Dissertations / Theses on the topic 'High Resolution Scanning Electron Microscopy (HRSEM)'

Epitaxially grown SiGe(C) materials have a great importance for many device applications. In these applications, (strained or relaxed) SiGe(C) layers are grown either selectively on the active areas, or on the entire wafer. Epitaxy is a sensitive step in the device processing and choosing an appropriate thermal budget is crucial to avoid the dopant out–diffusion and strain relaxation. Strain is important for bandgap engineering in (SiGe/Si) heterostructures, and to increase the mobility of the carriers. An example for the latter application is implementing SiGe as the biaxially strained channel layer or in recessed source/drain (S/D) of pMOSFETs. For this case, SiGe is grown selectively in recessed S/D regions where the Si channel region experiences uniaxial strain.The main focus of this Ph.D. thesis is on developing the first empirical model for selective epitaxial growth of SiGe using SiH2Cl2, GeH4 and HCl precursors in a reduced pressure chemical vapor deposition (RPCVD) reactor. The model describes the growth kinetics and considers the contribution of each gas precursor in the gas–phase and surface reactions. In this way, the growth rate and Ge content of the SiGe layers grown on the patterned substrates can be calculated. The gas flow and temperature distribution were simulated in the CVD reactor and the results were exerted as input parameters for the diffusion of gas molecules through gas boundaries. Fick‟s law and the Langmuir isotherm theory (in non–equilibrium case) have been applied to estimate the real flow of impinging molecules. For a patterned substrate, the interactions between the chips were calculated using an established interaction theory. Overall, a good agreement between this model and the experimental data has been presented. This work provides, for the first time, a guideline for chip manufacturers who are implementing SiGe layers in the devices.The other focus of this thesis is to implement SiGe layers or dots as a thermistor material to detect infrared radiation. The result provides a fundamental understanding of noise sources and thermal response of SiGe/Si multilayer structures. Temperature coefficient of resistance (TCR) and noise voltage have been measured for different detector prototypes in terms of pixel size and multilayer designs. The performance of such structures was studied and optimized as a function of quantum well and Si barrier thickness (or dot size), number of periods in the SiGe/Si stack, Ge content and contact resistance. Both electrical and thermal responses of such detectors were sensitive to the quality of the epitaxial layers which was evaluated by the interfacial roughness and strain amount. The strain in SiGe material was carefully controlled in the meta–stable region by implementingivcarbon in multi quantum wells (MQWs) of SiGe(C)/Si(C). A state of the art thermistor material with TCR of 4.5 %/K for 100×100 μm2 pixel area and low noise constant (K1/f) value of 4.4×10-15 is presented. The outstanding performance of these devices is due to Ni silicide contacts, smooth interfaces, and high quality of multi quantum wells (MQWs) containing high Ge content.The novel idea of generating local strain using Ge multi quantum dots structures has also been studied. Ge dots were deposited at different growth temperatures in order to tune the intermixing of Si into Ge. The structures demonstrated a noise constant of 2×10-9 and TCR of 3.44%/K for pixel area of 70×70 μm2. These structures displayed an improvement in the TCR value compared to quantum well structures; however, strain relaxation and unevenness of the multi layer structures caused low signal–to–noise ratio. In this thesis, the physical importance of different design parameters of IR detectors has been quantified by using a statistical analysis. The factorial method has been applied to evaluate design parameters for IR detection improvements. Among design parameters, increasing the Ge content of SiGe quantum wells has the most significant effect on the measured TCR value.
QC 20110405

This thesis concerns the application of aberration corrected scanning transmission electron microscopy (STEM) to the quantitative analysis of industrial Pd-Pt core-shell catalyst nanoparticles. High angle annular dark field imaging (HAADF), an incoherent imaging mode, is used to determine particle size distribution and particle morphology of various particle designs with differing amounts of Pt coverage. The limitations to imaging, discrete tomography and spectral analysis imposed by the sample’s sensitivity to the beam are also explored. Since scattered intensity in HAADF is strongly dependent on both thickness and composition, determining the three dimensional structure of a particle and its bimetallic composition in each atomic column requires further analysis. A quantitative method was developed to interpret single images, obtained from commercially available microscopes, by analysis of the cross sections of HAADF scattering from individual atomic columns. This technique uses thorough detector calibrations and full dynamical simulations in order to allow comparison between experimentally measured cross section to simulated ones and is shown to be robust to many experimental parameters. Potential difficulties in its applications are discussed. The cross section approach is tested on model materials before applying it to the identification of column compositions of core-shell nanoparticles. Energy dispersive X-ray analysis is then used to provide compositional sensitivity. The potential sources of error are discussed and steps towards optimisation of experimental parameters presented. Finally, a combination of HAADF cross section analysis and EDX spectrum imaging is used to investigate the core-shell nanoparticles and the results are correlated to findings regarding structure and catalyst activity from other techniques. The results show that analysis by cross section combined with EDX spectrum mapping shows great promise in elucidating the atom-by-atom composition of individual columns in a core-shell nanoparticle. However, there is a clear need for further investigation to solve the thickness / composition dualism.

