Shatabda Bhattacharya

Conductivity relaxation in some fast ion-conducting AgI-Ag2O-V2O5 glasses

Abstract We report the conductivity relaxation of some fast ion-conducting AgI–Ag 2 O–V 2 O 5 glasses in the frequency range from 10 Hz to 2 MHz and in the temperature range from 93 to 323 K. Using modulus formalism, we have shown that the conductivity relaxation is highly non-exponential. We have observed that the conductivity relaxation mechanism in these glasses is independent of temperature but dependent on the composition. We have further shown that the motion of Ag + ions is decoupled more and more from the viscous motion of the glassy matrix with the increase of AgI content in the composition, giving rise to the increase of the conductivity.

Role of trap states on storage capacity in a graphene/MoO3 2D electrode material

The trap state in graphene has been recently reported, however its role on charge transport and charge storage capacity in a graphene based electrode material has not yet been explored. In semiconducting devices trap states have a negative role in charge transport and current–voltage characteristics. Therefore, exploiting the huge trap states in chemically synthesized graphene to improve the desired material property is a unique idea. In the present work, trap states have been analyzed in detail using capacitance–voltage and current–voltage characteristics in chemically synthesized graphene/MoO3 composites. The effect of trap states on charge transport and on enhancing the dielectric properties are also demonstrated. Finally, we have shown that storage capacitance of the graphene/MoO3 electrode material increases remarkably to about 160% due to the increase in trap states in the material. We believe that this study will reveal a new way of exploiting trap states to improve the material’s characteristics, such as storage capacity etc.

Formation of ZnO nanoparticles and α-AgI nanocrystals embedded in superionic glass nanocomposites

We have reported the formation of ZnO nanoparticles and α-AgI nanocrystals embedded in the glass matrix in the 0.70AgI–0.15Ag2O–0.15[xZnO–(1−x)MoO3], x=0.05–0.30, glass nanocomposites from x-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy patterns. Fourier transform infrared spectra have revealed the strong partial covalency due to the presence of Ag+ and MoO42− ions. It is observed that the variation of the conductivity of the composites is well correlated to their structural behavior. A structural model to account for the electrical properties has been proposed.

Relaxation dynamics in AgI-doped silver vanadate superionic glasses.

Relaxation dynamics of Ag+ ions in several series of AgI-Ag2O-V2O5 superionic glasses has been studied in the frequency range from 10 Hz to 2 MHz and in the temperature range from 93 to 323 K. The composition dependence of the dc conductivity and the activation energy of these glasses has been compared with those of AgI-doped silver phosphate and borate glasses. The frequency-dependent electrical data have been analyzed in the framework of conductivity formalism. We have obtained the mobile ion concentration and the power-law exponent from the analysis of the conductivity spectra. We have observed that the concentration of Ag+ ions is independent of temperature and the conductivity is primarily determined by the mobility. A fraction of the Ag+ ions in the glass compositions are involved in the dynamic process. We have also shown that the power-law exponent is independent of temperature. The results are also supported by the temperature and composition independence of the scaling of the conductivity spectra.

Anomalous enhancement in the magnetoconductance of graphene/CoFe2O4 composite due to spin–orbit coupling

To understand the effect of charge transfer from the d-orbital of transition metal (TM) to the graphene p-orbital at the graphene/TM interface, magnetoconductance measurements have been carried out in graphene/CoFe2O4 composites over the temperature range from 20–300 K. A transition from positive to negative magnetoconductance is observed at 80 K. Below 80 K, magnetoconductance increases with decreasing temperature in the usual way; however, above 80 K it increases unusually with increasing temperature and reaches about 65% at 300 K. This anomalous enhancement in magnetoconductance at the higher temperature region has been explained on the basis of spin–orbit coupling acting at the interface. The nanocomposite containing large interfaces between graphene and CoFe2O4 nanoparticles exhibits a superior magnetodielectric effect with a 22% change in dielectric permittivity for an applied magnetic field of 1.8 T as a result of the combined effect between the Maxwell–Wagner polarization at the interface and a positive magnetoconductance of CoFe2O4.

Antiferro-ferromagnetic transition in ultrathin Ni(OH)2 layer grown on graphene surface and observation of interlayer exchange coupling in Ni(OH)2/graphene/Ni(OH)2 nanostructures

The major limitation of using graphene as a potential spacer element in interlayer exchange coupling (IEC) might be due to destruction of ferromagnetism as a result of the charge transfer effect at the interface if a transition metal based ferromagnetic layer is grown on the graphene surface. To overcome this problem, we have used the antiferromagnetic Ni(OH)2 layer grown on the graphene surface to convert it ferromagnetic due to the charge transfer effect. By growing thin layers of Ni(OH)2 on both sides of the graphene surface, strong antiferromagnetic IEC with ultra-low coercivity (7 Oe) is observed. By lowering the nickel content, an ultrathin layer of Ni(OH)2 is grown on either side of graphene and shows complete ferromagnetism with a giant coercivity of 4154 Oe. Ab initio calculations have been done to substantiate this kind of charge transfer effect at the interface of Ni(OH)2 and graphene. Magnetotransport of the composite material is also investigated to understand the role of IEC in transport pro...

Ferromagnetism in graphene due to charge transfer from atomic Co to graphene

The charge transfer effect at the graphene/transition metal interface has been studied extensively during the last few years; however, the experimental results are very poor. In the present work, a Co atom capped with porphyrin is attached on the graphene surface to realize the induced magnetic properties arising due to the charge transfer effect at the interface. Ferromagnetic ordering with fairly large coercivity (516 Oe) is observed as a result of this induced magnetism in graphene due to the presence of a transition metal atom on the graphene surface. Temperature dependent magnetotransport has also been investigated to understand the effect of spin-orbit coupling arising due to the electric field generated at the interface as a result of this charge transfer effect.