6,993 research outputs found
Cation Alloying as a Pathway to Reproducible olution Based Preparation of Efficient Metal Halide Perovskites Solar Cells with Increased Stability
With a certain amount of serendipity, research on dye sensitized solar cells led to the discovery of metal halide perovskite semiconductors as a solar energy conversion material1,2 that has inspired world wide research activities leading to efficiency increases from about 10 in 20123,4 to now 22 .5 8 Apart from their use as a single junction solar cell technology, metal halide perovskites can be processed as an add on layer onto other solar cells to realize efficient and low cost tandem architectures. The perovskite band gap can be tuned continuously from 1.1 to 3.0 eV by chemical engineering which makes them particularly relevant for multi junction devices.9 12 Experimental demonstrations of efficient tandem devices comprising metal halide perovskites as one13 15 or both10,16 components in a tandem stack highlight the potential for scalable and low cost multi junction devices17 with efficiencies approaching 30 is considered feasible.18,12,19,20 To reach this performance in a perovskite silicon tandem, the ideal band gap of the absorber material in top device should be about 1.7 eV18,12,21 sine silicon has a band gap of 1.1 eV .11,22,23 In this review, we dedicate section I to the discussion of the chemical tunability of metal halide perovskite with a particular focus on absorber materials with absorption onsets around 1.6 eV. Section II reflects on the benefit and role of including cesium Cs highlighting the work by Saliba et al.5,6 as this approach demonstrates a reliable pathway to obtain efficient solution processed metal halide perovskite absorbers with high reproducibility and extended operational device stability. Section III focuses on the state of the art of perovskite photovoltaics as a function of the absorption onset of the absorber layer highlighting materials with band gaps between 1.6 eV 1.75 eV as these are of great importance to the development of efficient tandem solar cells.12,18 20 In the conclusion section IV, we reflect on more general trends in metal halide alloys partially iscussed elsewhere23 highlighting cation alloying as an approach to obtain highly efficient devices in the band gap range between 1.6 and 1.7 eV. The data shown and discussed here is dominantly based on three of our recent publications and the interested reader is kindly referred to Unger et al.23 and Saliba et al.5,6 for more in depth discussions and experimental details
Unjamming of Granular Packings due to Local Perturbations: Stability and Decay of Displacements
We study the mechanical response generated by local deformations in jammed
packings of rigid disks. Based on discrete element simulations we determine the
critical force of the local perturbation that is needed to break the mechanical
equilibrium and examine the generated displacement field. Displacements decay
as a power law of the distance from the perturbation point. The decay exponent
and the critical force exhibit nontrivial dependence on the friction: Both
quantities are nonmonotonic and have a sharp maximum at the friction
coefficient 0.1. We find that the mechanical response properties are closely
related to the problem of force-indeterminacy where similar nonmonotonic
behavior was observed previously. We establish direct connection between the
critical force and the ensemble of static force networks.Comment: 4 pages, 4 figure
Frictional coupling between sliding and spinning motion
We show that the friction force and torque, acting at a dry contact of two
objects moving and rotating relative to each other, are inherently coupled. As
a simple test system, a sliding and spinning disk on a horizontal flat surface
is considered. We calculate, and also measure, how the disk is slowing down,
and find that it always stops its sliding and spinning motion at the same
moment. We discuss the impact of this coupling between friction force and
torque on the physics of granular materials.Comment: 4 pages, 5 figures; submitte
Survival Analysis of Complete Veneer Crowns vs. Multisurface Restorations: A Dental School Patient Population
The purpose of this study was to compare the longevity of crowns versus large multisurface restorations in posterior teeth. The investigation used the treatment database at Virginia Commonwealth University School of Dentistry. The inclusion criteria for the final data set used for analysis were: only one restored tooth per patient, premolars with three or more restored surfaces, molars with four or more restored surfaces, molars and premolars restored with complete veneer metal crowns, or crowns veneered with metal and porcelain. The Kaplan-Meier approach was used to visualize the survival curves, and the Cox proportional hazards model was used for analysis of predictor variables. The investigation indicates crowns survive longer than large restorations and premolar restorations survive longer than molar restorations. The median survival for crowns exceeded 16.6 years, with the median survival of premolar restorations being 4.4 years and molar restorations 1.3 years. An interaction between age and treatment was discovered, with overall survival decreasing as patient age increases. The doctor supervising the treatment also affected survival with treatment supervised by specialists lasting longer than treatment supervised by nonspecialists
