Search

Materials containing mixed anions, particularly, oxyhalides containing asymmetric functional building units, may lead to the discovery of excellent nonlinear optical (NLO) materials. In the present work, the geometric structure, mechanical properties, electronic structure and optical properties of two d 0– d 10 electron transition-metal oxyhalides Cs2CdV2O6Cl2 and Cs3CdV4O12Br have been systematically determined based on density functional theory. The asymmetric functional building units [V2O7], [V4O13], [CdO2Cl4] and [CdO4Br2] exhibit varying degrees of second-order Jahn−Teller distortions, contributing differently to the macroscopic nonlinearity. Mechanical properties reveal that the two oxyhalides are structurally and mechanically stable. Detailed electronic and optical properties of the two oxyhalides are provided. Optical anisotropy character is exhibited along different polarization vectors, giving a large birefringence for satisfying the phase-matching condition. Maximum absolute values of static second harmonic generation (SHG) coefficients are 4.47 pm V−1 for Cs2CdV2O6Cl2 and 3.72 pm V−1 for Cs3CdV4O12Br, suggesting that Cs2CdV2O6Cl2 and Cs3CdV4O12Br are potential NLO crystals with large SHG coefficients. In particular, unique 3D framework structures give a polar structure superposition of individual moments for asymmetric functional building units. Thus, maximum magnitudes of the total microscopic dipole are achieved, having the largest influence on the SHG response. This study elucidates the relationship between the structure and properties of transition-metal oxyhalides, providing valuable insights for designing NLO materials with excellent performance. [ABSTRACT FROM AUTHOR]

The computational study of the van der Waals hetero (vdW) bilayer GeI2/InTe has been carried out in present study. The isolated monolayer GeI2 and InTe have been studied first and the results were compared to the previous studies. The possible stackings are considered after the vdW interaction correction is applied in the structure relaxation. The vdW hetero bilayer stability has been checked from the phonon dispersion and ab initio Molecular Dynamics calculations. The charge transfer from InTe to GeI2 monolayer. Type-II indirect band gap (1.98, 2.01 eV) is verified by the projected band structure and band alignment calculations. The vdW hetero bilayer is a superior photocatalyst for the pH value up to pH = 0 to 11. The optical properties are calculated from the complex dielectric constant. The absorption coefficient shows the enhance absorption of light in the visible and ultraviolet regions. The vdW hetero bilayer has shown low reflectivity (37%) and a high refractive index (2.80) in the visible region. The enhanced optical properties have shown its possible applications in optoelectronic devices. [ABSTRACT FROM AUTHOR]

With the reported synthesis of a fully planar 2D silicon carbide (SiC) allotrope, the possibilities of its technological applications are enormous. Recently, several authors have computationally studied the structures and electronic properties of a variety of novel infinite periodic SiC monolayers, in addition to the honeycomb one. In this work, we perform a systematic first-principles investigation of the geometry, electronic structure, vibrational, and optical absorption spectra of several finite, but, fully planar structures of SiC, i.e. 0D quantum dots (QDs). The sizes of the studied structures are in the 1.20–2.28 nm range, with their computed HOMO(H)-LUMO(L) gaps ranging from 0.66 eV to 4.09 eV, i.e. from the IR to the UV region of the spectrum. The H-L gaps in the SiC QDs are larger as compared to the band gaps of the corresponding monolayers, confirming the quantum confinement effects. In spite of covalent bonding in the QDs, Mulliken charge analysis reveals that Si atoms exhibit positive charges, whereas the C atoms acquire negative charges, due to the different electron affinities of the two atoms. Furthermore, a strong structure property relationship is observed with fingerprints both in the vibrational and optical spectra. The wide range of H-L gaps in different SiC QDs makes them well-suited for applications in fields such as photocatalysis, light-emitting diodes, and solar cells. [ABSTRACT FROM AUTHOR]

