M. Premila

Pressure induced dimerisation of C70

Solid C70 has been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 750°C. X-ray diffraction measurements on the recovered samples indicate that the initial h.c.p. solid C70 transforms to a rhombohedral structure which recovers to an f.c.c. structure on annealing. The associated changes in the intra molecular vibrational modes have been probed through infrared (IR) and Raman measurements. The IR measurements on these HPHT samples show splitting of some of the pristine modes and occurrence of several new modes. These sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been attributed to the formation of C70 dimers.

Metal–insulator transition in V2O3: positron lifetime studies

Abstract Temperature dependence of positron lifetime has been studied across the metal–insulator transition at 150 K in V 2 O 3 . With the decrease of temperature, positron lifetime, 220 ps at room temperature is seen to decrease gradually to 190 ps at low temperature. The smaller value of lifetime in the low temperature insulating phase is surprising, and is possibly related to the changes in local structure across the rhombohedral to monoclinic structural transition.

Anodisation time dependent shift of photoluminescence excitation and emission spectra in porous silicon

The photoluminescence excitation and emission techniques are used to monitor the absorption and emission characteristics of silicon nanocrystals. Contributions from various critical points are identified and their shift as a function of reduction in particle size is deduced. A red shift in the direct band gap energy of silicon with decreasing crystal size is experimentally observed for the first time. The results are explained on the basis of quantum confinement in silicon nanocrystals.

Studies on Fullerenes Using Positron Annihilation Spectroscopy

ABSTRACT Results of positron lifetime measurements as a function of temperature and pressure are presented. With the application of pressure, the lifetime in solid C60 is observed to decrease and show signatures of the structural transition from fcc to the simple cubic (sc) phase. Positron lifetime measurements as a function of temperature, carried out in both C60 and C70, indicate an increase in lifetime at elevated temperatures. This is accounted for in terms of thermally activated trapping of positrons from the interstitial regions to the open regions within the cage structure. In addition to studies in the solid state, the annihilation characteristics of the ortho positronium (O-Ps) formed in organic solvents has been monitored with increasing concentration of dissolved fullerenes, C60 and C70. The observed reduction of the ortho positronium lifetime is understood in terms of the formation of O-Ps- Fullerene complex prior to annihilation.

Temperature and composition dependence of phonon modes in GdSr2Ru1−xCu2+xO8

Low temperature infrared absorption measurements have been carried out on GdSr2RuCu2O8 between 300 and 10 K. With the lowering of temperature, below the magnetic transition temperature, TM, the modes corresponding to vibrations of ruthenium and planar oxygen atoms are observed to harden significantly, over and above that determined by anharmonicity, and this is attributed to the spin–phonon interaction. The mode corresponding to the vibrations of the apical oxygen has an asymmetric (Fano) lineshape, and the asymmetry parameter is seen to decrease sharply below TM, and a similar behaviour is also seen for Cu-doped GdSr2Ru1−xCu2+xO8 (x = 0.0–0.20). This has been understood in terms of the coupling of this phonon mode with the charge carriers. A plot of the phonon linewidth with the square of the Fano asymmetry parameter shows a linear correlation, with a slope dependent on the density of carriers. This slope and the mode frequencies of the various Cu doped samples are seen to correlate with the observed non-monotonic variation of Tc with Cu content with a maximum for x = 0.1.

Structural studies on fullerenes

Amongst the other physical properties, the structure of fullerene solids, and the structural transformations with temperature, pressure and doping have all evoked considerable interest and during the last two years tremendous progress has been made in this area. This paper provides a brief review of the studies on the structural properties of pristine and doped fullerenes. The results of our own investigations are also presented.

Anodisation Time Dependence of Photoluminescence Properties of Porous Silicon

The photoluminescence excitation and emission techniques are used to monitor the absorption and emission characteristics of silicon nanocrystals. Contribution from various critical points are identified and their shift as a function of reduction in particle size is deduced. A red shift in direct band gap energy of silicon with decreasing crystal size is experimentally observed for the first time. The results are explained on the basis of quantum confinement in silicon nanocrystals.

Pressure-Induced Polymerisation of Fullerenes

Solid C60 and C70 have been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 800°C. X-ray diffraction measurements on the recovered samples indicate that the initial fee solid C60 transforms to an orthorhombic structure consisting of a polymerised linear chain of C60 molecules. The associated changes in the intra molecular vibrational modes have been probed through Raman and infrared (IR) measurements. In the case of solid C70, under similar conditions, the initial hcp structure is seen to transform to a rhombohedral structure, which recovers to an fee structure on annealing. The IR measurements on these HPHT samples show the splitting of pristine modes and occurrence of new modes. The sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been tentatively attributed to the formation of C70 dimers.