Structural and magnetic properties of sol-gel Co2xNi0.5-xZn0.5-xFe2O4 thin films

Nanocrystalline Co2xNi0.5-xZn0.5-xFe2O4 (x = 0-0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform with out microcracks . The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size in dependent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe2O4. The maximum absorption band shifted from 13 to 11GHz as cobalt content increased from x = 0.1 to 0.2.

Fabrication of cerium oxide nanoparticles: Characterization and optical properties

Ceria (CeO2) is a technologically important rare earth material because of its unique properties and various engineering and biological applications. A facile and rapid method has been developed to prepare ceria nanoparticles using microwave with the average size 7 nm in the presence of a set of ionic liquids based on the bis (trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The structural features and optical properties of the nanoparticles were determined in depth with X-ray powder diffraction, transmission electron microscope, N-2 adsorption-desorption technique, dynamic light scattering (DLS) analysis, FTIR spectroscopy, Raman spectroscopy, UV-vis absorption spectroscopy, and Diffuse reflectance spectroscopy. The energy band gap measurements of nanoparticles of ceria have been carried out by UV-visible absorption spectroscopy and diffuse reflectance spectroscopy. The surface charge properties of colloidal ceria dispersions in ethylene glycol have been also studied. To the best of our knowledge, this is the first report on using this type of ionic liquids in ceria nanoparticle synthesis. (C) 2011 Elsevier Inc. All rights reserved.

Epitaxial strain and electric boundary condition effects on the structural and ferroelectric properties of BiFeO3 films

The influence of both compressive and tensile epitaxial strain along with the electrical boundary conditions on the ferroelastic and ferroelectric domain patterns of bismuth ferrite films was studied. BiFeO3 films were grown on SrTiO3(001), DyScO3(110), GdScO3(110), and SmScO3(110) substrates to investigate the effect of room temperature in-plane strain ranging from -1.4% to +0.75%. Piezoresponse force microscopy, transmission electron microscopy, x-ray diffraction measurements, and ferroelectric polarization measurements were performed to study the properties of the films. We show that BiFeO3 films with and without SrRuO3 bottom electrode have different growth mechanisms and that in both cases reduction of the domain variants is possible. Without SrRuO3, stripe domains with reduced variants are formed on all rare earth scandate substrates because of their monoclinic symmetry. In addition, tensile strained films exhibit a rotation of the unit cell with increasing film thickness. On the other side, the presence of SrRuO3 promotes step flow growth of BiFeO3. In case of vicinal SrTiO3 and DyScO3 substrates with high quality SrRuO3 bottom electrode and a low miscut angle of approximate to 0.15 degrees we observed suppression of the formation of certain domain variants. The quite large in-plane misfit of SrRuO3 with GdScO3 and SmScO3 prevents the growth of high quality SrRuO3 films and subsequent domain variants reduction in BiFeO3 on these substrates, when SrRuO3 is used as a bottom electrode.

Correlation Between the Electrical and Optical Properties of an Atmospheric Pressure Plasma During Siloxane Coating Deposition

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.

Electronic, structural, and reactive properties of ultrathin aluminum oxide films on Pt(111)

Low-energy electron diffraction, X-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction spectrometry results are reported for the structural and reactive behavior of alumina films grown on Pt(111) as a function of thickness and oxidation temperature. Submonolayer Al films undergo compete oxidation at 300 K, annealing at 1100 K resulting in formation of somewhat distorted crystalline gamma-alumina, Thicker deposits require 800 K oxidation to produce Al2O3, and these too undergo crystallization at 800 K, yielding islands of apparently undistorted gamma-alumina on the Pt(111) surface. Oxidation of a p(2 x 2) Pt3Al surface alloy occurs only at>800 K, resulting in Al extraction, These alumina films on Pt(lll) markedly increase the coverage of adsorbed SO4 resulting from SO2 chemisorption onto oxygen-precovered surfaces. This results in enhanced propane uptake and subsequent reactivity relative to SO4/Pt(111). A bifunctional mechanism is proposed to account for our observations, and the relevance of these to an understanding of the corresponding dispersed systems is discussed.

Structural and electronic properties of Ce overlayers and low-dimensional Pt-Ce alloys on Pt{111}

The structural, thermal, chemisorptive, and electronic properties of Ce on Pt{111} are studied by photoemission, Auger spectroscopy, scanning tunnel microscope (STM), and low-energy electron diffraction (LEED). Stranski-Krastanov-like growth of low-density Ce layers is accompanied by substantial valence charge transfer from Ce to Pt: in line with this, the measured dipole moment and polarizability of adsorbed Ce at low coverages are 7.2 x 10(-30) C m and similar to 1.3x10(-29) m(3), respectively. Pt-Ce intermixing commences at similar to 400 K and with increasing temperature a sequence of five different ordered surface alloys evolves. The symmetry, periodicities, and rotational epitaxy observed by LEED are in good accord with the STM data which reveal the true complexity of the system. The Various bimetallic surface phases are based on growth of crystalline Pt5Ce, a hexagonal layer structure consisting of alternating layers of Pt2Ce and Kagome nets of Pt atoms. This characteristic ABAB layered arrangement of the surface alloys is clearly imaged, and chemisorption data permit a distinction to be made between the more reactive Pt2Ce layer and the less reactive Pt Kagome net. Either type of layer can appear at the surface as the terminating structure, thicker films exhibiting unit mesh parameters characteristic of the bulk alloy.

