Ab Initio Density Functional Theory Research Articles

Enantioseparation of novel anti-inflammatory chiral sulfoxides with two cellulose dichlorophenylcarbamate-based chiral stationary phases and polar-organic mobile phase(s)

Chiral sulfoxides represent a class of substances of great importance in many fields, including medicinal chemistry dealing with the synthesis of novel cyclooxygenase-2 (COX-2) inhibiting/NO donors. In the present study, two nitrooxyethyl sulfoxides along with their metabolites, hydroxyethyl derivatives have been successfully enantioresolved with two cellulose tris(3,5-dichlorophenylcarbamate)-based chiral stationary phases (CSPs), one with a coated (CSP 1) and the other (CSP 2) with an immobilized chiral selector. The immobilized selector in CSP 2 produced comparable-to-better performances than the coated one in CSP 1 (with α and RS values up to 1.94 and 6.32, respectively) running the analysis with a polar-organic phase made up with an ethanol/2-propanol (80:20, v/v) mobile phase. Electronic circular dichroism studies coupled to ab initio time-dependent density functional theory simulations allowed us to determine the elution order of three out of four compounds. For the two hydroxyethyl derivatives the same enantiomeric elution order [(S)<(R)] was obtained with both CSPs, while the compounds containing the −ONO2 group experienced a different elution order depending on the coated or immobilized nature of the chiral selector [(S)<(R) with CSP 1 and (R)<(S) with CSP 2]. A molecular modeling study based on docking simulations was performed to gain a deeper insight into the enantioseparation mechanism of the hydroxyethyl derivatives on both CSPs.

Structure and dynamics of dibutylphosphate/n-propylamine ionic liquid: A multi-scale theoretical study

Owing to the amphiphilic nature of their constituent molecules, binary mixtures of pure liquid surfactants are usually characterized by enhanced nano-segregation and thus can exhibit interesting transport properties and complex macroscopic behavior. In this ambit it was recently shown by Turco Liveri et al. (2018) that mixtures of short aliphatic chains compounds at room temperature, such as dibutyl phosphate (DBP) and n-propylamine (PA) liquids, display ionic liquid–like behavior ascribed to phosphate-to-amine proton transfer. To gain a detailed molecular picture of the system structure and dynamics and to understand the molecular mechanisms at the basis of the observed behavior, in the present study a combination of ab initio Density Functional Theory (DFT) calculation and ab initio Molecular Dynamics (AIMD) simulation has been carried out on DBP + PA clusters. It was found that in these systems the proton transfer is a cooperative process boosted by the concerted interactions of more than one DBP-PA pair. Moreover, classical molecular dynamics (MD) simulations of large DBP + PA systems showed that, triggered by steric effects due to the alkyl chains of both components, these interactions are able to build an extended network of polar domains embedded in an apolar environment of alkyl chains.

Electronic, mechanical, vibrational and thermodynamic properties of FeXSb (X = Hf and Nb) Half-Heusler alloys from first-principles approach

A comprehensive computational study of FeXSb (X = Hf and Nb) half-Heusler alloys have been performed by adopting ab initio density functional theory approach. Recent studies has shown that FeNbSb half-Heusler alloy is a potentially good and promising candidate for thermoelectric device when it is substitutionally doped with Hf. Therefore, the present study have investigated the electronic, elastic, mechanical, vibrational and thermodynamic properties of FeXSb (X = Hf and Nb) half-Heusler alloys for efficient and effective thermoelectric performance. The computed electronic properties affirmed the semiconductive nature of the FeNbSb alloy while FeHfSb alloy is predicted to be half metallic ferromagnetic compound with small magnetic moment of 0.9 μB. The contributions of the d-electrons of Fe Hf and Nb in the band structure is notable and significant as revealed in the projected density of states (PDOS). The mechanical stability of the two alloys was established when the computed elastic constants accurately satisfy the generalized mechanical stability conditions. Moreover, the mechanical properties revealed that our investigated materials are ductile and incompressible. Also, the alloys were predicted to be anisotropic in nature. The presence of positive frequencies obtained from the calculation of phonon dispersion throughout the Brillouin zone indicates that FeXSb alloys are dynamically stable. Moreso, LO-TO spliting is observed and reported in FeNbSb alloy. Various temperature dependent thermodynamic parameters such as debye temperature, sound velocity, internal energy, vibrational energy, entropy and constant volume heat capacity were computed within harmonic approximation. The obtained results pointed to the fact that the two alloys have impressive thermal response.

