Sandeep Chhoker

Structural, magnetic, and optical properties of Pr and Zr codoped BiFeO3 multiferroic ceramics

Single-phase (Bi1-xPrx)(Fe1-xZrx)O3 ceramics with x = 0.03, 0.06, 0.10 (named BPFZ-3, BPFZ-6, and BPFZ-10, respectively) were synthesized by solid state reaction method. X-ray diffraction suggested the pure phase formation with distorted rhombohedral structure in all samples. Raman spectroscopy also confirmed the rhombohedrally distorted perovskite structure with R3c symmetry. The x-ray photoelectron spectroscopy measurements showed the absence of Fe2+ ions indicating the suppression of oxygen vacancies due to Pr and Zr codoping. Room temperature magnetic hysteresis loops revealed that the spontaneous magnetization increases due to structural distortion and partial destruction of spiral spin structure caused by the codoping in BiFeO3 ceramics. The maximum remnant magnetization of 0.1234 emu/g was observed for BPFZ-6 sample. Optical studies showed absorption of light in the visible region, with a red shift in energy band gap with increasing concentration of Pr and Zr in BiFeO3 ceramics. The broad absorptio...

Substitution driven structural and magnetic transformation in Ca-doped BiFeO3 nanoparticles

Bi1−xCaxFeO3; (x = 0–0.20) nanoparticles were synthesized by tartaric acid based sol–gel route. X-ray diffraction and electron microscopy studies reveal the phase purity and nanocrystalline nature (45–90 nm) of Bi1−xCaxFeO3 samples. The Ca ion substitution driven structural transition from rhombohedral (space group R3c) to orthorhombic (Pnma) symmetry leads to enhancement in saturation magnetization due to the distorted cycloid spin structure (as also suggested by Mossbauer studies) and uncompensated surface spins which is accorded with electron paramagnetic resonance (EPR) studies as well. The ferromagnetic ordering contribution increases up to x = 0.15 samples with maximum saturation magnetization of 0.09 emu g−1 for x = 0.15 sample. The presence of high content orthorhombic phase for x = 0.20 sample results in the sharp decrease in the ferromagnetic component due to appearance of collinear antiferromagnetic ordering in agreement with EPR results. X-ray photoelectron spectroscopy confirmed the dominancy of Fe3+ oxidation states along with the shifting of the binding energy of Bi 4f orbital indicating the substitution of Ca2+ at Bi-site. Systematic change of Mossbauer parameters of nanoparticulate samples with Ca concentration are obtained by Mossbauer spectroscopy. The results of both one- and two-sextet fittings of the Mossbauer spectra provide evidence for destruction of the spin cycloid in Ca-doped BiFeO3 nanoparticles.

Field emission properties of carbon nanostructures: A review

Carbon nanotubes (CNTs), carbon nanoflakes (CNFs), carbon nanoparticles (CNPs) and other related structures (known as carbon nanostructures (CNSs)) have emerged as a promising class of materials as field electron emitters. They have a low threshold field and a high emission current density which make them attractive for technological applications. In this article, we review recent progress on understanding of CNS field emitters and discuss issues related to applications of CNS based cold cathodes in vacuum microelectronic devices. The emphasis is on the emission characteristics of macroscopic CNS cathodes and their relations with the underlying materials properties. In addition, some of realistic applications based on CNTs field emission have also been discussed.

Size dependent structural, vibrational and magnetic properties of BiFeO3 and core-shell structured BiFeO3@SiO2 nanoparticles

Bulk BiFeO3, BiFeO3 nanoparticles and core-shell structured BiFeO3@SiO2 nanoparticles were synthesized by solid state reaction method, sol-gel and Stober process (SiO2 shell) respectively. Transmission electron microscopy image confirmed the core-shell structure of BiFeO3@SiO2 nanoparticles with BiFeO3 core ∼50-90 nm and SiO2 shell ∼16 nm. X-ray diffraction and FTIR spectroscopy results showed the presence of distorted rhombohedral structure with R3c space group in all three samples. The magnetic measurement indicated the existence of room-temperature weak ferromagnetism in core-shell BiFeO3@SiO2 nanoparticles and BiFeO3 nanoparticles, whereas bulk BiFeO3 showed antiferromagnteic nature. Electron Spin Resonance results confirmed the enhancement in magnetic properties of coreshell structured BiFeO3@SiO2 nanoparticles in comparison with BiFeO3 nanoparticles and bulk BiFeO3.

