Foreword

Skutterudites Fe0.2Co3.8Sb12-xTex (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by induction melting at 1273 K and followed by annealing at 923 K for 144 h. X-ray diffraction confirmed the polycrystalline nature of the skutterudite phase as the main phase in all the samples with a small amount of (Sb) as a secondary phase. Since Te exceeds the solubility limit, additional secondary phases, CoSb2 and CoTe2, were observed in Co3.8Fe0.2Sb11.4Te0.6. Lattice constants and atomic positions are verified by Rietveld refinement using the composition from electron probe micro analysis (EPMA). The temperature dependence of transport properties has been measured for all samples between 300 K and 818 K. Whereas for the alloys up to x=0.2 (Co3.8Fe0.2Sb11.8Te0.2) the electrical resistivity initially increased by charge compensation, for x>0.2 it decreased with increasing Te content due to an increasing electron concentration. At room temperature, a positive Seebeck coefficient (holes are majority carriers) was obtained in single element (Fe) substituted Co4Sb12, whilst a negative Seebeck coefficient (electrons are majority carriers) was obtained in Fe0.2Co3.8Sb12-xTex. Thermally excited carriers change from n-type to p-type in Co3.8Fe0.2Sb11.9Te0.1 at 570 K while all other samples exhibit negative Seebeck coefficients in the entire temperature range measured. Lattice thermal conductivity decreased with increasing Te content due to mass fluctuation and point defect scattering. The maximum dimensionless figure of merit, zT = 1.04 at 818 K, was obtained with an optimized Te content for Co3.8Fe0.2Sb11.5Te0.5. Signature of novel Plasmonic Dicke effect in 2D quantum dot solids mediated by surface plasmons of embedded nanoparticles Jaydeep Basu Quantum dot ensembles and solids find numerous applications from solar cells to novel nano-lasers. Understanding their optical and electrical properties and the ability to tune them is thus of vital importance. Here, we will discuss our recent work on photoluminescence (PL) from two dimensional solids made of CdSe quantum dots lightly doped with metal nanoparticles. The emission from the quantum dot solid can be tuned by controlling the packing of the solid, spectral overlap between the quantum dots and the metal nanoparticles as well as through doping concentration. While the PL from the quantum dots show expected variation at low density and large doping of gold nanoparticles, unexpected strong enhancement in PL occurs at low and intermediate doping, especially when the quantum dots and metal nanoparticles are spectrally at resonance. A recently suggested model [3] of plasmon mediated superradiance, due to virtual plasmon exchange between proximal quantum dots, leading to such enhancements seems to be the likely cause of such an unusual effect. Acknowledgements: The work presented here has been done in collaboration with Laxminaryanan Tripathi, M. Praveena, M. Haridas. References: 1. L. N. Tripathi, M. Praveena, J. K. Basu, Plasmonics (2012). 2. M. Haridas, L. N. Tripathi and J. K. Basu, Applied. Physics. Letter, 98, 063305 (2011). 3. Vitaliy N. Pustovit and Tigran V. Shahbazyan, Phys Rev Lett. 102, 077401 (2009). Evidence of gradient in dynamics of confined polymers Sivasurender Chandran 1 , N. Begam 1 , J. K. Basu 1 and M. K. Mukhopadhyay 2 1 Department of Physics, Indian Institute of Science, Bangalore – 560012 2 Applied Materials Science Division, Saha Institute of Nuclear Physics, Kolkata – 700064 Particle segregation to surface/interface, surface mobility of the particles, interfacial viscosity and thereby the gradient in the dynamics of polymer thin films is a matter of debate, which generated lot of controversies over the last few years. We report [1] the evidence of gradient in dynamics by probing the diffusion coefficient of polymer grafted nanoparticles (PGNP) dispersed in polymer thin films of different thickness viz., 2.5Rg and 8Rg of the matrix chains (Rg is the radius of gyration). Using surface x-ray scattering, we observe a systematic vertical dispersion of PGNP from a pinned in substrate interface layer to the surface on thermal annealing. Even after annealing at high temperature (T>>Tg) and longer times, a fraction of PGNP pertain to stay at the substrate forming a stable interface layer. This hints about the low mobility of particles at the substrate interface and also emphasizes the presence of high viscous/gel-like interfacial layer. Real space microscopic images show the formation of lateral domains of the particles at air surface suggesting the higher surface mobility. In addition, it is also observed that the fraction of particles in the air surface is more in annealed thinner films compared to the thicker ones. Thus, we have correlated the observed lateral and vertical dispersion and its evolution with annealing, to the gradient in dynamics along the thickness of the thin films. [1] Sivasurender Chandran, J. K. Basu and M. K. Mukhopadhyay, in communication