A Chemical Map of Nasicon Electrode Materials for Sodium-Ion Batteries

Sodium (Na)-ion batteries are among the most promising devices to supersede the present Li-ion batteries in heavy-duty and smart grid applications. This is mostly encouraged by the lower cost of N-metal and its worldwide availability[1], as well as the possibility of replacing expensive copper current collectors by aluminium ones. In the recent years, many electrode materials have been discovered and designed to enhance the energy density of Na-ion batteries. Among those electrode materials, the Natrium Superionic CONductor (NaSICON)[2] prototype structure are known for their wide range of electrochemical potentials, high ionic conductivities, and most importantly their structural and thermal stabilities [3–5].In this talk we will explored the chemical space of NaSICON structured materials, NaxMM’(PO4)3 where M, M’(Ti, V, Cr, Mn, Fe, Co & Ni), using ab initio density functional theory (DFT) and thermodynamic approaches [6]. We have analysed the thermodynamic stabilities, Na+ intercalation voltages and corresponding redox pairs for 28 combinations of M & M’ and the full Na concentration range (i.e., 1 ≤ x ≤ 4). This study enabled us to pinpoint anomalies in exiting experimental reports as well as to identify new NaSICON compositions that can enable the extractions of three Na ions (e.g., NaxMn2(PO4)3 and NaxVCo(PO4)3, etc). Furthermore, the calculated quaternary phase diagrams Na-Mn-P-O could show the stability of NaxMn2(PO4)3, while analogues phase diagrams, i.e., Na-Ni-P-O and Na-Co-P-O explained the origin of the suspected instability of Ni and Co-based NaSICONs. We realized that the presence of Jahn-Teller active transition metals ions enhances the voltages but limits the practicality of entire chemical space for cathodic applications.[1] D. Larcher and J. M. Tarascon, Nat. Chem., 2015, 7, 19.[2] J. B. Goodenough , H. Y. P. Hong and J. A. Kafalas, Mater. Res. Bull., 1976, 11 , 203[3] W. Zhou , L. Xue , X. Lü , H. Gao , Y. Li , S. Xin , G. Fu , Z. Cui , Y. Zhu and J. B. Goodenough , Nano Lett., 2016, 16 , 7836[4] J. Wang , Y. Wang , D. Seo , T. Shi , S. Chen , Y. Tian , H. Kim and G. Ceder , Adv. Energy Mater., 2020, 10 , 1903968[5] Z. Deng , G. Sai Gautam , S. K. Kolli , J.-N. Chotard , A. K. Cheetham , C. Masquelier and P. Canepa , Chem. Mater., 2020, 32 , 7908[6] Baltej Singh, Z. Wang, S. Park, G. S. Gautam, J. N. Chotard, L. Croguennec, D. Carlier, A. K. Cheetham, C. Masquelier and P. Canepa. J. Mater. Chem. A, 9, 281-292 (2021)

Analysis of the combustion mechanism of diesel surrogate fuel under CO2/O2 atmosphere

Closed-cycle diesel engine (CCDE) can effectively control CO2 emissions and reduce the greenhouse gas effect, but diesel fuel combustion under an atmosphere with CO2 has the problem of misfire and unclear mechanism. In order to study the CO2 impact on diesel combustion under the CO2/O2 atmosphere, a quantum chemical (QC) model of diesel surrogate fuel (DSF) is proposed, which takes into account the CO2 pyrolysis effect (CO2⇆CO+12O2) and the new reaction of OH radical generation from pyrolysis products (CO+O2+H∙→CO2+OH∙). Firstly, the active sites of the DSF molecules are predicted by average local ionization energy (ALIE) and electrostatic potential (ESP). New chemical reaction paths are obtained using ab initio theory and density functional theory combined with thermodynamic combination algorithm. The QC model is then established by sensitivity analysis and simplification of the mechanism and computational fluid dynamics (CFD) simulation of constant volume combustion chamber (CVCC.) Secondly, the CVCC experiment bench and visualization system are set up. The flame propagation at different times (1.5 ms, 1.9 ms, 2.3 ms, 2.7 ms, 3.1 ms and 3.5 ms) are captured by high-speed cameras under four different CO2 concentrations (air condition, 50%CO2 + 50%O2, 43%CO2 + 57%O2 and 35%CO2 + 65%O2). Finally, the comparison between experiment and model shows that the QC model is suitable for studying combustion characteristics of diesel fuel under CO2/O2 atmosphere. Under maximum CO2 concentration (50%CO2 + 50%O2), The maximum errors of flame combustion limit length (FCLL) and Flame longitudinal section area (FLSA) are 8.54% and 4.94%, respectively. In addition, the CO2 pyrolysis reaction and the new reaction of generating OH radicals indicate that the CO2 chemical effect can promote the combustion of diesel fuel under CO2/O2 atmosphere.

