Nanosized inorganic particles are of great interest because their electronic properties can be easily tailored, providing a tremendous potential for applications in optoelectronic devices, light-emitting diodes, solar cells and hydrogen storage. Confinement of electrons and holes to dimensions comparable to their wavelength leads to quantum-well states with modified wavefunctions and density of states. Surface phenomena are crucial in determining nanoparticle properties in view of their large surface-to-volume ratio. Despite a wealth of information, many fundamental questions about the nature of the surface and its relationship with the electronic structure remain unsolved. Ab initio calculations on CdSe nanocrystals suggest that passivating the ligands does not produce the ideal wurtzite structure and that Se atoms relax outwards irrespective of passivation. Here we show that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons, monitor changes in composition and structure of the surface while hardly sensing the ligand molecules, and we thus unambiguously confirm the predicted strong surface relaxation.

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Spark discharge generation was used to synthesize Mg-Ti nanocomposites consisting primarily of a metastable body-centered-cubic (bcc) alloy of Mg and Ti. The bcc Mg-Ti alloy transformed upon hydrogenation into the face-centered-cubic fluorite Mg1-yTiyHx phase with favorable hydrogen storage properties. Both metal and metal hydride nanocomposites showed a fractal-like porous morphology, with a primary particle size of 10-20 nm. The metal content of 70 atom % (at %) Mg and 30 at % Ti, consistently determined by XRD, TEM-EDS, and ICP-OES, was distributed uniformly across the as-prepared sample. Pressure-composition isotherms for the Mg-Ti-H nanocomposites revealed large differences in the thermodynamics relative to bulk MgH2, with a much less negative enthalpy of formation of the hydride as small as -45 ± 3 kJ/molH2 as deduced from van't Hoff plots. The plateau pressures of hydrogenation were substantially higher than those for bulk MgH2 in the low temperature range from 150 to 250 °C. The reaction entropy was simultaneously reduced to values down to 84 ± 5 J/K mol H2, following a linear relationship between the enthalpy and entropy. Plausible mechanisms for the modified thermodynamics are discussed, including the effect of lattice strains, the presence of interfaces and hydrogen vacancies, and the formation of excess free volume due to local deformations. These mechanisms all rely on the finely interdispersed nanocomposite character of the samples which is maintained by grain refinement.

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This paper presents an inelastic neutron scattering study of the proper ferroelectric and elastic neutron scattering results on the satellite diffraction pattern which characterizes the modulated phase. The temperature dependences of the satellite intensities and modulation wavevector are in fair agreement with results from previous x-ray experiments. Close to the incommensurate-to-ferroelectric transition temperature , an unexpected intensity overshoot is observed, similar to that seen in birefringence and dilatation experiments. The relationship between the lattice dynamics and the observed phase transition sequence is examined. The dispersion of the ferroelectric soft optic phonon (-polarization) and of the acoustic phonons is followed along the and -directions. In the ferroelectric phase, the TO mode shows a considerable softening as the incommensurate phase is approached from below. In the paraelectric and incommensurate phases, the response from the TO (-polarization) and TA ( strain) branches has been investigated via a series of constant-q scans in the -direction (approximately the modulation wavevector direction). The combined inelastic line-shapes, as observed in a number of non-equivalent Brillouin zones, could all be analysed in terms of a coupled-mode damped harmonic oscillator model. In addition, a diverging, resolution-limited, central peak is observed close to . It is suggested that the TO-TA coupling lies at the origin of the incommensurate instability. A phenomenological free energy is developed, in the continuum approximation, in which the TO-TA interaction is included via a pseudo-Lifshitz term of the type .