Structural and magnetic properties of Ni1-xZnxFe2O4 (x=0, 0.5 and 1) nanopowders prepared by sol-gel method

A series of nanostructured Ni-Zn ferrites Ni1-xZnxFe2O4 (x=0, 0.5 and 1) with a grain size from 24 to 65 nm have been prepared with a sol-gel method. The effect of composition and sintering temperature on morphology, magnetic properties, Curie temperature, specific heating rate at 295 kHz and hysteresis loss have been studied. The highest coercivity of 50 and 40 Oe, were obtained for NiFe2O4 and Ni0.5Zn0.5Fe2O4 samples with the grain size of 35 and 29 nm, respectively. The coercivity of Ni and Ni-Zn mixed ferrites decreased with temperature. The Bloch exponent was 1.5 for all samples. As the grain size increased, the Curie temperature of NiFe2O4 increased from 849 to 859 K. The highest saturation magnetization of 70 emu/g at 298 K and the highest specific heating rate of 1.6 K/s under radiofrequency heating at 295 kHz were observed over NiFe2O4 calcined at 1073 K. Both the magnitude of the hysteresis loss and the temperature dependence of the loss are influenced by the sintering temperature and composition.

Fabrication and optical properties of gold nanotube arrays

Arrays of gold nanotubes with polypyrrole cores were grown on glass substrates by electrodeposition into thin film porous alumina templates. Measurements of optical transmission revealed strong extinction peaks related to plasmonic resonances, which were sensitive to the polarization state and angle of incidence. On prolonging the electrodeposition of gold, the polypyrrole core became fully encapsulated and this had a dramatic effect on the optical properties of the arrays, which was rationalized by finite element simulation of the local field intensities resulting from plasmon excitation.

Fabrication and optical properties of large-scale arrays of gold nanocavities based on rod-in-a-tube coaxials

Centimeter sized arrays of gold coaxial rod-in-a tube cavities have been fabricated using anodized aluminum oxide as a template. The etching process used to create the cavities enables the production of extremely small gaps between tube and rod, on the order of 5 nm, smaller than those created by standard fabrication techniques. Normal incidence spectroscopy reveals two extinction peaks in the visible and near infrared wavelength range associated with resonant plasmonic modes excited in the structure. Numerical simulations show that the modes are associated with in-phase and out-of-phase hybridization of transverse dipolar excitations in the nanorod and in the tube.

Sonochemical synthesis and measurement of optical properties of zinc sulfide quantum dots

A facile sonochemical method has been developed to prepare very small zinc sulfide nanoparticles (ZnS NPs) of extremely small size about 1. nm in diameter using a set of ionic liquids based on the bis (trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The structural features and optical properties of the NPs were determined in depth with X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS) analysis, and UV-vis absorption spectroscopy. The energy band gap measurements of ZnS NPs were calculated by UV-vis absorption spectroscopy. One of the interesting features of the present work is that the wide band gap semiconductor ZnS nanocrystals were prepared which are used in the fabrication of photonic devices.

Real-time approach to the optical properties of solids and nanostructures: Time-dependent Bethe-Salpeter equation

Many-body effects are known to play a crucial role in the electronic and optical properties of solids and nanostructures. Nevertheless, the majority of theoretical and numerical approaches able to capture the influence of Coulomb correlations are restricted to the linear response regime. In this work, we introduce an approach based on a real-time solution of the electronic dynamics. The proposed approach reduces to the well-known Bethe-Salpeter equation in the linear limit regime and it makes it possible, at the same time, to investigate correlation effects in nonlinear phenomena. We show the flexibility and numerical stability of the proposed approach by calculating the dielectric constants and the effect of a strong pulse excitation in bulk h-BN.

Structural and Electrical Investigation of C 60 –Graphene Vertical Heterostructures

Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60-graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 10(3). Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.

Experimental characterization of the nonlinear optical and magneto-optical properties of interfaces

An effective ellipsometric technique to determine parameters that characterize second-harmonic optical and magneto-optical effects in centrosymmetric media within the electric-dipole approximation is proposed and outlined in detail. The parameters, which are ratios of components of the nonlinear-surface-susceptibility tensors, are obtained from experimental data related to the state of polarization of the second-harmonic-generated radiation as a function of the angle between the plane of incidence and the polarization plane of the incident, linearly polarized, fundamental radiation. Experimental details of the technique are described. A corresponding theoretical model is given as an example for a single isotropic surface assuming polycrystalline samples. The surfaces of air-Au and air-Ni (in magnetized and demagnetized states) have been investigated ex situ in ambient air, and the results are presented. A nonlinear, least-squares-minimization fitting procedure between experimental data and theoretical formulas has been shown to yield realistic, unambiguous results for the ratios corresponding to each of the above materials. Independent methods for verifying the validity of the fitting parameters are also presented. The influence of temporal variations at the surfaces on the state of polarization (due to adsorption, contamination, or oxidation) is also illustrated for the demagnetized air-Ni surface. (C) 2005 Optical Society of America

Time-resolved optical spectroscopy of the pulsating DA white dwarf HS0507+0434B - New constraints on mode identification and pulsation properties

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, l, of the modes, most of which have l = 1. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of 2.6 +/- 1.0 kms(-1), which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673-1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni-Zn ferrite films was measured in the frequency range of 2-15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315-355 K was observed in the film close to the critical grain size.