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The interrelation between particle size, crystal structure and optical properties in semiconductor quantum dots has elicited widespread interest. We report the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prepared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic CdS nanoparticles was found to be almost 3.9 eV. We also suggest a simple mechanism (based on the periodic insertion of stacking faults) for the transformation from the cubic zinc blende structure to the hexagonal wurtzite structure.

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A direct measurement of the superconducting energy gap by point contact spectroscopy in nanostructured Nb films shows that the gap decreases with a reduction in the average particle size. The superconducting T(c), obtained from transport and magnetic measurements, also decreases with size and scales with the energy gap. The size dependence of the superconducting properties in this intermediate coupling type II superconductor is therefore governed by changes in the electronic density of states rather than by phonon softening. Consistent with the Anderson criterion, no T(c) was observed for sizes below 8 nm.