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Single transition metal (TM) atoms such as Fe, Co and Ni occupying a carbon divacancy in tetragonal graphene (TG) and bonded with four nitrogen atoms (TM@N4TG) as electrocatalysts are investigated by means of first‐principles calculations. To consider the effect of solvent species on the local configuration of the active single metal, a thermodynamical full‐landscape searching (TFLS) scheme is employed. The calculated thermodynamic overpotentials (ηtd) from our TFLS indicate that Co@N4TG displays high catalytic activity toward both oxygen evolution reaction (OER) and reduction reaction (ORR), with ηtd OER and ηtd ORR as 0.397 and 0.357 V, respectively. Its OER potential cannot be captured if only one four electron reaction loop (FERL) is considered. The actual active pathways do not always turn out to be the reactions starting from the bare site. Our findings demonstrate that TG is a promising support and TM confined TD can be used to design effective and cheap multifunctional electrocatalysts., Doped Tetragonal Graphene (TG) as electrocatalyst? The pre‐adsorption of solvent functional groups (OH, O and OOH) can tune the adsorption strength of species. A thermodynamic full‐landscape searching scheme is proposed where all the possible reactions were considered. Co@N4TG is confirmed to be a favorable bifunctional single‐atom electrocatalyst for both OER and ORR.

In the literature of heterogeneous water-splitting catalytic thermodynamic study, the computational hydrogen electrode (CHE) scheme is used in the majority of the cases. In this scheme, either the bare surface without O and OH decoration or a decorated phase chosen from a surface Pourbaix diagram is employed as a starting point in a four-electron reaction loop (FERL) to describe the oxygen evolution reaction (OER) process. The electrode potential that makes every step of this FERL exothermic is defined as the thermodynamic overpotential (η

The effects of isotropic strains on the magnetic properties in ZnO and GaN induced by cation vacancies are comparatively investigated by density functional theory calculations. The magnetic moments and the couplings between vacancies in different charged states are calculated as a function of strains. The modulation of strain on the magnetic properties relies on the materials and the charge states of cation vacancies in them. As the occurrence of charge transfer in ZnO:V Zn under compression, the coupling between $$V_{\rm{Zn}}^{0} $$ is antiferromagnetic (AFM) and it could be stabilized by strains. Tensions can strengthen the ferromagnetic (FM) coupling between $$V_{\rm{Zn}}^{0} $$ but weaken that of $$V_{\rm{Ga}}^{ - } $$ . The neutral V Ga are always AFM coupling under strains from −6 to +6% and could be stabilized by compressions. The interactions between $$V_{\rm{Ga}}^{ - } $$ are always FM with ignorable variations under strains; however, the FM couplings between $$V_{\rm{Ga}}^{2 - } $$ could be strengthened by compressions. These varying trends of magnetic coupling under strains are interpreted by the band coupling models. Therefore, strain-engineering provides a route to manipulate and design high Curie temperature ferromagnetism derived and mediated by intrinsic defect for spintronic applications.

The effects of isotropic strains on the doping and magnetic properties of nitrogen doped ZnO system are investigated by density functional theory calculations. The formation energy and the ionization energy of N O in ZnO are calculated each as a function of strains. We show that compressive strains favor the solubility of nitrogen in ZnO, while the tensile ones render the acceptor level shallower. The shallower energy level means that the acceptor wave function becomes delocalized, which is favorable for mediating long-range coupling between defects. The effects of strains on the coupling strength between two N O s are also studied. Our calculations indicate that the ferromagnetic coupling in ZnO:N can be enhanced by applying tensile strains, but weakened by compressive strains. These manipulation trends of the magnetic couplings under stains are interpreted by the band coupling models.

The effects of hydrostatic pressures on the ionization and formation energies of substitutional dopants N, As and Ga in ZnO and ZnTe are examined by means of first-principle total-energy calculations. The pressure effects on the energy level of dopants are related to their shallow or deep characteristics. The pressure derivatives of hydrogenic impurity levels are nearly one order of magnitude smaller than that of hostʼs band gap, while those of the localized are of the same order as that of hostʼs band gap. The pressure dependencies of formation energies are explained by the volume changes induced by dopants in combination with the bulk modulus B 0 of host semiconductor.

