Co-Cr共掺杂金红石型TiO2电子结构和光学性质的第一性原理研究

First Principles Study of Electronic and Optical Properties of Magnesium based Chalcogenides

We report the electronic structures and density of states (DOS) of perfect LiCAF and LiSAF crystals calculated from density functional theory (DFT) with local density approximation (LDA) using optimized lattice constants. DOS calculations reveal that the valence band is mainly derived from F 2p, thereby resulting to a very narrow valence band manifold. Meanwhile, the conduction band is mainly derived from Ca 4s or Sr 5s resulting to Sr having a broader band dispersion compared to Ca. Both fluoride compounds have indirect band gaps with LiCAF having a band gap of 8.02 eV and LiSAF a band gap of 7.92 eV. This is, to the best of our knowledge, the first report on the electronic structure of LiSAF calculated using DFT with LDA. Our results suggest that when doped with Ce3+, the shorter 5d-conduction band distance in Ce:LiSAF combined with the difficulty of growing high-purity crystals lead to the more pronounced excited state absorption (ESA) and solarization effect experimentally observed in Ce:LiSAF, limiting its potential as a laser material compared with Ce:LiCAF.

Water splitting for hydrogen production under sunlight using TiO2 as photo catalyst provides a better route for solar energy and attracts the attention of many researchers. The photo catalytic activity of TiO2 under sunlight irradiation depends on the band gap energy. The transition metal doped TiO2 shows an edge over TiO2 in optical absorbance and photo catalytic activity. Thin film of Cr doped TiO2was deposited by Electron beam evaporation deposition process on glass substrate, the band gap of sample was measured with UV-Vis spectroscopy. In order to provide theoretical ground to our experimental band gap results the first principle (ab-initio) calculation was performed using the computer code WIEN2K with Full Potential Linearized Augmented Plane Wave method (FP-LAPW) based on the density functional theory (DFT).

Ground-state electronic structure and optical properties of distorted perovskite DyMnO3 are investigated in the frame of density functional theory within generalized gradient approximation (GGA). Optimized lattice constants are reasonable in agreement with experimental data. Electronic band structure, density of states and partial density of states of elements are obtained. It shows that DyMnO3 exhibit an indirect band gap of 0.91 eV. Dielectric function, optical reflectivity, refractive index, extinction coefficient, electron energy loss, and absorption coefficient are calculated and analyzed for radiation up to 35 eV.

We study the electronic band structure and density of states (DOS) on ZnO material in various crystal structures : wurtzite (W), zincblende (ZB), and rocksalt (RS) phases. Calculations are based on Density Functional Theory (DFT) with Generalized Gradient Approximation (GGA) for exchange-correlation functional and Hubbard correction to consider the strong electron correlations in 3d orbitals. After structural optimization, GGA results show that wurtzite and zincblende structures have a direct band gap of 0.749 eV and 0.637 eV, respectively, whereas rocksalt structure has an indirect band gap of 0.817 eV. Symmetrical shape of total DOS for spin up and spin down electrons indicates a zero total magnetic moment. For all ZnO structures, the upper valence band is formed by hybridization among O 2p and Zn 3d orbitals, while lower valence and conduction band are primarily filled by O 2s and Zn 4s, respectively. The GGA+U approach is found to improve the calculated band gaps and correct the position of Zn 3d state below Valence Band Maximum (VBM). From GGA+U, the band gaps for W-ZnO, ZB-ZnO, and RS-ZnO are 1.12 eV, 1.00 eV, and 1.11 eV, respectively.

Segment of a periodic table depicting the elements that are molecular/atomic (blue) and that have extended network structures (gray) at STP.

