior and methods on improving tunability. However, there is a lack of thorough investigations on the nonlinear behavior, specifically the third order intermodulation distortion, and the parameters that impact it. There is also a research void that needs to be filled for nanoscale barium strontium titanate varactors as nanotechnology becomes increasingly prevalent in the design of RF and microwave components. This work aims to advance the understanding of nonlinear properties of barium strontium titanate varactors. Temperature and voltage impacts on the third order intermodulation distortion products of BST varactors are observed by two-tone measurements. The material properties of the films are correlated with the nonlinear behavior of the varactors. Additionally, size reduction capabilities are shown by fabricating planar barium strontium titanate interdigital varactors with nanoscale size gaps between the electrodes. Modeling techniques are also investigated.

"Recently, hydrogel have found to be promising biomaterials since their porous structure and hydrophilicity enables them to absorb a large amount of water. In this study the role of water on the mechanical properties of hydrogel are studied using ab-initio molecular dynamics (MD) and coarse-grained simulations. Condensed-Phased Optimized Molecular Potential (COMPASS) and MARTINI force fields are used in the all-atom atomistic models and coarse-grained simulations, respectively. The crosslinking process is modeled using a novel approach by cyclic NPT and NVT simulations starting from a high temperature, cooling down to a lower temperature to model the curing process. Radial distribution functions for different water contents (20%, 40%, 60% and 80%) have shown the crosslinks atoms are more hydrophilic than the other atoms. Diffusion coefficients are quantified in different water contents and the effect of crosslinking density on the water diffusion is studied. Elasticity parameters are computed by constant strain energy minimization in mechanical deformation simulations. It is shown that an increase in the water content results in a decrease in the elastic. Finally, continuum hyper elastic model of contact lens is studied for three different loading scenarios using Finite Element Model. "

The main objective of this work was to develop a novel drug delivery system exploiting special opportunities afforded by synthesis of nanoscale materials to be applied inside the colon. It must be robust enough to cope with the adverse conditions in the gastrointestinal tract (GI) and be able to reach and release “on demand” at the colon area at the right time. In this work, an oral capsule formulation with iron oxide nanoparticles (IONs) containing coating was used to transport drug and release drug in the colon. With that in mind, the synthesis of poly (alkylcyanoacrylate) nanocapsules by microemulsion polymerisation and magnetic iron oxide nanoparticles (IONs) via a coprecipitation method were conducted. The key physical properties of the materials were characterized employing standard techniques such as HPLC, FTIR, DSC, DLS, XRD, TEM and SEM. Hard capsules filled with model drug, paracetamol, were coated with IONs containing coatings (fatty acids and paraffin). The optimum composition for the formulation of the coating embedded with the nanoparticles was explored with respect to protection of the drug payload from conditions in the GI tract as well as for effective release “on demand” using radio-frequency hyperthermia. The optimum radiofrequency and the power level for heating the nanoparticles were also determined and melting the coating using magnetic nanoparticle hyperthermia. Results showed that paraffin-based coatings had appropriate properties for this application. Finally, taking into account all the results, a design of a novel drug delivery system, together with an experimental setup for testing the “release in demand” was proposed. The approach is generic, easy to set up and could also be applied to many other situations where delivery on demand is required.

Lipid bilayers form the basis of the membranes that serve as a barrier between a cell and its physiological environment. Their physical properties make them ideally suited for this role: they are extremely soft with respect to bending but essentially incompressible under lateral tension, and they are quite permeable to water but essentially impermeable to ions which allows the rapid establishment of the osmotic gradients. The function of membrane proteins, which are vital for tasks ranging from signal transduction to energy conversion, depends on their interactions with the lipid environment. Because of the complexity of natural membranes, model systems consisting of simpler lipid mixtures have become indispensable tools in the study of membrane biophysics. The objective of the work reported here is to develop a deeper understanding of the underlying physics of lipid bilayers through nanoscale measurements of the mechanical properties of mixed lipid systems including cholesterol, a key ingredient of cell membranes. Atomic force microscopy (AFM) has been used extensively to measure the topographical and elastic properties of supported lipid bilayers displaying complex phase behaviour and containing mixtures of important PC, PE lipids and cholesterol. Phase transformations have been investigated varying the membrane temperature, and the effects of cholesterol in controlling membrane fluidity, phase, and energetics have been studied. Elastic modulus measurements were correlated with phase behaviour observations. To aid in the nanoscale probing of lipid bilayers, AFM probes with a high aspect ratio and tip radii of $sim$4~nm were fabricated and characterised. These probes were used to investigate the phase boundary in binary and ternary lipid systems, leading to the discovery of a raised region at the boundary which has implications for the localisation of reconstituted proteins as well as the role of natural domains or lipid rafts. The electrical properties of the probes were examined to assess their potential application for combined structural and electrical measurements in liquid. A novel technique was developed to aid in the study of the physical properties of lipid bilayers. Membrane budding was induced above microfabricated substrates through osmotic pressure. Modification of the adhesion energy of the bilayer through biotin-avidin linking was successful in modulating budding behaviour of liquid disordered bilayers. The free energy of the system was modelled to allow quantitative information to be extracted from the data.

The evaluation of fracture risk in osteoporotic patients, which is still mostly based on bone mineral density (BMD) measurements, constitutes a major clinical challenge. Despite the fact that BMD is highly correlated with fracture risk in large populations, it unfortunately fails to be an accurate predictor for the individual. To increase the accuracy of fracture risk evaluation, a better understanding of all factors affecting bone fracture behaviour, including (but not limited to) BMD, is needed. This means a deeper understanding of bone’s material properties and structure-function relationship is required. Mammalian bones are composed of mineralized type I collagen fibrils immersed in a matrix of non-collagenous proteins (NCPs). This fundamental unit assembles into progressively larger features in a hierarchical manner, supplying bone with various “lines of defence” against catastrophic failure. Optimal load transfer and energy dissipation mechanisms have, to some extent, been discovered within bone’s nanostructure on which NCPs have been proposed to play a crucial role. Yet, it is largely unknown if integration of such mechanisms occurs to any other hierarchical level. This thesis attempts to answer this question for the osteonal level of cortical bone. The feature dominating this level is a hollow tubular structure of a few hundred micrometres in diameter, the osteons, consisting of concentric aligned lamellae. Lamellae range from 3 -10 μm in thickness and between them lie interlamellar areas (often referred to as thin lamellae). This thesis is the outcome of three studies. The first focuses on the aforementioned osteonal features, namely lamellae and interlamellar areas, studying their structure, composition and their mechanical behaviour during loading. For this purpose a series of experimental techniques were used including μ-RAMAN microscopy, atomic force microscopy (AFM), AFM cantilever-based nanoindentation and in-situ AFM analysis during microtensile testing. It was shown that interlamellar areas differ from lamellae by (a) being collagen-deficient and NCP-rich, (b) having a different arrangement of collagen fibres, (c) being more compliant when no load is applied to the bone and (d) exhibiting higher strains under loading conditions. Finally, stable crack propagation was for the first time captured in a time-lapsed fashion within the interlamellar areas by means of AFM, further proving their significant contribution towards fracture toughness. The second is a technical study for the development of a method capable for generating full fracture-resistance curves (R-curves) of small bone samples where crack propagation cannot easily be observed. The outcome was the development of a novel computer-aided videography method, “whitening-front tracking” method, which uses the whitening effect formed by the development of the damage in front of the crack-tip (frontal process zone) to indirectly track the crack propagation which is needed for the generation of the R-curve. The new method was then applied in the third study to correlate the ultimate toughness of human cortical bone, i.e. “fracture toughness” and “crack growth resistance” at the tissue-level, with the elasticity inhomogeneity between lamella and interlamellar areas at the osteonal-level and the damage-formation resistance at the micro-level. In this study the mechanical properties of bone in tissue-, micro- and osteonal-level were measured by means of the “whitening front tracking”, reference point indentation (RPI) and AFM cantilever-based nanoindentation methods respectively. The results revealed a correlation of toughness and crack-growth resistance at the tissue-level with the elastic inhomogeneity between the sub-osteonal features. That is, the higher the difference between the moduli of lamellae and interlamellar areas the higher the toughness of the tissue. Furthermore, toughness and crack-growth resistance correlated with bone’s “resistance to damage” as it was characterised by RPI at the micro-level. Finally, these measured suggested age-related degradation of the mechanical properties at all three levels measured. Overall, the results presented in this thesis propose that osteons are the principal component of a previously unknown proactive mechanism which transfers load and movement in a manner analogous to engineered “elastomeric bearing pads”. This ability originates from the elastic inhomogeneity between the lamellae and the interlamellar areas which allows osteons to adapt to high stresses without damage formation.

