M. S. Tomar

Structural and ferroelectric studies of Bi3.44La0.56Ti3O12 films

Bi4−xLaxTi3O12 materials were synthesized by the sol-gel process and thin films were prepared on Pt (i.e., Pt/TiO2/SiO2/Si) substrate by spin coating. Structural properties of the films were examined by x-ray diffraction and Raman spectroscopy. Dielectric and ferroelectric response was studied for 0.63-μm-thick Bi3.44La0.56Ti3O12 films. Butterfly dielectric behavior and remnant polarization of up to Pr=41 μC/cm2 has been achieved. These films also showed fatigue free response up to 109 switching cycles. On the basis of these studies, large polarization in Bi3.44La0.56Ti3O12 films is attributed to dipole formation which may tilt TiO6 octahedra to Bi2O2 interlayer.

Tuning of magnetic properties in cobalt ferrite nanocrystals

Cobalt ferrite (CoFe2O4) possesses excellent chemical stability, good mechanical hardness, and a large positive first order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to precise control of the composition and structure of CoFe2O4, its practical application will require the capability to control particle size at the nanoscale. The results of a synthesis approach in which size control is achieved by modifying the oversaturation conditions during ferrite formation in water through a modified coprecipitation approach are reported. X-ray diffraction, transmission electron microscopy (TEM) diffraction, and TEM energy-dispersive x-ray spectroscopy analyses confirmed the formation of the nanoscale cobalt ferrite. M-H measurements verified the strong influence of synthesis conditions on crystal size and hence, on the magnetic properties of ferrite nanocrystals. The room-temperature coercivity values increased from 460 up to 4626Oe under optimum...

Structural, optical, and magnetic characterization of monodisperse Fe-doped ZnO nanocrystals

The results on the synthesis and characterization of highly monodisperse Fe-doped ZnO nanocrystals are presented. Stable suspensions of these materials were produced in an ethanol solution at room temperature. To promote crystal growth, the suspensions of nanocrystals were aged in contact with their mother liquors. X-ray diffraction characterization of doped systems at various Fe-atomic fractions x confirmed the exclusive formation of the host ZnO with the wurtzite structure. High resolution transmission electron microscopy analyses of the suspensions revealed the high monodispersity and crystallinity of the 6–8nm nanocrystals. Ultraviolet-visible measurements confirmed not only the nanocrystalline nature of the samples but also evidenced the continuous growth of the crystals when aged in their mother liquors. Room-temperature magnetic measurements indicated that the ferromagnetic behavior of doped ZnO was dependent on composition and crystal size of produced nanocrystals. Room-temperature ferromagnetism ...

Multiferroic properties of CoFe2O4/Bi3.4La0.6Ti3O12 bilayer structure at room temperature

Bi3.4La0.6Ti3O12/CoFe2O4 bilayer films were synthesized by chemical solution method and deposited by spin coating on Pt (Pt/TiO2/SiO2/Si) substrate. X-ray diffraction of the bilayer system revealed the composite-like structure. The leakage current is less than 10−7 A at electric field below100 kV/cm, and it shows the ohmic behavior. Dielectric constant decreases with increasing frequency and reaches to 140 at 10−6 Hz. Bi3.4La0.6Ti3O12/CoFe2O4 system shows the co-existence of ferroelectric polarization (Pr) = 51 μC/cm2 and magnetization (Mr) = 206 emu/cm3 at room temperature. Observed ferromagnetic and ferroelectric responses in bilayer system may be useful for bi-functional devices.