Cyclo[18]carbon has received considerable attention thanks to its novel geometric configuration and special electronic structure. Superalkalis have low ionization energy. Doping a superalkali in cyclo[18]carbon is an effective method to improve the optical properties of the system because considerable electron transfer occurs. In this paper, the geometry, bonding properties, electronic structure, absorption spectrum, and nonlinear optical (NLO) properties of superalkaline M 3O (M = Li, Na)-doped cyclo[18]carbon were studied by using density functional theory. M 3O and the C18 rings are not coplanar. The C18 ring still exhibits alternating long and short bonds. The charge transfer between M 3O and C18 forms stable [ M 3O]+[C18]− ionic complexes. C18 M 3O (M = Li, Na) shows striking optical nonlinearity, i.e., their first- and second-order hyperpolarizability (β vec and γ ||) increase considerably at λ = 1907 nm and 1460 nm. [ABSTRACT FROM AUTHOR]

Here we present the optoelectronic properties of pure inorganic lead-free halide perovskites in the form of Cs _2 AgBi X _6 ( X = Br, Cl, F, I) using the density functional theory calculations on cubic phase (Fm $\mathop{3}\limits^{̅}$ m) and tetragonal phase (I4/m). First, all the structures of the two phases were optimized at the PBE level. Structural, electronic, optical properties, phonon, and thermal properties of Cs _2 AgBi X _6 in cubic (Fm $\mathop{3}\limits^{̅}$ m) and tetragonal phases (I4/m) were obtained using the VASP code. Tetragonal phases of all compounds of the form Cs _2 AgBi X _6 , except Cs _2 AgBiBr _6, are reported here for the very first time. Among all the Cs _2 AgBi X _6 ( X = F, Cl, Br, I) structures, the cubic phase of Cs _2 AgBiBr _6 was seen to have the highest absorption coefficient along with prominent electronic features that are favorable for optoelectronic applications. Thus, the cubic phase of Cs _2 AgBiBr _6 was selected as the host lattice and bromine atoms were partly replaced with chlorine and iodine atoms. Electronic and optical properties of these mixed halide compounds of Cs _2 AgBiBr _6−x F _x , Cs _2 AgBiBr _6−x Cl _x, and Cs _2 AgBiBr _6−x I _x where x = 1, 2, 3, 4, 5 are investigated with hybrid functional HSE06 level. The electronic structure revealed that these mixed compounds exhibited indirect band gap nature regardless of the halide substitution (different x concentration) and the band gap of Cs _2 AgBiBr _6 could be varied with the substitutions of fluorine, chlorine, and iodine atoms. Our in-depth analysis shows that Cs _2 AgBiBr _6 and their mixed halides have the potential to become active double perovskite materials for photovoltaic applications and as photocatalysts for water splitting.

Using density functional theory, we investigated the geometrical properties, electronic structures, carrier mobilities, piezoelectric coefficients, and optical absorption behaviors of three O-functionalized β -phase AsP structures (b-AsPO-FO, b-AsPO-As-SO and b-AsPO-P-SO). It is shown that three O-functionalized monolayers are all indirect semiconductors with bandgaps of 0.21, 0.67, and 0.80 eV, respectively. Our calculations demonstrated that the pristine AsP monolayer and these O-functionalized AsP monolayers have strongly anisotropic carrier mobilities, allowing their potential applications for in-plane anisotropic electronic device. The bandgaps of three functionalized nanomaterials exhibit non-monotonic variations under the biaxial strains changing from âˆ'0.10 to +0.10, all experiencing metal-indirect bandgap-direct bandgap transition. The calculated in-plane Young’s modulus results suggest that they are fairly flexible to allow the application of large elastic strains on the chemically functionalized AsP monolayers. Furthermore, the b-AsPO-FO monolayer exhibits excellent anisotropic light-harvesting behavior (absorption peak: 2.36 and 2.76 eV along x and 2.37 eV along y direction) in visible light region. The b-AsPO-As-SO and b-AsPO-P-SO monolayers have strong absorption peak at 2.60 eV and 2.87 eV, respectively. The tunable electronic structures, anisotropic carrier mobility, and excellent optical absorption properties may facilitate practical applications of O-functionalized b-AsP monolayers in nanoelectronics and photovoltaics. [ABSTRACT FROM AUTHOR]

