Energy Mismatch Research Articles

Single-crystalline aluminum grown on MgAl2O4 spinel using molecular-beam epitaxy

Al thin films were grown on MgAl2O4 spinel substrates using solid-source molecular-beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation and growth temperature. Scanning electron microscopy and atomic force microscopy show that low growth temperatures lead to smoother and more coalesced Al films. X-ray diffraction and electron backscatter diffraction (EBSD) measurements demonstrate that Al layers are single crystalline and coherently grown on both (100)- and (111)-oriented spinel substrates. EBSD data also clearly reveal a high density of twin domain structures in Al films grown on (111) spinel substrates, and all twin boundaries were determined to be Σ3 boundaries. High-resolution transmission electron microscopy was used to confirm the presence of twin structures. Al grown on (001) spinel roughens more easily than Al grown on (111) spinel at higher growth temperatures. It is suggested that Al surface energy and thermal expansion mismatch play a critical role in the evolution of surface morphology of Al thin films grown on MgAl2O4 spinel.

Tb3+–Er3+ couples as spectral converters in NaYF4 for GaAs solar cells

In order to reduce the thermal loss due to spectral mismatch of solar cell absorption, the quantum cutting with Tb3+–Er3+ couples as spectral converters is experimentally observed. One high-energy ultraviolet photon (Tb3+ 7F6 → 5L1) is quantumly cut into two lower energy photons: one in the near-infrared region (Er3+ 4I9/2 → 4I15) and the other in the blue region (Tb3+ 5D4 → 7F6), both of which can be efficiently absorbed by solar cells. A quantum efficiency, ηQE, of up to 188% is calculated, which is close to the theoretical limit of 200%. The energy mismatch in the energy transfer from Tb3+ (5L1 → 5D4) to Er3+ (4I9/2 → 4I15/2) is 237 cm−1, less than the phonon energy of 400 cm−1 in NaYF4, making the energy transfer nearly resonant. The energy migration among Tb3+ donors is treated approximately by the diffusion model and the initial process of energy transfer among the Tb3+–Er3+ couples is found to be dipole–dipole interactions.

Capitalizing on energy supply: Western China's opportunity for development

Two kinds of disparities pervade China and threaten its well-being. The first, regional disparities focus on levels of economic development, which vary considerably across China. The second is largely a corollary of the first, referring to mismatch in energy supply and demand, with some places suffering severe shortages while others are blessed with significant surpluses. Western China enjoys the dubious distinction of recording the country's lowest levels of economic development while, paradoxically, being blessed with plentiful reserves of energy and non-energy minerals. Turning those surplus resources to good account through transferring them to minerals and energy-hungry Eastern China is seen by policy-makers as something of a panacea. Not only will such a strategy significantly boost Western China's economic prospects, but it will eliminate the resource shortages currently constraining the East's vibrant growth. The issues of regional disparities, energy mismatches and transfers of these resources are discussed, with attention given to both spatial and time perspectives. The paper concludes with a cautious endorsement of the policy initiatives that promote the strategy of mineral transfers.

Zero energy buildings and mismatch compensation factors

This paper takes an overall energy system approach to analysing the mismatch problem of zero energy and zero emission buildings (ZEBs). The mismatch arises from hourly differences in energy production and consumption at the building level and results in the need for exchange of electricity via the public grid even though the building has an annual net-exchange of zero. This paper argues that, when looked upon from the viewpoint of the overall electricity supply system, a mismatch can be both negative and positive. Moreover, there are often both an element of levelling out mismatches between individual buildings and an element of economy of scale. For these three reasons mismatches should be dealt with at the aggregated level and not at the individual level of each building. Instead, this paper suggests to compensate the mismatch of a building by increasing (or decreasing) the capacity of the energy production unit. Based on historical data for the electricity supply area in western Denmark, this paper makes a first attempt to quantify mismatch compensation factors. The results indicate that such compensation factors are a little below one for buildings with photovoltaics (PV) and a little above one for buildings with wind turbines.

