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To overcome environmental problems and the energy crisis, water splitting through solar energy for the production of hydrogen fuel is a promising technique. Efficient and low-cost photocatalyst are highly desired. In the recent era, van der Waal (vdW) heterostructures have gained considerable attention due to their promising properties. Hybrid density functional theory is used to calculate the electronic and photocatalytic properties of BX-MSSe (X = P, As and M = Mo, W) vdW heterostructures. Most stable stacking is figured out among different stacking from their relaxation energy. The stability of the stable stacking is confirmed through phonon calculations. Calculated band structure shows that BAs-MoSSe and BP-WSSe have direct band gap nature while the BP-MoSSe and BAs-WSSe have indirect band gap nature. BAs-MoSSe has Type-I band alignment while BP-MoSSe, BAs-WSSe and BP-WSSe have type-II band alignment. The creation of heterostructures broadened the optical absorption from the infrared to the ultraviolet region of the solar spectrum. To tune the properties of these heterostructures compressive and tensile strain is applied. Switching in band alignment is predicted in these heterostructures with the applications of strain. Based on valence and conduction band edges potentially the photocatalytic behavior for pH = 0 is investigated, which confirms that in unstrained conditions BP-MSSe (M = Mo, W) heterostructures are efficient photocatalyst. [ABSTRACT FROM AUTHOR]

In this study, all-inorganic two-dimensional Dion–Jacobson halide perovskites (DJ-HPs) CsMX 2 Y 2 (M = Sb, Bi; and X/Y= I/I, I/Br, Br/Br, I/Cl, Br/Cl, Cl/Cl) have been studied for photovoltaic, photoemission and other potential applications in optoelectronic devices using first principle calculations within the framework of density functional theory (DFT). The ground state crystal symmetry is tetragonal with a P 4/ mmm (No. 123) space group. In these compounds, Cs+ is a spacer cation, which separates the octahedral layers and balances the charge. The corner sharing M-centered (Sb, Bi) [MX 2 Y 4 ]3– unit constructs the 2D [MX 2 Y 2 ] n n– plane, where the X and Y (halogens) atoms occupy the out of plane apical and in-plane bridging sites, respectively. The DJ-HPs with P 4/ mmm crystal symmetry exhibit pseudo-direct band gap semiconducting nature and the spin orbit (SO) effect reduces the band gaps as well as splits the Sb/Bi-p orbital in the bottom of conduction band (CB) and X/Y-p orbital at the top of valence band (VB). Among these DJ-HPs, the band gap energies of CsSbI 2 Cl 2 (0.87 eV), CsSbBr 2 Cl 2 (1.00 eV), CsSbCl 4 (1.40 eV), CsBiI 4 (1.10 eV), CsBiI 2 Br 2 (1.40 eV) and CsBiBr 4 (1.60 eV) lie in a band gap energy range (0.9–1.6 eV) suitable for photovoltaic applications. The M-s (Sb-5s and Bi-6s) and X/Y-p (Cl-3p, Br-4p and I-5p) orbitals are responsible for the electron transition from the valence to conduction band, while the spacer cation (Cs+) has a limited effect on the charge carrier mobility. The effective masses of the charge carriers and exciton binding energies of these materials are suitable for solar cell and optoelectronic applications. From the optical coefficients it is clear that these DJ-HPs have excellent response to the incident photons in the visible and ultraviolet light region. • New Dion–Jacobson two dimensional halide are investigated for optoelectronic applications. • These compounds belongs to tetragonal symmetry having space group P 4/ mmm. • Band gap of these semiconductors are pseudo direct and spin orbit (SO) effect increases from Sb to Bi and Cl to I. • The effective masses of the carriers are increasing linearly with the increasing band gaps. [ABSTRACT FROM AUTHOR]

