Abhinav Pratap Singh

Structural, electronic, and magnetic properties of Co doped SnO2 nanoparticles

We present a detailed study on the structural, electronic, and magnetic properties of chemically synthesized Sn1−xCoxO2 (x=0.00 to 0.05) nanoparticles. X-ray diffraction and transmission electron microscope measurements were performed to analyze the crystal structure and morphology of Sn1−xCoxO2 nanoparticles. The energy dispersive x-ray analysis measurements were performed to check the possible presence of any impurity elements in the nanocrystals. The near edge x-ray absorption fine structure (NEXAFS) experiments at Sn M5,4-edge and Co L3,2-edge were performed to probe the local environment of Sn and Co ions in the SnO2 matrix. The NEXAFS at Co L3,2-edge, along with multiplet calculations, indicate that the Co is substituted at the Sn site in SnO2 matrix with +2 charge state and do not form metallic clusters and other oxide phases. The ferromagnetic nature of these materials was confirmed by x-ray magnetic circular dichroism and room temperature magnetization hysteresis loop measurements.

Ferromagnetism and metal-semiconducting transition in Fe-doped ZnO thin films

We report the room temperature ferromagnetism and a metal–semiconductor transition at 227 K in 200 MeV Ag15+-ion irradiated thin films of Fe-implanted ZnO. The single phase nature of Fe-doped ZnO after ion irradiation is confirmed by x-ray diffraction. Magneto-resistance measurements show spin polarization below 150 K. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies at room temperature reveal that Fe is oxidized in a mixed valence (Fe2+ and Fe3+) state and the magnetic signal is due to the Fe2+ state. The observations are explained on the basis of the combined effect of carrier doping by Fe3+ and oxygen vacancies.

Modifications in structural and electronic properties of TiO2 thin films using swift heavy ion irradiation

We report on the structural and electronic properties of swift heavy ion (SHI) irradiated pristine TiO2 thin films, deposited by radio frequency magnetron sputtering on sapphire substrates. The high resolution x-ray diffraction and Raman measurements show a structural phase transition from anatase to admixture of brookite and rutile phases of TiO2 with increasing SHI fluence followed by a significant distortion in the TiO6 octahedra. The modification in the electronic structure stimulated by SHI irradiation has been investigated using x-ray absorption (XAS) experiments at the O K and Ti L3,2 absorption edges. The O K edge spectra clearly indicate the splitting of the pre-edge spectral features having t2g and eg symmetry bands due to structural disorder/distortion induced by irradiation. The intensity of the SHI generated components at the O K edge increases monotonically, which can be correlated to the modification in unoccupancies associated with O 2 p orbitals hybridized with Ti 3 d states. The XAS spec...

Irradiation induced ferromagnetism at room temperature in TiO2 thin films: X-ray magnetic circular dichroism characterizations

We report on the room temperature ferromagnetism in the swift heavy ion (SHI) irradiated TiO2 thin films by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) experiments at the O K and Ti L3,2 absorption edges. The XAS/XMCD measurements provide direct evidence of magnetic polarization of the O 2p and Ti 3d orbitals. The unquenched orbital magnetic moment within the O 2p shell is ferromagnetically coupled to the neighboring Ti moments, which illustrates the intense hybridization of the O 2p and Ti 3d orbitals induced by SHI irradiation.

Evidence of quantum correction to conductivity in strained epitaxial LaNiO3 films

We have deposited epitaxial thin films of LaNiO3 (LNO) on LaAlO3 (001) single crystals by rf-magnetron sputtering. Further, we studied the effect of systematically varied swift heavy ion irradiation induced strain on structural, electrical, and magnetotransport properties of the films. Deposited films were irradiated at varying fluence (1×1011, 1×1012, and 5×1012 ions/cm2) using 200 MeV Ag15+ beam. X-ray diffraction results reveal c-axis oriented epitaxial growth of the LNO film which is maintained even up to the highest fluence. All the films, except the one irradiated with highest fluence, show metallic behavior along with a resistivity upturn at lower temperatures. Film irradiated with the highest fluence value exhibits semiconducting behavior in the studied temperature range. Low temperature resistivity of the metallic films has been explained by quantum corrections to conductivity and it is observed that localization increases with the disorder. Presence of weak localization in metallic films is also...

