Nanoparticles under a few nanometres in size have structures and material functions that differ from the bulk because of their distinct geometrical shapes and strong quantum confinement. These qualities could lead to unique device applications. Our mass spectral analysis of CdSe nanoparticles reveals that (CdSe)(33) and (CdSe)(34) are extremely stable: with a simple solution method, they grow in preference to any other chemical compositions to produce macroscopic quantities. First-principles calculations predict that these are puckered (CdSe)(28)-cages, with four- and six-membered rings based on the highly symmetric octahedral analogues of fullerenes, accommodating either (CdSe)(5) or (CdSe)(6) inside to form a three-dimensional network with essentially heteropolar sp(3)-bonding. This is in accordance with our X-ray and optical analyses. We have found similar mass spectra and atomic structures in CdS, CdTe, ZnS and ZnSe, demonstrating that mass-specified and macroscopically produced nanoparticles, which have been practically limited so far to elemental carbon, can now be extended to a vast variety of compound systems.

Blue shifts corresponding to the absorption edge in the UV-A (310–400 nm) range are obtained from ultraviolet (UV) absorption spectra of monodisperse CeO2−x (0<x<0.5) nanoparticles, which are produced as toluene suspensions by a successive microemulsification method. The blue shift of the nanoparticles is inversely proportional to the 2.2 power of the particle size for a direct transition, but no relation is found for an indirect transition. In contrast, the shift for thin films of 100–200 nm thickness exhibits a relation nearly inverse proportional to the square root of the thickness for the direct transition. These relations are different from that for the quantum size effect, though the difference is small for the nanoparticles. A possible explanation for the blue shift is a valence change of the Ce ions.

Anomalous lattice expansions have been measured for the first time in monodisperse CeO2-x nanoparticles and in BaTiO3 single nanoparticles by electron diffraction. X-ray photoelectron spectroscopy studies on CeO2-x nanoparticles and ab initio computer simulation on BaTiO3 clusters show that the origin of expansion is the decrease of electrostatic force caused by valence reduction of Ce ions and the increase in ionicity of Ti ions, respectively. The lattice constant change of oxide (ionic) nanoparticles with the increase in ionicity would depend on the structure of the particles. Hence, first-principles calculations of large ionic clusters are indispensable.

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