Experimental studies of strong dipolar interparticle interaction in monodisperse Fe3O4 nanoparticles

Interparticle interaction of monodisperse Fe3 O4 nanoparticles has been experimentally investigated by dispersing the nanoparticles in solvents. With increasing the interparticle distances to larger than 100 nm in a controlled manner, the authors found that the blocking temperature (TB) of the nanoparticles drops continuously and eventually gets saturated with a total drop in TB of 7-17 K observed for 3, 5, and 7 nm samples, compared with their respective nanopowder samples. By carefully studying the dependence of TB on the interparticle distance, the authors could demonstrate that the experimental dependence of TB follows the theoretical curve of the dipole-dipole interaction. © 2007 American Institute of Physics.open313

Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd_{0.7}Sr_{0.3}MnO_{3}

The magnetocaloric effect in single crystalline Nd_{0.7}Sr_{0.3}MnO_{3} is investigated by measuring the field-induced adiabatic change in temperature which reveals a single negative peak around 130 K well below the Curie temperature (T_C=203 K). In order to understand this unusual magnetocaloric effect, we invoke the reported {55}^Mn spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on Nd_{0.7}Sr_{0.3}MnO_{3}. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of Nd_{0.7}Sr_{0.3}MnO_{3} near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields.Comment: Accepted for publication in Phys. Rev.

Silk and its composites for humidity and gas sensing applications

Silk fibroin (SF) is a natural protein largely used in the textile industry with applications in bio-medicine, catalysis as well as in sensing materials. SF is a fiber material which is bio-compatible, biodegradable, and possesses high tensile strength. The incorporation of nanosized particles into SF allows the development of a variety of composites with tailored properties and functions. Silk and its composites are being explored for a wide range of sensing applications like strain, proximity, humidity, glucose, pH and hazardous/toxic gases. Most studies aim at improving the mechanical strength of SF by preparing hybrids with metal-based nanoparticles, polymers and 2D materials. Studies have been conducted by introducing semiconducting metal oxides into SF to tailor its properties like conductivity for use as a gas sensing material, where SF acts as a conductive path as well as a substrate for the incorporated nanoparticles. We have reviewed gas and humidity sensing properties of silk, silk with 0D (i.e., metal oxide), 2D (e.g., graphene, MXenes) composites. The nanostructured metal oxides are generally used in sensing applications, which use its semiconducting properties to show variation in the measured properties (e.g., resistivity, impedance) due to analyte gas adsorption on its surface. For example, vanadium oxides (i.e., V2O5) have been shown as candidates for sensing nitrogen containing gases and doped vanadium oxides for sensing CO gas. In this review article we provide latest and important results in the gas and humidity sensing of SF and its composites

Polymer-stable magnesium nanocomposites prepared by laser ablation for efficient hydrogen storage

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ per kg), great variety of potential sources (for example water, biomass, organic matter), and low environmental impact (water is the sole combustion product). However, due to its light weight, the efficient storage of hydrogen is still an issue investigated intensely. Various solid media have been considered in that respect among which magnesium hydride stands out as a candidate offering distinct advantages. Recent theoretical work indicates that MgH2 becomes less thermodynamically stable as particle diameter decreases below 2 nm. Our DFT (density functional theory) modeling studies have shown that the smallest enthalpy change, corresponding to 2 unit-cell thickness (1.6 {\AA} Mg/3.0{\AA} MgH2) of the film, is 57.7 kJ/molMg. This enthalpy change is over 10 kJ per molMg smaller than that of the bulk. It is important to note that the range of enthalpy change for systems that are suitable for mobile storage applications is 15 to 24 kJ permolH at 298 K. The important key for the development of air/stable Mg/nanocrystals is the use of PMMA (polymethylmethacrylate) as an encapsulation agent. In our work we use laser ablation, a non-electrochemical method, for producing well dispersed nanoparticles without the presence of any long range aggregation. The observed improved hydrogenation characteristics of the polymer/stable Mg-nanoparticles are associated to the preparation procedure and in any case the polymer laser ablation is a new approach for the production of air/protected and inexpensive Mg/nanoparticles.Comment: Hydrogen Storage, Mg - Nanoparticles, Polymer Matrix Composites, Laser Ablation, to appear in International Journal of Hydrogen Energy, 201

Pyramidal nanostructures of zinc oxide

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 °C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 ± 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere

