1,720,967 research outputs found
Diffraction line profiles of spherical hollow nanocrystals
No full textAn analytical expression of diffraction line profiles of spherical hollow nanocrystals (NCs) is derived. The particular features of the profile lines, enhanced peak tail intensity, are analyzed and discussed as a function of the NC size parameters (outer and inner radius, shell thickness). The explicit formula for the integral breadth, the Fourier particle size, and the Scherrer constants are also obtained and discussed in detail. The diffraction line profiles of hollow CdS NCs of zincblende and wurtzite crystallographic structure are calculated and compared with Debye scattering profiles. The diffraction profiles of both approaches exhibit an enhanced peak tail intensity that can be considered as a fingerprint of the hollow NC structure
Surface states and electronic properties for small Cadmium Sulfide nanocluster
No full textIn this work total and partial density of states were calculated for small single wurtzite CdS nanoparticle, quantum dots, with diameter below 2 nm, performing DFT pseudopotentials calculations with generalized gradient approximation. The cluster was previously heated at different temperatures ranging from 100 to 600 K-, by means of Car-Parrinello ab-initio molecular dynamics, finding a stable phase after 340 K. Another stable phase is reached at higher temperature, more than 480 K, where surface migrations play an important role in the determination of final atomic configuration. Density of states was obtained for the structures heated at 100 K, 280 K 330 K, 340 K, 480 K and 570 K and substantial modifications were found after 340 K where the superficial effects cause some alterations of the intermediate electronic states around the band gap. From the partial density of states it has been clearly showed that the atoms on the surface have a main role on the formation of the intermediate states on the band gap. This effect is meaningful for small particles where the fraction of the surface atoms is greater than the fraction of the core atoms.Copyright © 2013 American Scientific Publishers
Optical investigation of degradation of graphene oxide in alkaline environment: Evidence of two distinct photon-emitting phases in visible region
Full text linkIn this work we show a procedure of treating of the graphene oxide in alkaline environment as a function of the treatment time in order to obtain novel structures with strong luminescence properties, water-stable, useful as potential replacement for critical raw materials employed as example in optical and optoelectronic devices or for diagnostic and therapeutic technology. These structures have distinct blue and green-luminescence properties which derived most likely from different structural conformations, one associable with that of carbon quantum dots (or as an alternative to that of the Oxidative Debris), the other, lighter and more similar to organic compounds, reported in literature as fulvic-like molecules, but whose nature has to be further investigated. We show that the lighter fraction has a dual mechanism of photoemission: the excitation-independent PL for excitation wavelength within 350 nm and the excitation-dependent component for excitation wavelength ranging in the visible spectrum. The PL dual behaviour could depend on fluorescent nanoclusters composed by specific organic fluorophores with a carbonaceous core. FTIR analysis shows reasonably the same functional groups unless of some difference discussed in the text, meanwhile UV–Vis and PL analysis clearly highlight two distinct emissions (450 nm and 530 nm) in the visible region of the electromagnetic spectrum. Excitation-dependent photoluminescence, water stability and organic fluorescent nanostructures are issues particularly required for application in the biological field but also in materials science
New approach in Auger elemental relative sensitive factor calculation by using TEM-EDS analysis based on bi-layers of pure elements
No full textIn fabrication of microelectronic devices two important steps are often recognized: i) all the processes performed on the wafer in order to build the active part of the devices and, ii) the assembly and packaging processes, typically performed on a chip, in order to fabricate interconnections between active part and exterior. The wafer back side is an active part of power devices and is normally coated with a stack of Ti-Ni-Au or Ti-Ni-Ag layers to ensure the best electrical contact with the frame on which the device is attached prior to the packaging. An important failure mechanism related to this particular process step is related to the diffusion of Ni to the surface of the stack that causes its oxidation on the back metal surface, inhibiting the correct connection to the metallic frame. Auger Electron Spectroscopy (AES) is a powerful analytical technique that can be used to detect this failure mechanism for its very high sensitivity in the characterization of surface layers. Unfortunately, its results