The efficiency of low voltage scanning electron microscopy, which presents near-surface information, has been well known for a long time. However, it is not widely known that the high resolution capability can only be achieved when the surface reveals the original characteristics of the materials without any deterioration due contamination. Therefore, initial attention in this study is directed at clarifying the efficient use of the ultra high resolution, low voltage SEM (UHRLV SEM), (Ultra55, Zeiss). The SEM images and the selected electrons for detection, and damage that occurs through UHRVL SEM observation are also researched. Subsequently, the most efficient specimen preparation technique, which is appropriate for the characteristics of the individual materials, is investigated for galvanized steel, ultrasonically welded alloys of Al6111 and AZ31 alloy, Ti6Al4V alloy honeycomb structure and a ceramic sensor. The outcomes of appropriate specimen preparation technique and use of the extremely Low-Voltage below 2.0 kV, are presented in the results section. The study also presented the challenge of improving the low compositional contrast for the dissimilar materials of aluminium and magnesium, and to reduce charging effects in an insulating material comprising a ceramic sensor. As an application of the surface prepared by the process in this study, 3D tomography is also introduced.

This thesis considers the theory and calculations of image formation mechanisms for various modes of three-dimensional imaging in aberration-corrected scanning transmission electron microscopy. Discrete tomography is used to determine and refine the three-dimensional structure of molecular nanowire bundles. The structure determination is expedited by the use of annular dark-field imaging, an incoherent imaging mode which provides directly interpretable images. The development of spherical aberration correctors and the subsequent reduction in probe sizes, including the depth of field, has made optical depth sectioning a feasible technique. The localisation in three dimensions of substitutional impurity atoms in zone-axis imaging is discussed. Both the channelling of the probe and the pre-focussing effect of the atomic column play an important role in determining the depth response of the impurity atom. Interband scattering within a sample is shown to be influential in imaging crystals containing dislocations and optical depth sectioning is explored as a possible option for overcoming surface relaxation effects in the imaging of screw dislocations end-on. The possibility of extending the optical depth sectioning approach using aberration-corrected scanning confocal electron microscopy is discussed. The coherent and incoherent imaging modes, involving elastically and inelastically scattered electrons respectively, are investigated.