Sub 20 nm Short Channel Carbon Nanotube Transistors
Carbon nanotube field-effect transistors with sub 20 nm long channels and
on/off current ratios of > 1000000 are demonstrated. Individual single-walled
carbon nanotubes with diameters ranging from 0.7 nm to 1.1 nm grown from
structured catalytic islands using chemical vapor deposition at 700 degree
Celsius form the channels. Electron beam lithography and a combination of HSQ,
calix[6]arene and PMMA e-beam resists were used to structure the short channels
and source and drain regions. The nanotube transistors display on-currents in
excess of 15 microA for drain-source biases of only 0.4 Volt.Comment: Nano Letters in pres
On the Nature of Intrinsic Absorption in Reddened Seyfert 1 Galaxies
We discuss the origin of the ``dusty lukewarm absorber'', which we previously
identified in the reddened Seyfert 1 galaxies NGC 3227 and Akn 564. This
absorber is characterized by saturated UV absorption lines (C IV, N V) near the
systemic velocity of the host galaxy, and is likely responsible for reddening
both the continuum and the emission lines (including those from the narrow-line
region) from these Seyferts. From a large sample of Seyfert 1 galaxies, we find
that continuum reddening (as measured by UV color) tends to increase with
inclination of the host galaxy. Furthermore, reddened, inclined Seyfert
galaxies observed at moderate to high spectral resolution all show evidence for
dusty lukewarm absorbers. We suggest that these absorbers lie in the plane of
the host galaxy at distances > 100 pc from the nucleus, and are physically
distinct from the majority of intrinsic absorbers that are outflowing from the
nucleus.Comment: 14 pages, including 2 figures, accepted for publication in the
Astrophysical Journal (Letters
Unjamming due to local perturbations in granular packings with and without gravity
We investigate the unjamming response of disordered packings of frictional
hard disks with the help of computer simulations. First, we generate jammed
configurations of the disks and then force them to move again by local
perturbations. We study the spatial distribution of the stress and displacement
response and find long range effects of the perturbation in both cases. We
record the penetration depth of the displacements and the critical force that
is needed to make the system yield. These quantities are tested in two types of
systems: in ideal homogeneous packings in zero gravity and in packings settled
under gravity. The penetration depth and the critical force are sensitive to
the interparticle friction coefficient. Qualitatively, the same nonmonotonic
friction dependence is found both with and without gravity, however the
location of the extrema are at different friction values. We discuss the role
of the connectivity of the contact network and of the pressure gradient in the
unjamming response.Comment: 12 pages, 13 figure
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Enrollment of adolescents and young adults onto SWOG cancer research network clinical trials: A comparative analysis by treatment site and era.
BackgroundFew adolescents and young adults (AYAs, 15-39 years old) enroll onto cancer clinical trials, which hinders research otherwise having the potential to improve outcomes in this unique population. Prior studies have reported that AYAs are more likely to receive cancer care in community settings. The National Cancer Institute (NCI) has led efforts to increase trial enrollment through its network of NCI-designated cancer centers (NCICC) combined with community outreach through its Community Clinical Oncology Program (CCOP; replaced by the NCI Community Oncology Research Program in 2014).MethodsUsing AYA proportional enrollment (the proportion of total enrollments who were AYAs) as the primary outcome, we examined enrollment of AYAs onto SWOG therapeutic trials at NCICC, CCOP, and non-NCICC/non-CCOP sites from 2004 to 2013 by type of site, study period (2004-08 vs 2009-13), and patient demographics.ResultsOverall, AYA proportional enrollment was 10.1%. AYA proportional enrollment decreased between 2004-2008 and 2009-2013 (13.1% vs 8.5%, P < .001), and was higher at NCICCs than at CCOPs and non-NCICC/non-CCOPs (14.1% vs 8.3% and 9.2%, respectively; P < .001). AYA proportional enrollment declined significantly at all three site types. Proportional enrollment of AYAs who were Black or Hispanic was significantly higher at NCICCs compared with CCOPs or non-NCICC/non-CCOPs (11.5% vs 8.8, P = .048 and 11.5% vs 8.6%, P = .03, respectively).ConclusionNot only did community sites enroll a lower proportion of AYAs onto cancer clinical trials, but AYA enrollment decreased in all study settings. Initiatives aimed at increasing AYA enrollment, particularly in the community setting with attention to minority status, are needed
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