Bakhtiar Ul Haq

Non-local exchange correlation functionals impact on the structural, electronic and optical properties of III–V arsenides

Exchange correlation (XC) energy functionals play a vital role in the efficiency of density functional theory (DFT) calculations, more soundly in the calculation of fundamental electronic energy bandgap. In the present DFT study of III-arsenides, we investigate the implications of XC-energy functional and corresponding potential on the structural, electronic and optical properties of XAs (X = B, Al, Ga, In). Firstly we report and discuss the optimized structural lattice parameters and the band gap calculations performed within different non-local XC functionals as implemented in the DFT-packages: WIEN2k, CASTEP and SIESTA. These packages are representative of the available code in ab initio studies. We employed the LDA, GGA-PBE, GGA-WC and mBJ-LDA using WIEN2k. In CASTEP, we employed the hybrid functional, sX-LDA. Furthermore LDA, GGA-PBE and meta-GGA were employed using SIESTA code. Our results point to GGA-WC as a more appropriate approximation for the calculations of structural parameters. However our electronic bandstructure calculations at the level of mBJ-LDA potential show considerable improvements over the other XC functionals, even the sX-LDA hybrid functional. We report also the optical properties within mBJ potential, which show a nice agreement with the experimental measurements in addition to other theoretical results.

Electronic structure engineering of ZnO with the modified Becke–Johnson exchange versus the classical correlation potential approaches

In this study, we report investigations of structural and electronic properties of ZnO in wurtzite (WZ), rock salt (RS) and zinc-blende (ZB) phases. Calculations have been done with full-potential linearized augmented plane wave plus local orbital method developed within the frame work of Density Functional Theory (DFT). For structural properties investigations, Perdew and Wang proposed local density approximations (LDA) and Perdew et al. proposed generalized gradient approximations (GGA) have been applied. Where for electronic properties in addition to these, Tran–Blaha modified Becke–Johnson (mBJ) potential has been used. Our computed band gap values of ZnO in WZ and ZB phases with mBJ potential are significantly improved compared to those with LDA and GGA; however, in RS phase, energy gap is significantly overestimated compared to experimental measurements. The Zn-d band was found to be more narrower with mBJ potential than that of LDA and GGA. On the other hand, our evaluated crystal field splitting energy values overestimate the experimental values.

First principle investigations of the physical properties of hydrogen-rich MGH2

Hydrogen being a cleaner energy carrier has increased the importance of hydrogen-containing light metal hydrides, in particular those with large gravimetric hydrogen density like magnesium hydride (MgH 2 ). In this study, density functional and density functional perturbation theories are combined to investigate the structural, elastic, thermodynamic, electronic and optical properties of MgH 2 . Our structural parameters calculated with those proposed by Perdew, Burke and Ernzerof generalized gradient approximation (PBE-GGA) and Wu‐Cohen GGA (WC-GGA) are in agreement with experimental measurements, however the underestimated band gap values calculated using PBE-GGA and WC-GGA were greatly improved with the GGA suggested by Engle and Vosko and the modified Becke‐Johnson exchange correlation potential by Trans and Blaha. As for the thermodynamic properties the specific heat values at low temperatures were found to obey the T 3 rule and at higher temperatures Dulong and Petit’s law. Our analysis of the optical properties of MgH 2 also points to its potential application in optoelectronics.

A COMPREHENSIVE DFT STUDY OF ZINC OXIDE IN DIFFERENT PHASES

A density functional study for structural and electronic properties of Zinc Oxide (ZnO), in wurtzite, rock salt and zinc-blende phases has been performed using full potential-linearized augmented plane wave/linearized augmented plane wave plus local ideal orbital (FP-LAPW/L(APW+lo) approach as realized in WIEN2k code. To approximate exchange correlation energy and corresponding potential, a special GGA parameterized by Wu–Cohen has been implemented. Our results of lattice constants, bulk moduli as well as for internal parameter with GGA-WC are found to be more reliable. This study reveals that value of internal parameter decreases with increasing volume whereas computed electronic band structure confirms the direct band gap behavior of ZnO in B4 and B3 phases while indirect band gap behavior in B1 phase. Moreover, two fold degeneracy at the maxima of valence band for B4 and B1 phases whereas three fold for B3 is observed. A detailed comparison with experimental and other first principles studies is also made.

