193 research outputs found
State-of-the-Art Scanning Electron Microscope for Multiple Applications: From Solar-Cells to Megaspores
The University of Oklahoma (OU) recently acquired a Zeiss Neon, a dual beam
SEM/FIB Cross beam machine. This thesis focuses on its utilization via two
independent projects, one involving the characterization of solar-cells semicon-
ductor materials while the other focuses on the FIB to provide information
about a preserved megaspore.
Part I: Solar-Cells
Third generation photovoltaics (PVs) seek to increase the efficiency of PVs and
reduce their cost. One candidate to this third generation involves the use of
quantum dot (QD) structures to create an intermediate band-gap. This de-
vice can overcome the Shockley-Queisser efficiency limit (about 30%) by better
utilizing the solar spectrum. However, the current results are far from the the-
oretical limit, partly because of the material quality of the QDs structure. This
project involves high-density self-assembled InAs/GaAs 1−x Sb x QDs grown by
molecular beam epitaxy at the University of Oklahoma (OU). Ultimately, size,
shape, density, composition, and crystalline quality of the QDs as well as sim-
ilar details for the matrix layers play an important role in the PV efficiency.
Hence, the characterization of these properties is important. Characterizing
multiple layers of QDs will ultimately require cross-sectional transmission microscopy; however, along the way, atomic force microscopy (AFM) and field-
emission scanning electron microscopy (FE-SEM) will more easily yield details
about the uncapped top QD layer. For example AFM accurately determines
QD density, and quantitatively describes QD shape. On the other hand, AFM
is sensitive to tip/probe-shape artifacts. Cross-sectional FE-SEM can clear up
some of these artifacts and indicate crystalline quality; cross-sectional FE-SEM
was found as a means of giving better lateral resolution than our typical AFM
results. Ultimately, cross-sectional images from a transmission electron micro-
scope (TEM), would provide the best information, however pitfalls and machine
breakage delayed us in our work; while SEM/FIB in situ TEM preparation is
now possible, it could not be operated
Part II: Megaspores
Optical and electron microscopy are critical tools for studying preserved and fos-
silized organisms. Due to its early development and refinement, light microscopy
has dominated the study of organic-walled fossils. Taxonomic identification and
discrimination is almost exclusively based on features visible under transmitted
light. However electron microscopy, and in particular TEM, has been applied to
investigations of the ultrastructure of the walls and ornament that are beyond
the resolution of light microscopy. Hypotheses for the assembly of the walls have
been formulated based on these data. Recently new techniques, such as FIB,
SEM, and synchrotron radiation tomography, have expanded the boundaries of
imaging fossils. This thesis part focuses on investigating Arcelites Hexapartitus
species megaspore, using an SEM with FIB to obtain three dimensional infor-
mation about their inner exine channels. Channels were readily imaged through
the inner exine using a number of cross-beam geometries. The channels seem
to be simply connected running through the inner part of the exine layers, however, the channels/pores are found to stop before exiting this inner exine layer
at both its inner and outer surfaces. Additionally, it was found that they are
not continuous this inner exine layer. Investigations concerning the material
obstructing the channels have begun. So far, there is no evidence of a foreign
material present. It appears that the pores simply end before exiting the layers
and have occasional nano sized regions of sporopollenin, the megaspore wall
material, blocking the channels
ENSO and interdecadal climate variability over the last century documented by geochemical records of two coral cores from the South West Pacific
The south west Pacific is affected by climatic phenomena such as ENSO (El Niño Southern Oscillation) or the PDO (Pacific Decadal Oscillation). Near-monthly resolution calibrations of Sr/Ca, U/Ca and <span style="font-family: 'Symbol, Times New Roman, Times'; font-size: 16px;">δ</span><sup>18</sup>Oc were made on corals taken from New Caledonia and Wallis Island. These geochemical variations could be linked to SST (sea surface temperature) and SSS (sea surface salinity) variations over the last two decades, itselves dependent on ENSO occurrences. On the other hand, near-half-yearly resolution over the last century smoothes seasonal and interannual climate signals, but emphasizes low frequency climate variability
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