Zongyan Zhao

First-principles study on the doping effects of nitrogen on the electronic structure and optical properties of Cu2O

Recent experimental literature (Sol. Energ. Mat. Sol. Cells, 2012, 105, 192) has reported that nitrogen doped Cu2O is a possible material for novel intermediate band solar cells. The doping effects of nitrogen on the crystal structure, electronic structure, and optical properties of Cu2O have been studied by an ultrasoft pseudopotential plane wave method based on first-principles calculations. The results show that nitrogen doping slightly widens the band gap of Cu2O, and form an intermediate band in the gap located at about 0.9 eV from the VBM (or CBM). This intermediate band is predominantly formed by hybridization between the N-2p states and the Cu-3d states. N-doped Cu2O is very likely to absorb at a maximum across the solar light spectrum, from the near infrared region to the ultraviolet region. Based on these results, N-doped Cu2O is considered to be a perfect intermediate band material for a novel kind of solar cells.

Analysis of sulfur modification mechanism for anatase and rutile TiO2 by different doping modes based on GGA + U calculations

In order to confirm the doping effects of sulfur on the photocatalytic performance of TiO2, the crystal structure, electronic structure, and optical properties of S-doped anatase and rutile TiO2, in which sulfur impurity occupies the lattice oxygen or titanium sites and the interstitial sites, were calculated by density functional theory within GGA + U method. If sulfur impurity incorporates into anatase and rutile lattice by the same doping mode, it exhibits similar doping effects (including lattice distortion, variation of band gap, localized defect states, and type and position of impurity energy levels). However, there are some subtle different doping effects between different crystal structures or different doping modes. According to the calculated results and basic relationship between electronic structure and photocatalytic performance, it is determined that sulfur interstitial doping in anatase TiO2 processes optimal modification effects due to its shallow impurity energy levels, located at the top of the valence and the bottom of the conduction band, which could not only improve the absorption of visible light, but also suppress the recombination of photo-generated electron–hole pairs.

Structural, Electronic, and Optical Properties of BiOX 1− x Y x (X, Y = F, Cl, Br, and I) Solid Solutions from DFT Calculations

Six BiOX1−xYx (X, Y = F, Cl, Br, and I) solid solutions have been systematically investigated by density functional theory calculations. BiOCl1−xBrx, BiOBr1−xIx, and BiOCl1−xIx solid solutions have very small bowing parameters; as such, some of their properties increase almost linearly with increasing x. For BiOF1−xYx solid solutions, the bowing parameters are very large and it is extremely difficult to fit the related calculated data by a single equation. Consequently, BiOX1−xYx (X, Y = Cl, Br, and I) solid solutions are highly miscible, while BiOF1−xYx (Y = Cl, Br, and I) solid solutions are partially miscible. In other words, BiOF1−xYx solid solutions have miscibility gaps or high miscibility temperature, resulting in phase separation and F/Y inhomogeneity. Comparison and analysis of the calculated results and the related physical–chemical properties with different halogen compositions indicates that the parameters of BiOX1−xYx solid solutions are determined by the differences of the physical–chemical properties of the two halogen compositions. In this way, the large deviation of some BiOX1−xYx solid solutions from Vegard’s law observed in experiments can be explained. Moreover, the composition ratio of BiOX1−xYx solid solutions can be measured or monitored using optical measurements.

Electronic structure and optical properties of Si–O–N compounds with different crystal structures

In order to explore the relationship between crystal structure and optical properties, five Si–O–N compounds with different crystal structures, including: α-quartz SiO2, β-quartz SiO2, Si2N2O, α-Si3N4, β-Si3N4 were considered in the present work. Using density functional theory with the GGA+U method, their crystal structure, electronic structure, and optical properties have been systematically investigated. The electronic structure of α and β phases of SiO2 (or Si3N4) are similar, but with some subtle differences that can be attributed to the different local bonding structure, and the electronic structure of Si2N2O shows the fundamental features of the electronic properties of SiO2 and Si3N4. Based on the calculated results, it is found that the optical properties not only are determined by the components of the Si–O–N compounds, but also are determined by the microstructure of the Si–O–N compounds. These calculated results will be useful as reference data for analyzing the optical properties of more complicated SiOxNy compounds. According to this principle, one could design novel Si–O–N compounds for specific optoelectronic applications, via tuning the composition and crystal structure.

DFT study on microstructures and electronic structures of Pt mono-/bi-doped anatase TiO2 (101) surface

