Debolina Das

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.

MAGNETIC PROPERTIES OF GLASS?METAL NANOCOMPOSITES PREPARED BY THE SOL?GEL ROUTE AND HOT PRESSING

Glass‐metal nanocomposite powders in the systems Fe/SiO2 and Ni/SiO2 have been prepared by the sol‐gel technique followed by reduction treatment. Bulk nanocomposites are then fabricated by hot pressing these powders. The metal particle diameters range from 8.9 to 14.8 nm. The materials show enhanced coercivities, e.g., a maximum of 82 Oe in the case of Ni/SiO2 and a maximum of 474 Oe in the case of Fe/SiO2 systems. The Mossbauer spectra of Fe/SiO2 samples are comprised of a ferromagnetic component superposed on a superparamagnetic doublet.

Synthesis of nanocrystalline nickel oxide by controlled oxidation of nickel nanoparticles and their humidity sensing properties

Nickel–silica nanocomposites were prepared by the sol-gel route. By subjecting these to an oxidation treatment in the temperature range 723–1023 K, nickel oxide films of estimated thicknesses in the range 0.5–1.5 nm were grown on the nickel nanoparticles. dc electrical resistivity was measured in the temperature range 300–570 K. The data indicate that an amorphous phase in the interfacial region of the oxide-coated nickel nanoparticles determines the electrical conduction. A small polaron hopping conduction is found to be operative. The humidity sensing properties of these nanocomposites were measured at 300 K in the range of relative humidity (RH) from 3% to 87%. Electrical conductance in all specimens showed an increase of about three orders of magnitude as the RH was raised. Specimens subjected to higher oxidation treatment showed a higher conductance for a specific value of RH. This is ascribed to the presence of a larger number of Ni3+ ions in such specimens.

Preparation of sol-gel nano-composites containing copper oxide and their gas sensing properties

Sol-gel derived composite materials were prepared with nano-sized copper particles in silica matrix. Nano-sized oxide coatings were grown on the nano-particles of copper by subjecting the composites to a controlled oxidation treatment at different temperatures. The current response of samples with oxide layers of different thickness were studied at fixed temperatures in the range 350–450°C in the presence of different concentrations of CO and NO2. The nano-composites were sensitive to both the gases. The operating temperature and the oxide thickness were found to have significant effect on the sensing properties.

Interfacial conduction in silica gels containing nanocrystalline copper oxide

Nanometer-sized copper particles have been grown within a gel derived glass in the system 60 CuO, 40u200aSiO2u200a(moleu200a%). By heat treatment at temperatures in the range of 450–850u200a°C, copper oxide shells of thickness varying from 1.1 to 1.7 nm have been produced. DC resistivity measurements carried out over the temperature range of 30–300u200a°C show a drastically reduced activation energy as compared to that of a reference sample with the above composition. This is ascribed to the presence of an interfacial amorphous phase generated by the assembly of nanosized copper oxide particles.

Iron nanoparticles in copper matrix prepared by sol‐gel route

Iron particles having diameters around 8 nm and loosely packed with nanosized copper particles have been prepared by a sol‐gel route. The samples exhibit coercivities in the range 200 to 500 Oe that are typical of single‐domain iron grains. The Mossbauer spectrum is consistent with the presence of α‐Fe particles in the system. However, a finite value of the isomer shift is obtained that is ascribed to possible electron transfer between the iron atoms and the surrounding copper matrix.

Interface controlled electrical and magnetic properties in Fe–Fe3O4–silica gel nanocomposites

Iron nanoparticles with a shell of Fe3O4 phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. dc electrical resistivity measurements were carried out in the temperature range 80–300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5–300 K both in zero field cooled and field cooled states. A peak in the m...

Nanodot to nanowire: A strain-driven shape transition in self-organized endotaxial CoSi2 on Si(100)

We report a phenomenon of strain-driven shape transition in the growth of nanoscale self-organized endotaxial CoSi2 islands on Si(100) substrates. Nanodots of CoSi2 grow in the square shape following the four fold symmetry of the Si(100) substrate, up to a critical size of 67u2009×u200967u2009nm2, where a shape transition takes place. Larger islands grow as nanowires with ever increasing length and the width decreasing to an asymptotic value of ∼25u2009nm. This produces long nanowires of nearly constant width. The endotaxial nanostructures grow into the Si substrate with a small extension above the surface.

Self-organized one-atom thick fractal nanoclusters via field-induced atomic transport

We report on the growth of a monolayer thick fractal nanostructures of Ag on flat-top Ag islands, grown on Si(111). Upon application of a voltage pulse at an edge of the flat-top Ag island from a scanning tunneling microscope tip, Ag atoms climb from the edge onto the top of the island. These atoms aggregate to form precisely one-atom thick nanostructures of fractal nature. The fractal (Hausdorff) dimension, DHu2009=u20091.75u2009±u20090.05, of this nanostructure has been determined by analyzing the morphology of the growing nanocluster, imaged by scanning tunneling microscopy, following the application of the voltage pulse. This value of the fractal dimension is consistent with the diffusion limited aggregation (DLA) model. We also determined two other fractal dimensions based on perimeter-radius-of-gyration (DP) and perimeter-area (D′P) relationship. Simulations of the DLA process, with varying sticking probability, lead to different cluster morphologies [P. Meakin, Phys. Rev. A 27, 1495 (1983)]; however, the value of ...

Negative differential resistance in electron tunneling in ultrathin films near the two-dimensional limit

We report on our observation of negative differential resistance (NDR) in electron tunneling conductance in atomic-scale ultrathin Ag films on Si(111) substrates. NDR was observed by scanning tunneling spectroscopy measurements. The tunneling conductance depends on the electronic local density of states (LDOS) of the sample. We show that the sample bias voltage, at which negative differential resistance and peak negative conductance occur, depends on the film thickness. This can be understood from the variation in the LDOS of the Ag films as a function of film thickness down to the two-dimensional limit of one atomic layer. First principles density functional theory calculations have been used to explain the results.