Biswarup Satpati

Synthesis and characterization of mn-doped zno nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

Understanding the quantum size effects in ZnO nanocrystals

In the present work, we report the synthesis of high quality ZnO nanocrystals with sharp absorption edges in four different sizes, namely 3.0, 3.5, 4.7 and 5.4 nm, characterized by X-ray and electron diffraction, as well as transmission electron microscopy. The bandgaps of these samples, in conjunction with further data from the published literature, exhibit a systematic dependence on the nanocrystal size. In absence of any prior reliable theoretical results in the literature to understand this dependence quantitatively, we have analyzed for the first time, the electronic structure of bulk ZnO obtained from the full potential linearized augmented plane wave method using fatbands, density of states and partial density of states. The crystal orbital Hamiltonian population is obtained from linearized Muffin-Tin orbital band structure calculations to understand the range of hopping interactions relevant for an accurate description of the electronic structure. Using these analyses, a realistic tight binding model is proposed. Based on this model, we calculate the variation of the bandgap with the size of ZnO nanocrystals. These theoretical results agree well with all available data over the entire range of sizes, establishing the effectiveness of this approach.

Simultaneous and sensitive determination of ascorbic acid, dopamine, uric acid, and tryptophan with silver nanoparticles-decorated reduced graphene oxide modified electrode.

In this paper, we report the synthesis of silver nanoparticle-decorated reduced graphene oxide composite (AgNPs/rGO) by heating the mixture of graphene oxide and silver nitrate aqueous solution in the presence of sodium hydroxide. This material was characterized by means of X-ray diffraction, UV-vis spectroscopy, and transmission electron microscopy. AgNPs/rGO based electrochemical sensor was fabricated for the simultaneous determination of ascorbic acid, dopamine, uric acid, and tryptophan. Electrochemical studies were carried out by using cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. AgNPs/rGO modified electrode exhibited excellent electrocatalytic activity, stability, sensitivity, and selectivity with well-separated oxidation peaks toward ascorbic acid, dopamine, uric acid, and tryptophan in the simultaneous determination of their quaternary mixture. The analytical performance of this material as a chemical sensor was demonstrated for the determination of ascorbic acid and dopamine in commercial pharmaceutical samples such as vitamin C tablets and dopamine injections, respectively. The applicability of this sensor was also extended in the determination of uric acid in human urine samples.

High-temperature ferromagnetism in manganese-doped indium-tin oxide films

High-temperature ferromagnetism is demonstrated in Mn-doped indium–tin oxide (ITO) films deposited using reactive thermal evaporation. These films were grown on sapphire (0001), Si∕SiO2 as well as Si (100) substrates with the highest magnetic moment observed around 0.8μB∕Mn in 5% Mn-doped ITO films. The electrical conduction is n type and the carrier concentration is ∼2.5×1019cm−3 for 5% Mn doping. An anomalous Hall effect is observed in magnetotransport measurements, showing that the charge carriers are spin polarized, revealing the magnetic interaction between itinerant electrons and localized Mn spins. The carrier concentration can be varied independent of the Mn concentration in this transparent ferromagnetic semiconductor for its easy integration into magneto-optoelectronic devices.

Microstructural and magnetic properties of ZnO:TM (TM=Co,Mn) diluted magnetic semiconducting nanoparticles

We have investigated the structural and the magnetic properties of 3d transition metal (TM) doped Zn1−xTMxO (TM=Co,Mn) diluted magnetic semiconducting nanoparticles for different doping concentrations (0⩽x⩽0.4) synthesized by chemical “pyrophoric reaction process.” From x-ray diffraction measurements the solubility limits of Co and Mn in ZnO nanoparticles are found to be strongly dependent on growth (calcinations) temperature (Tg). The highest solubility limit of both Co2+ and Mn2+ in ZnO at Tg∼300°C is found to be ∼30%. High resolution transmission electron microscopy studies show that Zn1−xTMxO particles are single crystalline of high quality with a wide particle size distribution in nanometric regime. The non-mean-field-like very strong concave nature of temperature dependent magnetization curves is observed at very low temperature in both the systems without showing any distinct magnetic transition. The magnetic behaviors of those Mn2+ and Co2+ doped ZnO semiconducting nanoparticles are observed to be...

A high-temperature ferromagnetic topological insulating phase by proximity coupling

Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin–orbit interaction and the spin–momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

Synthesis of Dicationic Ionic Liquids and their Application in the Preparation of Hierarchical Zeolite Beta

Piperidine- and imidazole-based dicatoinic ionic liquids have been developed for the synthesis of zeolite Beta. Hierarchical Beta has a larger surface area and pore volume than conventional Beta. Beta derived from a dicationic ionic liquid exhibited remarkably higher catalytic activity than the conventional Beta. Experimental evidence and DFT calculations suggest that only a suitable conformation of such dicationic ionic liquids is able to form zeolite Beta (see scheme).

