Matching Items (12)

137692-Thumbnail Image.png

A Study of an Inclusion Observed Under Transmission Electron Microscopy in Synechocystis sp. PCC 6803

Description

Transmission electron microscopy has been used to identify poly-3-hydroxybutyrate (PHB) granules in cyanobacteria for over 40 years. Electron-transparent (sometimes containing a slightly electron-dense area in the inclusions) or slightly electron-dense

Transmission electron microscopy has been used to identify poly-3-hydroxybutyrate (PHB) granules in cyanobacteria for over 40 years. Electron-transparent (sometimes containing a slightly electron-dense area in the inclusions) or slightly electron-dense spherical inclusions found in transmission electron micrographs of cyanobacteria are often assumed to be PHB granules. The aim of this study was to test this assumption in Synechocystis sp. PCC 6803, and to determine whether all inclusions of this kind are indeed PHB granules. Based on the results gathered, it is concluded that not all of the slightly electron-dense spherical inclusions are PHB granules in Synechocystis sp. PCC 6803. This result is potentially applicable to other cyanobacteria.

Contributors

Agent

Created

Date Created
  • 2013-05

137352-Thumbnail Image.png

Effect of Defects on Calculation of Electron Energy Loss Spectra of Ceria and Titania

Description

Climate change is one of the biggest challenges facing today's society.Since the late 19th century, the global average temperature has been rising. In order to minimize the temperature increase of

Climate change is one of the biggest challenges facing today's society.Since the late 19th century, the global average temperature has been rising. In order to minimize the temperature increase of the earth, it is necessary to develop alternative energy technologies that do not depend on fossil fuels. Solar fuels are one potential energy source for the future. Solar fuel technologies use catalysts to convert low energy molecules into fuels via artificial photosynthesis. TiO2, or titania, is an important model photocatalyst for studying these reactions. It is also important to use remaining fossil fuel resources efficiently and with the lowest possible greenhouse gas emissions. Fuel cells are electrochemical devices that aim to accomplish this goal and CeO2, or ceria, is an important material used in these devices. One way to observe the atomic structure of a material is with a transmission electron microscope (TEM). A traditional transmission electron microscope employs a beam of fast electrons to form atomic resolution images of a material. While imaging gives information about the positions of the atoms in the material, spectroscopy gives information about the composition and bonding of the material. A type of spectroscopy that can be performed inside the transmission electron microscope is electron energy loss spectroscopy (EELS), which provides a fundamental understanding of the electronic structure of a material. The energy loss spectrum also contains information on the chemical bonding in the material, and theoretical calculations that model the spectra are essential to correctly interpreting this bonding information. FEFF is a software that performs EELS calculations. Calculations of the oxygen K edges of TiO2 and CeO2 were made using FEFF in order to understand the changes that occur in the spectrum when oxygen vacancies are introduced as well as the changes near a grain boundary.

Contributors

Agent

Created

Date Created
  • 2013-12

137353-Thumbnail Image.png

社交網站對於國際經濟的影響

Description

隨著全球一體化,社交網絡在國際平台上擁有了相當出色的表現,影響全球人民的生活,特別有助於國際經濟發展,因此引起了各國的關注。雖然如此,它們具體的效果是難以去評估的。本文在前人研究的基礎上探討社交網絡的經營模式以及它對於國際經濟的影響。在微觀層面可以看到社交網的存在對市場有著直接的影響。在宏觀層面上,社交網站通過其網站設計推銷其他公司產品,提高廣告商的知名度,刺激消費。這兩方面,本文討論社交網的影響為何跟傳統公司有差別。

Contributors

Agent

Created

Date Created
  • 2013-12

158372-Thumbnail Image.png

Advanced Characterization of Aerogel Films Deposited via Aerosol Impaction-Driven Assembly

Description

A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the

A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the long aging and drying steps in the sol-gel method. Electron microscopy, pore size analysis, thermal conductivity, and optical measurements show the nanoparticle (NP) films to be similar to typical silica aerogel. Haze of nanoparticle films modeled as scattering sites correlates strongly with pore size distribution. Supporting evidence was obtained from particle sizes and aggregates using electron microscopy and small-angle X-ray scattering. NP films showed interlayers of higher porosity and large aggregates formed by tensile film stress.