Enhancement in electron field emission in ultrananocrystalline and microcrystalline diamond films upon 100 MeV silver ion irradiation

Enhanced electron field emission (EFE) behavior was observed in ultrananocrystalline diamond (UNCD) and microcrystalline diamond (MCD) films upon irradiation with 100 MeV Ag9+-ions in a fluence of 5×1011 ions/cm2. Transmission electron microscopy indicated that while the overall crystallinity of these films remained essentially unaffected, the local microstructure of the materials was tremendously altered due to heavy ion irradiation, which implied that the melting and recrystallization process have occurred along the trajectory of the heavy ions. Such a process induced the formation of interconnected nanocluster networks, facilitating the electron conduction and enhancing the EFE properties for the materials. The enhancement in the EFE is more prominent for MCD films than that for UNCD films, reaching a low turn-on field of E0=3.2 V/μm and large EFE current density of Je=3.04 mA/cm2 for 5×1011 ions/cm2 heavy ion irradiated samples.

Irradiation induced modification in transport properties of LaNiO3 thin films: An x-ray absorption study

Highly c-axis oriented LaNiO3 thin films are grown on LaAlO3 (001) single crystal substrates using rf-magnetron sputtering. Swift heavy ion irradiation induced variations in structural and electrical transport properties of deposited films are studied. Pristine film shows unusual insulating character while the irradiated film exhibits metallic behaviour. X-ray diffraction study indicates that irradiation improves the crystalline character of films while retaining the oriented single phase growth. Electronic structure measurements performed using x-ray absorption spectroscopy at O K, La M5,4, and Ni L3,2-edges reveal that Ni-O hybridization-controlled localization of charge carriers is responsible for the observed transport behaviour.

On the optical properties of Ag+15 ion-beam-irradiated TiO2 and SnO2 thin films

The effects of 200-MeV Ag+15 ion irradiation on the optical properties of TiO2 and SnO2 thin films prepared by using the RF magnetron sputtering technique were investigated. These films were characterized by using UV-vis spectroscopy, and with increasing irradiation fluence, the transmittance for the TiO2 films was observed to increase systematically while that for SnO2 was observed to decrease. Absorption spectra of the irradiated samples showed minor changes in the indirect bandgap from 3.44 to 3.59 eV with increasing irradiation fluence for TiO2 while significant changes in the direct bandgap from 3.92 to 3.6 eV were observed for SnO2. The observed modifications in the optical properties of both the TiO2 and the SnO2 systems with irradiation can be attributed to controlled structural disorder/defects in the system.

High transmittance contrast in amorphous to hexagonal phase of Ge2Sb2Te5: Reversible NIR-window

Ge2Sb2Te5 (GST) is one of the best phase change materials because of its splendid set of properties, viz., high thermal stability, fast crystallization speed, good endurance, scalability, and reliability. Phase transition [amorphous → face centered cubic (fcc) → hexagonal close packed (hcp)] of GST thin films with annealing was studied using X-ray diffraction. Thin films in amorphous, fcc, and hcp phases are highly, medium, and negligible transparent in the near infra-red region, respectively. The optical transmission in amorphous, fcc, and hcp phases is ∼92%, ∼46%, and ∼2%, respectively, at the wavelength of 2740 nm. At 2740 nm, a high transmission contrast (∼90%) is observed with phase transition from the amorphous to hcp phase. By utilizing large transmission contrast, it is demonstrated that GST can be availed as a potential candidate for reversible near infra-red-window. The sharp change in optical transmission with phase transition can be understood from the change in density of states in the valenc...