99 research outputs found
Single-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancy like defects with thermal treatments
No full textSi p-type (100) samples were coimplanted at room temperature with He+ ions at 30 keV with a dose of 1×1016 ions/cm2 and successively with H+ ions at 24 keV with a dose of 1×1016 ions/cm2. A series of samples was thermally treated for 2 h from 100 to 900 °C at 100 °C steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to 300 °C annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to 400–450 nm. Starting from 300 °C annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around 280 nm, where extended defects are expected; this indicates complete filling of the defects by H and He. From 300 to 600 °C decorated vacancy clusters of different sizes appear progressively in the region below 280 nm, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities
A new approach for the study of cell monolayers with Positron and Positronium Annihilation Spectroscopy leveraging on cell-sheet technology.
No full textA method for estimating a local dose in biological tissues after implantation of a positron flux
No full textSingle-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancylke defects with thermal treatments
No full textSi p-type 100 samples were coimplanted at room temperature with He+ ions at 30 keV with a dose of 1
1016 ions/cm2 and successively with H+ ions at 24 keV with a dose of 11016 ions/cm2. A series of
samples was thermally treated for 2 h from 100 to 900 °C at 100 °C steps to study the evolution of pointlike
and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission
electron microscopy. Depth profiling the samples with a positron beam led to the identification of five
different traps and the evolution of their profile distributions with thermal treatments. The positron traps were
identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case
by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to 300 °C annealing temperature
positrons probe three distributions of different decorated defects covering regions of the sample down to
400–450 nm. Starting from 300 °C annealing temperature no defects were revealed by positrons in the region
next to the peak of the implanted ions distributions positioned around 280 nm, where extended defects are
expected; this indicates complete filling of the defects by H and He. From 300 to 600 °C decorated vacancy
clusters of different sizes appear progressively in the region below 280 nm, with a distribution moving deeper
into the sample. Comparison with previous measurements on He-implanted samples points out the chemical
action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike
defects distributed through the damage region of the sample. Electron microscopy shows the transformation of
the extended defects from platelets to blisters and cavities
Combined spectroscopic and ab initio investigation of monolayer-range Cr oxides on Fe(001): The effect of ordered vacancy superstructure
No full textWe investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated Fe(001)−p(1×1)O surface and characterized by the formation of an ordered array of Cr vacancies producing a (√5×√5)R27∘ superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of O2p states and (ii) the orbital and spin character of Cr3d states. We show that the experimental O2p dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate
Nano-structures in Al-based alloys
No full textThe usefulness of first-principles calculations for studying solute-atom clustering in metal alloys is discussed. This usefulness stems directly from the properties predicted by the calculations or via a related interpretation of experimental results. In this paper we review the results of our computational studies on small solute clusters in Al-based alloys. The predicted coincidence Doppler broadening spectra of the positron annihilation method are used to analyse experimental results. The calculated binding energies of small solute atom clusters explain why Cu atoms form two-dimensional platelets on the (100) planes in Al whereas Zn forms three-dimensional clusters.Peer reviewe
Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculation
No full textElectron-positron momentum distributions measured by the coincidence Doppler broadening method can be
used in the chemical analysis of the annihilation environment, typically a vacancy-impurity complex in a solid.
In the present work, we study possibilities for a quantitative analysis, i.e., for distinguishing the average
numbers of different atomic species around the defect. First-principles electronic structure calculations selfconsistently
determining electron and positron densities and ion positions are performed for vacancy-solute
complexes in Al-Cu, Al-Mg-Cu, and Al-Mg-Cu-Ag alloys. The ensuing simulated coincidence Doppler broadening
spectra are compared with measured ones for defect identification. A linear fitting procedure, which uses
the spectra for positrons trapped at vacancies in pure constituent metals as components, has previously been
employed to find the relative percentages of different atomic species around the vacancy A. Somoza et al.
Phys. Rev. B 65, 094107 2002. We test the reliability of the procedure by the help of first-principles results
for vacancy-solute complexes and vacancies in constituent metals
Positron localization effects on the Doppler broadening of the annihilation line: aluminum as a case study.
No full textThe coincidence Doppler broadening CDB technique is widely used to measure one-dimensional momentum
distributions of annihilation photons, with the aim of obtaining information on the chemical environment
of open-volume defects. However, the quantitative analysis of CDB spectra needs to include also purely
geometrical effects. A demonstration is given here, on the basis of CDB spectra measured in quenched and in
deformed pure aluminum. The comparison of the experimental results with ab initio computations shows that
the observed differences come from the difference in free volume seen by positrons trapped in quenched-in
vacancies or in vacancylike defects associated to dislocations. The computation reproduces accurately all
details of CDB spectra, including the peak near the Fermi break, which is due to the zero-point motion of the
confined positron
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