Kajari Das

Enhanced Magnetic and Dielectric properties of Eu and Co co-doped BiFeO3 nanoparticles

Bi1−xEuxFe1−yCoyO3 (x = 0, 0.01; y = 0, 0.01) nanoparticles, having an average size of 13 nm, were prepared by a simple sol gel route. Strong electronegativity of Eu3+ and smaller oxidation-reduction potential of Co3+/Co2+ (0.55 eV) than Fe3+/Fe2+ (1.3 eV) increase the concentration of Fe3+ ions with doping. Distinct magnetic hysteresis and complete saturation of magnetisation indicate the presence of ferromagnetic phase. The successful co-doping of Eu and Co into BiFeO3 (BFO) lattice dramatically enhances the saturation magnetization (Ms) and coercivity (Hc) by about 20 times than that of pure BiFeO3. A large value of dielectric constant of about 650, low loss (<0.001), and small leakage current density (1.79 × 10−8 A/cm2) are observed for the co-doped sample.

Magnetic and dielectric properties of Eu-doped BiFeO3 nanoparticles by acetic acid-assisted sol-gel method

The BiFeO3 nanoparticles, having an average size of 13 nm, were synthesized by a simple sol-gel method. The samples possess single phase up to 2% Eu doping at the Bi site. The uncompensated spin moments on the surface and the modification of cycloidal spin structure due to the small size (13 nm) result in a ferromagnetic phase of the BiFeO3 nanoparticles. The successful doping of magnetically active Eu3+ ions in BiFeO3 nanoparticles improves the ferromagnetic property. The similar dependency of saturation magnetization, coercive field, and dielectric constant on Eu doping concentration reveals that a correlation between magnetic and dielectric properties exists in Eu-doped BiFeO3 nanoparticles.

Bright white light emitting Eu and Tb co-doped monodisperse In2O3 nanocrystals

Bright light emitting monodisperse In2−(x+y)EuxTbyO3 (x = 0.05, 0.1, 0.15, 0.2 and y = 0.05, 0.1, 0.15) nanocrystals were successfully synthesized by a versatile hot-injection colloidal route. The X-ray diffraction and X-ray photoelectron spectroscopy studies clearly showed that Eu and Tb were incorporated into the In2O3 lattice. Surface adsorbed NO3− and Cl− anions of Eu3+ and Tb3+ precursors assist the formation of flower like morphology through the oriented attachment process. Optical absorption spectra have been interpreted in terms of 4f → 5d (Eu, Tb) and 2p(O) → 5d (Eu, Tb) interband transitions. The observation of characteristic emission peaks related to Eu3+ and Tb3+ by indirect excitation suggests that In2O3 is an efficient sensitizer. Proper tuning of the compositions, 15% of Eu and 10% of Tb, gives the most intense white light and excellent chromaticity co-ordinates (0.35, 0.35). The shortening of lifetime with Eu and Tb doping improves the energy transfer from the host In2O3 to dopant Eu3+ and Tb3+ ions.

Template free synthesis of SnO2 nanoflower arrays on Sn foil

The well aligned poppy-flower-like SnO2 nanoflower array on Sn-foil was successfully prepared by a simple solvothermal method without using any template or catalyst. The mixed solvent of aqueous sodium hydroxide and hydrazine hydrate plays a significant role for obtaining the crystalline pure SnO2 nanoflower array. Each petal of the nanoflower consists of the (110) plane of SnO2. The growth of the SnO2 nanoflowers has been described by the formation of layered Sn-hydrazine complexes. The surface defects and the oxygen vacancies of the SnO2 hierarchical nanoflowers are identified by Raman and electron paramagnetic resonance (EPR) spectroscopy and are analyzed in detail by steady state and time resolved photoluminescence spectra.

Interfacial magnetism and exchange coupling in BiFeO3–CuO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 9 nm were used to form a composite with antiferromagnetic CuO nanosheets, with the composition (x)BiFeO3/(100-x)CuO, x = 0, 20, 40, 50, 60, 80 and 100. The dispersion of BiFeO3 nanocrystals into the CuO matrix was confirmed by x-ray diffraction and transmission electron microscopy. The ferromagnetic ordering as observed in pure BiFeO3 occurs mainly due to the reduction in the particle size as compared to the wavelength (62 nm) of the spiral modulated spin structure of the bulk BiFeO3. Surface spin disorder of BiFeO3 nanocrystals gives rise to an exponential behavior of magnetization with temperature. Strong magnetic exchange coupling between the BiFeO3 nanocrystal and the CuO matrix induces an interfacial superparamagnetic phase with a blocking temperature of about 80 K. Zero field and field cooled magnetizations are analyzed by a ferromagnetic core and disordered spin shell model. The temperature dependence of the calculated saturation magnetization exhibits three magnetic contributions in three temperature regimes. The BiFeO3/CuO nanocomposites reveal an exchange bias effect below 170 K. The maximum exchange bias field HEB is 1841 Oe for x = 50 at 5 K under field cooling of 50 kOe. The exchange bias coupling results in an increase of coercivity of 1934 Oe at 5 K. Blocked spins within an interfacial region give rise to a remarkable exchange bias effect in the nanocomposite due to strong magnetic exchange coupling between the BiFeO3 nanocrystals and the CuO nanosheets.

