With the depletion of fossil fuels and the environmental problems, the development and utilization of new energy resources is imminent. Hydrogen energy is one of the main new energy sources in the 21st century. Finding stable and efficient hydrogen storage materials is the key to achieving the hydrogen economy. Transition metal (TM)-decorated graphenes have been widely studied as hydrogen storage materials theoretically, but they suffer metal agglomeration and H2 dissociation. Our calculations show that the reconstruction energy of Sc, Ti, V decorated pristine graphenes in the process of adsorption and desorption of hydrogen molecules are only 0.00, 0.12 and 0.08 eV, respectively. The adsorption energy values of the first H2 dissociation adsorption on the Sc, Ti, V decorated pristine graphenes are –1.34, –1.34, and –1.16 eV, respectively. So, some hydrogen molecules are difficult to desorb at room temperature and medium pressure. In this paper, the stability and hydrogen storage properties of Sc, Ti, V decorated monovacancy graphene are also investigated based on density functional theory. The results show that the binding energy values between Sc, Ti, V and themonovacancy graphene are –6.93, –8.82, –9.30 eV, respectively, which indicate monovacancy can effectively avoid metal aggregation. The Sc, Ti and V atoms decorated on the monovacancy graphene would transfer more electrons to the carbon material with charge of +1.24|e|–+1.37|e|. They can adsorb 7, 3 and 4 hydrogen molecules through electrostatic interaction. When a monovacancy is introduced, all of the hydrogen molecules are adsorbed in molecular form. The average adsorption energy values of H2 are –0.13, –0.20 and –0.18 eV, respectively, which are in the best energy range for the adsorption/desorption process at room temperature and medium pressure. The most important thing is that their deformations in the adsorption/desorption process are very small, which is conducive to the rapid hydrogen adsorption/desorption. The calculated results show that the monovacancy introduction can effectively solve the two major problems, i.e. metal agglomeration and hydrogen molecular dissociation during hydrogen storage on Sc, Ti, V decorated pristine graphenes. The research in this paper will be helpful to further understand the hydrogen storage mechanism of 3d TM-decorated carbon nanomaterials.
Doping is one of the most important methods to improve the electronic conductivity and modify its electrochemical performance of LiFePO4. Rare earth elements have become an effective selection for doping modification due to their high electronic charges, large ion radii and strong self-polarization ability. In this work, we study the structural, electronic and ionic diffusion properties of LiFePO4 with rare earth (RE) doping (La, Ce, Pr) by using first-principles calculation based on density functional theory. The calculated results show that the lattice constant and cell volume of LiFePO4 increase to a different degree after RE doping. In the delithiation process, the volume change rate of the material after RE doping is significantly reduced, indicating the cycle performance of the material is improved, on the other hand, the energy density is reduced. The calculated density of states suggests that RE-doped LiFePO4 exhibits metallic characteristics, which is different from the undoped one with semiconductor characteristics. As a result, the RE-doping can increase the electronic conductivity of the material. The calculation of elastic modulus demonstrates the increase of ductility for REdoped LiFePO4, and it can be predicted that the cycle performance and the rate performance of the RE-doped battery have great improvement. In addition, La and Ce doped LiFePO4 materials exhibit that the complex energy barrier can change during the Li ion migration, and the migration barriers vary considerably, depending on different paths, which is related to the variation of potential energy surface caused by the doping of rareearth elements. The Li-ions are far from the RE ions, the migration barriers are obviously lower than the undoped one, while the Li-ions are closest to RE ions, the migration barriers increase essentially. Compared with Ce doping, the change of the Li-ion migration barrier caused by La doping is great, indicating that RE ion doping has a greater influence on the local structure of the system.
