Guangzhao Wang

Hybrid Density Functional Study on Mono- and Codoped NaNbO3 for Visible-Light Photocatalysis.

Monodoping with Mo, Cr, and N atoms, and codoping with Mo-N and Cr-N atom pairs, are utilized to adjust the band structure of NaNbO3 , so that NaNbO3 can effectively make use of visible light for the photocatalytic decomposition of water into hydrogen and oxygen, as determined by using the hybrid density functional. Codoping is energetically favorable compared with the corresponding monodoping, due to strong Coulombic interactions between the dopants and other atoms, and the effective band gap and stability for codoped systems increase with decreasing dopant concentration and the distance between dopants. The molybdenum, chromium, and nitrogen monodoped systems, as well as chromium-nitrogen codoped systems, are unsuitable for the photocatalytic decomposition of water by using visible light, because defects introduced by monodoping or the presence of unoccupied states above the Fermi level, which promotes electron-hole recombination processes, suppress their photocatalytic performance. The Mo-N codoped NaNbO3 sample is a promising photocatalyst for the decomposition of water by using visible light because Mo-N codoping can reduce the band gap to a suitable value with respect to the water redox level without introducing unoccupied states.

BiOX/BiOY (X, Y = F, Cl, Br, I) superlattices for visible light photocatalysis applications

The BiOX/BiOY (X, Y = F, Cl, Br, I, X ≠ Y) systems have been investigated as possible visible light photocatalysts in contrast with the BiOX (X = F, Cl, Br, I) systems by using hybrid density functional calculations. All the BiOX/BiOY systems have indirect bandgaps, and all the bandgaps of BiOX/BiOY systems we considered are between the bandgaps of BiOX and BiOY systems. The calculated bandgaps for BiOF/BiOCl, BiOF/BiOBr, BiOF/BiOI, BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI are respectively 3.86, 3.41, 2.74, 2.99, 2.30, and 2.23 eV. The maximum absorption wavelength increases in the order of BiOF, BiOF/BiOCl, BiOCl, BiOF/BiOBr, BiOCl/BiOBr, BiOBr, BiOF/BiOI, BiOCl/BiOI, BiOBr/BiOI, and BiOI. The conduction band edges for all the BiOX/BiOY systems originate from Bi 6p states, but the valance band edges are contributed by different electronic states. Besides, the relative positions of X p states and Y p states for BiOX/BiOY systems are different, which should be attributed to the different p orbital energies of X and Y atoms. Due to the conduction band maximum is lower than the hydrogen reduction potential, all the BiOX and BiOX/BiOY systems are thermodynamically unfavorable for hydrogen production. Meanwhile, owing to the suitable bandgaps and band edge positions, the BiOF/BiOI, BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI superlattices are possible visible light photocatalysts for degradation of organic pollutants.

High-performance giant-magnetoresistance junction with B2-disordered Heusler alloy based Co2MnAl/Ag/Co2MnAl trilayer

The current-perpendicular-to-plane giant magnetoresistance (MR) devices with full-Heulser Co2MnAl (CMA) electrodes and a Ag spacer have been simulated to investigate the relationship between the transport properties and the structural disordering of electrodes by performing first-principles electronic structure and ballistic transport calculations. The CMA electrode has nearly negligible interfacial roughness in both L21 and B2-types. The transmission coefficient Tσ(E,k→//) is found strongly dependent on the structures of the trilayers for different structural CMA electrodes. High majority-spin electron conductance in the magnetization parallel configuration turns up in the entire k→-plane and the MR ratio reaches as high as over 90% for the B2-based CMA/Ag/CMA magnetic trilayers. In contrast, the L21-based one has ∼60% MR ratio resulting from much lower bulk spin-asymmetry coefficient (β), which might be caused by the vibrational spin-polarization in each atomic layer adjacent to the interfaces in the co...

Mechanism for bipolar resistive switching memory behaviors of a self-assembled three-dimensional MoS2 microsphere composed active layer

A self-assembled three-dimensional (3-D) MoS2 microsphere-based memristor with a favorable ON/OFF resistance ratio of ∼104, endurance, and retention time is demonstrated at room temperature. The formation and rupture of a localized Ag metallic filament, establishment and destruction of a boundary-based hopping path, and charge trapping and detrapping from the space charge region co-contribute to the bipolar resistive switching memory behaviours observed in the device of Ag/MoS2/ITO. This work may give insight into the mechanism of the resistive switching memory behaviours of a device with a 3-D micro-scale.

Edge magnetism modulation of graphene nanoribbons via planar tetrahedral coordinated atoms embedding

Using first-principles calculations, we have explored the magnetic properties of zigzag edge graphene nanoribbons (ZGNR) with planar tetrahedral coordinated carbon (ptC) or silicon (ptSi). It is observed that the ptC can convert antiferromagnetic-coupled ZGNR into ferromagnetic one, while the ptSi cannot. These contrast results can attribute to the differences between magnetic ptC and non-magnetic ptSi atoms. Moreover, both ZGNR-ptC and ZGNR-ptSi structures are metallic and stable. Thus, the unique magnetic properties and high stabilities of these systems strongly suggest the feasibility of designing novel ferromagnetic devices.

ZnO/MoX2 (X = S, Se) composites used for visible light photocatalysis

Hybrid density functional has been adopted to investigate the structural, electronic, and optical properties of ZnO/MoS2 and ZnO/MoSe2 composites as compared with the results of ZnO, MoS2, and MoSe2 monolayers. The results indicate that MoS2 and MoSe2 monolayers could contact with monolayer ZnO to form ZnO/MoS2 and ZnO/MoSe2 heterostructures through van der Waals (vdW) interactions. The calculated bandgap of ZnO/MoS2 (ZnO/MoSe2) is narrower than that of ZnO or MoS2 (MoSe2) monolayers, facilitating the shift of light absorption edges of the composites towards visible light in comparison with bare ZnO and MoX2 monolayers. Through the application of strain, the ZnO/MoS2 and ZnO/MoSe2 composites which own suitable bandgaps, band edge positions, efficient charge separation, and good visible light absorption will be promising for visible light photocatalytic water splitting. These results provide a route for design and development of efficient ZnO/MoS2 and ZnO/MoSe2 photocatalysts for water splitting.

New insights into the electronic structures and optical properties in the orthorhombic perovskite MAPbI3: a mixture of Pb and Ge/Sn

As a new type of promising photovoltaic material, organic–inorganic hybrid perovskites (OIHPs) have attracted particular attention owing to their rapid increase in power conversion efficiency (PCE). It is quite remarkable that within several years PCEs have speedily exceeded the value of 20% from the original 3.8% in 2009. Nevertheless, the existence of heavy metal lead in the prototype perovskite MAPbI3 poses a great threat to human health and the living environment. To explore potential alternatives, we choose the congener elements of Ge and Sn as partial substituents for lead. By virtue of density functional theory calculations, the electronic structures and optical properties of Ge/Sn-doped perovskite compounds have been investigated involving GGA-PBE and DFT-SOC levels. The simulations indicate that the optical absorption coefficient is significantly enhanced due to the use of Ge/Sn dopants. Moreover, it is revealed that the absorption spectra shift towards the near infrared and even the middle infrared, and meanwhile, the band gap can be tuned by means of the Ge/Sn doping. Additionally, optical property analyses imply that the effect of spin–orbit coupling (SOC) plays a positive role for the application of OIHP materials in the photovoltaic field. These results shed a new light on a deep understanding of the nature of a range of functional materials constituted of OIHPs.