X.T. Zu

Adsorption-induced magnetic properties and metallic behavior of graphene

Magnetic properties and electronic structures of graphene with Cl, S, and P adsorption have been investigated using ab initio calculations. The adsorption of Cl leads to Fermi level shifting to valence band, which results in metallic graphene. A band gap of 0.6 eV emerges in a S-absorbed graphene, leading to the semiconducting graphene. The unpaired electrons in the absorbed P atom are polarized and thus exhibit a magnetic moment of 0.86μB, while no magnetic moment has been observed after Cl and S adsorption. This demonstrates that the magnetic properties and conductive behavior of graphene can be modified via atom adsorption. Specially, P-absorbed graphene may be useful for spintronic applications, such as tunneling magnetoresistance.

Effect of proton and Ne irradiation on the microstructure of Zircaloy 4

Proton irradiation at temperatures of 310 and 350°C has been explored as a method of studying radiation effects in Zircaloy 4 under power-reactor conditions. The dislocation-loop and hardness evolution are similar to those observed under neutron irradiation. Amorphization and Fe loss at Zr(Cr,Fe)2 precipitates are observed. Energy-filtered images indicate Fe accumulation in the matrix near the precipitate, a phenomenon not reported in neutron-irradiated alloys.

Theoretical investigation of structural, energetic and electronic properties of titanate pyrochlores

Ab initio total energy calculations using the plane-wave pseudopotential method based on density functional theory were carried out to investigate the structural, energetic and electronic properties of A2Ti2O7 (A = La, Gd and Yb) pyrochlores. It turned out that the formation energies of antisite defects are not linearly dependent on the ratio of the cation radii, and, for the three compositions, the cation antisite formation energy is largest for Gd2Ti2O7 pyrochlore. It was indicated that Gd2Ti2O7 compound is the least likely to form defect fluorite structure, which gives rise to the least resistance to radiation-induced amorphization. DOS analysis showed that stronger interaction exists in the Gd2Ti2O7 compound, and its electronic structure is very different from that of La2Ti2O7 and Yb2Ti2O7. Our calculations suggested that the electronic structure of the A cation and bond type should be taken into account when explaining the response behavior of A2Ti2O7 (A = La, Gd, Yb) pyrochlores to ion irradiation-induced amorphization.

Optical properties of metallic nanoparticles in Ni-ion-implanted α-Al2O3 single crystals

64 keV Ni ion implantation was performed at room temperature up to a dose of 1×1017 cm−2 in α-Al2O3 single crystals. The charge states, structure, and optical properties of metallic embedded Ni nanoparticles were studied by using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and optical spectroscopy, respectively. XPS analysis showed that implanted Ni ions are mainly in charge state of metallic Ni0. Nanoparticles distributed from the surface to 30 nm below the surface were observed in a high-angle annular dark-field image. The size of nanoparticles ranges from 1 to 5 nm in diameter. A high-resolution electron microscopy image indicated the Ni-implanted area had been entirely amorphized. A new broad absorption band centered at 400 nm appeared in the optical absorption spectrum of the as-implanted crystal, due to surface plasma resonance of Ni nanoparticles. We did not find any emission band in the as-implanted sample under a Xe lamp excitation wavelength of 250–430 nm in a spect...

Threshold displacement energy in GaN; Ab initio molecular dynamics study

Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies Ed along five specific directions and to determine the defect configurations created during low energy events. The Ed shows a significant dependence on direction. The minimum Ed is determined to be 39 eV along the ⟨1¯010⟩ direction for a gallium atom and 17.0 eV along the ⟨1¯010⟩ direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average Ed values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N–N dumbbell configuration), but various configurations for Ga defects are formed in GaN.