Introduction:The use of Density Functional Theory (DFT) simulations to predict the chemical-physical properties of nanostructured compounds and heterogeneous interactions (solid-gas) has become almost essential today, both to predict the properties of technologically advanced nanomaterials and to overcome the limits of experimental characterizations.Specifically, the use of DFT calculations can likewise be used in chemoresistive gas sensor applications, to investigate the physical-chemical properties of nanostructured semiconductors and their possible catalytic activity. Among the various gas sensing materials used, the Metal Oxide Semiconductors (MOXS) are the most investigated since the excellent sensitivity, good stability and low cost of production [1,2]. Nevertheless, besides all of these advantages they present also some drawbacks such as signal drift over the time, lack of selectivity and high operating temperature.Investigators have tried to address these problems in several manners, including the synthesis of solid solutions composed of different metal oxides, doping these materials with deferent kinds of elements and also with controlled concentrations of oxygen vacancies. The latter is a field still little explored, but preliminary experimental and theoretical works highlighted interesting results [3,4].When the sensor is exposed to a reducing (oxidizing) gas, the gas molecules interact with the chemisorbed oxygen species from the surrounding atmosphere and then decrease (increase) the resistance of a n-type based gas sensor, and vice versa for a p-type semiconductor. This change in resistance is due to the exchange of electrons between the surface and the conduction band. For a stoichiometric semiconductor, the electrons involved in this reaction that are in the conduction band are coming from the valence band after heating the material, in this case they need a high energy to blow up from the valence band to the conduction band. But when one introduces some defects, new energy level will be created and then they will be a new electrons source, thus the energy that the sensor needs to be thermoactivated decrease; that is energy that electrons need to jump to the conduction band. Scope and Methods: The purpose of this work is to investigate the impact of oxygen vacancies inside MOXS on their physical-chemical properties in general and on the electrons exchange in particular. Indeed, the creation of oxygen vacancies influences the physical-chemical properties of metal oxides. i.e. The bridging oxygen vacancies lead to the formation of energy level in the band gap region (Fig.1), these energy levels will behave as electrons donors that decrease the energy (ε1) needed for the electrons to be in the conduction band. While creation of oxygen vacancies from in-plane positions drives to the formation of impurity levels located slightly below the conduction band minimum (CBM), in this case the energy (ε2) that they need to jump to the conduction band is lower than ε1. Therefore, we can say that in-plane oxygen vacancies can overcome the disadvantage of high working temperatures for MOXS; as much as the electrons resource is close to the conduction band, as much as the ionization of these electron levels will be easy and then enhance the reactions (adsorption process) on the surface of the active element at low temperature.Tin dioxide is a typical n-type semiconducting material with a wide band gap of 3.6 eV. It has attracted the attention of many researchers because of its broad spectrum of physicochemical properties. It has also been used in several fields such as optoelectronic devices, electrocatalysis, ceramics and gas sensors. It is the semiconductor most studied as active element for the production of chemoresistive gas sensors. Then, it represents the best candidate for the proposed innovative work.In this work, DFT was used to compute the physical-chemical properties of the considered material. The Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA), which improve the LDA results taking into account the electron density gradient due to the non-homogeneity of the electron density, are the principal basis of the exchange-correlation functional within the framework of DFT. In this work, the GGA parameterized PBE (Perdew-Burke-Ernzerhof) has been used for structural properties prediction. This approximation has demonstrated, beside the LDA, their ability to predict the structural properties of materials. Nevertheless, their applications underestimate the band gap energy of materials compared to experimental results. To enhance the results concerning the band gap the Tran-Blaha’s modified Becke-Johnson exchange potential model (TB-mBJ) has been applied, this approximation has proved its ability to predict the band gap of materials.A series of first-principles studies has been carried out using the Full Potential Linearized Augmented Plane Wave (FPLAPW) method within the framework of the DFT, as implemented in the Wien2k code. This method is the most effective method to solve the Khon Sham equation.