In the latest experimental success, NbOI2 two-dimensional (2D) crystals with anisotropic electronic and optical properties have been fabricated (Adv. Mater. 33 (2021), 2101505). In this work inspired by the aforementioned accomplishment, we conduct first-principles calculations to explore the mechanical, electronic, and optical properties of NbOX2 (X = Cl, Br, I) nanosheets. We show that individual layers in these systems are weakly bonded, with exfoliation energies of 0.22, 0.23, and 0.24 J mâˆ'2, for the isolation of the NbOCl2, NbOBr2, and NbOI2 monolayers, respectively, distinctly lower than those of the graphene. The optoelectronic properties of the single-layer, bilayer, and bulk NbOCl2, NbOBr2, and NbOI2 crystals are investigated via density functional theory calculations with the HSE06 approach. Our results indicate that the layered bulk NbOCl2, NbOBr2, and NbOI2 crystals are indirect gap semiconductors, with band gaps of 1.79, 1.69, and 1.60 eV, respectively. We found a slight increase in the electronic gap for the monolayer and bilayer systems due to electron confinement at the nanoscale. Our results show that the monolayer and bilayer of these novel 2D compounds show suitable valence and conduction band edge positions for visible-light-driven water splitting reactions. The first absorption peaks of these novel monolayers along the in-plane polarization are located in the visible range of light which can be a promising feature to design advanced nanoelectronics. We found that the studied 2D systems exhibit highly anisotropic mechanical and optical properties. The presented first-principles results provide a comprehensive vision about direction-dependent mechanical and optical properties of NbOX2 (X = Cl, Br, I) nanosheets. [ABSTRACT FROM AUTHOR]

Herein, we carry out a density functional theory based investigation to understand the structural and electrical properties, such as band structure, partial density of states, electron density difference and optical properties of the ZnTe/CdSe/GaSb heterotrilayer. The results show that the stacking order and interlayer distance will affect the stability of the structure. The stacking order of monolayer ZnTe, CdSe and GaSb is the most stable (C structure), with the direct bandgap is only 0.210 eV. The heterostructure has a strong binding force which may be due to the local atomic orbital hybridization. In addition, the ZnTe/CdSe/GaSb heterotrilayer has excellent optical properties in ultraviolet, visible and infrared regions, and has stronger absorption ability for visible light compared to each monolayer. Finally, compared with the ZnTe/CdSe, ZnTe /GaSb and CdSe/GaSb, heterobilayers show that the heterotrilayer has smaller direct bandgap, and has better photoelectric properties. The ZnTe/CdSe/GaSb heterotrilayer has a stable structure and excellent photoelectric performance, which has good potential and wide application prospects in nano electronic devices, photoelectric devices, visible light absorption, photocatalysis, infrared light conversion and detection. [ABSTRACT FROM AUTHOR]

Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 1012 cm−2 which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in the x-direction is accompanied with that in the visible region along the y-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications. [ABSTRACT FROM AUTHOR]

Substitutional doping has traditionally been used to modulate the existing properties of semiconductors and introduce new exciting properties, especially in two-dimensional materials. In this work, we have investigated the impact of substitutional doping (using group III, IV, V, and VI dopants) on the structural, electronic, spin, and optical properties of GeSe monolayer by using first-principles calculations based on density functional theory. Our calculated binding energies, formation energies and phonon dispersion curves of the doped systems support their stability and hence the feasibility of physical realization. Our results further suggest that switching between metallic and semiconducting states of GeSe monolayer can be controlled by dopant atoms with a different number of valence electrons. The band gap of the semiconducting structures can be tuned within a range of 0.2864 eV to 1.17 eV by substituting with different dopants. In addition, most of the doped structures maintain the low effective mass, 0.20 to 0.59 for electron and 0.21 to 0.52 for hole, which ensures the enhanced transport properties of GeSe based electronic devices. Moreover, when Ge is substituted with group V dopants, a magnetic moment is introduced in an otherwise non-magnetic GeSe monolayer. The optical absorption coefficient of the doped structures can be significantly improved (>2×) in the visible and infrared regions. These intriguing results would encourage the applications of doped GeSe monolayer in next-generation electronic, optoelectronic and spintronic devices. [ABSTRACT FROM AUTHOR]