Femtosecond laser interaction with dielectrics has unique characteristics for micromachining, notably non-thermal interaction with materials, precision and flexibility. The nature of this interaction is highly nonlinear due to multiphoton ionization, so the laser energy can be nonlinearly absorbed by the material, leading to permanent change in the material properties in a localized region of Mu-m3. This dissertation demonstrated the potential of these nonlinear interactions induced changes (index modification and ablation for machining) in the dielectrics and explored several practical applications. We studied femtosecond laser ablation of Poly-methayl methacrylate (PMMA) under single and multiple pulse irradiation regimes. We demonstrated that the onset of surface ablation in dielectric surface is associated with surface swelling, followed by material removal. Also, the shape of the ablation craters becomes polarization dependent with increasing fluence, except for circular polarization. The morphology of the damaged/ablated material was examined by optical and scanning electron microscopy. The dynamics of laser ablation of PMMA was simulated using a 2 dimensional Molecular Dynamics model and a 3 dimensional Finite Difference Time Domain model. The results from numerical simulations agreed well with experimental results presented in this thesis. We also demonstrated the formation of nano-pillar within the ablation crater when the surface of bulk-PMMA was irradiated by two femtosecond pulses at a certain delay with energies below single shot ablation threshold. With increasing fluence, the nano-pillar vanished and the structure within the ablation crater resembled volcanic eruption. At higher fluences we demonstrated nanoscale porosity in PMMA. For application, a novel in-line fiber micro-cantilever was fabricated in bend insensitive fiber, that provides details of in-line measurement of frequency and amplitude of vibration, and can be further extended to be used as chemical/bio and temperature sensors. By modifying the refractive index at random spacing within the single mode fiber core, a unique quasi-random micro-cavities fiber laser was fabricated, which exhibits comparable characteristics with a commercial fiber laser in terms of narrow linewidth and frequency stability.

Las microscopías de campo cercano en general (SPM) y la microscopia de fuerzas (SFM) en particular, se han convertido en una poderosa herramienta en nanotecnología, ya que permiten tanto caracterizar como manipular las superficies de los materiales en la nanoescala. En el presente trabajo de investigación se han estudiado las propiedades morfológicas, mecánicas, electrostáticas y de conducción de sistemas autoensamblados y nanoestructurados que incluyen películas delgadas orgánicas y superficies inorgánicas mediante estas técnicas de SPM, SFM y microscopía de efecto túnel (STM), en condiciones de ambiente controlado. Nos centramos principalmente en el uso de técnicas de SFM en modos de operación de contacto, dinámico, microscopía de fuerzas de fricción (FFM), conductividad con SFM (CSFM) y microscopía de sonda Kelvin (KPFM). El manuscrito de la tesis está organizado del siguiente modo: en el capítulo 1 se exponen las motivaciones del trabajo y en el capítulo 2 se hace una pequeña introducción al concepto de autoensamblado y a los sistemas nanoestructurados en los sistemas bajo estudio, películas delgadas orgánicas y superficies inorgánicas. En el capítulo 3 se introducen las distintas técnicas y procedimientos experimentales. Se explican las características generales del SPM, haciendo particular hincapié en los modos de operación de SFM empleados. En el mismo capítulo se explican los procedimientos empleados para el crecimiento de películas delgadas orgánicas, incluyendo los métodos químicos de solución molecular, la litografía “microcontact printing” ( μCP) y la deposición de moléculas orgánicas por haces moleculares (OMBD). En el capítulo 4 investigamos el impacto de la estructura supramolecular de las capas orgánicas autoensambladas (SAMs) en las propiedades morfológicas, electrostáticas y de conducción de superficies funcionalizadas. Con este propósito y para poder realizar un análisis comparativo basado en el uso de referencias in-situ, estudiamos la SAM formada por dos fases supramoleculares de la misma molécula CH3(C6H4)2(CH2)4SH) (BP4) coexistiendo en la superficie Au(111). Mostramos como la organización supramolecular (estructura interna de la película orgánica) es un factor decisivo que determina las propiedades de la superficie y demostramos como la técnica FFM puede emplearse, por ejemplo, para diferenciar dominios moleculares de distinta orientación cristalina. Además, gracias al uso combinado del STM y CSFM en medidas de transporte electrónico, interpretamos la diferencia en la altura aparente medida por una u otra técnica en películas orgánicas inhomogéneas. En el capítulo 5 estudiamos las propiedades de la superficie SrTiO3 (001) nanoestructurada. La nanoestructuración en este caso viene dada por la coexistencia de las dos posibles terminaciones, TiO2 y SrO, lateralmente diferenciadas, que empleamos como plantilla para la adsorción selectiva de SAMs. Demostramos que la molécula de ácido esteárico (con funcionalidad COOH) se adsorbe selectivamente en la superficie TiO2 y estudiamos el impacto de su adsorción sobre las propiedades mecánicas y electrostáticas de la superficie. Así, describimos las principales características de la superficie SrTiO3 (001) nanoestructurada, la adsorción de la SAM en la superficie TiO2 y discutimos el impacto de esta adsorción en las propiedades de la superficie. Finalmente, en el capítulo 6 presentamos dos efectos inducidos por la punta del SFM susceptibles de usarse para la manipulación local y controlada de películas orgánicas. Mostramos el crecimiento de multicapas de pentaceno inducido mecánicamente y un efecto de pelado de capas moleculares al aplicar voltajes entre una punta conductora y materiales moleculares conductores, un efecto a tener en cuenta en el diseño de futuros dispositivos en electrónica molecular.
The present work lies within the scope of the morphological, mechanical, electrostatic and conductive characterization of self-assembled and nanostructured systems, including organic thin films and inorganic surfaces. Scanning probe microscopy (SPM) techniques, in general, and scanning force microscopy (SFM), in particular has become one of the most powerful tools in nanotechnology because they offer the combined capability of surface properties characterization and manipulation of material surfaces in the nanoscale. In this work we make use of SPM techniques, both SFM and scanning tunneling microscopy (STM), under controlled ambient conditions for the characterization and manipulation of different self-assembled and nanostructured systems. We mainly focus on the use of SFM in contact, dynamic, friction force microscopy (FFM), conductive scanning force microscopy (CSFM) and Kelvin probe force microcopy (KPFM) operating modes for such a purpose. The thesis is organized in the following way: the motivations for this work are presented in chapter 1, and a short introduction to the self-assembled concept and nanostructured systems in organic thin films and inorganic surfaces is done in chapter 2. Chapter 3 introduces the fundamental description of the experimental techniques and procedures used. The main experimental characterization SPM techniques are introduced and a particular attention is devoted to explain the different SFM techniques used. In the same chapter, the growth techniques of organic thin film are explained, including, the solution based methods, the soft lithography μ-contact printing (μCP) and the organic molecular beam deposition (OMBD). In chapter 4 we investigate the influence of the supramolecular structure of self-assembled monolayers (SAMs) into the morphological, mechanical, electrostatic and conductive properties of a functionalized surface. For this purpose we study the CH3(C6H4)2(CH2)4SH) (BP4) molecule SAM on the Au(111) surface presenting two different coexisting supramolecular arrangements. We show how the supramolecular order of the SAM is a decisive factor influencing the nanoscale properties of the surface and also demonstrate how FFM can be employed to differentiate SAM domains with different orientation. In addition, based on electron current measurements, the combined use of STM and CSFM allows us interpreting the differences in apparent height as measured by one or the other technique in non-homogeneous organic layers. In chapter 5 we study the properties of the nanopatterned SrTiO3 (001) surface and explore its use as template for the selective adsorption of SAMs. We find that stearic acid molecules (containing a COOH headgroup) selectively chemisorb on the TiO2 surface. This fact allows us to investigate SAMs adsorption influence on the mechanical and electrostatic properties of this oxide surface. We address the main characteristics of the nanopatterned SrTiO3 (001) surface and we describe the selective adsorption of SAMs on the TiO2 surface, discussing how this influences the local mechanical and electrostatic properties of the surface. Finally, in chapter 6 we present two different tip-induced effects which can be use to manipulate organic thin film materials. We address the mechanical induced growth of pentacene molecular layers, a phenomena that can be used as a local nanolithography approach for nanostructuration. And we also provide a way for peeling a layered organic molecular material when a voltage is applied between the conducting system and the conducting probe of the SFM, which is important to take into account for the design of organic electronic devices.

