4,313 research outputs found
Formation and structural characterization of Ni nanoparticles embedded in SiOā
Face-centered cubic Ni nanoparticles were formed in SiOā by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of statesThis work was financially supported by the Australian
Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication
Facility
NNLO QCD corrections to event shape variables in electron positron annihilation
Precision studies of QCD at electron-positron colliders are based on
measurements of event shapes and jet rates. To match the high experimental
accuracy, theoretical predictions to next-to-next-to-leading order (NNLO) in
QCD are needed for a reliable interpretation of the data. We report the first
calculation of NNLO corrections O(alpha_s^3) to three-jet production and
related event shapes, and discuss their phenomenological impact.Comment: Contributed to 2007 Europhysics Conference on High Energy Physics,
Manchester, England 19-25 July 200
Pt nanocrystals formed by ion implantation: a defect-mediated nucleation process
The influence of ion irradiation of SiOā on the size of metalnanocrystals (NCs) formed by ion implantation has been investigated. Thin SiOā films were irradiated with high-energy Ge ions then implanted with Pt ions. Without Geirradiation, the largest Pt NCs were observed beyond the Pt projected range. With irradiation, Ge-induced structural modification of the SiOā layer yielded a decrease in Pt NC size with increasing Ge fluence at such depths. A defect-mediated NC nucleation mechanism is proposed and a simple yet effective means of modifying and controlling the Pt NC size is demonstrated.The authors thank the Australian Research Council for
financial support
Nano-porosity in GaSb induced by swift heavy ion irradiation
Nano-porous structures form in GaSb after ion irradiation with 185 MeV Au ions. The porous layer formation is governed by the dominant electronic energy loss at this energy regime. The porous layer morphology differs significantly from that previously reported for low-energy, ion-irradiated GaSb. Prior to the onset of porosity, positron annihilation lifetime spectroscopy indicates the formation of small vacancy clusters in single ion impacts, while transmission electron microscopy reveals fragmentation of the GaSb into nanocrystallites embedded in an amorphous matrix. Following this fragmentation process, macroscopic porosity forms, presumably within the amorphous phase.The authors thank the Australian Research Council for
support and the staff at the ANU Heavy Ion Accelerator
Facility for their continued technical assistance. R.C.E. acknowledges the support
from the Office of Basic Energy Sciences of the U.S. DOE
(Grant No. DE-FG02-97ER45656)
Amorphization of embedded Cu nanocrystals by ion irradiation
While bulk crystalline elemental metals cannot be amorphized by ion irradiation in the absence of
chemical impurities, the authors demonstrate that finite-size effects enable the amorphization of
embedded Cu nanocrystals. The authors form and compare the atomic-scale structure of the
polycrystalline, nanocrystalline, and amorphous phases, present an explanation for the extreme
sensitivity to irradiation exhibited by nanocrystals, and show that low-temperature annealing is
sufficient to return amorphized material to the crystalline form
Swift heavy-ion irradiation-induced shape and structural transformation in cobalt nanoparticles
The shape and structural evolution of Co nanoparticles embedded in SiOā and subjected to swift heavy-ion irradiation have been investigated over a wide energy and fluence range. Modifications of the nanoparticle size and shape were characterized with transmission electron microscopy and small-angle x-ray scattering.Nanoparticles below a threshold diameter remained spherical in shape and progressively decreased in size under irradiation due to dissolution.Nanoparticles above the threshold diameter transformed into nanorods with their major dimension parallel to the incident ion direction. Modifications of the atomic-scale structure of the Co nanoparticles were identified with x-rayabsorption spectroscopy. Analysis of the x-rayabsorption near-edge spectra showed that prior to irradiation all Co atoms were in a metallic state, while after irradiation Co atoms were in both oxidized and metallic environments, the former consistent with dissolution. The evolution of the nanoparticle short-range order was determined from extended x-ray absorption fine structure spectroscopy. Structural changes in the Co nanoparticles as a function of ion fluence included an increase in disorder and asymmetric deviation from a Gaussian interatomic distance distribution coupled with a decrease in bondlength. Such changes resulted from the irradiation-induced decrease in nanoparticle size and subsequent dissolution.This work
