55 research outputs found
Auger Recombination Lifetime Scaling for Type-I and Quasi-Type-II Core/Shell Quantum Dots
Having already achieved near-unity quantum yields, with promising properties
for light-emitting diode, lasing, and charge separation applications, colloidal
core/shell quantum dots have great technological potential. The shell thickness
and band alignment of the shell and core materials are known to influence the
efficiency of these devices. In many such applications, a key to improving the
efficiency requires a deep understanding of multiexcitonic states. Herein, we
elucidate the shell thickness and band alignment dependencies of the biexciton
Auger recombination lifetime for quasi-type-II CdSe/CdS and type-I CdSe/ZnS
core/shell quantum dots. We find that the biexciton Auger recombination
lifetime increases with the total nanocrystal volume for quasi-type-II CdSe/CdS
core/shell quantum dots and is independent of the shell thickness for type-I
CdSe/ZnS core/shell quantum dots. In order to perform these calculations and
compute Auger recombination lifetimes, we developed a low-scaling approach
based on the stochastic resolution of identity. The numerical approach provided
a framework to study the scaling of the biexciton Auger recombination lifetimes
in terms of the shell thickness dependencies of the exciton radii, Coulomb
couplings, and density of final states in quasi-type-II CdSe/CdS and type-I
CdSe/ZnS core/shell quantum dots.Comment: 7 pages, 4 figure
Electron-hole correlations govern Auger recombination in nanostructures
The fast nonradiative decay of multiexcitonic states via Auger recombination
is a fundamental process affecting a variety of applications based on
semiconductor nanostructures. From a theoretical perspective, the description
of Auger recombination in confined semiconductor nanostructures is a
challenging task due to the large number of valance electrons and exponentially
growing number of excited excitonic and biexcitonic states that are coupled by
the Coulomb interaction. These challenges have restricted the treatment of
Auger recombination to simple, noninteracting electron-hole models. Herein we
present a novel approach for calculating Auger recombination lifetimes in
confined nanostructures having thousands to tens of thousands of electrons,
explicitly including electron-hole interactions. We demonstrate that the
inclusion of electron-hole correlations are imperative to capture the correct
scaling of the Auger recombination lifetime with the size and shape of the
nanostructure. In addition, correlation effects are required to obtain
quantitatively accurate lifetimes even for systems smaller than the exciton
Bohr radius. Neglecting such correlations can result in lifetimes that are 2
orders of magnitude too long. We establish the utility of the new approach for
CdSe quantum dots of varying sizes and for CdSe nanorods of varying diameters
and lengths. Our new approach is the first theoretical method to postdict the
experimentally known universal volume scaling law for quantum dots and makes
novel predictions for the scaling of the Auger recombination lifetimes in
nanorods
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Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks.
The goal of this work is to identify favored pathways for preparation of defect-resilient attached wurtzite CdX (X = S, Se, Te) nanocrystals. We seek guidelines for oriented attachment of faceted nanocrystals that are most likely to yield pairs of nanocrystals with either few or no electronic defects or electronic defects that are in and of themselves desirable and stable. Using a combination of in situ high-resolution transmission electron microscopy (HRTEM) and electronic structure calculations, we evaluate the relative merits of atomic attachment of wurtzite CdSe nanocrystals on the {11Μ
00} or {112Μ
0} family of facets. Pairwise attachment on either facet can lead to perfect interfaces, provided the nanocrystal facets are perfectly flat and the angles between the nanocrystals can adjust during the assembly. Considering defective attachment, we observe for {11Μ
00} facet attachment that only one type of edge dislocation forms, creating deep hole traps. For {112Μ
0} facet attachment, we observe that four distinct types of extended defects form, some of which lead to deep hole traps whereas others only to shallow hole traps. HRTEM movies of the dislocation dynamics show that dislocations at {11Μ
00} interfaces can be removed, albeit slowly. Whereas only some extended defects at {112Μ
0} interfaces could be removed, others were trapped at the interface. Based on these insights, we identify the most resilient pathways to atomic attachment of pairs of wurtzite CdX nanocrystals and consider how these insights can translate to the creation of electronically useful materials from quantum dots with other crystal structures
Molecular van der Waals fluids in cavity quantum electrodynamics
Intermolecular van der Waals interactions are central to chemical and
