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In this review, we discuss new developments and recent trends in both amorphous and crystalline Al2O3 oxide nanofluids related to their phase dependent characteristics in detail. Nowadays, nanofluids have gained significant attention with the enhanced energy/heat efficiency, which is highly desirable to improve the performance of any energy based devices and technology. This review article systematically describes the various chemical synthesis routes followed by bottom-up approaches, surface morphologies, and detailed microstructure characteristics, and phase dependent thermal as well as optical properties for potential use of such materials in various applications.

A facile method to produce conformal coated reduced graphene oxide (rGO) on vertically aligned titanium oxide (TiO2) nanotubes three dimensional (3D) arrays (NTAs) is demonstrated for enhanced field emission display applications. These engineered nano arrays exhibit efficient electron field emission properties such as high field emission current density (80 mA/cm2), low turn-on field (1.0 V/μm) and field enhancement factor (6000) with high emission current stability. Moreover, these enhancements observed in nano arrays attribute to the contribution of low work function with non-rectifying barriers, which allow an easy injection of electrons from the conduction band of TiO2 into the Fermi level of reduced graphene oxide under external electric field. The obtained results are extremely advantageous for its potential application in field emission devices.

A long persistent, transparent, flexible, and water resistant phosphorescent layer of sunlight-activated green luminescent Sr1-x-yAl2O4:Eux 2+/Dyy3+ (x = 0.05—0.3, y = 0.1—0.6) is fabricated via solid state reaction/solution casting of SrCO3, Al2O3, Eu2O3, and Dy2O3 (10% H2/N2 atmosphere, alumina crucible, 1300 °C, 3 h).

As the Bluetooth technology is growing there is a need of secure communication and to keep there data secret where Bluetooth is used as primary option. There are many attacks possible on Bluetooth technology and associated devices such as denial of service attack, passive eavesdropping and active eavesdropping, man-in-the-middle attacks, message modification, and resource misappropriation. So it is a challenge to detect the attacker. This paper proposed an idea for those Bluetooth enabled devices which uses Secure Simple Pairing (SSP) association models not having Strong security against passive eavesdropping. In this paper we proposed an idea which provides security against passive eavesdropping with the help of BlipTrack Bluetooth Detector (BBD) which is a sensor that detects the Bluetooth within the coverage area.

In this paper, we have carried out investigations on admixing carbon nanotubes (CNT) to increase the desorption rate of hydrogen in NaAlH 4 . So far only transition metal Ti has been attempted as the suitable dopant for making NaAlH 4 (purified version) a viable hydrogen storage material by increasing the hydrogen desorption rate. Out of the various materials corresponding to NaAlH 4 – x mol% CNT ( x = 2, 4, 6, 8 and 12), we have found that the material with x = 8 mol% is the optimum material. It shows highest desorption rate leading to 3.3 wt.% of H 2 at ∼160 °C within 2 h for the first dissociation reaction. The CNT admixed NaAlH 4 has also been found to exhibit good rehydrogenation characteristics (reversibility on hydrogenation up to ∼4.2 wt.%). A feasible mechanism for the improvement of hydrogen desorption characteristics has been put forward.

The nano-variants of carbon including graphitic nanofibres (GNF) have recently been considered to be exotic (light weight, high storage capacity) hydrogen storage materials. In the present paper, we report growth of aligned bundles of GNF. The length and width of cross-section of the bundles is ∼50 and ∼25 μm, respectively. The length of individual GNF is ∼50 μm and diameter ∼0.25 μm. The GNFs have been synthesized through thermal decomposition of acetylene using palladium (Pd) sheets as catalyst. This represents a new form of catalyst. The GNFs bundles grown by the present method are easier to hydrogenate. They adsorb hydrogen at a lower pressure of ∼80 atm as against ∼120 atm for the GNF grown in the earlier studies. The storage capacity obtained in the present investigation is ∼17 wt.%. Electron microscopic investigations reveal that as against the as grown GNF, the hydrogenated version embodies microstructures exhibiting fragmentation of graphitic layer bundles. The reasons for the growth of GNF in the form of aligned bundles, the ease of hydrogenation and relevance of GNF fragmentation after hydrogenation have been outlined.