Phonon‐assisted decoherence and tunneling in quantum dot molecules

AbstractWe study the influence of the phonon environment on the electron dynamics in a doped quantum dot molecule. A non‐perturbative quantum kinetic theory based on correlation expansion is used in order to describe both diagonal and off‐diagonal electron‐phonon couplings representing real and virtual processes with relevant acoustic phonons.We show that the relaxation is dominated by phonon‐assisted electron tunneling between constituent quantum dots and occurs on a picosecond time scale. The dependence of the time evolution of the quantum dot occupation probabilities on the energy mismatch between the quantum dots is studied in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

用于GaAs太阳能电池的NaYF4中Tb3+-Er3+耦合对的光谱转换

In order to reduce thermal loss due to spectral mismatch of the solar cell absorption,the quantum cutting with Tb3+-Er3+ couples as spectral converters is experimentally observed.One high energy ultraviolet photon(7F6→5L1 of Tb3+) is quantumly cut into two lower energy photons:one is in near-infrared(4I9/2→4I15 of Er3+) and the other in blue region(5D4→7F6 of Tb3+),both of which can be efficiently absorbed by solar cells.A quantum efficiency,ηQE of up to 188% is calculated which is close to the theoretical limit of 200%.The energy mismatch in the energy transfer from Tb3+(5L1→5D4)to Er3+(4I9/2→4I15/2) is 237 cm-1,less than a phonon energy of 400 cm-1 in NaYF4,making the energy transfer nearly resonant.The energy migration among Tb3+ donors is treated approximately by the diffusion model and the initial process of energy transfer among the Tb3+-Er3+ couples is found to be dipole-dipole interactions.

Growths of InAs/GaAs and InAs/In x Ga1-x As/GaAs nanowire heterostructures

InAs/GaAs and InAs/In xGa1-xAs/GaAs nanowire heterostructures are grown by metal organic chemical vapor deposition via Au-assistant vapor-liquid-solid mechanism. We find that the InAs nanowires grow directly on GaAs nanowires in a random way, or they grow along the sidewall of the GaAs nanowires, and thet InAs nanowires grow vertically on GaAs nanowires by using an In x Ga1-xAs (0≤x≤1) buffer segment. It can be concluded that the influences of crystal lattice mismatch and difference in interfacial energy can be eliminated by inserting a ternary compound semiconductor buffer segment, thereby improving the crystal quality and the capability to control the growth of nanowire heterostructure.

Ionization of decaborane with controlled hydrogen content by charge transfer from ambient gas

ABSTRACTWe present the ionization of decaborane (B10H14) and formation of hydrogen- and boron-contents-controlled B10-yHx+ through the charge transfer from ambient gas ion to decaborane molecules in an external quadrupole static attraction ion trap. The charge transfer energy is estimated from the experimentally observed products. PBE0/6-311+G(d)//B3LYP/6-31G(d) level of DFT calculations are conducted to investigate the mechanism of charge transfer from ambient gas ion. The calculation of the difference of ionization energies and mismatch of orbital energies between decaborane and ambient gas reveals the mechanism of ionization.

Biological inspired secure autonomous routing mechanism for wireless sensor networks

The field of wireless sensor network (WSN) is an important and challenging research area today. Advancements in sensor networks enable a wide range of environmental monitoring applications. Multihop routing in WSN is affected by new nodes constantly entering/leaving. Moreover, secure routing is a difficult problem due to the resource limitations in WSN. Thus, biological inspired algorithms are reviewed and enhanced to tackle the problems. Ant routing and human security system have shown excellent performance. Certain parameters as energy level, velocity, packet reception, dropping, mismatch rates and packet sending power are considered while making decision. The decision will come up with the optimal route and also to take best action against security attacks. In this paper, the design and initial work of BIOlogical Inspired Secure Autonomous Routing Protocol (BIOSARP) is presented. The proposed bio-inspired mechanism will meet the enhanced WSN requirements, including better delivery ratio, less energy consumption and routing overhead.

