Swapna S. Nair

Lead free heterogeneous multilayers with giant magneto electric coupling for microelectronics/microelectromechanical systems applications

Lead free magneto electrics with a strong sub resonant (broad frequency range) magneto electric coupling coefficient (MECC) is the goal of the day which can revolutionise the microelectronics and microelectromechanical systems (MEMS) industry. We report giant resonant MECC in lead free nanograined Barium Titanate–CoFe (Alloy)-Barium Titanate [BTO-CoFe-BTO] sandwiched thin films. The resonant MECC values obtained here are the highest values recorded in thin films/multilayers. Sub-resonant MECC values are quite comparable to the highest MECC reported in 2-2 layered structures. MECC got enhanced by two orders at a low frequency resonance. The results show the potential of these thin films for transducer, magnetic field assisted energy harvesters, switching devices, and storage applications. Some possible device integration techniques are also discussed.

Study of structural and magnetoelectric properties of 1−x(Ba 0.96 Ca 0.04 TiO 3 )–x(ZnFe 2 O 4 ) ceramic composites

Multiferroic ceramic composites of (1−x)Ba0.96Ca0.04TiO3–(x)ZnFe2O4 (BCT-ZF) were prepared from ferroelectric (FE) barium calcium titanate (BCT) and ferromagnetic (FM) zinc ferrite (ZF) by using the solid state reaction method with different mol% fractions of x (x = 0.1 and 0.2). The preliminary structural studies carried out by X-ray diffraction at room temperature reveals that the samples have a tetragonal structure along with the cubic spinel ferrite phase. Raman spectra of the composites also confirm the existence of BCT phase and ZF phase. The room temperature ferroelectric polarization measurements as a function of magnetic field show the existence strong magnetoelectric coupling of 10.85 (mV/(cm.Oe).

Synthesis of copper quantum dots by chemical reduction method and tailoring of its band gap

Metallic copper nano particles are synthesized with citric acid and CTAB (cetyltrimethylammonium bromide) as surfactant and chlorides as precursors. The particle size and surface morphology are analyzed by High Resolution Transmission Electron Microscopy. The average size of the nano particle is found to be 3 - 10 nm. The optical absorption characteristics are done by UV-Visible spectrophotometer. From the Tauc plots, the energy band gaps are calculated and because of their smaller size the particles have much higher band gap than the bulk material. The energy band gap is changed from 3.67 eV to 4.27 eV in citric acid coated copper quantum dots and 4.17 eV to 4.52 eV in CTAB coated copper quantum dots.

High voltage generation from lead-free magnetoelectric coaxial nanotube arrays and their applications in nano energy harvesters.

Harvesting energy from surrounding vibrations and developing self-powered portable devices for wireless and mobile electronics have recently become popular. Here the authors demonstrate the synthesis of piezoelectric energy harvesters based on nanotube arrays by a wet chemical route, which requires no sophisticated instruments. The energy harvester gives an output voltage of 400 mV. Harvesting energy from a sinusoidal magnetic field is another interesting phenomenon for which the authors fabricated a magnetoelectric energy harvester based on piezoelectric-magnetostrictive coaxial nanotube arrays. Piezoelectric K0.5Na0.5NbO3 (KNN) is fabricated as the shell and magnetostrictive CoFe2O4 (CFO) as the core of the composite coaxial nanotubes. The delivered voltages are as high as 300 mV at 500 Hz and at a weak ac magnetic field of 100 Oe. Further tailoring of the thickness of the piezoelectric and magnetic layers can enhance the output voltage by several orders. Easy, single-step wet chemical synthesis enhances the industrial upscaling potential of these nanotubes as energy harvesters. In view of the excellent properties reported here, the lead-free piezoelectric component (KNN) in this nanocomposite should be explored for eco-friendly piezoelectric as well as magnetoelectric power generators in nanoelectromechanical systems (NEMS).

Strain induced giant magnetoelectric coupling in KNN/Metglas/KNN sandwich multilayers

A lead-free magnetoelectric composite with sandwich layers of K0.5Na0.5NbO3 (KNN)/Co76Fe14Ni4Si5B (Metglas)/KNN is fabricated as a cantilever and it is characterized for its magnetic, ferroelectric, and magnetoelectric properties. Giant magnetoelectric (ME) coupling is recorded under both resonant and sub resonant conditions and the data are presented here. The observed magnetoelectric coupling coefficient reaches a maximum of 1321 V/cm Oe at resonance (750 Hz) and 9.5 V/cm Oe at a sub-resonant frequency of 50 Hz. The corresponding theoretical calculations are provided for comparison. High magnetostriction as well as initial permeability, fairly good piezoelectric properties, and low dielectric constant cumulatively contribute to the giant magnetoelectric properties in the present system. The high resonance and sub resonance ME coupling voltages make the system ideal for transducers and energy harvesting device applications.

Enhanced photoconductivity in CdS/betanin composite nanostructures

Development of novel materials for thin film solar cells are gaining significant attention due to their tunable wide bandgap and extensive application potential in flexible energy harvesters. CdS is a known window material for thin film solar cells. Tuning of the photoconductivity of CdS by doping, substitution and grain size tailoring is widely attempted by researchers. Inorganic core/shell structures like CdS/CdSe, CdS/ZnS etc. are other possible candidates with band gap tailorability. However, such attempts are rare in tailoring the photoconductivity by providing an organic shell over the inorganic core. Here the authors synthesised CdS/betanin core/shell structures using wet chemical routes. Spectroscopic studies show that the composite structure is core/shell like, with CdS as the core and betanin (a natural dye), as the outer shell with an average core particle size of 10 nm. The absorption spectra of the composite system show the signature of an additional band in the lower wavelength region and it is redshifted with increase in betanin percentage. The intermediate band observed in the energy of ∼1.75 eV, helps CdS to enhance the rate of absorption. Simultaneous absorbance of lower and higher energy photons from the solar radiation can increase the efficiency of CdS based solar cells. A huge enhancement in conductivity is observed in CdS/betanin composites on illumination with white light due to the transfer of photogenerated electrons from the conduction band of betanin dye to the conduction band of CdS.

Experimental investigation of optical and magneto optical effects of chemically synthesized cobalt nanocolloids

Surfactant coated (oleic acid) cobalt nanocolloids are synthesized using the chemical reduction technique. The colloidal cobalt particles are characterized by x-ray diffraction, transmission electron microscopy and a superconducting quantum interference device. Structural characterization of the samples confirms the formation of cobalt in fcc form. The magnetic field induced optical anisotropy is probed through Faraday rotation for both concentrated and diluted colloids. The magneto optical signals for different concentrations are analysed and it is observed that the behaviour can be fitted for a Langevin curve for diluted suspensions, while the deviation is higher for concentrated suspensions. Optical absorption spectra show quantum confinement of nanoparticles leading to a blue shift in the electronic energy band gap. The band gap varied from 2 eV to 4 eV showing semiconductor like behaviour.

Structural, morphological and optical properties of chromium oxide nanoparticles

Chromium oxide nanoparticles are synthesized by reduction route from chloride precursors with surfactant, trioctylphosphine oxide (TOPO). Structural and morphological characterization are analyzed using X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Transmission Electron micrographs show that the average grain size lies in the range 5nm to 10nm. Optical characterization has been done by UV-VIS spectrophotometer. Distinct optical absorptions of Cr3+ ions show hinting towards the presence of Cr2O3. Presence of oxygen is also confirmed from Electron Energy Loss Spectroscopy (EELS) studies.