We performed first-principle total-energy calculations to investigate the mechanism for the realization of high quality p -type ZnO codoped with lithium and nitrogen. We find that the higher hole concentrations measured in the codoped ZnO is related to decreased ionization energy of acceptors and reduction of compensations. The dual acceptor N O –Li Zn complex proposed in experiments is unstable. While in the (Li I –N O )–Li Zn complex, where acceptor Li Zn binds to the passivated (Li I –N O ) complex is stable and acts as a single acceptor. The activation energy of this complex is about 60 meV lower than that of Li Zn in Li-monodoped ZnO. The formation of inactive (Li I –N O ) complexes creates a fully occupied impurity band just above the valence band maximum of ZnO. Thus Li atoms binding to this complex is activated by the electrons from the complex state rather than from the host states, accounting for decreased activation energy. Besides, Li I + and N O − bind tightly through the Coulomb interaction. Such binding will suppress the amount of compensating donor Li I and limit the compensation for the desired acceptor Li Zn .

The electronic properties of PbSe quantum dots containing a nonstoichiometric Pb:Se ratio are investigated by ab initio density functional theory. We take five nearly spherical PbSe nanocrystals wi...

The electronic and magnetic properties of the singly charged oxygen vacancy in undoped ZnO under biaxial strains are investigated by density functional theory calculations. A net magnetic moment (MM) of 0.561 μB is obtained for in ZnO under no strains, but the magnetic interaction between them is antiferromagnetic. The formation energy of V O and the MM induced by as well as the coupling type and strength between vary with the application of biaxial strains. Compressive strains can enhance the concentrations of V O and enlarge the MM, and strengthen the antiferromagnetic interactions between them at lower V O concentrations. However, at higher V O concentrations, the coupling varies from sizable antiferromagnetic to negligible weak ferromagnetic, and then becomes paramagnetic with the increase of compression. Antiferromagnetic results are further confirmed by the local density approximation with Hubbard U (LDA + U) calculations.

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.

To improve the photoelectrochemical activity of TiO2 for hydrogen production through water splitting, the band edges of TiO2 should be tailored to match with visible light absorption and the hydrogen or oxygen production levels. By analyzing the band structure of TiO2 and the chemical potentials of the dopants, we propose that the band edges of TiO2 can be modified by passivated codopants such as (Mo+C) to shift the valence band edge up significantly, while leaving the conduction band edge almost unchanged, thus satisfying the stringent requirements. The design principle for the band-edge modification should be applicable to other wide-band-gap semiconductors.

Using first-principles total energy methods, we systematically calculated the formation energies and transition energy levels of intrinsic and extrinsic defects in cubic boron nitride (c-BN). We employed a mixed k-point sampling approach and brought the band-gap error into consideration. We find that under B-rich conditions boron vacancy (VB) and nitrogen vacancy (VN) act as the dominant compensating centers for additional dopants, while at N-rich limits, boron antisite (NB) becomes the main acceptor 'killer'. The result is inaccessible if the impact of band-gap correction on the formation energies is ignored. Besides, all the calculated defect levels are too deep to account for the observed n- or p-type conductivity in as-grown samples. Among all the attempted dopants, substitutional Be (BeB) is the desirable acceptor in terms of lower formation energy and energy levels under N-rich conditions. However, n-type c-BN seems difficult to obtain under equilibrium conditions, because of the low solubility of dopants and the abundance of the compensating center VB3−.

Thermally stimulated luminescence spectroscopy has been applied to study the deep centres in unintentionally doped high resistivity GaN epilayers grown by the metal organic chemical vapour deposition method on c-sapphire substrates. Two trap states with activation energies of 0.12 and 0.62 eV are evaluated from two luminescence peaks at 141.9 and 294.7 K in the luminescence curve. Our spectroscopy measurement, in combination with more accurate first-principles studies, provided insights into the microscopic origin of these levels. Our investigations suggest that the lower level at 0.12 eV might originate from C-N, which behaves as a hole trap state; the deeper level at 0.62 eV can be correlated with V-Ga that corresponds to the yellow luminescence band observed in low-temperature photoluminescence spectra.

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is sm...

Our Raman measurement indicates that the intensity of the peaks (510 and 645 cm(-1)) related to nitrogen concentration is enhanced in MgZnO compared with that in ZnO. Using first-principles band structure methods, we calculated the formation energy and transition energy level for nitrogen acceptor in ZnO and random MgxZn1-xO (with x=0.25) alloy. Our calculations show that the incorporation of nitrogen can be enhanced as Mg is alloyed into ZnO, which agrees with our experiments. The acceptor energy level deeper in the alloy ascribes to the downward shift of the valence-band maximum edge in the presence of magnesium. (c) 2008 American Institute of Physics.