The thermal properties of nanocrystal arrays and large unit cell molecular crystals are studied using experimental and computational techniques. The major objective is to understand the mechanisms of thermal transport through three-dimensional organic-inorganic superstructured materials that are built from superatoms. The frequency domain thermoreflectance technique is applied to measure thermal conductivity in thin films and nanoliter sized single crystals. Molecular dynamics simulations, lattice dynamics calculations, and density functional theory calculations are employed to interpret the measurements and to explore experimentally-inaccessible nanoscale phenomena. A superatom is a cluster of atoms that behaves as a stable or metastable unit with emergent properties distinct from its elemental atoms. Superatoms can self-assemble into three-dimensional hierarchical materials with each superatom occupying a lattice site to form a periodic solid (i.e., a superlattice). By varying geometry and composition, the resulting solid can have tunable electrical and optical properties. This work presents the first investigation of the thermal properties of solids built from two classes of superatoms: (i) monodispersed nanocrystals that form a nanocrystal array (NCA) and (ii) inorganic-organic superatoms of precise stoichiometric composition that form a molecular crystal (LUMC). The thermal conductivity of NCAs was measured to be between 0.1 to 0.3 W/m-K. Experiments revealed that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces as well as the volume fractions of nanocrystal cores and surface ligands. The NCA thermal conductivity trends upward then plateaus with increasing temperature suggesting elastic scattering events dominate transport at the organic-inorganic interfaces. The onset temperature of the plateau is dependent on the overlap of the vibrational states in the core and ligand. Atomistic computational analysis of the thermal transport explored experimentally inaccessible trends that provided new insights for controlling heat flow in NCAs. A decreasing interfacial thermal conductance trend for the organic-inorganic interface with increasing nanocrystal diameter was discovered. This trend can be related to the interfacial thermal conductance of a self-assembled monolayer (SAM) interface through a geometrical scaling law. Changing the atomic mass of the nanocrystal core to vary its vibrational states resulted in a non-monotonic trend in both the thermal conductivity and interfacial thermal conductance. Peaks in both properties occur at the same small atomic mass and are related to the overlap and coupling of the organic and inorganic vibrational states. Preliminary measurements of LUMCs indicate that the thermal conductivity is between 0.2 and 0.4 W/mK at the temperature of 300 K, comparable to that of an amorphous polymer. A slight increase in thermal conductivity is observed for the binary-species LUMCs that contain fullerene derivatives over their corresponding mono-species LUMCs composed of inorganic core superatoms. This increase may be attributed to the stronger ionic intermolecular bonds in the binary LUMCs. The presence of a larger chalcogenide element in the inorganic core superatom decreases the thermal conductivity of the LUMC. This decrease is consistent with lower frequency vibrational modes that have a lower group velocity in the crystal. The temperature dependent thermal conductivity of a mono-species CoSe LUMC has a crystalline-like behavior, unlike most low thermal conductivity materials. Nanoscale superstructured organic-inorganic materials self-assemble from solutions and can be scalable to replace single crystal semiconductors for many technologies. Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, opto-electronic, and energy-related applications. The low thermal conductivity in the NCA presents a challenge for thermal management but a boon for thermoelectric waste heat scavenging. The class of large unit cell molecular crystals investigated here have low thermal conductivity and a moderate electrical conductivity, making them novel candidates for thermoelectricity.

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.

