Anna Maria Giovanna Musinu

Correlated electron–hole plasma in organometal perovskites

Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.

IR and NMR study of nanoparticle-support interactions in a Fe2O3-SiO2 nanocomposite prepared by a sol-gel method

The interaction of iron oxide with the silica matrix in a Fe2O3-SiO2 nanocomposite prepared by a sol-gel method has been investigated using Near-, Mid-, and Far-IR, and 29Si MAS-, and 1H NMR techniques. Samples of nanocomposites and of pure silica obtained by the same preparation procedure and subjected to the same thermal treatments have been examined. Spectroscopic data indicate that the Fe2O3 nanoparticles interact with the silica or silanol groups at the surface of the cavities in which they form. This result allowed us to propose a model for the nanoparticle/silica interface.

Characterization of Nanocrystalline γ–Fe 2 O 3 Prepared by Wet Chemical Method

Homogeneous maghemite (γ–Fe 2 O 3 ) nanoparticles with an average crystal size around 5 nm were synthesized by successive hydrolysis, oxidation, and dehydration of tetrapyridino-ferrous chloride. Morphological, thermal, and structural properties were investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD) techniques. Rietveld refinement indicated a cubic cell. The superstructure reflections, related to the ordering of cation lattice vacancies, were not detected in the diffraction pattern. Kinetics of the solid-state phase transition of nanocrystalline maghemite to hematite (α–Fe 2 O 3 ), investigated by energy dispersive x-ray diffraction (EDXRD), indicates that direct transformation from nanocrystalline maghemite to microcrystalline hematite takes place during isothermal treatment at 385 °C. This temperature is lower than that observed both for microcrystalline maghemite and for nanocrystalline maghemite supported on silica.

Cationic distribution and spin canting in CoFe2O4 nanoparticles

CoFe(2)O(4) nanoparticles (D(NPD) ~6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and (57)Fe Mössbauer spectrometry under high applied magnetic field. Despite the small particle size, the value of saturation magnetization at 300 K (M(s) ͠= 70 A m(2) kg(-1)) and at 5 K (M(s) ͠= 100 A m(2) kg(-1)) are rather close to the bulk values, making the samples prepared with this method attractive for biomedical applications. Neutron diffraction measurements indicate the typical ferrimagnetic structure of the ferrites, showing an inversion degree (γ(NPD) = 0.74) that is in very good agreement with cationic distribution established from low temperature (10 K) Mössbauer measurements in high magnetic field (γ(moss) = 0.76). In addition, the in-field Mössbauer spectrum shows the presence of a non-collinear spin structure in both A and B sublattices. The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.

Magnetic properties of cobalt ferrite-silica nanocomposites prepared by a sol-gel autocombustion technique

The magnetic properties of cobalt ferrite-silica nanocomposites with different concentrations (15, 30, and 50 wt %) and sizes (7, 16, and 28 nm) of ferrite particles have been studied by static magnetization measurements and Mossbauer spectroscopy. The results indicate a superparamagnetic behavior of the nanoparticles, with weak interactions slightly increasing with the cobalt ferrite content and with the particle size. From high-field Mossbauer spectra at low temperatures, the cationic distribution and the degree of spin canting have been estimated and both parameters are only slightly dependent on the particle size. The magnetic anisotropy constant increases with decreasing particle size, but in contrast to many other systems, the cobalt ferrite nanoparticles are found to have an anisotropy constant that is smaller than the bulk value. This can be explained by the distribution of the cations. The weak dependence of spin canting degree on particle size indicates that the spin canting is not simply a surface phenomenon but also occurs in the interiors of the particles.

Spin-Canting and Magnetic Anisotropy in Ultrasmall CoFe2O4 Nanoparticles

The magnetic properties of cobalt ferrite nanoparticles dispersed in a silica matrix in samples with different concentrations (5 and 10 wt% CoFe2O 4) and same particle size (3 nm) were studied by magnetization, DC and AC susceptibility, and Mossbauer spectroscopy measurements. The results indicate that the particles are very weakly interacting. The magnetic properties (saturation magnetization, anisotropy constant, and spin-canting) are discussed in relation to the cation distribution.

Structural properties of lead-iron phosphate glasses by X-ray diffraction☆

Abstract The structural properties of lead-iron phosphate glasses have been investigated by X-ray diffraction in an attempt to understand the chemical and mechanical characteristics presented by lead metaphosphate glass as the result of the addition of iron oxide. Information on FeO bond lengths and coordination numbers was obtained by a comparison of radial distribution functions of glass samples containing iron oxide and of the iron-free glass matrix. Stable structural arrangements around Fe(III) atoms were detected, which could be responsible for the stabilizing effect of Fe2O3 on lead metaphosphate glass.

Superparamagnetic behaviour of γ-Fe2O3 nanoparticles dispersed in a silica matrix

The structural and magnetic properties of two Fe2O3–SiO2 nanocomposites, containing respectively 16.9 and 28.5 wt.% Fe2O3, were investigated. The samples were synthetized by a sol–gel method, using ethylene glycol as a solvent, and heating the gels gradually to 900°C. The procedure allowed us to obtain γ-Fe2O3 nanoparticles homogeneously dispersed in the amorphous silica matrix. The particles have a narrow size distribution and mean sizes from 3 to 6 nm depending on the iron oxide content. The magnetic properties of the samples were investigated by static and dynamic susceptibility measurements. All the samples showed superparamagnetic behaviour. The superparamagnetic relaxation was investigated also by Mossbauer spectroscopy. Hysteresis loops were measured at 2.5 K and both samples showed high values of coercive field. The role of magnetic interparticle interactions on the magnetic properties is discussed.

Magnetism in nanoparticles: beyond the effect of particle size.

A set of investigations on selected samples of nanosized cobalt ferrite are reviewed, aimed at studying the various factors affecting the magnetic properties of nanoparticles. Specifically, the effects of inter-particle interactions, of structural and magnetic order, both in the core and on the surface of the particle, have been examined. All factors render the control of the magnetic properties of nanosystems quite difficult, but, at the same time, they also offer the opportunity of tuning them properly, so that materials for specific applications may be created.

XRD, TEM and 29Si MAS NMR study of sol-gel ZnO-SiO2 nanocomposites

X-Ray amorphous ZnO nanoparticles homogeneously dispersed in a silica matrix were evidenced in ZnO-SiO 2 nanocomposites obtained by a sol-gel method and heated to 700°C. TEM observations indicated that the particle size slowly increases with temperature and zinc oxide content, reaching an upper limit of 12 nm. Through a comparison of the 29 Si MAS NMR data of the nanocomposites and silica samples, obtained by the same preparation method, it was possible to observe that reaction occurs between ZnO and silica on heating, which causes a depolymerization of the host matrix with the formation of low condensation groups. This result is discussed in terms of interactions between nanoparticles and the silica matrix at the nanoparticle/matrix interface. A further increase in temperature (900°C) results in the formation of the β-Zn 2 SiO 4 crystalline phase.