The present paper deals with the synthesis of MmNi 5− x Fe x alloys ( x =0.05, 0.1, 0.3, 0.5). It has been found that Fe substitution, because of the higher electron attractive power of the Fe when substituted, leads to enhancement of the hydrogen storage capacity. Thus, the material MmNi 4.9 Fe 0.1 shows a storage capacity of ∼1.66 wt.%, which is one of the highest capacities for any as synthesised AB 5 type alloy. The P – C isotherms at various temperatures have been simulated by a suitable mathematical model. For this, the randomised variables Δ H and Δ S have been evaluated experimentally. These have been used as input parameters in the modelling. The simulated and experimental curves have been found to match closely. It is suggested that mathematical modelling is useful for predicting P – C – T behaviours when the hydride is used at different temperatures.

Hydrogen is a promising alternative energy vector, but its use at an appropriate site requires storage, which is a crucial aspect. Hydrogen storage (HS) in the form of metal hydrides represents an attractive possibility, and is being investigated worldwide. La(Ni0.95Fe0.05)5 (LNF) has achieved significant attention as a HS media due to its suitable thermodynamics. However, its use as an effective storage material is hindered due to burning of hydrogenated LNF (LNFH) on exposure to air. The pristine LNFH catches fire rapidly on exposure to atmosphere. Here, a breakthrough strategy is demonstrated for design of hydrogenated air-stabilized hybrid material by encapsulating LNF inside reduced graphene oxide-ethyl cellulose. This novel hybrid material does not ignite upon exposure to air. This proposed hybrid material could be the ultimate choice for air-stable and safe storage for fuel cell/internal combustion engine-based vehicles. Further, the effectiveness of this hydrogen storage material is demonstrated for fuel cells.

Herein, we have demonstrated the high yield facile growth of Al2O3 nanowires of uniform morphology with different polymorph phases (e.g. γ, δ and θ) via a hydrothermal method with varying calcination temperatures. The synthesized θ-Al2O3 nanowires were well characterized by XRD, FTIR, SEM/EDAX, AFM and HRTEM techniques. Microstructural analysis confirmed that the dimensions of the individual θ-Al2O3 nanowires are approximately in the ranges 5-20 nm in width and 40-150 nm in length, and the aspect ratio is up to 20. AFM results evidenced the uniform distribution of the nanowires with controlled morphology. Furthermore, UV-vis spectroscopic data reveal that the estimated optical band gap of the θ-Al2O3 nanowires was ~5.16 eV. The photoluminescence spectrum exhibits blue emission upon excitation at a wavelength of 252 nm. Time-resolved spectroscopy demonstrates that these nanowires illustrate a decay time of ~2.23 nanoseconds. The obtained photoluminescence results with a decay time of nanoseconds suggest that the θ-Al2O3 phase could be an exceptional choice for next generation fast optical sensors.

Herein, we report a new concept for garnet materials in terms of the synthesis of nanocrystalline structure at low temperatures and its multimodal luminescence processes. Terbium- and ytterbium-ion-codoped yttrium gallium garnet nanophosphors have been synthesized via solution combustion technique; nearly pure phase nanophosphor samples were obtained. The synthesized nanophosphor shows efficient multimodal upconversion (UC), downshifting (DS), and quantum cutting (QC)/downconversion (DC) luminescence, which is a new paradigm in garnet material. The garnet nanophosphor shows strong green emission through DS and UC processes both. Furthermore, cooperative energy transfer (CET) has been described in detail, and a possible mechanism for the QC process is also proposed. A UV/blue photon absorbed by Tb(3+) ion splits into two near-infrared photons (wavelength range 900-1040 nm), emitted by a Yb(3+) ion pair, with an efficiency of more than 100%. The Yb(3+) concentration dependent ET from Tb(3+) to Yb(3+) has been verified using time domain analysis. An ET efficiency as high as 28% and a corresponding QC efficiency of about 128% (for 15 mol % of Yb(3+) concentration) have been attained. Such a multimode emitting nanophosphor could be very useful in display devices and for enhancing the conversion efficiency of next generation solar cells via spectral modification etc.