Copper–zinc–tin-sulfide (Cu 2 ZnSnS 4 , abbreviated as CZTS) is a direct band gap p-type semiconductor material with high absorption coefficient. Using oleylamine as solvent/stabilizing agent and metal chlorides and sulfur particles as chemical precursors, CZTS based nanoparticles were produced and subsequently deposited as thin films on glass substrates via spin coating of the nanoinks. The effect of temperature on crystallite size and phase composition was assessed after the solution mixture was undercooled by 30, 70 or 90 °C. Upon cooling the solution from 230 to 140 °C i.e. by 90 °C, maximum refinement in the nanoparticles size was noticed with average size on the order of few nanometers. The morphological and compositional studies of the nanoparticles were performed by means of scanning electron microscope, X-ray diffraction and Fourier transform infrared spectroscopy techniques. Phase-pure CZTS formation was confirmed from fast Fourier transform (FFT) patterns and lattice fringes observed during HR-TEM examination. Characterization of the annealed spin coated films, made from nanoink containing ultrafine nanoparticles, indicated morphological changes in the film surface during air annealing at 350°C that can be attributed to depression of CZTS phase decomposition temperature. Spectrophotometric studies of the annealed films suggested quantum confinement effect through an associated increase in the band gap value from 1.34 to 2.04 eV upon reduction in the nanoparticle size caused by increasing the degree of undercooling to 90 °C. [ABSTRACT FROM AUTHOR]

Wide band gap semiconductor alloys, Mg x Z 1− x Te (Z=Zn, Cd and Hg), are investigated over a full range of Mg compositions (0≤ x ≤1) using density functional theory (DFT). The variation in the lattice constant of Mg x Z 1− x Te is linear with the composition x, and all these alloys obey Vegrd's law. The CdTe (6.50 Å) and MgTe (6.44 Å) are lattice matched compounds, therefore the lattice constant of MgCdTe decreases slightly with the concentration x , whereas the lattice constant also decreases for MgHgTe but increases for MgZnTe. It is due to the fact that Mg has larger size than Zn and smaller size than Cd and Hg. The band gap of these compounds are calculated using the modified Becke–Johnson (mBJ) exchange potential as LDA and GGA are not effective in producing the experimental band gap of a strongly correlated electron system. The calculated band gaps of these compounds cover the range 0–3.5 eV and are consistent with the experimental band gaps. The band gaps exhibit nonlinear behavior or bowing effect with the change in concentration. The frequency dependent optical properties like dielectric functions, and indices of refraction of these ternary systems are also calculated and discussed. [ABSTRACT FROM AUTHOR]