Recent advances in the synthesis and characterization techniques of atomic-scale thicknesses of crystals have enabled the fabrication of various two-dimensional lateral heterostructures (LHSs). In this paper, we systematically investigate the geometrical structures, electronic properties, band alignments, and optical properties of phosphorene/arsenene LHSs using first-principles calculations based on density functional theory. First, we constructed four stable phosphorene/arsenene LHSs with different edge contacts, named A–Z, Z–A, A–A and Z–Z phosphorene/arsenene LHSs. We then investigated the electronic structures of the different edge contacts. Interestingly, the calculated electron localization functions show that the most stable six-membered rings and previously reported five-membered and seven-membered rings are formed at the interface of the blue phosphorene/arsenene LHSs. Moreover, the band edge alignments and optical absorption properties show that A–A and Z–A phosphorene/arsenene LHSs have band-edge conditions suitable for photocatalytic water decomposition, which provides a possibility for their application in the area of photocatalytic water decomposition. These results provide a route for applications of these heterostructures in future optoelectronic and semiconductor devices, and also pave the way for the design of new blue phosphorene/arsenene LHSs and the exploration of their potential applications in optoelectronic devices. [ABSTRACT FROM AUTHOR]

The problem of atoms and molecules caged inside fullerenes has attracted renewed interests since a new endohedral species has been experimentally realized (Bloodworth et al 2019 Angew. Chem., Int. Ed. 58 5038). In this sense, detailed theoretical studies on the spectroscopic properties of atoms and ions spatially confined in fullerene-like structures are convenient. Here we perform density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to investigate the electronic, vibrational and optical properties of two-electron atomic systems, X, caged in C20 and C20H20 endohedral complexes; i.e. X@C20 and X@C20H20 (X = He, Li+, and Be++). Among these endohedral complexes, only the encapsulated Be++ ion gives rise to strongly bound complexes, whereas the encapsulated Li+ ion depends on the confining environment, and the encapsulated He atom seems to be highly repulsive in both types of cages. Our calculated excitation energies indicate that the lowest-lying singlet states strongly depend on both the nature of the endohedral atom/ion and the type of the carbon cage. Although He@C20H20 and He@C20 are obtained as repulsive complexes, they produce a small effect in the absorption spectra of the complexes. However, the presence of Li+ or Be++ in the endohedral complexes dramatically changes the electronic absorption profile of these cages. Overall, this study shows that the confinement of a Be++ ion in a very restricted space is energetically favorable, being its quantum states controllable by the confining environment. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are continuously growing in terms of both crystalline and composition varieties. They have received significant attention in science and technology. The new members of MXenes with in-plane ordered double transition metals have been named as iMXenes. In this study, we have investigated the electronic structures and optical properties of 2D (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 iMXene monolayers, using a set of density functional theory calculations. We found that the (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 2CO2 are semiconductors with indirect bandgaps of 0.477 eV and 0.655 eV, respectively. To investigate the optical properties, we calculated the absorption spectrum and reflectivity percentage of these structures along x and z directions using the real and imaginary parts of the dielectric function. It is observed that the real and imaginary parts of their dielectric functions possess many peaks in the energy region of less than 3.1 eV. Interestingly, they show high absorption in the visible and UV regions, implying the potential applications of these semiconducting iMXenes in solar cells and optical nanodevices. [ABSTRACT FROM AUTHOR]