We report a strategy for fabrication of 3D triangular GaN nano prism islands (TGNPI) grown on Ga/Si(553) substrate at low temperature by N2(+) ions implantation using a sputtering gun technique. The annealing of Ga/Si(553) (600 °C) followed by nitridation (2 keV) shows the formation of high quality GaN TGNPI cross-section. TGNPI morphology has been confirmed by atomic force microscopy. Furthermore, these nano prism islands exhibit prominent ultra-violet luminescence peaking at 366 nm upon 325 nm excitation wavelength along with a low intensity yellow luminescence broad peak at 545 nm which characterizes low defects density TGNPI. Furthermore, the time-resolved spectroscopy of luminescent TGNPI in nanoseconds holds promise for its futuristic application in next generation UV-based sensors as well as many portable optoelectronic devices.

We have grown high quality epitaxial GaN films on sapphire (0001) substrates using a ultra-high vacuum laser molecular beam epitaxy (MBE) system at different growth temperatures, deposition rate and nitrogen species. The HVPE grown GaN solid target was ablated at laser energy density ~5 J/cm2 with laser frequency ~ 5 Hz (low flux) and 10 Hz (high flux) in presence of r.f. nitrogen plasma. Structural properties of the epitaxial GaN films were characterized using high resolution x-ray diffraction, atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). At high flux, the full width at half maximum (FWHM) of x-ray diffraction rocking curve of GaN (0002) peak decreases with increasing growth temperature from 500 to 720 °C. The GaN film grown at 700 °C with low flux shows a large FWHM (368 arc sec) with small grain sizes in comparison to the GaN film grown with high flux (FWHM: 110 arc sec). We have also studied the effect of high pressure nitrogen ambient during ablation of GaN target for growth of GaN films on sapphire with and without pre-nitridation of sapphire at growth temperature 500 °C. The typical PL measurement on the GaN film grown on sapphire using laser MBE system shows the high quality of GaN film with minimum defects. The obtained results suggest that the present growth technique could be an alternative for fabrication of high quality GaN based devices.

The present study is aimed at microstructural characterization and hydrogenation behaviour of graphitic nanofibres (GNF) prepared by ethylene gas. The GNF were prepared by thermal cracking of ethylene gas at a temperature of ∼600°C for time durations of 2 h . The structural characterizations have been evaluated by X-ray diffraction and transmission electron microscopy. It was found that the typical length of GNF was in the range of ∼1 to ∼6 μm and most of GNF sample exhibits coil like configuration. The microstructural characteristics as studied by transmission electron microscopy (TEM) techniques after hydrogenation/dehydrogenation were found to consist of curved voids and were significantly different as compared to as grown GNF. The graphitic nanofibres were hydrogenated at 120 atm for 24 h and then dehydrogenated upto 1 atm . The P-C-T curves drawn based on several dehydrogenation runs have revealed the hydrogen storage capacity of present GNF as ∼15 wt % . In addition to hydrogen storage capacity, we have also investigated the desorption kinetics of GNF samples, the hydrogen was desorbed at the rate of ∼57 ml 3 / min .

The present paper deals with the synthesis and hydrogenation behaviour of graphitic nanofibres. The GNF were grown by thermal cracking of hydrocarbon gases (ethylene, acetylene) at a temperature of 600°C for a time duration of 2 h. The structural characterizations have been evaluated by X-ray diffraction and transmission as well as scanning electron microscopy. It was found that the as grown graphitic nanofibres, unlike graphite, have preferred orientations corresponding to (101). The hydrogenation behaviour was studied through charging at a pressure of 120 atm. and then discharging at a pressure up to 1 atm. The P–C–T curve drawn based on several dehydrogenation runs has revealed the GNF to have hydrogen storage capacity of ∼10 wt%.

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat can cause head/disk contacts, resulting in thermal asperities and pole tip damage. The damage at the head–disk interface due to protruded pole tips and alumina overcoat may degrade the drive mechanical performance when flying height is below 10 nm. In this study the change in pole tin recession (PTR) with temperature and current in the writer coil, are measured using an optical profiler and an atomic force microscope for heads having a stack design with single and dual layers of writer coils. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with a single layer of writer coils exhibit significantly lower thermal PTR than those with dual layers of coils. The ABS profiles at elevated temperature generated using the finite element modeling of the differential thermal expansion of various layers in the head stack are in close agreement with the measured profiles. The thermal PTR and alumina overcoat protrusion can be reduced by optimizing the thermal expansion coefficient of the alumina basecoat and overcoat, the height of the head stack, and by replacing alumina by SiO2 and SiC.

