Modified Becke Johnson Potential Research Articles

Structural and optoelectronic properties of the zinc titanate perovskite and spinel by modified Becke–Johnson potential

Structural and electronic properties of the cubic perovskite ZnTiO3 and spinel Zn2TiO4 are theoretically studied by the modified Becke–Johnson (mBJ) potential within the framework of density functional theory (DFT). The calculated lattice constants are found to be consistent with the experimental results. The electronic band structures of both the materials reveal that ZnTiO3 is an indirect band gap while Zn2TiO4 is a direct band gap semiconductor. The calculated fundamental band gaps of these compounds are 2.7eV and 3.18eV, which are consistent with the experimental band gaps of 2.9eV and 3.1eV, respectively. Zn2TiO4 is a wide and direct band gap compound and hence is an attractive material for optoelectronic applications, especially in near ultraviolet (UV) optoelectronics. Keeping in view the importance of Zn2TiO4 in low frequency UV devices its optical properties like dielectric functions, refractive index, reflectivity and energy loss function are also evaluated and discussed in detail.

Electronic Band Structure and Optical Response of Spinel SnX2O4 (X = Mg, Zn) through Modified Becke—Johnson Potential

The modified Becke—Johnson exchange potential approximation is applied to predict the band structure, optical parameters and electron density of SnMg2O4 and SnZn2O4. The local density approximation, generalized gradient approximation (GGA), EV-GGA significantly underestimate the direct band gap values compared to modified Becke—Johnson approximation. The band gap dependent optical parameters such as dielectric constant, index of refraction, reflectivity, and optical conductivity are calculated and analyzed. A prominent feature of cation replacement is observed and analyzed for these studied compounds. The replacement of the cation Mg by Zn leads to a significant reduction in the value of band gap and consequently affects its dependant optical parameters. This variation is of crucial importance for device fabrication in different regions of the spectrum.

Electronic structure, charge density, and chemical bonding properties of C11H8N2O o-methoxydicyanovinylbenzene (DIVA) single crystal

A comprehensive theoretical density functional theory (DFT) study of the electronic crystal structure, bonding properties, electron charge density of C11H8N2O o-methoxydicyanovinylbenzene (DIVA) single crystals were performed. The exchange and correlation potential was described within a framework of the local density approximation (LDA) by Ceperley-Alder and gradient approximation (GGA) based on exchange–correlation energy optimization to calculate the total energy. In addition, we have used Engel–Vosko generalized gradient approximation (EV-GGA) and the modified Becke–Johnson potential (mBJ) for the electronic crystal structure, bonding properties, electron charge density calculations. There is systematically increasing in the energy gap from 2.25 eV (LDA), 2.34 eV (GGA), 2.50 eV (EV-GGA), 2.96 eV (mBJ). Our calculations show that this crystal possess direct energy gap. Furthermore, the electronic charge density space distribution contours in the (1 1 0) crystallographic plane clarifies the nature of chemical bonding.

Contrast of LiFeAs with Isostructural, Isoelectronic, and Non-superconducting MgFeGe

Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while isoelectronic MgFeGe is not, despite their extremely similar electronic structures. To investigate possible sources of this distinctively different superconducting behavior, we quantify the differences using first principles density functional theory, establishing first that the Fe total 3d occupations are identical in the two compounds. Individual 3d orbital occupations also differ very little ($\sim 0.01$). The differences in Fermi surfaces (FSs) do not seem significant; however a redistribution of bands just above the Fermi level does represent a possibly significant distinction. Because the bands and FSs of LiFeAs are less in agreement with experiment than for other iron-pnictides, we study the effects of additional exchange-correlations effects beyond GGA (the generalized gradient approximation) by applying the modified Becke-Johnson potential (mBJ) exchange potential, which gives much improved bandgaps in insulators compared to GGA and might be useful for semimetals such as the Fe-based superconductors. Overall, we conclude that the mBJ corrections do not improve the description of LiFeAs as compared to experiment.

Electronic structure, density of electronic states, and the chemical bonding properties of 2,4-dihydroxyl hydrazone crystals (C13H11N3O4)

Electronic crystal structure, bonding properties, and the electron charge densities of 2,4-dihydroxybenzaldehyde-4-nitrophenylhydrazone (2,4-DHNPH,C13H11N3O4) crystal are theoretically investigated. Calculations are performed with local density approximation, generalized gradient approximation, the Engel–Vosko generalized gradient approximation, and modified Becke–Johnson potential. We present the results of the total and partial (C, N, O, H atoms) density of states. Furthermore, the electronic charge density space distribution contours in the (1 1 0) crystallographic plane, which gives better insight picture of chemical bonding were calculated to understand the effect of hydrogen bonding on the crystal structure of 2,4-DHNP.

