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Pushan Ayyub

Bio: Pushan Ayyub is an academic researcher from Tata Institute of Fundamental Research. The author has contributed to research in topics: Thin film & Nanocrystalline material. The author has an hindex of 36, co-authored 173 publications receiving 5130 citations. Previous affiliations of Pushan Ayyub include University of Florida & Institute of Physics, Bhubaneswar.


Papers
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TL;DR: In this article, a detailed study of the size dependence of the ferroelectric transition in an ensemble of nanoparticles produced by coprecipitation was performed. But the phase transition was monitored by dielectric measurements, variable temperature x-ray diffraction, and differential scanning calorimetry (DSC).
Abstract: We report a detailed study of the size dependence of the ferroelectric transition in an ensemble of ${\mathrm{PbTiO}}_{3}$ nanoparticles produced by coprecipitation. The phase transition was monitored by dielectric measurements, variable temperature x-ray diffraction, and differential scanning calorimetry (DSC). Size effects were found to become important only below \ensuremath{\approxeq}100 nm (coherently diffracting x-ray domain size). The tetragonal distortion of the unit cell (which is related to the spontaneous polarization) decreases exponentially with size and vanishes at 7 nm. The ${\mathit{T}}_{\mathit{c}}$ decreases gradually but the transition becomes increasingly diffuse with a decrease in the size from 80 to 30 nm, below which there is no peak in the dielectric constant or the heat flow (DSC), though ferroelectric ordering probably persists down to \ensuremath{\approxeq}7 nm.

347 citations

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TL;DR: In this article, size-induced changes in the crystal symmetry have been observed in partially covalent oxides, and it was shown that the size dependence of many important physical properties can be attributed directly to the lattice distortion.
Abstract: Size-induced changes in the crystal symmetry have been observed in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{PbTiO}}_{3}$, ${\mathrm{PbZrO}}_{3}$, ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, and ${\mathrm{Bi}}_{2}$${\mathrm{CaSr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$. In these systems---and presumably in a large majority of partially covalent oxides---the crystal lattice tends to transform into a structure of higher symmetry with a decrease in the crystal size. We also find that the size dependence of many important physical properties can be ascribed directly to the lattice distortion. The physical mechanism responsible for these phenomena is intriguing and their complete understanding essential since they effectively define the practical lower limit to the miniaturization of devices based on these materials.

326 citations

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TL;DR: In this paper, the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap was made.
Abstract: The interrelation between particle size, crystal structure and optical properties in semiconductor quantum dots has elicited widespread interest. We report the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prepared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic CdS nanoparticles was found to be almost 3.9 eV. We also suggest a simple mechanism (based on the periodic insertion of stacking faults) for the transformation from the cubic zinc blende structure to the hexagonal wurtzite structure.

322 citations

Journal ArticleDOI
V. Pillai1, Pradeep Kumar1, M.J. Hou1, Pushan Ayyub1, Dinesh O. Shah1 
TL;DR: In this article, the authors reviewed several articles pertaining to the synthesis of nanoparticles in micro-emulsions and described in some detail their research efforts in the past decade in the field of synthesis of nano-reactors using water-in-oil micro emulsions as nano- reactors.

244 citations

Journal ArticleDOI
TL;DR: In this article, a succession of crystal-size-induced structural transitions was observed, which were related to the increase in the unit-cell volume that occurs as the particle size is decreased.
Abstract: Microcrystalline particles of Fe2O3 having different sizes (varying between 70 and 5 nm) have been synthesised using a novel three-component micro-emulsion technique. A succession of crystal-size-induced structural transitions was observed. While alpha -Fe2O3 was found to nucleate for a particle size above 30 nm, gamma -Fe2O3 was preferentially formed for a size below 30 nm, whereas amorphous Fe2O3 was formed at a particle size of 5 nm. These structural transformations have been related to the increase in the unit-cell volume that occurs as the particle size is decreased. The size dependence of the lattice parameter is shown to arise from a coupling of the surface energy to the dilatational lattice mode. A model Hamiltonian which incorporates this interaction and displays size-induced phase transitions is defined. The Mossbauer hyperfine field in the microcrystalline samples at 4.2 K was found to be substantially smaller than in the 'bulk'. The hyperfine parameters of the amorphous sample were found to be similar to those pertaining to samples prepared by conventional techniques such as melt quenching. A large anisotropy in the ionic vibrational amplitudes was detected in samples with particles smaller than about 10 nm.

217 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI
TL;DR: Practical Interests of Magnetic NuclearRelaxation for the Characterization of Superparamagnetic Colloid, and Use of Nanoparticles as Contrast Agents forMRI20825.
Abstract: 1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inflammation, Apoptose, etc.)20866.2.

5,915 citations

Journal ArticleDOI
TL;DR: In this paper, photo-induced superhydrophilicity was used on the surface of a wide-band gap semiconductor like titanium dioxide (TiO 2 ) for photocatalytic activity towards environmentally hazardous compounds.

4,241 citations

Journal ArticleDOI
TL;DR: This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials, and highlights crucial issues that should be addressed in future research activities.
Abstract: Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

3,265 citations

Journal ArticleDOI
25 Feb 2010-Nature
TL;DR: The results suggest that the doping-induced structural and size transition, demonstrated here in NaYF4 upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
Abstract: Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

2,835 citations