H. Y. Xiao

Adsorption of hydrogen on boron-doped graphene : A first-principles prediction

The doping effects of boron on the atomic adsorption of hydrogen on graphene have been investigated using density functional theory calculations. The hydrogen adsorption energies and electronic structures have been considered for pristine and B-doped graphene with the adsorption of hydrogen on top of carbon or boron atom. It is found that the B-doping forms an electron-deficient structure and decreases the hydrogen adsorption energy dramatically. For the adsorption of hydrogen on top of other sites, similar results have also been found. These results indicate that the hydrogen storage capacity is improved by the doping of B atom.

Atomistic simulation of helium-defect interaction in alpha-iron

Molecular dynamics (MD) methods are utilized to study the formation of vacancy clusters created by displacement cascades in α‐Fe containing different concentrations of substitutional He atoms. Primary knock-on atom energies, Ep, from 500eV to 20keV are considered at a temperature of 100K, and the results are compared with those performed in pure α‐Fe. There are distinct differences in the number and size of vacancy clusters within displacement cascades with and without substitutional helium atoms. It is found that many large vacancy clusters can be formed within cascade cores in α‐Fe with helium atoms, in contrast to a few small vacancy clusters observed in pure α‐Fe. The number and size of helium/vacancy clusters generally increase with increasing helium concentration and PKA energy. One of the striking results is that the number of self-interstitial atoms (SIAs) and the size of interstitial clusters are much smaller than those in pure α‐Fe.

Electronic and magnetic properties of graphene absorbed with S atom: A first-principles study

Stable configuration, electronic structures, and magnetic behaviors for S adsorption on graphene have been investigated by first-principles calculations. It is found that the adsorption site of S on graphene is coverage dependent. As the increase in coverage from 0 to 0.5 ML, the preferred site is changed from bridge to hollow site. For the adsorption of S at bridge site, no magnetic moment is detected, and the adsorption is characterized by strong hybridization between the S 2s state and graphene sigma states. For the adsorption of S at hollow site, a magnetic moment of 1.98 mu(B) was induced. In this case, the hybridization occurs between S 2p states and graphene pi states. Furthermore, from the investigation of the surface potential energy curve, we find that graphene is a suitable candidate for the S storage.

First-principles study of energetic and electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlore

First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that ⟨Gd-O48f⟩ bond is less covalent than ⟨Sm-O48f⟩ and ⟨Er-O48f⟩ bonds, while ⟨Ti-O48f⟩ bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A=Sm, Gd, Er) pyrochlores, ⟨Ti-O48f⟩ bonds may play more significant role in determining their radiation resistance to amorphization.

Hydrogen adsorption, dissociation and diffusion on the α-U(001) surface

First-principles pseudopotential plane-wave calculations based on density functional theory and the generalized-gradient approximation have been used to study the adsorption, dissociation, and diffusion of hydrogen on the α-U(001) surface. Weak molecular chemisorption was observed for H2 approaching with its molecular axis parallel to the surface. The optimization of the adsorption geometries on the threefold hollow sites yields final configurations with H2 molecules moving towards the top site at both coverages considered, 0.25 and 0.5 monolayers. A low dissociation barrier of 0.081 eV was determined for H2 dissociated from the onefold top site with the H atoms falling into the two adjacent threefold hollow sites. The analysis of the density of states along the dissociation paths shows that the hybridization of U 5f and H 1s states only occurs when the H2 molecule is dissociated.

First-principles calculation of defect formation energies and electronic properties in stannate pyrochlores

The electronic structures and defect formation energies for a series of stannate pyrochlores Ln2Sn2O7 (Ln=La, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu, and Y) have been investigated using the first-principles total energy calculations. The calculated results show that Ln-site cation ionic radius, x-O48f, lattice constant and the covalency of the ⟨Sn–O48f⟩ bond have a significant affect on the defect formation energies. The cation-antisite defect has the lowest formation energy, as compared with that of other defects, indicating that cation disorder causes local oxygen disordering. The present studies suggest that Lu2Sn2O7 is the most resistant to ion beam-induced amorphization. The electronic structure calculations reveal that Ln2Sn2O7 compounds have direct band gaps of 2.64–2.95 eV at the Γ point in the Brillouin zone.