Thin film arrays of molecules or supramolecules are active subjects of investigation because of their potential value in electronics, chemical sensing, catalysis, and other areas. Scanning probe microscopes (SPMs), including scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs) are commonly used for the characterization and metrology of thin film arrays. As opposed to transmission electron microscopy (TEM), SPMs have the advantage that they can often make observations of thin films in air or liquid, while TEM requires highly specialized techniques if the sample is to be in anything but vacuum. SPM is a surface imaging technique, while TEM typically images a 2D projection of a thin 3D sample. Additionally, variants of SPM can make observations of more than just topography; for instance, magnetic force microscopy measures nanoscale magnetic properties. Thin film arrays are typically two-dimensionally periodic. A perfect, infinite two-dimensionally periodic array is mathematically constrained to belong to one of only 17 possible 2D plane symmetry groups. Any real image is both finite and imperfect. Crystallographic Image Processing (CIP) is an algorithm that Fourier transforms a real image into a 2D array of complex numbers, the Fourier coefficients of the image intensity, and then uses the relationship between those coefficients to first ascertain the 2D plane symmetry group that the imperfect, finite image is most likely to possess, and then adjust those coefficients that are symmetry-related so as to perfect the symmetry. A Fourier synthesis of the symmetrized coefficients leads to a perfectly symmetric image in direct space (when accumulated rounding and calculation errors are ignored). The technique is, thus, an averaging technique over the direct space experimental data that were selected from the thin film array. The image must have periodicity in two dimensions in order for this technique to be applicable. CIP has been developed over the past 40 years by the electron crystallography community, which works with 2D projections from 3D samples. Any periodic sample, whether it is 2D or 3D has an "ideal structure" which is the structure absent any crystal defects. The ideal structure can be considered one average unit cell, propagated by translation into the whole sample. The "real structure" is an actual sample containing vacancies, dislocations, and other defects. Typically the goal of electron and other types of microscopy is examination of the real structure, as the ideal structure of a crystal is already known from X-ray crystallography. High resolution transmission electron microscope image based electron crystallography, on the other hand, reveals the ideal crystal structure by crystallographic averaging. The ideal structure of a 2D thin film cannot be easily in a spatially selective fashion examined by grazing incidence X-ray or low energy electron diffraction based crystallography. SPMs straightforwardly observe thin films in direct space, but SPM accuracy is hampered by blunt or multiple tips and other unavoidable instrument errors. Especially since the film is often of a supramolecular system whose molecules are weakly bonded (via pi bonds, hydrogen bonds, etc.) both to the substrate and to each other, it is relatively easy for a molecule from the film to adhere to the scanning tip during the scan and become part of the tip during subsequent observation. If the thin film array has two-dimensional periodicity, CIP is a unique and effective tool both for image enhancement (determination of ideal structure) and for the quantification of overall instrument error. In addition, if a sample of known 2D periodicity is scanned, CIP can return information about the contribution of the instrument itself to the image. In this thesis we show how the technique is applied to images of two dimensionally periodic samples taken by SPMs. To the best of our knowledge, this has never been done before. Since 2D periodic thin film arrays have an ideal structure that is mathematically constrained to belong to one of the 17 plane symmetry groups, we can use CIP to determine that group and use it for a particularly effective averaging algorithm. We demonstrate that the use of this averaging algorithm removes noise and random error from images more effectively than translational averaging, also known as "lattice averaging" or "Fourier filtering". We also demonstrate the ability to correct systematic errors caused by hysteresis in the scanning process. These results have the effect of obtaining the ideal structure of the sample, averaging out the defects crystallographically, by providing an average unit cell which, when translated, represents the ideal structure. In addition, if one has recorded a scanning probe image of a 2D periodic sample of known symmetry, we demonstrate that it is possible to use the Fourier coefficients of the image transform to solve the inverse problem and calculate the point spread function (PSF) of the instrument. Any real scanning probe instrument departs from the ideal PSF of a Dirac delta function, and CIP allows us to quantify this departure as far as point symmetries are concerned. The result is a deconvolution of the "effective tip", which includes any blunt or multiple tip effects, as well as the effects caused by adhesion of a sample molecule to the scanning tip, or scanning irregularities unrelated to the physical tip. We also demonstrate that the PSF, once known, can be used on a second image taken by the same instrument under approximately the same experimental conditions to remove errors introduced during that second imaging process. The preponderance of two-dimensionally periodic samples as subjects of SPM observation makes the application of CIP to SPM images a valuable technique to extract a maximum amount of information from these images. The improved resolution of current SPMs creates images with more higher-order Fourier coefficients than earlier, "softer" images; these higher-order coefficients are especially amenable to CIP, which can then effectively magnify the resolution improvement created by better hardware. The improved resolution combined with the current interest in supramolecular structures (which although 3D usually start building on a 2D periodic surface) appears to provide an opportunity for CIP to significantly contribute to SPM image processing.

Transmission Electron Microscopy (TEM) on light element materials and soft matters is problematic due to electron irradiation damage and low contrast. In this doctoral thesis techniques were developed to address some of those issues and successfully characterize these materials at high resolution. These techniques were demonstrated on graphene flakes, DNA/magnetic beads and a number of water containing biomaterials. The details of these studies are given below. A TEM based method was presented for thickness characterization of graphene flakes. For the thickness characterization, the dynamical theory of electron diffraction is used to obtain an analytical expression for the intensity of the transmitted electron beam as a function of thickness. From JEMS simulations (experiments) the absorption constant λ in a low symmetry orientation was found to be ~ 208 nm (225 ± 9 nm). When compared to standard techniques for thickness determination of graphene/graphite, the method has the advantage of being relatively simple, fast and requiring only the acquisition of bright-field (BF) images. Using the proposed method, it is possible to measure the thickness change due to one monolayer of graphene if the flake has uniform thickness over a larger area. A real-space TEM study on magnetic bead-DNA coil interaction was conducted and a statistical analysis of the number of beads attached to the DNA-coils was performed. The average number of beads per DNA coil was calculated around 6 and slightly above 2 for samples with 40 nm and 130 nm beads, respectively. These results are in good agreement with magnetic measurements. In addition, the TEM analysis supported an earlier hypothesis that 40 nm beads are preferably attached interior of the DNA-coils while 130 nm beads closer to the exterior of the coils. A focused ion-beam in-situ lift-out technique for hydrated biological specimens was developed for cryo-TEM. The technique was demonstrated on frozen Aspergillus niger spores which were frozen with liquid nitrogen to preserve their cellular structures. A thin lamella was prepared, lifted out and welded to a TEM grid. Once the lamella was thinned to electron transparency, the grid was cryogenically transferred to the TEM using a cryo-transfer bath. The structure of the cells was revealed by BF imaging. Also, a series of energy filtered images was acquired and C, N and Mn elemental maps were produced. Furthermore, 3 Å lattice fringes of the underlying Al support were successfully resolved by high resolution imaging, confirming that the technique has the potential to extract structural information down to the atomic scale. The experimental protocol is ready now to be employed on a large variety of samples e.g. soft/hard matter interfaces.

