Physical properties of semiconductors in their nanocrystalline state have been predicted to be different from those of their bulk counterpart [1]. Detailed investigation on the optical absorption characteristics of semiconductor clusters of several II-VI and IV-VI compounds has shown the absorption edge to be blueshifted as the crystallite size is reduced [2-5]. In such studies nanoclusters of these compounds have been prepared by the precipitation of the crystallites in a colloidal solution. Recently we have reported on the synthesis of nanocrystalline PbS within a polyacrylamide matrix [6]. This method produced percolative chains of nanosized PbS particles. The d.c. electrical measurements on these systems have shown that the band gap for these particles is much higher than that of bulk PbS. We have now prepared ultrafme particles of PbS by a simple chemical method with sizes varying from 12.9 to 32,0 nm. Electrical measurements carried out on the cold pressed pellets using these powders show an increase in the band gap. The details are briefly described in this letter.
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