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat result increased thermal asperities and can interfere with the higher disc asperities and be damaged due to high contact. The damage at the head-disk interface due to protruded pole tips and alumina overcoat may become a major roadblock in the drive mechanical performance when flying height is below 10 nm. In this study, the thermal PTR defined as change in PTR with temperature is measured using an optical profiler and an AFM for heads having stack design with single and dual layers of writer coil. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with single layer of writer coil exhibit significantly lower thermal PTR than those with dual layers of coils.

The exponential increase in areal density of recording of magnetic storage devices has reduced the recorded bit size to nano length scale. This has profound implications on all aspects of the storage system including recording device modeling, materials, fabrication, metrology, characterization, and tribology of the head-disk interface. In this paper, the impact of nanotechnology in extending the data storage device storage systems is explored with an emphasis on the fabrication and characterization of nanolayers and structures.

Highly-luminescent nanophosphors have a decisive role in solid-state lighting (SSL) as well as in field emission display (FED) applications due to their potential use in fabrication of nanophosphor based FED and solid state display devices. Herein, the red emitting highly-luminescent Eu(3+)-Li(+) co-doped magnesium aluminate (MgAl2O4) nanophosphors were synthesized by a customized sol-gel route with an average particle size of 18 nm, which can be easily scaled up in a large quantity. The resulting nanophosphor exhibits hypersensitive red emission, peaking at 615 nm upon 394 nm excitation. Furthermore, comparative photoluminescence (PL) studies have been carried out for Eu(3+) doped and Eu(3+) doped-Li(+) co-doped magnesium aluminate (Li(+) co-doped MgAl2O4:Eu(3+)) nanophosphors, which indicated that Li(+) co-doping significantly improves luminescence intensity along with good crystallinity. Moreover, the charge compensation by addition of Li(+) co-activator in MgAl2O4:Eu(3+) lattice led to the two fold enhancement of PL intensity. The obtained results suggest that this nanophosphor could be an ultimate choice for next generation advanced luminescent nanomaterials for solid state lighting and portable FED devices.

Organic conductive polymers are at the forefront of materials science research because of their diverse applications built around their interesting and unique properties. This work reports for the first time a correlation between the structural, electrical, and electromagnetic properties of polyaniline (PANI)–tetragonal BaTiO3 (TBT) nanocomposites prepared by in-situ emulsion polymerization. XRD studies and HRTEM micrographs of these nanocomposites clearly revealed the incorporation of TBT nanoparticles in the conducting PANI matrix. EPR and XPS measurements reveal that increase in loading level of BaTiO3 results in a reduction of the doping level of PANI. The Ku-Band (12.4–18 GHz) network analysis of these composites shows exceptional microwave shielding response with absorption dominated total shielding effectiveness (SET) value of −71.5 dB (blockage of more than 99.99999% of incident radiation) which is the highest value reported in the literature. Such a high attenuation level, which critically depends on the fraction of BaTiO3 is attributed to optimized dielectric and electrical attributes. This demonstrates the possibility of using these materials in stealth technology and for making futuristic radar absorbing materials (RAMs).

This review article mainly focused on the recent progress on the synthesis and characterization of emerging artificially engineered nanostructures of oxide materials as well as their potential applications. A fundamental understanding about the state-of-the-art of the synthesis for different size, shape and morphology, which can be tuned to the desired properties of oxide nanomaterials have discussed in details in this review. The present review covers the a wide range of artificially engineered oxide nanomaterials such as cadmium-, cupric-, nickel-, magnesium-, zinc-, titanium-, tin-, aluminium-, and vanadium-oxides and their useful applications in sensors, optical displays, nanofluids and defence.

A dual mode rare-earth based oxide phosphor (Y(0.977)Yb(0.02)Er(0.003)NbO(4)), demonstrating both down conversion (DC) and upconversion (UC) emission, has been developed using a facile solid state reaction method which can be easily scaled-up for large quantities. In the DC studies, the material exhibits a strong blue emission with a long decay time (4.36 μs), corresponding to the charge transfer band of [NbO(4)](3-) under UV excitation (262 nm), and a green emission of the Er(3+) ions under blue (379 nm) excitation. Furthermore, it has been noticed that under infrared (976 nm) excitation, this phosphor shows strong green and red emission along with relatively weak emission bands in the UV-blue and IR regions, which confirm the compatibility of this phosphor for UC too. In the UC emission, the (2)H(11/2) → (4)I(15/2) and (4)S(3/2) → (4)I(15/2) transitions of the Er(3+) ion portray a temperature dependent behaviour and have been used for temperature sensing (optical thermometry) using the fluorescence intensity ratio (FIR) method. The maximum sensitivity is found to be 0.0073 K(-1) at 473 K, which is better in comparison with other host matrixs reported so far. The results suggest that this dual mode phosphor could be an exceptional choice for next generation luminescence-based temperature sensing devices as well as in advanced display devices.

