Energy Mismatch Research Articles

Carrier flow and nonequilibrium superconductivity in superconductor-based LEDs

Transport properties of superconductor-based LEDs are quantitatively investigated. In the gate-controlled region, we confirm the realization of a new type of Josephson field effect transistor performance, in which the channel cross-sectional area of the junction can be directly modulated by the gate voltage. In the current-injected region, a superconducting critical current of the order of a few microamperes is found to be modulated by the steady current injection of the order of a few picoamperes. This ultrahigh monitoring sensitivity is considered to be due to the large energy mismatch between generated photons and superconducting pairs in the radiative recombination process, which causes the carrier flows and nonequilibrium superconductivity in the active layer.

Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+-Yb3+ Co-doped ZnO Thin Films

Embedding luminescent rare earth ions into transparent oxides such as ZnO is a well-known approach to functionalize the material by adding photon-management properties. In this paper we present a detailed study of the luminescence properties and energy level structure of Pr3+ and Yb3+ ions embedded in ZnO thin films deposited by magnetron reactive sputtering. Careful study of the photoluminescence and excitation spectra allowed identifying and locating almost all excited levels of Pr3+ and Yb3+ ions. Thus, an almost complete electronic energy level diagram of these ions in the ZnO crystal lattice can be drawn for the first time. In particular, we show that the crystal field of ZnO strongly modifies the energy level structure of the Pr3+ and Yb3+ ions creating energy mismatches between the transitions useful for cooperative down conversion. Finally, we demonstrate that light emission from Pr3+ ions can be induced both by direct excitation of the ions and indirectly by energy transfer from the ZnO matrix.

Determining the Chiral Index of Semiconducting Carbon Nanotubes Using Photoconductivity Resonances

We utilize photoconductivity spectroscopy to identify the unique chiral structure of individual carbon nanotubes (CNTs). Peaks in photoconductivity are measured throughout the visible and near-IR wavelength ranges. Photoconductivity peaks associated with individual CNTs are referenced against existing Rayleigh scattering measurements to uniquely identify chiral indices. We find close agreement between our assigned exciton resonances and the previously published exciton resonances. The typical net energy mismatch is ≤20 meV. By enabling chiral identification of CNTs after the completion of device fabrication, the technique offers a facile method for investigating relationships between CNT structure and electronic/optoelectronic properties.

Cascaded multilevel converter based superconducting magnetic energy storage system for frequency control

The Super conducting magnetic energy storage (SMES), owing to high energy density and capacity, has been widely applied in different stages of power systems. One of these applications is the frequency control of the electric power systems. Frequency of a power system depends on the balance of produced and demanded energy in any instant of time. Subsequent to a sudden change in the system, which causes produced and demanded energy mismatch, frequency oscillates. According to standards, the permissible variation band of the frequency is very restricted. Larger swings of frequency may result in instability and undesirable trips. As a result, suitable frequency control mechanisms should be implemented in the system. SMES is well-suited for this application because of high energy density and fast response. SMES is attached to system by a power conditioning system (PCS) which include power electronic converters, mainly a dc–dc chopper and an inverter. This paper, studies the application of a cascaded H-bridge (CHB) multilevel converter for frequency control. As far as the authors' knowledge, such a study has not been done before. The design procedure of the converter is presented. Simulation results on a sample system are presented to verify the performance of the proposed PCS.

Head–Disk Interface Materials Issues in Heat-Assisted Magnetic Recording

In this paper, some issues concerning the reliability of heat-assisted magnetic recording (HAMR) media are highlighted. The large roughness of the grain structure originates from a surface energy mismatch between the FePt grains and the graphene-like segregant material. A simple roughness model, based on interfacial energies, is proposed that can quantitatively predict media grain structure and roughness. The thermal behavior of the disk lubricant is reviewed both experimentally as well as theoretically using molecular dynamics (MD) and density functional theory (DFT). The lubricant film can be subjected to evaporation and oxidation, both of which follow an Arrhenius reaction rate. MD also predicts that the disk carbon overcoat can undergo structural changes under thermal transient exposure in the nanosecond time frame, and Raman imaging performed on a disk zone that was HAMR written shows small but unequivocal changes, consistent with an increase in carbon sp2 cluster size.

