Venkat Narayana - Academia.edu (original) (raw)

Papers by Venkat Narayana

Journal of Alloys and Compounds, Oct 1, 2016

In this manuscript, we propose a method to prepare small flakes of DyCu 2. On top of that we also... more In this manuscript, we propose a method to prepare small flakes of DyCu 2. On top of that we also report on the magnetocaloric effect and nature of magnetic transition of a strongly anisotropic DyCu 2 in its low dimension. Magnetization measurements were carried out in the temperature range of 5-100 K and up to the maximum magnetic field strength of 50 kOe. Magnetic entropy change (ΔS M) is estimated using the well-known Maxwell's equations and is found to be-4.31 J/kg-K. Indeed, the ΔS M peak broadened marginally compared with its bulk DyCu 2 and such a broadening can be attributed to significant increase in the total grain boundary volume. As these small flakes consists larger ΔS M values at temperatures higher than the Nѐel temperature (T N), one can use them as a magnetic refrigerant material in a broad temperature range. We also plotted the M 2 vs. H/M (which are called as the Arrott plots) in order to find the nature of magnetic transition. Arrott plots infer that indeed there exists nonlinearity in M 2 vs.

We demonstrate on the effect of the negative cooling magnetic field on the exchange bias properti... more We demonstrate on the effect of the negative cooling magnetic field on the exchange bias properties of the graphene nanoribbons (GNRs).

physica status solidi (a), 2017

We report on the effect of the strong spinorbit coupling and the Lorentz force on the efficiency ... more We report on the effect of the strong spinorbit coupling and the Lorentz force on the efficiency of TiO 2 based dye sensitized solar cells. Upon inclusion of Ho 2 O 3 , due to the strong spinorbit coupling of the rare earth Ho 3+ ion, we do see 13% enhancement in the efficiency. We attribute such an enhancement in power conversion efficiency to the increased lifetime of the photoexcited excitons. Essentially, a Ho 3+ ion accelerates the phenomenon of the spin-rephasing or the intersystem crossing of the excitons in a photosensitizer. Increase in the absorbance and decrease in the photoluminescence intensity suggests a decrease in the recombination rate, hinting an enhanced charge transport and is in accordance with our electrochemical impedance spectra and the J-V characteristics. From the above we strongly believe that enhanced efficiency of the device is due to increased intersystem crossing which would accelerate the exciton dissociation. On top of spinorbit interaction, a configuration where the electric and magnetic fields are perpendicular to each other helped in enhancing the efficiency by 16%, suggesting that the Lorentz force also plays a dominant role in controlling the charge transport of the photo-generated charge carriers. We strongly believe that this simple and novel strategy of improving the efficiency may pave the way for realizing higher efficiency dye sensitized solar cells.

Journal of Materials Science and Engineering A

We demonstrate the evidenced exchange bias properties of graphene nanoribbons (GNRs) with the neg... more We demonstrate the evidenced exchange bias properties of graphene nanoribbons (GNRs) with the negative magnetic field cooling. Upon the negative field cooling from 300 K to 5 K, the hysteresis loop shifts along the negative magnetic field axis, that coincides with the cooling field direction. This observation indicates that there exists a positive exchange bias in GNRs. Furthermore, enhanced exchange bias was observed when the polarity of field cooling is negative as compared with positive field cooling, hinting that there might be complex interplay between orbital and spin degrees of freedom. In addition, the variation of exchange bias and the coercive field as a function of negative cooling field is also studied.

Graphene, 2016

We report on the effect of compressive stress on the optical properties of graphene oxide using a... more We report on the effect of compressive stress on the optical properties of graphene oxide using a wet ball milling technique. For this purpose, graphene oxide was prepared using the modified Hummer's method and subsequently processed with wet ball milling. X-ray diffraction infers a peak at 9.655˚ which is the allowed reflection for the graphene oxide. The Williamson-Hall method is used to quantify the strain on the 10 hrs and 20 hrs ball milled graphene oxide samples and is found to be 4.2% and 4.8% respectively. Although we applied strain on the graphene oxide, it actually helped to reduce the defects which are confirmed by the intensity drop-off of D-peak in Raman spectroscopy. Indeed there exists a band gap alteration of 0.14 eV for an applied compressive strain of ~4.8%, hinting that the reduction in oxygen functional groups and the same is confirmed with the Fourier Transform Infrared Spectroscopy (FTIR). The present results would be helpful in developing graphene oxide based flexible memories and optoelectronic devices.

