127 research outputs found
Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals
For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger
recombination (AR) and carrier multiplication (CM) lifetimes are computed
exactly in a three-dimensional real space grid using empirical pseudopotential
wave functions. Our results in support of recent experimental data offer new
predictions. We extract simple Auger constants valid for NCs. We show that both
Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due
to the fact that under an optical excitation exceeding twice the band gap
energy, the electrons gain lion's share from the total excess energy and can
cause a CM. We predict that CM becomes especially efficient for hot electrons
with an excess energy of about 1 eV above the CM threshold.Comment: 4 pages, 6 figures (Published version
Determination of critical nondimensional parameters in aircraft dynamic response to random input
The critical parameters of subsonic jet aircraft response in a random atmospheric environment are determined. Equations of motion are presented for semirigid aircraft with a flexible primary airfoil. However, the analysis is easily extendable to include additional appendage flexibility. The analysis establishes the mechanical admittance values for pitching, plunging, and the first mode effects from wing elastic bending and torsion. Nondimensional parameters are established which allow the representation of all subsonic jet transport aircraft with one nondimensional model. The critical parameters for random forcing are found to be aircraft relative mass, reduced natural and forcing frequencies, and Mach number. Turbulence scale lengths are found to be directly related to the critical values of reduced forcing frequency. Results are given for subsonic craft traveling at constant altitude. Specific values of admittance functions are tabulated at Mach numbers of 0.2, 0.5, and 0.7. The relative mass range covers all aircraft currently in operation
Antiprotozoal activity of natural products from Nigerien plants used in folk medicine
In the course of the screening of plants from Niger for antiprotozoal activity, the methanol extract of Cassia sieberiana, and the dichloromethane extracts of Ziziphus mauritiana and Sesamun alatum were found to be active against protozoan parasites, namely Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani and/or Plasmodium falciparum. Myricitrin (1), quercitrin (2) and 1-palmitoyl-lysolecithin (3) were isolated from C. sieberiana. From Z. mauritiana, the three triterpene derivatives 13, 15, and 16 are described here for the first time. Their chemical structures were determined by 1D and 2D NMR experiments, UV, IR and HRESIMS data. The absolute configurations were assigned via comparison of the experimental and calculated ECD spectra. In addition, eight known cyclopeptide alkaloids (4, 5, 7-12), and five known triterpenoids (6, 14, 17-19) were isolated. The antiprotozoal activity of the isolated compounds, as well as of eleven quinone derivatives (20-30) previously isolated from S. alatum was determined in vitro. The cytotoxicity in L6 rat myoblast cells was also evaluated. Compound 18 showed the highest antiplasmodial activity (IC(50) = 0.2 microm) and compound 24 inhibited T. b. rhodesiense with an IC(50) value of 0.007 microM. However, it also displayed significant cytotoxicity in L6 cells (IC(50) = 0.4 microm)
Isolation and structural elucidation of compounds from Pleiocarpa bicarpellata and their in vitro antiprotozoal activity
Species of the genus Pleiocarpa are used in traditional medicine against fever and malaria. The present study focuses on the isolation and identification of bioactive compounds from P. bicarpellata extracts, and the evaluation of their antiprotozoal activity. Fractionation and isolation combined to LC-HRMS/MS-based dereplication provided 16 compounds: seven indole alkaloids, four indoline alkaloids, two secoiridoid glycosides, two iridoid glycosides, and one phenolic glucoside. One of the quaternary indole alkaloids (7) and one indoline alkaloid (15) have never been reported before. Their structures were elucidated by analysis of spectroscopic data, including 1D and 2D NMR experiments, UV, IR, and HRESIMS data. The absolute configurations were determined by comparison of the experimental and calculated ECD data. The extracts and isolated compounds were evaluated for their antiprotozoal activity towards Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani, and Plasmodium falciparum, as well as for their cytotoxicity against rat skeletal myoblast L6 cells. The dichloromethane/methanol (1:1) root extract showed strong activity against P. falciparum (IC50 value of 3.5 microg/mL). Among the compounds isolated, tubotaiwine (13) displayed the most significant antiplasmodial activity with an IC50 value of 8.5 microM and a selectivity index of 23.4. Therefore, P. bicarpallata extract can be considered as a source of indole alkaloids with antiplasmodial activity
Evaluation of Retinal Image Quality Assessment Networks in Different Color-spaces
Retinal image quality assessment (RIQA) is essential for controlling the
quality of retinal imaging and guaranteeing the reliability of diagnoses by
ophthalmologists or automated analysis systems. Existing RIQA methods focus on
the RGB color-space and are developed based on small datasets with binary
quality labels (i.e., `Accept' and `Reject'). In this paper, we first
re-annotate an Eye-Quality (EyeQ) dataset with 28,792 retinal images from the
EyePACS dataset, based on a three-level quality grading system (i.e., `Good',
`Usable' and `Reject') for evaluating RIQA methods. Our RIQA dataset is
characterized by its large-scale size, multi-level grading, and multi-modality.