To better understand film stress and NP adhesion, chemical bonding analyses were performed for samples annealed up to 900 °C. Analysis revealed that about 50% of the NP surfaces are functionalized by hydroxyl (-OH) groups, providing for hydrogen bonding. Ellipsometric porosimetry was used to further understand the mechanical properties by providing a measure of strain upon capillary pressure from filling pores. Upon annealing to 200 °C, the films lost water resulting in closer bonding of NPs and higher Young’s modulus. Upon further annealing up to 900 °C, the films lost hydroxyl bonds while gaining siloxane bonds, reducing Young’s modulus. The application of ellipsometric porosimetry to hydrophilic coatings brings into question the validity of pore size distribution calculations for materials that hold onto water molecules and result in generally smaller calculated pore sizes.

Doped hydrogenated microcrystalline silicon was grown on crystalline silicon NPs, as a test case of an application for NP films to reduce parasitic absorption in silicon heterojunction solar cells. Parasitic absorption of blue light could be reduced because microcrystalline silicon has a mix of direct and indirect bandgap, giving lower blue absorption than amorphous silicon. Using Ultraviolet Raman spectroscopy, the crystallinity of films as thin as 13 nm was determined rapidly (in 1 minute) and non-destructively. A mono-layer of nanocrystals was applied as seeds for p-doped microcrystalline silicon growth and resulted in higher crystallinity films. Applications of the method could be explored for other nanocrystalline materials.

Contributors

Agent

Created

Date Created
  • 2020

158158-Thumbnail Image.png

Transmission Electron Microscopy Characterization of Photovoltaic Semiconductor Materials

Description

The research of this dissertation has primarily involved using transmission electron microscopy (TEM) techniques to study several semiconductor materials considered promising for future photovoltaic device applications.

Layers of gallium phosphide (GaP)

The research of this dissertation has primarily involved using transmission electron microscopy (TEM) techniques to study several semiconductor materials considered promising for future photovoltaic device applications.

Layers of gallium phosphide (GaP) grown on silicon (Si) substrates were characterized by TEM and aberration-corrected scanning transmission electron microscopy (AC-STEM). High defect densities were observed for samples with GaP layer thicknesses 250nm and above. Anti-phase boundaries (APBs) within the GaP layers were observed at interfaces with the Si surfaces which were neither atomically flat nor abrupt, contradicting conventional understanding of APB formation.

Microcrystalline-Si (μc-Si) layers grown on crystalline-Si (c-Si) substrates were investigated. Without nanoparticle seeding, an undesired amorphous-Si (a-Si) layer grew below the μc-Si layer. With seeding, the undesired a-Si layer grew above the μc-Si layer, but μc-Si growth proceeded immediately at the c-Si surface. Ellipsometry measurements of percent crystallinity did not match TEM images, but qualitative agreement was found between TEM results and Ultraviolet Raman spectroscopy.

TEM and Xray spectroscopy were used to study metal-induced crystallization and layer exchange for aluminum/ germanium (Al/Ge). Only two samples definitively exhibited both Ge crystallization and layer exchange, and neither process was complete in either sample. The results were finally considered as inconclusive since no reliable path towards layer exchange and crystallization was established.

Plan-view TEM images of indium arsenide (InAs) quantum dots with gallium arsenide antimonide (GaAsSb) spacer layers revealed the termination of some threading dislocations in a sample with spacer-layer thicknesses of 2nm, while a sample with 15-nm-thick spacer layers showed a dense, cross-hatched pattern. Cross-sectional TEM images of samples with 5-nm and 10-nm spacer-layer thicknesses showed less layer undulation in the latter sample. These observations supported photoluminescence (PL) and Xray diffraction (XRD) results, which indicated that GaAsSb spacer layers with 10-nm thickness yielded the highest quality material for photovoltaic device applications.

a-Si/c-Si samples treated by hydrogen plasma were investigated using high-resolution TEM. No obvious structural differences were observed that would account for the large differences measured in minority carrier lifetimes. This key result suggested that other factors such as point defects, hydrogen content, or interface charge must be affecting the lifetimes.