An Improved Study of Electronic Band Structure and Optical Parameters of X-Phosphides (X=B, Al, Ga, In) by Modified Becke—Johnson Potential

We report the electronic band structure and optical parameters of X-Phosphides (X=B, Al, Ga, In) by first-principles technique based on a new approximation known as modified Becke—Johnson (mBJ). This potential is considered more accurate in elaborating excited states properties of insulators and semiconductors as compared to LDA and GGA. The present calculated band gaps values of BP, AlP, GaP, and InP are 1.867 eV, 2.268 eV, 2.090 eV, and 1.377 eV respectively, which are in close agreement to the experimental results. The band gap values trend in this study is as: Eg(mBJ-GGA/LDA) > Eg(GGA) > Eg(LDA). Optical parametric quantities (dielectric constant, refractive index, reflectivity and optical conductivity) which based on the band structure are also presented and discussed. BP, AlP, GaP, and InP have strong absorption in between the energy range 4–9 eV, 4–7 eV, 3–7 eV, and 2–7 eV respectively. Static dielectric constant, static refractive index and coefficient of reflectivity at zero frequency, within mBJ-GGA, are also calculated. BP, AlP, GaP, and InP show significant optical conductivity in the range 5.2–10 eV, 4.3–8 eV, 3.5–7.2 eV, and 3.2–8 eV respectively. The present study endorses that the said compounds can be used in opto-electronic applications, for different energy ranges.

Elastic and optoelectronic properties of RbMF3 (M=Zn, Cd, Hg): A mBJ density functional calculation

Bonding nature as well as the structural, elastic, electronic and optical properties of cubic fluoroperovskites RbMF3 (M=Zn, Cd and Hg) compounds have been calculated using a full-potential augmented plane (FPLAPW) method within the density functional theory. The exchange-correlation potential was treated with the generalized gradient approximation of Wu and Cohen (WC-GGA) to calculate the total energy. Moreover, the modified Becke–Johnson potential (TB-mBJ) was also applied for the electronic and optical properties. It is found that lattice constant increases while bulk modulus decreases with the change of cation (M) in going from Zn to Hg in RbMF3 in accordance with the experimental results. The calculations of the electronic band structure, density of states and charge density show that these compounds have an indirect energy band gap (M–Г) with a mixed ionic and covalent bonding. The optical properties (namely: the absorption coefficient and the reflectivity) were calculated for radiation up to 45.0eV.

Performance of the modified Becke-Johnson potential for semiconductors

Very recently, in the 2011 version of the wien2k code, the long-standing shortcoming of the codes based on density functional theory, namely, its impossibility to account for the experimental band-gap value of semiconductors, was overcome. The novelty is the introduction of a new exchange and correlation potential, the modified Becke-Johnson potential (mBJLDA). In this paper, we report our detailed analysis of this recent work. We calculated using this code, the band structure of 41 semiconductors and found an important improvement in the overall agreement with experiment as Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] did before for a more reduced set of semiconductors. We find, nevertheless, within this enhanced set, that the deviation from the experimental gap value can reach even much more than 20$%$, in some cases. Furthermore, since there is no exchange and correlation energy term from which the mBJLDA potential can be deduced, a direct optimization procedure to get the lattice parameter in a consistent way is not possible as in the usual theory. We analyze the consequences of this problem. Furthermore we found that using the experimental lattice parameter as input, surprisingly high deviations of the predicted band-gap value from experiment occur. This is an odd result. A closer look at the obtained band structures reveals that, in some cases, important differences occur that might not be negligible. The overall implementation of the calculation of the band structure of semiconductors with the wien2k code using this new potential is quite empirical, although it mimics well the results obtained by other methods, such as the GW approximation, which give better results and are theoretically well founded. We conclude that, in spite of the very important improvement in the band-gap agreement with experiment using the mBJLDA potential, there are issues that point to the fact that this problem is not yet totally closed.