La création de matériaux multifonctionnels en ayant recours à l’auto-assemblage de molécules spécifiques est d’un grand intérêt dans le domaine des nanotechnologies. Dans ce contexte un réel effort a été produit afin d’obtenir le contrôle total de l’auto-organisation de molécules π-conjuguées ; ceci dans le but de créer des architectures supramoléculaires anisotropes et de créer des nano-objets électriquement actifs. Il est en effet connu que l’ordre, à un niveau supramoléculaire, affecte de façon importante les propriétés électroniques d’assemblages moléculaires. Dans ce travail de thèse nous nous sommes intéressé à l’auto-assemblage de différents systèmes moléculaires menant à la formation de nanostructures supramoléculaires, ces dernières résultant d’un équilibre entre les interactions intramoléculaires, intermoléculaires ainsi que celles existant à l’interface. Les propriétés structurelles, dynamiques, électriques et électroniques de surfaces solides ont été étudiées à l’échelle du nanomètre en utilisant les techniques de microscopie en champs proche (ou SPM pour Scanning Probe Microscopy). Les techniques utilisées durant cette thèse vont du microscope à effet tunnel (STM) et spectroscopie à effet tunnel (STS) aux techniques dérivées des microscopies à force atomique(SFM) telles que Kelvin Probe Microscopy (KPFM) et le Conducting Probe Force Microscopy (C-AFM). Les nanostructures qui ont été développées dans le cadre de ce travail ne sont pas intéressantes uniquement en tant que nano-constructions sur des surfaces solides mais aussi en tant que candidats en vue d’applications dans le domaine de l’électronique (supra)moléculaire. Afin d’explorer l’utilisation d’interactions de faible intensité menant à la constitution d’assemblages supramoléculaires fonctionnels, nous avons étudié les systèmes suivants:i)Un composant thio-tryophénylène alkylé connu pour former une phase cristal liquide discotique grâce à la présence d’interactions π-π entre les molécules; ce composant ayant de plus des propriétés électroniques potentiellement intéressantes. Ii)Un hexaazatriphenylène fonctionnalisé par des amides, intéressant comme potentiel porteur d’électrons aux vues de la nature chimique de son noyau conjugué. Ce composant montre de plus une phase cristal liquide discotique résultant de la formation de liaisons hydrogènes entre ses fonctions amides. Iii)Une diade HBC-PérylèneMonoImide formant un système Donneur-Accepteur monomoléculaire. Iv)Des composant HBC fonctionnalisés à l’aide de nucléotides, afin d’exploiter les propriétés directionnelles et de reconnaissance des liaisons hydrogènes entre les nucléotides. V)Des composants « cavitands » en vue de former des assemblages hôte-invité dont la courbure sera contrôlable. En ce qui concerne les dérivés de triphenylène, nous avons porté notre attention, en collaboration avec le groupe du Prof. Yves Geerts de l’université Libre de Bruxelles (Belgique), sur des composants hexaazatriphenylenes et des thio-triphenylène alkylés. Ces derniers sont des molécules de cristaux liquides discotiques possédant d’intéressantes propriétés électroniques. Nous avons suivi leur auto-assemblage à l’interface liquide –solide et nous avons observé la formation de monocouches ayant deux motifs coexistants sur la surface basale du graphite. De plus, l’évolution temporelle des joints de domaine au sein d’une monocouche polycrystalline a révélé un « Ostwal ripening » en 2D. Les propriétés électriques ont été investiguées, à la fois à l’échelle de la molécule à l’aide de STS et à l’échelle des ensembles supramoléculaires à l’aide de C-AFM. D’autre part les expériences utilisant un microscope à force atomique ont montré la formation de nanofils allongés sur la surface de substrats isolants, comme par exemple des substrats de mica muscovite et de SiOx. Des approches ont été développées afin de créer des auto-assemblages utilisant les nanofils cités précédemment en les incluant entre deux nano-électrodes, ceci en vue de l’étude de leur comportement électrique dans une telle configuration. D’autre part, les molécules de hexathialkylhexaazatriphénylènes ne montrent pas de phase cristal liquide colonnaire comme escompté, ceci étant probablement du à leur grande charge négative portée par les atomes d’azote; ce qui pourrait entraîner une répulsion entre les noyaux des molécules plus proches voisines. En conséquence, la formation de liaisons hydrogènes entre les différentes fonctions amide a été utilisée afin de contrebalancer et surpasser les répulsions coulombiennes. Nous avons ensuite étudié l’auto-organisation et les propriétés électroniques de molécules d’azatriphénylène spécialement synthétisées pour l’occasion. . Ces études ont révélé la formation d’architectures colonnaires dans différents environnements. Les molécules discales sont alors maintenues ensemble grâce aux liaisons hydrogène entre les fonctions amide. En particulier, un réseau de fibres a été formé, ce dernier exhibant une hauteur caractéristique comparable au diamètre d’une molécule. Les propriétés électroniques aux échelles de la molécule et de grands ensembles ont été étudiées sur des substrats conducteurs (HOPG). Les investigations STM à l’interface liquide-solide mettent en avant la formation de monocouches ordonnées. De plus, l’analyse des niveaux d’énergie contribuant au contraste dans les images STM, en complément de calculs de chimie quantique (effectués en collaboration avec le groupe du Dr. Jérôme Cornil de l’université de Mons, Belgique) , montre de façon remarquable la contribution des niveaux électroniques localisés au niveau des groupes amides. Finalement,des mesures utilisant le KPFM ont permis de déterminer le potentiel de surface des couches auto-organisées. . En collaboration avec le groupe du Professeur Klaus Müellen du Max-Planck Institut à Mainz, nous avons étudié de nombreux dérivés de HBC. En exploitant les propriétés directionnelles ainsi que les propriétés de reconnaissance des liaisons hydrogènes, nous avons obtenu des architectures branchées à partir de composants hybrides HBC-nucléotide, et ceci sur des surfaces isolantes (mica et SiOx) mais aussi conductrices (HOPG). De plus, grâce à la combinaison d’études STM et SFM nous avons investigué une diade HBC-PerylèneMonoImide sur graphite à l’interface liquide-solide et sur films secs, ce système étant un système Donneur Accepteur monomoléculaire. Dans une autre partie, en collaboration avec le groupe du Prof Enrico Dalcanale à l’université de Parme (Italie), nous avons étendu notre étude à l’auto-arrangement sur les surfaces de « cavitands », ces molécules pouvant former au travers d’une reconnaissance hôte-invité des polymères supramoléculaires ayant une courbure contrôlée. Les « cavitands » se sont avérés être des systèmes versatiles pour la formation des assemblages hôte-invité. Les études en solution ont montré la formation de polymères formés par ces « cavitands », et leur capacité d’agrégation a été quantifiée. Nous avons ensuite transféré ces nano-structures de la solution sur des surfaces atomiquement planes où nous avons pu visualiser des architectures en bâtonnet. Un recourbement en deux dimensions de bâtonnets uniques a pu être observé et a pu être corrélé au degré de courbure des polymères obtenu par simulation. En résumé, nous avons étudié l’auto-organisation d’une large gamme de systèmes moléculaires et nous en avons déterminé les propriétés structurelles électriques et électroniques de l’échelle mésoscopique jusqu’à l’échelle de la molécule. Les composants dérivés des triphénylènes ont montré des propriétés les rendant intéressant pour de potentielles applications tirant partie de leur conduction quasi unidimensionnelle. De plus, nous avons vu que la reconnaissance entre les nucléotides pouvait être exploitée afin de contrôler l’auto-assemblage de HBC. Finalement, les « cavitands » se sont montrés être de bons candidats pour la formation de polymères ayant une courbure contrôlée
The generation of multifunctional materials by self-assembly of specifically designed molecules, through supramolecular chemistry approaches, currently gathers a great interest in nanotechnology. In this context a great deal of effort has been devoted to achieve a full control of the self-organization of -conjugated molecules into highly ordered, anisotropic supramolecular architectures as spatially confined electrically active nano-objects. It is indeed well known that the order at the supramolecular level strongly affects the electronic properties of molecular based assemblies. In this thesis we report on the self-assembly on surfaces of different molecular systems into supramolecular nanostructures, as obtained by balancing the interplay of intramolecular, intermolecular, and interfacial interactions. On solid substrates structure, dynamics, electrical and electronic properties have been investigated on the nanometer scale making use primarily of Scanning Probe Microscopies, in particular Scanning Tunneling Microscopy and Scanning Force Microscopy (SFM) based approaches, such as Scanning Tunneling Spectroscopy (STS), Kelvin Probe Force Microscopy (KPFM) and Conducting Probe Force Microscopy (C-AFM). The nanostructures that have been developed are not only of interest as nano-constructions on solid surfaces, but exhibit properties that make them candidates for applications in the field of (supra)molecular electronics. In order to explore the use of weak interactions to for functional supramolecular assemblies at surfaces, we have studied the following systems:i)an alkylated thio-triphenylene compound known to from a discotic liquid crystal phase through  stacking, with potentially interesting electronic properties;ii)an amide functionalized hexaazatriphenylene, of interest as potential electron carrier in view of the nature of its conjugated core, and forming a discotic liquid crystal phase through the formation of Hydrogen bonds between amide units;iii)a HBC-PeryleneMonoImide dyad, as a single molecule Donor-Acceptor system;iv) HBC-nucleotide functionalized compounds, to exploit directionality and recognition properties of hydrogen bonds between nucleotides;v)Cavitand compounds as molecular building blocks for host-guest assemblies with controlled curvature. Among triphenylene derivatives, we have centered our attention, in collaboration with the group of Prof. Yves Geerts of the Université Libre de Bruxelles (Belgium), on hexaazatriphenylenes, and on an alkylated thio-triphenylene compound. This latter discotic is a liquid crystal forming molecule holding interesting electronic properties. We followed its self-organization at the solid-liquid interface observing a packing into monolayers bearing two coexisting structural motifs on the basal plane of graphite. The temporal evolution of domain boundaries in a polycrystalline monolayer, explored by recording series of subsequent STM images, revealed an Ostwald ripening phenomenon, i. E. Coarsening in two-dimensional molecular polycrystals. The electrical properties have been investigated at both the single molecule level, using STS, and at the ensemble (supramolecular) level by means of C-AFM. On the other hand SFM microscopy experiments highlighted the formation of edge-on nanowires on the surface of electrically insulating substrates, such as muscovite mica and SiOx. Approaches have been explored to drive the self-assembly of the aforementioned nano-wires embedded in the gap between nano- electrodes, and to study their electrical behavior in such a configuration. In a different way, hexathialkylhexaazatriphenylenes do not form columnar liquid crystalline phases like the corresponding triphenylene derivatives, probably due to the large negative charges on the nitrogen atoms that lead to a repulsion of adjacent cores. Hydrogen bonds between peripheral amide units have been therefore used to counterbalance and overcome this Coulombic repulsion. We have studied the self-organization and electronic properties of a specially designed and synthesized azatriphenylene. Such studies revealed in different environments the formation of columnar architectures, where the single molecular discs are held together by H-bonds between amide moieties. In particular a network of fibers was formed, exhibiting a height comparable to the molecular diameter. The electronic properties both at the single molecule as well as at the ensemble level have been investigated. On electrically conductive substrates such as highly oriented pyrolitic graphite (HOPG), STM investigations at the solid-liquid interface highlighted the formation of ordered monolayers and the analysis of the energy levels contributing to the contrast in the current STM images, supported by quantum-chemical calculations (pursued in collaboration with the group of Dr. Jérôme Cornil of the Université de Mons (Belgium)), revealed a significant contribution of the electronic levels localized on the amide groups. Furthermore KPFM measurements allowed us to determine the surface potential of the self-organized layers. In collaboration with the group of Prof. Klaus Müllen of the MPI of Maintz (Germany), we have studied various HBC-derivatives. Exploiting the directionality and recognition properties of hydrogen bonds between nucleotides, branched architectures have been obtained from hybrid solutions of HBC-nucleotide functionalized compounds on both insulating (mica and SiOx) and conductive (HOPG) surfaces, employing different room temperature deposition methods. Furthermore through combined STM and SFM studies we have investigated the behaviour of a HBC-PeryleneMonoImide dyad on HOPG at the solid liquid interface, and on dry films, as a single molecule Donor-Acceptor system. In addition, in collaboration with the group of Prof. Enrico Dalcanale at the University of Parma (Italy), we have extended our studies of self assembly on surfaces to cavitand compounds as system that can from, through host-guest recognition, supramolecular polymers with controlled curvature. Cavitands are indeed versatile molecular building blocks for host-guest assemblies. The design, crystal structure determination and self-assembly in solution and on surfaces of supramolecular polymers from cavitand based compounds have been subject of investigation. Studies in solution provided evidence for the formation of polymers from specifically designed cavitands, and the aggregation behavior was quantified. The successful translation of the given self-assembly procedure from solution to surfaces required a comprehensive understanding and control over various boundary conditions. Rod-like architectures have been in this way visualized on surface. A bending in two dimensions of the single rods has also been observed and it has been correlated to the degree of curvature of the polymer as obtained form simulation studies. In summary, we have studied the self organization of a variety of molecular systems into supramolecular architectures on surfaces, exploring structural, electrical and electronic properties down to the nanoscale. Triphenylene based compounds have proved to exhibit properties that make them interesting for potential applications as quasi-1D charge carrier systems for electrical conduction. Moreover, we have shown how nucleoside recognition can be exploited to drive the self-assembly of HBCs from solution to surfaces. In addition cavitand compounds have shown to be versatile molecular building blocks to form polymers with potentially controlled curvature