Ab initio molecular dynamics simulation of the effects of stacking faults on the radiation response of 3C-SiC.

In this study, an ab initio molecular dynamics method is employed to investigate how the existence of stacking faults (SFs) influences the response of SiC to low energy irradiation. It reveals that the C and Si atoms around the SFs are generally more difficult to be displaced than those in unfaulted SiC, and the corresponding threshold displacement energies for them are generally larger, indicative of enhanced radiation tolerance caused by the introduction of SFs, which agrees well with the recent experiment. As compared with the unfaulted state, more localized point defects are generated in faulted SiC. Also, the efficiency of damage production for Si recoils is generally higher than that of C recoils. The calculated potential energy increases for defect generation in SiC with intrinsic and extrinsic SFs are found to be higher than those in unfaulted SiC, due to the stronger screen-Coulomb interaction between the PKA and its neighbors. The presented results provide a fundamental insight into the underlying mechanism of displacement events in faulted SiC and will help to advance the understanding of the radiation response of SiC with and without SFs.

Effects of surface defects on two-dimensional electron gas at NdAlO3/SrTiO3 interface

Density functional theory calculations of NdAlO3/SrTiO3 heterostructure show that two-dimensional electron gas (2-DEG) is produced at the interface with a built-in potential of ~0.3 eV per unit cell. The effects of surface defects on the phase stability and electric field of 2-DEG have been investigated. It is found that oxygen vacancy is easily to form on the NdAlO3(001) surface, with a low threshold displacement energy and a low formation energy. This point defect results in surface reconstruction and the formation of a zigzag -Al-O-Al- chain, which quenches the built-in potential and enhances the carrier density significantly. These results will provide fundamental insights into understanding how surface defects influence the electronic behavior of 2-DEG and tuning their electronic properties through surface modification.

Dehydrogenation: a simple route to modulate magnetism and spatial charge distribution of germanane

Two-dimensional (2D) materials recently emerged as a new type of nanostructures exhibited large potential for application in nanoscale devices. Nevertheless, many proposed applications require efficient modulation of magnetism and spatial charge distribution within these 2D nanostructures. Here we, via density functional theory (DFT), demonstrated that both magnetism and spatial charge distribution of recently experimentally realized germanane can be effectively modulated via a simple dehydrogenating process. Both single-sided and double-sided H vacancy clusters due to the unpairing of Ge-4p electrons can make germanane obtain magnetism with designated magnitudes. Charges of valence band maximum (VBM) and conduction band minimum (CBM) in germanane that contains single-sided H vacancy clusters can be effectively separated, and charges of VBM and CBM in germanane that contains double-sided H vacancy clusters can be effectively assembled. Thus, our results provide a new perspective on precisely modulating magnetism and spatial charge distribution of germanane, which are fundamentally important for application of germanane in quantum information and optoelectronic fields.

A comparative study of low energy radiation response of AlAs, GaAs and GaAs/AlAs superlattice and the damage effects on their electronic structures

In this study, the low energy radiation responses of AlAs, GaAs and GaAs/AlAs superlattice are simulated and the radiation damage effects on their electronic structures are investigated. It is found that the threshold displacement energies for AlAs are generally larger than those for GaAs, i.e., the atoms in AlAs are more difficult to be displaced than those in GaAs under radiation environment. As for GaAs/AlAs superlattice, the Ga and Al atoms are more susceptible to the radiation than those in the bulk AlAs and GaAs, whereas the As atoms need comparable or much larger energies to be displaced than those in the bulk states. The created defects are generally Frenkel pairs, and a few antisite defects are also created in the superlattice structure. The created defects are found to have profound effects on the electronic properties of GaAs/AlAs superlattice, in which charge transfer, redistribution and even accumulation take place, and band gap narrowing and even metallicity are induced in some cases. This study shows that it is necessary to enhance the radiation tolerance of GaAs/AlAs superlattice to improve their performance under irradiation.