Effect of Zinc Oxide Nanocrystals in Media Containing E. coli and C. xerosis Bacteria

The present investigation is centered on the study of the growth curves of E. coli and C. xerosis bacteria in the presence of nanosize particles of Zinc Oxide. Previous works demonstrated the sensitivity of the bacteria, when these were reproduced in media that contain nanoparticles of luminescent silicon and Cobalt Ferrite. Doped ZnO nanocrystals were synthesized by conventional precipitation in ethanol solutions as reported by Spanhel and Anderson for bare ZnO. In our case, the syntheses were carried out under room-temperature conditions. The experimental results of E. coli bacteria in contact with a stable suspension of nanoparticles of Zinc Oxide, shows a growth curve without adaptation period. Moreover a short and slowly logarithmic stage has been observed, reaching the stationary stage after approximately four hours compared with one in absence of the nanoparticles (standard curve). During the observations, a change in the lifetime of the bacteria (metabolism) with particulate was noticed,as well as the beginning of the mortality stage. However, different results were recorded for silicon and ferrite. For the case of the bacteria C. xerosis , the curve with particles is above its standard curve, for all times with none of the oscillations which occured in the nanometer silicon. For these bacteria the beginning of the mortality stage is observed when they have particles. For both bacteria with Zinc Oxide nanoparticles this occurs approximately after nine hours.

Structural, Optical and Luminescent Properties of ZnO:Eu 3+ Nanocrystals Prepared by Modified Sol-Gel Method

Highly monodisperse ZnO:Eu 3+ nanocrystals have been synthesized by modified sol-gel method from ethanolic solutions. The effect of Eu 3+ ions (x=0.05-0.30) concentration on the structural, optical and luminescent properties has been evaluated. No other than the ZnO-wurtzite phase was observed at all dopant levels, which was confirmed by FT-IR and Raman spectroscopy techniques. A blue shift of the exciton peak and the increase on the corresponding band gap were observed at increasing Europium contents, which would indicate an interaction between Eu 3+ ions and the development of the ZnO host structure. The luminescence properties were also dependent on Europium contents; a systematic blue shift and enhancement of the intensity of visible luminescence peak, attributed to an increment of surface defects, was observed by a rising Europium concentration. The red luminescence band, representing the 5 D 0 → 7 F 0 transition, was clearly observed in nanocrystals after annealing at 300 O C for one hour. The presence of this band could be considered as an evidence of the effective energy transfer from ZnO to Eu 3+ ions.

Structural, Optical and Magnetic Properties of Co and Fe Doped ZnO Thin Films Grown by Radio Frequency Magnetron Sputtering

Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O targets were used to grow thin films by rf magnetron sputtering. XRD patterns of the films showed a strong preferred orientation along c-axis. Zn0.90Co0.10O film showed a transmittance above 75% in the visible range, while the transmittance of the Zn0.85[Co0.50Fe0.50]0.15O film was about 45%; with three absorption peaks attributed to d -d transitions of tetrahedrally coordinated Co. The band gap values for Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films were 2.95 and 2.70 eV respectively, which are slightly less than ZnO bulk. The Zn0.90Co0.10O film showed a relatively large positive magnetoresistance (MR) at the high magnetic field in the temperature range from 7 to 50 K, which reached 11.9% a 7K for the magnetoresistance. The lowest MR was found at 100 K. From M-H curve measured at room temperature shown a probable antiferromagnetic behavior, although was possible to observe little coercive field of 30 Oe and 40 Oe for Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films, respectively. INTRODUCTION There is interest in dilute magnetic oxide semiconductors for spin based devices. The ZnO is a II-VI compound semiconductor with the hexagonal wurtzite structure with band gap energy ~3.1 eV. It has interesting properties from the viewpoint of forming a transparent ferromagnetic material [1]. Several groups have reported room temperature ferromagnetism in transition-metaldoped ZnO [3-8]. ZnO films doped with Co and Fe have been reported to be ferromagnetic with a Curie temperature of about 280 K [7]. But, Kim et al. pointed out the presence of Co clusters in their films as a reason for room temperature ferromagnetism. Therefore, the origin and clear understanding of ferromagnetism in Coand Fe-doped ZnO has not been clearly elucidated. It is known that rf magnetron sputtering offers higher density plasma and films of better Mater. Res. Soc. Symp. Proc. Vol. 957