Techniques that measure physical properties at the nanoscale with high sensitivity are significantly limited considering the number of new nanomaterials being developed. The development of atomic force microscopy (AFM) has lead to significant advancements in the ability to characterize physical properties of materials in all areas of science: chemistry, physics, engineering, and biology have made great scientific strides do to the versatility of the AFM. AFM is used for quantification of many physical properties such as morphology, electrical, mechanical, magnetic, electrochemical, binding interactions, and protein folding. This work examines the electrical and mechanical properties of materials applicable to the field of nano-electronics. As electronic devices are miniaturized the demand for materials with unique electrical properties, which can be developed and exploited, has increased. For example, discussed in this work, a derivative of tetrathiafulvalene, which exhibits a unique loss of conductivity upon compression of the self-assembled monolayer could be developed into a molecular switch. This work also compares tunable organic (tetraphenylethylene tetracarboxylic acid and bis(pyridine)s assemblies) and metal-organic (Silver-stilbizole coordination compounds) crystals which show high electrical conductivity. The electrical properties of these materials vary depending on their composition allowing for the development of compositionally tunable functional materials. Additional work was done to investigate the effects of molecular environment on redox active 11-ferroceneyl-1 undecanethiol (Fc) molecules. The redox process of mixed monolayers of Fc and decanethiol was measured using conductive probe atomic force microscopy and force spectroscopy. As the concentration of Fc increased large, variations in the force were observed. Using these variations the number of oxidized molecules in the monolayer was determined. AFM is additionally capable of investigating interactions at the nanoscale, such as ligand-receptor interactions. This work examines the interactions between the enzyme dihydrofolate reductase (DHFR), a widely investigated enzyme targeted for cancer and antimicrobial pharmaceutical, and methotrexate (MTX), a strong competitive inhibitor of DHFR. The DHFR was immobilized on a gold substrate, bound through a single surface cysteine, and maintained catalytic activity. AFM probe was functionalized with MTX and the interaction strength was measured using AFM. This work highlights the versatility of AFM, specifically force spectroscopy for the quantification of electrical, mechanical, and ligand-receptor interactions at the nanoscale.

Le but de cette thèse était tout d'abord de comprendre les théories physiques qui décrivent la dynamique des polymères à l'échelle macroscopique. Le modèle de Rouse et la théorie d'enchevêtrement de De Gennes décrivent la dynamique des polymères non enchevêtrés et enchevêtrés, respectivement. Nous avons étudiés les différentes transitions entre ces deux régimes en utilisant deux techniques expérimentales: Broadband Dielectric Spectroscopy (BDS) et rhéologie. Les effets d'enchevêtrement sur les spectres diélectriques ont été discutés. Un test complet du modèle de Rouse à été effectué sur en comparant les prédictions de ce modèle pour la dépendance en fréquence de la permittivité diélectrique et du module de cisaillement aux données expérimentales. Ensuite nous avons développés des méthodes bas"s sur la microscopie à force électrostatique afin d'étudier les propriétés diélectriques locales. En utilisant la simulation numérique de la Méthode des Charges Equivalentes la constante diélectrique a été quantifiée à partir de la mesure du gradient de force crée par un potentiel statique entre une pointe et un diélectrique. Cette méthode permet d'imager la constante diélectrique avec une résolution spatial de 40 nm. Le retard de phase de la composante en 2ω de la force ou du gradient de force crée par un voltage alternatif est relié aux pertes diélectriques. En mesurant cette quantité nous avons montré que la dynamique était plus rapide proche d'une interface libre et nous avons développé un mode d'imagerie des pertes diélectriques. Cette méthode simple pourrait être appliqué en biologie ou matière molle en générale afin d'étudier des variations locales de constantes diélectriques
The aim of this thesis was first to understand the physical theories that describe the dynamics of linear polymers at the macroscopic scale. Rouse and the reptational tube theory describe the large scale dynamics of unentangled and entangled polymers respectively. Using Broadband Dielectric Spectroscopy (BDS) and rheology we have studied the different transition between these two regimes. Effects of entanglement on dielectric spectra will be discussed (Rheologica Acta. 49(5):507-512). Avoiding the segmental relaxation contribution and introducing a distribution in the molecular weight we have been able to perform a comparison of the Rouse model with experiment dielectric and rheological data (Macromolecules 42(21): 8492-8499) Then we have developed EFM-based methods in order to study the local dynamics. Using the numerical simulation of the Equivalent Charge Method, the value of the static dielectric permittivity has been quantified from the measurement of the force gradient created by a VDC potential between a tip and a grounded dielectric (Journal of Applied Physics 106(2):024315). This method allows a quantitative mapping of dielectric properties with a 40 nm spatial resolution and is therefore suitable for the study of nano-defined domains (Physical Review E 81(1): 010801). The electrical phase lags in the 2ω components of the force or force gradient created by VAC voltage, ΔΦ2ω, are related with dielectric losses. Measuring the frequency dependence of ΔΦ2ω Crieder et al (Applied Physics Letters 91(1):013102) have shown that the dynamics at the near free surface of polymer films is faster than the one in bulk. We have used this method in order to visualize the activation of the segmental relaxation with temperature and frequency (Applied Physics Letters 96(21): 213110). All this measurements can be achieved using standard Atomic Force Microscope (and a lock-in) for VAC measurements

This thesis describes the effects of a post-growth hydrogenation on as-grown samples and device structures based on III-N-V and III-V semiconductor compounds. The spectral response of quantum wells (QWs) or superlattices (SLs) are tuned by the control dissociation of N-H complexes using a focused laser beam (photon assisted dissociation) or by thermal annealing. These approaches could be implemented in other materials and heterostructure devices, and offer the advantage of enabling an accurate control of the spectral response of a device using a layer compound with a single N- concentration. A focused laser beam is also used to diffuse hydrogen from the p-type contact layer towards the III-N-V superlattice in the intrinsic region of a p-i-n diode, thus creating preferential injection paths for the carriers and creating nanoscale light emitting diodes. Opportunities for realizing a movable micron size-light emitting diode (-LED) are also demonstrated. Moreover, room temperature electroluminescence from semiconductor junctions formed from combinations of n-InSe, p-InSe, p-GaSe and n-In2O3 is demonstrated. These p-n junctions are fabricated using mechanical exfoliation of Bridgman-grown crystals and a simple mechanical contact method or thermal annealing. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct band gap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum. These layered crystals could be combined in different sequences of layer stacking, thus offering exciting opportunities for new structures and devices.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, June 2014.
Cataloged from PDF version of thesis. "June 2014."
Includes bibliographical references (pages 172-194).
Gallium nitride (GaN) and indium gallium nitride (InGaN) nanowires promise potential for further improving the electricity-to-light energy conversion efficiencies in light emitting diodes due to strain relaxation, reduced density of structural defects, and easier light extraction. Material quality and effective band engineering of such III-nitride nanowires are crucial for the design and fabrication of their optoelectronic applications such as LEDs, lasers and photodetectors. In this thesis, we first demonstrate effective control over GaN nanowire size, growth rate and structural quality through careful choice of metal seed particles. The differences in morphology, structural defects and optical properties of GaN nanowires grown by metalorganic chemical vapor deposition were studied systematically by electron microscopy and photoluminescence, and related to supersaturation in different seed particles and nanowire nucleation mechanisms. These results also demonstrate that systematic screening of seed materials is essential for synthesizing nanostructures with defect-free structures and other functional heterostructures. Next, challenges for nanoscale mapping of band engineering were successfully addressed through direct spatial correlation of optical properties to a variety of III-nitride heterostructures grown by molecular beam epitaxy, including GaN p-n junction nanorods, InGaN nanodisks, and GaN quantum disks and quantum wires. We demonstrate that effective doping, alloying and quantum confinement can be readily achieved in nanowire heterostructures, by cathodoluminescence in scanning transmission electron microscopy. P-n junction position and carrier diffusion lengths inside a single GaN nanorod were determined with nanometer spatial resolution. InGaN disk compositional uniformities were quantified from their optical emissions, which revealed substantial compositional inhomogeneity in bottom-up synthesized nanostructures. The studies on optical properties of individual GaN quantum structures demonstrated that small differences in the degree of quantum confinements resulted in substantial changes in the optical band gap. More importantly, reduced light emissions are directly correlated to regions containing grain boundaries, dislocations and stacking faults, which were formed as a result of nanorod coalescence and fluctuations in growth environment during nanostructure synthesis. Our findings demonstrate that controlling compositional and structural homogeneity, understanding defect formation mechanism and their effects on materials properties are key challenges to be addressed for developing large scale functional devices based on bottom-up synthesized nanostructured materials.
by Xiang Zhou.
Ph. D.

This research developed a CrN/NbN coating with promising properties for tribological applications where corrosion plays a big role. Building on this knowledge base a novel approach to the surface treatment was selected, in which multiple layers, each chosen for a specific purpose, were optimised to combine the best properties of each. In this approach a careful consideration of macro- and microstructure of each layer is required in order to extract the good properties of each layer while eliminating the negative ones. It was shown in the work that, if such consideration is neglected, a catastrophic failure may follow. For example poor adhesion may cause a total failure of the coating. As the number of layers and interfaces increase a good understanding on the structure and the properties of each layer becomes very important as the number of parameters and possible combinations increase many times. In this whole work the intention was to take a very practical approach to the coating. The objective was to combine different approaches, such as duplex treatments and multi layering and investigate the specific interactions that are not otherwise apparent. The results of this work show that such an approach is viable and should lead to excellent results as long as the wear mechanisms of the coating are understood and the coating is correctly engineered for the application.

