Showing papers by "Jingbo Li published in 2009"

Revised ab initio natural band offsets of all group IV, II-VI, and III-V semiconductors

Abstract: Using an all-electron band structure approach, we have systematically calculated the natural band offsets between all group IV, III-V, and II-VI semiconductor compounds, taking into account the deformation potential of the core states This revised approach removes assumptions regarding the reference level volume deformation and offers a more reliable prediction of the "natural" unstrained offsets Comparison is made to experimental work, where a noticeable improvement is found compared to previous methodologies

Possible origin of ferromagnetism in undoped anatase TiO2

Abstract: Using first-principles electronic structure calculations we find that the titanium vacancy and divacancy may be responsible for the unexpected ferromagnetism in undoped anatase TiO2. An isolated titanium vacancy produces a magnetic moment of 3.5 mu(B), and an isolated titanium divacancy produces a magnetic moment of 2.0 mu(B). The origin of the collective magnetic moments is the holes introduced by the titanium vacancy or divacancy in the narrow nonbonding oxygen 2p(pi) band. At the center of the divacancy, an O-2 dimer forms during the relaxation, which lowers the total energy of the system and leads to the decrease in the total magnetic moment due to a hole compensation mechanism. For both the two native defects, the ferromagnetic state is more stable than the antiferromagnetic state.

The bipolar doping of ZnS via native defects and external dopants

Abstract: By employing first-principle total-energy calculations, a systematic study of the dopability of ZnS to be both n- and p-types compared with that of ZnO is carried out. We find that all the attempted acceptor dopants, group V substituting on the S lattice site and group I and IB on the Zn sites in ZnS, have lower ionization energies than the corresponding ones in ZnO. This can be accounted for by the fact that ZnS has relative higher valence band maximum than ZnO. Native ZnS is weak p-type under S-rich condition, as the abundant acceptor V-Zn has rather large ionization energy. Self-compensations by the formation of interstitial donors in group I and IB-doped p-type ZnS can be avoided when sample is prepared under S-rich condition. In terms of ionization energies, Li-Zn and N-S are the preferred acceptors in ZnS. Native n- type doping of ZnS is limited by the spontaneous formation of intrinsic V-Zn(2-); high efficient n-type doping with dopants is harder to achieve than in ZnO because of the readiness of forming native compensating centers and higher ionization energy of donors in ZnS. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3103585]

Design of shallow acceptors in ZnO through compensated donor-acceptor complexes: A density functional calculation

Abstract: The intrinsic large electronegativity of O 2p character of the valence-band maximum (VBM) of ZnO renders it extremely difficult to be doped p type. We show from density functional calculation that such VBM characteristic can be altered by compensated donor-acceptor pairs, thus improve the p-type dopability. By incorporating (Ti+C) or (Zr+C) into ZnO simultaneously, a fully occupied impurity band that has the C 2p character is created above the VBM of host ZnO. Subsequent doping by N in ZnO: (Ti+C) and ZnO: (Zr+C) lead to the acceptor ionization energies of 0.18 and 0.13 eV, respectively, which is about 200 meV lower than it is in pure ZnO.

Controlling electronic structures by irradiation in single-walled SiC nanotubes: a first-principles molecular dynamics study.

Abstract: Using first-principles molecular dynamics simulations, the displacement threshold energy and defect configurations are determined in SiC nanotubes. The simulation results reveal that a rich variety of defect structures (vacancies, Stone-Wales defects and antisite defects) are formed with threshold energies from 11 to 64 eV. The threshold energy shows an anisotropic behavior and exhibits a dramatic decrease with decreasing tube diameter. The electronic structure can be altered by the defects formed by irradiation, which suggests that the electron irradiation may be a way to use defect engineering to tailor electronic properties of SiC nanotubes.

Stone–Wales defects created by low energy recoils in single-walled silicon carbide nanotubes

Abstract: The defect creation at low energy events was studied using density functional theory molecular dynamics simulations in silicon carbide nanotubes, and the displacement threshold energies determined exhibit a dependence on sizes, which decrease with decreasing diameter of the nanotubes. The Stone-Wales (SW) defect, which is a common defect configurations induced through irradiation in nanotubes, has also been investigated, and the formation energies of the SW defects increase with increasing diameter of the nanotubes. The mean threshold energies were found to be 23 and 18 eV for Si and C in armchair (5,5) nanotubes. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3238307]

Crystal structure and magnetic properties of Nd(Mn 1-x Fe x ) 2 Si 2 compounds

Abstract: X-ray powder diffraction, resistivity and magnetization studies have been performed on polycrystalline Nd(Fe(x)Mn(1-x))(2)Si(2) (0 <= x <= 1) compounds which crystallize in a ThCr(2)Si(2)-type structure with the space group 14/mmm. The field-cooled temperature dependence of the magnetization curves shows that, at low temperatures, NdFe(2)Si(2) is antiferromagnetic, while the other compounds show ferromagnetic behaviour. The substitution of Fe for Mn leads to a decrease in lattice parameters a, c and unit-cell volume V. The Curie temperature of the compounds first increases, reaches a maximum around x = 0.7, then decreases with Fe content. However, the saturation magnetization decreases monotonically with increasing Fe content. This Fe concentration dependent magnetization of Nd(Fe(x)Mn(1-x))(2)Si(2) compounds can be well explained by taking into account the complex effect on magnetic properties due to the substitution of Mn by Fe. The temperature's square dependence on electrical resistivity indicates that the curve of Nd(Fe(0.6)Mn(0.4))(2)Si(2) has a quasi-linear character above its Curie temperature, which is typical of simple metals.

Crystal structure determination of K2Zn(PO3)(4)

Abstract: The crystal structure of K2Zn(PO3)(4) was determined and refined using the Rietveld method based on the isostructure model of K2Cu(PO3)(4). This compound belongs to the monoclinic system with space group Cc and lattice parameters of a=11.0941(2) angstrom, b=12.5215(3) angstrom, c=7.6597(2) angstrom, and, beta=102.47(2)degrees. The chemical formula unit per unit cell is Z=4 and the calculated density is 2.938(3) g/cm(3). Zigzag [PO3](infinity) chains formed along the a axis, and their period contains eight PO4 tetrahedrons. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3076081]