Highly luminescent-paramagnetic nanophosphors have a seminal role in biotechnology and biomedical research due to their potential applications in biolabeling, bioimaging, and drug delivery. Herein, the synthesis of high-quality, ultrafine, europium-doped yttrium oxide nanophosphors (Y(1.9)O(3):Eu(0.1)(3+)) using a modified sol-gel technique is reported and in vitro fluorescence imaging studies are demonstrated in human breast cancer cells. These highly luminescent nanophosphors with an average particle size of ≈6 nm provide high-contrast optical imaging and decreased light scattering. In vitro cellular uptake is shown by fluorescence microscopy, which visualizes the characteristic intense hypersensitive red emission of Eu(3+) peaking at 610 nm ((5)D(0)-(7)F(2)) upon 246 nm UV light excitation. No apparent cytotoxicity is observed. Subsequently, time-resolved emission spectroscopy and SQUID magnetometry measurements demonstrate a photoluminescence decay time in milliseconds and paramagnetic behavior, which assure applications of the nanophosphors in biomedical studies.

El pdf del artículo es la versión de autor.-- et al., Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties., A.R. acknowledges financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG -Proposal No. 267374), Spanish MICINN (FIS2010-21282-C02-01), ACI Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco″ (IT-319-07), the EU project THEMA (Contract Number 228539), and the Ikerbasque Foundation. Y.Y. acknowledges funding from the National Natural Science Foundation of China (51042013, 51003056), the Shanghai City Committee of Science and Technology Project (10170502400), and the National Oceanic Administration Project (201005028-4). J.T.-T. acknowledges the support from PGE and CONACYT (213780).

and core–shell nanocomposites. [ 4 ] All of these materials are either composites or hybrid structures combining luminescent and magnetic materials individually. In these materials, organic dyes or metal complexes were immobilized on a silica layer, which leads to critical problems of leaching and photobleaching. [ 4a ] Alternatively, semiconductor quantum dots such as CdS, CdSe, and CdTe, have been demonstrated to be highly effective for cellular and animal imaging. [ 5 ] However, the use of such colloids for bioimaging applications suffers from additional issues such as toxicity, harmful solvents, and additives, [ 6 ] low light penetration depth, surface-ligand

The exponential increase in areal density of recording of magnetic storage devices has reduced the recorded bit size to nano length scale. This has profound implications on all aspects of the storage system including recording device modeling, materials, fabrication, metrology, characterization, and tribology of the head-disk interface. In this paper, the impact of nanotechnology in extending the data storage device storage systems is explored with an emphasis on the fabrication and characterization of nanolayers and structures.

A facile method to produce high-quality ZnO nanostructures; either tetrapod (TP), nanotetraneedle (NTN) or multipod (MP) with a high degree of homogeneity for advanced field emission (FE) applications is presented. Among these nanostructures, NTN has been successfully employed to demonstrate enhanced current densities (2.6 mA cm(-2)), turn-on field (1.5 V microm(-1)) and field-enhancement factors (6930) over conventional multiwalled carbon nanotubes (MWCNTs), TP, MP and ZnO-spheroids. A comparative study of FE from various ZnO nanostructures, morphologies and site densities has lead to the conclusion that diameter of the tip is one of the vital parameters in enhancing the overall FE properties.

We report a simple method for the synthesis of ultra-fine Eu(3+)-doped yttria (Y(2)O(3)) nanophosphors with an average diameter of approximately 5 nm for development of a transparent colloid that could be used as a luminescent security ink. This has been achieved by suitably substituting Eu(3+) ions at the favorable C(2) symmetry sites of Y(3+) ions and quantum mechanically confining the growth of the nanophosphor using a novel acid-catalyzed sol-gel technique. This is one of the few reports that depict the development of a transparent aqueous-stable Y(2)O(3):Eu(3+) colloidal solution for strategic applications related to security codes. High resolution transmission electron microscopy images showed excellent lattice fringes that in turn support the presence of better crystal quality and enhanced photoluminescence (PL) emission from the Y(1.9)O(3)Eu(0.1)(3+) nanophosphor system. Time resolved emission spectroscopy measurement indicated a PL decay time in the range of a few milliseconds, suitable for making luminescent security ink and other advanced applications in optoelectronic devices and bio-labeling.

