D. Chakravorty

Synthesis of nanocrystalline nickel-zinc ferrite by the sol-gel method

Ni0.5Zn0.5Fe2O4 crystals having dimensions varying from 13 to 35 nm have been synthesized within a silica glass matrix by a sol-gel method. Particle sizes in the above range can be controlled by a suitable heat treatment schedule at temperatures varying from 973 to 1323 K. The coercivity values for different heat-treated samples are found to be in the range 20–95 Oe, which are significantly larger than that for samples having grain sizes of the order of a few microns. The Mossbauer spectra of the samples show the presence of ultrafine particles exhibiting superparamagnetic relaxation.

Glass-metal nanocomposite synthesis by metal organic route

Glass-metal nanocomposites incorporating ultrafine particles of iron, nickel, cobalt and manganese, respectively, in a silica glass matrix have been prepared by heat treatment of a gel derived from a sol containing silicon tetraethoxide and a suitable metal organic compound. Metal particles in all the nanocomposites are isolated and spherical-shaped with diameters ranging from 3 to 10 nm. Films of these nanocomposites with thickness of the order of a few micrometres have been prepared on glass slides by a simple dip-and-pull technique. Optical absorption spectra of the nanocomposite films have been measured over the wavelength range 200 to 2000 nm. Effective medium theories of Maxwell-Garnett and Bruggeman, respectively, have been used to calculate theoretically the optical absorption of these materials. The Maxwell-Garnett theory gives results which are in better agreement with experimental data than those obtained from Bruggeman formalism. The filling factor f as estimated from the least-squares fit of the experimental results with the Maxwell-Garnett theory has a value in the range 1 to 4%.

Nanowire formation in a polymeric film

Silver nanowires of diameter ∼2 nm and length ∼2 mm have been grown by an electrodeposition method within a polyvinyl alcohol film subjected to a two-stage treatment, viz., in ammonium persulphate and pyrrole solutions, respectively. A staircase current–voltage characteristic has been measured in this composite. Also, the material shows a dielectric constant ∼104. Both these effects are observed when the applied electric field is in a direction perpendicular to that of the electrodeposition process.

Silver nanowires grown in the pores of a silica gel

Silver nanowires of diameter ∼40 nm and length ∼0.3 mm have been grown by electrodeposition within the pores of silica gels which were heat treated in the temperature range 523 to 823 K and, subsequently, soaked in a silver nitrate solution. A staircase current–voltage characteristic was observed in the direction of electrodeposition after nanowires were disrupted by the application of a dc voltage pulse. Such gels containing interrupted nanowires of silver showed a dielectric constant value ∼104 both in directions parallel and perpendicular to that of electrodeposition.

Review Synthesis of conducting nanowires

Work reported on the synthesis of nanowires of different metals/alloys and semiconductors respectively in recent years is reviewed. The methods used mostly belong to one of the following categories: chemical, electrodeposition, physical and filling of carbon nanotubes. Electrical properties investigated for some of these nanowires indicate quantum mechanical effects to be present. Nanodevice fabrication using doped semiconducting nanowires has also been reported.

Mössbauer spectra of nanocrystalline Fe and Fe‐Cr particles in sol‐gel‐derived SiO2 glass

Ultrafine iron particles prepared by a sol‐gel route are characterized by Mossbauer spectroscopy and transmission electron microscopy. The Mossbauer absorption patterns consist of a ferromagnetic component superposed on a superparamagnetic doublet. The intensity of the superparamagnetic doublet is found to be larger for particles having smaller average diameter. For very fine particles a diffused electron diffraction pattern is observed. It is also shown that the sol‐gel technique could be used to prepare fine particles of a Fe‐Cr alloy.

Stability of cubic phase in nanocrystalline ZrO2

Stable cubic ZrO 2 up to a temperature of 1173 K has been synthesized by a chemical precipitation technique. Such a stability appears to be driven by particle size. The critical value below which cubic ZrO 2 has been found to be stable is 17 nm. The x-ray diffraction pattern of such ultrafine cubic particles is similar to that obtained by stabilization of ZrO 2 by the addition of 20 mole % CaO.

Electrical resistivity of nanocrystalline PbS grown in a polymer matrix

PbS particles of average diameters ranging from 9.9 to 18.0 nm have been synthesized within a polyacrylamide matrix. The percolative chains of these particles exhibit intrinsic semiconducting behavior at temperatures higher than 340 K. The estimated band gap for these particles are found to be much higher than that of bulk PbS being in the range 1.03 to 1.49 eV. The low‐temperature conductivity of these composites appears to be controlled by an electron tunneling mechanism.

Surface-modified sulfur nanoparticles: an effective antifungal agent against Aspergillus niger and Fusarium oxysporum

Surface-modified sulfur nanoparticles (SNPs) of two different sizes were prepared via a modified liquid-phase precipitation method, using sodium polysulfide and ammonium polysulfide as starting material and polyethylene glycol-400 (PEG-400) as the surface stabilizing agent. Surface topology, size distribution, surface modification of SNPs with PEG-400, quantitative analysis for the presence of sulfur in nanoformulations, and thermal stability of SNPs were determined by atomic force microscopy (AFM), dynamic light scattering (DLS) plus high-resolution transmission electron microscopy (HR-TEM), fourier transform infrared (FT-IR) spectroscopy, energy dispersive X-ray (EDX) spectroscopy, and thermogravimetric analysis (TGA), respectively. A simultaneous study with micron-sized sulfur (S0) and SNPs was carried out to evaluate their fungicidal efficacy against Aspergillus niger and Fusarium oxysporum in terms of radial growth, sporulation, ultrastructural modifications, and phospholipid content of the fungal strains using a modified poisoned food technique, spore-germination slide bioassay, environmental scanning electron microscopy (ESEM), and spectrometry. SNPs expressed promising inhibitory effect on fungal growth and sporulation and also significantly reduced phospholipid content.

Size-dependent magnetic properties of Mn0.5Zn0.5Fe2O4 nanoparticles in SiO2 matrix

Mn0.5Zn0.5Fe2O4 ferrite nanoparticles (<100 nm) in SiO2 matrix have been prepared by the sol-gel method. The particle size was varied by changing the duration of heat treatment above crystallization temperature. An x-ray diffraction study indicates the presence of single-phase spinel ferrite in the sample. The particle size was estimated by the x-ray diffraction method as well as from the micrograph taken by a transmission electron microscope. The magnetic properties of the samples were studied by a vibrating sample magnetometer. The samples show superparamagnetic behavior when the particle size is below 20 nm, which is confirmed by Mossbauer spectroscopy measurements. The average particle size in the superparamagnetic state was also estimated from the low-field magnetization measurement by considering the samples as consisting of noninteracting single domain particles.