Novel semiconductor nanostructures possess a range of notable properties that have the potential to be harnessed in the next generation of optical devices. Electron microscopy is uniquely suited to characterising the complex microstructure, the results of which may be related to the growth conditions and optical properties. This thesis investigates three such novel materials: (1) GaN/InGaN core/shell nanowires, (2) n-GaN/InGaN/p-GaN core/multi-shell microrods and (3) Zn₃N₂ nanoparticles, all of which were grown at Sharp Laboratories of Europe. GaN nanowires were grown by a Ni-catalysed VLS process and were characterised by various techniques before and after InGaN shells were deposited by MOCVD. The majority of the core wires were found to have the expected wurtzite structure and completely defect free – reflected in the strong strain-free photoluminescence peak –with a- and m- axis orientations identified with shadow imaging. A small component, <5%, were found to have the cubic zinc-blende phase and a high density of planar faults running the length of the wires. The deposited shells were highly polycrystalline, partially attributed to a layer of silicon at the core shell interface identified through FIB lift-out of cross section samples, and accordingly the PL was very broad likely due to recombination at defects and grain boundaries. A high throughput method of identifying the core size indirectly via the catalyst particle EDX signal is described which may be used to link the shell microstructure to core size in further studies. An n-GaN/InGaN/p-GaN shell structure was deposited by MOCVD on the side walls of microrods etched from c-axis GaN film on sapphire, which offers the possibility of achieving non-polar junctions without the issues due to non-uniformity found in nanowires. Threading dislocations within the core related to the initial growth on sapphire were shown to be confined to this region, therefore avoiding any harmful effect on the junction microstructure. The shell defect density showed a surprising relationship to core size with the smaller diameter rods having a high density of unusual 'flag' defects in the junction region whereas the larger diameter sample shells appeared largely defect free, suggesting the geometry of the etched core has an impact on the strain in the shell layers. The structure of unusual 'flag' defects in the m-plane junctions was characterised via diffraction contrast TEM, weak beam and atomic resolution ADF STEM and were shown to consist of a basal plane stacking faults meeting a perfect or partial dislocation loop on a pyramidal plane, the latter likely gliding in to resolve residual strain due to the fault formed during growth. Zn₃N₂ has the required bandgap energy to be utilised as a phosphor with the additional advantage over conventional materials of its constituent elements not being toxic or scarce. The first successful synthesis of Zn₃N₂ nanoparticles appropriate to this application was confirmed via SAD, EDX and HRTEM, with software developed to fit experimental polycrystalline diffraction patterns to simulated components suggesting a maximum Zn₃N₂ composition of ~30%. There was an apparent decrease in crystallinity with decreasing particle size evidenced in radial distribution function studies with the smallest particles appearing completely amorphous in 80kV HRTEM images. A rapid change in the particles under the electron beam was observed, characterised by growth of large grains of Zn₃N₂ and ZnO which increased with increasing acceleration voltage suggesting knock-on effects driving the change. PL data was consistent with the bandgap of Zn₃N₂ blue shifted from 1.1eV to around 1.8eV, confirming the potential of the material for application as a phosphor.