Structural, electronic, magnetic and mechanical properties of the perovskites BiFeO3 (BFO) and BaTiO3 (BTO) are investigated using density functional theory (DFT). Structural and mechanical parameters are calculated using generalized gradient approximation (GGA) and the results consistent with the available literature. The stable magnetic phases are achieved by optimizing total energies versus volumes of the cells in different magnetic configurations such as nonmagnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM). BTO is found to be NM while BFO favors G-type AFM (G-AFM) phase. The electronic properties are investigated using GGA, GGA with Hubbard potential (GGA + U) and modified Becke–Johnson (GGA-mBJ) exchange–correlation functionals. BFO is found to be a direct bandgap semiconductor having gap energy value 3.0 eV whereas BTO is an indirect semiconductor with bandgap energy 2.9 eV. Spin–orbit coupling effect is dominant in BFO due to the larger size of A-site cation. The electrical polarization shows that both the compounds are ferroelectric materials with significant spontaneous polarization of 144.1  μ C/cm2 and 27.9  μ C/cm2 for BFO and BTO respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fluorine derivatives of piceatannol are studied for photobiological applications. • DFT approach and PBE-GGA exchange correlation potentials are used. • Narrow band gap semiconductors were identified for fluorine derivatives. • Anisotropic behaviors are observed in the optical spectra in the low energies. In the present work, we have calculated the electrical and optical properties of pure piceatannol (PI) and its six fluorinated derivatives (F1-F6). The first-principles calculations are performed in the framework of density functional theory employing the full-potential linearized augmented plane wave (FP-LAPW) method using PBE-GGA. It is found that the PI and its F-substituted derivatives are semiconductors with a HOMO-LUMO bandgap of 0.38 to 2.29 eV. Calculated partial density of states (DOS) of carbon and oxygen atoms show that the nature of C-O bonds is polar at PI. The effect of F substitution on the bandgap of PI are analyzed by comparing the DOS spectra. The size of the band gap can be controlled by changing the F sites in the PI. It is found that the band gap value decreases by substation of F at the F1, F2, F3, and F4 sites along with the F-O bonds at the LUMO state. However, in F5 and F6 sites, the change of the band gap is not significant. The dielectric behavior of the PI is observed at the y-direction. The calculated dielectric function spectra show the optical anisotropy at the x and the y-directions. The optical spectra revealed that F substitution in PI led to reduction of the optical band gap values and a red-shift occurs in the absorption spectra. The calculated absorption spectra of PI are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Two-dimensional materials have demonstrated outstanding electronic, optical, and photocatalytic characteristics. In this article, the physical properties of pentagonal-like structure penta-PtPX (X = S, Se, Te) monolayers are systematically investigated by employing density functional theory. Phonon spectra and ab initio molecular dynamics simulations confirmed that these systems are thermodynamically stable. The hybrid HSE06 functional demonstrated that the PtPX monolayers are indirect band gap semiconductors, having band gap of 2.85 eV, 2.69 eV, and 2.14 eV for PtPS, PtPSe, and PtPTe respectively. The optical properties calculated using GW 0 +BSE approach reveal strong anisotropic absorption along x- and y-directions and the optical transitions are dominated by excitons. High absorbance in the visible and ultraviolet light region is observed, and the excitonic peaks are red shifted for heavy chalcogens. Furthermore, the photocatalytic study show that the PtPX monolayers can oxidize H 2 O/O 2 to O 2 and reduce H+ to H 2 , similar to the PdPX (X = S, Se, Te) monolayers. Our results suggest that the PdPX and PtPX (X = S, Se, Te) monolayers have potential for optoelectronic and photovoltaic devices, and are promising photocatalysts for water splitting reaction. [Display omitted] • Square-planar pentagonal structure of penta-PdPS, -PdPSe, -PdPTe, -PtPS, -PtPSe, -PtPTe monolayers. • Electronic and optical properties penta-PtPX and -PdPX (X = S, Se, Te) monolayers. • High visible light absorbance by penta-PtPX and -PdPX (X = S, Se, Te monolayers. • Water splitting ability of penta-PtPX and -PdPX (X = S, Se, Te monolayers. [ABSTRACT FROM AUTHOR]

In this paper, we explore the structural, electronic and optical properties of ZnX and CdX (X = S, Se and Te) compounds in the two-dimensional (2D) graphene-like structure using the full potential augmented plane waves plus local orbitals ( FP-APW + l o ) method. Unlike their bulk phase, they are optically inactive because of their indirect band gap nature except CdS and ZnS. These two compounds maintain their direct band gap nature and hence are optically active. The static dielectric constants for these monolayers illustrate increasing trend with decrease in the band gap values. Furthermore, an acceptable description of electron transitions in these monolayers is accomplished according to the imaginary parts of the dielectric functions and absorption spectra in ZnS and CdS as examples of each group of CdX and ZnX. The results presented in this article revealed that ZnS and CdS in the 2D structure can be effectively used in optoelectronic devices such as solar cell materials and so forth. [ABSTRACT FROM AUTHOR]

DFT is used to study various transition metal based ceramics LiAA′O 6 (A = Nb, Ta, and A′ = W, Mo) in tetragonal phase with space group 421 m (No. 113). The calculated structural and geometrical parameters are found in closed agreement with the experiments. Electronic clouds explain the chemical bonding and reveal that Li atom occupy central position and form ionic bond. Other bonds in these compounds are significantly covalent due to the sharing of electrons between O and A/A′. The electronic properties demonstrate that these compounds are wide bandgap semiconductors in the energy range of 2.18–2.60 eV. These bandgap energies confirm the suitability of these oxides in optoelectronic devices operating in the visible range of the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