The microstructures and electronic structures of Pt mono- and bi-doped anatase TiO2 (101) surfaces were investigated by density functional theory calculations to elucidate the surface doping effects and the initial stages of Pt cocatalyst formation on a TiO2 photocatalyst surface. Several substitutional and interstitial configurations for the Pt impurity on the surface were studied, and the relative stability of different doping configurations was compared by the impurity formation energy. Under reducing conditions, surface interstitial (in other words, surface adsorbed) doping for Pt is more stable than any competitors (substitutional, bulk or subsurface doping) on an anatase TiO2 (101) surface. Compared with bulk doping, the metal induced gap states were very localized and outstanding in the case of Pt surface interstitial doping onto a TiO2 (101) surface. In the case of Pt substitutional doping, the surface doping effects were harmful for photocatalysis due to the metal induced gap states in the middle of the band gap. However, in the case of Pt interstitial doping, the surface doping effects were very favorable for photocatalysis due to the overlapping of metal induced gap states with the top of the valance band. Systematic calculations revealed that Pt doping is prone to occupy the interstitial sites on the (101) surface. In particular, in the case of Pt bi-doping, two Pt impurity atoms can be co-adsorbed on the surface to form a stable configuration due to the strong Pt–Pt atomic interaction. Therefore, Pt mono-/bi-doping on an anatase TiO2 (101) surface can be considered as the initial stage (nucleation process) of Pt cocatalyst loading onto a TiO2 photocatalyst surface. Therefore, the calculated results can form the basis for further investigations about Ptn cluster loadings or Pt/TiO2 interface formation as well as water molecule adsorption or decomposition on a Pt/TiO2 composite photocatalyst.

Synthesis and Near-Infrared Fluorescent Properties of Nd3+-Yb3+ Co-Doped Lanthanum Phosphate

In the present work, LaPO₄: Ln (Ln = Nd, Yb) samples were prepared via a simple hydrothermal method and characterized by X-ray diffraction, UV-Vis spectrophotometer, and fluorescence spectrophotometer. The XRD patterns show that the crystal structure of samples was lanthanum phosphate phase that belongs to monoclinic system with a space group of P2₁/n. Nd³⁺, as a sensitizer, was excited to ⁴I_9/2 by 808 nm laser diode, and then transfer the energy to Yb³⁺, resulting in the fluorescent emission at 980 nm. This fluorescent emission arises from the ²F_5/2→²F_7/2 transition. And the luminescent intensity depends on the concentration of Yb³⁺ ions. This implies that the concentration quenching is observed in the present work. The optimal Yb³⁺ ions concentration is 4 mol %.

Systematic studies on YbxBi1−xVO4:Tm3+ solid solutions: experiments and DFT calculations on up-conversion photoluminescence properties

Compound solid solutions have attracted intensive attention due to their adjustable structure, electronic structure, and optical properties. Despite tremendous advances in compound solid solution preparations, combining a rare metal compound and a bismuth compound with the same crystal phase by forming a compound solid solution is still challenging but fascinating. For example, RE1−xBixVO4 with a zircon-type structure exhibits tunable band gaps and photoluminescence performance with varying RE compositions. Herein, Yb1−xBixVO4 solid solutions with continuous monophasic phase prepared by a facile synthesis strategy that is combined with co-precipitation and hydrothermal methods are reported. By doping a small amount of Tm3+, YbxBi1−xVO4 solid solutions can achieve a broad range up-conversion photoluminescence from UV-light to NIR-light. Combined with DFT calculations, the underlying mechanism of experimental observations is explained. In these up-conversion processes, the existence of Tm3+ is an essential factor. In particular, the NIR-to-UV up-conversion photoluminescence of YbxBi0.98−xVO4: 2 mol% Tm3+ solid solution is very interesting and a worthy phenomenon for further studies. As such, designing compound solid solutions may provide a new avenue for controllable up-conversion efficiencies in semiconductor nanocrystals and also a novel insight into the rational tunable up-conversion process for applications in biological labeling and imaging.

Monophasic zircon-type tetragonal Eu{sub 1−x}Bi{sub x}VO{sub 4} solid-solution: synthesis, characterization, and optical properties

Highlights: • Complete Eu{sub 1−x}Bi{sub x}VO{sub 4} with zircon-type structure was successfully synthesized. • The band gap of the samples could be adjusted and controlled by bismuth content. • Eu{sub 1−x}Bi{sub x}VO{sub 4} show strong red emission under both near UV and visible-light excitation. - Abstract: By combining the methods of co-precipitation and hydrothermal synthesis methods, the complete solid-solution of Eu{sub 1−x}Bi{sub x}VO{sub 4} with monophasic zircon-type structure was successfully synthesized. The zircon-type structure was determined by X-ray diffractometer and Raman scattering, and the optical properties were characterized by ultraviolet-visible diffuse reflectance and photoluminescence spectrophotometer. The results indicate that the band gap of Eu{sub 1−x}Bi{sub x}VO{sub 4} could be adjusted and controlled by bismuth content in the range of x = 0–0.9. Meanwhile, the Eu{sub 1−x}Bi{sub x}VO{sub 4} solid-solution phosphors show strong red light emission was shown in 619 nm under both near UV-light and visible-light excitation. Notably, the emission intensity of Eu{sub 1−x}Bi{sub x}VO{sub 4} (x = 0.4) is the strongest in all samples.

Hydrothermal Synthesis Nano FAP : Nd3+ as Biological Probe with Near-Infrared to Near-Infrared Luminescence

Biocompatible nano Nd3+ doped Ca5(PO4)3F( FAP) were successfully synthesized through hydrothermal method. Nearly pure near-infrared to near-infrared (NIR-to-NIR) luminescence can be realized. Moreover, the excitation and the emission at 808 and 1060 nm are away from the visible region. These are beneficial to deeper tissue penetration and reduced autofluorescence. This material exhibits an excellent NIR-to-NIR emission performance and is thus potentially applicable as an high-contrast in vitro and in vivo imaging.