ZnO(101) films by pulsed reactive crossed-beam laser ablation

We have employed pulsed reactive crossed-beam laser ablation (PRCLA) to deposit a (101) oriented ZnO film. In this method, a supersonic jet of oxygen pulse is made to cross the laser plume from a zinc metal target while being carried to the Si(111) substrate. The obtained deposit was nanocrystalline ZnO as confirmed by a host of characterization techniques. When the substrate was held at varying temperatures, from room temperature to 900°C, the crystallinity of the obtained films increased as expected, but importantly, the crystallographic orientation of the films was varied. High substrate temperatures produced the usual (001) oriented films, while lower substrate temperatures gave rise to increasingly (101) oriented films. The substrate held at room temperature contained only the (101) orientation. The film morphology also varied with the substrate temperature, from being nanoparticulate to rod-like deposits for higher deposition temperatures. Surprisingly, the (101) orientation showed reactivity with acetone forming carbonaceous nanostructures on the surface

Magnetic Pd nanoparticles: Effects of surface atoms

We have investigated the magnetic properties of trioctylphosphine (TOP)-stabilized monodisperse palladium nanoparticles of 2, 3, 5 and 10 nm in size, in order to study the possible effects of surface Pd atoms. These nanoparticles display clear signatures of ferromagnetism such as hysteresis and saturation magnetization over the entire temperature range studied here from 2 to 380 K. The magnetization of the nanoparticles increases with decreasing particle size, indicating a possibly important role played by Pd atoms on the surface of the nanoparticles. More importantly, we also found that the magnetization of our TOP-stabilized Pd nanoparticles is one order of magnitude smaller than those of other Pd nanoparticles reported so far, which is most likely to be due to the weak nature of interface interaction between TOP ligands and Pd nanoparticles compared to other ligands. This observation is consistent with the view that the magnetism of Pd nanoparticles is strongly influenced by the interaction of surface atoms with the ligands. We discuss our experimental findings in terms of a charge transfer mechanism due to a covalent bond of Pd atoms with the protective TOP ligand, which would increase the 4d density of states of Pd atoms due to localization by the bonded P atoms.

Films and dispersions of reduced graphene oxide based Fe2O3 nanostructure composites: Synthesis, magnetic properties and electrochemical capacitance

Films and dispersions of reduced graphene oxide (rGO) composites with Fe2O3 nanostructures have been synthesized by liquid/liquid interface and co-precipitation methods, respectively. Gamma phase, Fe2O3 nanoparticles with rGO are obtained as aqueous dispersions by co-precipitation method while Fe2O3 nanostructures consisting of a mixture of alpha and gamma phases are obtained in the form of freestanding thin films at the liquid/liquid interface. Different morphologies of Fe2O3 such as nano particles and nanorods are obtained by employing a modified or bare liquid/liquid interface. The nucleation and growth in this case is controlled by the density of oxygen functional groups on rGO. A comparison of the magnetic properties of dispersion and films of nanocomposites and their constituents are presented. rGO-gamma Fe2O3 dispersions show superparamagnetic nature while films exhibit extremely low magnetic moments confirming the presence of mixed phases of Fe2O3. Electrochemical capacitance studies of nanocomposite films reveal contributions due to electrical double layer capacitance of rGO and pseudocapacitance of Fe2O3 nanostructures and a specific capacitance 64.5 Fig at 2 mV/s is estimated. These films in microgram quantities without the aid of any binders exhibit good adhesion on carbon electrodes with excellent recyclability and less internal resistance and are promising for applications as supercapacitors

Influence of Iodine Doping on the Structure, Morphology, and Physical Properties of Manganese Phthalocyanine Thin Films

Doping with halide ions is a popular method toalter the properties of metal phthalocyanines (MPcs),particularly magnetic and electrical nature of organic semiconductorsfor applications in spintronic or electronic devices.Doping can cause a structural rearrangement in MPc packing,and the physical properties may be correlated with molecularpacking. Films of a planar and magnetic MPc, manganese-(II)phthalocyanine (MnPc), are chosen for iodine dopingstudy. The optical, magnetic, and electrical properties ofpristine- and iodine-doped MnPc thin films are investigatedand can be directly associated with their molecular structure.Two-dimensional grazing incidence synchrotron X-ray diffractionreveals structural disorder in MnPc films upon iodineinfusion induced by the reorientation of ordered, edge-on molecular configuration to tilted and face-on configurations in arandom fashion. The film morphology changes accordingly, where in the uniform crystallites reorganize in a disordered manner.The ferromagnetic nature of the pristine film gets weakened because of iodine species and favors antiferromagnetic coupling.The study of electrical properties at room temperature by conducting atomic force microscopy reveals that the conductance isenhanced independently of the film thickness because of the disorder induced by iodine inclusion