Ruifeng Lu

Half-metallicity in organic single porous sheets.

The unprecedented applications of two-dimensional (2D) atomic sheets in spintronics are formidably hindered by the lack of ordered spin structures. Here we present first-principles calculations demonstrating that the recently synthesized dimethylmethylene-bridged triphenylamine (DTPA) porous sheet is a ferromagnetic half-metal and that the size of the band gap in the semiconducting channel is roughly 1 eV, which makes the DTPA sheet an ideal candidate for a spin-selective conductor. In addition, the robust half-metallicity of the 2D DTPA sheet under external strain increases the possibility of applications in nanoelectric devices. In view of the most recent experimental progress on controlled synthesis, organic porous sheets pave a practical way to achieve new spintronics.

Generation of isolated sub-10-attosecond pulses in spatially inhomogenous two-color fields.

We present a theoretical investigation of high-order harmonic generation in spatially inhomogeneous two-color laser fields by solving three dimensional time dependent Schrödinger equation. The cutoff in the harmonic spectra can be significantly extended by means of our proposed method (i.e., from helium interacting with the plasmon-enhanced two-color laser fields), and an ultrabroad supercontinuum up to 1.5 keV is generated by selecting proper carrier-envelope phase of the controlling field. Moreover, classical trajectory extraction, time-dependent ionization and recombination rates, and time-frequency analyses are used to explain the generation of this ultrabroadband supercontinuum. As a result, an isolated 8.8 attosecond pulse can be generated directly by the superposition of the supercontinuum harmonics.

Efficient band structure tuning, charge separation, and visible-light response in ZrS2-based van der Waals heterostructures

As a fast emerging topic, van der Waals heterostructures can modify two-dimensional (2D) layered materials with desired properties, thus greatly extending the applications of these materials. Via state-of-the-art first-principles calculations, we systematically study four types of van der Waals heterostructures formed by monolayer graphene, h-BN, g-C3N4, and polyphenylene on ZrS2 nanosheets. A direct band gap can be obtained in the graphene/ZrS2 heterostructure, endowing graphene with the real ability to be applied in nanoelectronics, whereas the van der Waals interactions of graphene significantly broadens the optical absorption of ZrS2. The conduction band and valence band of the four heterostructures are contributed by the ZrS2 layer and the other layer, respectively, meaning good charge separation is achieved. We proposed that the strained h-BN/ZrS2 and g-C3N4/ZrS2 heterostructures satisfy fundamental aspects for photocatalytic water splitting, with the reduction and oxidation levels well inside their band gaps. By forming heterostructures with ZrS2, the optical properties of h-BN, g-C3N4 and polyphenylene show a remarkable improvement in the visible-light region. The findings in this study will be of broad interest in van der Waals heterostructure research and in the photocatalysis field.

Lithium-doped MOF impregnated with lithium-coated fullerenes: A hydrogen storage route for high gravimetric and volumetric uptakes at ambient temperatures

We theoretically demonstrated that by the impregnation of Li-decorated IRMOF-10 with Li-coated C(60), the hydrogen storage capacity is improved to be 6.3 wt% and 42 g L(-1) at 100 bar and 243 K. Both the gravimetric and volumetric hydrogen uptakes reach the 2015 DOE target at near ambient conditions.

Intense attosecond pulse generated from a molecular harmonic plateau of H2+ in mid-infrared laser fields

High-order harmonic generation from the molecular ion H-2(+) exposed to intense laser fields is investigated by the time-dependent quantum wave packet method. Molecular and atomic plateaus of harmonic spectra are effectively distinguished at large internuclear distances, where the harmonic efficiency of the molecular plateau is several orders of magnitude higher than that of the latter. We report on a physical model of the origin of the molecular supercontinua and reveal that the creation of this plateau directly results from the interference of the intramolecular electronic wave packet localized in two potential wells following the laser field. This is our first effort in utilizing the efficient molecular plateau to generate intense isolated attosecond pulses by controlling the dynamics of the nucleus and electrons with a mid-infrared laser. Further, we show that the harmonic plateau is enhanced at the macroscopic level by solving the Maxwell wave equation coupled with the Schrodinger equation.

Ultrahigh energy storage and ultrafast ion diffusion in borophene-based anodes for rechargeable metal ion batteries

Since the turn of the new century, the increasing demand for high-performance energy storage systems has generated considerable interest in rechargeable ion batteries (IBs). However, current IB technologies are not entirely satisfactory, especially the electrodes. We report here, via density functional theory calculations and first principles molecular dynamics simulations, that a borophene anode material has the fascinating properties of ultrahigh energy storage and ultrafast ion diffusion in metal (Li, Na, K, Mg, Al) IBs. Particularly for Li IBs with a borophene anode, a specific density of 3306 mA h g−1 and a high charging voltage of 1.46 V can be maintained at room temperature. Furthermore, non-ideal borophene anodes, including those with defects or oxidation and nanoribbon samples, still possess good properties for practical applications. This theoretical exploration will provide helpful guidance in searching for available or novel boron nanosheets as promising anode materials to advance commercial IB technology.

Interference effects on harmonic generation from H 2 + in nonhomogeneous laser field

By solving the time-dependent Schrödinger equation both in simplified one-dimensional coordinate and three-dimensional cylindrical coordinate systems, the high-order harmonic generation from H2 + in spatially symmetric and asymmetric nonhomogeneous laser fields was studied. At large internuclear distances, minima were clearly observed in high energy part of harmonic spectra, which can be attributed to two-center interference in diatomic molecule. Compared with previous studies, the minima in nonhomogeneous laser field are more distinct. Remarkably, the positions of the minima are different in these two types of fields, which demonstrate that interference effects are greatly influenced by laser parameters. Besides, the asymmetric nonhomogeneous field leads to an asymmetric recollision of the ionized electron, and both odd and even order harmonics could be emitted, which is explained in detail based on quantum dynamics calculations.

Isolated few-attosecond emission in a multi- cycle asymmetrically nonhomogeneous two- color laser field

By solving the three-dimensional time-dependent Schrodinger equation, we present a theoretical investigation of helium-based high-order harmonic generation (HHG) in both symmetric and asymmetric spatially nonhomogeneous laser fields. It is determined that the harmonic cutoff is extended in both symmetric and asymmetric nonhomogeneous laser fields, and the spatial symmetry of the nonhomogeneous field greatly influences the movement of electronic wave packets, which subsequently affect harmonic emission. In particular, the spatially asymmetric distribution of the laser field will induce asymmetric recombination of electronic wave packets between the two sides of a helium atom, which could reduce the interference in HHG emission for every optical cycle. In combination with the two-color technique, multi-cycle laser pulses with asymmetric spatially nonhomogeneous characteristics are capable of generating a coherent extreme ultraviolet supercontinuum, which can be used to produce isolated few-attosecond pulses.

Enhancing magnetic vacancies in semiconductors by strain

Although cation-vacancies can induce localized magnetic moments in semiconductors, the collective magnetism is impeded by low vacancy concentration. To improve the vacancy concentration, we study the effect of external hydrostatic strain on the vacancy formation energy. Our first-principles calculations discover that vacancy formation energy is significantly reduced in ionic semiconductors with the monotonic volume contraction, while only slightly decreased in covalent semiconductors. Especially for ZnO, the equilibrium concentration of cation-vacancies has been improved by 109 times. We predicted that strain can be used to produce “d0 magnetism” in ionic semiconductors much easier in experiments.

The theoretical search for half-metallic material: The non-stoichiometric peroskite oxide Sr2FeCoO6−δ

The non-stoichiometric peroskite oxide Sr2FeCoO6−δ is investigated and predicted to be half-metallic material using density-functional calculations. The results reveal that Sr2FeCoO5 shows antiferromagnetic half-metallic behavior and exhibits magnetic moment ordering with the magnetic moments of 2.97 and 3.72 μB on Fe(1) and Fe(2) sites, 2.96 and 3.20 μB on Co(1) and Co(2) sites antiparallel to those of Fe, and about 0.05 μB on O sites parallel to those of Co, respectively. Moreover, Sr2FeCoO4 and Sr2FeCoO6 both have half-metallic character. This hints that Sr2FeCoO6−δ possesses half-metallic nature in a large range of δ.