Resonant Coherence in Photosynthetic Electronic Energy Transfer by Site-Dependent Pigment–Protein Interactions

We have numerically examined the effect of site-dependent reorganization on the dynamics of coherent electronic excitation energy transfer in a donor and acceptor pair of photosynthetic pigment-protein complex. Using the quasi-adiabatic propagator path integral method (QUAPI), we have found that a specific proportionality between the site-energy mismatch ϵ and the site-reorganization energy mismatch λ simultaneously increases the length and robustness of the quantum coherence. This behavior is associated with a Rabi type resonance that is manifested in the amplitude and frequency in the coherent portion of the population dynamics. Impact of the resonance on robustness of the coherence under static disorder is also discussed.

Thermodynamic Analysis on a Pre-dried Lignite-fired Power System: Comparison on Energy Supply Systems for Dryer

Pre-drying is a potential way to improve the electric generation efficiency of lignite-fired power plant, and steam is a main kind of drying heat source. In conventional pre-dried lignite-fired power system, the steam extraction of regenerative system is led to the dryer directly. The steam extraction must be used by the dryer after throttling, if the energy mismatch exists between the supply of steam extraction and the need of dryer. Two systems of energy supply for the dryer, that the energy of the steam extraction is supplied to the dryer via compressor or ejector, were proposed in this paper to further increase the energy utilization efficiency. These systems were thermodynamic analyzed, and the electric generation efficiencies of these systems were compared. The energy saving boundary was gotten.

Competitive absorption of Eu3+ and Tb3+codoped in NaGd(PO3)4 phosphors

Energy transfer has been generally utilized for the sensitization of luminescence in phosphors. It is common that excitation of donor can be transferred efficiently to acceptor. However, energy transfer should be investigated in association with the transfer efficiency from donors to activators. To accomplish this, energy transfer from Gd3+ to Eu3+ and Tb3+ was investigated. It was found that the transfer efficiency from Gd3+ to Eu3+ is higher than that from Gd3+ to Tb3+ because there are less energy mismatches between the emission of Gd3+ and the absorption of Eu3+ than that between Gd3+ and Tb3+ in the host NaGd(PO3)4.

Experimental study of the phase change and energy characteristics inside a cylindrical latent heat energy storage system: Part 2 simultaneous charging and discharging

The time mismatch between energy availability and energy demand with solar domestic hot water (SDHW) systems is often solved using energy storage. Energy storage systems typically employ water for thermal energy storage, however, water storage takes up considerable space and weight due to the large volumes required under certain conditions. A latent heat energy storage system (LHESS) may provide a valuable solution to the space and weight issue, while also correcting the energy mismatch by storing energy in phase change materials (PCMs) when it is available, dispensing energy when it is in demand, and acting as a heat exchanger when there is supply and demand simultaneously. PCMs are advantageous as energy storage materials due to their high energy density which reduces the space requirements for energy storage. However, heat transfer problems arise due to the inherently low thermal conductivity of PCMs. Simultaneous charging and discharging has not been addressed in literature making questionable the ability of a LHESS to operate as a heat exchanger during the mode of operation. The main objective of this research is to study the heat transfer processes and phase change behavior of a PCM during simultaneous charging and discharging of a LHESS.In Part 2 of this paper, experiments are performed using a vertical cylindrical LHESS which is charged and discharged simultaneously to replicate latent heat energy storage paired with a SDHW system with simultaneous energy supply and demand. Dodecanoic acid is used as the PCM. Experimental results for simultaneous operations are presented, under various scenarios and flow rates for both the hot and cold heat transfer fluids. The ability of the system to directly transfer heat between the hot and cold heat transfer fluids is studied, and the results found during consecutive, or separate, charging and discharging, presented in Part 1 of this paper, are compared to the results found during simultaneous charging and discharging. It was found that natural convection in the melted PCM clearly provides an advantage towards direct heat exchange between the hot and cold heat transfer fluid; while the low thermal conductivity of solid PCM provides a barrier to this direct energy exchange.