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).

Restoring both anterior and posterior teeth with resin-composite materials is now an established clinical procedure with almost universal acceptance. The clinical performance of these restorations in the patient’s mouth is determined by a number of factors including the clinical techniques involved in their placement, the patient’s oral habits, and the physical and mechanical properties of the restorative materials themselves. These materials are being increasingly used in load-bearing areas of the posterior dentition and are therefore inevitably subject to masticatory forces of varying magnitude. The success of different resin-composites in different applications is understood through their clinical performance and laboratory-based experimental evaluation.My research was divided into two parts; the first part was concerned with the examination of different types of contemporary restorative resin-composites and in the second part, I compared different methods of examination. In the first part, I investigated and compared different sets of varied types of resin-composites, such as flowable resin-composites, bulk-fill resin-composites and conventional resin-composites. Using different sets of these materials, I examined a number of properties that affect their clinical performance and durability.In the second part, I studied and compared the conventional (macroscopic) methods of investigation with nanoindentation method. Both methods were applied to examine and characterise different properties for some types of resin-composites.The flowable and the bulk-fill resin-composites exhibited satisfactory results comparable with conventional resin-composites. The properties investigated included strength properties, modulus of elasticity, hardness and viscoelastic time-dependent creep deformation. The results obtained by nanoindentation confirmed that this method of examination is a valuable experimental tool to investigate and characterise some mechanical properties of resin-composites.

The complex of properties including the structure, adhesive strength, internal stresses, tribological properties, microhardness and crack-resistance of nanoscale carbon coatings obtained by the pulsed vacuum-arc method on single-crystal silicon substrates was investigated. Two types of samples of the carbon coating: type (i) formed at the normal location of the substrate relative to the geometric axis of the plasma flow (θ = 0°); type (ii) obtained at an angle θ = 70° were studied. The analysis of the experimental results showed, that the angle of plasma flow incidence relative to the substrate drastically affects the properties of carbon coatings. The structure, adhesion, internal stresses, wear resistance, crack resistance are interrelated and determined by the radiation-diffusion sealing during the process of carbon coating deposition from the carbon plasma flow. Nanoscale carbon coatings can significantly improve the strength and tribological properties of different tools, parts and products. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35305

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.

A negative index material (NIM), which possesses simultaneously negative permittivity and permeability, is an emerging material that has caught many researchers attention after it was first demonstrated in 2001. It has been shown that electromagnetic waves propagating in NIMs have some remarkable properties such as negative phase velocities and negative refraction and hold enormous promise for applications in imaging and optical communications. This dissertation is centered on investigating the unique aspects of the radiative properties of NIMs. Photon tunneling, which relies on evanescent waves to transfer radiative energy, has important applications in thin-film structures, microscale thermophotovoltaic devices, and scanning thermal microscopes. With multilayer thin-film structures, photon tunneling is shown to be greatly enhanced using NIM layers. The enhancement is attributed to the excitation of surface or bulk polaritons, and depends on the thicknesses of the NIM layers according to the phase matching condition. A new coherent thermal emission source is proposed by pairing a negative permittivity (but positive permeability) layer with a negative permeability (but positive permittivity) layer. The merits of such a coherent thermal emission source are that coherent thermal emission occurs for both s- and p-polarizations, without use of grating structures. Zero power reflectance from an NIM for both polarizations indicates the existence of the Brewster angles for both polarizations under certain conditions. The criteria for the Brewster angle are determined analytically and presented in a regime map. The findings on the unique radiative properties of NIMs may help develop advanced energy conversion devices. Motivated by the recent advancement in scanning probe microscopy, the last part of this dissertation focuses on prediction of the radiation heat transfer between two closely spaced semi-infinite media. The objective is to investigate the dopant concentration of silicon on the near-field radiation heat transfer. It is found that the radiative energy flux can be significantly augmented by using heavily doped silicon for the two media separated at nanometric distances. Large enhancement of radiation heat transfer at the nanoscale may have an impact on the development of near-field thermal probing and nanomanufacturing techniques.

In the past two decades much effort has been put in the characterization of the mechanical

and surface properties at the nano-scale in order to conceive reliable N/MEMS

(Nano and Micro ElectroMechanical Systems) applications. Techniques like nanoindentation,

nanoscratching, atomic force microscopy have become widely used to measure

the mechanical and surface properties of materials at sub-micro or nano scale. Nevertheless,

many phenomena such us pile-up and pop-in as well as surface anomalies

and roughness play an important role in the accurate determination of the materials

properties. The first goal of this report is to study the infulence of these sources of data

distortion on the experimental data. The results are discussed in the first experimental

chapter.

On the other hand, conceptors would like to adapt/tune the mechanical and surface

properties as a function of the required application so as to adapt them to the industrial

need. Coatings are usually applied to materials to enhance performances and reliability

such as better hardness and elastic modulus, chemical resistance and wear resistance.

In this work, the magnetron sputtering technique is used to deposit biocompatible thin

layers of different compositions (titanium carbide, titanium nitride and amorphous

carbon) over a titanium substrate. The goal of this second experimental part is the

study of the deposition parameters influence on the resulting mechanical and surface

properties.

New materials such as nanocrystal superlattices have recently received considerable

attention due to their versatile electronic and optical properties. However, this new

class of material requires robust mechanical properties to be useful for technological

applications. In the third and last experimental chapter, nanoindentation and atomic

force microscopy are used to characterize the mechanical behavior of well ordered lead

sulfide (PbS) nanocrystal superlattices. The goal of this last chapter is the understanding

of the deformation process in order to conceive more reliable nanocrystal

superlattices.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Nanoscale characterization of the piezoelectric and polarization related properties of III-Nitrides by piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) resulted in the measurement of piezoelectric constants, surface charge and surface potential. Photo-electron emission microscopy (PEEM) was used to determine the local electronic band structure of a GaN-based lateral polarity heterostructure (GaN-LPH). Nanoscale characterization of the imprint and switching behavior of ferroelectric thin films by PFM resulted in the observation of domain pinning, while nanoscale characterization of the spatial variations in the imprint and switching behavior of integrated (111)-oriented PZT-based ferroelectric random access memory (FRAM) capacitors by PFM have revealed a significant difference in imprint and switching behavior between the inner and outer parts of capacitors. The inner regions of the capacitors are typically negatively imprinted and consequently tend to switch back after being poled by a positive bias, while regions at the edge of the capacitors tend to exhibit more symmetric hysteresis behavior. Evidence was obtained indicating that mechanical stress conditions in the central regions of the capacitors can lead to incomplete switching. A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively) has been used to map the out-of-plane and in-plane polarization distribution, respectively, of integrated (111)-oriented PZT-based capacitors, which revealed poled capacitors are in a polydomain state.

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).

The surface properties of minerals have important implications in geology, environment, industry and biotechnology and for certain aspects in the research on the origin of life. This research project aims to widen the knowledge on the nanoscale surface properties of chlorite and phlogopite by means of advanced methodologies, and also to investigate the interaction of fundamental biomolecules, such as nucleotides, RNA, DNA and amino acid glycine with the surface of the selected phyllosilicates. Multiple advanced and complex experimental approaches based on scanning probe microscopy and spatially resolved spectroscopy were used and in some cases specifically developed. The results demonstrate that chlorite exposes at the surface atomically flat terraces with 0.5 nm steps typically generated by the fragmentation of the octahedral sheet of the interlayer (brucitic-type). This fragmentation at the nanoscale generates a high anisotropy and inhomogeneity with surface type and isomorphous cationic substitutions determining variations of the effective surface potential difference, ranging between 50-100 mV and 400-500 mV, when measured in air, between the TOT surface and the interlayer brucitic sheet. The surface potential was ascribed to be the driving force of the observed high affinity of the surface with the fundamental biomolecules, like single molecules of nucleotides, DNA, RNA and amino acids. Phlogopite was also observed to present an extended atomically flat surface, featuring negative surface potential values of some hundreds of millivolts and no significant local variations. Phlogopite surface was sometimes observed to present curvature features that may be ascribed to local substitutions of the interlayer cations or the presence of a crystal lattice mismatch or structural defects, such as stacking faults or dislocation loops. Surface chemistry was found similar to the bulk. The study of the interaction with nucleotides and glycine revealed a lower affinity with respect to the brucite-like surface of chlorite.

