Influence of tungsten doping concentration on the electronic and optical properties of anatase TiO2

The structural, electronic and optical properties of tungsten-doped TiO₂ have been investigated using density functional theory with plane wave basis sets and ultrasoft pseuodopotential. Substitutional W doping at Ti sites create W 5d states just below the conduction band minimum while interstitial W doping gives isolated W 5d states in the middle of forbidden region. Averaged bond lengths show that W doping at Ti sites produce minimum structural distortion as compared to the interstitial W-doped TiO₂. Substitutional W-doped TiO₂ has better visible light absorption compared to interstitial W-doped TiO₂ and has stable configuration which provide reasonable explanation for the experimental findings. Tungsten doping in TiO₂ with different doping concentrations is investigated as an enabling concept for enhancing the visible light absorption. Optical properties show that optimal W doping concentration would improve the visible light absorption. 2.08% W doping concentration gives strong visible and ultraviolet light absorption among all doped models found consistent with experiments.     

Structural and electronic properties of the TiC nanotubes: Density functional-based tight binding calculations

Atomic models of the hypothetical single- and multi-walled cylindrical- and prismatic-like TiC nanotubes have been constructed and their structural and electronic properties have been studied by means of density functional-based tight binding (DFTB) method. The electronic bands, densities of states and binding energies are analyzed as a function of the TiC tubes sizes. Our calculations showed that TiC nanotubes are semiconducting, in contrary to the metallic-like crystalline TiC, and the band gaps tend to vanish as the number of tube walls increase.     

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO₂ in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO₂. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO₂, are observed by analyzing the results of optical properties, which locate in the region of 400-1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO₂ shift to the long wavelength region compared with the pure TiO₂. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO₂. Our results show that the visible light response of TiO₂ can be modulated by substitutional doping of Cu, Ag, and Au.     

Density functional investigation of structural, electronic and optical properties of Ge-doped ZnO

Recent experiments reported fascinating phenomenon of photoluminescence (PL) blueshift in Ge-doped ZnO. To understand it, we examined the structural, electronic and optical properties of Ge-doped ZnO (ZnO:Ge) systematically by means of density functional theory calculations. Our results show that Ge atoms tend to cluster in heavily doped ZnO. Ge clusters can limit the conductivity of doped ZnO but reinforce the near-band-edge emission. The substitutional Ge for Zn leads to Fermi level pinning in the conduction band, which indicates Ge-doped ZnO is of n-type conductivity character. It is found that the delocalized Ge 4s states hybridize with conduction band bottom, and is dominant in the region near the Fermi level, suggesting that Ge-4s states provides major free carriers in ZnO:Ge crystal. The observed blueshift of PL in Ge-doped ZnO originates from the electron transition energy from the valence band to the empty levels above Fermi level larger than the gap of undoped ZnO. The electron transition between the gap states induced by oxygen vacancy and conduction band minimum may be the origin of the new PL peak at 590 nm.     

First-principles study of the structural, elastic, electronic and optical properties of the monoclinic BiScO3

The structural, elastic, electronic and optical properties of the monoclinic BiScO₃ are investigated in the framework of the density functional theory. The calculated structural parameters are in agreement with the experimental values. Moreover, the structural stability of BiScO₃ system has been confirmed by the calculated elastic constants. The band structure, density of states, charge transfers and bond populations are also given. The results indicate that BiScO₃ has a direct band gap of 3.36 eV between the occupied O 2p states and unoccupied Bi 6p states, and its bonding behavior is a combination of covalent and ionic nature. Finally, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss function and dielectric function of the monoclinic BiScO₃ are calculated. In addition, the variation of the static dielectric constants ε₁(0) as a function of pressure for BiScO₃ is also discussed.     

Ab-initio calculations of electronic structure and optical properties of TiAl alloy

The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1-3.5 eV.     

Concentration-dependent electronic structure and optical absorption properties of B-doped anatase TiO2

The concentration-dependent electronic structures and optical properties of B-doped anatase TiO₂ have been calculated using the density functional theory. The calculated results indicate that the electronic structures of B-doped TiO₂ have changed compared with those of pure TiO₂, which is mainly due to the new midgap states induced by B doping. As to the optical properties, we calculate the imaginary part of dielectric function ε₂(ω) and optical absorption spectra of pure and B-doped TiO₂. Two transitions E₁ and E₂ emerged after B doping. The intensity of absorption is enhanced by B doping both in the UV and visible regions. According to the results of imaginary part of dielectric function ε₂(ω) and DOS, it can be concluded that the two optical transitions correspond to the transitions from the O 2p states in the top of valence band to the midgap states and from the midgap states to the Ti 3d states in the bottom of conduction band, respectively. These results have important implications for the further development of photocatalytic materials.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

First-principles study of structural,electronic and optical properties of ZnF_2

The structural, electronic, and optical properties of rutile-, CaC12-, and PdF2-ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile-, CaCl2-, and PdF2-ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.     

