427 research outputs found
Multiple exciton generation in VO2
Multiple exciton generation (MEG) is a widely studied phenomenon in
semiconductor nanocrystals and quantum dots, aimed at improving the energy
conversion efficiency of solar cells. MEG is the process wherein incident
photon energy is significantly larger than the band gap, and the resulting
photoexcited carriers relax by generating additional electron-hole pairs,
rather than decaying by heat dissipation. Here, we present an experimental
demonstration of MEG in a prototype strongly correlated material, VO2, through
photocurrent spectroscopy and ultrafast transient reflectivity measurements,
both of which are considered the most prominent ways for detecting MEG in
working devices. The key result of this paper is the observation of MEG at room
temperature (in a correlated insulating phase of VO2), and the estimated
threshold for MEG is 3Eg. We demonstrate an escalated photocurrent due to MEG
in VO2, and quantum efficiency is found to exceed 100%. Our studies suggest
that this phenomenon is a manifestation of expeditious impact ionization due to
stronger electron correlations and could be exploited in a large number of
strongly correlated materials.Comment: 6 pages, 5 figures, Physical Review
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Erratum: Consortium biology in immunology: The perspective from the Immunological Genome Project
Tissue Microenvironments Define and Get Reinforced by Macrophage Phenotypes in Homeostasis or during Inflammation, Repair and Fibrosis
Current macrophage phenotype classifications are based on distinct in vitro culture conditions that do not adequately mirror complex tissue environments. In vivo monocyte progenitors populate all tissues for immune surveillance which supports the maintenance of homeostasis as well as regaining homeostasis after injury. Here we propose to classify macrophage phenotypes according to prototypical tissue environments, e.g. as they occur during homeostasis as well as during the different phases of (dermal) wound healing. In tissue necrosis and/or infection, damage- and/or pathogen-associated molecular patterns induce proinflammatory macrophages by Toll-like receptors or inflammasomes. Such classically activated macrophages contribute to further tissue inflammation and damage. Apoptotic cells and antiinflammatory cytokines dominate in postinflammatory tissues which induce macrophages to produce more antiinflammatory mediators. Similarly, tumor-associated macrophages also confer immunosuppression in tumor stroma. Insufficient parenchymal healing despite abundant growth factors pushes macrophages to gain a profibrotic phenotype and promote fibrocyte recruitment which both enforce tissue scarring. Ischemic scars are largely devoid of cytokines and growth factors so that fibrolytic macrophages that predominantly secrete proteases digest the excess extracellular matrix. Together, macrophages stabilize their surrounding tissue microenvironments by adapting different phenotypes as feed-forward mechanisms to maintain tissue homeostasis or regain it following injury. Furthermore, macrophage heterogeneity in healthy or injured tissues mirrors spatial and temporal differences in microenvironments during the various stages of tissue injury and repair. Copyright (C) 2012 S. Karger AG, Base
Transverse Electronic Transport through DNA Nucleotides with Functionalized Graphene Electrodes
Graphene nanogaps and nanopores show potential for the purpose of electrical
DNA sequencing, in particular because single-base resolution appears to be
readily achievable. Here, we evaluated from first principles the advantages of
a nanogap setup with functionalized graphene edges. To this end, we employed
density functional theory and the non-equilibrium Green's function method to
investigate the transverse conductance properties of the four nucleotides
occurring in DNA when located between the opposing functionalized graphene
electrodes. In particular, we determined the electrical tunneling current
variation as a function of the applied bias and the associated differential
conductance at a voltage which appears suitable to distinguish between the four
nucleotides. Intriguingly, we observe for one of the nucleotides a negative
differential resistance effect.Comment: 19 pages, 7 figure
Correlation-Driven Transient Hole Dynamics Resolved in Space and Time in the Isopropanol Molecule
The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or hole) dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump–x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first tentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics
Some empirical issues in research on academic departments: Homogeneity, aggregation, and level of analysis
This study investigated the appropriateness of the academic departmental level of analysis. Homogeneity of faculty members' responses to measures of organizational structure, environmental uncertainty, and task routineness was examined to determine the legitimacy of aggregating those responses to create departmental level variables. Analysis of variance suggested that the structure and environmental uncertainty subscales were measuring departmental level phenomena, but that the task routineness scales were not. Results demonstrate the importance of empirically testing, rather than assuming, levels of analysis in studies of academic departments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43590/1/11162_2004_Article_BF00974806.pd
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