Silicon technologyhas been seekingfor a monolithic solution for a chip where data processing and data communication is performed in the CMOS part and the photonic component, respectively. Traditionally, silicon has been widely considered for electronic applications but not for photonic applications due to its indirect bandgap nature. However, band structure engineering and manipulation through alloying Si with Ge and Sn has opened new possibilities. Theoretical calculations show that it is possible to achieve direct transitions from Ge ifit is alloyed with Sn. Therefore, a GeSn system is a choice to get a direct bandgap. Extending to ternary GeSnSi and quaternary GeSnSiCstructures grown on Si wafers not only makes the bandgap engineering possible but also allowsgrowing lattice matched systems with different strain and bandgaps located in the infrared region. Different heterostructures can be designed and fabricated for detecting lightas photonic sensing oremitting the light as lasers. Alloying not only makes engineering possible but it also induces strain which plays an important role for electronic applications. Theoretical and experimental works show that tensile strain could increase the mobility, which is promising for electronic devices where high mobility channels for high performance MOSFETs is needed to speed up the switching rate. On the other hand, high n-doping in tensile strains in p-i-n structures makesΓ band transitions most probable which is promising for detection and emission of the light. As another benefit of tensile strain, the direct bandgap part shrinks faster than the indirect one if the strain amount is increased. To get both electronic and photonic applications of GeSn-based structures, two heterostructures (Ge/GeSn(Si)/GeSi/Ge/Si and Ge/GeSn/Si systems), including relaxed and compressive strained layers used to produce tensile strained layers, were designed in this thesis. The low temperature growth is of key importance in this work because the synthesis of GeSn-based hetrostructures on Si wafers requires low thermal conditions due tothe large lattice mismatch which makes them metastable. RPCVD was used to synthesize theseheterostructures because not only it offers a low temperature growth but also because it is compatible with CMOS technology. For utilization of these structures in devices, n-type and p-type doping of relaxed and compressive strained layers were developed. HRRLMs, HRTEM, RBS, SIMS, and FPP techniques were employed to evaluatestrain, quality, Sn content and composition profile of the heterostructures. The application of GeSn-based heterostructures is not restricted to electronics and photonics. Another application investigated in this work is photovoltaics. In competition with Si-based solar cells, which have, or areexpected to have,high stability and efficiency, thirdgeneration solar cells offer the use of low cost materials and production and can therefore be an alternative for future light energy conversion technology. Particularly, quantum dot sensitized solar cells are associated with favorable properties such as high extrinsic coefficients, size dependent bandgaps and multiple exciton generation and with a theoretical efficiencyof 44%. In this work, two categories of QDs, Cd-free and Cd-based QDs were employed as sensitizers in quantum dot sensitized solar cells (QDSSCs). Cd-based QDs have attracted large interest due to high quantum yield,however, toxicityremains still totheir disadvantage. Mn doping as a bandgap engineering tool for Cd-based type IIZnSe/CdS QDs wasemployed to boostthe solar cell efficiency. Theoretical and experimental investigations show that compared to single coreQDSSCs,typeII core-shells offer higher electron-hole separation due to efficient band alignment where the photogenerated electrons and holes are located in the conduction band of the shell and valence band of the core, respectively. This electron-hole separation suppresses recombination and by carefully designing the band alignment in the deviceit can increase the electron injection and consequently the power conversion efficiency of the device. Considering eco-friendly and commercialization aspects, three different “green” colloidal nanostructures having special band alignments, which are compatible for sensitized solar cells, were designed and fabricated by the hot injection method. Cu2GeS3-InP QDs not only can harvest light energy up to the infraredregion but can also be usedastypeII QDs. ZnS-coating was employed as a strategy to passivate the surface of InP QDs from interaction with air and electrolyte. In addition, ZnS-coating and hybrid passivation was applied for CuInS2QDs to eliminate surface traps.

QC 20151125

Les semi-conducteurs nitrures (AlN, GaN, InN) focalisent une activité de recherche intense en raison de nombreuses applications comme les diodes électroluminescentes, les composants de puissance ou hyperfréquence. Dans cette recherche, nous avons abordé le travail sous deux angles: a) la conduction électrique dans les couches d'InN produites par croissance épitaxiale aux jets moléculaires assistée par plasma (PAMBE) et une recherche sur l'origine de la forte émission bleue dans les puits de quantiques d'InGaN/GaN. L'accumulation d'électron en surface dans les couches d'InN constitue une limitation importante pour la fabrication de composants. Au cours de ce travail, nous avons exploré l'utilisation des mesures de bruit de basse fréquence sur les couches d'InN et pu accéder à leur conductivité électrique en volume. L'étude des puits quantiques d'InGaN/GaN, obtenue par croissance épitaxiale aux jets moléculaires (MBE) ou épitaxie en phase vapeurs aux organométalliques (MOVPE) , a été effectuée par analyses de la microstructure par microscopie électronique en transmission (MET, HRTEM et STEM) en corrélation avec les propriétés optiques d'un grand nombre d'échantillons provenant de conditions de croissance différentes. Ce travail nous a permis d'acquérir une vision plus critique du rôle des conditions de fabrication et des paramètres comme la morphologie, les fluctuations de composition et la présence des défauts en V sur les explications actuellement avancées pour la forte efficacité d'émission dans les puits quantiques d' InGaN/GaN
The nitride semiconductors (AlN, GaN, InN) are subject to a large research effort due to their numerous applications, such as light emitting diodes, high power and high frequency components. Following the trend, the aim of this dissertation has been twofold: first, we have probed the bulk electrical conduction in InN layers, second, we investigated the origin of the high emission efficiency in InGaN/GaN Quantum Wells (QWs). The surface electron accumulation in InN layers is still an important limitation to device applications. W have explored this point using low frequency noise measurements on Plasma Assisted Molecular Beam Epitaxy (PAMBE) InN layers and we demonstrated that the bulk electrical conductivity of InN can be accessed. The investigation of quantum wells produced by molecular beam epitaxy (MBE) or matalorganic vapour phase epitaxy (MOVPE), has been carried out through microstructural analyses by transmission electron microscopy techniques(TEM, HRTEM, STEM) in correlation with optica properties on a large number of samples grown in different growth conditions. This experimental work has allowed us to obtain a critical view on the role of the growth conditions and such parameters as the well morphology, composition fluctuations, as well as the V shaped defects on the current explanations of high emission efficiency in InGaN/GaN QWs

