Multicast capacity and delay trade‐offs in ad hoc networks with random iid mobility model

In this paper, multicast capacity and delay trade‐offs of mobile ad hoc networks are considered under random independently and identically distributed (iid) mobility model. Compared with unicast, multicast can reduce the overall network load by a factor with high probability (whp) in static random ad hoc networks, where k is the number of destination nodes in a multicast session. So we firstly discuss whether the law still holds in mobile random ad hoc networks, and in this case what delay should be tolerated. Through the relation between average retransmissions and multicast capacity, we prove that order of multicast capacity is not achievable whp, and delay for multicast capacity is , where is the number of ad hoc nodes in the whole networks, and and c is a positive constant. Then achievable throughput whp is considered. The nearest neighbor transmission strategy is introduced by Grossglauser and Tse to achieve throughput which is so far the highest achievable unicast capacity. So the multicast capacity of mobile ad hoc networks under this strategy is studied. The analysis shows that under any multicast routing scheme based on the nearest neighbor transmission strategy, the upper bound on multicast capacity is whp. Then we propose a multicast routing and scheduling scheme by which mobile ad hoc networks can achieve throughput whp, and must tolerate total network delay. Copyright © 2009 John Wiley & Sons, Ltd.     

Engineering of Facets,Band Structure,and Gas‐Sensing Properties of Hierarchical Sn2+‐Doped SnO2 Nanostructures

Hierarchical SnO₂ nanoflowers, assembled from single‐crystalline SnO₂ nanosheets with high‐index (11$ \bar 3 $ ) and (10$ \bar 2 $ ) facets exposed, are prepared via a hydrothermal method using sodium fluoride as the morphology controlling agent. Formation of the 3D hierarchical architecture comprising of SnO₂ nanosheets takes place via Ostwald ripening mechanism, with the growth orientation regulated by the adsorbate fluorine species. The use of Sn(II) precursor results in simultaneous Sn²⁺ self‐doping of SnO₂ nanoflowers with tunable oxygen vacancy bandgap states. The latter further results in the shifting of semiconductor Fermi levels and extended absorption in the visible spectral range. With increased density of states of Sn²⁺‐doped SnO₂ selective facets, this gives rise to enhanced interfacial charge transfer, that is, high sensing response, and selectivity towards oxidizing NO₂ gas. The better gas sensing performance over (10$ \bar 2 $ ) compared to (11$ \bar 3 $ ) faceted SnO₂ nanostructures is elucidated by surface energetic calculations and Bader analyses. This work highlights the possibility of simultaneous engineering of surface energetics and electronic properties of SnO₂ based materials.     

Highly Ordered Fe and Nb Stripe Arrays on Facetted α‐Al2O3 (10$ \bar 1 $0)

A process is described whereby highly ordered arrays of epitaxial thin‐film nano‐ and mesostripes can be grown using molecular beam epitaxy (MBE) techniques on M‐plane sapphire α‐Al₂O₃ (10 0) substrates. The planar sapphire substrate surface is unstable, and spontaneously forms primarily ( 101) and (1 02) nanofacets upon annealing at a high temperature. By employing this nanofacetted sapphire as a substrate for MBE growth at controlled shallow incident angles, perfect nano‐ and mesostripes can be produced by means of geometrical shadowing in conjunction with partial de‐wetting of the epilayer on the facets. Advantages over other stripe fabrication strategies include: epitaxial quality, tunable width, and the ability to grow superconducting and rare earth nanowires using well‐established MBE techniques. The process is demonstrated by the growth of regular arrays of 100 nm wide Nb nanostripes. Additionally, we have determined the epitaxy of Nb (111) on the Al₂O₃ ( 101) facet. The applicability of the periodic defect structure of Nb layers of uniform thickness on the facetted surface is exemplarily demonstrated for the study of the vortex dynamics of type II superconductors.     

