Effect of the size factor on the magnetic properties of manganite La0.50Ba0.50MnO3

Nanocrystalline manganite La_0.50Ba_0.50MnO₃ was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ～30 nm to ～7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ～3.787 to ～3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ～30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T _C from ～264 to ～331 K and in the spontaneous magnetic moment σ _S from ～1.52 to ～3.31 μ_B/f.u. The anomalous behavior of the magnetic properties of the manganite La_0.50Ba_0.50MnO₃ obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn³⁺-O-Mn⁴⁺ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.     

Studies on luminescence properties and energy transfer in Ce/Dy co-doped CaS nano-phosphors

Luminescence properties of CaS:Ce co-doped with dysprosium has been studied. Ce/Dy co-doped CaS nanophosphors (CaS:Ce_0.25Dy_0.75, CaS:Ce_0.50Dy_0.50, CaS:Ce_0.75Dy_0.25) were synthesized using the solid state diffusion method. The phase purity of the samples was confirmed using XRD data. The particle size was calculated using Debye–Scherrer formula and was found to be varying between 50 and 60 nm for all the three samples (CaS:Ce_0.25Dy_0.75, CaS:Ce_0.50Dy_0.50 and CaS:Ce_0.75Dy_0.25). TEM image analysis of CaS:Ce_0.50Dy_0.50 shows nearly spherical particles with diameter varying between 50 and60 nm. One way energy transfer from Dy³⁺ to Ce³⁺ in CaS host has been investigated using photoluminescence studies. Thermoluminescence of these nanophosphors has been studied for 0.5 Gy–21 kGy dose of gamma rays and the dose linearity of CaS:Ce_0.50Dy_0.50 has been compared with CaSO₄:Dy (standard TL dosimeter). Linear behavior over a large dose range between 0.5 Gy and 21 kGy was found for CaS:Ce_0.50Dy_0.50 as compared to CaSO₄:Dy (nanocrystalline and microcrystalline) but it is found to be less sensitive than microcrystalline CaSO₄:Dy. To identify the peaks of Ce³⁺ and Dy³⁺ in CaS, the TL spectra of CaS, CaS:Ce, CaS:Dy and CaS:Ce_0.50Dy_0.50 were recorded. The addition of dopants does not add new peaks in CaS but aid to enhance the TL emission. The peaks in CaS may be associated to intrinsic traps in the host lattice.     

In situ TEM study of stability of TaRhx diffusion barriers using a novel sample preparation method

The atomic diffusion mechanisms associated with metallurgical failure of TaRh_x diffusion barriers for Cu metallizations were studied by in situ transmission electron microscopy (TEM). The issues related to in situ heating of focused ion beam (FIB) prepared cross-sectional TEM samples that contain Cu thin films are discussed. The Cu layer in Si/(13 nm)TaRh_x/Cu stacks showed grain growth and formation of voids at temperatures exceeding 550 °C. For Si/(43 nm)TaRh_x/Cu stacks, grain growth of Cu was delayed to higher temperatures, i.e., 700 °C, and void formation was not observed. Extensive surface diffusion of Cu, however, preceded bulk diffusion. Therefore, a 10 nm film of electron beam evaporated C was deposited on both sides of the TEM lamellae to limit surface diffusion. This processing technique allowed for direct observation of atomic diffusion and reaction mechanisms across the TaRh_x interface. Failure occurred by nucleation of orthorhombic RhSi particles at the Si/TaRh_x interface. Subsequently, the barrier at areas adjacent to RhSi particles was depleted in Rh. This created lower density areas in the barrier, which facilitated diffusion of Cu to the Si substrate to form Cu₃Si. The morphology of an in situ annealed lamella was compared with an ex situ bulk annealed sample, which showed similar reaction morphology. The sample preparation method developed in this study successfully prevented surface diffusion/delamination of the Cu layer and can be employed to understand the metallurgical failure of other potential diffusion barriers.     

Redox reactions and inward cationic diffusion in glasses caused by CO and H2 gases

We find that inward diffusion of network-modifying cations can occur in an iron-containing silicate glass when it is heat-treated in CO/CO₂ (98/2 v/v) or H₂/N₂ (1/99 v/v) gases at temperatures around the glass transition temperature. The inward diffusion is caused by the reduction of ferric to ferrous ions and this diffusion leads to formation of a silica-rich surface layer with a thickness of 200–600 nm. The diffusion coefficients of the network-modifying divalent cations are calculated and they are different for the glasses treated in the CO and H₂ gases. At the applied partial pressures of CO and H₂, the H₂-bearing gas creates the silica-rich layer more effectively than the CO-bearing gas. The layer increases the hardness and chemical durability of the glass due to the silica network structure in the surface layer.     

