T. Mohanty

Color center formation in sapphire by swift heavy ion irradiation

Abstract Single crystals of sapphire (α-Al2O3) were irradiated at room temperature with 100 and 190 MeV silver (Ag) ions from the 15 UD Pelletron at the Nuclear Science Centre, New Delhi. The pristine as well as irradiated sapphires are characterized by photoluminescence at room temperature under 2.8 eV blue excitation by a He–Cd laser, ultraviolet (UV)–Visible absorption and electron paramagnetic resonance (EPR). Photoluminescence studies show a peak at 2.2 eV , due to formation of a F22+ center. The intensity of this peak increases with fluence. A sudden increase in peak intensity has been observed after 5×10 12 ions / cm 2 where the defect zones started overlapping due to multiple ion impact. UV–Visible and EPR studies show formation of defects like F and F+ centers.

Fermi level shifting of TiO2 nanostructures during dense electronic excitation

Scanning Kelvin probe microscopy has been used to understand the modification of work function of TiO2 with swift heavy ion irradiation. The observed increase in contact potential difference (CPD) indicates a shift in Fermi level towards the valence band, which is due to the development of defects during the bombardment of high energy heavy ions. The change in CPD values on ion irradiation is attributed to electronic excitation induced defect concentration and surface roughness.

200 MeV silver ion irradiation induced structural modification in YBa2Cu3O7-y thin films at 89 K: An in situ x-ray diffraction study

We report in situ x-ray diffraction (XRD) study of 200 MeV Ag ion irradiation induced structural modification in c-axis oriented YBa2Cu3O7−y (YBCO) thin films at 89 K. The films remained c-axis oriented up to a fluence of 2×1013 ionscm−2, where complete amorphization sets in. The amorphous ion tracks, the strained region around these tracks, and irradiation induced point defects are shown to control the evolution of the structure with ion fluence. Secondary electrons emanating from the ion paths are shown to create point defects in a cylindrical region of 97 nm radius, which corresponds to their maximum range in the YBCO medium. The point defects are created exclusively in the CuO basal planes of fully oxygenated YBCO, which has not been possible, by other techniques including low energy ion irradiation and thermal quenching. The point defects led to a faster decrease in the integral intensity of XRD peaks at very low fluences of irradiation (Φ≤3×1010 ionscm−2) than what can be expected from amorphous tra...

Nanoprecipitation in transparent matrices using an energetic ion beam

Silicon nanophases are grown in fused silica using an ion implantation technique followed by a thermal and swift heavy ion irradiation induced annealing process. From an estimation of the track cooling time and nanoprecipitation nucleation time for the present experiment, it is inferred that nanoprecipitation occurs inside the swift heavy ion induced latent tracks. The blue shift of the photoluminescence peak as well as the UV/visible absorption band edge for swift heavy ion induced annealing indicate the occurrence of nanoprecipitation inside the track.

Effect of secondary electrons from latent tracks created in YBCO by swift heavy ion irradiation

Abstract Swift heavy ions (SHI) with electronic energy loss exceeding a value of 14.4 keV nm −1 create amorphized latent tracks in YBCO type superconductors. In the low fluence regime of an ion beam where tracks do not overlap, a decrease of the superconducting transition temperature as probed through resistivity studies, is not expected due to availability of percolating current paths. The present study however shows Tc decrease by about 1– 3 K in thin films of YBCO when irradiated by 250 MeV Ag ions at 79 K at a fluence of 5×1010– 1×10 12 ions cm −2 . The highest fluence used in the present study is three times less than the fluence where track overlapping becomes significant. The Tc tends to increase towards the preirradiation value on annealing the films at room temperature. To explain this unusual result, we consider the effect of ion irradiation in inducing materials modification not only through creation of amorphized latent tracks along the ion path, but also through creation of atomic disorder in the oxygen sublattice in the Cu–O chains of YBCO by the secondary electrons. These electrons are emitted radially from the tracks during the passage of the SHI. Considering the correlation between the charge state of copper and its oxygen coordination, we show in particular that the latter process is a consequence of the inelastic interaction of the SHI induced low-energy secondary electrons with the YBCO lattice, which result in chain oxygen disorder and Tc decrease.