CH3NH3PbI3 is one of the most promising candidates for high-performance hybrid organic-inorganic perovskite solar cells. The CH3NH3PbI3 single crystal and polycrystalline thin film exhibit the unique features of long carrier lifetimes and diffusion lengths, however, their carrier mobilities are in fact rather modest in a range from 1 cm2·V–1·s–1 to 100 cm2·V–1·s–1. Experimentally, the temperature dependence of mobility is described as T–1.3 to T–1.6 due to the acoustic phonon scattering. To be sure, the rotating CH3NH\begin{document}${}_3^+ $\end{document} cations are disadvantageous to the carrier transport and performance for CH3NH3PbI3 solar cells. The effect of the rotating CH3NH\begin{document}${}_3^+ $\end{document} cations on high-performance CH3NH3PbI3 solar cells remains an open question. The Gaussian 09 software has been utilized to optimize the geometrical structures of CH3NH3 dimer, trimer, tetramer, and pentamer in isolated state at the MP2 level with using the cc-PVTZ basis set. For CH3NH3 polymer, the mean distance between two centroids of neighboring CH3NH3 decreasing with the number of CH3NH3 is slightly smaller than the lattice constant 6.28 Å of tetragonal CH3NH3PbI3, which is advantageous to structural stability and higher structural order of inorganic [PbI3]– framework. It signifies that the long range order of electrically neutral CH3NH3 is easily formed for room-temperature CH3NH3PbI3. The total dipole moment linearly increases with the number of CH3NH3 for CH3NH3 polymer, and attains a large value 19.7 Debye for CH3NH3 pentamer, which may be the origin of strong polarization in CH3NH3PbI3 heterojunction. The molecular orbitals of five unpaired electrons for CH3NH3 pentamer are distributed around NH3-sides of five different CH3NH3 pentamers respectively, and these orbital energies are in a range from –4.4 eV to –3.2 eV. The unpaired electrons in CH3NH3 polymer have an electrostatic attraction on the CH3-side of neighboring CH3NH3, which is the key cause of forming the ordered CH3NH3 polymer. Hence it can be inferred that the orbital energies of unpaired electrons are getting closer when the longer range order of CH3NH3 are formed in room-temperature CH3NH3PbI3 through the interfacial electron injection. The vector field map of electrostatic potential (ESP) shows that CH3NH\begin{document}${}_3^+ $\end{document} has strong electrophilic character, and the NH3-side has a stronger electrophilic character than CH3-side, however, CH3NH3 monomer and polymer have weak electrophilic and nucleophilic character. Thus, the forming of CH3NH3 polymer at the CH3NH3PbI3 heterojunction leads the organic and inorganic portions to be decoupled, which can effectively reduce the anharmonic phonon modes. Under an applied electric field, the unpaired electrons in CH3NH3 pentamer can transfer along the C-N axis through the hopping mechanism. According to these results, we can draw three useful conclusions below. i) The electrons under an applied electric field are easily injected into the CH3NH3PbI3 material through the heterojunction, the CH3NH3 polymer is easily formed, and the unpaired electrons in polymer are transferred between two neighboring CH3NH3 through hopping mechanism. ii) The decoupling between organic CH3NH3 and inorganic [PbI3]– framework can effectively reduce the anharmonic phonon modes, which can lead the carrier scattering decrease and the efficiency of carrier separation and transport to improve; iii) The ordered CH3NH3 polymer at the CH3NH3PbI3 heterojunction can enhance the order of inorganic [PbI3]– framework. Our researches may help to further understand the origin of high power conversion efficiency (PCE) for hybrid organic-inorganic perovskite solar cells.
Heterostructure engineering is an effective strategy to improve the optoelectronic properties of semiconductor materials. We propose a van der Waals (vdW) heterostructure based on perovskite CsPbX3 (X = Cl, Br, I) and two-dimensional penta-graphene (PG), and investigate the stabilities of two kinds of interface contacts (Pb-X and Cs-X) by first-principles calculations. And we also study the electronic structures and optoelectronic properties of CsPbX3-PG heterostructures with stabler Pb-X interface. Our results show that all the CsPbX3 (X = Cl, Br, I)-PG heterostructures possess the type-II band arrangement, that the energy level gap is gradually narrowed from Cl to I, and that there are good photogenerated carrier separation ability and charge transport property. Moreover, the absorption spectrum of CsPbX3-PG heterostructures can be broadened and the optical absorption ability is effectively improved. The power conversion efficiency (PCE) of CsPbX3-PG can increase up to 21% given by theoretical estimation. These results indicate that the optoelectronic properties of the all-inorganic metal halide perovskite CsPbX3-PG heterostructures can be effectively improved, which would become a potential candidate for high-performance photoelectric conversion devices.s.
