M. G. Proietti

Step by step capping and strain state of GaN/AlN quantum dots studied by grazing incidence diffraction anomalous fine structure

The investigation of small-size embedded nanostructures, by a combination of complementary anomalous diffraction techniques, is reported. GaN quantum dots (QDs), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode, are studied in terms of strain and local environment, as a function of the AlN cap layer thickness, by means of grazing-incidence anomalous diffraction. That is, the x-ray photon energy is tuned across the Ga absorption K edge which makes diffraction chemically selective. Measurement of hkl scans, close to the AlN (3030) Bragg reflection, at several energies across the Ga K edge, allows the extraction of the Ga partial structure factor, from which the in-plane strain of GaN QDs is deduced. From the fixed-Q energy-dependent diffracted intensity spectra, measured for diffraction-selected isostrain regions corresponding to the average in-plane strain state of the QDs, quantitative information regarding the composition and out-of-plane strain has been obtained. We recover the in-plane and out-of-plane strains in the dots. The comparison to the biaxial elastic strain in a pseudomorphic layer indicates a tendency to an overstrained regime.

Strain assisted inter-diffusion in GaN/AlN quantum dots

The structural and optical properties of high temperature-annealed superlattices of GaN quantum dots embedded in AlN barrier have been studied by a combination of X-ray techniques (reciprocal space mapping, multiwavelength anomalous diffraction, and diffraction anomalous fine structure), high resolution transmission electron microscopy, and photoluminescence spectroscopy. Taking advantage of the disentangling of the chemical and structural information provided by the simultaneous use of X-ray absorption and diffraction data obtained in a synchrotron environment, we provide quantitative determination of strain and composition for each different region of the nanostructures. Eventually, it is shown that strain driven dot/barrier intermixing is present, mostly on top of the dots. These observations have been confirmed by high resolution electron microscopy. A blue shift of photoluminescence peak has been furthermore observed and assigned to GaN/AlN intermixing suggesting a new path for engineering the emissi...

Glancing-angle diffraction anomalous fine structure of InAs quantum dots and quantum wires.

We have performed Diffraction Anomalous Fine Structure measurements at the As K-edge of self-growth InAs/InP(001) Quantum Wires and InAs/GaAs(001) Quantum Dots. The samples have been grown by Molecular Beam Epitaxy and their equivalent thickness is of 2.5 monolayers. We have measured the (440) and (420) Bragg reflections in glancing-angle scattering geometry, at incidence angles close to the substrate critical angle. We demonstrate the feasibility of the experiment reporting, for the first time. Diffraction Anomalous Fine Structure spectra of such low coverage epitaxial layers, and we show that the analysis of the Diffraction Anomalous Fine Structure lineshape together with the analysis of oscillatory part of the signal, can provide information about composition and strain of the nanostructures.

Local structure of Ni2+ ions in fluorochloro- and fluorobromo-zirconate glasses

Nickel doped fluorochloro- and fluorobromo-zirconate glasses have been prepared following and modifying standard methods. Electron probe microanalysis has been carried out to obtain the actual chlorine and bromine content of the samples. X-ray and optical absorption measurements have been performed in order to study the Ni2+ neighborhood. Extended x-ray absorption fine structure and x-ray absorption near edge structure spectra at the Ni K edge show the progressive substitution of the fluorine ions in the first coordination shell of Ni2+ by chlorine or bromine ions when their concentration increases in the glasses. From the analysis of the absorption features due to the 3A2g(F)→1Eg(D) spin forbidden transition, up to four Ni2+ environments have been identified and assigned to: 6F−, 5F− and 1Cl− or 1Br−, 1F− and 5Cl− or 5Br− and 6Cl− or 6Br−. It is deduced that less than 10% of fluorine ions substitution is enough to have all the Ni2+ ions hexacoordinated to chlorine or bromine ions. NiCl2 and NiBr2 crystal...