Efficient non-resonant energy transfer in Nd^3+-Yb^3+ codoped Ba-Al-metaphosphate glasses

An efficient Nd3+→Yb3+ energy transfer in a new series of alkali-free barium-alumino-metaphosphate glasses with a transfer efficiency reaching up to 95% has been reported here. It is due to the effective phonon assistance arising with the excellent matching of the present host phonon energy with the energy mismatch between Nd3+(4F3/2) and Yb3+(2F5/2) excited levels. The energy transfer microparameters for Nd3+→Yb3+ forward and back energy transfers are estimated from the spectral data analysis. A parameter, ΦET(=CDANd-Yb/CDANd-Nd), is proposed as a quantitative measure of sensitization ability per unit loss of the donor. The parameter is found to be highest for the presently reported barium-alumino-metaphosphate glasses.

Mixed alkali effect in xK 2O-(30 − x) Na 2O-30P 2O 5-40ZnO glasses

The mixed alkali effect in xK 2O-(30 − x) Na 2O-30P 2O 5-40ZnO was firstly observed. Density, DSC, DC electrical conductivity, chemical durability and IR have been carried out for xK 2O-(30 − x) Na 2O-30P 2O 5-40ZnO glasses. The DC electrical conductivity was measured in the temperature range between 100 °C and < T g . The strength of the MAE in T g , DC electrical conductivity, and activation energy has been determined. It is observed that the strength of MAE in DC electrical conductivity is less pronounced with the increase of temperature. The extremum of the glass transition temperature, the DC electrical conductivity, the activation energy, and the DR were observed along with the variation of K alkali ratio [K]/[K + Na] (alkali dosage is constant). The results showed that extremum was caused by the large energy mismatch for ions jumping between dissimilar alkalis' sites as Swenson's structural model suggested, generally shown as the K + vibration pathway blocking by the Na + due to the difference of ions' diameter. IR convinced the good correlation between MAE of xK 2O-(30 − x) Na 2O-30P 2O 5-40ZnO system glasses and the origination of metaphosphate structure, secondarily pyrophosphate, and [ZnO 6] octahedral.

Mode-specific intermolecular vibrational energy transfer. II. Deuterated water and potassium selenocyanate mixture

Vibrational energy transfer from the first excited state (2635 cm(-1)) of the O-D stretch of deuterated water (D(2)O) to the 0-1 transition (2075 cm(-1)) of the CN stretch of potassium selenocyanate (KSeCN) in their 2.5:1 liquid mixture was observed with a multiple-mode two dimensional infrared spectroscopic technique. Despite the big energy mismatch (560 cm(-1)) between the two modes, the transfer is still very efficient with a time constant of 20 ps. The efficient energy transfer is probably because of the large excitation coupling between the two modes. The coupling is experimentally determined to be 176 cm(-1). An approximate analytical equation derived from the Landau-Teller formula is applied to calculate the energy transfer rate with all parameters experimentally determined. The calculation results are qualitatively consistent with the experimental data.

Magnetic gap effect on the tunneling conductance in a topological insulator ferromagnet/superconductor junction

The tunneling conductance on the surface of a topological-insulator-based ferromagnet/superconductor ( F / S ) structure is studied where S is an s-wave superconductor with superconducting order parameter ∼Δ. The conductance is calculated based on the BTK formalism. The magnetization in F is applied along the z-direction ( m → = 〈 0 , 0 , M 〉 ) in order to induce the energy-mass gaps ( m) for the Dirac electrons in the F-region. In this work, the influence of energy gap due to the magnetic field in the F-region on the conductance is emphasized. The Fermi energy mismatch between F ( E FF = E F ) and S ( E FS = E F + U ), where the gate potential U is applied to the electrode on top of S, is also considered. As a result, a biased voltage V can cause the conductance switch at e V = Δ , depending on the value of the magnetic field. The conductance is found to be linearly dependent on either m or U. The slope of the curve can also be adjusted. This linear behavior in a topological-insulator-based F / S structure may be valuable for electronic applications of the linear-control-current devices. The tunneling conductances of the quasi-Dirac-particle in a topological-insulator-based F / S junction are quite different from those of a graphene-based F / S junction.