Scaled nano-and microrelief of ordering regions in Al x Ga1 − x As/GaAs(100) epitaxial heterostructures

The Al _x Ga_{1 ? x} As/GaAs(100) heterostructures grown by MOS hydride epitaxy were studied using atomic force and scanning electron microscopy. Regions with an ordered nanorelief with a period of approximately 115 nm were discovered on the surface of the sample with x ～ 0.50. An AlGaAs₂ superstructural phase appears in these regions.     

Nanostructures of sodium titanate/zirconium oxide

In this work is reported the synthesis of nanotubes and nanoribbons from mixed oxides (Ti_1−x Zr _x O₂·nH₂O), employing hydrothermal treatment in a highly alkaline medium. The morphology and crystal structure of the products obtained via hydrothermal treatment depend on the value of x. For example, for x equal to 0 and 0.50 were observed the presence of nanotubes (diameter around 9 nm) and nanoribbons (diameter around 200 nm), respectively. However, for x values above 0.50, there was no morphological change. Regarding the crystalline structure of these samples, for x equal to 0 was observed the sodium titanate phase; already for x values up to 0.50, we observed the presence of two crystalline phases: sodium titanate and tetragonal ZrO₂. For x values above 0.50, only tetragonal ZrO₂ was observed. Furthermore, only the product obtained from x equal to 0.15 was observed the presence of three-dimensional flower-like arrangements. The results obtained by the characterization techniques showed the segregation of zirconium after hydrothermal treatment of precursors with x less or equal to 0.50. Thus, we describe the important role that Ti/Zr molar ratio of the precursor plays on the morphology and crystalline phase of the products formed by hydrothermal treatment.     

X-ray spectral analysis of the interface of a thin Al<Subscript>2</Subscript>O<Subscript>3</Subscript> film prepared on silicon by atomic layer deposition

The extent and phase chemical composition of the interface forming under atomic layer deposition (ALD) of a 6-nm-thick Al₂O₃ film on the surface of crystalline silicon (c-Si) has been studied by depthresolved, ultrasoft x-ray emission spectroscopy. ALD is shown to produce a layer of mixed Al₂O₃ and SiO₂ oxides about 6–8 nm thick, in which silicon dioxide is present even on the sample surface and its concentration increases as one approaches the interface with the substrate. It is assumed that such a complex structure of the layer is the result of interdiffusion of oxygen into the layer and of silicon from the substrate to the surface over grain boundaries of polycrystalline Al₂O₃, followed by silicon oxidation. Neither the formation of clusters of metallic aluminum near the boundary with c-Si nor aluminum diffusion into the substrate was revealed. It was established that ALD-deposited Al₂O₃ layers with a thickness up to 60 nm have similar structure.     

Structural investigations of synthetic ferrihydrite nanoparticles doped with Si

X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoacoustic/Fourier transform infrared spectroscopy (PA/FTIR) are employed to monitor the changes in the structural aspects of two-line ferrihydrite (FHYD) nanoparticles doped with silica viz. Si_xFe_1−xOOH·nH₂O for x=0, 0.02, 0.05, 0.10, 0.15, 0.20, 0.26, 0.40 and 0.50. In XRD, crystallinity decreases and d-spacings increase with increase in x. TEM studies show that the particle size increases systematically with increase in x, from 3.7 nm for x=0 to 5.7 nm for x=0.50. In PA/FTIR, two new bands appear, band F near 3700 cm⁻¹ identified with the surface Si-O-H group and band A near 900 cm⁻¹ identified with Si-O-Fe group, which shifts to higher wavenumbers with increase in x. These results are used to propose a model in which doped Si⁴⁺ ions do not displace Fe³⁺ ions but are chemisorbed on the FHYD surface making a shell of silica for higher doping. This model is consistent with the reported changes in the magnetic properties of FHYD with Si doping.     

Bi2MoO6 nanofilms on the stainless steel mesh by PS-PED method: Photocatalytic degradation of diclofenac sodium as a pharmaceutical pollutant

For the first time, Bi₂MoO₆ nanofilms were successfully synthesized by simultaneous pulse sonication-pulse electrodeposition (PS-PED) on the stainless steel mesh surface. Bismuth molybdate films were formed under various combinations of electrodeposition and sonication (sono-electrodeposition) in continuous and pulse modes. Porous Bi₂MoO₆ films synthesized by PS-PED method and showed the highest efficiency in photocatalytic degradation in comparison with other films. Bi₂MoO₆ film obtained from PS-PED had a thickness of 13.78 nm while, the thickness for the electrodeposition method was 39.52 nm. The high photocatalytic efficiency is attributed to the high surface roughness and low thickness of film synthesized by PS-PED method. Indeed, ultrasound played a key role in the synthesis of films with high surface roughness. On the other hand, shock waves and micro-jets could be dissolved diffusion problems and reduced the dendrite like structures in deposition process. Simultaneous application of pulse modes for both combined methods led to more growth of crystallographic planes. This is due to reaction of ions on the surface in interval relaxation times and produce more nuclei for growth. In order to obtain a high efficiency, response surface methodology was used for optimization of effective variable parameters (t_on, t_off and sonication amplitude) in film preparation.     