With the continued down-scaling of the integrated circuits, the conventional dielectric oxide, SiO2, have been replaced by new dielectric materials, such as the high-κ oxides in CMOS transistors to avoid excess leakage current and the low-κ silicate insulating layers in-between copper wires to reduce the interconnect capacitances. To further control or improve their performance, a detailed understanding of the structural, electronic, dielectric, and the mechanical properties of these new dielectric materials are strongly in demand. In this thesis, we have employed first-principles density functional theory calculations to investigate the important properties of these new dielectric oxide materials in many fundamental aspects. For gate dielectrics in CMOS transistors, we have investigated the he structural, electronic, and dielectric properties of the Si- and La-doped HfO2, as well as the O vacancy formation and the relevant induced defect states in band gaps over a wide range of chemical compositions. For Si-doped HfO2, i.e. Hf-silicates (a-Hf1-xSixO2), our results show that the Si-rich Hf-silicates possess distinct structural characteristics from the Hf-rich ones and the electronic band gaps were found to vary nonlinearly with the Si content. Different from the previous theoretical studies, our results support a nonlinear dependence of the dielectric constants on the composition of silicates, which is mainly attributed to the rapid reduction of the low-frequency vibrational motions of Hf atoms with increasing the Si content. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device. For the La-doped HfO2 (a-Hf1-xLaxO2-0.5x, x = 0 ~ 0.5), the mass density was found to decrease linearly with the La content, but the electronic band gap only reduces slightly by 0.29 eV as the La content increases from x = 0 to 0.5. Our Bader charge analysis shows that the average atomic charge on Hf (La) atoms can decrease from 2.31 (2.12) to 2.26 (2.06) as the La content increases from 0 to 0.5, which indicates that the ionicity of the Hf (La) atom tends to decrease as the La content increases. This results is consistent with our calculations for the Hf 4f core level energy, which reveal a binding energy shift of around 0.3 ~ 0.6 eV as the La content increases from 0 to 0.5, in good agreement with a recent experimental measurement. Our calculations also show that the dielectric constant of a-Hf1-xLaxO2-0.5x increases rapidly with the La content from x = 0 to 0.25 but it appears to be nearly unchanged as x ranges between 0.25 and 0.5, which is consistent with the recent experiments. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in a-Hf1-xLaxO2-0.5x. Similar to Hf-silicates, the formation energies are largely dependent on the local bonding configurations rather than on the compositions. However, O vacancy was found to be more likely to stay in positively charged states with lower formation energies than that in HfO2, implying that the flat band voltage shift and the threshold voltage instability could be enhanced due to La-doping. Furthermore, the trap energy level was found to decrease gradually with increasing the La contents, which indicates that the trap-assisted conduction can become more seriously in a-Hf1-xLaxO2-0.5x. For the interlayer insulating dielectrics, we have investigated the structure and mechanical properties of the methyl- and ethyl-bridged organosilicate hybrid glasses (OSG) using first-principles calculations. Our results show that the mass density of OSG can gradually decrease with increasing the carbon content, but the porosity may not necessarily increase with carbon-bridging units in the amorphous bond network. Our results also reveal that the elastic moduli and fractured surface energies of OSG models tend to decrease with increasing the alkylene-bridged units in the silicate bond network, which imply that the crack-resistance of the alkylene-bridged OSG can be much worse than fused silica based on the Griffith theory of brittle fracture. According to the calculations of decohesive energy, the mechanical strength of a-SiO2 still outperforms the methyl-bridged OSG models, indicating that the crack-resistance of these two materials is largely determined by the chemical bond strength. However, the decohesion energy of the ethyl-bridged OSG models turns out to be higher than a-SiO2 though their critical stress for crack propagation predicted by Griffith theory is simply half of that in a-SiO2. Our results further show that the enhanced fracture toughness in the ethyl-bridged OSG model can be mainly attributed to the plastic deformation and bonding reconfiguration induced by the ethyl-bridged units during fracture, which was not observed for the methyl-bridged OSG models and a-SiO2.