Phonon-assisted energy up-conversion process of [formula omitted] ions doped in [formula omitted] crystal

Er 3+ ions doped in KY F 4 crystal emit blue photoluminescence around 406 nm by energy up-conversion process when excited by 532 or 488 nm laser. By measuring the temperature dependence of up-converted luminescence, we found that the up-conversion process is enhanced with increasing temperature, which is an indication that phonons are assisting the process. This observation is supported by the fact that the energy transfer responsible for the up-conversion involves two transitions that have an energy mismatch.

Mode-specific intermolecular vibrational energy transfer. I. Phenyl selenocyanate and deuterated chloroform mixture

Vibrational energy transfer from the first excited state (2252 cm−1) of the C–D stretch of deuterated chloroform (DCCl3) to the 0-1 transition (2155 cm−1) of the CN stretch of phenyl selenocyanate (C6H5SeCN) in their 1:1 liquid mixture was observed with a pump/probe two-color two dimensional infrared spectroscopic technique. The mode-specific energy transfer can occur mainly because of the long vibrational lifetime of the CN stretch first excited state (∼300 ps) and the relatively strong hydrogen-bond between the C–D and CN (calculated H-bond formation energy in gas phase ∼−5.4 kcal/mol). The mode-specific energy transfer is relatively low efficient (only ∼2%), which is mainly because of the relatively short vibrational lifetime (∼9 ps) of the C–D stretch first excited state and the big donor/acceptor energy mismatch (97 cm−1) and the slow transfer kinetics (1/kCD→CN=330 ps).

Cavity quantum electrodynamics with semiconductor quantum dots: Role of phonon-assisted cavity feeding

For a semiconductor quantum dot strongly coupled to a microcavity, we theoretically investigate phonon-assisted transitions from the exciton to a cavity photon, where the energy mismatch is compensated by phonon emission or absorption. By means of a Schrieffer-Wolff transformation we derive an effective Hamiltonian, which describes the combined effect of exciton-cavity and exciton-phonon coupling, and compute the scattering rates within a Fermi golden rule approach. The results of this approach are compared with those of a recently reported description scheme based on the independent Boson model [U. Hohenester et al., Phys. Rev. B 80, 201311(R) (2009)], and a numerical density matrix approach. All description schemes are shown to give very similar results. We present results for the spontaneous emission lifetime of a quantum dot initially populated with a single exciton or biexciton, and for the spectral properties of an optically driven dot-cavity system operating in the strong coupling regime. Our results demonstrate that phonon-assisted feeding plays a dominant role for strongly coupled dot-cavity systems, when the detuning is of the order of a few millielectron volts.

Calculations of nuclear excitation by electron capture (NEET) in nonlocal thermodynamic equilibrium plasmas

The nuclear excitation by electron capture (NEET) process may occur when the energy differences between two nuclear levels and between two electronic states are nearly equal, provided the quantum selection rules are fulfilled. These resonant conditions drastically limit the number of possible candidates, even though thermodynamic conditions encountered in hot dense plasmas do modify the orbital electronic binding energy and the resonance conditions. $^{201}\mathrm{Hg}$, with a low-lying isomeric state located at $1.565$ keV, can be excited by NEET process in a laser-created plasma. However, its correct calculation requires nonlocal thermodynamic equilibrium (non-LTE) atomic physics treatment because current laser-created plasmas do not reach high-enough temperature in the area at LTE. In this article, we describe the calculation leading to an estimated excitation rate and discuss the influence of LTE$/$non-LTE physics with an average-atom model and the use of a Gaussian variance calculation to estimate the broadening around the mean energy mismatch.