was financially supported by the Australian Synchrotron and
the Australian Research Council with access to equipment
provided by the Australian Nanofabrication Facility. ChemMatCARS
Sector 15 is principally supported by the NSF/
DOE under Grant No. NSF/CHEā0822838
Scanning spreading resistance microscopy of two-dimensional diffusion of boron implanted in free-standing silicon nanostructures
B implants of 1keV, 1Ć10Ā¹āµat.cmā»Ā² into 125-nm-wide, free-standing Si nanostructures have been characterized using scanning spreading resistancemicroscopy following a 0s, 1050Ā°Canneal in Nā. A curved diffusion front has been observed. B in the center of the ridge diffuses further than at the sides. A similar effect has been observed in SUPREM-IV simulations. It is attributed to a reduction in transient enhanced diffusion close to the vertical surfaces due to recombination of ion-implantation-induced excess Si self-interstitials
Energy dependent saturation width of swift heavy ion shaped embedded Au nanoparticles
The transformation of Aunanoparticles (NPs) embedded in SiOā from spherical to rod-like shapes induced by swift heavy ion irradiation has been studied. Irradiation was performed with Ā¹ā¹ā·Au ions at energies between 54 and 185 MeV. Transmission electron microscopy and small angle x-ray scatteringmeasurements reveal an energy dependent saturation width of the NP rods as well as a minimum size required for the NPs to elongate. The NP saturation width is correlated with the ion track diameter in the SiOā. NP melting and in-plane strain in the irradiatedSiOā are discussed as potential mechanisms for the observed deformation.P.K. and M.C.R. thank the Australian Research Council
for support. P.K., R.G., D.J.S., and M.C.R. were supported
by the Australian Synchrotron Research Program, funded by
the Commonwealth of Australia via the Major National Research
Facilities Program
Congenital diaphragmatic hernia: the impact of embryological studies
In recent years, a substantial research effort within the specialty of pediatric surgery has been devoted to improving our knowledge of the natural history and pathophysiology of congenital diaphragmatic hernias (CDH) and pulmonary hypoplasia (PH). However, the embryological background has remained elusive because certain events of normal diaphragmatic development were still unclear and appropriate animal models were lacking. Most authors assume that delayed or inhibited closure of the diaphragm will result in a diaphragmatic defect that is wide enough to allow herniation of the gut into the fetal thoracic cavity. However, we feel that this assumption is not based on appropriate embryological observations. To clarify whether it was correct, we restudied the morphology of pleuroperitoneal openings in normal rat embryos. Shortly before, a model for CDH and PH had been established in rats using nitrofen (2,4-di-chloro-phenyl-p-nitrophenyl ether) as teratogen. We used this model in an attempt to answer the following questions: (1) When does the diaphragmatic defect appear? (2) Are the pleuroperitoneal canals the precursors of the diaphragmatic defect? (3) Why is the lung hypoplastic in babies and infants with CDH? In our study we made following observations: (1) The typical findings of CDH and PH cannot be explained by inhibited closure of the pleuroperitoneal "canals". In normal development, the pleuroperitoneal openings are always too small to allow herniation of gut into the thoracic cavity. (2) The maldevelopment of the diaphragm starts rather early in the embryonic period (5th week). The lungs of CDH rats are significantly smaller than those of control rats at the end of the embryonic period (8th week). (3) The maldevelopment of the lungs in rats with CDH is "secondary" to the defect of the diaphragm. (4) The defect of the lungs is "structural" rather than "functional". Complete spontaneous correction of these lung defects is unlikely even after fetal intervention. (5) The "fetal lamb model" does not completely mimic the full picture of CDH, because the onset of the defect lies clearly in the fetal period. We believe that our rat model is better. It is especially useful for describing the abnormal embryology of this lesion
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