We perform the first-principles investigations of the structural, elastic, electronic, and optical properties of SrBO3 (B = Cr, Fe) perovskites under pressure based on density functional theory (DFT). This is the first detailed pressure-dependent study of the physical properties for these compounds. The calculated structural parameters are consistent with the existing experimental results and slightly decrease with the application of pressure. The mechanical properties are discussed in detail and reveal that the SrCrO3 is harder than SrFeO3. Without pressure, these compounds behave like half-metals, confirmed by their band structure and density of states. Although the SrCrO3 retains its half-metallic nature under pressure, SrFeO3 becomes metallic for both up-spin and down-spin configuration. Both charge density and bond overlap population reveal the covalent nature of Cr–O bond and Fe–O bond in the studied compounds. The optical properties of SrBO3, also discussed for the first time, reveal some interesting results. [ABSTRACT FROM AUTHOR]

Due to their characteristic geometry, TiO2 nanotubes (TNTs), suitably doped by metal-substitution to enhance their photocatalytic properties, have a high potential for applications such as clean fuel production. In this context, we present a detailed investigation of the magnetic, electronic, and optical properties of transition-metal doped TNTs, based on hybrid density functional theory. In particular, we focus on the 3d, the 4d, as well as selected 5d transition-metal doped TNTs. Thereby, we are able to explain the enhanced optical activity and photocatalytic sensitivity observed in various experiments. We find, for example, that Cr- and W-doped TNTs can be employed for applications like water splitting and carbon dioxide reduction, and for spintronic devices. The best candidate for water splitting is Fe-doped TNT, in agreement with experimental observations. In addition, our findings provide valuable hints for future experimental studies of the ferromagnetic/spintronic behavior of metal-doped titania nanotubes. [ABSTRACT FROM AUTHOR]

We perform ab initio density functional theory calculations along with the frozen phonon approach to study the renormalization of the electronic structure of diamondoids induced by coupling to nuclear vibrations. We obtain an energy gap reduction between 0.1 and 0.3 eV across nine different diamondoids. The energy renormalization of the highest occupied molecular orbital state varies with the sizes and the symmetry of the diamondoids while that of the lowest unoccupied molecular orbital state is nearly constant. This behavior is explained using the localization of the electronic states. The energy gap renormalization is shown to originate mainly from bending C-H and rotation/shear H-C-H modes with frequencies between 800 and 1400 cm ^−1 . We calculate the band gap renormalization due to excitonic effects using a screened configuration interaction approach and find excitonic binding energies of around 2 eV in excellent agreement with experiment for our largest diamondoids.

The structure, electronic and optical properties of 3C–SiC nano films were calculated according to first principles based on density functional theory. It is manifested that the nano films were structurally stable when the number of layers was increased to 12. Compared with bulk counterparts, nano films displayed narrower band gaps, and their Fermi energy levels were brought into the conduction band. The ε2(ω) of dielectric function, extinction coefficient, and absorption coefficient of the nano films were greater than that of the bulk in the ultraviolet range. With the layer numbers decreased, the values increased in the range. The results provide a guideline for 3C–SiC nano films to work for UV detectors. [ABSTRACT FROM AUTHOR]

Heterostructures (HSs) have attracted significant attention because of their interlayer van der Waals interactions. The electronic structures and optical properties of stacked GaN–MoS2 HSs under strain have been explored in this work using density functional theory. The results indicate that the direct band gap (1.95 eV) of the GaN–MoS2 HS is lower than the individual band gaps of both the GaN layer (3.48 eV) and the MoS2 layer (2.03 eV) based on HSE06 hybrid functional calculations. Specifically, the GaN–MoS2 HS is a typical type-II band HS semiconductor that provides an effective approach to enhance the charge separation efficiency for improved photocatalytic degradation activity and water splitting efficiency. Under tensile or compressive strain, the direct band gap of the GaN–MoS2 HS undergoes redshifts. Additionally, the GaN–MoS2 HS maintains its direct band gap semiconductor behavior even when the tensile or compressive strain reaches 5% or -5%. Therefore, the results reported above can be used to expand the application of GaN–MoS2 HSs to photovoltaic cells and photocatalysts. [ABSTRACT FROM AUTHOR]