Anisotropic in distribution in InGaN core-shell nanowires

In this work, we investigate the local atomic structure of defect-free homogeneous and self-organized core-shell structure nanowires by means of X-ray Absorption Fine Structure (XAFS) Spectroscopy at the In LIII and K edges and Multiwavelength Anomalous Diffraction. The results are interpreted by comparison of the experimental data with X-ray absorption calculations carried out with ab initio structural models. Extended-XAFS data analysis at In K-edge shows an anisotropic In distribution in the second nearest neighbors pointing out to a deviation from randomness in In distribution for the core-shell sample.

Mn K-edge XANES spectra of manganites measured by Kbeta emission.

The electronic state of Mn atoms in mixed valence manganites has been studied by means of X-ray absorption spectroscopy at the Mn K-edge. Higher resolution than in conventional measurements has been achieved by measuring the Mn Kbeta fluorescence line. We have found a unique resonance at the edge in the XANES spectra of intermediate composition RE1-xCa(x)MnO3 samples. The features of these XANES spectra do not depend on small changes in the local structure around the Mn atom. However, the spectra of the intermediate composition samples can not be reproduced by a linear combination of REMnO3 and CaMnO3 spectra. Accordingly, the electronic state of Mn atoms in these compounds can not be considered as a mixture of Mn3+ and Mn4+ pure states.

Local structure around Mn atom in the LaMnO3+δ system

Abstract Mn K-edge X-ray absorption spectroscopy was used to determine the electronic and geometrical local structure at the Mn site in LaMnO3 + δ (δ = 0, 0.07, and 0.15) series. The XANES spectra of LaMnO3 + δ samples show a mixed valence oxidation state for the Mn atom similar to that found in La1 − xCaxMnO3 series. EXAFS spectra show a distorted Jahn—Teller octahedra for LaMnO3 which is drastically reduced as δ increases. Otherwise, an anomalous structural disorder, reported for the paramagnetic phase in magnetoresistive compounds, is also present at low temperatures in this series.

Structural study of TiIV exchanged K10-montmorillonite by XRD, EXAFS and XANES

Abstract Structural and catalytic properties of TiIV/K10-montmorillonite and fine size anatase activated at different temperatures are compared. EXAFS spectra at the Ti K edge and X-ray diffraction patterns show the formation of small TiO2 particles in the preparative process, which grow up with the calcination temperature. The low catalytic activity of the fine size anatase clearly shows that the activity of the exchanged clay is due to the exchanged TiIV into the K10-montmorillonite. XANES indicates that some of the titanium exchanged atoms would present a tetrahedral structure and consequently behave as a strong Lewis acid.

Structural and physical characterization of NdNiO3 synthesized by a sol-gel method

Abstract NdNiO 3− x has been synthesized at low oxygen pressure using a sol-gel process. This method gives an oxygen deficient phase with a value of x around 0.1 as determined by thermogravimetric analysis. X-ray diffraction powder data and Ni K-edge EXAFS measurements show that this compound belongs to the orthorhombic crystalline system and not to the rhombohedral one as it has been reported elsewhere. Different sintering processes give samples with different grain sizes that show similar magnetic and electrical behaviour. All these samples have a strong metal-insulator transition at 210 K and their resistivity curves show a large thermal hysterisis that could be related to the lack of oxygen stoichiometry. No contribution of magnetic Ni ion have been deduced from the magnetic susceptibility measurements. Moreover, no magnetic anomaly has been detected at the metal-semiconductor phase transition.

Has charge ordering actually been observed in manganites by resonant X-ray scattering?

The comparison between X-ray resonant scattering experiments and X-ray absorption data shows that no real Mn3+/Mn4+ charge ordering occurs in manganites with Mn3+/Mn4+=1 ratio. However, these experiments demonstrate the presence of two different types of manganese atoms with a different local geometrical structure. A structural model, which accounts for both X-ray absorption and resonant scattering experiments, is proposed.