Theory of the Miscibility of Fullerenes in Random Copolymer Melts

We combine polymer integral equation theory and computational chemistry methods to study the interfacial structure, effective interactions, miscibility and spatial dispersion mechanism of fullerenes dissolved in specific random AB copolymer melts characterized by strong noncovalent electron donor–acceptor interactions with the nanofiller. A statistical mechanical basis is developed for designing random copolymers to optimize fullerene dispersion at intermediate copolymer compositions. Pair correlation calculations reveal a strong sensitivity of interfacial packing near the fullerene to copolymer composition and adsorption energy mismatch. The potential of mean force between fullerenes displays rich trends, often nonmonotonic with copolymer composition, reflecting a nonadditive competition between direct filler attractions and polymer-mediated bridging and steric stabilization. The spinodal phase diagrams are in qualitative agreement with recent solubility limit experimental observations on three systems, and testable predictions are made for other random copolymers. The distinctive nonmonotonic variation of miscibility with copolymer composition is found to be primarily a consequence of composition-dependent, spatially short-range attractions between the A and B monomers with the fullerene and nontrivial pair correlations. A remarkably rich, polymer-specific temperature dependence of the spinodal diagram is predicted, which reflects the thermal sensitivity of spatial correlations which can result in fullerene miscibility either increasing or decreasing with cooling. The calculations are contrasted with a simpler effective homopolymer model and the random structure Flory–Huggins model. The former appears to be qualitatively reasonable but can incur large quantitative errors since it misses preferential packing of monomers near nanoparticles, while the latter appears to fail qualitatively due to its neglect of all spatial correlations.

Effect of strain on doped graphene-based N/I/S junction with d-wave superconductivity

We investigate the effect of tensional strain on tunneling conductance in graphene-based normal/insulator/superconductor junction taking into account the anisotropic (d-wave asymmetry) superconductor pairing potential. By applying strain in the zigzag direction to graphene sheet, the highly asymmetric velocity of massless Dirac fermions can be provided. To study the conductance behavior based on Blonder–Thinkham–Klapwijk formalism in the d-wave pair coupling case, we must restrict ourselves to the large Fermi energy in the superconductor region, so that the incident angle of quasiparticles of superconductor region can be possible to calculate exactly in terms of modified wavevectors, kx and ky. In particular, investigation of the effect of Fermi energy mismatch in interface shows that it (EFS+U0) causes a reduction in conductance of structure only in the strain direction. In addition, we illustrate how d-type of pairing asymmetry affects the tunneling conductance in quite different behaviors in parallel and perpendicular directions of strain.

Disorder-Induced Rectification in a Molecular System

We study on the electron transport of an ensemble of coupled sites that simulates an array of quantum dots or a molecular system. By using the Green's function technique, we calculate current and shot noise for linear and disordered site arrays. While in the linear case the characteristic I-V curve reveals no current rectification, in the disordered con- figurations a robust rectification is found, thus indicating an operational regime typical of molecular diodes. Addi- tionally, a negative differential resistance is observed due to the drop of the bias voltage along the structure, which yields to an energy mismatch of neighboring sites. Finally, the Fano factor reveals a stronger transport correlation for positive than for negative bias voltages in the disordered site configuration.