Latent track creation in fused silica by silver beam

Abstract Studies of formation of latent tracks in swift heavy ion irradiated SiO 2 are presented. Fused silica (SiO 2 ) were irradiated with 200 MeV silver (Ag) ion beam at varying fluences. Radiation-induced effects were studied by ultraviolet(UV)/Visible optical absorption spectroscopy and transmission electron microscopy (TEM). UV/Visible absorption study indicated E′ centers and oxygen deficiency centers having characteristic absorption occurred at 5 eV . The density of these color centers calculated from the absorption peak intensity showed Poisson-type variation with irradiation fluence. The defects are thus entirely confined to the latent tracks created by swift heavy ions in SiO 2 . The track radius estimated from optical absorption study was found to be 5.1 nm . Similar results were obtained from TEM studies of the irradiated samples.

Composition dependent Fermi level shifting of Au decorated MoS2 nanosheets

In the present work, shifting of Fermi level of MoS2 nanosheets due to decoration of Au nanoparticles (Au NPs) is reported. Au NPs are grown on MoS2 nanosheets by chemical reduction method. The structural analysis of pristine MoS2 and Au NPs decorated MoS2 has been done using X-ray diffraction and transmission electron microscopy. The effect of Au NPs decoration on the Fermi energy level of MoS2 nanosheets have been monitored by scanning Kelvin probe microscopy, which measures the work function in terms of contact potential difference. The work function of pristine MoS2 is found to be 4.994 eV, and it increases linearly for Au-MoS2 with increasing concentration of Au NPs. The gradual increase in the work function values indicate a systematic shifting of Fermi energy level of MoS2 towards valence band due to decoration of Au NPs.

Probing the Förster resonance energy transfer between fluorescent copper nanoclusters and cobalt complex

In the present study, we report the Forster resonance energy transfer (FRET) from fluorescent copper nanoclusters (Cu NCs) as donor to cobalt complex (nitrate (Co (NO3)2)) as acceptor. Fluorescent Cu NCs have been synthesized on bovine serum albumin template by wet chemistry method; these NCs show fluorescence maxima at 435 nm. The fluorescence intensity of Cu NCs is quenched in proximity presence of acceptors, and subsequently, energy is transferred. In such type of system, these Cu NCs are found to be efficient donor with Forster distance (R0) 8.9 A and FRET efficiency (E) up to 42%. The Forster distance obtained is found to be the lowest among other reported values for donor/acceptor pair till today.

Coexistence of interfacial stress and charge transfer in graphene oxide-based magnetic nanocomposites

AbstractIn this paper, the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite–graphene oxide (CoFe2O4/GO) nanocomposites has been established. Transmission electron microscopy results reveal the decoration of CoFe2O4 nanoparticles on GO sheets. Magnetic response of nanocomposites was confirmed from superconducting quantum interference device magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. The interfacial compressive stress involved in this system has been evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in the full-width half-maximum value as well as upshift in D and G peaks is clear indications of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi energy level of these nanocomposites. This is evaluated from variation in contact surface potential difference using scanning Kelvin probe microscopy. XRD spectra of CoFe2O4/GO confirm the polycrystalline nature of CoFe2O4 nanoparticles. Lattice strain estimated from XRD peaks is correlated with the observed Raman shift.

GRANULARITY CONTROLLED IRRADIATION RESPONSE OF CUPRATE SUPERCONDUCTORS

Abstract Confining to an energy range where ions can neither create defects through elastic energy loss nor they can create defects through latent track formation, we study the effect of 140 MeV Si-ion irradiation in YBa2Cu3O7−x (YBCO). We show that the evolution of superconducting and normal state properties in such situation is largely governed by the initial defects structure, particularly the grain boundary characteristics of the YBCO system. Both intra- and inter-granular defect structure in films of two batches were made widely different by having Ag as composite and substituent in one and by aging the other prior to irradiation. Evolution of the resistivity vs temperature characteristics in these films with ion fluence reveals the importance of Ag in bringing about both inter- and intra-granular modifications and making the films insensitive to ion irradiation.