A detailed understanding of the corneal biomechanical response is an important prerequisite to understanding corneal diseases such as keratoconus and for placing the empirical equations used in refractive surgery on a physical basis. We have assembled a combined nonlinear optical microscopy (NLOM) and optical coherence microscopy (OCM) imaging system to simultaneously capture coregistered volumetric images of corneal morphology and biochemistry. Fudicial markers visible in the OCM volume enabled the calculation of strains for multiple depth layers in rabbit cornea. The results revealed a depth dependent strain distribution, with smaller strains in the anterior stroma and larger strains in the posterior stroma. The stress-strain curves can be grouped readily by depth into three groups: anterior (~20%), transitional mid (~40%), and posterior (~40%). Cross-sectional images of collagen lamellae, visible in NLOM, showed inhomogeneous collagen structure and its response to intraocular pressure along the anterior-posterior direction. The inhomogeneities correlate well with the noted heterogeneous corneal mechanical properties. The combined NLOM-OCM system can measure corneal microstructure and mechanical response uniquely, thus providing a microstructural understanding of corneal response to changes of collagen structure.

Fatigue crack growth in ±-Zirconium phosphate/epoxy nanocomposites was investigated. A new fatigue testing technique was implemented for miniature samples. Two different methods “strength of materials and Rayleigh-Ritz - were used in determining the bending stress. The fatigue stress and fatigue life of different nanocomposite specimens were plotted in a traditional stress-life (S-N) curve. It was inferred from the S-N plot that the values obtained from both the methods compare well. The experimental results showed that fatigue life of filled epoxy nanocomposite is more than that of the unfilled epoxy composite. A model for bending stresses, ultimate strength and the number of cycles to failure was obtained to predict a component service life without conducting elaborate tests. Scanning electron examination of the fractured surfaces revealed that the crack takes a tortuous path during its propagation course, indicating crack blunting and crack deflection roles of ZrP and CSR nanofillers, which consequently improve the fracture resistance. In case of the M-ZrP-epoxy systems, delamination of ZrP platelets from surrounding epoxy matrix was proposed as the reason behind crack growth. The improved fracture resistance of these nanocomposites was attributed to the delamination of ZrP platelets and deflection of crack direction. The superior behavior of CSR-ZrP-epoxy composites was attributed to the cavitation process and void coalescence due to CSR particles delamination.

