Abstract: There has been a rapidly increasing interest in the synthesis and characterization of Si- nanostructures embedded in a dielectric matrix, as it can lead to energy-efficient and cost-effective Complementary Metal-Oxide-Semiconductor (CMOS)-compatible Si-based light sources for optoelectronic integration. In the present contribution, first an overview of the SiOx as a dielectric matrix and its limitations are discussed. We then review the literature on hydrogenated amorphous silicon nitride (a-SiNx:H) as a dielectric matrix for Si-nanostructures, which have been carried out using silane (SiH4) and ammonia (NH3) as the reactant gases. Our studies demonstrate that the least amount of hydrogen in the as-deposited (ASD) a-SiNx:H films not only allows in-situ formation of Si-nanostructures but also stabilizes silicon nitride (Si3N4) phase. The recent advances made in controlling the shape and size of Si-nanostructures embedded in a-SiNx:H matrix by swift heavy ion (SHI) irradiation are briefly discussed.
Abstract: ZnO doped with a few per cent of magnetic ions such as Ni, Fe, Co exhibits room temperature ferromagnetism (RTFM), transforming it into a very promising candidate for future spintronic applications. Two samples i.e. ZnO doped with Ni and Cr (5% each) have been investigated in the present work. The samples were characterized by Rietveld refinement of X-ray diffraction (XRD) patterns and the superconducting quantum interference device (SQUID) magnetometry. Rietveld analysis confirms that both the polycrystalline samples possess wurtzite structure with no evidence of any secondary phase. The SQUID measurements exhibit a diamagnetic state for the pristine ZnO and a paramagnetic state for the as-synthesized (Cr and Ni)-doped ZnO samples. However, the post annealing in H2 and vacuum drive them to a remarkable ferromagnetic state at room temperature. No element specific signature for ferromagnetism was seen. Then the X-ray photoelectron spectroscopic (XPS) measurements were performed to investigate their electronic structure and exploring the origin of ferromagnetism in these diluted magnetic semiconductor materials. The XPS results confirm the creation of oxygen vacancies upon Hydrogen/ vacuum annealing, owned to the (Ni/Cr) 3d¬−O 2p hybridization. The findings suggest oxygen vacancies as the intrinsic origin for ferromagnetism in doped ZnO. The important feature of this work is that the ferromagnetism and the consequent electronic property changes are found to be reversible with regard to re-heating the samples in air, showing a switch “on” and “off” ferromagnetic ordering in the ZnO matrix.
Abstract: Organic Light Emitting Diodes (OLED) are receiving increased attention due to tremendous application potential these devices hold in the areas of large area displays and lighting applications. However, the problems of efficiency, stability and shelf life are major challenges for making OLEDs an attractive alternative. The simple device structure involving anode, emissive layer and cathode is no longer the norm. Recently, various buffer layers like Hole Injection Layer (HIL), Hole transport Layer (HTL), Electron Injection Layer (EIL), Electron Transport Layer (ETL) etc. are being widely used as integral parts of the OLED architecture to enhance the performance parameters. The nomenclature of these layers is often confusing and sometimes used by different authors to mean different layers and a common and universal nomenclature for layers is still wanting. Applying a buffer layer, often called as the hole injecting layer (HIL) between anode and emissive layer is a general technique for increasing the efficiency and stability of organic light emitting diodes. Poly- (3,4-ethyhylene dioxythiophene): poly- (styrenesulphonate) (PEDOT:PSS) is a very common and popular such HIL used in OLEDs. In this chapter, a basic structure of OLEDs has been discussed in perspective with this HIL material and the effect of annealing this PEDOT: PSS layer on the characteristics of the device at different temperatures ranging from 100°C to 300°C in vacuum. Devices fabricated in clean room conditions are characterized for their electrical and optical properties. Equivalent circuits of the devices are deduced using impedance spectroscopy and discussed. Surface morphology of the HIL layers using atomic force microscopy (AFM) provides reasons for the variation of the device properties with the annealing of HIL.
Abstract: Amorphous alloys, in general, exhibit superior mechanical and chemical properties as compared to their crystalline counterparts, which is attributed to their chemical homogeneity and to the absence of crystal-like structural defects. Nanocrystalline and fully crystallized forms of these alloys can be easily obtained by a suitable thermal annealing treatment. It is important to have the knowledge of corrosion/oxidation behavior of amorphous and nanocrystalline alloys for various possible applications. In contrast to many investigations on corrosion and oxidation behavior of amorphous alloys reported in the literature, only limited studies have been carried out on comparison of corrosion/oxidation behavior of amorphous and nanocrystalline states of the same alloy. With this motivation potentiodynamic polarization studies were carried out on amorphous and nanocrystalline states of the alloy Ti60Ni40 in several aqueous media at room temperature. The oxidation in air was also investigated in the temperature range 2800C-3800C using a thermogravimetric analyzer. It was found from these investigations that nanocrystalline state exhibits the maximum corrosion/oxidation resistance in comparison to amorphous and crystalline states. The better corrosion/oxidation resistance of nanocrystalline state can be explained in terms of the nature of the nanocrystalline phase/phases and the size of the crystallites. The results of the present study are supported by other similar studies reported in the literature. A short review on comparison of corrosion/oxidation behaviour of amorphous and nanocrystalline Ti-based alloy is also presented in the paper.