Electronic and magnetic properties of metal-doped BN sheet: A first-principles study

The electronic and magnetic properties of a BN sheet doped with 3d transition metals (Fe, Co and Ni) have been investigated using ab initio calculations. Our calculations show many interesting physical properties in a metal-doped BN sheet. A Fe-doped BN sheet is a half-metal with the magnetic moment of 2.0 micro(B), and Co-doped BN sheet becomes a narrow-gap semiconductor with a magnetic moment of 1.0 micro(B). However, no magnetic moment is induced on a Ni-doped BN sheet, which has the same band gap as a pristine BN sheet. Furthermore, Fe atom easily forms an isolated particle on the BN sheet, while Ni and Co atoms are likely to form a sheet-supported metal nanotemplate. These results are useful for spintronics application and could help in the development of magnetic nanotructures and metallic nanotemplate at room temperature.

Electronic and magnetic properties of substituted BN sheets: A density functional theory study

Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: V(B), V(N), and V(B+N). We show that some embedded configurations, except TM atoms in V(N) vacancy, are stable in BN sheets and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheets can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in V(B+N), Co in V(B), and Ni in V(B) leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (C(Mn)) embedments, a regular 1D structure can be formed in BN sheets as an electron waveguide, a metal nanometre wire with a single atom thickness.

First-principles study of electronic properties of La2Hf2O7 and Gd2Hf2O7

The structural and electronic properties of A2Hf2O7 (A=La and Gd) pyrochlore compounds are investigated by means of first-principles total energy calculations. Also, the formation energies of defects are calculated, and the results can be used to explain the stability of pyrochlores. Hybridizations between A 5p and O 2s and between A 5d and O 2p states are observed, but the interaction between A 5p and O 2s orbitals is much stronger in Gd2Hf2O7 than that in La2Hf2O7. Gd2Hf2O7 shows a density of state distribution much different from that of La2Hf2O7. Mulliken overlap population analysis shows that the A–O48f and A–O8b bonds in Gd2Hf2O7 are more ionic than the corresponding bonds in La2Hf2O7, while the Hf–O48f bond in Gd2Hf2O7 is more covalent. These calculations suggest that A–O48f and A–O8b bonds may play important roles in their response to irradiation-induced amorphization observed experimentally.

Optical and magnetic properties of Ni nanoparticles in rutile formed by Ni ion implantation

Crystalline Ni nanoparticles in the near surface of TiO2 (rutile) have been synthesized by Ni ion beam implantation at room temperature to a fluence of 1×1017∕cm2. Transmission electron microscopy, optical absorption spectroscopy, and a superconducting quantum interference device magnetometer have been utilized to characterize the nanostructure, optical and magnetic properties of Ni particles in TiO2. Crystalline Ni nanoparticles with dimensions ranging 3–20 nm formed in the near surface of rutile, which caused a broad absorption band from 700 nm in the optical absorption spectrum. Magnetic measurement indicated that the coercive force of Ni nanoparticles was about 210 Oe at 10 K. The superparamagnetism of the nanoparticles was observed above blocking temperature T=85K.

First-principles study of the adsorption of cesium on Si(001)(2×1) surface

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed on cesium adsorption on Si(001)(2x1) surface. The optimized geometries and adsorption energies have been obtained and the preferred binding sites have been determined for the coverage (Theta) of one monolayer and half a monolayer. At Theta = 0.5 ML the most stable adsorption site is shown to be T3 site. At Theta = 1 ML two Cs atoms are adsorbed at HH and T3 sites, respectively. It was found that the saturation coverage of Cs for the Si(001)(2x1)-Cs surface is one monolayer instead of half a monolayer. This finding supports the majority of experimental observations but does not support recent coaxial impact collision ion scattering spectroscopy investigations [Surf. Sci. 531, L340 (2003)] and He+ Rutherford backscattering spectroscopy studies [Phys. Rev. B 62, 4545 (2000)]. Mulliken charge and overlap population analysis showed that the Cs-Si bond is indeed ionic rather than polarized covalent as generally assumed for alkali metal (AM) on Si(001)(2x1) surface. Geometrical structure analysis seems to have limitations in determining the nature of AM-substrate bond. We also found that the silicon surface is metallic and semiconducting for the coverages of 0.5 and 1 ML, respectively. (c) 2005 American Institute of Physics.