Journal of Magnetism and Magnetic Materials

Journal of Alloys and Compounds, Oct 1, 2016

In this manuscript, we propose a method to prepare small flakes of DyCu 2. On top of that we also... more In this manuscript, we propose a method to prepare small flakes of DyCu 2. On top of that we also report on the magnetocaloric effect and nature of magnetic transition of a strongly anisotropic DyCu 2 in its low dimension. Magnetization measurements were carried out in the temperature range of 5-100 K and up to the maximum magnetic field strength of 50 kOe. Magnetic entropy change (ΔS M) is estimated using the well-known Maxwell's equations and is found to be-4.31 J/kg-K. Indeed, the ΔS M peak broadened marginally compared with its bulk DyCu 2 and such a broadening can be attributed to significant increase in the total grain boundary volume. As these small flakes consists larger ΔS M values at temperatures higher than the Nѐel temperature (T N), one can use them as a magnetic refrigerant material in a broad temperature range. We also plotted the M 2 vs. H/M (which are called as the Arrott plots) in order to find the nature of magnetic transition. Arrott plots infer that indeed there exists nonlinearity in M 2 vs.

We demonstrate on the effect of the negative cooling magnetic field on the exchange bias properti... more We demonstrate on the effect of the negative cooling magnetic field on the exchange bias properties of the graphene nanoribbons (GNRs).

physica status solidi (a), 2017

We report on the effect of the strong spinorbit coupling and the Lorentz force on the efficiency ... more We report on the effect of the strong spinorbit coupling and the Lorentz force on the efficiency of TiO 2 based dye sensitized solar cells. Upon inclusion of Ho 2 O 3 , due to the strong spinorbit coupling of the rare earth Ho 3+ ion, we do see 13% enhancement in the efficiency. We attribute such an enhancement in power conversion efficiency to the increased lifetime of the photoexcited excitons. Essentially, a Ho 3+ ion accelerates the phenomenon of the spin-rephasing or the intersystem crossing of the excitons in a photosensitizer. Increase in the absorbance and decrease in the photoluminescence intensity suggests a decrease in the recombination rate, hinting an enhanced charge transport and is in accordance with our electrochemical impedance spectra and the J-V characteristics. From the above we strongly believe that enhanced efficiency of the device is due to increased intersystem crossing which would accelerate the exciton dissociation. On top of spinorbit interaction, a configuration where the electric and magnetic fields are perpendicular to each other helped in enhancing the efficiency by 16%, suggesting that the Lorentz force also plays a dominant role in controlling the charge transport of the photo-generated charge carriers. We strongly believe that this simple and novel strategy of improving the efficiency may pave the way for realizing higher efficiency dye sensitized solar cells.

Journal of Materials Science and Engineering A

We demonstrate the evidenced exchange bias properties of graphene nanoribbons (GNRs) with the neg... more We demonstrate the evidenced exchange bias properties of graphene nanoribbons (GNRs) with the negative magnetic field cooling. Upon the negative field cooling from 300 K to 5 K, the hysteresis loop shifts along the negative magnetic field axis, that coincides with the cooling field direction. This observation indicates that there exists a positive exchange bias in GNRs. Furthermore, enhanced exchange bias was observed when the polarity of field cooling is negative as compared with positive field cooling, hinting that there might be complex interplay between orbital and spin degrees of freedom. In addition, the variation of exchange bias and the coercive field as a function of negative cooling field is also studied.

Graphene, 2016

We report on the effect of compressive stress on the optical properties of graphene oxide using a... more We report on the effect of compressive stress on the optical properties of graphene oxide using a wet ball milling technique. For this purpose, graphene oxide was prepared using the modified Hummer's method and subsequently processed with wet ball milling. X-ray diffraction infers a peak at 9.655˚ which is the allowed reflection for the graphene oxide. The Williamson-Hall method is used to quantify the strain on the 10 hrs and 20 hrs ball milled graphene oxide samples and is found to be 4.2% and 4.8% respectively. Although we applied strain on the graphene oxide, it actually helped to reduce the defects which are confirmed by the intensity drop-off of D-peak in Raman spectroscopy. Indeed there exists a band gap alteration of 0.14 eV for an applied compressive strain of ~4.8%, hinting that the reduction in oxygen functional groups and the same is confirmed with the Fourier Transform Infrared Spectroscopy (FTIR). The present results would be helpful in developing graphene oxide based flexible memories and optoelectronic devices.

Journal of Magnetism and Magnetic Materials