The 15 μm emission spectrum corresponding to the 4I13/2→4I15/2 transition and upconversion of Er3+ in tellurite glass in the temperature range from 8 to 300K is studied. The emission spectrum of Er3+: 4I13/2→4I15/2 transition is also analyzed using a peak-fit routine, and an equivalent four-level system is proposed to estimate the Stark splitting for the 4I15/2 and 4I13/2 levels of Er3+ in the tellurite glass. The results indicate that the 4I13/2→4I15/2 emission of Er3+ has considerable broadening due to a significant enhancement of the a' and b' emission peaks. Temperature-dependent FWHM are investigated, the results show that a monotonic increase of FWHM is observed for temperatures from 8 to 300K. Intense upconversion emission signals around 529, 545 and 669nm corresponding to the 2H11/2, 4S3/2, and 4F9/2 transitions, respectively, to the 4I15/2 ground state are generated and are measured as functions of temperature in the 8 to 300K range. The most remarkable result is observed in the green upconversion signal around 546 and 669nm which presented a maximum intensity enhancement of 2198 and 1556 times around 80K compared with that around 300K. Monotonic decrease is observed at temperatures above 80K. In the same temperature range the signal at 529nm diminishes to zero with lowering temperature.
We propose a scheme for implementing the Grover search algorithm with two superconducting quantum interference devices (SQUIDs) in a cavity. Our scheme only requires single resonant interaction of the SQUID-cavity system and the required interaction time is very short. The simplicity of the process and the reduction of the interaction time are important for restraining decoherence.
This paper reports that the high-K HfO2 gate dielectrics are fabricated on n-germanium substrates by sputtering Hf on Ge and following by a furnace annealing. The impacts of sputtering ambient, annealing ambient and annealing temperature on the electrical properties of high-K HfO2 gate dielectrics on germanium substrates are investigated. Experimental results indicate that high-K HfO2 gate dielectrics on germanium substrates with good electrical characteristics are obtained, the electrical properties of high-K HfO2 gate dielectrics is strongly correlated with sputtering ambient, annealing ambient and annealing temperature.
Radiation effects of the floating gate read-only-memory (FG ROM) and the static random access memory (SRAM) have been evaluated using the 14 MeV neutron and 31.9MeV proton beams and Co-60 gamma-rays. The neutron fluence, when the first error occurs in the FG ROMs, is at least 5 orders of magnitude higher than that in the SRAMs, and the proton fluence, 4 orders of magnitude higher. The total dose threshold for Co-60 gamma-ray irradiation is about 10(4) rad (Si) for both memories. The difference and similarity are attributed to the structure of the memory cells and the mechanism of radiation effects. It is concluded that the FG ROMs are more reliable as semiconductor memories for storing data than the SRAMs, when they are used in the satellites or space crafts exposed to high energy particle radiation.
In the context of microwave cavity QED, this paper proposes a new scheme for teleportation of an arbitrary pure state of two atoms. The scheme is very different from the previous ones which achieve the integrated state measurement, it deals in a probabilistic but simplified way. In the scheme, no additional atoms are involved and thus only two atoms are required to be detected. The scheme can also be used for the teleportation of arbitrary pure states of many atoms or two-mode cavities.
This paper reports on the canard phenomenon occurring in a rheodynamic model of cardiac pressure pulsations. By singular perturbation techniques the corresponding parameter value at which canards exist is obtained. The physiological significance of canards in this model is given.
Long-term prediction of chaotic time series is very difficult, for the chaos restricts predictability. In this paper a new method is studied to model and predict chaotic time series based on minimax probability machine regression (MPMR). Since the positive global Lyapunov exponents lead the errors to increase exponentially in modelling the chaotic time series, a weighted term is introduced to compensate a cost function. Using mean square error (MSE) and absolute error (AE) as a criterion, simulation results show that the proposed method is more effective and accurate for multistep prediction. It can identify the system characteristics quite well and provide a new way to make long-term predictions of the chaotic time series.
This paper reports that the ultraviolet and visible upconversion luminescence from the (4)S(3/)2, (2)G(9/2) and P-2(3/2) levels have been observed in Er3+: YAG following 647.2 nm excitation of the F-4(9/2) multiple. Upconversion luminescence intensity dependence on pump power was recorded. The measured decay profiles were theoretically fitted by kinetics theory and the basically good agreements were achieved. The results indicate that some energy transfer processes proposed to explain the observed upconversion phenomena are reasonable.
This paper studies the evolution of wave in the system of a pure anharmonic lattice with a double well on-site potential by numerical calculation. It finds that an initial distribution of static or moving wave can evolve into two travelling soliton-like trains with contrary directions and a region of oscillation in this lattice system. It presents that some cases with cosine-square-shape and Gaussian-shape initial distribution of static or moving wave will produce ordered soliton-like train. Careful numerical observation shows that the centre oscillation region in this system may act as a resource of generating soliton-like train.
