@article { ISI:000260119300077, title = {Transverse susceptibility study of the effect of varying dipolar interactions on anisotropy peaks in a three-dimensional assembly of soft ferrite nanoparticles}, journal = {Journal of Applied Physics}, volume = {104}, number = {6}, year = {2008}, month = {SEP}, pages = {Article No. 063901}, publisher = {AMER INST PHYSICS}, type = {Article}, address = {CIRCULATION \& FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USA}, abstract = {

Collective magnetization dynamics in nanoparticle assemblies is of current interest as it forms the basis of high density storage media. It is important to understand how interparticle interactions in a three-dimensional (3D) arrangement of superparamagnetic nanoparticles would affect the overall effective magnetic anisotropy of the system. We have studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese zinc ferrite nanoparticles using reversible transverse susceptibility. We track the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The blocking temperature shows an increase from 28 to 32 K and the coercive field (at 10 K) shows an increase from 144 to 192 Oe as the system changes from the case of weakly interacting to strongly interacting 3D assembly of the particles. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2977592]

}, issn = {0021-8979}, doi = {10.1063/1.2977592}, author = {Poddar, Pankaj and Morales, Marienette B. and Frey, Natalie A. and Morrison, Shannon A. and Carpenter, Everett E. and Srikanth, Hariharan} } @article { ISI:000280070900009, title = {Temperature-dependent raman and dielectric spectroscopy of BiFeO3 nanoparticles: signatures of spin-phonon and magnetoelectric coupling}, journal = {Journal of Physical Chemistry C}, volume = {114}, number = {29}, year = {2010}, month = {JUL}, pages = {12432-12439}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report the temperature-dependent Raman and dielectric spectroscopy of chemically synthesized BiFeO3 nanoparticles (average size similar to 50-60 nm). The Raman spectra (90-700 K) show two sets of transitions in the lowest Raman E mode, associated with Bi-O bond motion situated in close proximity to the spin reorientation transitions reported for BiFeO3, thereby indicating the existence of possible coupling between magnons and phonons for particle size below the helical order parameter (62 nm). These transitions are slightly shifted in temperature in comparison to the bulk single crystals. We also observe a step-like behavior in Raman peak position around the Neel temperature, suggesting that the phonons are influenced by the magnetic ordering in nanosized BiFeO3. The heat-flow measurements show two sharp endothermic peaks at 1094 and 1223 K representing rhombohedral to orthorhombic or monoclinic transition followed by transition into the cubic phase above 1200K. The low temperature (20-325 K), frequency-dependent (1-10(6) Hz) dielectric constant and loss tangent measurements show that the loss tangent (similar to 10(-3)) and ac conductivity values (similar to 10(-8) Ohm(-1)-cm(-1)) are orders of magnitude lower than the reported values for BiFeO3 ceramics, indicating high levels of ionic purity of our samples. The real part of the permittivity shows a slight reduction in its value (similar to 30) in comparison to the bulk single crystals. Similar to the Stokes Raman shift, its temperature-dependent dielectric constant also shows four weak anomalies at similar to 85, 168, 205, and 230 K situated in close proximity to the spin reorientation transitions, indicating magnetoelectric coupling.

}, issn = {1932-7447}, doi = {10.1021/jp102450z}, author = {Jaiswal, Adhish and Das, Raja and Maity, Tuhin and Vivekanand, K. and Adyanthaya, Suguna D. and Poddar, Pankaj} } @article { ISI:000312519900003, title = {Temperature-dependent spectroscopic evidences of curcumin in aqueous medium: a mechanistic study of its solubility and stability}, journal = {Journal of Physical Chemistry B}, volume = {116}, number = {50}, year = {2012}, month = {DEC}, pages = {14533-14540}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