physical phenomena ranging from biomolecule binding to soft-matter phase
transitions. However, there are currently very limited approaches to manipulate
van der Waals interactions. In this work, we demonstrate that strong
light-matter coupling can be used to tune van der Waals interactions, and,
thus, control the thermodynamic properties of many-molecule systems. Our
analysis reveals orientation-dependent intermolecular interactions between van
der Waals molecules (for example, H) that depend on the distance between
the molecules as and . Moreover, we employ non-perturbative
\textit{ab initio} cavity quantum electrodynamics calculations to develop
machine learning-based van der Waals interaction potentials for molecules
inside optical cavities. By simulating fluids of up to H
molecules, we demonstrate that strong light-matter coupling can tune the
structural and thermodynamic properties of molecular fluids. In particular, we
observe collective orientational order in many-molecule systems as a result of
cavity-modified van der Waals interactions. These simulations and analyses
demonstrate both local and collective effects induced by strong light-matter
coupling and open new paths for controlling the properties of condensed phase
systems.Comment: 6 pages and 3 figures in main text - 15 pages and 12 figures
supplemental materia
Charge Carrier Dynamics in Photocatalytic Hybrid Semiconductor-Metal Nanorods: Crossover from Auger Recombination to Charge Transfer
Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique, synergistic electronic and optical properties as a result of combining semiconductor and metal physics via a controlled interface. These structures can exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. The combination of the photocatalytic activity of the metal domain with the ability to generate and accommodate multiple excitons in the semiconducting domain can lead to improved photocatalytic performance because injecting multiple charge carriers into the active catalytic sites can increase the quantum yield. Herein, we show a significant metal domain size dependence of the charge carrier dynamics as well as the photocatalytic hydrogen generation efficiencies under nonlinear excitation conditions. An understanding of this size dependence allows one to control the charge carrier dynamics following the absorption of light. Using a model hybrid semiconductor-metal CdS-Au nanorod system and combining transient absorption and hydrogen evolution kinetics, we reveal faster and more efficient charge separation and transfer under multiexciton excitation conditions for large metal domains compared to small ones. Theoretical modeling uncovers a competition between the kinetics of Auger recombination and charge separation. A crossover in the dominant process from Auger recombination to charge separation as the metal domain size increases allows for effective multiexciton dissociation and harvesting in large metal domain HNPs. This was also found to lead to relative improvement of their photocatalytic activity under nonlinear excitation conditions
Informing the design of a national screening and treatment programme for chronic viral hepatitis in primary care: qualitative study of at-risk immigrant communities and healthcare professionals
n Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain
Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article,
unless otherwise statedThis paper presents independent research funded by the National Institute
for Health Research (NIHR) under the Programme Grants for Applied
Research programme (RP-PG-1209-10038).
Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals
Nonradiative processes limit optoelectronic functionality of nanocrystals and
curb their device performance. Nevertheless, the dynamic structural origins of
nonradiative relaxations in nanocrystals are not understood. Here, femtosecond
electron diffraction measurements corroborated by atomistic simulations uncover
transient lattice deformations accompanying radiationless electronic processes
in semiconductor nanocrystals. Investigation of the excitation energy
dependence shows that hot carriers created by a photon energy considerably
larger than the bandgap induce structural distortions at nanocrystal surfaces
on few picosecond timescales associated with the localization of trapped holes.
On the other hand, carriers created by a photon energy close to the bandgap
result in transient lattice heating that occurs on a much longer 200 ps
timescale, governed by an Auger heating mechanism. Elucidation of the
structural deformations associated with the surface trapping of hot holes
provides atomic-scale insights into the mechanisms deteriorating optoelectronic
performance and a pathway towards minimizing these losses in nanocrystal
devices.Comment: 17 pages, 4 figure
Illness perceptions and explanatory models of viral hepatitis B & C among immigrants and refugees: a narrative systematic review.