Few-layer GaSe is one of the latest additions to the family of two-dimensional semiconducting crystals whose properties under strain are still relatively unexplored. Here, we study rippled nanosheets that exhibit a periodic compressive and tensile strain of up to 5%. The strain profile modifies the local optoelectronic properties of the alternating compressive and tensile regions, which translates into a remarkable shift of the optical absorption band-edge of up to 1.2 eV between crests and valleys. Our experimental observations are supported by theoretical results from density functional theory calculations performed for monolayers and multilayers (up to seven layers) under tensile and compressive strain. This large band gap tunability can be explained through a combined analysis of the elastic response of Ga atoms to strain and the symmetry of the wave functions. [ABSTRACT FROM AUTHOR]

We study the modification of the electronic structure in the strong spin–orbit coupled Sr2IrO4 by epitaxial strain using density-functional methods. Structural optimization shows that strain changes the internal structural parameters such as the Ir–O–Ir bond angle, which has an important effect on the band structure. An interesting prediction is the Γ−X crossover of the valence band maximum with strain, while the conduction minimum at M remains unchanged. This in turn suggests strong strain dependence of the transport properties for the hole-doped system, but not when the system is electron doped. Taking the measured value of the Γ−X separation for the unstrained case, we predict the Γ−X crossover of the valence band maximum to occur for the tensile epitaxial strain exx ≈ 3%. A minimal tight-binding model within the Jeff = 1/2 subspace is developed to describe the main features of the band structure. The optical absorption spectra under epitaxial strain are computed using density-functional theory, which explains the observed anisotropy in the optical spectra with the polarization of the incident light. We show that the optical transitions between the Ir (d) states, which are dipole forbidden, can be explained in terms of the admixture of Ir (p) orbitals with the Ir (d) bands. [ABSTRACT FROM AUTHOR]

According to density functional theory, we investigate the effects of BF3, BF4, BCl3, AlF3, AlCl3, AlBr3, BeF3, GaF3, GaCl3, GaBr3, NO3, BS2, BSO, BO2, F2, PF5, PCl5, and ASF5 molecules on the geometric, electronic, linear, and nonlinear optical properties of an Mg12O12 nanocage. The thermodynamic stability and feasibility of the adsorption process are investigated by analyzing the free energy. It is shown that the adsorptions of almost all molecules on the Mg12O12 surface are exothermic. The calculations of the polarizability of these nanoclusters show that among the studied molecules, BeF3 has the largest influence on the polarizability value (α ≈ 315 a.u., the unit a.u. is short for atomic unit). The static first hyperpolarizability (β0) value is increased in the presence of these superhalogens. This increase is greatest for BeF3 and BF4 of which the highest value of the first hyperpolarizability (β0 ≈ 5775 a.u.) is related to a BeF3c(e@Mg12O12) nanocluster. The adsorption position is a key to estimating the value of increasing the first hyperpolarizability. [ABSTRACT FROM AUTHOR]

According to first principle simulations, we theoretically predict a type of stable single-layer graphene oxide (C2O). Using density functional theory (DFT), C2O is found to be a direct gap semiconductor. In addition, we obtain the absorption spectra of the periodic structure of C2O, which show optical anisotropy. To study the optical properties of C2O nanostructures, time-dependent density functional theory (TDDFT) is used. The C2O nanostructure has a strong absorption near 7 eV when the incident light polarizes along the armchair-edge. Besides, we find that the optical properties can be controlled by the edge configuration and the size of the C2O nanostructure. With the elongation strain increasing, the range of light absorption becomes wider and there is a red shift of absorption spectrum. [ABSTRACT FROM AUTHOR]