Multi-metallic aerogels have emerged as a promising unsupported, high surface area-based metal material for different applications in heterogeneous catalysis and electrochemistry. The fabrication of these multi-metallic aerogels is based on a complex gelation process which is characterized by controlled aggregation of metallic nanoparticles to form a macroscopic network structure in aqueous solution. However, achieving structural homogeneity of the multi-metallic aerogels in terms of diameter of the nanochains and chemical composition at the nano- as well as at the macro-scale is still a great challenge. In this paper, we show the characterization of two Pd-Pt aerogels prepared by the two-step gelation method. The structural homogeneity and chemical distribution of both metals (Pd and Pt) inside the aerogels were analyzed using high-resolution (scanning) transmission microscopy (HR(S)TEM), energy-dispersive X-ray spectroscopy (EDX), extended X-ray absorption fine structure (EXAFS) spectroscopy, and cyclic voltammetry. Based on the microscopic and spectroscopic results, the Pd-Pt aerogels show the presence of Pd/Pt-rich domains inside the long-range framework. It is evident that the initial monometallic features dominate over alloying during the gelation process. Although the same synthetic approach for Pd-Pt aerogels with different atomic ratios was used, we observed that the sizes of these monometallic domains strongly varied between the Pd-rich and Pt-rich aerogels. For instance, the Pd-rich aerogels showed larger clusters with a size range from few nanometers up to several tens of nanometers, while the dimension of the clusters of the Pt-rich aerogels varies from the sub-nanometers to a few nanometers. The presence of the metal clusters strongly influenced the electrochemical robustness of these Pd-Pt aerogels. Electrochemical durability investigations revealed that the aerogels with a high content of Pd are less stable due to the gradual dissolution of the less noble metal in particular inside the Pd-rich domains. A better chemical and structural homogeneity might improve the life-time of the Pd-Pt aerogels under electrochemical conditions. In this work, we provide a better understanding about the structure and chemical distribution of the bimetallic aerogel framework prepared by the two step gelation process.

This thesis describes investigations into the role of non-stoichiometry in the surface and bulk properties of SrTiO₃ single crystals. A family of (n×n) reconstructions, where n = 2, 3, 4, 5, 6 are produced by argon ion sputtering of the SrTiO₃ (111) single crystals and subsequent annealing in UHV or in an oxygen rich environment. The sputtering process introduces defects or oxygen vacancies in the surface region of the sample, whilst the annealing gives rise to surface reconstructions. The surface preparation conditions such as sputtering time, annealing temperature and environment are optimized to obtain various reconstructions in a controlled and reproducible manner. High resolution STM images of these reconstructions are also obtained and utilized in the investigation of the surface reactivity. Fullerene molecules are deposited on the reconstructed surfaces to elucidate the surface reactivity through template assisted growth. Fullerene molecules are first deposited with substrate surfaces held at room temperature. Being the most highly reduced among the (n×n) family, the 5×5 reconstruction significantly influenced the growth of fullerenes. Both C₆₀ and C₇₀ adsorb as individual molecules and produce clusters with magic numbers. The 4×4 and 6×6 reconstructed surfaces encourage the formation of close-packed structures upon the deposition at room temperature. When the surface covered with fullerenes is heated to a temperature of around 200 °C, epitaxial islands are observed. The 6×6 reconstructed surface appeared to be less reactive than the 4×4. Electrical transport, cathodoluminescence (CL) and electron spin resonance (ESR) experiments are also carried out to investigate the effect of oxygen vacancies on the bulk properties of UHV annealed SrTiO₃ single crystals. Thermal reduction leads to carrier doping of the material, which not only gives rise to electrical conduction but also induces room temperature luminescence. Both the electrical conductivity and CL intensity increases with annealing time. The work presented in this thesis provides insight into the defect driven properties in both the surface and bulk of SrTiO₃ single crystals, which could play an important role in the development of oxide-based electronic devices.

