2,617 research outputs found
[Review of] Karl H. Schlesier. The Wolves of Heaven: Cheyenne Shamanism, Ceremonies, and Prehistoric Origins
Schlesier has a necessary footnote advisory to readers explaining the way in which he has structured The Wolves of Heaven. In the advisory Schelesier [Schlesier] writes that the book is a slow read on purpose so as to develop the story of how it was that the Tsistsistas (Cheyenne), came out of the boreal forest to become hunters of the northern plains, evolving eventually into the 19th century Tsistsistas bison hunting horse nomads
Exciton Trapping Is Responsible for the Long Apparent Lifetime in Acid-Treated MoS2
Here, we show that deep trapped "dark" exciton states are responsible for the
surprisingly long lifetime of band-edge photoluminescence in acid-treated
single-layer MoS2. Temperature-dependent transient photoluminescence
spectroscopy reveals an exponential tail of long-lived states extending
hundreds of meV into the band gap. These sub-band states, which are
characterized by a 4 microsecond radiative lifetime, quickly capture and store
photogenerated excitons before subsequent thermalization up to the band edge
where fast radiative recombination occurs. By intentionally saturating these
trap states, we are able to measure the "true" 150 ps radiative lifetime of the
band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature.
These experiments reveal the dominant role of dark exciton states in
acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at
room temperature in trap states below the band edge. We hypothesize that these
states are associated with native structural defects, which are not introduced
by the superacid treatment; rather, the superacid treatment dramatically
reduces non-radiative recombination through these states, extending the exciton
lifetime and increasing the likelihood of eventual radiative recombination
Can disorder enhance incoherent exciton diffusion?
Recent experiments aimed at probing the dynamics of excitons have revealed
that semiconducting films composed of disordered molecular subunits, unlike
expectations for their perfectly ordered counterparts, can exhibit a
time-dependent diffusivity in which the effective early time diffusion constant
is larger than that of the steady state. This observation has led to
speculation about what role, if any, microscopic disorder may play in enhancing
exciton transport properties. In this article, we present the results of a
model study aimed at addressing this point. Specifically, we present a general
model, based upon F\"orster theory, for incoherent exciton diffusion in a
material composed of independent molecular subunits with static energetic
disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule
transition rates which we demonstrate has two important consequences related to
exciton transport. First, the distribution of local site-specific diffusivity
is broadened in a manner that results in a decrease in average exciton
diffusivity relative to that in a perfectly ordered film. Second, since
excitons prefer to make transitions that are downhill in energy, the steady
state distribution of exciton energies is biased towards low energy molecular
subunits, those that exhibit reduced diffusivity relative to a perfectly
ordered film. These effects combine to reduce the net diffusivity in a manner
that is time dependent and grows more pronounced as disorder is increased.
Notably, however, we demonstrate that the presence of energetic disorder can
give rise to a population of molecular subunits with exciton transfer rates
exceeding that of subunits in an energetically uniform material. Such
enhancements may play an important role in processes that are sensitive to
molecular-scale fluctuations in exciton density field.Comment: 15 pages, 3 figure
Nonequilibrium dynamics of localized and delocalized excitons in colloidal quantum dot solids
Self-assembled quantum dot (QD) solids are a highly tunable class of
materials with a wide range of applications in solid-state electronics and
optoelectronic devices. In this perspective, we highlight how the presence of
microscopic disorder in these materials can influence their macroscopic
optoelectronic properties. Specifically, we consider the dynamics of excitons
in energetically disordered QD solids using a theoretical model framework for
both localized and delocalized excitonic regimes. In both cases, we emphasize
the tendency of energetic disorder to promote nonequilibrium relaxation
dynamics and discuss how the signatures of these nonequilibrium effects
manifest in time-dependent spectral measurements. Moreover, we describe the
connection between the microscopic dynamics of excitons within the material and
the measurement of material specific parameters, such as emission linewidth
broadening and energetic dissipation rate.Comment: 4 figure
Geological mapping of an area along the Frisco Railroad between Cuba and Saint James
The chief object of the work has been the location of the Roubidoux-Jefferson City contact, if it occurred within a mile of the railroad. Distances and positions along the track were obtained by the use of a map and by counting telephone poles. Other locations were made by obtaining their bearing and distance from these points by the use of a compass and by pacing. Elevations were carried by aneroid barometers, using the railroad elevations at mile points as bench marks. An attempt has been made to locate all important outcrops; but these are shown only on the field map. Extreme accuracy has not been attempted; however, the chief drainage courses have been fairly accurately located, and the contours give a fair idea of the topography and elevations --Introduction, Methods of Field Work, page 1
Attendance Feedback In An Academic Setting: Preliminary Results
In the fall of 2005, the attendance behavior of 118 business students at Northern State University (NSU) was monitored in 4 classes. After 10 weeks of classes Absenteeism Feedback was given to these students. Examination of the data indicated a strong correlation between attendance and subsequent course grade
Calibrating field sprayers (1997)
Pesticides are most cost-effective and perform optimally when applied at labeled rates. Too much pesticide can injure crops; too little can give inadequate or unreliable control. Careful calibration is essential to proper sprayer operation, and the economic return on the small investment of time required can be substantial
Effects of intra- and inter-laminar resin content on the mechanical properties of toughened composite materials
Composite materials having multiphase toughened matrix systems and laminate architectures characterized by resin-rich interlaminar layers (RIL) have been the subject of much recent attention. Such materials are likely to find applications in thick compressively loaded structures such as the keel area of commercial aircraft fuselages. The effects of resin content and its interlaminar and intralaminar distribution on mechanical properties were investigated with test and analysis of two carbon-epoxy systems. The RIL was found to reduce the in situ strengthening effect for matrix cracking in laminates. Mode 2 fracture toughness was found to increase with increasing RIL thickness over the range investigated, and Mode 1 interlaminar toughness was negligibly affected. Compressive failure strains were found to increase with increasing resin content for specimens having no damage, holes, and impact damage. Analytical tools for predicting matrix cracking of off-axis plies and damage tolerance in compression after impact (CAI) were successfully applied to materials with RIL
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