This report presents a first-principles investigation of the structural, electronic, and optical properties of perovskite oxynitrides BaTaO2N by means of density functional theory (DFT) calculations using the full-potential linearized augmented plane wave (FP-LAPW) method. Three possible structures (P4mm, I4/mmm, and Pmma) are considered according to the TaO4N2 octahedral configurations. The calculated structural parameters are found to be in good agreement with the previous theoretical and experimental results. Moreover, the electronic band structure dispersion, total, and partial densities of electron states are investigated to explain the origin of bandgaps and the contribution of each orbital’s species in the valence and the conduction bands. The calculated minimum bandgaps of the P4mm, I4/mmm, and Pmma structures are 1.83 eV, 1.59 eV, and 1.49 eV, respectively. Furthermore, the optical properties represented by the dielectric functions calculated for BaTaO2N show that the I4/mmm phase absorbs the light at a large window in both the visible and UV regions, whereas the other two structures (P4mm and Pmma) are more active in the UV region. Our investigations provide important information for the potential application of this material. [ABSTRACT FROM AUTHOR]

The effect of alkali metal superoxides M3O (M = Li, Na, K) on the electronic and optical properties of a Be12O12 nanocage was studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The energy gaps (Eg) of all configurations were calculated. Generally, the adsorption of alkali metal superoxides on the Be12O12 nanocage causes a decrease of Eg. Electric dipole moment μ, polarizability α, and static first hyperpolarizability β were calculated and it was shown that the adsorption of alkali metal superoxides on Be12O12 increases its polarizability. It was found that the absorption of M3O on Be12O12 nanocluster improves its nonlinear optical properties. The highest first hyperpolarizability (β ≈ 214000 a.u.) is obtained in the K3O–Be12O12 nanocluster. The TD-DFT calculations were performed to investigate the origin of the first hyperpolarizabilities and it was shown that a higher first hyperpolarizability belongs to the structure that has a lower transition energy. [ABSTRACT FROM AUTHOR]

The searches for large-gap quantum spin Hall insulators are important for both practical and fundamental interests. In this work, we present a theoretical observation of the two-dimensional fully fluorinated stanene (SnF) by means of density functional theory. Remarkably, a significant spin-orbit coupling is observed for the SnF monolayer in the valence band at the Γ point, with a considerable indirect band gap of 278 meV. The direct gap of the SnF monolayer is at the Γ point, which is slightly larger by as much as 381 meV. In addition, the elastic modulus of the SnF monolayer is about 20 J/m2, which is comparable with the in-plane stiffness of black phosphorus monolayer along the x-direction (∼28.94 J/m2). Finally, the optical properties of stanene, SnF monolayer and stanene/SnF bilayer are calculated, in which the stanene/SnF bilayer is supposed to be an attractive sunlight absorber. [ABSTRACT FROM AUTHOR]

In order to deepen the understanding of the relationship between fundamental properties (including: microstructure and composition) and photocatalytic performance, four bismuthate compounds, including: LiBiO3, NaBiO3, KBiO3, and AgBiO3, are regarded as research examples in the present work, because they have particular crystal structures and similar compositions. Using density functional theory calculations, their structural, electronic, and optical properties are investigated and compared systematically. First of all, the calculated results of crystal structures and optical properties are in agreement with available published experimental data. Based on the calculated results, it is found that the tunneled or layered micro-structural properties lead to the stronger interaction between bismuth and oxygen, and the weaker interaction between alkaline-earth metal and [BiO6] octahedron, resulting in the feature of multi-band gaps in the cases of LiBiO3, NaBiO3, and KBiO3. This conclusion is supported by the case of AgBiO3, in which the feature of multi-band gaps disappears, due to the stronger interaction between the noble metal and [BiO6] octahedron. These properties have significant advantages in the photocatalytic performance: absorbing low energy photons, rapidly transferring energy carriers. Furthermore, the features of electronic structures of bismuthate compounds are well reflected by the absorption spectra, which could be confirmed by experimental measurements in practice. Combined with the calculated results, it could be considered that the crystal structures and compositions of the photocatalyst determine the electronic structures and optical properties, and subsequently determine the corresponding photocatalytic performance. Thus, a novel Bi-based photocatalyst driven by visible-light could be designed by utilizing specific compositions to form favorable electronic structures or specific micro-structures to form a beneficial channel for energy carriers. [ABSTRACT FROM AUTHOR]