Tailoring ferromagnetism in chromium-doped zinc oxide

The simultaneous manipulation of both charge and spin has made diluted magnetic semiconductors (DMS) a potential material for the fabrication of spintronic devices. We report DMSs based on ZnO doped with Cr in wurtzite (WZ) and zinc-blend (ZB) geometries. The injection of Cr impurities at a concentration of 6.25% has successfully tuned ferromagnetism in ZnO. To recognize the nature of magnetic interactions, two spatial configurations are investigated, where the impurity atoms are placed at minimum and maximum separation distances. The material favors the short-range magnetic coupling and has a tendency towards Cr clustering. The injection of a Cr impurity into ZnO strongly influences the electronic properties in terms of band structure, dependent on the impurity spatial distribution. It is half metallic for both structural geometries when impurity atoms have minimum separation and is metallic when they are placed far apart. Moreover, replacing Zn with Cr does not show a significant effect on the structural geometries. Our results endorse that Cr:ZnO can be efficiently used for spin-polarized transport and other spin-dependent applications in both hexagonal and cubic phases.

Structural and electronic properties of ni-doped ZnO in zinc-blende phase: A DFT investigations

In the present work investigations of structural and electronic properties of nickel doped ZnO in zinc-blende phase have been done in the framework of density functional theory. In doping process 25% cations (Zn atoms) have been replaced by Ni atoms. Wu-Cohen parameterized Generalized Gradient Approximation (GGA-WC) is used for exchange and correlation energy functional treatment. Our calculations for structural properties reveal a reduction in lattice constant with Ni doping. Whereas the spin polarized electronic structures show metallic behavior of ZnO in the presence of Ni impurity atoms for both up and down spin configuration. Moreover we present calculated density of states to understand the effect of Ni doping on ZnO.

Structural, Electronic and Nonlinear Optical Properties of Novel Derivatives of 9,12-Diiodo-1,2-dicarba-closo-dodecaborane: Density Functional Theory Approach

Abstract Using density functional theory (DFT) methods, we shed light on the structural, optical, electronic, and nonlinear optical (NLO) properties of three derivatives of 9,12-diiodo-1,2-dicarba-closo-dodecaborane(12) (C2H10B10I2). The DFT and time-dependent DFT methods are considered very precise and practical to optimize the ground and excited state geometries, respectively. A vibrant intramolecular charge transfer from highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbitals (LUMOs) was observed in all compounds. The geometrical parameters of the experimental crystal structure, i.e. bond lengths/angles, have been successfully reproduced. The HOMO and LUMO energies, as well as their energy gaps (Eg), were also calculated and compared with each other for all derivatives. The effect of attached groups on electronic, optical, and NLO properties along with detailed structure-property relationship was discussed. For NLO response, the CAM-B3LYP functional along with relatively larger basis set 6-31+G** (for hydrogen, carbon, boron, and oxygen atoms) and LANL2DZ (for iodine atoms) have been used to optimize the compounds at ground states. The calculation of second-order NLO polarizabilities (βtot) shows that compounds 2 and 3 possess the βtot amplitudes of 3029 and 4069 a.u., respectively, with CAM-B3LYP method that are reasonably larger than similar prototype molecules. Owing to their unique V-shapes, the nonlinear anisotropy values are found to be 0.63, 0.34, and 0.44 for compounds 1–3, respectively, which show the significant two-dimensional character of these compounds. Thus, the NLO amplitudes as well as the nonlinear anisotropies indicate that the above-entitled compounds are good contenders for optical and NLO applications.