This thesis focuses on the synthesis and characterisation of organoclays. X-ray diffraction has been used to study the changes in the basal spacings of montmorillonite clay and surfactant-intercalated organoclays. Variation in the d-spacing was found to be a step function of the surfactant concentration. Three different molecular environments for surfactant octadecyltrimethylammonium bromide (ODTMA) within the surface-modified montmorillonite are proposed upon the basis of their different decomposition temperatures. High-resolution thermogravimetric analysis (HRTG) shows that the thermal decomposition of montmorillonite modified with ODTMA takes place in four steps attributing to dehydration of adsorbed water, dehydration of water hydrating metal cations, loss of surfactant and the loss of OH units respectively. In addition, it has shown that the decomposition procedure of DODMA and TOMA modified clays are very different from that of ODTMA modified ones. The surfactant decomposition takes place in several steps in the DODMA and TOMA modified clays while for ODTMA modified clays, it shows only one step for the decomposition of surfactant. Also TG was proved to be a useful tool to estimate the amount of surfactant within the organoclays. A model is proposed in which, up to 0.4 CEC, a surfactant monolayer is formed between the montmorillonite clay layers; up to 0.8 CEC, a lateral-bilayer arrangement is formed; and above 1.5 CEC, a pseudotrimolecular layer is formed, with excess surfactant adsorbed on the clay surface. While for dimethyldioctadecylammonium bromide (DODMA) and trioctadecylmethylammonium bromide (TOMA) modified clays, since the larger sizes of the surfactants, some layers of montmorillonite are kept unaltered because of steric effects. The configurations of surfactant within these organoclays usually take paraffin type layers. Thermal analysis also provides an indication of the thermal stability of the organoclay as shown by different starting decomposition temperatures. FTIR was used as a guide to determine the phase state of the organoclay interlayers as determined from the CH asymmetric stretching vibration of the surfactants to provide more information on surfactant configurations. It was used to study the changes in the spectra of the surfactant ODTMA upon intercalation into a sodium montmorillonite. Surfaces of montmorillonites were modified using ultrasonic and hydrothermal methods through the intercalation and adsorption of the cationic surfactant ODTMA. Changes in the surfaces and structure were characterized using electron microscopy. The ultrasonic preparation method results in a higher surfactant concentration within the montmorillonite interlayer when compared with that from the hydrothermal method. Both XRD patterns and TEM images demonstrate that SWy-2-Namontmorillonite contains superlayers. TEM images of organoclays prepared at high surfactant concentrations show alternate basal spacings between neighboring layers. SEM images show that modification with surfactant will reduce the clay particle aggregation. Organoclays prepared at low surfactant concentration display curved flakes, whereas they become flat with increasing intercalated surfactant. Fundamentally this thesis has increased the knowledge base of the structural and morphological properties of organo-montmorillonite clays. The configurations of surfactant in the organoclays have been further investigated and three different molecular environments for surfactant ODTMA within the surface-modified montmorillonite are proposed upon the basis of their different decomposition temperatures. Changes in the spectra of the surfactant upon intercalation into clay have been investigated in details. Novel surfactant-modified montmorillonite results in the formation of new nanophases with the potential for the removal of organic contaminants from aqueous media and for the removal of hydrocarbon spills on roads.