Phase-transformation-induced microstructures, including twin domains, anti-phase domains and inversion domains have been analyzed using the scanning and transmission electron microscopy for BaTiO3, BaCeO3 and CaTiO3 of the perovskite structure. Differential etching rate was taken to identify the ferroelectric domains in tetragonal (t-) BaTiO3. Space group Pbnm (No. 62) usually adopted for the orthorhombic crystals by materials scientists is assumed throughout this research to avoid confusion of the plane and direction indices. Traditional contrast analysis was adopted for determining dislocation Burgers vectors (b) and fault vectors (R) in deformed and phase-transformed perovskites, synthetic ceramics (BaTiO3, BaCeO3 and CaTiO3) as well as natural minerals (CaTiO3), polycrystalline (BaTiO3, BaCeO3 and CaTiO3) as well as single crystal (CaTiO3). Atomic images for the structures of twin boundaries and anti-phase boundaries were taken by high resolution technique and image contrast enhancement was performed using fast Fourier transform. Failure of Friedel¡¦s law is adopted for determining if the crystal belongs to non-centrosymmetric point groups. Whether the twins are £_-, £\- or £k-type (i.e. anti-phase domain boundaries) is analysed from the contrast of extreme fringe patterns. Tilting experiments were performed on selected area diffraction patterns containing un-split row of reflections to ensure that the twin boundaries are the reflection or rotation type. Transformation twinning in all perovskites studied here follows the prediction by the relation of point group symmetries between the high- and low-symmetry phases, assuming continuous, diffusionless, second-order transitions that obey the restrictions imposed by the Landau theory of phase transition. Although such predictions of transformation-induced twinning are only permitted when crystallographic group-subgroup relationship exists and structural coherence retains between the high- and low-symmetry phases, experimental observations for r (rhombohedral) ¡÷ o-BaCeO3 and t ¡÷ o in CaTiO3 that are not related by group-subgroup, c (cubic) ¡÷ t (tetragonal) in CaTiO3 and and c (cubic) ¡÷ t (tetragonal) in BaTiO3 that are related by group-subgroup, are all consistent with the predictions from loss of point group symmetry elements and change of unit cell volume. In order that the Landau theory is conformed, however, an intermediate phase of either the lowest common supergroup (cubic Pm m) or highest common subgroup (monoclinic C2/c), with phase transition experiencing multistage pathways suggested by Christy and assumption of non-disruption conditions proposed by Guymont, was identified to bridge between two structures, such as rhombohedral and orthorhombic that are not group-subgroup related. Both the 90o and 180o ferroelectric twin domains in t-BaTiO3 are the reflection type and have been identified in pressureless-sintered ceramics. Further, fault vectors (R = £`<110]) for such domain boundaries were determined, boundary planes of {110) for the former, {100) and {220) for the latter deduced accordingly. The polar c-direction between adjacent domains was determined by differential etching rate across domain boundaries, convergent beam electron diffraction was also adopted for identification and confirmation of the c-axis for two types of domains in t-BaTiO3. Plastic deformation resulting from the thermodynamic driving force for sintering (?p) intensified by a multiplication factor £p) was evidenced microstructurally from analysing dislocations in pressureless-sintered BaTiO3 where a Frank-Read source was observed. Slip systems are activated for the effective stress acting on the slip plane along the slip direction has exceeded the critical value of resolved shear stress (£nCRSS) and yielding occurs. It has contributed to densification, i.e. the overall system shrinkage of a green powder compact, although if such contribution is at all significant requires studies of sintering kinetics to ascertain. Dislocation dissociation into the scallop-shaped half partials according to the following reactions is determined from analysing corresponding Burgers vectors. [010] + [001] ¡÷ [011] [001] + [10 ] ¡÷ [100] [001] + [110] ¡÷ [111] Both transformation twins lying in {110) and {112) and anti-phase domain boundaries with R = 1/2<111> are detected in o-BaCeO3. For orthorhombic (o-) BaCeO3, fault vectors of the latter R = 1/2<111> determined by contrast analysis was confirmed by high-resolution imaging, but on the contrary, fault vectors the former R = £`<110] and £`<021], respectively, could not be determined from such images. Utilizing the technique of large-angle convergent beam electron diffraction, such fault vectors and dislocation Burgers vectors determined by traditional contrast analysis have been confirmed. Both twinning and dislocations were observed in hot-pressed CaTiO3 prepared in a multi-anvil apparatus. Such twins are deformation twins since hot-pressing was conducted in the orthorhombic stable phase field at 1000oC under 8 GPa. Since fault vectors R = £`<110] determined for {112) and {110) twins are different from the transformation-induced twins in o-CaTiO3, R = £`<021] determined for the {112) twinning in natural perovskite may serve as a diagnostic feature for the deformation twins. Plastic deformation in hot-pressured sample was contributed by both slip and twinning. Slip occurred via slip systems with dislocations of b = [110] gliding in (110) is therefore {110}o <1 0>o (equivalent to {100}pc <001>pc, where pc for pseudo-cubic) often found in perovskites deformed at high temperatures. Another set of dislocations with b = [001] in screw orientation was also determined. APB with R = 1/2<111> detected in natural minerals suggests that the phase transition sequence in CaTiO3 is better described by: (c) ¡÷ t (I4/mcm) ¡÷ o (Pbnm) and such APB are generated from loss of the lattice point at I-centre (1/2,1/2,1/2) in the absence of a second orthorhombic Cmcm between t-I4/mcm and o-Pbnm reported before from neutron and X-ray powder diffraction studies.

Many proteins are known to actively interact with biological, as well as inorganic and synthetic surfaces that are widely used in nano- and bio-technology as biosensing platforms and in tissue engineering. Amyloid fibrils are insoluble protein aggregates in beta-sheet conformation that are implicated in at least 20 diseases for which no cure is currently available. The molecular mechanism of fibril formation, as well as the mechanism of fibril clusters interacting with lipid membrane surfaces is currently unknown. The lipid membrane surface has a complex biochemical composition and is also electrostatically non-homogeneous. Currently, the experimental data available for amyloid fibril formation both on lipid and artificial surfaces is limited. The goal of our study is to investigate how the physical properties of the surfaces affect binding of amyloid peptides and affect the fibril formation. We seek to elucidate the effect of electrostatic interactions of amyloid peptides with surfaces using Atomic Force Microscopy (AFM) and Kelvin probe force microscopy (KPFM). We show using KPFM that electrostatic domains readily form within biological systems such as lung surfactant and lipid monolayers. We compared three different implementations of KPFM to demonstrate that frequency modulated (FM-) KPFM provides significant advantages over other modes. We also present a study of Amyloid beta (1-42) fibril formation on model surfaces, which are uniformly charged or possess periodicity of charges and hydrophobic functionality based on thiol self-assembly. Effect of membrane composition, surface charge, and presence of steroids will be discussed.