Carbon precipitation and diffusion in SiGeC alloys under silicon self-interstitial injection

In this work we investigate the diffusion and precipitation of supersaturated substitutional carbon in 200-nm-thick SiGeC layers buried under a silicon cap layer of 40 nm. The samples were annealed in either inert (N₂) or oxidizing (O₂) ambient at 850 °C for times ranging from 2 to 10 h. The silicon self-interstitial (I) flux coming from the surface under oxidation enhances the C diffusion with respect to the N₂-annealed samples. In the early stages of the oxidation process, the loss of C from the SiGeC layer by diffusion across the layer/cap interface dominates. This phenomenon saturates after an initial period (2–4 h), which depends on the C concentration. This saturation is due to the formation and growth of C-containing precipitates that are promoted by the I injection and act as a sink for mobile C atoms. The influence of carbon concentration on the competition between precipitation and diffusion is discussed. Received: 19 October 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002     

Microscopic observation of lateral diffusion at Si-SiO2 interface by photoelectron emission microscopy using synchrotron radiation

The lateral surface diffusion at Si-SiO₂ interface has been observed at nanometer scale using photoelectron emission microscopy (PEEM) combined with synchrotron soft X-ray excitation. The samples investigated were Si-SiO_x micro-patterns prepared by O₂⁺ ion implantation in Si (0 0 1) wafer using a mask. The lateral spacial resolution of the PEEM system was about 41 nm. The brightness of each spot in the PEEM images changed depending on the photon energy around the Si K-edge, in proportion to the X-ray absorption intensity of the corresponding valence states. It was found that the lateral diffusion occurs by 400-450 °C lower temperature than that reported for the longitudinal diffusion at the Si-SiO₂ interface. It was also found that no intermediate valence states such as SiO (Si²⁺) exist at the Si-SiO₂ interface during the diffusion. The observed differences between lateral and longitudinal diffusion are interpreted by the sublimated property of silicon monoxide (SiO).     

Electrode reaction and microstructure of La0.6Sr0.4CoO3−δ thin films

Dense La_0.6Sr_0.4CoO_3−δ film electrodes were deposited by pulsed laser deposition (PLD) on Ce_0.9Gd_0.1O_1.95 electrolytes. The grain size of one film was 300–500 nm, and the other was 30–50 nm. DC polarization and AC impedance measurements were performed at 873 K–1073 K in O₂–Ar gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion.     

Atomic force microscopy study of growth kinetics: Scaling in TiN-TiB2 nanocomposite films on Si(1 0 0)

We used the reactive unbalanced close-field dc-magnetron sputtering growth of TiN-TiB₂ on Si(1 0 0) at room temperature to determine if scaling theory provides insight into the kinetic mechanisms of two-phase nanocomposite thin films. Scaling analyses along with height-difference correlation functions of measured atomic force microscopy (AFM) images have shown that the TiN-TiB₂ nanocomposite films with thickness ranging from 70 to 950 nm exhibit a kinetic surface roughening with the roughness increasing with thickness exponentially. The roughness exponent α and growth exponent β are determined to be ∼0.93 and ∼0.25, respectively. The value of dynamic exponent z, calculated by measurement of the lateral correlation length ξ, is ∼3.70, agreeing well with the ratio of α to β. These results indicate that the surface growth behavior of sputter-deposited TiN-TiB₂ thin films follows the classical Family-Vicseck scaling and can be reasonably described by the noisy Mullins diffusion model, at which surface diffusion serves as the smoothing effect and shot noise as the roughening mechanism.     

Barrier-height non-uniformities of PtSi/Si(111) Schottky diodes

The forward current-voltage characteristics of PtSi Schottky contacts on epitaxial n-type Si (111) is studied in the temperature range from 100 to 300 K. A current contribution in excess to that predicted by thermionic emission diffusion theory is caused by a few thousand patches of reduced Schottky barrier height in a device area of 3.14 mm². The typical lateral extent of these patches is 70–250 nm. It correlates with the size of surface bumps observed. The number of patches is reduced upon increasing the silicidation temperature up toT _siI=550°C. Possible non-uniformities of the typical crystallite grain size of 20 nm are not resolved due to effective pinch-off.     

Effect of the granule size in porous silicon on the photosensitization efficiency of molecular oxygen on the surface of silicon nanocrystals

Photoluminescence is used to study the effect of the granule size in porous silicon on the generation efficiency of the excited state of molecular oxygen (¹O₂) on the surface of silicon nanocrystals. The generation efficiency is found to increase as the granule size becomes smaller than 100 nm, which can be explained by a change in the conditions of exciton diffusion along a network of silicon nanocrystals.     