are mainly qualitative. Quantitative extrapolations can be inaccurate using library Elemental Relative Sensitive Factor (ERSF) because they are mainly referred to a silicon substrate and could be not valid for a different matrix. A most accurate evaluation of the ERSF is based on the analysis, under identical experimental condition, of standard materials (with known concentration) that should be similar to the unknown sample and having the same matrix. However, the production of this kind of standard is not easy due to the mobility of Ni in Au and Ag. Another commonly used technique is the Energy Dispersive X-ray Spectrometry (EDS) which is less sensitive than the Auger and not sufficiently adequate for a quantitative analysis due to the limitation of the matrix correction methods. Recently, a new method to perform quantitative analysis by using Transmission Electron Microscopy (TEM) EDS was proposed, starting from bi-layers of pure elements. In this work we show how the use of TEM-EDS quantification of Ni in Ag could be a successful method for ERSF evaluation in order to overcome matrix effect in Auger quantification. For this purpose suitable foils of Ag/Al and Ni/Al were used. The validation of the method was performed on a sample with a tri-metal stack of Ti/Ni/Ag previously stimulated by means of a thermal budget to induce Ni migration on Ag surface. The quantitative analysis allowed us to use this characterized sample as AES standard for ERSF calculation. © 201
Mechanical properties and oxidation behavior of silicon carbide-molybdenum silicides composites
No full textSiCMoSi2 porous preforms with different SiC/MoSi2 weight ratios were densified by means of the melt infiltration method. Mixture of SiC-MoSi2Mo≤5Si3C≤1 was used as infiltrant. The resultant infiltrated composites showed high density, good mechanical properties and oxidation resistance. In particular, fracture toughness determined at 1773 K was 6.80 MPa m1/2 and 6.28 MPa m 1/2 with SMI-80 (preform with SiC/MoSi2 weight ratio 80/20) and SMI-50 (preform with SiC/MoSi2 weight ratio 50/50) composites, respectively. The same composites showed high temperature (1773 K) flexural strength values: 214 MPa (SMI-80) and 242 MPa (SMI-50). Long term oxidation behavior was also tested at 1773 K and results confirmed the refractoriness of these materials. © 2012 Elsevier Ltd and Techna Group S.r.l
Lifetime estimation of a zirconia-alumina composite for biomedical applications
No full textObjectives In this work long term stability of a zirconia toughened alumina (ZTA) composite was investigated. Methods Accelerated aging tests under hydrothermal environment, in autoclave and hot water, at different temperature, was conducted on material sample. Tetragonal to monoclinic transformation was evaluated by XRD analysis and the monoclinic content was plot as a function of the exposure time. The kinetic of transformation was studied by means Mehl-Avrami-Johnson (MAJ) nucleation and growth model. Results An activation energy for tetragonal to monoclinic transformation of 99 kJ/mol was found by the Arrhenius plot of reaction rate, value in agreement with other bibliography works regarding Y-TZP and alumina-zirconia composites. The in vivo hydrothermal stability simulation, estimated by the obtained activation energy, predicts in 65 years the time necessary to reach 25 vol% of monoclinic phase. Significance These results support the material suitability in biomedical field, especially in dentistry applications as implantology. © 2013 Academy of Dental Materials
Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE
Full text linkThe fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs1−xPx/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80–150 nm) GaAs1−xPx epilayers appear partially relaxed (within 1−12% of the initial misfit) through a network of misfit dislocations along the sample (Formula presented.) and (Formula presented.) in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews–Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs1−xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy EA = 1.41 ± 0.04 eV over the entire alloy compositional range
Pressureless sintered silicon carbide with enhanced mechanical properties obtained by the two-step sintering method
No full textThe Two-step sintering (TSS) method was applied to the pressureless sintering of commercial silicon carbide powder doped with boron and carbon. The microstructural and mechanical properties of TSS-SiC were compared to those of sintered SiC obtained with the conventional thermal cycle (CS-SiC). TSS-SiC was densified (97.7% T.D.) at 2050 C instead of 2200 C needed for CS-SiC (97% T.D.). Furthermore, TSS-SiC showed finer microstructure and enhanced mechanical properties. In particular, flexural strength of the TSS-SiC materials greatly increased up to 556 MPa, much higher than 341 MPa reached by CS-SiC. © 2013 Elsevier Ltd and Techna Group S.r.l
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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