Optical Spectroscopy of Rare Earth Ion-Doped TiO2 Nanophosphors

Trivalent rare-earth (RE3+) ion-doped TiO2 nanophosphors belong to one kind of novel optical materials and have attracted increasing attention. The luminescence properties of different RE3+ ions in various TiO2 nanomaterials have been reviewed. Much attention is paid to our recent progresses on the luminescence properties of RE3+ (RE = Eu, Er, Sm, Nd) ions in anatase TiO2 nanoparticles prepared by a sol-gel-solvothermal method. Using Eu3+ as a sensitive optical probe, three significantly different luminescence centers of Eu3+ in TiO2 nanoparticles were detected by means of site-selective spectroscopy at 10 K. Based on the crystal-field (CF) splitting of Eu3+ at each site, C2v and D2 symmetries were proposed for Eu3+ incorporated at two lattice sites. A structural model for the formation of multiple sites was proposed based on the optical behaviors of Eu3+ at different sites. Similar multi-site luminescence was observed in Sm(3+)- or Nd(3+)-doped TiO2 nanoparticles. In Eu(3+)-doped TiO2 nanoparticles, only weak energy transfer from the TiO2 host to the Eu3+ ions was observed at 10 K due to the mismatch of energy between the TiO2 band-gap and the Eu3+ excited states. On the contrary, efficient host-sensitized luminescences were realized in Sm(3+)- or Nd(3+)-doped anatase TiO2 nanoparticles due to the match of energy between TiO2 band-gap and the Sm3+ and Nd3+ excited states. The excitation spectra of both Sm(3+)- and Nd(3+)-doped samples exhibit a dominant broad peak centered at approximately 340 nm, which is associated with the band-gap of TiO2, indicating that sensitized emission is much more efficient than direct excitation of the Sm3+ and Nd3+ ions. Single lattice site emission of Er3+ in TiO2 nanocrystals can be achieved by modifying the experimental conditions. Upon excitation by a Ti: sapphire laser at 978 nm, intense green upconverted luminescence was observed. The characteristic emission of Er3+ ions was obtained both in the ultraviolet-visible (UV-vis) and near-infrared regions through the high-resolution experiments at 10 K. The CF experienced by Er3+ in TiO2 nanocrystal was systematically studied by means of the energy level fitting.

Transition states in Ei reactions

AbstractThe pyrolysis of amine oxides, sulfoxides, selenoxides, and esters to form alkenes is believed to be a concerted reaction with a cyclic transition state. Phosphine oxides, sulfones, and nitro compounds are unreactive. This study seeks to identify reasons for the lack of reactivity of the latter. Transition states were located for all substrates progressing from RHF/3‐21G* to the MP2/6‐31+G(d,p) level (in certain cases). For sulfones and nitro compounds, two possible reasons for lack of reactivity were considered: (1) Atoms approaching one another in the transition state may be considered to participate in a local nO → σ*CH interaction. The second oxygen in the sulfone or nitro compound lowers the energy of the ‘non‐bonded electrons’ at oxygen leading to a greater mismatch in energy with the antibonding CH orbital (in comparison to the sulfoxide or amine oxide). (2) The sulfone and nitro ‘lone pairs’ are not really ‘alone’, and available to react. In fact, complex bonding arrangements exist in the SO2 and NO2 groups. The situation is less clear for phosphine oxides, although ground state stability of bonds is important. Implications concerning the Hammond postulate are covered. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Study of Energy Transfer and Triplet Exciton Diffusion in Hole‐Transporting Host Materials

AbstractA device structure is used in which the hole‐transporting layer (HTL) of an OLED is doped with either fluorescent or phosphorescent emitters, that is, anode/HTL‐host/hole blocker/electron‐transporting layer/cathode. The HTL hosts have higher HOMO energy allowing holes to be transported without being trapped by dopant molecules, avoiding direct recombination on the dopant. The unconventional mismatch of HOMO energies between host and dopant allow for the study of energy transfer in these host/guest systems and triplet exciton diffusion in the HTL‐host layers of OLED devices, without the complication of charge trapping at dopants. The host materials examined here are tetraaryl‐p‐phenylenediamines. Data shows that Förster energy transfer between these hosts and emissive dopant in devices is inefficient. Triplet exciton diffusion in these host materials is closely related to molecular structure and the degree of intermolecular interaction. Host materials that contain naphthyl groups demonstrate longer triplet exciton diffusion lengths than those with phenyl substituents, consistent with DFT calculations and photophysical measurements.