Absolute band structure determination on naturally occurring rutile with complex chemistry: Implications for mineral photocatalysis on both Earth and Mars

Studies have been carried out on aqueous colloidal suspensions of semiconductors, principally by using laser flash-photolysis techniques, in order to scrutinize in detail the dynamics of light-induced charge transfer reactions occuring at the solid-electrolyte interface. Preparation of sols of good transparency allows the use of all fast spectroscopic methods usually reserved for homogeneous systems and enables the monitoring of the temporal behavior of transient species involved during light absorption and electron transfer events. Under these conditions, one was able to observe the following reactions : a) the reduction of different acceptors in solution by conduction band electrons of titanium dioxide under band-gap irradiation, b) the oxidation of halides by valence band holes of TiO2 and α-Fe2O3, and c) the charge injection in the solid from visible light excited sensitizers. Results have lead in most cases to the development of simple models describing the dynamics of interfacial electron transfer. The direct spectroscopic observation of trapped electrons in the conduction band of the semiconductor has afforded a handle for quantitative evaluation of electron-hole pairs recombination kinetics in pure or doped TiO2 particles. Dynamics of charge carriers out of equilibrium has also been entered upon by the measurement of its influence on the electron transfer quantum yield.

This paper investigated the electronic properties and photoresponse of two-dimensional SnX2 (X = Cl, Br, I) monolayer binary materials using computational techniques. The calculated band structure and density of states indicate that these are large band gap semiconducting materials with an indirect band gap. The studied chemical bonding mechanism shows the existence of the hybrid bonding of ionic and covalent bonds in these dihalide materials. The valence band (VB) and conduction band (CB) edge positions are also estimated, using the concept of electronegativity and band gap, to investigate the photocatalytic activity of SnX2. Next, we investigated the polarization and energy-dependent dielectric and optical functions along the crystallographic axes of these materials in the linear response approach of the perturbing incident oscillating light field. These materials exhibit an anisotropic behavior of these functions, especially in the high-energy visible and low-energy ultraviolet (UV) regions. The absorption of incident light photons is very fast in SnI2 than SnBr2 and SnCl2 in the low-energy UV region. It demonstrates the higher absorption coefficient and optical conductivity in Snl2. The obtained average static refractive index of SnCl2 is comparable to that of glass (1.5), showing its application as transparent material. The low reflection coefficient, less than 20%, makes them superior for antireflection coating materials in the infrared and visible regions. The prominent energy loss peaks show the existence of plasmon resonances in these materials. The most of losses occur in the UV region. The investigated electronic and photoresponse properties indicate that these Sn-based dihalide materials are excellent for electronic devices and optoelectronic applications. Also, the calculated VB and CB edge positions with respect to the normal hydrogen electrode show the favorable water-splitting capability of these materials.

Theoretical studies on electronic structure and optical properties of Bi2WO6

Titanium dioxide (TiO2) thin films were deposited on Si wafer and glass substrates by direct current (dc) magnetron sputtering, by which the photocatalytic phase, anatase, was obtained under the optimum deposition conditions of O2/(O2+Ar) flow ratios and substrate biases. TiO2 thin films doped with Sn or Nb were prepared on glass substrates by dc/rf (radio frequency) magnetron co-sputtering, in which dc and rf were utilized for Ti and Sn or Nb targets, respectively, and the samples were post-annealed at temperatures ranging from 473 to 773 K for 1 h in ambient air. Microstructure, surface morphology, chemistry, and optical properties of the un-doped, Sn-doped, and Nb-doped TiO2 films were characterized in details by glancing incidence X-ray diffraction (GIXD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS), differential thermal analysis/thermogravimetry (DTA/TG), atomic force microscopy (AFM), UV/visable spectrophotometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). In addition, the photocatalytic activity was evaluated by measurement of the degeneration of methylene blue under UV/visible irradiation. The analytical results of GIXD show that the anatase phase is the dominant phase at an O2/(O2+Ar) flow ratio of over 40 %, whereas the films exhibit the rutile phase dispersed in amorphous matrix at an O2/(O2+Ar) flow ratio of 20 %. Microstructure and optical properties of the TiO2 films are dependent closely upon the rf power applied to the substrates as a bias. The anatase phase with columnar structure was obtained for the films prepared at rf powers of 0 and 5 W; whereas the films exhibit a rutile phase with equi-axial crystals at rf powers of 15 and 25 W. The rf-bias-assisted TiO2 films have a smoother surface than that without rf bias, and the absorbance edges shift to long wavelengths. The TiO2 films with a rutile phase have a higher refractive index, whereas better photocatalytical properties were observed for the films having anatase phase. TiO2 films with various Sn or Nb doping contents were investigated on the microstructure, optical properties, and photocatalytic activities. All the as-deposited TiO2 films on glass substrate were amorphous. The concentration of Sn in rf powers of 100 and 200 W and Nb in rf powers of 200, 300, and 400 W were 6.65 and 16.4 at.% for Sn and 1.98, 3.56, and 4.9 at.% for Nb, respectively, by X-ray photoelectron spectroscopy. GIXD revealed a polycrystalline phase for the Nb-doped films post-annealed at temperature 523 K, in contrast to the un-doped one that has to be annealed at temperature 723 K, indicating that Nb dopant can enhance the crystallization of amorphous TiO2. Furthermore, the Nb-doped film post-annealed at 673 K was found to have an anatase-dominanted phase with a fine grain microstructure, while the Sn-doped film post-annealed at 673 K was found to have a coexistence of anatase/rutile duplex and amorphous structure, as observed by using TEM. Heat treatment also induces a change in the surface morphology of the TiO2 films examined by FE-SEM. The optical properties of the TiO2 films were characterized by UV/visible spectrophotometry. The average transmittance of the Sn-doped and Nb-doped films is higher than 85%. Absorption zone in visible regime and optical band gap of the Sn-doped or Nb-doped TiO2 films increase with the doping contents. Under visible light irradiation, all the Nb-doped TiO2 films exhibit better photocatalytic activity than that of un-doped and Sn-doped ones. Among them, the Nb-doped TiO2 film with rf power of 400 W post-annealed at temperature 673 K shows the best photocatalytic performance.

