Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites

The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal-host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the cluster's optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging

Doubling of biomass production in European boreal forest trees by a four-year suppression of background insect herbivory

Background insect herbivory, i.e. the minor but chronic plant damage caused by insects, is usually considered `negligible' for plants when compared with the severe defoliation associated with forest pest outbreaks. We experimentally tested the hypothesis that the impacts of background herbivory on tree growth and mortality accumulate over years, resulting in much larger effects than usually assumed. In boreal taiga forests near Arkhangelsk (Northern Russia), application of insecticide at 10-day intervals for four growth seasons (June-September of 2014-2017) decreased foliage losses to insects in our study species (early successional deciduous Betula pubescens and Populus tremula; late successional coniferous Picea abies and Pinus sylvestris) from 2.04-6.35% to 0.72-1.18%. The magnitude of the insecticide treatment effect on plant losses to defoliating insects varied considerably among the study species, with the highest effect observed in white birch and the lowest effects in the two late successional species. Across all tree species, insecticide treatment nearly doubled the increase in tree biomass relative to control plots, demonstrating that background insect herbivory has major negative impacts on tree growth and productivity. The insecticide-treated plots showed the largest increase in biomass in Norway spruce and the smallest increase in European aspen when compared to the control plots. The changes in birch growth following the release from insect herbivory were three times greater than the effects of the same level of simulated herbivory in an earlier experiment, thereby hinting at the importance of herbivore-specific elicitors in the growth suppression of trees damaged by insects. The mortality of late successional species in the treatment plots increased nearly three-fold, whereas the mortality of early successional species did not change relative to controls, suggesting an increase in competitiveness of the early successional trees released from herbivory. Thus, in agreement with an earlier modelling study, we conclude that minor herbivore damage, over the long term, substantially reduces biomass production in North European forest trees. Due to differential effects on coexisting tree species, this damage has a pronounced impact on plant competitiveness and affects both the productivity and the structure of boreal forests

Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

The metastable Ne(

The fluorescence excitation spectra of metastable3 P2 Ne atoms in superfluid helium have been studied for the first time. Ne spectral lines show a surprisingly small broadening and shift. We have also studied emission spectra and decay times related to the strongly forbidden N(2D → 4S) transition in the impurity-helium solid phase (IHSP) containing metastable N atoms, N2 molecules and - in some cases - rare gas atoms (Rg = Ne or Kr). We have shown that the central cores of solid He clusters correspond to single metastable N*(2D) atoms or to N*-N2 and N*-Rg pairs as well as to triple N*-N2-Rg and exciton-like N*Rgn complexes, as evidenced by their luminescence spectra

Quantum cutting in Li (770 nm) and Yb (1000 nm) co-dopant emission bands by energy transfer from the ZnO nano-crystalline host

Li-Yb co-doped nano-crystalline ZnO has been synthesized by a method of thermal growth from the salt mixtures. X-ray diffraction, transmission electron microscopy, atomic absorption spectroscopy and optical spectroscopy confirm the doping and indicate that the dopants may form Li-Li and Yb3+-Li based nanoclusters. When pumped into the conduction and exciton absorption bands of ZnO between 250 to 425 nm, broad emission bands of about 100 nm half-height-width are excited around 770 and 1000 nm, due to Li and Yb dopants, respectively. These emission bands are activated by energy transfer from the ZnO host mostly by quantum cutting processes, which generate pairs of quanta in Li (770 nm) and Yb (1000 nm) emission bands, respectively, out of one quantum absorbed by the ZnO host. These quantum cutting phenomena have great potential for application in the down-conversion layers coupled to the Si solar cells. (C)2011 Optical Society of Americastatus: publishe