Les alliages métalliques complexes (CMAs) sont des composés intermétalliques dont la structure cristallographique diffère de celle des alliages conventionnels par le nombre conséquent d'atomes dans la maille (jusqu'à plusieurs milliers d'atomes), généralement arrangés sous forme d'agrégats atomiques de haute symétrie. Ils sont prometteurs pour un certain nombre d'applications technologiques, en particulier les revêtements fonctionnels, en raison de leurs propriétés de surface uniques. Cette thèse a pour objectif, à la fois la détermination de la structure et des propriétés électroniques d’une surface d’un CMA de la famille des clathrates intermétalliques, et des propriétés de mouillage intrinsèques de plusieurs CMAs à base d’aluminium. Dans une première partie, nous nous sommes intéressés aux surfaces de bas indice (100) et (110) du clathrate Ba8Au5.25Ge40.75. Leurs structures atomiques et électroniques ont été déterminées en combinant des expériences de sciences des surfaces et des calculs basés sur la théorie de la fonctionnelle de la densité. La structure tridimensionnelle de Ba8Au5.25Ge40.75, formée d'un réseau de deux types de cages (structure hôte) à base de germanium et d’or, qui emprisonnent les atomes de Ba, induit une nanostructuration de la surface contrôlée par son orientation, puisque le type de cages préservées à la surface diffère pour les surfaces (100) et (110). Dans les deux cas, les atomes de Ba qui protrudent à la surface, ont un rôle primordial pour la stabilité de surface : ils assurent un transfert de charge qui sature les liaisons pendantes des atomes de germanium en surface. Dans une seconde partie, les propriétés intrinsèques de mouillage de plusieurs CMAs à base d’aluminium, ont été déterminées par une approche couplant des mesures de microscopie et des calculs ab initio. Expérimentalement, les angles de contact de gouttes de plomb (métal sonde) sur plusieurs surfaces de CMAs ont été systématiquement mesurés. Les angles précédents étant fonction, entre autres, de l’énergie interfaciale, des calculs d'énergie interfaciale ont été menés, d’une part avec un substrat d’un métal simple, Al(111), et d’autre part sur un substrat de CMA, Al13Co4(100). Les résultats obtenus mettent en évidence une forte influence de la structure de l’interface sur l’énergie interfaciale
Complex metallic alloys (CMAs) are intermetallic compounds possessing a large unit cell containing several tens to hundreds of atoms. Their structure can be described alternatively by the packing of highly symmetric atomic clusters. Clathrate (or cage) compounds are a new class of CMAs having a crystal structure described by a complex arrangement of covalently-bonded cages. The Ba8Au5.25Ge40.75 type-I clathrate is one such cage compound, whose bulk properties have been (and still are) extensively explored for thermoelectric applications. In fact, it is possible to tune the compound electronic structure by a fine control of its bulk composition. Regarding the properties of the Ba8Au5.25Ge40.75 surface, information remains scarce if not inexistent. However, it is known that the surfaces of CMAs often exhibit interesting surface properties. To this end, we have studied two low-index surfaces: BaAuGe(100) and BaAuGe(110) by a combination of experimental (XPS; LEED; STM) and computational (DFT) methods. Experimental results show no evidence for surface segregation and LEED patterns are consistent with (1x1) bulk terminations with no surface reconstruction. The interplay between the 3D nano-caged structure and 2D surfaces is investigated. We demonstrate that the surface structures of the two surfaces considered preserve the bulk structure cages in addition to an ordered arrangement of surface Ba atoms. The two surfaces are formed by a breakage of highly directional covalent bonds present within the framework, hence leading to destabilizing dangling bonds. Ab initio calculations show that the surface structure is stabilized through electron charge transfer from protruding Ba to surface Ge and Au atoms, saturating the dangling bonds. This charge-balance mechanism lifts the possible surface reconstruction envisaged. We reveal how the surface nanostructuration is surface orientation dependent. The results indicate that the surface electronic structure of BaAuGe(110) is impacted by the Au surface concentration. The surface models for BaAuGe(100) and BaAuGe(110) present a metallic character and low work function values, useful for further applications. Such structurally complex surfaces may also be used as templates for novel nanoscale architectures. Further in this work, we also applied the state-of-the-art surface science techniques to investigate the wetting properties of Al-based CMAs. In these experiments, chemically inert Pb element was used as a metal probe. Systematic analysis is done to find the correlation between the wetting properties and the electronic structure properties of these CMAs. Interfacial energy calculations have been performed to model the Pb/CMA interface based on few approaches reported in literature. We have tested these approaches on a moiré patterned Pb(111)/Al(111) interface. This interface is found to be controlled by geometric factors. Hence, an acquired understanding was applied to Pb deposited on Al13Co4(100) (Al-rich side) interface

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.
Includes bibliographical references (leaves 123-125).
Polymer nano-clay composites have been observed to exhibit dramatic enhancements in mechanical properties with relatively low filler loadings (1-4 percent volume fraction). These property enhancements have been speculated to be a result of the change in polymer morphology and properties within the polymer/particle interfacial regions, due to the nanometer length scale and the large interface area/unit volume of the nanoparticles. In this work, the potential contribution of composite-level effects on the observed enhancements is explored. Two-dimensional models of various representative volume elements (RVEs) of the underlying structure of the polymer nano-clay composite are constructed. These models are characterized by clay particle volume fraction and micro/nano scale morphological features such as clay particle aspect ratio (length/thickness, L/t), clay particle distribution (random vs. regular patterns) and clay particle orientation distribution. Macroscopic moduli of these RVEs are predicted as a function of these geometrical parameters as well as particle and matrix stiffness parameters through FEM simulations. Effective properties of intercalated clay particles have been estimated in terms of characteristic clay structural parameters (interlayer spacing and number of layers), with additional information from molecular dynamics simulations of silicate layer stiffness. The predictions of macroscopic stiffness from these two-dimensional micromechanical models, based on structure-dependent particle volume fraction and properties, are consistent with experimental observations. Furthermore, studies of the local stress/strain fields show that the stiffness enhancement comes through the efficient load transfer mechanism in the high aspect ratio fillers, modulated by the strain shielding effect in the matrix. These results suggest that physically-based composite level interpretations may explain the stiffness enhancement mechanism of polymer nanocomposites to a large degree. The adopted methodology offers promise for study of related properties in polymer/layered-silicate nanocomposites.
by Nuo Sheng.
S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 109-119).
The laser-based transient grating technique was used to study phonon mediated thermal transport in bulk and nanostructured semiconductors and surface wave propagation in a monolayer of micron sized spheres. In the transient grating technique two picosecond pulses are crossed to generate a spatially periodic intensity profile. The spatially periodic profile generates a material excitation with a well-defined wave vector. The time dependence of the spatially periodic material response is measured by monitoring the diffracted signal of an incident probe beam. Non-diffusive thermal transport was observed in thin Si membranes as well as bulk GaAs at relatively short (micron) transient grating periods. First-principles calculations of the phonon mean free paths in Si and GaAs were compared with experimental results and showed good agreement. Preliminary measurements on promising thermoelectric materials such as PbTe and Bi2Te3 are presented showing evidence of non-diffusive transport at short length scales. The transient grating technique was used to measure the thermal conductivity of Si membranes with thickness ranging from 15 nm to 1518 nm. Using the Fuchs-Sondheimer suppression function along with first-principles results, the thermal conductivity as a function of membrane thickness was calculated. The calculations showed excellent agreement with experimental measurements. A convex optimization algorithm was employed to reconstruct the phonon mean free path distribution from experimental measurements. This marks the first experimental determination of the mean free path distribution for a bulk material. Thermal conductivity measurements at low temperatures in a 200 nm Si membrane indicate the breakdown of the diffuse boundary scattering approximation. The transient grating technique was used to generate surface acoustic waves and measure their dispersion in a monolayer of 0.5 - 1 [mu]m diameter silica spheres. The measured dispersion curves show "avoided crossing" behavior due to the interaction between an axial contact resonance of the microspheres and the surface acoustic wave at a frequency of -200MHz for the 1 [mu]m spheres and -700 MHz for the 0.5 [m spheres. The experimental measurements were fit with an analytical model in which the contact stiffness was the only fitting parameter. Preliminary results of surface acoustic wave propagation in microsphere waveguides, transmission through a microsphere strip, and evidence of a nonlinear response in a 2D array of microspheres are presented.
by Jeffrey Kristian Eliason.
Ph. D.