Oxidation behaviors of CrNb, CrNbTi, and CrNbTaTi concentrated refractory alloys

The 1200 °C oxidation behaviors of CrNb, CrNbTi and CrNbTaTi are reported. All alloys formed some combination of BCC and Laves phases. However, additions of Ti produced a unique inflection point where the microstructure inverts to a Ti-rich BCC matrix with Laves phase precipitates. Further additions of Ta to CrNbTi were found to increase the overall phase fraction of BCC phase. The oxidation kinetics for CrNb and CrNbTi were similar and nearly parabolic throughout the duration of exposure. CrNbTaTi exhibited inferior oxidation kinetics and mixed mode behavior throughout oxidation testing. All alloys formed intricate multi-phase oxide structures, consisting of a combination of both simple and complex oxides. The resulting microstructures are characterized using a variety of analytical techniques and the results are discussed relative to modeling predictions using ab initio density functional theory (DFT).

Pressure-induced ferromagnetism in the topological semimetal EuCd2As2

The antiferromagnet and semimetal $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$ axis in $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$. In the search for such phases, we investigate here the effects of hydrostatic pressure in $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$. For that, we present specific heat, transport, and $\ensuremath{\mu}\mathrm{SR}$ measurements under hydrostatic pressure up to $\ensuremath{\sim}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, combined with ab initio density functional theory (DFT) calculations. Experimentally, we establish that the ground state of $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ changes from in-plane antiferromagnetic $({\mathrm{AFM}}_{ab})$ to ferromagnetic at a critical pressure of $\ensuremath{\approx}2$ GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane $({\mathrm{FM}}_{ab})$. The ${\mathrm{AFM}}_{ab}\text{\ensuremath{-}}{\mathrm{FM}}_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our theoretical results indicate that $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ moves closer to the sought-for ${\mathrm{FM}}_{c}$ state (moments $\ensuremath{\parallel}c$) with increasing pressure further. We predict that a pressure of $\ensuremath{\approx}23$ GPa will stabilize the ${\mathrm{FM}}_{c}$ state if Eu remains in a $2+$ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

Sensing strain-induced symmetry breaking by reflectance anisotropy spectroscopy

Intentional breaking of the lattice symmetry in solids is a key concept to alter the properties of materials by modifying their electronic band structure. However, the correlation of strain-induced effects and breaking of the lattice symmetry is often indirect, resorting to vibrational spectroscopic techniques, such as Raman scattering. Here, we demonstrate that reflectance anisotropy spectroscopy (RAS), which directly depends on the complex dielectric function, enables the direct observation of electronic band structure modulation. Studying the strain-induced symmetry breaking in copper, we show how uniaxial strain lifts the degeneracy of states in the proximity of the both L and X symmetry points, thus altering the matrix element for interband optical transitions, directly observable in RAS. We corroborate our experimental results by analyzing the strain-induced changes in the electronic structure based on ab initio density functional theory calculations. The versatility to study breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to gain a direct insight on the band structure of other strain-engineered materials, such as graphene and two-dimensional transition metal dichalcogenides.

Solvent Effect on the Structure and Properties of RGD Peptide (1FUV) at Body Temperature (310 K) Using Ab Initio Molecular Dynamics.

The structure and properties of the arginine-glycine-aspartate (RGD) sequence of the 1FUV peptide at 0 K and body temperature (310 K) are systematically investigated in a dry and aqueous environment using more accurate ab initio molecular dynamics and density functional theory calculations. The fundamental properties, such as electronic structure, interatomic bonding, partial charge distribution, and dielectric response function at 0 and 310 K are analyzed, comparing them in dry and solvated models. These accurate microscopic parameters determined from highly reliable quantum mechanical calculations are useful to define the range and strength of complex molecular interactions occurring between the RGD peptide and the integrin receptor. The in-depth bonding picture analyzed using a novel quantum mechanical metric, the total bond order (TBO), quantifies the role played by hydrogen bonds in the internal cohesion of the simulated structures. The TBO at 310 K decreases in the dry model but increases in the solvated model. These differences are small but extremely important in the context of conditions prevalent in the human body and relevant for health issues. Our results provide a new level of understanding of the structure and properties of the 1FUV peptide and help in advancing the study of RGD containing other peptides.