We theoretically study the structural, electronic, and optical properties of lithium niobate under pressure using the plane-wave pseudopotential density functional theory by CASTEP code. It was found that there is a phase transition from the R3c structure to the Pnma structure at a pressure of 18.7 GPa. The Pnma structure was dynamically stable according to the calculation of phonon dispersion. From the charge density distributions, there exist covalent interactions along the Nb–O bond. The hybridization between O 2p and Nb 4d orbital in the Pnma phase increases with increasing pressure, while it is not changed in the R3c phase. With increasing pressure, the average Nb–O bond length decreases and the Nb–O bond population increases, indicating the increased covalent character between Nb and O atoms under high pressure at Pnma phase, which leads to the increased hybridization between O 2p and Nb 4d orbitals. Furthermore, the optical dielectric function, refractive index, extinction coefficient, electron energy, loss and reflectivity are calculated. [ABSTRACT FROM AUTHOR]

We report a study of the electronic structure and optical properties of uranium dioxide (UO2) based on the ab-initio density-functional theory and using the generalized gradient approximation. To correctly describe the strong correlation between 5f electrons of a uranium atom, we employ the on-site Hubbard U correction term and optimize the correlation parameter of the bulk uranium dioxide. Then we give the structural and electronic properties of the ground state of uranium dioxide. Based on the accurate electronic structure, we calculate the complex dielectric function of UO2 and the related optical properties, such as reflectivity refractive index, extinction index, energy loss spectra, and absorption coefficient. [ABSTRACT FROM AUTHOR]

The geometric structure, electronic structure, and optical properties of CdHg(SCN)4 crystal are calculated by using the density functional perturbation theory and Green function screening Coulomb interaction approximation. The band gap of CdHg(SCN)4 crystal is calculated to be 3.198 eV, which is in good agreement with the experimental value 3.265 eV. The calculated second-order nonlinear optical coefficients are d14 = 1.2906 pm/V and d15 = 5.0928 pm/V, which are in agreement with the experimental results (d14 = (1.4 ± 0.6)pm/V and d15 = (6.0 ± 0.9 pm/V). Moreover, it is found that the contribution to the valence band mainly comes from Cd-4d, Hg-5d states, and the contributions to the valence band top and the conduction band bottom predominantly come from C-2p, N-2p, and S-3p states. The second-order nonlinear optical effect of CdHg(SCN)4 crystal results mainly from the internal electronic transition of (SCN)−. [ABSTRACT FROM AUTHOR]

Based on systematic first-principles density functional calculations, we predict that Li-decorated monolayer black phosphorus (MBP) is a hydrogen storage medium with great potential. It is found that pure MBP has only weak H2 adsorption energies. Surprisingly, its hydrogen storage properties can be significantly improved after lithium decoration. The calculated Li-binding energy on MBP is higher than the cohesive energy of bulk Li, ruling out the possibility of cluster formations of Li on the MBP surface. Our study shows that Li-decorated MBP can achieve a hydrogen storage capacity as high as 8.11 wt% . The calculated H2 adsorption energies fall within the range of 0.13–0.18 eV. This is a remarkable result indicating another important application of MBP. [ABSTRACT FROM AUTHOR]

Based on the generalized gradient approximation (GGA) in density functional theory (DFT) and using the first-principle plane wave ultrasoft pseudopotential method, we construct and optimize the structures of intrinsic and oxygen vacancy (VO) ZnO bulks and nanowires (NWs) in the Castep module. Moreover, the calculation of band structures and the optical properties are carried out. The calculated results exhibit that the oxygen vacancy exerts a more significant influence on the electronic structures of the ZnO bulks instead of the NWs. What is more, the influences of the VO on the optical properties are mainly embodied in the ultraviolet region, and the main optical parameters of ZnO bulks and NWs with VO are anisotropic. [ABSTRACT FROM AUTHOR]