Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

Nuclear Sciences 532.4.1.3 Defects and morphological features of ZnS nanostructures as revealed frompositron annihilation studiesNanostructures of ZnS, both particles and rods, were synthesized through solvothermal processesand characterized by X-ray diffraction and high resolution transmission electron microscope. Further,positron lifetime and Doppler broadening measurements were made to study the featuresrelated to the defect nanostructures present in the samples. The nanocrystalline grain surfacesand interfaces, which trapped significant fractions of positrons, gradually disappeared during graingrowth, as indicated by the decreasing fraction of orthopositronium atoms. The crystal vacanciespresent within the grains also trapped positrons. The positron lifetime within these vacancieswas estimated from model analysis and the lifetime increased with increasing sizes of the particles.This was due to the agglomeration of the small vacancies into clusters during the thermal treatmentgiven to effect grain growth. The positron lifetime was remarkably large at very small grain sizes( 1.5 nm) and was attributed to quantum confinement effects, as verified through optical absorptionmeasurements. Positron lifetimes in ZnS nanorods increased with increasing content of cubic phaseand this observation is assigned to the annihilation of positrons in sites with increased cubic unitcell volume. The Doppler broadened spectra also indicated qualitative changes consistent withthese observations.Soumitra Kar†, Subhajit Biswas†, PMG Nambissan, Subhadra Chaudhuri†NAP2.4.1.4 Defects-related aspects of Mn x Zn 1−x S nanorods: Results from positron annihilationstudiesPositron lifetime and Doppler broadening measurements were carried out on Mn-doped ZnSnanorods and the change in the defects and morphologies were studied with complementary evidencesfrom other spectroscopic studies like optical absorption. Doping by Mn ions transforms thehexagonal lattice to a cubic one, perhaps to minimize the strain created by the Mn ions of slightlylarger radius. Beyond a dopant concentration of 20 atomic%, the nanostructure collapses and Mn-Mn clustering starts dominating, leading to the formation of pyramid-like MnS crystals. Theseare verified from transmission electron microscopy. Positron lifetimes and the Doppler broadenedlineshape parameters indicated these changes vividly and the results are reasonably interpreted.Subhajit Biswas†, Soumitra Kar†, PMG Nambissan, Subhadra Chaudhuri†NAP2.4.1.5 Thermal evolution of boron irradiation induced defects in pre-doped Si revealedby positron annihilation experimentsThe isochronal annealing behaviour of high energy (25 to 72 MeV) boron ion irradiation induceddefects in boron-doped silicon was monitored through measurements of positron lifetimes and threedistinct defect-evolution stages were identified. The initial boron doping created a defect environmentwhere positrons could sensitively annihilate with the boron electrons, suggesting borondecoratedSi monovacancies as potential trapping sites. The irradiation resulted in the dissolutionof boron from these sites and positrons were then trapped by the empty divacancies of Si. Chargeneutralization of divacancies through interaction with boron atoms led to enhanced positron trappingin the initial stages of isochronal annealing. The divacancies started annealing above 673K.