196 research outputs found
Energy-Resolved Femtosecond Hot Electron Dynamics in Single Plasmonic Nanoparticles
Efficient excitation and harvesting of hot carriers are central to a variety
of emerging nanoplasmonic applications, but ballistic carrier extraction
remains a challenge. To elucidate the relevant dynamics as a function of
nanoscale geometry, we perform femtosecond two-color pump-probe photoemission
studies of single gold nanorods and gold/silica nanoshells with simultaneous
time, energy, and vector momentum resolution. Angle-resolved photoelectron
momentum distributions elucidate the dominant intraband photoexcitation
mechanism and subsequent ballistic dynamics within the gold nanoparticle
volume, as verified via Monte Carlo photoemission modeling. Energy-averaged hot
electron lifetimes around 30 fs are measured in the ~1-2 eV excitation energy
range, while energy-resolved measurements reveal good agreement with Fermi
liquid theory behavior based on electron-electron inelastic scattering, as well
as more detailed kinetic Boltzmann modeling including the effects of electron
cascading from higher energy levels and quasi-elastic electron phonon
scattering. These results directly demonstrate the predominance of bulk-like
hot electron dynamical behaviors (including volume-like excitation and bulk
inelastic scattering rates) in metal nanoparticles with dimensions as small as
10 nm, which should contribute to the design of more efficient hot carrier
devices
Oscillator-Based Volatile Detection System Using Doubly- Clamped Micromechanical Resonators
AbstractIn this paper, we demonstrate a functionalized and resonant piezo-actuated volatile sensor which is interfaced by electronics for frequency shift detection. Enhanced signal sensing is achieved via the effective feed-through capacitance cancellation scheme. The closed-loop oscillator, realized with off-the-shelf components, attains a frequency stability of 2.7Hz for the 1.8MHz resonant mode of the gas sensor. The sensor was exposed to pulses of water and ethanol vapor mixtures, yielding a temporary dip in resonance frequency as well as volatile-specific recovery times
Influence of soil minerals on chromium(VI) reduction by sulfide under anoxic conditions
The effects of soil minerals on chromate (Cr(VI)O(4)(2-), noted as Cr(VI)) reduction by sulfide were investigated in the pH range of 7.67 to 9.07 under the anoxic condition. The examined minerals included montmorillonite (Swy-2), illite (IMt-2), kaolinite (KGa-2), aluminum oxide (γ-Al(2)O(3)), titanium oxide (TiO(2), P-25, primarily anatase), and silica (SiO(2)). Based on their effects on Cr(VI) reduction, these minerals were categorized into three groups: (i) minerals catalyzing Cr(VI) reduction – illite; (ii) minerals with no effect – Al(2)O(3); and (iii) minerals inhibiting Cr(VI) reduction- kaolinite, montmorillonite, SiO(2 )and TiO(2 ). The catalysis of illite was attributed primarily to the low concentration of iron solubilized from the mineral, which could accelerate Cr(VI) reduction by shuttling electrons from sulfide to Cr(VI). Additionally, elemental sulfur produced as the primary product of sulfide oxidation could further catalyze Cr(VI) reduction in the heterogeneous system. Previous studies have shown that adsorption of sulfide onto elemental sulfur nanoparticles could greatly increase sulfide reactivity towards Cr(VI) reduction. Consequently, the observed rate constant, k(obs), increased with increasing amounts of both iron solubilized from illite and elemental sulfur produced during the reaction. The catalysis of iron, however, was found to be blocked by phenanthroline, a strong complexing agent for ferrous iron. In this case, the overall reaction rate at the initial stage of reaction was pseudo first order with respect to Cr(VI), i.e., the reaction kinetics was similar to that in the homogeneous system, because elemental sulfur exerted no effect at the initial stage prior to accumulation of elemental sulfur nanoparticles. In the suspension of kaolinite, which belonged to group (iii), an inhibitive effect to Cr(VI) reduction was observed and subsequently examined in more details. The inhibition was due to the sorption of elemental sulfur onto kaolinite, which reduced or completely eliminated the catalytic effect of elemental sulfur, depending on kaolinite concentration. This was consistent with the observation that the catalysis of externally added elemental sulfur (50 μM) on Cr(VI) reduction would disappear with a kaolinite concentration of more than 5.0 g/L. In kaolinite suspension, the overall reaction rate law was: -d[Cr(VI)]/dt = k(obs)[H(+)](2)[Cr(VI)][HS(-)](0.70