Vibrational spectroscopy and ionic conductivity of polyethylene oxide-NaClO4-CuO nanocomposite.

Structure, morphology and thermal properties of polyethylene oxide (PEO) with sodium perchlorate (NaClO(4)) as electrolytic salt have been investigated by incorporating cupric monoxide (CuO) nanoparticles. Monoclinic CuO affects melting and glass transition temperatures of PEO-NaClO(4). Crystallinity and free ion concentration change with the variation of CuO concentration. The maximum ionic conductivity is observed for 10 wt.% CuO. Ionic conductivity follows Arrhenius type behavior as a function of temperature.

Highly efficient photocatalytic activity of CuO quantum dot decorated rGO nanocomposites

CuO quantum dots (QD) of size 4.5 nm decorated on a rGO sheet to form nanocomposites with different weight percentages via a simple soft chemical route was reported here. Tuning of CuO QD absorption towards the visible region from the UV region in the presence of rGO was also observed. The luminescence of rGO was found to be quenched in rGO–CuO nanocomposites due to charge transfer from the lowest unoccupied molecular orbital of the rGO layer to the conduction band of CuO. Systematic and concise studies of photocatalytic performance towards degradation of methylene blue (MB) dye by CuO QD along with rGO–CuO nanocomposites were presented in this work. A nanocomposite with an equal weight percentage of rGO and CuO degrades almost 99% of MB under irradiation of visible light for 50 min, showing maximum degradation efficiency.

Luminescence properties of the solvothermally synthesized blue light emitting Mn doped Cu2O nanoparticles

The Cu2O nanoparticles having average crystallite diameters ∼8–16 nm were synthesized by a simple solvothermal method. The Mn was doped in the Cu2O sample of crystallite size ∼8 nm. The effects of the size and doping concentration on the crystal structures of the nanoparticles were investigated. The x-ray photoelectron spectroscopy studies clearly showed that the Mn was incorporated into the Cu2O lattice as Mn2+ due to the substitution of the Cu+ ions by Mn2+ ions. The quantum confinement effects were observed in the nanoparticles. The multiple emissions from the Cu2O were quenched in the Mn doped nanoparticles and only blue light emitting Cu2O nanoparticles were obtained due to the transition T42→A61 of Mn. The effects of the doping concentration and the particle size on the relaxations dynamics of the Cu2O nanoparticles were mainly investigated using photoluminescence decay.

Exchange bias effect in BiFeO3-NiO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 11 nm were used to form nanocomposites (x)BiFeO3/(100 − x)NiO, x = 0, 20, 40, 50, 60, 80, and 100 by simple solvothermal process. The ferromagnetic BiFeO3 nanocrystals embedded in antiferromagnetic NiO nanostructures were confirmed from X-ray diffraction and transmission electron microscope studies. The modification of cycloidal spin structure of bulk BiFeO3 owing to reduction in particle size compared to its spin spiral wavelength (62 nm) results in ferromagnetic ordering in pure BiFeO3 nanocrystals. High Neel temperature (TN) of NiO leads to significant exchange bias effect across the BiFeO3/NiO interface at room temperature. A maximum exchange bias field of 123.5 Oe at 300 K for x = 50 after field cooling at 7 kOe has been observed. The exchange bias coupling causes an enhancement of coercivity up to 235 Oe at 300 K. The observed exchange bias effect originates from the exchange coupling between the surface uncompensated spins of BiFeO3 nanocrystals and...

THE ORIGIN OF AGING IN AL-SIO2-SI TUNNEL DIODES

The origin of aging of Al–SiO2–Si tunnel diodes is investigated. A theoretical model is proposed based on the assumption that aluminum undergoes a chemical reaction with the SiO2 layer and produces silicon which introduces active centers for trapping electrons in the oxide layer. The model shows that the negative charge density that develops at the interface is responsible for the time dependence of the barrier height observed in Al–SiO2–Si tunnel devices.