Dispersion of Linear, Nonlinear Optical Susceptibilities and Hyperpolarizability of C11H8N2O (o-Methoxydicyanovinylbenzene) Crystals

Linear and nonlinear optical susceptibility dispersion and hyperpolarizability of C(11)H(8)N(2)O, o-methoxydicyanovinylbenzene, crystals were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation (CA-LDA) and gradient approximation (GGA) based on exchange-correlation energy optimization to calculate the total energy. Engel-Vosko generalized gradient approximation (EV-GGA) and the modified Becke-Johnson potential (mBJ) were used for the electronic crystal structure and optical susceptibility dispersion calculations. We have established systematically increasing energy gap from 2.25 eV (LDA), 2.34 eV (GGA), 2.50 eV (EVGGA), and 2.96 eV (mBJ). The crystal possesses a direct band gap which is an important key factor to make the crystal optically active with high linear and nonlinear optical susceptibilities and hyperpolarizability. Additionally, the studied crystal has a considerable anisotropy of birefringence which is necessary for phase matching conditions during the optical second harmonicexperiments. We have found that the theoretically evaluated second harmonic generation achieves about 5.8 × 10(-8) esu in good agreement with the experimental value (4.9 × 10(-8) esu). Additionally, we have found that our calculated value of β(222) is about 2.3 × 10(-30) esu at zero energy and 6.6 × 10(-30) esu at λ = 1.064 μm. We should emphasize that our calculated value of β(222) (6.60 × 10(-30) esu at λ = 1.064 μm) shows better agreement with the experimental data (5.04 × 10(-30) esu at λ = 1.064 μm) than other calculations.

Dispersion of the linear and nonlinear optical susceptibilities of disilver germanium sulfide from DFT calculations

The dispersion of the linear and nonlinear optical susceptibilities is calculated for disilver germanium sulfide (Ag2GeS3) using the all-electron full potential linearized augmented plane wave (FP-LAPW) method. Calculations are performed with four exchange correlations namely local density approximation (LDA), general gradient approximation (GGA), Engel–Vosko generalized gradient approximation (EVGGA), and modified Becke–Johnson potential (mBJ). Our calculations give a band gap of 0.40 eV (LDA), 0.42 eV (GGA), 1.03 eV (EVGGA), and 1.30 eV (mBJ) in comparison with our measured gap (1.98 eV). The mBJ exchange correlation gives the best agreement with experiment. We find that the calculated linear optical susceptibilities of Ag2GeS3 show considerable anisotropy which is useful for second harmonic generation and optical parametric oscillation. To analyze the spectra of the calculated χ 113 (2) (ω), χ 232 (2) (ω), χ 311 (2) (ω), χ 322 (2) (ω), and χ 333 (2) (ω), we have correlated the features of these spectra with the features of ɛ2(ω) spectra as a function of ω/2 and ω. From the calculated dominant component |χ 333 (2) (ω)|, we find that the microscopic second-order hyperpolarizability, β333, the vector components along the dipole moment direction is 41.2 × 10−30 esu at static limit and 222.9 × 10−30 esu at λ = 1064 nm.

Structural, electronic properties and charge density distribution of the LiNaB4O7: Theory and experiment

The title compound was synthesized by employing high-temperature solution reaction methods at 840 °C. Single-crystal XRD analysis showed that it crystallizes in the orthorhombic noncentrosymmetric space group Fdd2, with unit cell parameters a = 13.326(3) Å, b = 14.072(3) Å, c = 10.238(2) Å, Z = 16, and V = 1919.9(7) Å3. It has two independent and interpenetrating 3D frameworks consisting of [B4O9]6− groups bridged by O atoms, with intersecting channels occupied by Na+ and Li+ cations. The IR spectrum further confirmed the presence of both BO3 and BO4 groups. UV–vis diffuse reflectance spectrum showed a band gap of about 3.88 eV. Solid-state fluorescence spectrum exhibited the maximum emission peak at around 337.8 nm. Furthermore we have performed theoretical calculations by employing the state-of-the-art all-electron full potential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. We have optimized the atomic positions taken from our XRD data by minimizing the forces. The optimized atomic positions are used to calculate the electronic band structure, the atomic site-decomposed density of states, electron charge density and the chemical bonding features. The calculated electronic band structure and densities of states suggested that this single crystal possesses a wide energy band gap of about 2.80 eV using the local density approximation, 2.91 eV by generalized gradient approximation, 3.21 eV for the Engel–Vosko generalized gradient approximation and 3.81 eV using modified Becke–Johnson potential (mBJ). This compares well with our experimentally measured energy band gap of 3.88 eV. From our calculated electron charge density distribution, we obtain an image of the electron clouds that surround the molecules in the average unit cell of the crystal. The chemical bonding features were analyzed and the substantial covalent interactions were observed between O and O, B and O, Li and O as well as Na and O atoms.