We communicate theoretical results of the structural, electronic, and optical properties of ZnOxTe1-x (0≤x≤1) in the zincblende structure. The calculations are performed using full potential linearized augmented plane waves (FP-LAPW) method, based on density functional theory (DFT). The structural properties are calculated with simple GGA (PBEsol), while the electronic and optical properties are calculated using mBJ-GGA. The mBJ-GGA is used to properly treat the active d-orbital in their valence shell. The ZnOTe alloy is highly lattice mismatched and consequently the lattice constants and bulk moduli largely deviate from the linear behavior. The calculated bandgaps are in agreement with the experimentally measured values, where the nature of bandgaps is direct for the whole range of x except at x=0.25. We also calculate the bandgap bowing parameter from our accurate bandgaps and resolve the existing controversy in this parameter. [ABSTRACT FROM AUTHOR]

The electronic properties of complex binary skutterudites, MX 3 (M = Co, Rh, Ir; X = P, As, Sb) are explored, using various density functional theory (DFT) based theoretical approaches including Green's Function (GW) as well as regular and non-regular Tran Blaha modified Becke Jhonson (TB-mBJ) methods. The wide range of calculated bandgap values for each compound of this skutterudites family confirm that they are theoretically as challenging as their experimental studies. The computationally expensive GW method, which is generally assume to be efficient in the reproduction of the experimental bandgaps, is also not very successful in the calculation of bandgaps. In this article, the issue of the theoretical bandgaps of these compounds is resolved by reproducing the accurate experimental bandgaps, using the recently developed non-regular TB-mBJ approach, based on DFT. The effectiveness of this technique is due to the fact that a large volume of the binary skutterudite crystal is empty and hence quite large proportion of electrons lie outside of the atomic spheres, where unlike LDA and GGA which are poor in the treatment of these electrons, this technique properly treats these electrons and hence reproduces the clear electronic picture of these compounds. [ABSTRACT FROM AUTHOR]

Application of strain is an important technique to tune the physical properties of materials and make them promising in renewable energy resources and nanoelectronics. Production of hydrogen from water splitting is a possible solution to the energy crises. Low cost and efficient photocatalyst is highly desired to be explored. Through first principle investigation the electronic properties, band alignment and the photocatalytic response of GaN-MoSSe and GaN-WSSe heterostructures are calculated. Different stacking of the materials are modeled and from relaxation energies the most stable material is investigated. Type-II band alignments are confirmed in these heterostructures. Switching of band alignment from type-II to Type-III is predicted in GaN-MoSSe with compressional strain. The photocatalytic behavior of these systems based on valence and conduction band edge potentials confirms that GaN-WSSe is energetically favorable at pH ​= ​0 for water splitting. [Display omitted] • Theoretical investigation of GaN-MSSe (M ​= ​Mo, W) van der Waals heterostructures is performed by DFT calculations. • These van der Waals heterostructures are found to be energetically stable. • GaN-MSSe (M ​= ​Mo, W) exhibit indirect band gap nature. • Type-II band alignment is observed through PBE calculations in these materials. • GaN-WSSe is energetically favorable for water splitting. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new class of MX 2 (M = Mo, W; X = S, Se) monolayers with general formula MXY or XMY, have been successfully synthesized by chemical vapor deposition of Se(S) in MoS 2 (MoSe 2). • Optimized ZrSSe, HfSSe monolayers and ZrSSe/HfSSe vdW heterostructure are indirect band gap semiconductors. • A transition from an indirect to direct band gap is achieved for tensile strain of 6(8)% for ZrSSe(HfSSe) monolayer. • ZrSSe, HfSSe monolayers and type-II ZrSSe/HfSSe vdW heterostructure heterostruture under tensile strain are efficient photocatalysts. We investigated the effects of biaxial strain on electronic structure of ZrS 2 , ZrSe 2 , HfS 2 , HfSe 2 , ZrSSe and HfSSe monolayers. Similar to ZrS 2 , ZrSe 2 , HfS 2 , HfSe 2 monolayers, Janus ZrSSe and HfSSe monolayers are indirect bandgap semiconductors. Tensile strain of 6(8)% transform ZrSSe(HfSSe) monolayer to direct bandgap semiconductor. Based on the calculation of binding energies and interlayer distance staking-(c) is found to be the most stable configuration for ZrSSe/HfSSe vdW heterostructure. Unstrained ZrSSe/HfSSe vdW heterostructure in staking-(c) is a type-II indirect bandgap semiconductor. Valence and conduction band edges show that under tensile strain ZrSSe, HfSSe and ZrSSe/HfSSe vdW heterostructure are efficient photocatalysts. [ABSTRACT FROM AUTHOR]

