Search

We applied an adaptive genetic algorithm (AGA) to search for low-energy crystal structures of Fe3S. A number of structures with energies lower than that of the experimentally reported Pnma and I-4 structures have been obtained from our AGA searches. These low-energy structures can be classified as layer-motif and column-motif structures. In the column-motif structures, Fe atoms self-assemble into rods with a bcc type of underlying lattice, which are separated by the holes terminated by S atoms. In the layer-motif structures, the bulk Fe is broken into slabs of several layers passivated by S atoms. Magnetic property calculations showed that the column-motif structures exhibit reasonably high uniaxial magnetic anisotropy. In addition, we examined the effect of Co doping to Fe3S and found that magnetic anisotropy can be enhanced through Co doping.

Structures, magnetic moments, and magnetocrystalline anisotropy energies of the Fe3−x Co x C intermetallic compounds are systematically investigated using adaptive genetic algorithm (AGA) crystal-structure predictions and first-principles calculations. Besides reproducing the known cementite (Pnma) structure of Fe3C, i.e. x = 0, the AGA searches also capture several new metastable phases within the room-temperature range. In particular, a bainite (P6322) structure exhibits the largest magnetic moment among all low-energy structures, and its energy is only 4 meV/atom higher than the cementite (Pnma) phase. The atomic structure of the Pnma Fe2CoC phase, i.e. x = 1, is also identified, and the calculated x-ray diffraction spectrum, magnetocrystalline anisotropy energy, and saturation magnetization based on the structure from our theoretical study are in good agreement with experiment.

The structure and magnetic properties of new magnetic Fe3Co3 X 2 (X = Ti, Nb) compounds are studied by genetic algorithm, first-principles density functional theory (DFT) calculations, and experiments. The atomic structure of a hexagonal structure with P-6m2 symmetry is determined. The simulated x-ray diffraction (XRD) spectra of the P-6m2 structures agree well with experimental XRD data for both Fe3Co3Ti2 and Fe3Co3Nb2. The magnetic properties of these structures as well as the effect of the disorder of Fe and Co on their magnetic properties are also investigated. The magnetocrystalline anisotropy energy is found to be very sensitive to the occupancy disorder between Fe and Co.

We report a first-principles study of the electronic structure of functionalized graphene nanoribbon (aGNRs-f) by an organic functional group (CH2C6H5) and find that CH2C6H5 functionalized group does not produce any electronic states in the gap and the band gap is direct. By changing both the density of the organic functional group and the width of the aGNRs-f, a band gap tuning exhibits a fine three-family behaviour through the side effect. Meanwhile, the carriers at the conduction band minimum and the valence band maximum are located in both CH2C6H5 and aGNR regions when the density of CH2C6H5 is big, while they distribute dominantly in aGNR conversely. The fine band gap modulation effects make aGNRs-f good candidates with high quantum efficiency and a significantly wider wavelength range from 750 to 93 924 nm for lasers, light-emitting diodes and photodetectors due to the direct band gap and small carrier effective masses.

Electronic structures of (4, n) and (m, 4) (the NW has m layers parallel to the {1 1 1} facet and n layers parallel to {1 1 0}) Si/Ge core/shell nanowires (NWs) along the [1 1 2] direction with cross-sectional aspect ratio (m/n) from 0.36 to 2.25 are studied by first-principles calculations. An indirect to direct band gap transition is observed as m/n decreases, and the critical values of m/n and diameter for the transition are also estimated. The size of the band gap also depends on the aspect ratio. These results suggest that m/n plays an important role in modulating the electronic properties of the NWs.

We report a first-principles study of Ge/Si and Si/Ge core/shell nanowires (NWs) along the [1 1 2] direction with a diameter of ∼20 Å using density-functional theory. Our results show that for both NW structures the band gaps are indirect and are significantly larger than the gaps of the bulk crystalline Si and Ge. The quantum well confinement effect in these NWs is found to be modified by a type II lineup of band structures. Moreover, the carriers on the conduction band minimum are strongly localized in the Si region while the carriers on the valence band maximum are located mainly in the Ge region. The charge separation and localization characters make the NWs good candidates for nanochannels in field effect devices, solar cells with higher efficiency and high mobility heterostructures due to the spatial separation of one-dimensional electron gas and one-dimensional hole gas.