Mg diffusion in K(Ta0.65Nb0.35)O3 thin films grown on MgO evidenced by Auger electron spectroscopy investigation

The diffusion of Mg in pulsed laser deposited K(Ta_0.65Nb_0.35)O₃ thin films epitaxially grown on (1 0 0) MgO single crystal substrate were investigated by Auger electron spectroscopy (AES). A diffusion of Mg from the substrate into the whole thickness (400 nm) of the as-deposited K(Ta_0.65Nb_0.35)O₃ films was observed with an accumulation of Mg at the surface. Ex situ post-annealing (750 °C/2 h) has led to a homogeneous distribution of Mg in all the ferroelectric coating. This strong reaction between film and substrate promotes a doping effect, responsible for the reduction of K(Ta_0.65Nb_0.35)O₃ dielectric losses in comparison with films grown on other substrates.     

Critical dimensions for YBCO islands grown by pulsed laser deposition

Islands of constant width at half maximum of approximately 100 nm have been observed during the pulsed laser deposition of films of nominal thickness from 0.7 to 3.0 nm of the material YBa₂Cu₃O_7-δ(YBCO) on substrates of SrTiO₃. The critical island dimensions of width, height and spacing were analyzed with classical kinetic and thermodynamic theories. Analytically it was calculated that the equilibrium island width for a 29-nm-high island should be 111 nm. The analysis also predicted that islands of smaller height should be wider, and higher islands should be narrower. Islands of height from 3.5 nm to 29 nm were observed with an atomic force microscope to have a constant width at half maximum of approximately 100 nm. There are several possible differences between experiment and analysis that could explain the difference in the results: the islands formed in a non-equilibrium phase of YBCO, the island strain relaxed from the base to the top, and smaller islands may not have reached their equilibrium width. Larger islands had significant roughening at the top. Calculations predict that these islands would be unstable with respect to surface perturbations. Calculations of relaxation strain energy and surface energy showed that there was excess strain energy available for island heights above 9 nm to provide the extra surface energy that would be necessary for surface perturbations to develop. The minimum observed interisland spacing of 36 nm agreed with calculations of the average atom diffusion length (40.6 nm). Received: 2 April 2001 / Accepted: 6 September 2001 / Published online: 20 December 2001     

The effects of vacuum annealing on the structure and surface chemistry of iron nanoparticles

In order to increase the longevity of contaminant retention, a method is sought to improve the corrosion resistance of iron nanoparticles (INP) used for remediation of contaminated water and thereby extend their industrial lifetime. A multi-disciplinary approach was used to investigate changes induced by vacuum annealing (<5 × 10^?8 mbar) at 500 °C on the bulk and surface chemistry of INP. The particle size did not change significantly as a result of annealing but the surface oxide thickness decreased from an average of 3–4 nm to 2 nm. BET analysis recorded a decrease in INP surface area from 19.0 to 4.8 m² g^?1, consistent with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations which indicated the diffusion bonding of previously discrete particles at points of contact. X-ray diffraction (XRD) confirmed that recrystallisation of the metallic cores had occurred, converting a significant fraction of poorly crystalline iron to bcc α-Fe and Fe₂B phases. X-ray photoelectron spectroscopy (XPS) indicated a change in the surface oxide stoichiometry from magnetite (Fe₃O₄) towards wüstite (FeO) and the migration of boron and carbon to the particle surfaces. The improved core crystallinity and the presence of passivating impurity phases at the INP surfaces may act to improve the corrosion resistance and reactive lifespan of the vacuum annealed INP for environmental applications.     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

Study on triplet exciton diffusion length of mCP in phosphorescent organic light-emitting devices using electroluminescent spectra

Electroluminescent (EL) spectra was employed to probe the triplet exciton diffusion length (L_T) of a commonly used host material of N,N′-dicarbazolyl-3,5-benzene (mCP) in phosphorescent organic light-emitting devices (OLEDs). By varying the film thickness of bis [2-(4-tertbutylphenyl) benzothiazolato-N,C²], iridium (acetylacetonate) [(t-bt)₂Ir(acac)] phosphor doped layer within 30 nm thick mCP layer, a series of devices were fabricated to investigate the EL characteristics. The results showed that with the increasing doped layer thickness (d), both (t-bt)₂Ir(acac) emission peaks at 562 nm and mCP emission centered at 403 nm were observed. Moreover, the relationship between mCP EL intensity and d was detected. The L_T was induced by an abrupt decrease in variation of mCP EL intensity when d is increased from 10 to 15 nm, and the reason to cause this phenomenon was investigated. The L_T of mCP approximately to 15 nm was perfectly consistent to the result of 16±1 nm, which was calculated by the traditional steady-state diffusion model.