Herein, we have investigated the various characteristics of two structurally different phases of Al2O3 nanowires: (i) γ-Al2O3 (cubic) and (ii) θ-Al2O3 (monoclinic). These nanowires were synthesized via hydrothermal treatment followed by calcination. The structural and luminescent properties were analyzed by scanning electron microscopy, high-resolution transmission electron microscopy and time-resolved photoluminescence spectroscopy. A plausible formation mechanism was proposed on the basis of experimental observations and analysis. These nanowires of both phases exhibit blue emission at ~466 nm (2.65 eV) upon 253 nm UV excitation wavelength. The time-resolved photoluminescence spectroscopy indicated that the decay time (τ) of θ-Al2O3 nanowires (τ = 273 picoseconds) is longer than that of γ-Al2O3 nanowires (τ = 198 picoseconds). Moreover, these Al2O3 nanowires were dispersed in ferroelectric liquid crystal (FLC) to observe their effect on the electro-optical characteristics. The electro-optical response of the composite devices showed faster electro-optical response, thereby suggesting potential applications in electro-optical shutters and modulators.

It is time to invent new nanomaterials and in this regards graphitic nanofibres is one of the new break through in the field of hydrogen storage materials. The unique properties of graphitic nanofibres (GNFs) have generated intense interest in the application of these new carbon materials toward a number of applications including selective absorption, hydrogen storage, polymer reinforcement and catalytic support.

We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 108 cm−2 at 750 °C) than that of the low temperature grown sample (1.1 × 109 cm−2 at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

We report photoluminescence and time-resolved spectroscopy in bismuth ferrite excited with a 325 nm source. The direct-bandgap recombinations near 2.55 eV and indirect-bandgap transitions near 2.67 eV are presented as functions of applied in-plane electric field with recombination time in the microsecond regime. The applied field moves some conduction electrons away from the Brillouin zone center, increasing significantly the intensity of indirect-gap recombination. An aging phenomenon is manifest in specimens stored for more than twelve months under ambient conditions. Effect of external magnetic field on the surface phase transition is negligible up to H = 0.5 T.

Leather industry produces huge quantities of bio-waste that can be used as raw material for the bulk synthesis of carbonaceous materials. Here we report the synthesis of multifunctional carbon nanostructures from pristine collagen wastes by a simple high temperature treatment. Our studies reveal that the nanocarbons derived from the bio-waste have a partially graphitized structure with onion-like morphology and are naturally doped with nitrogen and oxygen, resulting in multifunctional properties. This synthetic route from bio-waste raw material provides a cost-effective alternative to existing chemical vapor deposition methods for the synthesis of functional nanocarbon materials and presents a sustainable approach to tailor nanocarbons for applications such as battery electrodes.

We report an interesting observation on strong enhancement in green luminescence from hybrid ZnO/multi-walled carbon nanotubes (MWCNTs). The hybrid structures were synthesized via a high temperature sintering method. The strong green emission at 510 nm has been attributed to surface defects of ZnO, originating from interactions between ZnO and the MWCNT surface, which has been confirmed by high resolution transmission electron microscopy and x-ray photoelectron spectroscopy. Furthermore, the two-dimensional (2D) layer of this hybrid material shows a high degree of homogeneity and 82% transparency. Time resolved emission spectroscopy measurement shows a photoluminescence decay time in microseconds, which is suitable for making optoelectronic devices.