In this paper high-order harmonic generation (HHG) spectra and the ionization probabilities of various charge states of small cluster Na-2 in the multiphoton regimes are calculated by using time-dependent local density approximation (TDLDA) for one-colour (1064 nm) and two-colour (1064 nm and 532 nm) ultrashort (25 fs) laser pulses. HHG spectra of Na2 have not the large extent of plateaus due to pronounced collective effects of electron dynamics. In addition, the two-colour laser field can result in the breaking of the symmetry and generation of the even order harmonic such as the second order harmonic. The results of ionization probabilities show that a two-colour laser field can increase the ionization probability of higher charge state.
Taking the actual operating condition of complementary metal oxide semiconductor (CMOS) circuit into account, conventional direct current (DC) stress study on negative bias temperature instability (NBTI) neglects the detrapping of oxide positive charges and the recovery of interface states under the `low' state of p-channel metal oxide semiconductor field effect transistors (MOSFETs) inverter operation. In this paper we have studied the degradation and recovery of NBTI under alternating stress, and presented a possible recovery mechanism. The three stages of recovery mechanism under positive bias are fast recovery, slow recovery and recovery saturation.
Complex networks have been applied to model numerous interactive nonlinear systems in the real world. Knowledge about network topology is crucial to an understanding of the function, performance and evolution of complex systems. In the last few years, many network metrics and models have been proposed to investigate the network topology, dynamics and evolution. Since these network metrics and models are derived from a wide range of studies, a systematic study is required to investigate the correlations among them. The present paper explores the effect of degree correlation on the other network metrics through studying an ensemble of graphs where the degree sequence (set of degrees) is fixed. We show that to some extent, the characteristic path length, clustering coefficient, modular extent and robustness of networks are directly influenced by the degree correlation.
Er3+-doped lithium-potassium mixed alkali aluminophosphate glasses belonging to the oxide system xK(2)O-(15x)Li2O-4B(2)O(3)-11Al(2)O(3)-5BaO-65P(2)O(5) are obtained in a semi-continuous melting quenching process. Spectroscopic properties of Er3+-doped glass matrix have been analysed by fitting the experimental data with the standard Judd-Ofelt theory. It is observed that Judd-Ofelt intensity parameters-Omega(t)(t=2, 4 and 6) of Er3+ change when the second alkali is introduced into glass matrix. The variation of line strength S-ed[I-4(13/2),I-4(15/2)] follows the same trend as that of the Omega(6) parameter. The effect of mixed alkali on the spectroscopic properties of the aluminophosphate glasses, such as absorption cross-section, stimulated emission cross-section, spontaneous emission probability, branching ratio and the radiative lifetime, has also been investigated in this paper.
With the help of ab initio full-potential linearized augmented plane wave (FPLAPW) method, calculating the electronic structure and linear optical properties is carried out for XCd2(SO4)3 (X =Tl, Rb). The results show that Tl2Cd2(SO4)3 (TlCdS) has a larger band gap than Rb2Cd2(SO4)3 (RbCdS) and the energy bands for RbCdS are more dispersive than those of TlCdS. From their partial densities of states (PDOS), we have observed that the hybridization between S ionic 2p and O atomic 2p orbitals forms SO4 ionic groups. The remarkable difference between RbCdS and TlCdS is, however, the degree of hybridization between cation (Tl and Rb) and its surrounding oxygen atoms. In the view of quantum chemistry, the strong p-d hybridization indicates the existence of their cation ionic bonds (Cd-O, Rb-O, and Tl-O). The calculations of TlCdS and RbCdS show their optical properties to be less anisotropic. Their anisotropies in the optical properties mainly occur in a low photon energy region of 5-16 eV.
The evolution of shock waves produced by 7 ns laser pulses in air is investigated by time-resolved shadowgraph. A nodular structure of the shock wave is observed. It is found that the origin of the structure is the multi-longitudinal-microfocus caused by the astigmatism of the laser beam. The spherical shock waves formed by each microfocus expand gradually and collide with each other, resulting in the nodular structure of the shock wave.
The fundamental and second order strongly nonlocal solitons of the nonlocal nonlinear Schrodinger equation for several types of nonlocal responses are calculated by Ritz's variational method. For a specific type of nonlocal response, the solutions of the strongly nonlocal solitons with the same beam width but different degrees of nonlocality are identical except for an amplitude factor. For a nonlocal case where the nonlocal response function decays in direct proportion to the mth power of the distance near the source point, the power and the phase constant of the strongly nonlocal soliton are in inverse proportion to the (m+ 2)th power of its beam width.