Hierarchical MnO2/SnO2 Heterostructures for a Novel Free-Standing Ternary Thermite Membrane

We report the synthesis of a novel hierarchical MnO2/SnO2 heterostructures via a hydrothermal method. Secondary SnO2 nanostructure grows epitaxially on the surface of MnO2 backbones without any surfactant, which relies on the minimization of surface energy and interfacial lattice mismatch. Detailed investigations reveal that the cover density and morphology of the SnO2 nanostructure can be tailored by changing the experimental parameter. Moreover, we demonstrate a bottom-up method to produce energetic nanocomposites by assembling nanoaluminum (n-Al) and MnO2/SnO2 hierarchical nanostructures into a free-standing MnO2/SnO2/n-Al ternary thermite membrane. This assembled approach can significantly reduce diffusion distances and increase their intimacy between the components. Different thermite mixtures were investigated to evaluate the corresponding activation energies using DSC techniques. The energy performance of the ternary thermite membrane can be manipulated through different components of the MnO2/SnO2 heterostructures. Overall, our work may open a new route for new energetic materials.

Femtosecond OH Bending Dynamics of Water Nanopools Confined In Reverse Micelles

The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump-probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/DOPC reverse micelles, the OH bending lifetime T1 decreases from T1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w0 = 3) to T1 = 345 fs for a 16:1 ratio (w0 = 16). In contrast, H2O in Brij-30 reverse micelles shows a much shorter T1 = 400 fs at w0 = 2 which decreases to T1 = 250 fs at w0 = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small w0 result in a larger mismatch and longer T1 times. In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.

Theory of Core Hole Decay Dynamics of Adsorbate on Metal Surfaces

A comparative study of a core hole decay dynamics is made for an adsorbate on a metal surface. On the basis of the available experimental results for a CO molecule chemisorbed on Cu(110) as a prototype system, we investigate the elementary processes of the core hole decay via various new Auger channels open for adsorbates, i.e., the crossed Auger transition and the participation of an electron in the initially unoccupied level of the adsorbate created by charge transfer from the substrate before core hole decay or resonant core excitation. We calculate the valence Auger spectrum and deexcitation spectrum following resonant excitation. These spectra are compared with the direct valence photoemission, in particular with a single valence hole state screened by charge transfer from the metal. Absence of shake-up satellite in the deexcitation spectrum following resonant core to bound (e.g., 2π* of a CO molecule) level excitation and the mismatch of the binding energy of the screened final state between direct photoemission or normal Auger spectrum and resonantly excited Auger spectrum are explained in terms of the relaxation processes of the excited state before the Auger decay of the core hole.

Surface Plasmon Coupling of Compositionally Heterogeneous Core-Satellite Nanoassemblies.

Understanding plasmon coupling between compositionally heterogeneous nanoparticles in close proximity is intriguing and fundamentally important because of the energy mismatch between the localized surface plasmons of the associated nanoparticles and interactions beyond classical electrodynamics. In this Letter, we explore surface plasmon coupling between silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), assembled in the form of core-satellite structures. A recently developed assembly method allows us to prepare ultrapure core-satellite nanoassemblies in solution, where 50 nm AgNPs are surrounded by 13 nm AuNPs via alkanedithiol linkers. We observe changes in the plasmon coupling between the AgNP core and AuNP satellites as the core-to-satellite gap distance varies from 2.3 to 0.7 nm. Comparison with theoretical studies reveals that the traditional hybridized plasmon modes are abruptly replaced by charge-transfer plasmons at a ∼1 nm gap. Changes with the number of satellites are also discussed.

Quantum transmission across a low-dimensional nanoscale diffuse junction

The transmission across a nano-scale diffuse junction between two metals is studied numerically for 1D wires and 2D ribbons within a tight-binding model. In such a situation, a junction electric potential must appear to equilibrate the electrochemical potential on each side of the junction. We compute the transmission under two very different potential distributions and we find that the typical transmission does not depend on the details of this distribution. The transmission is found to follow a scaling law. The variations of the 1D transmission are explained in a semi-classical way by considering the multiple reflections caused by the barriers due to the on-site energy mismatch between the two metals. Both the 1D and 2D results can also be partly interpreted in the frame of the Anderson theory of localisation.