Contributors

Agent

Created

Date Created
  • 2020

157793-Thumbnail Image.png

Investigation into a laser welded interconnection method for Interdigitated Back Contact (IBC) solar cell modules

Description

Interconnection methods for IBC photovoltaic (PV) module integration have widely been explored yet a concrete and cost-effective solution has yet to be found. Traditional methods of tabbing and stringing which

Interconnection methods for IBC photovoltaic (PV) module integration have widely been explored yet a concrete and cost-effective solution has yet to be found. Traditional methods of tabbing and stringing which are still being used today impart increased stress on the cells, not to mention the high temperatures induced during the soldering process as well. In this work and effective and economical interconnection method is demonstrated, by laser welding an embossed aluminum (Al) electrode layer to screen-printed silver (Ag) on the solar cell. Contact resistivity below 1mΩ.cm2 is measured with the proposed design. Cross-sectional analysis of interfaces is conducted via Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS) methods. Typical laser weld phenomenon observed involves Al ejection at the entrance of the weld, followed by Al and Ag fusing together mid-way through the weld spot, as revealed by cross-sectional depth analysis. The effects of voltage and lamp intensity are also tested on the welding process. With the range of voltages tested, 240V seems to show the least process variability and the most uniform contact between Al and Ag layers, upon using an Ethylene-Vinyl Acetate (EVA) encapsulant. Two lamp intensities were also explored with a Polyolefin (POE) encapsulant with Al and Ag layers seen welded together as well. Smaller effect sizes at lamp 2 intensity showed better contact. A process variability analysis was conducted to understand the effects of the two different lamps on welds being formed. Lamp 2 showed a bi-modal size distribution with a higher peak intensity, with more pulses coupling into the sample, as compared to lamp 1.

Contributors

Agent

Created

Date Created
  • 2019

158507-Thumbnail Image.png

Excursions in Electron Energy-Loss Spectroscopy

Description

Recent improvements in energy resolution for electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) allow novel effects in the low-loss region of the electron energy-loss spectrum to be

Recent improvements in energy resolution for electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) allow novel effects in the low-loss region of the electron energy-loss spectrum to be observed. This dissertation explores what new information can be obtained with the combination of meV EELS energy resolution and atomic spatial resolution in the STEM. To set up this up, I review nanoparticle shape effects in the electrostatic approximation and compare the “classical” and “quantum” approaches to EELS simulation. Past the electrostatic approximation, the imaging of waveguide-type modes is modeled in ribbons and cylinders (in “classical" and “quantum" approaches, respectively), showing how the spatial variations of such modes can now be imaged using EELS. Then, returning to the electrostatic approximation, I present microscopic applications of low-loss STEM-EELS. I develop a “classical” model coupling the surface plasmons of a sharp metallic nanoparticle to the dipolar vibrations of an adsorbate molecule, which allows expected molecular signal enhancements to be quantified and the resultant Fano-type asymmetric spectral line shapes to be explained, and I present “quantum” modelling for the charged nitrogen-vacancy (NV-) and neutral silicon-vacancy (SiV0) color centers in diamond, including cross-sections and spectral maps from density functional theory. These results are summarized before concluding.

Many of these results have been previously published in Physical Review B. The main results of Ch. 2 and Ch. 4 were packaged as “Enhanced vibrational electron energy-loss spectroscopy of adsorbate molecules” (99, 104110), and much of Ch. 5 appeared as “Prospects for detecting individual defect centers using spatially resolved electron energy loss spectroscopy” (100, 134103). The results from Ch. 3 are being prepared for a forthcoming article in the Journal of Chemical Physics.

Contributors

Agent

Created

Date Created
  • 2020

153378-Thumbnail Image.png

Fluctuation electron microscopy of amorphous and polycrystalline materials

Description

Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO

Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy between the computed kinematical variance and the experimental variance, which previously was attributed to source incoherence. Although high-brightness, high coherence, electron guns are now routinely available in modern electron microscopes, they have not eliminated this discrepancy between theory and experiment. The main objective of this thesis was to explore, and to reveal, the reasons behind this conundrum.

The study was started with an analysis of the speckle statistics of tilted dark-field TEM images obtained from an amorphous carbon sample, which confirmed that the structural ordering is sensitively detected by FEM. This analysis also revealed the inconsistency between predictions of the source incoherence model and the experimentally observed variance.

FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples was carried out in an attempt to explore the conundrum. Electron probe and sample parameters were varied to observe the scattering intensity variance behavior. Results were compared to models of probe incoherence, diffuse scattering, atom displacement damage, energy loss events and multiple scattering. Models of displacement decoherence matched the experimental results best.

Decoherence was also explored by an interferometric diffraction method using bilayer amorphous samples, and results are consistent with strong displacement decoherence in addition to temporal decoherence arising from the electron source energy spread and energy loss events in thick samples.