Elucidating the First-Principles Calculations of SnO 2 Within DFT Framework and Beyond: A Library for Optimization of Various Pseudopotentials

In the present work detailed electronic, structural and optical properties of rutile-type SnO2 are presented based on plane-wave ultrasoft pseudopotential technique within Density Functional Theory (DFT) and beyond using LDA, GGA, HSE03, HSE06, LDA(HSE03), LDA(HSE06), GGA(HSE03) and GGA(HSE06) respectively. The results show that the calculated lattice constants and volumes are very close to the experimental values. The bandgap obtained from LDA(HSE06) is quite close to the experimental values. Conversely, the bandgaps calculated by HSE03 and HSE06 are also close to 3.6 eV. However, density of state and optical properties calculated from each type of potential is mostly alike in qualitative investigations, and the numerical values have a little difference. The graphs have been plotted for eight exchange correlation potentials to depict the properties of SnO2 in detail. These studies elucidate the first principles calculations of SnO2 using various pseudopotentials and provide a complete library for their optimization.

Engineering the electronic band structures of novel cubic structured germanium monochalcogenides for thermoelectric applications

Germanium mono-chalcogenides have received considerable attention for being a promising replacement for the relatively toxic and expensive chalcogenides in renewable and sustainable energy applications. In this paper, we explore the potential of the recently discovered novel cubic structured (π-phase) GeS and GeSe for thermoelectric applications in the framework of density functional theory coupled with Boltzmann transport theory. To examine the modifications in their physical properties, the across composition alloying of π-GeS and π-GeSe (such as π-GeS1-xSex for x =0, 0.25, 0.50, 0.75, and 1) has been performed that has shown important effects on the electronic band structures and effective masses of charge carriers. An increase in Se composition in π-GeS1-xSex has induced a downward shift in their conduction bands, resulting in the narrowing of their energy band gaps. The thermoelectric coefficients of π-GeS1-xSex have been accordingly influenced by the evolution of the electronic band structures and effective masses of charge carriers. π-GeS1-xSex features sufficiently larger values of Seebeck coefficients, power factors and figures of merit (ZTs), which experience further improvement with an increase in temperature, revealing their potential for high-temperature applications. The calculated results show that ZT values equivalent to unity can be achieved for π-GeS1-xSex at appropriate n-type doping levels. Our calculations for the formation enthalpies indicate that a π-GeS1-xSex alloying system is energetically stable and could be synthesized experimentally. These intriguing characteristics make π-GeS1-xSex a promising candidate for futuristic thermoelectric applications in energy harvesting devices.Germanium mono-chalcogenides have received considerable attention for being a promising replacement for the relatively toxic and expensive chalcogenides in renewable and sustainable energy applications. In this paper, we explore the potential of the recently discovered novel cubic structured (π-phase) GeS and GeSe for thermoelectric applications in the framework of density functional theory coupled with Boltzmann transport theory. To examine the modifications in their physical properties, the across composition alloying of π-GeS and π-GeSe (such as π-GeS1-xSex for x =0, 0.25, 0.50, 0.75, and 1) has been performed that has shown important effects on the electronic band structures and effective masses of charge carriers. An increase in Se composition in π-GeS1-xSex has induced a downward shift in their conduction bands, resulting in the narrowing of their energy band gaps. The thermoelectric coefficients of π-GeS1-xSex have been accordingly influenced by the evolution of the electronic band structures and e...

DFT investigations of structural and electronic properties of gallium arsenide (GaAs)

First principles calculations for structural and electronic properties of GaAs have been reported using a full potential linearized augmented plane wave (FP-LAPW) scheme of calculations developed within density functional theory (DFT). We use in this study local density approximation (LDA), Perdew-Burke-Ernzerhof parameterized generalized gradient approximation (PBE-GGA), Wu-Cohen parameterized GGA (WC-GGA) executed in WIEN2k code. In addition, to calculate band structure with high accuracy we used modified Becke-Johnson exchange potential (MBJ) + LDA approach. Our calculated lattice constant with GGA-WC is in good agreement to experimental value than LDA and PBE-GGA. Whereas our calculations for the band structure show that MBJ+ LDA approach gives much better results for band gap value as compared to other exchange correlation approaches.