D. Sanyal

Temperature dependent reversible p-n-p type conduction switching with colossal change in thermopower of semiconducting AgCuS.

Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulfide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (ΔS of ~1757 μV K(-1)) at the superionic phase transition (T of ~364 K). In addition, its thermal conductivity is ultralow in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy, and positron annihilation spectroscopy measurements. Using first-principles calculations, we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulfur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.

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...

Identifying defects in multiferroic nanocrystalline BaTiO3 by positron annihilation techniques

Room temperature ferromagnetism in nanoparticles of otherwise nonmagnetic materials has been attributed to point defects at the surface of the nanoparticles. Here, we have employed positron annihilation spectroscopy to identify the nature of defects in multiferroic BaTiO(3) nanocrystalline materials with varying average particle sizes. Ratio curve analysis of the Doppler broadening profile to a reference profile suggests that the defect is an oxygen vacancy. The decrease of intensity of the intermediate lifetime component with increasing particle size indicates a decrease of surface defect concentration. The large defect concentration in nanocrystalline BaTiO(3) can explain the observed room temperature ferromagnetism.

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.

Ab initio calculation of magnetic properties of p-block element doped ZnO

Recently, extensive calculations based on density functional theory (DFT) have been carried out to understand the origin of magnetism by doping ZnO, as observed experimentally. Theoretically, it has been understood that the main source of the magnetic moment arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy. In the present work, we try to understand the reason for induced magnetic moment in ZnO by replacing O, using different p-block elements. We have studied the effective magnetic moments of the Zn54O53X (X = B, C, N, Al, Si, P, Ga, Ge, As) system in the framework of density functional theory. Our calculations suggest that partial substitution of the oxygen atom by a foreign element can induce magnetism in ZnO, and the amount of induced magnetic moment depends upon the electronegativity and the size of the doping element with respect to the oxygen atom.

Origin of the Order–Disorder Transition and the Associated Anomalous Change of Thermopower in AgBiS2 Nanocrystals: A Combined Experimental and Theoretical Study

Bulk AgBiS2 crystallizes in a trigonal crystal structure (space group, P3̅m1) at room temperature, which transforms to a cation disordered rock salt structure (space group, Fm3̅m) at ∼473 K. Surprisingly, at room temperature, a solution-grown nanocrystal of AgBiS2 crystallizes in a metastable Ag/Bi ordered cubic structure, which transforms to a thermodynamically stable disorded cubic structure at 610 K. Moreover, the order-disorder transition in nanocrystalline AgBiS2 is associated with an unusual change in thermopower. Here, we shed light on the origin of a order-disorder phase transition and the associated anomalous change of thermopower in AgBiS2 nanocrystals by using a combined experimental, density functional theory based first-principles calculation and ab initio molecular dynamics simulations. Positron-annilation spectroscopy indicates the presence of higher numbers of Ag vacancies in the nanocrystal compared to that of the bulk cubic counterpart at room temperature. Furthermore, temperature-dependent two-detector coincidence Doppler broadening spectroscopy and Doppler broadening of the annihilation radiation (S parameter) indicate that the Ag vacancy concentration increases abruptly during the order-disorder transition in nanocrystalline AgBiS2. At high temperature, a Ag atom shuttles between the vacancy and interstitial sites to form a locally disordered cation sublattice in the nanocrystal, which is facilitated by the formation of more Ag vacancies during the phase transition. This process increases the entropy of the system at higher vacancy concentration, which, in turn, results in the unusual rise in thermopower.

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.