Carla Cannas

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.

Spin-glass-like freezing and enhanced magnetization in ultra-small CoFe2O4 nanoparticles

The magnetic properties of ultra-small (3 nm) CoFe(2)O(4) nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization T(max) approximately 40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

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.

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.

How to tailor maghemite particle size in γ-Fe2O3–SiO2 nanocomposites

A sol–gel procedure for the tailoring of maghemite particle size in silica-based nanocomposites is proposed. The preparation method allows control of the gelation time, which is varied from 2 to 16 days in samples having the same concentration of iron oxide (25%). Superparamagnetic particles of γ-Fe2O3 with sizes in the 2.5–5.6 nm range were found in all the samples, as confirmed by TEM, Mossbauer spectroscopy and magnetic measurements. The particle size was independent of the porosity of the silica host matrix, but strongly dependent on the amount of solvent trapped inside the gels. The solvent plays an important role, favouring the formation of Fe3O4 nanoparticles as an intermediate step before the final oxidation to maghemite.

Synthesis, characterisation and optical properties of nanocrystalline Y2O3–Eu3+ dispersed in a silica matrix by a deposition–precipitation method

A Eu3+-doped-yttria–silica nanocomposite was synthesized through a deposition–precipitation technique. Yttria nanocrystalline particles with a mean size of 12 nm, dispersed in an amorphous silica matrix, were evidenced by XRD and TEM in a sample treated at 900 °C. The nanoparticles are coated with an amorphous layer visible in the HRTEM micrographs; this amorphous layer is likely to interact with the silica matrix through Si–O–Y bonds, which is consistent with 29Si NMR MAS results. The nanocomposite treated at 1000 °C partially evolves to give an α-Y2Si2O7 crystalline phase. The luminescence spectra of the nanocomposites indicate that the sites in which the Eu3+ ions are accommodated are disordered. On the other hand, the decay times of the 5D0 emission are rather long in the present nanocomposites studied herein, indicating that multiphonon relaxation is not effective in quenching the luminescence. The reduced coupling to OH vibrations in the materials under investigation could be ascribed to the presence of the amorphous layer coating the nanoparticles and effectively shielding the Eu3+ ion from the silanol groups.