Light-Driven Nanoscale Vectorial Currents
Controlled charge flows are fundamental to many areas of science and
technology, serving as carriers of energy and information, as probes of
material properties and dynamics, and as a means of revealing or even inducing
broken symmetries. Emerging methods for light-based current control offer
promising routes beyond the speed and adaptability limitations of conventional
voltage-driven systems. However, optical manipulation of currents at nanometer
spatial scales remains a basic challenge and a key step toward scalable
optoelectronic systems and local probes. Here, we introduce vectorial
optoelectronic metasurfaces as a new class of metamaterial in which ultrafast
charge flows are driven by light pulses, with actively-tunable directionality
and arbitrary patterning down to sub-diffractive nanometer scales. In the
prototypical metasurfaces studied herein, asymmetric plasmonic nanoantennas
locally induce directional, linear current responses within underlying
graphene. Nanoscale unit cell symmetries are read out via polarization- and
wavelength-sensitive currents and emitted terahertz (THz) radiation. Global
vectorial current distributions are revealed by spatial mapping of the THz
field polarization, also demonstrating the direct generation of elusive
broadband THz vector beams. We show that a detailed interplay between
electrodynamic, thermodynamic, and hydrodynamic degrees of freedom gives rise
to these currents through rapidly-evolving nanoscale forces and charge flows
under extreme spatial and temporal localization. These results set the stage
for versatile patterning and optical control over nanoscale currents in
materials diagnostics, nano-magnetism, microelectronics, and ultrafast
information science
Tratamento de osteoma osteóide de corpo vertebral da coluna lombar por ablação por radiofreqüência
Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere
Neoproterozoic (1,000–542 Myr ago) Earth experienced profound environmental change, including ‘snowball’ glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ82/76Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ82/76Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ82/76Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution
Treatment of osteochondral lesions of the talus: a systematic review
The aim of this study was to summarize all eligible studies to compare the effectiveness of treatment strategies for osteochondral defects (OCD) of the talus. Electronic databases from January 1966 to December 2006 were systematically screened. The proportion of the patient population treated successfully was noted, and percentages were calculated. For each treatment strategy, study size weighted success rates were calculated. Fifty-two studies described the results of 65 treatment groups of treatment strategies for OCD of the talus. One randomized clinical trial was identified. Seven studies described the results of non-operative treatment, 4 of excision, 13 of excision and curettage, 18 of excision, curettage and bone marrow stimulation (BMS), 4 of an autogenous bone graft, 2 of transmalleolar drilling (TMD), 9 of osteochondral transplantation (OATS), 4 of autologous chondrocyte implantation (ACI), 3 of retrograde drilling and 1 of fixation. OATS, BMS and ACI scored success rates of 87, 85 and 76%, respectively. Retrograde drilling and fixation scored 88 and 89%, respectively. Together with the newer techniques OATS and ACI, BMS was identified as an effective treatment strategy for OCD of the talus. Because of the relatively high cost of ACI and the knee morbidity seen in OATS, we conclude that BMS is the treatment of choice for primary osteochondral talar lesions. However, due to great diversity in the articles and variability in treatment results, no definitive conclusions can be drawn. Further sufficiently powered, randomized clinical trials with uniform methodology and validated outcome measures should be initiated to compare the outcome of surgical strategies for OCD of the talus
Treatment of lumbar degenerative disc disease-associated radicular pain with culture-expanded autologous mesenchymal stem cells: a pilot study on safety and efficacy
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