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Different from conventional ferromagnetic methods, producing a spin-polarized current through electrical ways in spintronic devices can greatly increase operating speed, reduce power consumption, and improve device integration. Inspired by recent experimental progress on the synthesis of a heptauthrene molecule, we investigate its spin-dependent transport contacted with Au electrodes through first-principles calculations. By applying a gate voltage, the transmission can be switched between completely spin-up and spin-down polarized states, achieving an electrically controlled dual-spin filter. Furthermore, a fine tuning on the spin polarization, between 100% and − 100%, can also be realized, where the transport with any ratio of spin-up to spin-down electron quantities can be realized beyond the traditional devices. The peculiar transmission spectra and their shift are found to play crucial roles, where transmission peaks distribute on both sides of the Fermi level with opposite spin components. Such a spin-polarization modulating effect is found to be robust to the molecule-electrode contacting site, indicating it is an intrinsic feature of such systems. Moreover, the dimension of the device is at the single-molecule level, suggesting great application potential. [ABSTRACT FROM AUTHOR]

Inspired by the recent experimental progress on the synthesization of a new type of molecule, a polyyne wire doped with the organometallic Ru (dppe) 2 fragment, which exhibits superior electronic properties to similar organic molecules reported before, we studied the electronic transport of it contacted with Au electrodes through first-principles calculations. Multiple striking negative differential resistance (NDR) behavior is observed, where the current could decrease to nearly zero. Moreover, such a phenomenon is found to be robust to the conformational change of the molecule, indicating it is the intrinsic feature of it. Further analysis shows that it is the suppression of the transmission peaks at the border of the bias window that results in the NDR. And, the suppression is resulted from the decay of the local density of states in the two-probe system, especially in the central axis of the molecule, where the states are found to originate from the p orbital of C atoms and the d orbital of Ru atoms. The multiple striking NDR behavior is achieved at the single-molecule level under low bias, showing great application potentials. These findings may throw light on the development of molecular devices. [ABSTRACT FROM AUTHOR]

Compared with conventional magnetic ways, modulating a spin-polarized current through electrical means could greatly reduce nano-devices' energy consumption and dimensions, which emerges as a new research area in spintronics. Inspired by the experimental progress on the synthesis of helically twisted graphene, we study the electronic transport of kekulene-like helical graphene nanoribbons through first-principles calculations. By applying a gate voltage, the system can be switched between spin-unpolarized and completely spin-polarized states, realizing an electrically controlled spin filter. Moreover, a fine modulation of the spin polarization can also be achieved, where transmission with any ratio of spin-up to spin-down electrons can be obtained, beyond the traditional spin filter. The analysis shows that it is the particular transmission spectra that play a key role, where two wide peaks with opposite spins reside partially overlapped around the Fermi level. They originate from the p orbitals of the zigzag edge part in the helical structure. Since the configuration only consists of carbon atoms, the electrical control of spin polarization is realized in a pure-carbon nano-system, showing great application potentials. [ABSTRACT FROM AUTHOR]

Carbon quantum dots (CDs) with favorable luminescent features for biphotonic applications have attracted much interest in modulating their photoluminescence (PL) efficiency. A surface state with various defects is believed to play a key role in the emissive intensity. Here, pressure-induced quenching of PL is observed in red emissive CDs (R-CDs) and is ascribed to defects in carbon cores upon compression. In the power-law fitting to the excitation power-dependent PL of R-CDs at high pressure, the coefficient k parameter related to the emissive mechanism decreases from 1 under ambient pressure to much less than 1 under the application of pressure, suggesting a transition from single exciton recombination to defect-related emission. With the k parameter decreasing to 0.69 at 1.6 GPa, the pressure-induced defects reduce the PL intensity by approximately one order of magnitude. Furthermore, the attenuation and broadening of the G band characterizing the sp2 hybrid structure of carbon cores in the Raman spectra for R-CDs at high pressure support that the pressure-induced lattice relaxation impairs the crystalline symmetry of the carbon core and results in the dramatic quenching of PL. Our results highlight the importance of the well-crystallized carbon core in designing CDs with high quantum yields. [ABSTRACT FROM AUTHOR]

This article explores the transmission effects of fiscal and credit policies on investment by renewable energy enterprises using a generalized method of moments model applied to enterprise-level panel data. Our empirical results demonstrate that subsidies and taxation rebates received from central and local governments have significantly positive impacts on renewable energy enterprises' investment efficiency. Short-run, long-run, and total credit loans obtained from financial institutions have significantly positive impacts on renewable energy enterprises' investment. Subsidies and credit loans and taxation rebates and credit loans represent significantly positive interactions on renewable energy enterprises' investment. Variations in subsidies, taxation rebates, and credit loans induce interactions to become more sensitive. Overall, the results demonstrate that fiscal and credit policies are able to effectively incentivize renewable energy enterprises to invest in more profitable renewable energy projects and then enhance investments in tangible assets. [ABSTRACT FROM AUTHOR]

Titanium nitride (TiN) exhibits extremely high wear resistance and corrosion resistance, leading to a broad range of applications. Despite the extensive theoretical efforts on the structural stability of TiN under pressure, a few experimental observations of the pressure-induced phase transition through x-ray diffraction are still in debate. Here, the Raman spectra of TiN under pressure are studied in a diamond anvil cell up to 36 GPa. The discontinuity in the pressure dependent phonon frequencies suggests a phase transition at around 5 GPa with the bonding between Ti and N hardly changing. The phase transition is ascribed to an isostructural transition or a vacancy filling mechanism. [ABSTRACT FROM AUTHOR]