The effects of morphology on electronic and optical properties in solution processable photovoltaic (PV) materials have been studied through two different approaches. One approach, scanning photocurrent (PC) and photoluminescence (PL) microscopy, involved mapping PC generation and PL in functional PV devices on the length scale of around 250-500 nm. Additionally, local diode characteristics were studied from regions of interest in the PV through local voltage-dependent photocurrent (LVPC) measurements. In a PV made from a Copper Indium Gallium Selenide (CIGS) nanocrystal (NC) "ink", two morphological features were found to cause the spatial heterogeneity in PC generation. Cadmium Sulfide (CdS) aggregates lowered PC generation by blocking incident light to the photoactive layer, and cracks in the CIGS-NC film enhanced PC generation through improved charge carrier extraction. LVPC measurements showed all regions to have similar diode characteristics with the main difference being the PC generated at zero bias voltage. For another PV made from a donor/acceptor blend of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl- 1,4-phenylenediamine (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), two incident laser wavelengths were used to selectively illuminate only one or both polymers. The results showed that when F8BT is illuminated, the PFB-rich regions produced the most PC and when both polymers are illuminated (but mostly PFB), the F8BT-rich regions produce the most PC; showing PC generation is more affective when less absorber material is present in the morphology. The other approach to study morphological effects on PV properties was to fabricate particles that mimicked morphological variations known to occur in solution-processable PVs. Through solution processing of an oligothiophene molecule, a range of weakly coupled H-aggregate particles were made. These particles, identifiable by shape, were shown to have a varying degree of energetic disorder (as gauged by the 0-0 vibronic band intensity in the emission spectrum), despite all particles showing a similarly high degree of molecular order from fluorescence dichroism (FD) measurements. A trend was observed correlating a decrease in energetic disorder with an increase in the local contact potential (LCP) difference as measured with Kelvin probe force microscopy (KPFM). The LCP difference was found to range by 70 mV between particles of moderate to low energetic disorder.

This dissertation will demonstrate the synergy of nanoscopic materials and surface metrology methods by the fabrication and implementation of CNT atomic force microscopy (AFM) tips, CNT scanning tunneling microscopy (STM) tips, Pt spike AFM tips, and Pt spike near-field scanning optical microscopy (NSOM) tips for the methods of critical dimension metrology, STM, AFM phase imaging, scanning surface potential AFM (SSPM), NSOM, and three-dimensional AFM. Chapter 1 provides a general overview of the information that will be discussed in this dissertation. Chapter 2 describes two methods for the simultaneous fabrication of carbon nanotube atomic force microscopy and scanning tunneling microscopy probes. The fabrication of these high resolution probes, as well as their imaging characteristics, is described in detail. Resolution standards were used to characterize their behavior and resolution limits. In Chapter 3, the effect of high aspect ratio probe length on AFM phase imaging is studied by fabricating highly controllable Pt spike AFM tips. By monitoring phase shifts on homogenous surfaces as a function of Pt spike length, it is shown that attractive forces at the tip are significantly reduced when high aspect ratio structures are added to conventional AFM probes. In Chapter 4, the effect of probe geometry on scanning surface potential microscopy (SSPM) is described. By studying the effect of scan height in SSPM, it was found that large surface area probe geometries, such as conventional Pt coated AFM tips, have lower surface potential resolution because of contributions from the sides of the tip as well as the cantilever. Spatial resolution standards were probed to evaluate the effect of probe geometry on SSPM sensitivity and resolution. Chapter 5 describes the fabrication of specialized probes for three-dimensional atomic force microscopy, scanning near-field optical microscopy, and scanning electrochemical -- atomic force microscopy (SECM-AFM). Using techniques described in Chapters 2-4, high aspect ratio structures were added to conventional probes used in 3D AFM, NSOM and SECM-AFM to solve limitations inherent to current probe designs for each method. Preliminary data indicates that each probe will have a significant beneficial effect on the resolution limit of its technique.

published_or_final_version / Anatomy / Doctoral / Doctor of Philosophy

No description available.

Epitaxial growth, or the oriented growth of a crystalline monolayer on an ordered substrate, appears in a wide range of systems and applications, from novel device fabrication to freshwater remediation. Despite this, methodical studies of the phenomenon are rare, and the mechanisms governing epitaxial growth are poorly understood. This investigation employs AFM techniques to monitor the epitaxial growth of ion crystal systems at the initial stages of growth. By using systems with well-known physical properties, we are able to relate growth modes to two key parameters, crystal lattice mismatch, Δr/r₀, and affinity between the overgrowth and the substrate ions, ξ. We found wetting growth occurs for systems in which Δr/r₀ is expansive (overgrowth lattice must expand to accommodate substrate) or mildly compressive (overgrowth compresses to accommodate substrate). Additionally, a strong affinity between the substrate and overgrowth ions, in combination with an expansive system, allows for epitaxial growth from undersaturated solutions. We also have observed several instances where the lateral force contrast on the growing film exhibits a strong dependence on the time of exposure to the growth solution and on the driving force for growth (solute concentration). We present results for three epitaxial growth systems in aqueous solutions: CaSO₃ on CaCO₃, PbSO₄ on BaSO₄, and BaSO₃ on BaSO₄. Chemically and topographically identical regions grown at higher concentrations exhibit higher friction than regions grown at lower concentrations. These observations suggest that epitaxial growth occurs by a fast condensation step incorporating a high defect density.

No description available.