Abstract: A detailed theoretical study of structural, electronic and vibrational properties of niobium carbide are carried out in rocksalt phase using the density functional theory implemented in ABINIT code. The calculated structural parameters like lattice constant and bulk modulus agree well with the available data. The Zener anisotropy factor (A), Poison's ratio (v), Young’s modulus (Y) and shear modulus (C’) are also presented. The electronic band structure and density of states are presented and discussed in light of bonding nature in NbC. The band structure indicates its metallic nature. The calculated phonon dispersion curves show that the NbC in rocksalt phase has all positive phonons throughout the Brillouin zone. The thermodynamical properties are also presented and discussed.
Abstract: Particle size has significant effect on the magnetic properties of fine particles. In this work, Cu0.2Ni0.8Fe2O4 nano-particles have been synthesized by the co-precipitation method. Different particle sizes were obtained by annealing the samples at various temperatures. The X-ray diffraction (XRD) patterns confirm the formation of cubic spinel structure. The particle size was found to enhance with increasing the annealing temperature. The saturation magnetization and the blocking temperature increase with particle size, which is a typical characteristic of the superparamagnetic behaviour. The dc magnetization measurements show that the samples are superparamagnetic above the blocking temperatures and the blocking temperature of the nanoparticles correlates with the size of the nanoparticles that is found to increase as the function of the particle size. The hysteresis curves show reduction in saturation magnetization in case of nanoparticles as compared to their bulk counterparts. This has been explained on the basis that the magnetic moments in the surface layers of a nanoparticle are in a state of frozen disorder. However, the saturation magnetization increases with particle size, which is a characteristic property of the single domain superparamagnetic particles.
Abstract: The aim of this article is to provide an overview on the effect of Co doping for Fe in Fe2P on the lattice parameters, magnetic transition point, saturation magnetic moment, paramagnetic Curie point and paramagnetic moment. Five alloys in the series (Fe1-xCox)2P (x= 0.03, 0.10, 0.30, 0.50, 0.70) were synthesized and investigated using X-ray diffraction, magnetization and paramagnetic susceptibility measurements. Alloys with x=0.03 and 0.10 crystallize in Fe2P type hexagonal structure while those for x= 0.30, 0.50, 0.70 crystallize in Co2P type orthorhombic structure. In the hexagonal phase, the substitution of Co for Fe leads to a very little enhancement in the unit cell volume but in the orthorhombic phase the increase in Co concentration leads to monotonic decrease in the unit cell volume. All the five alloys are ferromagnetically ordered and display nonlinearity in the -1-T curves above Tc. Interestingly, the compounds for x= 0.10, 0.30 and 0.50 shows a sharp first order type ferromagnetic to paramagnetic phase transition. The paramagnetic Curie temperature (θP) increases up to x=0.50. The saturation magnetizations and paramagnetic moments monotonically decrease with the increase in the Co concentration. Both Ni and Co substitution for Fe in Fe2P are known to strengthen the ferromagnetic ordering, however, the effect of Co substitution has stronger influence on the structural parameters and magnetic properties than the Ni. The observed nonlinearity in the -1-T curves above Tc and the Rhodes- Wohlfarth ratio (μP/μs) greater than one, gives a clear evidence of presence of large magnetic correlations far above Tc and itinerant magnetism in these compounds.
Abstract: The crystallization kinetics of metallic glass Ti50Cu20Ni30 has been studied using Differential Scanning Calorimetry (DSC). The DSC thermograms have been analysed using the model-free isoconversional methods and model dependent isokinetic methods. The activation energy(E) for the crystallization process has been determined utilizing; (i) various linear integral isoconversional methods, namely, Ozawa-Flynn-Wall, Kissinger-Akahira-Sunose, Li Tang method (ii) linear differential isoconversional method and (iii) different isokinetic methods. In the present work, we intend to the determination of true value of E. The above methods are found to give consistent results for E.
Abstract: Knowledge of glass forming ability (GFA) of amorphous metallic alloys is very important from both theoretical and practical point of view. Thermodynamically, the Gibbs free energy difference, ΔG between the undercooled liquid and the corresponding crystalline state is driving force for crystallization. As a consequence, it is a good indicator for glass forming ability of metallic glasses. A novel expression for ΔG has been used to estimate the GFA of recently developed Ca-based bulk metallic glasses viz. Ca53Mg23Cu24,Ca65Mg15Cu20,Ca40Mg25Cu35, Ca50Mg22.5Cu27.5 and Ca55Mg15Cu30. Different GFA criteria are also evaluated for systems taken up in the study and effect of addition of variation in composition of Ca-Mg-Cu system is also investigated. Present work suggests that among different GFA criteria, ΔG is the best criterion for the prediction of GFA for Ca-based bulk metallic glasses.