Inspired by recent progress of experimental fabrication of partial structure of the helical graphenes, we construct helical graphene nanoribbons and study the electronic structure and transport properties of them, through first-principles calculations. It is found that a metal-semiconductor transition occurs when the pitch of the configuration changes, and the whole transition process can be divided into three regimes, i.e., metallic, fast changing bandgap, and slowly changing bandgap ones. Further analysis shows that, interlayer transmission, interlayer interaction (bilayer graphenelike state) and intralayer interaction are the mechanisms behind, respectively. Moreover, such a transition and corresponding mechanisms show good robustness to the size of the helical graphene nanoribbons, suggesting great application potential. As the systems are pure-carbon and graphene-based ones, as well as an adjustable and reversible bandgap, such configurations might be quite useful in future nanodevices. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) semiconductor lead iodide (PbI2) has recently attracted considerable attention owing to its favorable properties in both applications as photodetectors and as a precursor for lead halide perovskite solar cells. Although earlier experiments have investigated the structural stability of PbI2 below 20 GPa, the electric structures at high pressure remain ambiguous, as does the crystal structure at higher pressures. Herein, a structural phase transition and a semiconductor-metal transition are revealed in PbI2 through high-pressure femtosecond optical pump-probe spectroscopy combined with Raman spectra, synchrotron x-ray diffraction (XRD), and resistance measurements up to 70 GPa. Two discontinuities appear in the pressure-dependent amplitude of the ultrafast spectroscopy at approximately 24.8 and 37.6 GPa. Raman spectra and in situ XRD patterns confirm a structural phase transition from orthorhombic Pnma to tetragonal I4/MMM symmetry at the first discontinuity. The second discontinuity is ascribed to the closure of the bandgap and the enhanced electron-phonon interaction across the semiconductor-metal transition, which is also revealed by the temperature dependencies of resistance for PbI2 under pressure. Our results not only help to design optical devices based on lead iodide but also highlight that ultrafast spectroscopy is an efficient noncontact tool to investigate the crystalline and electric phase transition under pressures simultaneously. [ABSTRACT FROM AUTHOR]

2H-type lead iodide (PbI2) is a two-dimensional (2D) layered semiconductor and has recently received considerable attention as a precursor for lead halide perovskite solar cells. Although the pressure-induced structural phase transitions of PbI2 have been investigated for several decades, the detailed crystalline structure of PbI2 at high pressure is still elusive because of lacking in situ x-ray diffraction (XRD) studies. In this report, in situ Raman spectra and XRD patterns of 2H-PbI2 under pressure have been investigated in a diamond-anvil cell. Both Raman spectra and XRD patterns provide consistent evidence for two structural phase transitions upon compression at approximately 0.58 and 2.6 GPa. The XRD patterns suggest that the complete pressure-driven phase transition sequence of PbI2 up to 20 GPa is two-dimensional hexagonal 2H- P 3 ¯ m 1 →polytype 4H-P3m1→three-dimensional (3D) orthorhombic Pnma. The uncovered 2D to 3D structural transition in 2H-PbI2 advances the understanding of structural stability of 2H-type layered semiconductors and lead halide perovskite. [ABSTRACT FROM AUTHOR]

Owing to the influence of uncontrollable factors (e.g., climate), wind and photovoltaic (PV) energy are highly fluctuating and undispatchable, thereby hindering their integration to the grid. To address this issue, complementary systems using hydropower as a regulating energy are gradually emerging. With the development of wind and PV power industries and the electricity spot market, wind and PV power subsidies are being reduced, and the bidding subjects are diverse. It is likely that wind, PV, and hydropower will participate in the market with the same quality and price. Wind–photovoltaic–hydropower complementary systems (WPHCSs) normally aim at minimum wind and PV power curtailment, which do not consider hydropower production aspects. This study considers the effect of complementary operation on hydropower in the context of energy indifference bidding in the future electricity spot market, establishes a comprehensive benefit index system to optimize WPHCS, and proposes a particle swarm optimization algorithm that introduces the dominant solution with an adjustable parameter to solve the maximizing comprehensive benefits of a complementary system model. Control experiments with different loads, storage capacities, inflows, and models were conducted to verify the superiority of the proposed model and to summarize the optimal operation rule of the complementary system. The results show that turbine efficiency is the main factor affecting the results, and the total benefit of the system could be improved by up to 69% comparing with the priority consumption of wind and the PV power model. This paper describes the WPHCS benefits more comprehensively, and the results can be used to improve the benefits of complementary systems. [ABSTRACT FROM AUTHOR]

Vacancy engineering can effectively modulate the optical and electronic properties of metal oxides. Here, we demonstrate that high-pressure could be a clean strategy to tune the vacancies in oxides with a high cationic vacancy content. By combining in situ synchrotron x-ray diffraction, Raman scattering, and charge transport measurements in a diamond anvil cell, we systematically study the structure and electrical properties of TiO with ∼16% ordered vacancies up to 50.2 GPa at room temperature. The monoclinic TiO transforms to the cubic phase at ∼37.8 GPa. After decompression to ambient conditions, the cubic phase survives. The vacancies are partially filled and become disordered with a concentration of approximately 12.5%. The charge transport of TiO at high pressure exhibits a metal-insulator transition, which originates from the ordered to disordered transition of vacancies under pressure. Molecular dynamics simulations suggest that the vacancies enhance the mobility of atoms in the lattice under pressure and lead to the pressure-induced amorphization and recrystallization. [ABSTRACT FROM AUTHOR]