Mahuya Chakrabarti

Defects and the optical absorption in nanocrystalline ZnO

The correlation between the structural and optical properties of mechanically milled high purity ZnO powder is reported in the present work. Reduction of average grain size and enhancement of strain as a result of milling have been estimated from the broadening of x-ray powder diffraction patterns. After milling, the optical bandgap, revealed from absorption spectroscopy, has been red-shifted and the width of the localized states, calculated from the analysis of the Urbach tail below the absorption edge, has been extended more and more into the bandgap. Moreover, the band tailing parameter is seen to vary exponentially with the inverse of the grain size. Finally, the positron annihilation technique has been employed to identify the nature of defects present (or generated due to milling) in the system and thereby to correlate the defect mediated modification of optical absorption in ZnO.

Defect dynamics in annealed ZnO by positron annihilation spectroscopy

As-supplied polycrystalline ZnO samples (purity 99.9% from Sigma-Aldrich, Germany) have been annealed at different temperatures and subsequently characterized by positron annihilation spectroscopy, x-ray-diffraction (XRD) analysis, thermogravimetric analysis (TGA), and resistivity measurements. Positron annihilation lifetime analysis and coincidence Doppler-broadened electron-positron annihilation γ-radiation (CDBEPAR) line-shape measurements have been employed at a time to identify the nature of defects in differently annealed ZnO materials. Annealing up to 300°C, an increase of defect lifetime (τ2) as well as shape parameter (S parameter) has been observed. Further annealing causes a large decrease of τ2 and S parameter. TGA study shows considerable mass loss from ZnO as the annealing temperature is increased above 300°C. This is possibly due to oxygen evaporation from the sample. The c-axis lattice parameter, extracted from the XRD spectra, shows an increase due to annealing above 600°C, which is a sig...

Grain size dependence of optical properties and positron annihilation parameters in Bi2O3 powder

Nanocrystalline Bi2O3 has been prepared by a ball milling process. The particle size of the ball-milled Bi2O3 powder has been determined by the x-ray powder diffraction method and transmission electron microscopy. The absorption spectra, in the spectral range 300–1300 nm, indicate an increase of the optical bandgap for both the direct and indirect transitions due to the reduction of grain size. The defects introduced in Bi2O3 during grinding have been investigated by the positron annihilation technique. Positron annihilation results indicate an increase of defects due to ball milling.

Observation of room temperature ferromagnetism in Mn–Fe doped ZnO

A room temperature ferromagnetic Fe doped ZnO bulk sample has been synthesized by co-doping 2 at% Mn with 2 at% Fe in ZnO. The final firing temperature for the preparation of the Zn0.96Mn0.02Fe0.02O sample is the same as the final firing temperature for the preparation of the Zn0.98Mn0.02O sample and in this case it is found to be 490 °C. It has been found that room temperature ferromagnetism in Zn0.96Mn0.02Fe0.02O is a factor of 1.6 times more than that observed in Zn0.98Mn0.02O samples. The size of the saturation magnetic field for the Zn0.96Mn0.02Fe0.02O sample is, however, only a factor of 7 times more than that observed for nanocrystalline undoped ZnO.

Interplay of 4f-3d magnetism and ferroelectricity in DyFeO3

DyFeO3 exhibits a weak ferromagnetism (TNFe ? 645?K) that disappears below a spin-reorientation (Morin) transition at TSRFe???50?K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy ions (TNDy ? 4.5?K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (TSRFe <T <TN,C) without applying a magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57Fe M?ssbauer studies show that a hyperfine field (Bhf) deviates from the mean-field?like behaviour that is observed in the weak ferromagnetic state and decreases below the onset of the spin-reorientation transition (80?K), implying that the Bhf above TSR had an additional contribution from Dy ions due to the induced magnetization by the weak ferromagnetic moment of the Fe sublattice and below TSR this contribution decreases due to collinear ordering of the Fe sublattice. These results clearly demonstrate the presence of magnetic interactions between Dy(4f) and Fe(3d) and their correlation with ferroelectricity in the weak ferromagnetic state of DyFeO3.

Surface defects induced ferromagnetism in mechanically milled nanocrystalline ZnO

Bulk ZnO is a diamagnetic material but ferromagnetism (FM) has been observed by several groups in its nanostructures. In order to elucidate the room temperature (RT) FM of ZnO nanostructures, magnetic property of mechanically milled and subsequently annealed nano-ZnO powder has been investigated. Sample that has been milled and then annealed at 200 °C in ambient condition shows highest value of saturation magnetization (Ms), whereas lowest value of Ms has been noticed for the sample pre-annealed at 500 °C before milling. The variation of Ms with annealing temperatures closely resembles with the variation of average positron lifetime (τav) and S-parameter reported earlier for these nano-systems. It has also been found that Ms decreases systematically for increasing average grain size of the ZnO nanoparticles. Room temperature photoluminescence of the as-milled sample shows broad defect related emission centered ∼2.23 eV. Enhancement of such emission has been observed due to 200 °C annealing. Results altoge...

Room temperature ferromagnetic ordering in 4?MeV Ar5+ irradiated TiO2

Room temperature ferromagnetic ordering has been observed in a rutile TiO2 polycrystalline sample after 4 MeV Ar5+ ion irradiation. The sheet resistance of the irradiated sample decreases from 107 to 3 × 103 Ω cm−2. Ab initio calculation in the density-functional theory indicates that both oxygen vacancy (VO) and titanium vacancy (VTi) can lead to ferromagnetism. However, the drastic lowering of resistance and change of colour (from white to black) indicate the formation of VO. Experimental results along with the theoretical calculation suggest that presence of VO in the irradiated sample plays the main role in inducing ferromagnetism.

Positron annihilation lifetime and photoluminescence studies on single crystalline ZnO

The room temperature positron annihilation lifetime for single crystalline ZnO has been measured as 164 ± 1 ps. The single component lifetime value is very close to but higher than the theoretically predicted value of ~154 ps. Photoluminescence study (at 10 K) indicates the presence of hydrogen and other defects, mainly acceptor related, in the crystal. Defects related to a lower open volume than zinc vacancies, presumably a complex with two hydrogen atoms, are the major trapping sites in the sample. The bulk positron lifetime in ZnO is expected to be a little less than 164 ps.

Particle size dependence of optical and defect parameters in mechanically milled Fe2O3

Fe2O3 of particle sizes ranging from 120 to 20 nm has been prepared by the ball-milling process using different milling hour. X-ray diffraction technique and transmission electron microscopy have been used for determining the average particle sizes of the prepared samples. Direct optical band gap for the unmilled and the ball-milled samples has been calculated from the optical absorption data. A red shift in the band gap due to the reduction of particle size has been observed. The coincidence Doppler broadening of the electron positron annihilation γ-radiation spectroscopy has been employed to identify the nature of defects generated due to the ball-milling process.

Defect driven ferromagnetism in SnO2: a combined study using density functional theory and positron annihilation spectroscopy

Room temperature ferromagnetic ordering has been observed in a high purity polycrystalline SnO2 sample due to irradiation of 96 MeV oxygen ions. Ab initio density functional theory calculation indicates that tin vacancies are mainly responsible for inducing the magnetic moment in SnO2 whereas oxygen vacancies in SnO2 do not contribute any magnetic moment. Positron annihilation spectroscopy has been employed to characterize the chemical identity of irradiation generated defects in SnO2. Results indicate the dominant presence of Sn vacancies in O ion irradiated SnO2. The irradiated sample turns out to be ferromagnetic at room temperature.