This thesis explores theoretical aspects of scanning transmission electron microscopy (STEM) and the comparison of simulation with experiment.
The long standing contrast mismatch problem between theory and experiment in conventional high resolution transmission electron microscopy (HRTEM) is examined using the principle of reciprocity and bright field scanning transmission electron microscopy (BFSTEM). It is found that quantitative agreement between theoretical and experimental images is possible provided that theory suitably accounts for the spatial incoherence of the source, and that experimental images are placed on an absolute scale with respect to the incident beam current. Agreement between theory and experimental image contrast is found to be independent of specimen thickness and probe defocus.
Core-loss electron energy-loss spectroscopy (EELS) is a powerful experimental tool with the potential to provide atomic-resolution information about the electronic structure at defects and interfaces in materials and nanostructures. Interpretation, however, is nonintuitive due to the nonlocal ionization potential. Novel improvements in microscope design and operating environment have enabled two dimensional chemical maps. This has permitted a more thorough theoretical analysis. This thesis compares experimental STEM EELS images of LaMnO3, BiSrMnO3 and Si samples to the relevant theoretical simulations. Image features which at first appear counter intuitive are discussed and explained with the accompanying theoretical simulations. It is demonstrated, using a sample of SrTiO3, that more direct interpretation of atomic resolution chemical maps is possible when using energy dispersive x-ray spectroscopy (EDS) in STEM.
This thesis considers extending chemical mapping in STEM EELS to three dimensions using depth sectioning. It explores, theoretically, the feasibility to depth section zone-axis aligned crystals that contain embedded impurities. In STEM EELS this is found to be possible for point defects but not for larger extended objects such as nanoparticles.
The theory describing the mechanism by which contrast is obtained in elastic scanning confocal electron microscopy (SCEM) is developed. It is shown that there is no first order phase contrast in SCEM and thus low image contrast. Finally, energy filtered scanning transmission electron microscopy (EFSCEM) is developed theoretically. The fundamental equation describing image formation is derived and an efficient computation method is developed to allow the rapid calculation of EFSCEM images.

碩士
國立東華大學
應用物理研究所
95
In this study we used low-resistivity indium tin oxide (ITO) substrate to obtain high resolution scanning electron microscopy (SEM) images of biological samples without any conductive coating. First, the macrophages of raw264.7 cells were immobilized on the ITO substrate and then observed by a field-emission scanning electron microscope (FESEM) at various accelerating voltages. The effect of accelerating voltage on the image resolutions of secondary electron (SE) and backscattering electron (BE) images is significantly large. For example, the morphologies of the macrophages in the BE images is corresponding to their atomic number contrasts at various accelerating voltages. Also, the macrophages of raw264.7 cells immobilized on glass substrates were examined by the FESEM at various accelerating voltages. The results of the macrophages on the glass substrate compared with those of the macrophages on the ITO substrate are much worse. Secondly, the macrophages were immobilized on substrates with varying resistivity to understand the effect of resistivity on the image resolution of SEM images. We found that the low-resistivity substrate helping acquire high resolution FESEM images for macrophages is much better than not only the insulating glass substrate, but also the good conductive copper substrates. This method confirms that the low-resistivity ITO substrate can supply high resolution for morphology of biological samples in SEM images without any conductive coating. Therefore, details of biological morphology are not obscured or distorted by conductive coating.

M.Sc.
Donor-acceptor (D-A) systems consist of a donor covalently or non-covalently linked to an acceptor. The simplest D–A system consists of a donor linked to an acceptor and is called a dyad system. Photoactive dyad systems are molecular devices designed to perform through the separation of charge separation states and the conversion of solar energy to chemical energy in analogy to photosynthesis.1 These dyad systems consist of a donor which is usually a chromophore and an acceptor. The design of these systems is guided to mimic photo-induced electron transfer (PET) and charge separation (CS), which are fundamental processes of photosynthesis. In nature, photosynthetic units are often built from dyads consisting of pigments like chlorophyll (donors), non–covalently linked to quinones, (acceptors). The donor harvests light energy and transfers the energy to the nearby pigment molecules until it eventually reaches a special region of the chlorophyll macrocycle called the reaction centre where this light energy is then converted to electrochemical energy. Photoactive dyad systems act as artificial photosynthetic models as they reproduce photo–induced electron transfer and charge separation of natural photosynthesis. In this project, dyad systems were made by covalently linking zinc pheophorbide, a modified chlorophyll derivative to double–walled carbon nanotubes (DWCNTs). Zinc pheophorbide acts as the donor and DWCNTs as the acceptors. Chlorophyll was modified by cutting the phytol chain and inserting zinc as the central metal to yield zinc pheophorbide. This derivative is stable against irradiation, has a good range of acceptor wavelength and is also a good light harvester. DWCNTs are one dimensional nanowires with two concentric tubes. They readily accept electrons because they have an extended π electron system. These electrons are then transported efficiently under ballistic conditions. DWCNTs were synthesised by catalytic chemical vapour deposition (CCVD) of methane over Mg0.99Co0.075Mo0.025O catalyst. In dyad system 1, amidated zinc pheophorbide molecules were covalently attached to oxidised DWCNTs in the presence of N–ethyl–N’–(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) \ and N–hydroxysuccinnimide (NHS) as a catalysts. Dyad system 2 was synthesized by attaching zinc pheophorbide molecules to amidated DWCNTs using the same catalysts.