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Tiwari V.K.,Banaras Hindu University | Avasthi D.K.,Inter University Accelerator Center | Maiti P.,Banaras Hindu University
ACS Applied Materials and Interfaces | Year: 2011

Ion flux dependent swift heavy ions (SHI) induced structural changes have been reported for pristine poly-(vinylidene fluoride) (PVDF). Ordering phenomena has been observed first followed by its transformation from α to β-form (polar metastable piezoelectric phase). The ordering of (020) plane become prominent at higher ion flux SHI irradiation and its further increase induces structural change from R to β phase as revealed by XRD and FTIR analyses. Structural changes are also supported by morphological evidence and thermal studies before and after SHI irradiation. © 2011 American Chemical Society.


Kumar P.,Indian Institute of Technology Delhi | Malik H.K.,Indian Institute of Technology Delhi | Ghosh A.,Indian School of Mines | Thangavel R.,Indian School of Mines | Asokan K.,Inter University Accelerator Center
Applied Physics Letters | Year: 2013

We propose Mg doping in zinc oxide (ZnO) films for realizing wider optical bandgap in highly c-axis oriented Zn1-xMgxO (0 ≤ x ≤ 0.3) thin films. A remarkable enhancement of 25% in the bandgap by 30% Mg doping was achieved. The bandgap was tuned between 3.25 eV (ZnO) and 4.06 eV (Zn0.7Mg0.3O), which was further confirmed by density functional theory based wien2k simulation employing a combined generalized gradient approximation with scissor corrections. The change of stress and crystallite size in these films were found to be the causes for the observed blueshift in the bandgap. © 2013 AIP Publishing LLC.


Mal S.,North Carolina State University | Nori S.,North Carolina State University | Narayan J.,North Carolina State University | Prater J.T.,U.S. Army | Avasthi D.K.,Inter University Accelerator Center
Acta Materialia | Year: 2013

We have introduced defects in ZnO epitaxial thin films by swift heavy 107Ag9+ ion irradiation and investigated systematically their magnetic, electrical and optical properties. Oxygen annealed ZnO films are epitaxial single crystals that exhibit no long-range magnetic order. However, in this paper it is shown that room-temperature ferromagnetism (RTFM) can be introduced in a controlled manner in these films using ion irradiation and that the magnetization increases with ion dose. This qualitatively agrees with earlier studies which showed that RTFM could be induced in ZnO films through either vacuum thermal annealing or pulsed laser annealing below energy densities that lead to melting. Raman studies of the ion irradiated samples revealed dramatic changes in the vibration modes that correlated with increases in the carrier concentration, indicative of lattice disorder and defect creation. We compare these results with those observed in laser irradiated and vacuum annealed samples, and then discuss these findings in the context of defects and defect complexes created during the high-energy heavy ion irradiation process. We propose a unified mechanism to explain RTFM and n-type conductivity enhancements during irradiation, and laser and vacuum annealing. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


Sultan K.,National Institute of Technology Srinagar | Ikram M.,National Institute of Technology Srinagar | Asokan K.,Inter University Accelerator Center
Vacuum | Year: 2014

Polycrystalline bulk samples of PrFe1-xMnxO 3 (x = 0.0, 0.1, 0.3, 0.5) were synthesized by solid state reaction method to understand their structural, optical and dielectric properties. X-ray diffraction (XRD) and Raman spectroscopy were investigated to confirm chemical phase and the orthorhombic pbnm structure. As the concentration of Mn increases, the lattice parameter b increases while the lattice parameters a and c/√2 decrease but the change of former is less than later. PrFe1-xMn xO3 exhibits O-type (a < c/√2 < b) orthorhombic pbnm structure upto x = 0.5. From XRD it is also evident that the peaks shift towards higher 2θ values with increase in Mn content indicating the development of strain in the crystal structure possibly due to Jahn-Teller distortion after the incorporation of Mn3+ ions in the parent compound PrFeO3. From the Raman study, the modes exhibit a blue shift with broadening of spectral features in the doped samples. The observed shift in wave number with doping clearly indicates change in the bond lengths of Fe-O/Mn-O as well as their impact on FeO6/MnO6 octahedra. The dielectric constant (É′) and dielectric loss (tan δ) are also studied as a function of frequency and temperature. The dielectric constant and ac conductivity increases with Mn doping. The variation of dielectric properties such as ac conductivity, tan δ and É′ suggests that small polarons contribute to the conduction mechanism. Activation energy (Eσ) and optical band gap (E g) decreases with the concentration of Mn. The observed higher values of these quantities reveals that there is hopping between Mn3+ to Mn4+ and Fe3+ to Fe2+ at the octahedral sites of the compound. Possible mechanism contributing to these processes has been discussed. © 2013 Elsevier Ltd. All rights reserved.


Siva Kumar V.V.,Inter University Accelerator Center
Vacuum | Year: 2016

We report studies on growth of nanocrystalline diamond films by electron cyclotron resonance chemical vapour deposition (ECR CVD) and their structural and photoconduction properties. Bias enhanced nucleation (BEN) under higher methane partial pressure has been used to grow diamond nanocrystals with sp2 carbon clusters at grain boundaries in an amorphous carbon matrix. Structural studies indicate the size of diamond nanocrystals decreased and sp2 content increased in the films with increase in methane partial pressure, which is explained using subplantation model. Current- Voltage (I-V) measurements under dark and deep ultraviolet (DUV) light illumination conditions showed higher photocurrent in films with smaller sized diamond nanocrystals (3 nm–8 nm) compared to films having diamond nanocrystals of larger size (∼15 nm). The results indicates that the photocurrent generated in the films has major contributions from photoexcited electrons in diamond nanocrystals and sp2 carbon nanoclusters, with minor contribution from amorphous carbon phase. © 2016 Elsevier Ltd

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