Role of Rare-Earth Element Doping in the Cycleability of LiMn2O4 Cathodes for Li-ion Rechargeable Batteries

We have synthesized spinel LiMn 1.99 Nd 0.01 O 4 and LiMn 1.99 Ce 0.01 O 4 powder by chemical synthesis method. The synthesized powders were used to prepare cathodes for Li ion coin cells. The structural and electrochemical properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry and charge-discharge studies, respectively. The cyclic voltammetry of the cathodes revealed the reversible nature of Li-ion intercalation in the cell. The charge-discharge characteristics for LiMn1.99Ce0.01O4 were obtained in 3.5 V – 4.8 V voltage range, while for LiMn 1.99 Nd 0.01 O 4 the charge-discharge were carried out in 3.4 V – 4.4 V range. The initial discharge capacities of LiMn 1.99 Ce 0.01 O 4 and LiMn 1.99 Nd 0.01 O 4 were obtained as 134mAh/g and 149 mAh/g, respectively. The coin cells were tested for up to 25 charge-discharge cycles and after 25 cycles the discharge capacities were determined to 79.5 mAh/g and 132 mAh/g for LiMn 1.99 Ce 0.01 O 4 and LiMn 1.99 Nd 0.01 O 4 cathodes respectively. However, by doping with a small concentration of rare earth materials, like Ce and Nd reduces the capacity fading in pure LiMn 2 O 4 cathodes making it suitable for Li-ion battery applications.

Effect of ZnO Coating on the Electrochemical Performance of LiMn 1.5 Ni 0.5 O 4 Cathode Material

LiMn 1.5 Ni 0.5 O 4 cathode material was prepared by sol-gel method and annealed at 850°C for 15 hrs. The prepared powder was coated with ZnO by dissolving zinc acetate in methanol and LiMn 1.5 Ni 0.5 O 4 powder was mixed in this solution followed by the continuous stirring for 4 hr. The LiMn 1.5 Ni 0.5 O 4 and ZnO coated LiMn 1.5 Ni 0.5 O 4 powder was characterized using X-ray diffraction, TEM and Raman spectroscopy. The coin cell was fabricated using LiMn 1.5 Ni 0.5 O 4 and ZnO coated LiMn 1.5 Ni 0.5 O 4 as cathode materials, LiPF 6 , dissolved in EC/DMC (1:1 wt ratio) as electrolyte, and Li foil as anode. The cyclic voltammetric and charge-discharge characteristics were carried out for the coin cell using LiMn 1.5 Ni 0.5 O 4 and ZnO coated LiMn 1.5 Ni 0.5 O 4 cathode materials. It was found that the ZnO coated LiMn 1.5 Ni 0.5 O 4 cathode materials showed improved discharge capacity (∼146mAh/g) as compared to the pure LiMn 1.5 Ni 0.5 O 4 (∼140mAh/g). The discharge capacity retention after 50 cycles was found to be about 94% and 97% for LiMn 1.5 Ni 0.5 O 4 and ZnO coated LiMn 1.5 Ni 0.5 O 4 cathode materials, respectively.

Synthesis and Characterization of Transition Metal ion Doped ZnS and ZnO Nanostructures

Nanocrystalline films and powders of semiconducting ZnS, with and without doping with Cr, Co and Ni ions, have been synthesized using a low‐temperature chemical bath deposition approach in aqueous solution. X‐ray diffraction analyses evidenced that ZnS structures consisted of cubic or a mixture of a cubic/hexagonal phase. Average crystallite size was estimated to be 3–4 nm using Scherrer’s equation. ZnS and transition metal‐doped ZnS were thermally oxidized into corresponding oxides. Magnetic and optical behavior of the sulfide films and powders were investigated using SQUID and UV‐visible spectrometry, respectively. Preliminary magnetic measurements suggest antiferromagnetic ordering in Zn0.9Cr0.1S.