We report the vibrational and optical properties of the 'defect' perovskites Cs 2 SnX 6 (X = Cl, Br, I) as well as their use as hole-transporting materials (HTMs) in solar cells. All three air-stable compounds were characterized using powder X-ray diffraction and Rietveld refinement. Far-IR reflectance, Raman, and UV−vis spectroscopy as well as electronic band structure calculations show that the compounds are direct band gap semiconductors with a pronounced effect of the halogen atom on the size of the energy gap and the vibrational frequencies. Scanning electron microscopy and atomic force microscopy confirmed that the morphology of the perovskite films deposited from N,N-dimethylformamide solutions on TiO 2 substrates also strongly depends on the chemical composition of the materials. The Cs 2 SnX 6 perovskites were introduced as hole-transporting materials in dye-sensitized solar cells, based on mesoporous titania electrodes sensitized with various organic and metal−organic dyes. The solar cells based on Cs 2 SnI 6 HTM and the Z907 dye performed best with a maximum power conversion efficiency of 4.23% at 1 sun illumination. The higher performance of Cs 2 SnI 6 is attributed to efficient charge transport in the bulk material and hole extraction at the perovskite-Pt interface, as evidenced by electrochemical impedance spectroscopy.

The magnetic properties of maghemite (gamma-Fe2O3) cubic and spherical nanoparticles of similar sizes have been experimentally and theoretically studied. The blocking temperature, T(B), of the nanoparticles depends on their shape, with the spherical ones exhibiting larger T(B). Other low temperature properties such as saturation magnetization, coercivity, loop shift or spin canting are rather similar. The experimental effective anisotropy and the Monte Carlo simulations indicate that the different random surface anisotropy of the two morphologies combined with the low magnetocrystalline anisotropy of gamma-Fe2O3 is the origin of these effects.

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide-manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles.

We have observed the magnetic behavior of nanostructured magnetic materials produced by co-depositing pre-formed Fe nanoclusters from a gas aggregation source and Ag vapor from a Knudsen cell. Films containing particle volume fractions from Ͻ1% ͑isolated clusters͒ to 100% ͑pure clusters with no matrix͒ have been prepared in UHV conditions and, after capping with a thin Ag layer for removal from the deposition chamber, have been studied at temperatures in the range 2-300 K by magnetometry and field-cooled/zero-field-cooled measurements. The results have been interpreted with the help of a Monte Carlo simulation of the clusterassembled films that includes exchange and dipolar interactions. At elevated temperatures (Ͼ50 K) the lowest concentration films display ideal superparamagnetism with an H/T scaling of the magnetization. With increasing cluster density the films pass through an interacting superparamagnetic phase in which the effective blocking temperature and the initial susceptibility above the blocking temperature increase, in contrast to predictions of nanoparticle systems interacting via dipolar forces only. It is concluded that the exchange interaction becomes important even at volume fractions of 10% as clusters that are in contact behave as a single larger particle. This is confirmed by the theoretical model. At high volume fractions, well above the percolation threshold, the cluster assemblies form correlated superspin glasses ͑CSSG's͒. At 2 K, the magnetization curves in all films, irrespective of cluster concentration, have a remanence of Ϸ50% and an approach to saturation that is characteristic of randomly oriented, particles with a uniaxial anisotropy, in agreement with the remanence. In the most dense Ag-capped films there appears to be a ''freezing out'' of the interparticle exchange interaction, which is attributed to temperature-dependent magnetoelastic stress induced by the capping layer. An uncapped 100% cluster film measured in UHV remains in the CSSG state at all temperatures and does not show the low-temperature decoupling of particles evident in the Ag-capped samples.

Magnetization and AC susceptibility measurements have been performed on ∼3 nm NiO nanoparticles in powder form. The results indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered surface shell. The core magnetic moments block progressively with decreasing temperature, according to the distribution of their anisotropy energy barriers, as shown by a broad maximum of the low field zero-field-cooled magnetization (M(ZFC)) and in the in-phase component χ' of the AC susceptibility, centred at ∼70 K. On the other hand, surface spins thermally fluctuate and freeze in a disordered spin-glass-like state at much lower temperature, as shown by a peak in M(ZFC) (at 17 K, for H = 50 Oe) and in χ'. The temperature of the high temperature χ' peak changes with frequency according to the Arrhenius law; instead, for the low temperature maximum a power law dependence of the relaxation time was found, τ = τ(0)(T(g)/(T(ν)-T(g)))(α), where α = 8, like in spin glasses, τ(0) = 10(-12) s and T(g) = 15.9 K. The low temperature surface spin freezing is accompanied by a strong enhancement of magnetic anisotropy, as shown by the rapid increase of coercivity and high field susceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with an antiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of the temperature dependence of the coercivity. Interparticle interactions lead to a shift to a high temperature of the distribution of the core moment blocking temperature and to a reduction of magnetization dynamics.