Surface activation of self assembled carbon nanoflakes

Single step growth of carbon nanoflakes over p-silicon (100) substrates in MPECVD system using acetylene, hydrogen and argon as reactant gases has been carried out. FESEM studies of the deposited carbon films revealed that the morphology is greatly influenced with deposition time. Transmission electron microscope (TEM) and Raman spectra analysis of these thin films confirmed presence of highly crystalline carbon films growth. Its surface activation properties have been studied by depositing ultra thin layer of RbF over 2D carbon nanoflake films and corresponding KP (Kelvin-Probe) measurements.

Effect of Ca and Ni co-substitution on structural and magnetic properties of BiFeO3 nanoparticles

Bi1-xCaxFe1-xNixO3 nanoparticles with x = 0.0, 0.05, 0.10 and 0.15 were effectively synthesized using a profitable route, tartaric acid based sol-gel one. In BiFeO3 at Bi and Fe-sites, the co-substitution of alkali earth metal Ca2+ ions and transition metal Ni2+ ions leads to structural distortion, reduction in average grain size and enhanced magnetic properties. The study of XRD and Raman scattering demonstrated the compressive lattice distortion in the samples occurred from the co-substitution of Ca2+ and Ni2+ ions. The measurements of XRD patterns and Raman spectroscopy also recommended the coexistence of rhombohedral and orthorhombic phases in co-substituting BiFeO3 samples. Room temperature enhanced ferromagnetic behavior demonstrated with magnetic study, by the co-substitution of Ca2+ and Ni2+ ions at Bi and Fe-site, respectively. The line shape of electron paramagnetic resonance spectra changes continuously due to the local distortion caused from substituting Ni.

Linking Catalyst Phase with CNT Morphology and its Subsequent Field Emission Characteristics: An Optimization Study

The effect of phase transition in catalyst nanoparticles due to increasing growth temperatures and its effect over the morphology and field emission of carbon nanotubes (CNT) films is reported here. Thin Fe film coated Si substrates are used to obtain Fe nanoparticles under hydrogen and argon plasma treatment. For this, three different microwave powers (400, 500 and 650 watts) in a microwave plasma enhanced CVD system is used. TEM images of plasma annealed films shows Fe nanoparticles of size ranging from 10 nm to 70 nm. The SAED images of these Fe nanoparticles shows additional diffraction rings for the films annealed at higher microwave power (i.e., higher growth temperatures). Acetylene and hydrogen plasma is used for 2 minutes to deposit the CNT films over three different sized Fe nanoparticles. High resolution TEM (HRTEM) images are used to further investigate any effect of growth temperature on catalyst encapsulated in the CNTs. The field emission studies and Kelvin probe measurements are carried out to examine the suitability of films for its field emission application. It is established that, only under specific optimized growth parameters CNT film must be grown for its usefulness as efficient field emitters.

Structural, raman, dielectric, magnetic and magnetoelectric properties of Ba and Mn doped BiFeO 3 nanoparticles

Ba and Mn doped BiFeO3 nanoparticles were synthesized by sol-gel method. X-ray diffraction and Raman spectroscopy results showed the presence of distorted rhombohedral structure for Bi0.85Ba0.15FeO3 nanoparticles and the substitution induced phase transition (rhombohedral to orthorhombic) phase for Bi0.85Mn0.15FeO3 nanoparticles. FESEM images of both the samples revealed grain size in the range from 50 to 100 nm. Magnetic measurement showed room temperature ferromagnetic behavior, which may be attributed to the antiferromagnetic core and the ferromagnetic surface of the nanoparticles, together with the structural distortion caused by Ba and Mn substitution in BiFeO3 samples. The magnetoelectric coupling was evidenced by the observation of the dielectric anomaly in the dielectric constant near antiferromagnetic Neel temperature in both the samples.