A numerical model have been developed in order to describe and achieve deeper understanding of experimentally obtained I-V curves from Cu2O/ZnO p-n heterojunctions for potential use as future solar cell material. The model was created using the simulation software COMSOL Multiphysics® and their semiconductor module. To experimentally study the samples two approaches were taken: (1) macro-electrical measurements and (2) local I-V measurements using conductive AFM. The final model is one-dimensional, time dependent and with the ability to study photovoltaic effects of samples with different layer thickness at different voltage ramping speeds and different light irradiance. The model is also able to study the effects of using different contact materials by treating the contacts as ideal Schottky contacts. The dynamic behavior of a Cu_2O/ZnO heterojunction was studied by considering the systems response to a voltage step and the effect of changing the voltage ramping speed. The output from the step response, the current as a function of time, is varying a short time after a step has occurred before settling on to a steady value. The response also shows an overshoot of the current in the direction of the voltage step and the final steady value depends on whether the junction is conducting or not. The effects of this behavior on the shape of the I-V curves are witnessed when studying the different voltage ramping speeds. The voltage is ramped from 2 V to -2 V and back again for different speeds (V/s). The I-V curves have different shapes when sweeping the voltage in different directions and the magnitude increases with increasing speed. The photovoltaic effects were studied by applying different light irradiances. The behavior of the model agrees well with the theory for an ideal diode solar cell. An investigation was done of how the work function of the metal in contact with the Cu_2O affects the shape of the I-V curve under dark and illuminated conditions. The metal work function was changed from 4.5 eV to 6.5 eV in steps of 0.4 eV and does not affect the shape of the I-V curves much in dark after increasing it above 4.5 eV. The effects are more visible under illuminated conditions where a "step"-behavior appears for the lower values of the work function. Only one of the physical samples show a noticeable light effect. The macro-electrical measurement on this sample is compared with simulated results and are in qualitative agreement with each other. The agreement between the local electrical measurements and the simulated results is not as good with the current model.

This thesis describes the nano-engineering of self-assembly processes, to accomplish nanoscale metal-organic framework (MOF) systems potentially useful for optoelectronic applications. The well-known fact underlying the large crystal size formation of common MOF compounds has been re-investigated here, not only to minimise the prolonged synthesis time but also to yield facile deposition in uniform thin film formats. This thesis presents the innovative concept of high concentration reaction (HCR) and its utility for making nano MOFs. Surprisingly, this method resulted in formation of a new kind of gel-like soft materials, for which I coined the term - supraMOFs - meaning supramolecular MOF hybrid materials. A detailed study of supraMOFs focussing on their constituent nano MOF elements, stimuli-responsive behaviours, and sol-gel conversion phenomena have been systematically performed. Akin to low molecular weight gels (LMWG), mechanical properties of supraMOFs displaying storage modulus (G') > loss modulus (G") have been confirmed by rheological experiments. The use of sol-gel MOF system to attain a uniform MOF film, has been demonstrated through an example of HKUST-1 thin coating of ~10 nm roughness and ~1 μm thickness. The HCR concept was further extended to develop advanced functional nano MOF systems, where one-step rapid synthesis of fluorescent MOF nanosheets has been accomplished. The old but effective concept of functionalisation of MOFs using porous coordination space and external guest species was implemented here, but with a twist. Particularly, the challenge of caging larger sized guest species into smaller pore apertures of MOFs, in parallel to controlling material growth in the nanoscale regime were solved by adopting the HCR approach. Furthermore, this thesis has demonstrated new interesting possibilities employing fluorescent nano MOF system to engineer smart sensors for detecting volatile organic compounds (VOCs), and mechanical stresses via a mechanochromic luminescent MOF nanoplate system.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 137-159).
Although InGaN/GaN-based quantum well (QW) heterostructures continue to set the industry standard for inorganic blue and green light emitting diodes (LEDs), these devices suffer from efficiency droop at high current densities and material quality degradation at longer emission wavelengths. Establishing rational process design principles to address such issues remains inhibited by ongoing controversy surrounding the impact of commonly observed defects such as well-width fluctuations or V-pit defects on carrier recombination. Organic-inorganic perovskites have begun to attract attention as a potential next-generation LED material, but these nascent materials suffer from rapid material degradation under device operating conditions. Understanding structure-property correlations will be necessary to improve incumbent InGaN/GaN technologies and evaluate the potential of organic-inorganic perovskites.
In InGaN/GaN QW heterostructures, we first employ aberration-corrected scanning transmission electron microscopy (STEM) to examine the impact of well-width fluctuations and QW period on measured EQE and find no significant correlation. Next, we observe time-delayed cathodoluminescence (CL) rise dynamics in droop-mitigating QW designs and propose a model linking rise behavior to carrier transport and deep level defects. Finally, we use CL-STEM to map radiative recombination around commonly observed V-pit defects with nanoscale spatial and spectral resolution. Furthermore, dark field diffraction contrast imaging elucidates the relationship between V-pit optical emission and threading dislocation character. These results provide a platform for evaluating the impacts of microstructural defects on LED device performance. In methylammonium lead iodide, we use STEM imaging to establish a direct correlation between local stoichiometry and CL intensity.
We demonstrate that areas of high CL intensity correspond to regions which are enriched in iodide content relative to lead. Furthermore, CL-STEM imaging reveals the presence of localized high-energy emissions which we attribute to beam-induced ion migration. The continuous evolution of such high-energy emissions under electron beam irradiation suggests these local spectral heterogeneities could reflect material evolution during device degradation.In summary, the current work demonstrates novel insights gained by the application of advanced electron imaging techniques to two vastly different materials systems. Our findings suggest that continued improvements in process design will hinge on controlling the distribution of structural defects in order to minimize undesirable recombination pathways.
by Zhibo Zhao.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Studying physical properties of nanoscale materials has gained a significant attention owing to their applications in the fields such as electronics, medicine, pharmaceutical industry, and materials science. However, owing to size constraints, number of techniques that measures physical properties of materials at nanoscale with a high accuracy and sensitivity is limited. In this context, development of atomic force microscopy (AFM) based techniques to measure physical properties of nanomaterials has led to significant advancements across the disciplines including chemistry, engineering, biology, material science and physics. AFM has recently been utilized in the quantification of physical-chemical properties such as electrical, mechanical, magnetic, electrochemical, binding interaction and morphology, which are enormously important in establishing structure-property relationship. The overarching objective of the investigations discussed here is to gain quantitative insights into the factors that control electrical and mechanical properties of nano-dimensional organic materials and thereby, potentially, establishing reliable structure-property relationships particularly for organic molecular solids which has not been explored enough. Such understanding is important in developing novel materials with controllable properties for molecular level device fabrication, material science applications and pharmaceutical materials with desirable mechanical stability. First, we have studied electrical properties of novel silver based organic complex in which, the directionality of coordination bonding in the context of crystal engineering has been used to achieve materials with structurally and electrically favorable arrangement of molecules for an enhanced electrical conductivity. This system have exhibited an exceptionally high conductivity compared to other silver based organic complexes available in literature. Further, an enhancement in conductivity was also observed herein, upon photodimerization and the development of such materials are important in nanoelecrtonics. Next, mechanical properties of a wide variety of nanocrystals is discussed here. In particular, an inverse correlation between the Young’s modulus and atomic/molecular polarizability has been demonstrated for members of a series of macro- and nano-dimensional organic cocrystals composed of either resorcinol (res) or 4,6-di-X-res (X = Cl, Br, I) (as the template) and trans-1,2-bis(4-pyridyl)ethylene (4,4’-bpe) where cocrystals with highly-polarizable atoms result in softer solids. Moreover, similar correlation has been observed with a series of salicylic acid based cocrystals wherein, the cocrystal former was systematically modified. In order to understand the effect of preparation method towards the mechanical properties of nanocrystalline materials, herein we have studied mechanical properties of single component and two component nanocrystals. Similar mechanical properties have been observed with crystals despite their preparation methods. Furthermore, size dependent mechanical properties of active pharmaceutical ingredient, aspirin, has also been studied here. According to results reduction in size (from millimetre to nanometer) results in crystals that are approximately four fold softer. Overall, work discussed here highlights the versatility of AFM as a reliable technique in the electrical, mechanical, and dimensional characterization of nanoscale materials with a high precision and thereby, gaining further understanding on factors that controls these processes at nanoscale.

In this thesis, the coal microstructure was investigated to better understand the related nanoscale rock-mechanical properties, and morphological changes for different effective stresses and during water and CO2 adsorption, including related porosity and permeability changes. Using in-situ reservoir condition x-ray micro-computed tomography imaging, it was observed for the first time how the micro cleat structure inside the coal matrix closed induced by supercritical CO2 flooding in-situ, and associated fracturing of the mineral phase.