ANNIHILATION CHARACTERISTICS OF POSITRONS IN A POLYMER CONTAINING SILVER NANOPARTICLES

Composites of silver particles of diameters in the range 16.4-33.3 nm and polyacrylamide were prepared by a chemical method. Positron lifeti me and Doppler broadening measurements of these samples were carried o ut. The positron lifetime spectra of all the samples could be decompos ed into three components having lifetimes around 200, 500, and 1800 ps . These are believed to arise due to vacancy clusters on the grain sur faces, the open spaces between the grain surface and the surrounding p olymer layer and the annihilation of orthopositronium at the free-volu me defects, respectively. The lifetime of positrons trapped at the gra in surface defects and the grain-polymer interface is found to decreas e as the grain size is increased. Doppler broadening measurements were carried out from 13 K to 300 K on a silver-polyacrylamide nanocomposi te containing silver particles of diameters in the range 2-20 nm. The line-shape parameter S is decomposed by a mixture rule to obtain the c ontribution of electrons from the nanoparticles. This shows a sharp in crease at around 80 K that is adduced as evidence for the splitting of the electron energy levels in the nanosized silver particles leading to a semiconductorlike behavior. [S0163-1829(98)03102-6].