Β© 2015 Owiti et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain
Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article,
unless otherwise stated.BACKGROUND: Hepatitis B and C (HBV, HCV) infections are associated with high morbidity and mortality. Many countries with traditionally low prevalence (such as UK) are now planning interventions (screening, vaccination, and treatment) of high-risk immigrants from countries with high prevalence. This review aimed to synthesise the evidence on immigrants' knowledge of HBV and HCV that might influence the uptake of clinical interventions. The review was also used to inform the design and successful delivery of a randomised controlled trial of targeted screening and treatment. METHODS: Five databases (PubMed, CINHAL, SOCIOFILE, PsycINFO & Web of Science) were systematically searched, supplemented by reference tracking, searches of selected journals, and of relevant websites. We aimed to identify qualitative and quantitative studies that investigated knowledge of HBV and HCV among immigrants from high endemic areas to low endemic areas. Evidence, extracted according to a conceptual framework of Kleinman's explanatory model, was subjected to narrative synthesis. We adapted the PEN-3 model to categorise and analyse themes, and recommend strategies for interventions to influence help-seeking behaviour. RESULTS: We identified 51 publications including quantitative (nβ=β39), qualitative (nβ=β11), and mixed methods (nβ=β1) designs. Most of the quantitative studies included small samples and had heterogeneous methods and outcomes. The studies mainly concentrated on hepatitis B and ethnic groups of South East Asian immigrants residing in USA, Canada, and Australia. Many immigrants lacked adequate knowledge of aetiology, symptoms, transmission risk factors, prevention strategies, and treatment, of hepatitis HBV and HCV. Ethnicity, gender, better education, higher income, and English proficiency influenced variations in levels and forms of knowledge. CONCLUSION: Immigrants are vulnerable to HBV and HCV, and risk life-threatening complications from these infections because of poor knowledge and help-seeking behaviour. Primary studies in this area are extremely diverse and of variable quality precluding meta-analysis. Further research is needed outside North America and Australia
Age-Dependent Maturation of Toll-Like Receptor-Mediated Cytokine Responses in Gambian Infants
The global burden of neonatal and infant mortality due to infection is
staggering, particularly in resource-poor settings. Early childhood vaccination
is one of the major interventions that can reduce this burden, but there are
specific limitations to inducing effective immunity in early life, including
impaired neonatal leukocyte production of Th1-polarizing cytokines to many
stimuli. Characterizing the ontogeny of Toll-like receptor (TLR)-mediated innate
immune responses in infants may shed light on susceptibility to infection in
this vulnerable age group, and provide insights into TLR agonists as candidate
adjuvants for improved neonatal vaccines. As little is known about the leukocyte
responses of infants in resource-poor settings, we characterized production of
Th1-, Th2-, and anti-inflammatory- cytokines in response to agonists of TLRs 1-9
in whole blood from 120 Gambian infants ranging from newborns (cord blood) to 12
months of age. Most of the TLR agonists induced TNFΞ±, IL-1Ξ², IL-6, and
IL-10 in cord blood. The greatest TNFΞ± responses were observed for TLR4, -5,
and -8 agonists, the highest being the thiazoloquinoline CLO75 (TLR7/8) that
also uniquely induced cord blood IFNΞ³ production. For most agonists,
TLR-mediated TNFΞ± and IFNΞ³ responses increased from birth to 1 month of
age. TLR8 agonists also induced the greatest production of the Th1-polarizing
cytokines TNFΞ± and IFNΞ³ throughout the first year of life, although the
relative responses to the single TLR8 agonist and the combined TLR7/8 agonist
changed with age. In contrast, IL-1Ξ², IL-6, and IL-10 responses to most
agonists were robust at birth and remained stable through 12 months of age.
These observations provide fresh insights into the ontogeny of innate immunity
in African children, and may inform development of age-specific adjuvanted
vaccine formulations important for global health
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