The 1.53 μm spectroscopic properties and thermal stability in Er3+/Ce3+ codoped TeO2–WO3–Na2O–Nb2O5 glasses

Er3+/Ce3+ codoped tellurite-based glasses with composition of TeO2–WO3–Na2O–Nb2O5 were prepared by high-temperature melt-quenching technique and a detailed study of the 1.53μm band spectroscopic properties and thermal stability was presented in this paper. The absorption spectra, visible upconversion spectra, 1.53μm band fluorescence spectra, fluorescence lifetimes, Raman spectra, differential scanning calorimeter (DSC) and X-ray diffraction (XRD) curves of glass samples were measured and investigated, together with the quantitative calculations and analyses of Judd–Ofelt intensity parameters, stimulated emission and absorption cross-sections, amplification quality factors, energy transfer rates and quantum transition efficiencies. It was found that the prepared glass samples have good thermal stability (Tg>425°C), large Judd–Ofelt intensity parameter (Ω6>0.80×10−20cm2) and bandwidth quality factor (σep×FWHM>360×10−21cm2nm) of Er3+. With the increasing of Ce3+ doped concentration, the visible upconversion emission decreased while the 1.53μm band fluorescence intensity improved significantly due to the enhanced energy transfer of Er3+:4I11/2+Ce3+:2F5/2→Er3+:4I13/2+Ce3+:2F7/2 owing to the moderate phonon energy (∼930cm−1) of glass host, which was beneficial in compensating for the existing energy mismatch between the Er3+:4I11/2→4I13/2 emission and Ce3+:2F5/2→2F7/2 absorption transitions. Compared with the Er3+ single-doped case, the 1.53μm band fluorescence intensity in Er3+/Ce3+ codoped glass sample could increase by about 30%. The results indicate that the prepared tellurite-based glass with a suitable Er3+/Ce3+ codoping concentration is an excellent gain medium applied for broadband EDFA pumped with a 980nm laser diode.

Tailoring nanobranches in three-dimensional hierarchical rutile heterostructures: a case study of TiO2–SnO2

We employed HNO3 as a hydrolysis control agent and developed three-dimensional (3D) hierarchical rutile heterostructures (TiO2–SnO2) with a high-density, uniform, and epitaxial TiO2 nanobranch tailored by a facile CBD method at a low temperature (∼50 °C). By investigating the influence of acids such as HNO3, HCl, H2SO4, and HCOOH on the formation of TiO2 nanobranches on SnO2 nanobelts (NBs), we demonstrate that the rate of crystal growth can be promoted by HNO3 and that the poor ligand affinity of NO3− and the total charge variation of the Ti complex by H+ alter the precipitation rates, eventually controlling the length of the TiO2 nanorods. Additionally, it was found that short and long TiO2 nanorods were generated via the mechanism of diffusional- and reaction-controlled growth under moderate and high concentrations of HNO3, respectively. TiO2 nanorods grew epitaxially along the [001] direction on all four sides of SnO2 NB like the spokes of a wheel. This was attributed to the minimization of surface energy and lattice mismatch. This facile, surfactant-free, and low-temperature synthetic strategy may provide an alternative route for fabricating uniform and length-controlled nanobranches in 3D heterostructures.

A Novel Self Adaptive Modification Approach Based on Bat Algorithm for Optimal Management of Renewable MG

In the new competitive electricity market, the accurate operation management of Micro-Grid (MG) with various types of renewable power sources (RES) can be an effective approach to supply the electrical consumers more reliably and economically. In this regard, this paper proposes a novel solution methodology based on bat algorithm to solve the op- timal energy management of MG including several RESs with the back-up of Fuel Cell (FC), Wind Turbine (WT), Photovoltaics (PV), Micro Turbine (MT) as well as storage devices to meet the energy mismatch. The problem is formulated as a nonlinear constraint optimization problem to minimize the total cost of the grid and RESs, simultaneously. In addition, the problem considers the interactive effects of MG and utility in a 24 hour time interval which would in- crease the complexity of the problem from the optimization point of view more severely. The proposed optimization technique is consisted of a self adaptive modification method compromised of two modification methods based on bat algorithm to explore the total search space globally. The superiority of the proposed method over the other well-known algorithms is demonstrated through a typical renewable MG as the test system.