Growth,Cathodoluminescence and Field Emission of ZnS Tetrapod Tree‐like Heterostructures

We report the growth mechanism, cathodoluminescence and field emission of dual phase ZnS tetrapod tree‐like heterostructures. This novel heterostructures consist of two phases: zinc blende for the trunk and hexagonal wurtzite for the branch. Direct evidence is presented for the polarity induced growth of tetrapod ZnS trees through high‐resolution electron microscopy study, demonstrating that Zn‐terminated ZnS (111)/(0001) polar surface is chemically active and S‐terminated ( )/(000 ) polar surface is inert in the growth of tetrapod ZnS trees. Two strong UV emissions centered at 3.68 and 3.83 eV have been observed at room temperature, which are attributed to the bandgap emissions from the zinc blende trunk and hexagonal wurtzite branch, indicating that such structures can be used as unique electromechanical and optoelectronic components in potential light sources, laser and light emitting display devices. In addition, the low turn‐on field (2.66 Vµm⁻¹), high field‐enhancement factor (over 2600), large current density (over 30 mAcm⁻² at a macroscopic field of 4.33 Vµm⁻¹) and small fluctuation (∼1%) further indicate the availability of ZnS tetrapod tree‐like heterostructures for field emission panel display. This excellent field‐emission property is attributed to the specific crystallographic feature with high crystallinity and cone‐shape patterned branch with nanometer‐sized tips. Such a structure may optimize the FE properties and make a promising field emitter.     

From Bulk to Monolayer MoS2: Evolution of Raman Scattering

Molybdenum disulfide (MoS₂) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of and peaks vary monotonously with the layer number of ultrathin MoS₂ flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS₂ decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS₂. The asymmetric Raman peak at 454 cm^?1, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS₂, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode ( ). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS₂ is scaled down from three‐ to two‐dimensional geometry.     

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

Here, the photophysics and performance of single‐layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae and where ppy, bpy, and phen are 2‐phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the N?N ligand has little effect on the emitting metal‐ligand to ligand charge‐transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m^?2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.     

Nanowire Structural Evolution from Fe3O4 to ϵ‐Fe2O3

The ?‐Fe₂O₃ phase is commonly considered an intermediate phase during thermal treatment of maghemite (γ‐Fe₂O₃) to hematite (α‐Fe₂O₃). The routine method of synthesis for ?‐Fe₂O₃ crystals uses γ‐Fe₂O₃ as the source material and requires dispersion of γ‐Fe₂O₃ into silica, and the obtained ?‐Fe₂O₃ particle size is rather limited, typically under 200 nm. In this paper, by using a pulsed laser deposition method and Fe₃O₄ powder as a source material, the synthesis of not only one‐dimensional Fe₃O₄ nanowires but also high‐yield ?‐Fe₂O₃ nanowires is reported for the first time. A detailed transmission electron microscopy (TEM) study shows that the nanowires of pure magnetite grow along [111] and <211> directions, although some stacking faults and twins exist. However, magnetite nanowires growing along the <110> direction are found in every instance to accompany a new phase, ?‐Fe₂O₃, with some micrometer‐sized wires even fully transferring to ?‐Fe₂O₃ along the fixed structural orientation relationship, (001) ∥ (111), [010] ∥ <110>. Contrary to generally accepted ideas regarding epsilon phase formation, there is no indication of γ‐Fe₂O₃ formation during the synthesis process; the phase transition may be described as being from Fe₃O₄ to ?‐Fe₂O₃, then to α‐Fe₂O₃. The detailed structural evolution process has been revealed by using TEM. 120° rotation domain boundaries and antiphase boundaries are also frequently observed in the ?‐Fe₂O₃ nanowires. The observed ?‐Fe₂O₃ is fundamentally important for understanding the magnetic properties of the nanowires.     

Synthesis of Nanohole‐Structured Single‐Crystalline Platinum Nanosheets Using Surfactant‐Liquid‐Crystals and their Electrochemical Characterization

Nanohole‐structured single‐crystalline Pt nanosheets have been synthesized by the borohydride reduction of Na₂PtCl₆ confined to the lyotropic liquid crystals (LLCs) of polyoxyethylene (20) sorbitan monooleate (Tween 80) with or without nonaethylene‐glycol (C₁₂EO₉). The Pt nanosheets of around 4–10 nm in central thickness and up to 500 nm or above in diameter have a number of hexagonal‐shaped nanoholes ∼1.8 nm wide. High‐resolution electron microscope images of the nanosheets showed atomic fringes with a spacing of 0.22 nm indicating that the nanosheets are crystallographically continuous through the nanoholed and non‐holed areas. The inner‐angle distributions for the hexagonal nanoholes indicate that the six sides of the nanoholes are walled with each two Pt (111), Pt (1 1) and Pt (010) planes. The formation mechanism of nanoholed Pt nanosheets is discussed on the basis of structural and compositional data for the resulting solids and their precursory LLCs, with the aid of similar nanohole growth observed for a Tween 80 free but oleic acid‐incorporated system. It is also demonstrated that the nanoholed Pt nanostructures loaded on carbon exhibit fairly high electrocatalytic activity for oxygen reduction reaction and a high performance as a cathode material for polymer‐electrolyte fuel cells, along with their extremely high thermostability revealed through the effect of electron‐irradiation.     