The structural, electronic and optical response of IIA–VIA compounds through the modified Becke–Johnson potential

The structural, electronic and optical properties of IIA–VIA compounds are performed, by using the full-potential linearized augmented plan wave (FP-LAPW) method within DFT, by using the (PBEsol-GGA 2008) version. We have compared the modified Becke–Johnson (mBJ) potential to LDA, GGA and EV-GGA approximations. The IIA–VIA compounds have rock salt structure (B1) and zinc-blend structure (B3). The results obtained for band structure using mBJ show a significant improvement over previous theoretical work and give closer values to the experimental results. The bandgaps less than 3.1eV are used in the visible light devices applications, while those with bandgaps bigger than 3.1eV, used in UV devices applications. Optical parameters, like the dielectric constant, refractive indices, reflectivity, optical conductivity and absorption coefficient are calculated and analyzed. Refractive index lesser than unity (vg=c/n) shows that the group velocity of the incident radiation is greater than the speed of light.

Doping and temperature dependence of thermoelectric properties in Mg2(Si,Sn)

We report the use of Boltzmann transport theory to investigate the electrical properties of thermoelectric Mg${}_{2}$Si, Mg${}_{2}$Sn, and a supercell model of the 50-50 alloy. The results are based on first-principles electronic structure calculations with the modified Becke-Johnson potential of Tran and Blaha, which yields band gaps in good accord with experiment. The calculated transport coefficients are discussed in relation to the thermoelectric performance of these materials. The results imply roughly symmetric behavior with respect to carrier type and the possibility of improvements in $ZT$, especially for $p$-type and lower temperatures.

Electronic Band Structure and Optical Parameters of Spinel SnMg2O4 by Modified Becke—Johnson Potential

The electronic band structure and optical parameters of SnMg2O4 are investigated by the first-principles technique based on a new potential approximation known as modified Becke—Johnson (mBJ). The direct band gap values by LDA, GGA and EV-GGA are underestimated significantly as compared to mBJ-GGA, which generally provides the results comparable to the experimental values. Similarly, the present band gap value (4.85 eV) using mBJ-GGA is greatly enhanced to the previous value by EV-GGA (2.823 eV). The optical parametric quantities (dielectric constant, index of refraction, reflectivity, optical conductivity and absorption coefficient) relying on the band structure are presented and examined. The first critical point (optical absorption's edge) in SnMg2O4 occurs at about 4.85 eV. A strong absorption region is observed, extending between 5.4 eV to 25.0 eV. For SnMg2O4, static dielectric constant ε1(0), static refractive index n(0), and the magnitude of the coefficient of reflectivity at zero frequency R(0) are 2.296, 1.515 and 0.0419, respectively. The optoelectronic properties indicate that this material can be successfully used in optical devices.

Optoelectronic Response of GeZn2O4 through the Modified Becke—Johnson Potential

A first-principles technique capable of describing the nearly excited states of semiconductors and insulators, namely the modified Becke—Johnson (mBJ) potential approximation, is used to investigate the electronic band structure and optical properties of spinel oxides: GeZn2O4. The predicted band gaps using the mBJ approximation are significantly more accurate than the proposed previous theoretical work using the common LDA and GGA. Band gap dependent optical parameters, like the dielectric constant, index of refraction, reflectivity and optical conductivity are calculated and analyzed. The results from the dielectric constant shows that the numerical value of the static dielectric, after dropping constantly, becomes less than zero and the material exhibits metallic behavior. The refractive index also drops below unity for photons higher than 18 eV, which indicates that the velocities of incident photons are greater than the velocity of light. However, these phenomena can be explained by the fact that a signal must be transmitted as a wave packet rather than a monochromatic wave. This comprehensive theoretical study of the optoelectronic properties predicts that these materials can effectively be used in optical devices.

Calculating energy loss spectra of NiO: Advantages of the modified Becke-Johnson potential

The density of states and the energy loss near-edge structure of the oxygen $K$ edge in NiO are calculated using different models for the exchange-correlation functional. The results are compared to each other and to experimentally acquired energy loss spectra. It is found that only when using the modified Becke-Johnson potential are the calculated spectra in good agreement with the experimental data. This can be achieved at much less computational costs than with more sophisticated calculation methods but with as good results.

Electronic structure of transparent oxides with the Tran–Blaha modified Becke–Johnson potential

We present electronic band structures of transparent oxides calculated using the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In2O3, ZnO, CdO and SnO2 along with the p-type conducting ternary oxides delafossite CuXO2 (X=Al, Ga, In) and spinel ZnX2O4 (X=Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated.