Tanveer Ahmad Dar

A systematic study of the dielectric hysteresis and temperature-dependent vibrational excitations of NiO containing antiferroelectric (AFE) NaNbO ₃ composites is reported. The Mott insulator NiO plays a significant role in the crystal structure of NaNbO ₃ . In particular, for low and moderate substitutions of NiO, a transition from orthorhombic to monoclinic symmetry is evident at room temperature. The temperature dependence of the relative dielectric permittivity, , exhibits an anomalous change in its magnitude across three distinct phase transitions of NaNbO ₃ at 449 K, 665 K and 751 K. These are, respectively, associated with a second-order phase transition occurring due to incommensurate phase between two orthorhombic AFE orderings, change from the orthorhombic P- to R-phase ( ) without altering the AFE structure and an orthorhombic R- to S-phase transition accompanied by an AFE to paraelectric transition. A large thermal hysteresis ( K) was evident in and ac resistivity ( ) between the temperatures 473 K and 673 K signifying the first-order nature of the transition. At room temperature, for higher NiO content, a new mode centred at 838 cm ⁻¹ was observed in the Raman spectra along with a two-magnon mode at 1496 cm ⁻¹ , a broad longitudinal optical mode (2LO) at 1096 cm ⁻¹ and a weak translational optical mode (2TO) at 726 cm ⁻¹ . At low temperatures, down to 80 K, several peculiar features are evident in the Raman spectra of NbO ₆ octahedra which are associated with the structural and antiferromagnetic transition of NaNbO ₃ and NiO, respectively.

We report the effect of screw dislocations and oxygen vacancy defects on the optical nonlinear refraction response of ZnO films grown on a sapphire substrate at various oxygen flow rates using the chemical vapor deposition technique. The nonlinear refraction response was investigated in the off-resonant regime using a CW He-Ne laser source to examine the role of the intermediate bandgap states. It has been observed that the structural defects strongly influence the optical nonlinearity in the off-resonant regime. Nonlinearity has been found to improve as the oxygen flow rate is lowered from 2 sccm to 0.3 sccm. From photoluminescence studies, we observe that the enhanced defect density of the electronic defect levels due to the increased concentration of structural defects (with the decrease in the oxygen flow rate) is responsible for this improved optical nonlinearity along with the thermal effect. This suggests that defect engineering is an effective way to tailor the nonlinearity of ZnO films and their utility for optoelectronic device applications.

We report the structural and electronic properties of single phase, polycrystalline rutile tetragonal SnO₂ thin film grown on Si (100) substrate by pulsed laser deposition technique. X-ray photoelectron and resonant photoemission spectroscopic (RPES) studies divulge that Sn is present in 4+ (∼91%) valence state with a very small involvement of 2+ (∼9%) valence state at the surface. Valence band spectrum of the film shows prominent contribution due to the Sn⁴⁺ valence state. RPES measurements were performed in the Sn 4d→5p photo absorption region. This study shows that O-2p, Sn-5s, and Sn-5p partial density of states are the main contributions to the valence band of this material. The resonance behavior of these three contributions has been analyzed. Constant initial state versus photon energy plots suggest that the low binding energy feature at ∼2.8 eV results from the hybridization of the O-2p and mixed valence states of Sn, while remaining features at higher binding energies are due to the hybridization between O-2p (bonding) orbitals and Sn⁴⁺ valence state.

We report a detailed study on structural and dielectric properties, Raman scattering, and ac-conductivity of ferroelectric KNbO ₃ and antiferromagnetic NiO two-phase composites [( 1 ​ ​ − ​ ​ x ) KNbO ₃ + x NiO]. Two major transitions were observed in the temperature dependent relative dielectric permittivity ϵ r ( T )—the first at 445   ∘ C ( T ∗ ) and the second at 220   ∘ C ( T ₁ )—respectively associated with the structural transitions from tetragonal to cubic and orthorhombic to tetragonal phases. In addition, a new anomaly across T ₂ ∼ 320   ∘ C has been observed in ϵ r ( T ), and the loss-tangent (Tan δ (T)), which may be associated with the combined effect of antiferromagnetic ordering of NiO and ferroelectric coupling of KNbO ₃ . The dispersive nature of T ∗ , and the Vogel–Fulcher temperature dependence of its characteristic relaxation frequency reveals the existence of relaxor-like behavior, which is further supported by Uchino–Nomura criteria (for x ⩽ 0.10 ) but departs from relaxor behavior for x > 0.10 . Temperature dependent ac-resistivity analysis ρ a c ( T ) provides strong evidence for a Mott’s Variable range hopping process of charge carriers between localized states, with average hopping length of 4.2 nm and hopping energy 0.72 eV. The variation of T ∗ , T ₁ and T ₂ as a function of x , in consonance with the crystal structure, has been discussed. Raman spectroscopic studies were carried out to study the local structure of the ( 1 ​ ​ − ​ ​ x ) K N b O 3 + x NiO two-phase composites, which shows significant increase of tilt-angle between adjacent N b O 6 octahedra as the divalent Ni occupies octahedral sites of the K N b O 3 matrix. All the Raman-active modes of these composites are indexed as lattice translations, specifically with the motion of K + (190 c m − 1 ), the core vibrations of the N b O 6 octahedra (220 c m − 1 ⩽ ν ⩽ 320 c m − 1 ) and their collective modes (50 c m − 1 ⩽ ν ⩽ 150 c m − 1 ), and Magnon modes (2M ∼ 1500 c m − 1 ) associated with the antiferromagnetic ordering of NiO.

Effect of annealing on optical characteristics of Nickel oxide thin films deposited by ion beam sputtering technique from a Ni target in a mixture of oxygen and argon gas on to a glass substrate has been studied. The deposited films were characterized in as deposited state(S1) and after annealing(S2) at temp of 523 K. Crystalline properties of films were investigated using X-ray diffraction technique from which we found that both S1 and S2 shows the polycrystalline nature with preferential growth along (111) plane. The transmittance of the S2 films was decreased. The surface morphology of the film was studied using atomic force microscopy (AFM). The nonlinear optical properties of the films were obtained using z-scan technique which reveals that the nonlinear absorption coefficient of S2 films is larger than that of S1 samples. Improved nonlinearity suggests the utility of the grown films for optoelectronic device application.

We report the structural, optical and electronic properties of polycrystalline ZnO and NiO thin films and amorphous ZnO/NiO bilayer film, prepared by pulsed laser deposition technique. Despite of the presence of both Zn and Ni in +2 state in the bilayer film, the grown bilayer shows no reflections (in XRD) corresponding to ZnO or NiO. The difference in crystal structure of ZnO and NiO leads to the strain in the grown bilayer film. An increase in the band gap has been observed in bilayer film which can be attributed to the amorphous nature of the structure.

Thermo-optic properties and optical limiting characteristics are investigated in pulsed laser deposited Mg-doped ZnO thin films by performing a Z-scan experiment in the off-resonant regime using a CW He–Ne laser source. Optical limiting studies in these thin films lead to a conclusion that reverse saturable absorption is the major mechanism for optical limiting properties in these films and nonlinear refraction is caused due to the thermal lensing effect. Negative thermo-optic coefficients were observed, indicating potentiality for athermalization, required to counter the effect of thermal expansion. Sufficiently large thermo-optic coefficients and good optical limiting response suggest the utility of MgZnO films for thermo-optic switches and optical limiting device applications.