Operational results of grid‐connected photovoltaic system with different inverter's sizing factors (ISF)

This paper presents operational results of a 11·07 kWp grid‐connected photovoltaic system. This system is made up by eight groups with different relationships between the inverter's rated power and the PV generator's maximum power (P / P) . The obtained results led to the verification that the different studied relationships, P / P between 55 and 102%, do not affect significantly the final yields (Y_F). Copyright © 2006 John Wiley & Sons, Ltd.     

On the electro-optic properties of single crystals of sodium,potassium and rubidium acid phthalates

The linear electro-optic (Pockels) effect in a series of alkali metal acid phthalate crystals has been studied. Single electro-optic coefficients r and r for sodium (NaAP), potassium (KAP) and rubidium (RbAP) acid phthalates have been measured by employing the modified Mach–Zehnder interferometric technique. The best electro-optic crystal in this series is RbAP with r = 9.10 × 10^?12 m V^?1, r = 3.05 × 10^?12 m V^?1 and a sizable figure of merit for electro-optic phase retardation, comparable with that of KDP. The dispersion properties of the electro-optic coefficients for KAP are discussed in detail.     

Simultaneous Electrical and Thermoelectric Parameter Retrieval via Two Terminal Current–Voltage Measurements on Individual ZnO Nanowires

The thermoelectric parameters, in particular the thermal conductivity and dimensionless figure of merit ZT, of ZnO nanowires, are estimated via two terminal current–voltage measurements. The measurements are carried out in situ in a transmission electron microscope and negative differential conductance is observed on individually suspended ZnO nanowires. From the low bias region of the current–voltage curve, the electrical parameters, including carrier concentration and mobility, are obtained by fitting the experimental data using a metal–semiconductor–metal model. The thermal conductivity is extracted from the high bias region of the same current–voltage curve using a self‐consistent method, which combines the self‐heating thermal conduction and electrical transport properties of ZnO nanowires. It is shown that the thermal conductivity of ZnO nanowires is suppressed significantly in comparison with that of bulk ZnO, which is attributed to the strong surface scattering of phonons. The thermal conductivity is also found to decrease more steeply than the expected $ {1 \mathord{\left/{\vphantom {1 T}} \right.} T} $ trend, but does obey a $ {1 \mathord{\left/{\vphantom {1 {\left({\alpha T + \beta T^2} \right)}}} \right. } {\left({\alpha T + \beta T^2} \right)}} $ relation; this is shown to result from four‐phonon processes at high temperatures. The dimensionless figure of merit ZT is determined to be about 0.1 at 970 K. Finally, the thermoelectric properties of individual ZnO nanowires are also discussed, indicating that ZnO nanowires are promising high temperature thermoelectric materials.     

Nano‐ and Mesoscale Structure of Na$_{1 \over 2}$Bi$_{1 \over 2}$TiO3: A TEM Perspective

The room‐temperature structure of Na Bi TiO₃ (NBT) ceramics was studied using several transmission electron microscopy (TEM) techniques. High‐angle annular dark field imaging in a scanning TEM confirmed an essentially random distribution of Bi and Na, while electron diffraction revealed significant disorder of the octahedral rotations and cation displacements. Diffraction‐contrast dark‐field and Fourier‐filtered high‐resolution TEM images were used to develop a model that reconciles local and average octahedral tilting in NBT. According to this model, NBT consists of nanoscale twin domains which exhibit a^?a^?c⁺ tilting. The coherence length of the in‐phase tilting, however, is limited to a few unit cells and is at least one order of magnitude shorter than that of anti‐phase tilting. Assemblages of such nanodomains are proposed to exhibit an average a^?a^?c^? tilt system. Diffuse sheets of intensity in electron diffraction patterns are attributed to local cation displacements correlated along both 〈111〉 and 〈100〉 chains and suggest partial polar ordering of these displacements. Overall, the TEM data indicate significant chemical, cation‐displacement and tilt disorder of the NBT structure at the nano and mesoscale and support the premise that the Cc symmetry recently proposed from powder diffraction refinements is an averaged “best fit” cell.     

Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers

The optical conductance of monolayer graphene is defined solely by the fine structure constant, α = (where e is the electron charge, is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero‐gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode‐locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.     

Cadmium(II) (8‐Hydroxyquinoline) Chloride Nanowires: Synthesis,Characterization and Glucose‐Sensing Application

Luminescent cadmium(II) (8‐hydroxyquinoline) chloride (CdqCl) complex nanowires are synthesized via a sonochemical solution route. The results of X‐ray photoelectron spectroscopy, energy dispersive X‐ray analysis, infrared spectroscopy, elemental analysis (EA), and atomic absorption spectroscopy demonstrate that the chemical composition of the product is Cd(C₉H₆NO)Cl. Transmission electron microscopy and scanning electron microscopy images show that the CdqCl product is wire‐like in structure, with a diameter of approximately 50 nm and an approximate length of 2–4 µm. The morphology and composition of the product can be transformed from Cdq₂ micrometer‐scaled flakes to CdqCl nanowires by increasing the ratio of CdCl₂/q. A new fluorescent sensing strategy for detecting H₂O₂ and glucose is developed and is based on the combination of the luminescent nanowires and the biocatalytic growth of Au nanoparticles. The quenching effects of Au nanoparticles and on the fluorescence of CdqCl nanowires are investigated. The dominant factor for the fluorescence quenching of CdqCl nanowires is that the Stern–Volmer quenching constant of Au nanoparticles is larger than that of .     

Mechanism of charge recombination and IPCE in ZnO dye‐sensitized solar cells having I−/I and Br−/Br redox couple

The influence of intensity and wavelength variation on the solar cell parameters of two different ZnO‐based liquid state DSSCs named as Cell (A) ZnO/EosinY/LiI and Cell (B) ZnO/EosinY/LiBr was studied. It was found that V_oc and I_sc depend logarithmically and linearly on light flux, respectively, which indicates that light absorption and carrier diffusion do not limit the solar cell efficiency. The data was analyzed to ascertain the charge recombination mechanism between conduction band electrons and the electrolytes. The regeneration of dye due to I⁻/I₃ and Br⁻/Br redox couple was examined by studying the wavelength dependence of IPCE. An estimation of series and shunt resistance is made using two methods: (i) different illumination method (ii) single I–V curve, for the two cells in order to understand the role of the electrolyte in controlling the solar cell parameters. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoarchitecturing of Activated Carbon: Facile Strategy for Chemical Functionalization of the Surface of Activated Carbon

Nanoarchitecturing of carbon nanospheres onto the surface of activated carbon (AC) gives birth to a new composite carbon material that features a hierarchical structure with macro‐ and nanometer dimensions of the respective carbon components and exhibits a remarkably enhanced adsorption capability for heavy‐metal ions ( and Fe³⁺) from aqueous solution as compared to AC. Thus, we first propose that nanoarchitecturing of AC can be utilized not only as a flexible method for the synthesis of novel, hybrid, nanostructured composite carbon materials but also as a new and “green‐route” strategy for functionalization of the surface of AC in an effective manner. Hence, there is scope for a possible new concept in the functionalization of industrial AC for specific applications.     

Defect‐Related Emissions and Magnetization Properties of ZnO Nanorods

A clear correlation between defect‐related emissions and the magnetization of ZnO nanorods synthesized by a one‐step aqueous chemical method is demonstrated. The relative contribution of the emission bands arising from various types of defects is determined and found to be linked with the size of the nanorods and annealing conditions. When the size of the nanorods and the annealing temperature are increased, the magnetization of pure ZnO nanorods decreases with the reduction of a defect‐related band originating from singly charged oxygen vacancies ($V_{\rm o}^ +$ ). With a sufficient increase of annealing temperature (at 900 °C), the nanorods show diamagnetic behavior. Combining with the electron paramagnetic resonance results, a direct link between the magnetization and the relative occupancy of the singly charged oxygen vacancies present on the surface of ZnO nanorods is established.     