Structural,electronic, and optical properties of ZnO:ZnSnN2 compounds for optoelectronics and photocatalyst applications

Semiconductors with suitable band gap are highly desirable for the applications in optoelectronic and energy conversion devices. In this work, using the recently developed strongly constrained and appropriately normed (SCAN) density functional calculations in conjunction with hybrid functional, we investigate the structural, electronic, and optical properties of earth abundant element based ZnO:ZnSnN₂ compounds formed through alloying. The proposed ZnO:ZnSnN₂ compounds in the low energy configurations possess band gaps of 2.28 eV-2.52 eV. The decrease in band gap compared to ZnO is mainly attributed to the p-d repulsion between N 2p+O 2p and Zn 3d electrons that lifts the top of valence band. For the ZnO:ZnSnN₂ compounds studied the band edges straddle the water redox potentials and the absorption onsets lie in the visible light range. Our studies are helpful for ZnO:ZnSnN₂ compounds' experimental synthesis and future application in optoelectronics and photocatalyst.     

Structural and electronic properties of boron-doped double-walled silicon carbide nanotubes

The effects of boron doping on the structural and electronic properties of (6,0)@(14,0) double-walled silicon carbide nanotube (DWSiCNT) are investigated by using spin-polarized density functional theory. It is found that boron atom could be more easily doped in the inner tube. Our calculations indicate that a Si site is favorable for B under C-rich condition and a C site is favorable under Si-rich condition. Additionally, B-substitution at either single carbon or silicon atom site in DWSiCNT could induce spontaneous magnetization.     

Density functional theory studies on the structural and physical properties of Cu-doped anatase TiO2(101) surface

Structure and physical properties of anatase TiO₂ (101) surface doped with copper have been studied by using density functional theory. Results show that Cu@Ti and Cu@O systems behave as p and n type semiconductors, respectively. Anatase TiO₂ (101) surface exhibits a blue shift in optical absorption spectra compared with pure TiO₂ bulk materials. Enhanced photocatalytic activity at wavelength around 400 nm could be contributed by the change in electronic structure.     

Stress and strain effects on the electronic structure and optical properties of ScN monolayer

Based on the density functional theory, electronic and optical properties of a monolayer scandium nitride structure have been studied under different strain conditions. Our results indicate that both biaxial compressive and tensile strain effects lead to change the band gap of this structure with different rates. Also, optical absorption spectrum peaks experience an obvious red and blue shifts with the exerting of tensile and compressive strains, respectively. Our results express that ScN monolayer can be the promising candidate for the future nano-base electrical and optical devices.     

Structural,electronic and optical properties of Ca3Bi2: First-principles investigation

In this work by applying first principles calculations structural, electronic and optical properties of Ca₃Bi₂ compound in hexagonal and cubic phases are studied within the framework of the density functional theory using the full potential linearized augmented plane wave (FP-LAPW) approach. According to our study band gap for Ca₃Bi₂ in hexagonal phase are 0.47, 0.96 and 1?eV within the PBE-GGA, EV-GGA and mBJ-GGA, respectively. The corresponding values for cubic phase are 1.24, 2.08 and 2.14?eV, respectively. The effects of hydrostatic pressure on the behavior of the electronic properties such as band gap, valence bandwidths and anti-symmetry gap are investigated. It is found that the hydrostatic pressure increases the band widths of all bands below the Fermi energy while it decreases the band gap and the anti-symmetry gap. In our calculations, the dielectric tensor is derived within the random phase approximation (RPA). The first absorption peak in imaginary part of dielectric function for both phases is located in the energy range 2.0–2.5?eV which are beneficial to practical applications in optoelectronic devices in the visible spectral range. For instance, hexagonal phase of Ca₃Bi₂ with a band gap around 1?eV can be applied for photovoltaic application and cubic phase with a band gap of 2?eV can be used for water splitting application. Moreover, we found the optical spectra of hexagonal phase are anisotropic along E||x and E||z.     