We report a simple method for the synthesis of Na(+) doped and stable zinc oxide quantum dots, using the quantum confinement atom method. An intense broad green photoluminescence (PL) was observed with a maximum located at approximately 535 nm when excited by UV radiation of 332 nm. The PL peak intensity is found to be highly dependent on the size of the quantum dots (QDs). Electron microscopy observation revealed that the radius of the QD was approximately 1 nm, which clearly indicated that the QDs are in the strong quantum confinement region (exciton Bohr radius, r(B), for bulk ZnO is 1.8 nm). Phase purity of ZnO and the presence of Na(+) was confirmed by x-ray diffraction (XRD) and atomic absorption spectroscopy (AAS), respectively. The results are well incremented by x-ray photoelectron spectroscopy (XPS) studies. Intentional ageing of QDs for several days under controlled experimental conditions such as temperature, relative humidity and pH etc, facilitated the formation of various nanostructures with a slight red shift in the PL peak position. Time resolved emission spectroscopy measurements indicated that PL decay time changes from 35 ns for QDs to 1660 micros for nanocrystals. The observed high-intensity and stable green PL emissions have been analyzed and thoroughly discussed.

Carbon nanofiber (CNF) with springlike and double-helix structures has been synthesized by catalytic thermal pyrolysis of an acetylene precursor at 850–950 °C using iron nanopowder and thiophene as catalyst and promoter, respectively. High resolution electron microscopy revealed a higher d-spacing (∼3.46 A) of (002) crystal plane than customary multiwalled carbon nanotube (MWCNT) (3.37 A) that helps in sustaining mechanical shocks better than MWCNTs. The large surface to volume ratio of springlike CNF does provide many delocalized free electrons to enhance the photoluminescence activity. Electron paramagnetic resonance signal showed a single narrow line having g-value 2.0024±0.0002 and spin contribution 3.4956×10−16 spins/g.

This paper presents an analytical study of dynamic characteristics of half-elliptical pressure dam bearings. One half (either upper or lower) of these bearings is elliptical, while the other half is circular. A step or pressure dam is cut in the upper half bearing surface. The lower half bearing surface is provided with a deep groove or relief-track. These bearings have been analyzed by the finite-element method. The results have been computed for dimensionless rotor flexibilities of 0.0, 0.5, 1.0, 2.0, and 4.0. Dynamic characteristics such as stiffness and damping coefficients, and stability threshold speed are presented in the form of graphs. It has been determined that half-elliptical pressure dam bearings are superior to circular pressure dam bearings in stability for bearings having same design parameters. Both these half-elliptical pressure dam bearings have almost identical dynamic characteristics. Rotor flexibility adversely affects the stability of these bearings and its effect is pronounced at h...

The stability of a system consisting of a flexible shaft with a single central rotor supported by two finite elliptical journal bearings was considered. The Reynolds equation was solved numerically for several values of the eccentricity ratio, the L D ratio and the dimensionless velocity of the journal centre. The resulting pressure profiles are used to determine the load capacity and the spring and damping coefficients. Limiting speeds of stable operation are obtained from the roots of characteristic equations for the corresponding bearing-rotor system. The operating load, ellipticity, L D ratio and shaft flexibility significantly affect the limit of stable operation. Elliptical bearings are suitable for stiff and moderately flexible rotors.

Journal bearings operating at high speeds often exhibit vibrational instability. Circular bearings are known to be prone to vibrations and, in general, a non-circular bearing geometry enhances shaft stability. Among the several possible configurations of non-circular bearings, elliptical, offset, three-lobe and tilted three-lobe bearings have attracted attention. The present analysis is a theoretical prediction of stability for a hybrid two-lobe bearing obtained by displacing the lobe centres of an elliptical bearing. It has been found that an orthogonally displaced bearing is more stable than the hitherto known bearings and at the same time it is easier to manufacture. Numerical results have been presented for nine combinations of horizontal and vertical displacements of lobes and three L D ratios so that a suitable profile and L D ratio can be selected by a designer in a specific application.

The stability of two-axial groove cylindrical journal bearing is improved by milling a step or pressure dam in the upper half surface and by cutting a deep groove or relief-track in the lower half surface. Dynamic analysis of elliptical bearings having similar construction of dam and relief-track, called elliptical pressure dam bearings, has revealed that these bearings are far more stable than ordinary cylindrical, cylindrical pressure dam, elliptical, three-lobe, etc., when supporting a rigid rotor. In this paper, the stability of finite elliptical pressure dam bearings supporting flexible rotors is discussed. The dimensionless rotor flexibilities selected are 0.5, 1.0, 2.0, and 4.0, which cover almost the entire range of practical interest. Effects of design parameters such as vertical preset, L/D ratio, Sommerfeld number, dam location and relief-track width ratio on the stability of these bearings, corresponding to various dimensionless rotor flexibilities are presented in the form of graphs. Also, th...