In curcumin, keto-enol-enolate equilibrium of the heptadiene-dione moiety determines its physiochemical and antioxidant properties. However, its poor solubility in water at neutral pH and room temperature decreases its bioavailability. Potential therapeutic applications have triggered an interest in manipulating the solubility of curcumin in water as its stability and solubility in water remains poorly understood. Here, the mechanism behind its solubility at various temperatures and the influence of interplay of temperature, intramolecular H-bonding, and intermolecular forces is reported, which leads to aggregation-disaggregation at various temperatures. Remarkable change is observed in temperature-dependent electronic transition behavior of curcumin, however, the absorption spectra after cooling and heating cycles remain unchanged, hinting much better thermal stability of curcumin in water than previously thought. This study indicates that it is perhaps the breaking of intramolecular hydrogen bonding which leads to exposure of polar groups and hence responsible for the dissolution of curcumin at higher temperature. The formation of intermolecular aggregates might be responsible behind a better room temperature stability of the molecules after cooling its aqueous suspension from 90 to 25 degrees C. These curcumin solubility studies have great application in biological research with reference to bioavailability and to understand target oriented mode of action of curcumin.

}, issn = {1520-6106}, doi = {10.1021/jp3050516}, author = {Jagannathan, Ramya and Abraham, Priya Mary and Poddar, Pankaj} } @article { ISI:000340809600062, title = {Template-free fabrication of highly-oriented single-crystalline 1D-rutile TiO2-MWCNT composite for enhanced photoelectrochemical activity}, journal = {Journal of Physical Chemistry C}, volume = {118}, number = {33}, year = {2014}, month = {AUG}, pages = {19363-19373}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Template-free synthesis of phase pure one-dimensional (1D), single crystalline rutile titania nanorods or wires at low temperature still remains a challenging task due to its complex nature of surface chemistry. In these ID structures, charge transport is highly favored. To further modify the electrical conductivity and optoelectronic properties of these 1D nanostructures, various methods such as doping of TiO2 with metal and nonmetal and synthesis of branched and hybrid structures are developed. If these hybrid structures can directly synthesize on the substrate, the transport of the electron will improve due to reduced grain boundary and exciton recombination. In this contribution, for the first time, we have simultaneously synthesized 1D-rutile TiO2-multiwalled carbon nanotube (MWCNT) composite film directly grown on fluorine dope conducting oxide (FTO) substrate along with 1D-rutile TiO2-MWCNT composite powder. The as-grown nanorods films were single-crystalline and oriented vertically with respect to the substrate, having an average height of similar to 2 mu m. The well connected network of TiO2 with MWCNTs was observed through electron microscopy. The composite film shows positive movement of the flat-band edge and increase in charge carrier density. The TiO2-MWCNT composite was successfully used as photoanode in a dye sensitized solar cell (DSSC) and exhibits a 60\% increase in energy-conversion efficiency compared with only TiO2 nanorods.

}, issn = {1932-7447}, doi = {10.1021/jp5023983}, author = {Sadhu, Subha and Poddar, Pankaj} } @article { ISI:000362460800031, title = {Temperature and magnetic field-assisted switching of magnetization and observation of exchange bias in YbCrO3 nanocrystals}, journal = {Inorganic Chemistry}, volume = {54}, number = {19}, year = {2015}, month = {OCT}, pages = {9509-9516}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

In this paper, we demonstrate an interesting feature in YbCrO3 (YCO) nanocrystals, in which the material shows temperature and external magnetic field-assisted switching (a complete sign reversal) of zero field cooled magnetization (M-ZFC) and observation of exchange bias (EB) as a result of competing spin interaction at low temperature. This feature can be applied in nonvolatile memories, where, simply by changing the magnitude of the H-ext and T, the polarity of the magnetization can be switched between negative and positive. We also observed negative magnetization in YCO. Our results showed that, below its Neel temperature (T-N approximate to 119 K), the M-ZFC crosses over to negative sign for H \< 1000 Oe. At 60 K, YCO showed a significant negative M-ZFC approximate to -0.05 emu/g (at 100 Oe) due to the competing effects of Yb3+, Cr3+ spins, thermal activation energy, and Hext. At further lower temperatures, the M-ZFC showed a crossover to positive values, and the crossover temperature showed the dependence on H-ext (similar to 19 K for 100 Oe curve). The YCO also showed H-ext T-dependent H-EB, which changed its sign with T. The observed T-dependent sign reversal in the EB was closely associated with the sign reversal of M-ZFC. The symmetric shift in field-cooled isothermal hysteresis curves confirmed that the observed EB was not due to the unsaturated minor loop. The training cycle further confirmed that the H-EB value decreased to similar to 2\% of the initial value of observed EB, which was very small compared to the observed H-EB in YCO, which indicated stable spin configuration at the locally formed ferromagnetic/antiferromagnetic interface.