To tune the physical properties of a material for promising application in nanoelectronics and renewable energy sources, stacking of two dimensional materials via weak van der Waals interaction and application of strain are important techniques. The electronic properties and the band alignment in Janus transition metal dichalcogenides heterostructures are investigated by performing first principle calculations. Different stacking are modeled and the most stable stacking are confirmed through binding energy. All these heterostructures are confirmed to have type-I band alignment. Switching from type-I to type-II or type-III band alignment is observed with application of strain in these heterostructures. Semi classical Boltzmann transport theory is used to investigate thermoelectric parameters of these heterostructures. The high Seebeck coefficient of these heterostructures confirms that these materials are suitable for thermoelectric devices. We have stacked the layers of Janus transition metal diachalcogenides in different stacking order, and find the most stable stacking. The structure and electronic properties of the most stable heterostructures are then calculated. [Display omitted] • Strain effect on structural and electronic properties of MXY (M=Zr, Hf and Pt; X/Y=S, Se) vdW heterostructures. • These van der Waals heterostructures are found to be energetically stable. • Like their monolayers these heterostructures exhibit the indirect band gap nature. • Switching from type-I to type-II or Type-III band alignment is observed with the application of strain. • The thermoelectric response of these heterostructures is also reported. [ABSTRACT FROM AUTHOR]

In the present work potassium inserted cadmium chalcogenides are investigated using density functional theory. This insertion is very important in the sense that it reduces dimensions of the compounds. The dimension reduction results in two dimension layers [Cd 3 Q 4 ]2- separated by K+ ions and blue shift in the band gaps of the binary compounds. The ternary compounds K 2 Cd 3 Q 4 (Q = S, Se and Te) exist in orthorhombic crystal structure with space group Pnma (no. 62). These investigated compounds are wide and direct band gap semiconductors. The conduction band minimum is highly dispersive and mainly composed of cadmium s-orbital whereas the maximum of valence band is less dispersive and dominating by chalcogen's p-orbital. The band gap decreases in the series Q = S, Se, and Te, which is due to the increasing extension of the chalcogen p-orbital. The band energies belong to the visible energy range, therefore optical and thermoelectric properties are also calculated. [ABSTRACT FROM AUTHOR]

Electronic band structure, spontaneous polarization, hyperfine fields and electric field gradients (EFG) of the desirable multiferroic BiFeO 3 (BFO) are explored in R3c space group, using density functional theory. Improved TB-mBJ approach resolves the long-standing inconsistency between the theoretical and experimental band gap nature and value of BFO. In this study the calculated direct band gap is 2.84 eV, which was reported indirect band structures in other theoretical studies. The ferroelectric spontaneous polarization 88 μC/cm2 by PBE-GGA and 81 μC/cm2 by TB-mBJ were obtained using the Berry phase approach. Furthermore, the hyperfine interaction and EFG of the compound are also investigated. Image 1 • Bismuth ferrite (BiFeO 3) is attractive ferroelectric material. • The long-standing controversy in its experimental and theoretical band-gap is resolved. • The hyperfine fields and electric field gradients are calculated for the first time. • The spontaneous polarization of the compound is also explored. [ABSTRACT FROM AUTHOR]

• Electronic and optical properties. • Effects of chemical bond strength and electronegativity difference on electronic properties. • Accuracy of improved TB-mBJ exchange potential. • Improved TB-mBJ approach results compared with experimental. The recently developed improved Tran and Blaha modified Becke-Johnson exchange potential (TB-mBJ) approach within the framework of DFT is used to calculate band gaps and optical properties of cubic perovskites ABO 3 (A = Ca, Sr, Ba and B = Ti, Zr, Hf). The bonding nature and electronegativity difference between B-cation and O atoms are considerably influencing the electronic band gaps of these compounds. Improved TB-mBJ treats the electronic density appropriately between them, producing accurate band gaps. The optical response shows wide range of light absorption, from visible to ultraviolet, in these compounds expecting their possible applications in various optoelectronic devices. [ABSTRACT FROM AUTHOR]