It is clear that decoherence plays an important role in the long-standing discrepancy between experimental FEM and its theoretical predictions.

Contributors

Agent

Created

Date Created
  • 2015

156673-Thumbnail Image.png

Physicochemical characterization of PZT-based ultrasonic transducer stacks

Description

A piezoelectric transducer, comprised of electroded and active pad PZT layer atop a backing PZT layer and protected with an acoustic matching layer, and operating under a pulse-echo technique for

A piezoelectric transducer, comprised of electroded and active pad PZT layer atop a backing PZT layer and protected with an acoustic matching layer, and operating under a pulse-echo technique for longitudinal ultrasonic imaging, acts as both source and detector.

Ultrasonic transducer stacks (modules), which had failed or passed during pulse-echo sensitivity testing, were received from Consortium X. With limited background information on these stacks, the central theme was to determine the origin(s) of failure via the use of thermal and physicochemical characterization techniques.

The optical and scanning electron microscopy revealed that contact electrode layers are discontinuous in all samples, while delaminations between electrodes and pad layer were observed in failed samples. The X-ray diffraction data on the pad PZT revealed an overall c/a ratio of 1.022 ratio and morphotropic boundary composition, with significant variations of the Zr to Ti ratio within a sample and between samples. Electron probe microanalysis confirmed that the overall Zr to Ti ratio of the pad PZT was 52/48, and higher amounts of excess PbO in failed samples, whereas, inductively coupled plasma mass spectrometry revealed the presence of Mn, Al, and Sb (dopants) and presence of Cu (sintering aid) in in this hard (pad) PZT. Additionally, three exothermic peaks during thermal analysis was indicative of incomplete calcination of pad PZT. Moreover, transmission electron microscopy and scanning transmission electron microscopy revealed the presence of parylene at the Ag-pad PZT interface and within the pores of pad PZT (in failed samples subjected to electric fields). This further dilutes the electrical, mechanical, and electromechanical properties of the pad PZT, which in turn detrimentally influences the pulse echo sensitivity.

Contributors

Agent

Created

Date Created
  • 2018

152386-Thumbnail Image.png

Combined photo- and thermionic electron emission from low work function diamond films

Description

In this dissertation, combined photo-induced and thermionic electron emission from low work function diamond films is studied through low energy electron spectroscopy analysis and other associated techniques. Nitrogen-doped, hydrogen-terminated diamond

In this dissertation, combined photo-induced and thermionic electron emission from low work function diamond films is studied through low energy electron spectroscopy analysis and other associated techniques. Nitrogen-doped, hydrogen-terminated diamond films prepared by the microwave plasma chemical vapor deposition method have been the most focused material. The theme of this research is represented by four interrelated issues. (1) An in-depth study describes combined photo-induced and thermionic emission from nitrogen-doped diamond films on molybdenum substrates, which were illuminated with visible light photons, and the electron emission spectra were recorded as a function of temperature. The diamond films displayed significant emissivity with a low work function of ~ 1.5 eV. The results indicate that these diamond emitters can be applied in combined solar and thermal energy conversion. (2) The nitrogen-doped diamond was further investigated to understand the physical mechanism and material-related properties that enable the combined electron emission. Through analysis of the spectroscopy, optical absorbance and photoelectron microscopy results from sample sets prepared with different configurations, it was deduced that the photo-induced electron generation involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. (3) Based on results from the first two studies, possible photon-enhanced thermionic emission was examined from nitrogen-doped diamond films deposited on silicon substrates, which could provide the basis for a novel approach for concentrated solar energy conversion. A significant increase of emission intensity was observed at elevated temperatures, which was analyzed using computer-based modeling and a combination of different emission mechanisms. (4) In addition, the electronic structure of vanadium-oxide-terminated diamond surfaces was studied through in-situ photoemission spectroscopy. Thin layers of vanadium were deposited on oxygen-terminated diamond surfaces which led to oxide formation. After thermal annealing, a negative electron affinity was found on boron-doped diamond, while a positive electron affinity was found on nitrogen-doped diamond. A model based on the barrier at the diamond-oxide interface was employed to analyze the results. Based on results of this dissertation, applications of diamond-based energy conversion devices for combined solar- and thermal energy conversion are proposed.

Contributors

Agent

Created

Date Created
  • 2013