Charged States and Band-Gap Narrowing in Codoped ZnO Nanowires for Enhanced Photoelectrochemical Responses

By means of first-principles calculations within the density functional theory, we study the structural and optical properties of codoped ZnO nanowires and compare them with those of the bulk and film. It is disclosed that the low negatively charged ground states of nitrogen related defects play a key role in the optical absorption spectrum tail that narrows the band-gap and enhances the photoelectrochemical response significantly. A strategy of uncompensated N, P and Ga codoping in ZnO nanowires is proposed to produce a red-shift of the optical absorption spectra further than the exclusive N doping and to get a proper formation energy with a high defect concentration and a suppressed recombination rate. In this way, the absorption of the visible light can be improved and the photocurrent can be raised. These observations are consistent with the existing experiments, which will be helpful to improve the photoelectrochemical responses for the wide-band-gap semiconductors especially in water splitting applications.

Spin transfer in a ferromagnet-quantum dot and tunnel barrier coupled Aharonov-Bohm ring system with Rashba spin-orbit interactions

The spin transfer effect in ferromagnet-quantum dot (insulator)-ferromagnet Aharonov-Bohm (AB) ring system with Rashba spin-orbit (SO) interactions is investigated by means of Keldysh nonequilibrium Green function method. It is found that both the magnitude and direction of the spin transfer torque (STT) acting on the right ferromagnet electrode can be effectively controlled by changing the magnetic flux threading the AB ring or the gate voltage on the quantum dot. The STT can be greatly augmented by matching a proper magnetic flux and an SO interaction at a cost of low electrical current. The STT, electrical current, and spin current are uncovered to oscillate with the magnetic flux. The present results are expected to be useful for information storage in nanospintronics.

Family of boron fullerenes: general constructing schemes, electron counting rule and ab initio calculations

A set of general constructing schemes is unveiled to predict a large family of stable boron monoelemental, hollow fullerenes with magic numbers 32+8k (k>=0). The remarkable stabilities of these new boron fullerenes are then studied by intense ab initio calculations. An electron counting rule as well as an isolated hollow rule are proposed to readily show the high stability and the electronic bonding property, which are also revealed applicable to a number of newly predicted boron sheets and nanotubes.

Half-Metallic Silicon Nanowires: Multiple Surface Dangling Bonds and Nonmagnetic Doping

By means of first-principles density functional theory calculations, we find that hydrogen-passivated ultrathin silicon nanowires (SiNWs) along [100] direction with symmetrical multiple surface dangling bonds (SDBs) and boron doping can have a half-metallic ground state with 100% spin polarization, where the half-metallicity is shown quite robust against external electric fields. Under the circumstances with various SDBs, the H-passivated SiNWs can also be ferromagnetic or antiferromagnetic semiconductors. The present study not only offers a possible route to engineer half-metallic SiNWs without containing magnetic atoms but also sheds light on manipulating spin-dependent properties of nanowires through surface passivation.

Face-Centered-Cubic B$_{80}$ Metal: Density functional theory calculations

By means of ab initio calculations within the density functional theory, we have found that B80 fullerenes can condense to form stable face-centered-cubic fcc solids. It is shown that when forming a crystal, B80 cages are geometrically distorted, the Ih symmetry is lowered to Th, and four boron-boron chemical bonds are formed between every two nearest neighbor B80 cages. The cohesive energy of B80 fcc solid is 0.23 eV/atom with respect to the isolated B80 fullerene. The calculated electronic structure reveals that the fcc B80 solid is a metal. The predicted solid phase would constitute a form of pure boron and might have diverse implications. In addition, a simple electron counting rule is proposed, which could explain the stability of B80 fullerene and the recently predicted stable boron sheet.

Structures, Electronic Properties, Spectroscopies and Hexagonal Monolayer Phase of a Family of Unconventional Fullerenes C64X4 (X = H; F;Cl;Br)

A systematic first-principles study within density functional theory on the geometrical structures and electronic properties of unconventional fullerene C64 and its derivatives C64X4 (X = H; F;Cl;Br) has been performed. By searching through all 3465 isomers of C64, the ground state of C64 is found to be spherical shape with D2 symmetry, which differs from the parent cage of the recently synthesized C64H4 that is pear-shaped with C3v symmetry. We found that the addition of the halogen atoms like F;Cl;Br to the pentagon-pentagon fusion vertex of C64 cage could enhance the stability, forming the unconventional fullerenes C64X4. The Mulliken charge populations, LUMO-HOMO gap energies and density of states are calculated, showing that different halogen atoms added to C64 will cause remarkably different charge populations of the C64X4 molecule; the chemical deriving could enlarge the energy gaps and affect the electronic structures distinctly. It is unveiled that C64F4 is even more stable than C64H4, as the C-X bond energy of the former is higher than that of the latter. The computed spectra of C64H4 molecules agree well with the experimental data; the IR, Raman, NMR spectra of C64X4 (X = F;Cl;Br) are also calculated to stimulate further experimental investigations. Finally, it is uncovered by total energy calculations that C64X4 could form a stable hexagonal monolayer.