Effects of aluminum vacancies on electronic structure and optical properties of Ta4AlC3: A first principles study

We investigated the effect of aluminum vacancies (V_Al) on the structural, electronic and optical properties of Ta₄Al_1−xC₃ (x=0, 0.25, 0.5, 0.75) based on the first-principle calculation using density functional theory. We found that the lattice constant a remains almost unchanged with the variation of V_Al concentration, while c and c/a ratio decrease with increasing V_Al concentration. Moreover V_Al induced local distortions have significant influence on the electronic and optical properties of Ta₄AlC₃, especially beyond the critical V_Al concentration (x=0.5). On the other hand, the presence of V_Al can improve the dielectric properties of Ta₄AlC₃. From the optical properties analysis, we predicted that Ta₄Al_1−xC₃ is not suitable as a coating material to avoid solar heating.     

Density functional theory study of electronic,optical and transport properties of magnesium based MgY2Z4 (Z = S and Se) spinels

Magnesium based spinel chalcogenides MgY₂Z₄ (Z = S and Se) have recently become a focus of renewed research interest owing to their high magnesium mobility; making them potential candidates for Mg batteries. In addition, the existence of a >1 eV band gaps in MgY₂Z₄ compounds also make them interesting for opto-electronic device operating in the visible to UV range of electromagnetic spectrum. Our calculations indicate that the electronic properties computed using the mBJ-LDA + SOC are in good agreement with earlier DFT calculations. The optical properties of the two compounds are examined as a function of incident photon energy, which indicate that these materials can be utilized in optical devices operating in visible and UV region of electromagnetic spectrum. The transport properties are also computed using BoltzTrap code where the variation of electrical conductivity, thermal conductivity and Seebeck coefficient with chemical potential and temperature is examined.     

Fe-N共掺杂锐钛矿相TiO2电子性质与光学性质的第一性原理研究

近年来,Fe和N掺杂锐钛矿相TiO2半导体在实验中发现许多优异性能,本文采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO2、Fe和N单掺杂及Fe和N共掺杂TiO2的能带结构、电荷布居、态密度和光学性质.分析发现:Fe掺杂引起杂质能带位于禁带中央,杂质能带最高点与导带相距大约0.6 eV而最低点与价带相距大约0.2 eV;N掺杂引起的杂质能带位于价带顶部附近. Fe和N共掺杂后杂质能带由两部分组成,位于价带顶上方0.62 eV和导带底下方0.22 eV处,其中一层杂质能带主要由N原子的2p轨道和Fe原子的3d轨道杂化形成,而另一条杂质能带主要由Fe原子的3d轨道形成,由于杂质能级的出现,使锐钛矿TiO2的禁带宽度变小.对光学性质分析发现:Fe和N共掺杂会使锐钛矿TiO2光学吸收带边红移,可见光区的光吸收系数明显增大,在低能区出现了新的吸收峰,对应能量为1.82 eV,与实验结果相符.     

第一性原理研究Eu/N共掺杂锐钛矿TiO2光催化剂的电子和光学性质

基于密度泛函理论的第一性原理平面波赝势方法, 运用Vasp方法计算了Eu, N掺杂及Eu/N共掺杂锐钛矿TiO₂的结构, 并分析了其电子及光学性质. 通过计算发现有一些Eu的4f态电子在Eu掺杂锐钛矿TiO₂的体系的费米能级附近出现杂质能级, 并且N掺杂会使得锐钛矿TiO₂的禁带宽度减小. 对于共掺杂体系而言, Eu/N共掺杂的协同效应能导致锐钛矿TiO₂的晶格畸变及禁带宽度减小. 与此同时, 计算得到的光吸收谱表明Eu/N混合掺杂锐钛矿TiO₂展现出了明显的光谱吸收边缘红移. 这些计算结果表明Eu/N共掺杂锐钛矿TiO₂具有优良的光催化活性. 关键词： 2')" href="#">TiO₂ 共掺杂 可见光催化剂 密度泛函理论     

Control of optical absorption edge of TiO2 through co-doped acceptors: The chemical trend

Tuning the optical adsorption edge of TiO₂ is attracting increasing attention as a potential solution to the worldwide energy shortage. A possible strategy to achieve high efficiency photocatalysis with TiO₂ is through dopants to modulate chemical composition. Based on first-principles calculations, we propose a hole-strain-mediated coupling mechanism between co-doped acceptors in anatase TiO₂. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor–acceptor codoping. The calculated absorption spectra show that acceptor–acceptor codopings could shift the absorption edge to the visible light region.