Nano sulfide and oxide semiconductors as promising materials for studies
by positron annihilation
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India
A number of wide band gap sulfide and oxide semiconducting nanomaterial systems
were recently investigated by us using the experimental techniques of positron
annihilation spectroscopy and the results helped to attribute several of their physical
properties to the presence of vacancy-type defects and their clusters (1-3). The samples
were prepared through chemical routes (i.e., either via solvothermal or sol-gel method)
and were characterized by x-ray diffraction and high resolution transmission electron
microscopy. In some cases, optical absorption and photoluminescence studies were
made to extract the precise values of the band gap, which has been found to have an
important bearing on the measured positron annihilation parameters.
The occurrence of quantum confinement effects was displayed in these studies as
remarkable changes in the positron lifetimes and Doppler broadened lineshape
parameters (1,4). The results obtained in CdS nanoparticles were supported by a
characteristic blue shift in the optical absorption spectra (1). We got similar observations
in NiO nanoparticles too (4). In a more recent work on yet another series of NiO
nanocrystalline samples, considerable enhancement in the band gap and significant rise
of the positron lifetimes were found occurring when the particle sizes were reduced to
below about 5 nm. Although ZnO continues to be an enigma to positron lifetime
spectroscopy, we got exciting results during doping of the semiconductor by other
transition metals such as Mn (3), Li etc. A very interesting observation was the reduction
in the diameters of ZnO nanocones when the Mn doping concentration was increased
from 0.1 to 15 at.%. This was explained as due to the necessity to increase the surface
to volume ratio of the nanoparticles in order to accommodate the increasing number of
Mn2+ ions on the surfaces. We also recently studied heat-treated TiO2 nanoparticles
and the results are being separately reported. Finally, the results of positron annihilation
and other measurements on carefully grown CeO nanoparticle systems and the
variations with Eu doping will be reported.
§ Corresponding author. E-mail : firstname.lastname@example.org
 Soumitra Kar, Subhajit Biswas, Subhadra Chaudhuri & P.M.G. Nambissan, Phys.
Rev. B 72, 075338 (2005).
 Subhajit Biswas, Soumitra Kar, Subhadra Chaudhuri & P.M.G. Nambissan, J. Chem.
Phys. 125, 164719 (2006).
 Tandra Ghoshal, Soumitra Kar, Subhajit Biswas, S.K. De & P.M.G. Nambissan, J.
Phys. Chem. C 113, 3419 (2009).
 Soumen Das, Tandra Ghoshal & P.M.G. Nambissan, Phys. Stat. Solidi C 6, 2569