}, issn = {0020-1669}, doi = {10.1021/acs.inorgchem.5b01448}, author = {Gupta, Preeti and Poddar, Pankaj} } @article { ISI:000351235800053, title = {Tunable band gap and coercivity of bismuth ferrite-polyaniline core-shell nanoparticles: the role of shell thickness}, journal = {RSC Advances}, volume = {5}, number = {30}, year = {2015}, month = {MAR}, pages = {23563-23568}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

We report a tunable band gap of bismuth ferrite-polyaniline core-shell nanoparticles from 2.24 to 1.98 eV and the variation of coercivity from 118 to 100 Oe, by varying the thickness of the polyaniline shell. Bismuth ferrite nanoparticles were synthesized via a chemical route and coating of polyaniline was achieved by a double surfactant layer technique. Coating of polyaniline introduced oxygen vacancies, local distortions and microstrains in the bismuth ferrite lattice which resulted in a change in the Fe-O-Fe bond angle of the bismuth ferrite lattice. This is confirmed with structural analysis tools i.e. X-ray Diffraction, Fourier Transform Infrared Spectroscopy and Raman spectroscopy. Moreover, the absorption of light around 600 nm is increased and photoluminescence emission around 430 nm is decreased, as the thickness of shell is increased, which also indicates the changes in optical properties due to variation in shell thickness. Thus tunability of the band gap and coercivity is achieved as a function of shell thickness in bismuth ferrite-polyaniline core-shell nanoparticles.

}, issn = {2046-2069}, doi = {10.1039/c5ra00933b}, author = {Chaturvedi, Smita and Das, Raja and Poddar, Pankaj and Kulkarni, Sulabha} } @article {50027, title = {Tuning the electric dipole transitions (5D0 {\textrightarrow} 7F2 and 5D0 {\textrightarrow} 7F4) in thin Eu-doped BiOCl nanosheets}, journal = {Journal of Physical Chemistry C}, volume = {130}, year = {2026}, month = {MAR}, pages = {3504-3517}, type = {Article}, abstract = {

The polarization and depolarization of layered ferroelectric materials can be tuned by altering the thickness of nanosheets, apart from controlling inter/intralayer distances, doping of ions, the surrounding dielectric environment, etc. In two-dimensional, single-crystalline ferroelectric nanosheets, the reduction in the thickness facilitates the internal electric field (IEF), which leads to enhanced depolarization. BiOCl is a layered ferroelectric material with repeating layers of [Bi2O2](2+), which exhibit anisotropy. However, the depolarization and polarization of BiOCl are significantly influenced by the thicknesses of its nanosheets. Herein, 1-dodecanethiol-capped europium (Eu3+)-doped BiOCl thin nanosheets resembling a two-dimensional structure have been synthesized using the solid-state grinding method at ambient temperature. As a result of the depolarization effect, the strength of the electric dipole transitions D-5(0) -\> F-7(2) and D-5(0) -\> F-7(4) increases in the Eu-doped BiOCl nanosheets. However, thiol capping helps in synthesizing uniform 2D nanosheets with reduced vertical dimensions (thickness near 15-30 nm). It is observed that all of the prepared samples with varying Eu ion concentrations show uniform nanosheet-like morphology, as confirmed by electron microscopy (FESEM and HRTEM). As the concentration of Eu ions increases in the BiOCl host lattice, the intensity of electric dipole transitions also increases (until 9 mol \%). Fourier transform infrared spectroscopy (FT-IR) reveals the coating of 1-dodecanthiol on the Eu-doped BiOCl molecule. Furthermore, the Eu-doped BiOCl samples showed prominent far-red emission at 700 nm, corresponding to the D-5(0) -\> F-7(4) transition. Moreover, this work emphasizes the synthesis of Eu-doped phosphor at an ambient temperature of 24 +/- 2 degrees C and generates a deeper understanding of the abnormal electric dipole (D-5(0) -\> F-7(4)) transition.

}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.5c07685}, author = {Singh, Dinesh and Poddar, Pankaj} }