Synthesis of NiCo2O4/Nano-ZSM-5 nanocomposite material with enhanced electrochemical properties for the simultaneous determination of ascorbic acid, dopamine, uric acid and tryptophan

NiCo2O4/Nano-ZSM-5 nanocomposite materials were prepared by the calcination of a physical mixture of NiCo2O4 and Nano-ZSM-5. The materials were characterized by the complementary combination of X-ray diffraction, N2-adsorption and electron microscopic techniques. An electrochemical sensor based on NiCo2O4/Nano-ZSM-5 nanocomposite was constructed for the simultaneous determination of ascorbic acid, dopamine, uric acid, and tryptophan. Among the materials investigated in this study, the NiCo2O4 (30%)/Nano-ZSM-5 modified glassy carbon electrode exhibited the highest electrocatalytic activity with excellent stability, sensitivity, and selectivity. Under the optimum conditions, wide linear ranges were obtained from 1–1200 μM for ascorbic acid, 0.6–900 μM for dopamine, 0.9–1000 μM for uric acid, and 0.9–1000 μM for tryptophan. The detection limits were found to be 0.8, 0.5, 0.7 and 0.7 μM for ascorbic acid, dopamine, uric acid, and tryptophan, respectively. The analytical performance of the developed sensor was demonstrated in the determination of ascorbic acid and dopamine in commercial pharmaceutical preparations (vitamin C tablets and dopamine injections) and uric acid in human urine samples.

Self Assembled InAs/InP Quantum Dots for Telecom Applications in the 1.55 µm Wavelength Range: Wavelength Tuning, Stacking, Polarization Control, and Lasing

Wavelength-tunable InAs quantum dots (QDs) embedded in lattice-matched InGaAsP on InP(100) substrates are grown by metalorganic vapor-phase epitaxy (MOVPE). As/P exchange, which causes a QD size and an emission wavelength that are very large, is suppressed by decreasing the QD growth temperature and V–III flow ratio. As/P exchange, QD size and emission wavelength are then reproducibly controlled by the thickness of ultrathin [0–2 monolayers (ML)] GaAs interlayers underneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs to quantum dashes for a low V–III flow ratio. It is the combination of reduced growth temperature and V–III flow ratio with the insertion of GaAs interlayers of greater than 1 ML thickness which allows the tuning of the emission wavelength of QDs at room temperature in the 1.55 µm wavelength range. Temperature-dependent photoluminescence (PL) measurements reveal the excellent optical properties of the QDs. Widely stacked QD layers are reproduced with identical PL emission to increase the active volume while closely stacked QD layers reveal a systematic PL redshift and linewidth reduction due to vertical electronic coupling, which is proven by the fact that the linear polarization of the cleaved-side PL changes from in-plane to isotropic. Ridge-waveguide laser diodes with stacked QD layers for their active regions exhibit threshold currents at room temperature in continuous-wave mode that are among the lowest threshold currents achieved for InAs/InP QD lasers operating in the 1.55 µm wavelength range.

Facile preparation of Ni(OH)2-MnO2 hybrid material and its application in the electrocatalytic oxidation of hydrazine.

A surfactant-free synthetic methodology is reported for the preparation of Ni(OH)2-MnO2 hybrid nanostructures. For comparative study, MnO2 and Ni(OH)2 were also synthesized. Materials were characterized by X-ray diffraction, nitrogen sorption, scanning electron microscopy, and transmission electron microscopy. Ni(OH)2-MnO2 modified electrode is fabricated for the determination of hydrazine. The electrochemical oxidation of hydrazine was investigated using cyclic, linear sweep voltammetries, and chronoamperometry methods. The Ni(OH)2-MnO2 modified electrode showed hydrazine oxidation with decrease in the over voltage and increase in the oxidation peak current, when compared to MnO2, Ni(OH)2, and bare GCE. pH was optimized to obtain the best peak potential and current sensitivity. Chronoamperometry was used to estimate the diffusion coefficient of hydrazine. The kinetic parameters such as overall number of electrons involved in the catalytic oxidation of hydrazine and the rate constant (k) for the oxidation of hydrazine at Ni(OH)2-MnO2 modified electrode were determined. The Ni(OH)2-MnO2 modified electrode exhibited good sensitivity, stability, and reproducibility in hydrazine sensing.