Then, we analyze the influences on RIQA of different color-spaces, and propose
a simple yet efficient deep network, named Multiple Color-space Fusion Network
(MCF-Net), which integrates the different color-space representations at both a
feature-level and prediction-level to predict image quality grades. Experiments
on our EyeQ dataset show that our MCF-Net obtains a state-of-the-art
performance, outperforming the other deep learning methods. Furthermore, we
also evaluate diabetic retinopathy (DR) detection methods on images of
different quality, and demonstrate that the performances of automated
diagnostic systems are highly dependent on image quality.Comment: Accepted by MICCAI 2019. Corrected two typos in Table 1 as: (1) in
training set, the number of "Usable + All" should be '1,876'; (2) In testing
set, the number of "Total + DR-0" should be '11,362'. Project page:
https://github.com/hzfu/Eye
Boron Nitride Monolayer: A Strain-Tunable Nanosensor
The influence of triaxial in-plane strain on the electronic properties of a
hexagonal boron-nitride sheet is investigated using density functional theory.
Different from graphene, the triaxial strain localizes the molecular orbitals
of the boron-nitride flake in its center depending on the direction of the
applied strain. The proposed technique for localizing the molecular orbitals
that are close to the Fermi level in the center of boron nitride flakes can be
used to actualize engineered nanosensors, for instance, to selectively detect
gas molecules. We show that the central part of the strained flake adsorbs
polar molecules more strongly as compared with an unstrained sheet.Comment: 20 pages, 9 figure
Processing of ultrafine-size particulate metal matrix composites by advanced shear technology
Copyright @ 2009 ASM International. This paper was published in Metallurgical & Materials Transactions A 40A(3) and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Lack of efficient mixing technology to achieve a uniform distribution of fine-size reinforcement within the matrix and the high cost of producing components have hindered the widespread adaptation of particulate metal matrix composites (PMMCs) for engineering applications. A new rheo-processing method, the melt-conditioning high-pressure die-cast (MC-HPDC) process, has been developed for manufacturing near-net-shape components of high integrity. The MC-HPDC process adapts the well-established high shear dispersive mixing action of a twin-screw mechanism to the task of overcoming the cohesive force of the agglomerates under a high shear rate and high intensity of turbulence. This is followed by direct shaping of the slurry into near-net-shape components using an existing cold-chamber die-casting process. The results indicate that the MC-HPDC samples have a uniform distribution of ultrafine-sized SiC particles throughout the entire sample in the as-cast condition. Compared to those produced by conventional high-pressure die casting (HPDC), MC-HPDC samples have a much improved tensile strength and ductility.EP-SR
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
Simulation of dimensionality effects in thermal transport
The discovery of nanostructures and the development of growth and fabrication
techniques of one- and two-dimensional materials provide the possibility to
probe experimentally heat transport in low-dimensional systems. Nevertheless
measuring the thermal conductivity of these systems is extremely challenging
and subject to large uncertainties, thus hindering the chance for a direct
comparison between experiments and statistical physics models. Atomistic
simulations of realistic nanostructures provide the ideal bridge between
abstract models and experiments. After briefly introducing the state of the art
of heat transport measurement in nanostructures, and numerical techniques to
simulate realistic systems at atomistic level, we review the contribution of
lattice dynamics and molecular dynamics simulation to understanding nanoscale
thermal transport in systems with reduced dimensionality. We focus on the
effect of dimensionality in determining the phononic properties of carbon and
semiconducting nanostructures, specifically considering the cases of carbon
nanotubes, graphene and of silicon nanowires and ultra-thin membranes,
underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
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