Continuous Aspect‐Ratio Tuning and Fine Shape Control of Monodisperse α‐Fe2O3 Nanocrystals by a Programmed Microwave–Hydrothermal Method

A rapid and economical route based on an efficient microwave–hydrothermal process has been developed to synthesize monodisperse α‐Fe₂O₃ nanocrystals with continuous aspect‐ratio tuning and fine shape control, which takes advantage of microwave irradiation and hydrothermal effects. This method easily programs the experimental conditions (e.g., temperature and time) and significantly shortens the synthesis time to minutes. It allows the creation of numerous recipes for optimizing and scaling up production. The effects of experimental conditions including reaction temperature and reactant concentration on the morphology of α‐Fe₂O₃ have been investigated systematically. Results reveal that the initial molar ratio of Fe³⁺ to PO plays a crucial role in the final morphology of the α‐Fe₂O₃ products. Several morphologies, which include ellipsoids/spindles with aspect ratios that range from 1.1 to 6.3, nanosheets, nanorings, and spheres can be obtained. The as‐formed α‐Fe₂O₃ exhibits shape‐dependent infrared optical properties. The growth process of colloidal α‐Fe₂O₃ crystals in the presence of phosphate ions is discussed. The products have been characterized by using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy. This work presents an efficient and cost‐effective approach that is potentially competitive for scaling‐up industrial production. The as‐formed α‐Fe₂O₃ crystals with controllable morphologies not only provide flexible building blocks for advanced functional devices, but are also ideal candidates for studying their nanoarchitecture‐dependent performance in optical, catalytic, and magnetic applications.     

Electrical Transport Properties of Large,Individual NiCo2O4 Nanoplates

Understanding the electrical transport properties of individual semiconductor nanostructures is crucial to advancing their practical applications in high‐performance nanodevices. Large‐sized individual nanostructures with smooth surfaces are preferred because they can be easily made into nanodevices using conventional photolithography procedures rather than having to rely on costly and complex electron‐beam lithography techniques. In this study, micrometer‐sized NiCo₂O₄ nanoplates are successfully prepared from their corresponding hydroxide precursor using a quasi‐topotactic transformation. The Co/Ni atomic arrangement shows no changes during the transformation from the rhombohedral LDH precursor (space group R$ \bar 3 $ m) to the cubic NiCo₂O₄ spinel (space group Fd $ \bar 3 $ m), and the nanoplate retains its initial morphology during the conversion process. In particular, electrical transport within an individual NiCo₂O₄ nanoplate is further investigated. The mechanisms of electrical conduction in the low‐temperature range (T < 100 K) can be explained in terms of the Mott's variable‐range hopping model. At high temperatures (T > 100 K), both the variable‐range hopping and nearest‐neighbor hopping mechanisms contribute to the electrical transport properties of the NiCo₂O₄ nanoplate. These initial results will be useful to understanding the fundamental characteristics of these nanoplates and to designing functional nanodevices from NiCo₂O₄ nanostructures.     

The capacity of multi‐channel multi‐interface wireless networks with multi‐packet reception and directional antenna

The capacity of wireless networks can be improved by the use of multi‐channel multi‐interface (MCMI), multi‐packet reception (MPR), and directional antenna (DA). MCMI can provide the concurrent transmission in different channels for each node with multiple interfaces; MPR offers an increased number of concurrent transmissions on the same channel; DA can be more effective than omni‐DA by reducing interference and increasing spatial reuse. This paper explores the capacity of wireless networks that integrate MCMI, MPR, and DA technologies. Unlike some previous research, which only employed one or two of the aforementioned technologies to improve the capacity of networks, this research captures the capacity bound of the networks with all the aforementioned technologies in arbitrary and random wireless networks. The research shows that such three‐technology networks can achieve at most capacity gain in arbitrary networks and capacity gain in random networks compared with MCMI wireless networks without DA and MPR. The paper also explored and analyzed the impact on the network capacity gain with different , θ, and k‐MPR ability. Copyright © 2012 John Wiley & Sons, Ltd.