Monolayer molybdenum diselenide (MoSe₂), a notable two-dimensional transition metal dichalcogenide, has garnered significant attention for its favorable electronic structure and potential applications in optoelectronic and photovoltaic-related fields. In this work, the intrinsic electronic properties of monolayer MoSe₂ are systematically investigated using first-principles calculations based on density functional theory. The electronic band structure and total density of states are calculated to provide a comprehensive description of the fundamental electronic behavior of the material. The results indicate that monolayer MoSe₂ is a direct band-gap semiconductor, with both the valence band maximum and the conduction band minimum located at the K point of the Brillouin zone. The calculated band gap is consistent with previously reported theoretical results, confirming the reliability of the computational methods employed. Analysis of the band dispersion reveals relatively flat valence bands and more dispersive conduction bands near the K point, suggesting distinct effective masses for holes and electrons. Furthermore, the density of states analysis shows that the electronic states near the band edges are predominantly contributed by Mo-d and Se-p orbitals, highlighting the key role of orbital hybridization in determining the electronic structure. Overall, this study provides a clear and consistent understanding of the electronic properties of monolayer MoSe₂ and offers a useful theoretical reference for future investigations of two-dimensional transition metal dichalcogenides and their optoelectronic-related applications.

We have studied the structural, electronic and dynamic properties of γ- Li4SiO4(lithium orthosilicate) using density functional theory (DFT) with the generalized gradient approximation (GGA). The crystal structure is fully relaxed. The electronic band structure and Density of States (DOS) calculations indicate that γ- Li4SiO4is an insulator with an indirect band gap of 5.19 eV and it has a conduction band with the width of 5.92 eV and two valance bands with the width of 4.45 eV and 0.57 eV, respectively. In the partial DOS, Li and Si electronic densities increase more sharply than O atoms. Comparing with previous works, the phonon dispersion curves without negative frequencies are calculated along high symmetry points. By adding the Born effective charges in the phonon calculation, the LO–TO splittings are also calculated which indicate that γ- Li4SiO4is polar and anisotropic. The optical modes of phonon frequencies at Γ point are assigned as Raman and Infrared-active modes. Additionally, the thermodynamic functions (entropy, internal energy, Helmholtz free energies and constant-volume specific heats) were determined by using the phonon DOS. The calculated results may provide useful guidance of γ- Li4SiO4for future experimental studies in some degree.