Boron adsorption and its effect on stability and CO activation of χ-Fe5C2 catalyst: An ab initio DFT study

Developing advanced Fischer-Tropsch synthesis (FTS) technology to acquire fuel and valuable chemicals from non-petroleum sources more efficiently is one of the important tasks in heterogeneous catalysis and chemical engineering research. With the purpose of improving iron-based FTS catalyst, we explored the modification effect of boron on the χ-Fe5C2 catalyst by density functional theory (DFT) calculations. Boron atom adsorption on χ-Fe5C2 surfaces is considered to modulate the stability as well as the activity of CO activation on FTS process. Anti-coking can be observed when boron atom adsorbs on χ-Fe5C2 catalyst due to the competitive adsorption and repulsive interactions between carbon and boron atoms. More open and active planes, such as (311), (021), and (312̅) facets, can expose in the morphology of χ-Fe5C2 when boron atom adsorbs on χ-Fe5C2. It is found that co-adsorbed boron atoms have little effect on the barrier of CO direct dissociation in most cases, while there is also particular situation that CO is adsorbed and activated on boron with carbon termination. Our work shed light on the promoting effects of boron adsorption on CO activation, which can provide useful guide for the design of novel iron-based FTS catalysts.

Nitrogen Rejection from Methane via a "Trapdoor" K-ZSM-25 Zeolite.

Nitrogen (N2) rejection from methane (CH4) is the most challenging step in natural gas processing because of the close similarity of their physical-chemical properties. For decades, efforts to find a functioning material that can selectively discriminate N2 had little outcome. Here, we report a molecular trapdoor zeolite K-ZSM-25 that has the largest unit cell among all zeolites, with the ability to capture N2 in favor of CH4 with a selectivity as high as 34. This zeolite was found to show a temperature-regulated gas adsorption wherein gas molecules' accessibility to the internal pores of the crystal is determined by the effect of the gas-cation interaction on the thermal oscillation of the "door-keeping" cation. N2 and CH4 molecules were differentiated by different admission-trigger temperatures. A mild working temperature range of 240-300 K was determined wherein N2 gas molecules were able to access the internal pores of K-ZSM-25 while CH4 was rejected. As confirmed by experimental, molecular dynamic, and ab initio density functional theory studies, the outstanding N2/CH4 selectivity is achieved within a specific temperature range where the thermal oscillation of door-blocking K+ provides enough space only for the relatively smaller molecule (N2) to diffuse into and through the zeolite supercages. Such temperature-regulated adsorption of the K-ZSM-25 trapdoor zeolite opens up a new approach for rejecting N2 from CH4 in the gas industry without deploying energy-intensive cryogenic distillation around 100 K.

Computational Modelling of SO2 and H2S gas interacting with small diameter Au-doped (5, 0) zig-zag single-walled carbon nanotubes

An ab initio density functional theory (DFT) is employed to explore the effect of adsorption of SO2 and H2S gases on gold-doped small diameter (5,0) zig-zag single-walled carbon nanotubes. Au-SWCNT nanostructures are extremely sensitive to the presence of both SO2 and H2S gases. Charge is transferred from the Au-SWCNT to the SO2 upon the adsorption of the gas molecules onto the Au-SWCNTs surface. There is a decrease in the energy bandgap of the Au-SWCNT. As a consequence, the electrical conductivity of Au-SWCNTs/SO2 is extremely enhanced. On the contrary, the adsorption of H2S gas onto the surface of the gold-doped nanotube provokes charge transfer from H2S to the Au-SWCNT. Another interesting observation is that the energy bandgap of AuSWCNT increases upon the adsorption of the H2S gas molecules onto the Au-SWCNT surface, thereby decreasing the electrical conductivity of the Au-SWCNT/H2S nanostructure. Citation: Elloh, V. W., Boadu, E. O., Ofosuhene, D. F. and Arhin, I. (2021). Computational Modelling of SO2 and H2S gas interacting with small diameter Au-doped (5, 0) zig-zag single-walled carbon nanotubes.International Journal of Technology and Management Research (IJTMR), Vol. 6 (2): Pp.87-95. Received: March 20, 2021Accepted: September 1, 2021

Interlayer Coupling Dependent Discrete H → T' Phase Transition in Lithium Intercalated Bilayer Molybdenum Disulfide.

In this work, the interlayer coupling dependent lithium intercalation induced phase transition in bilayer MoS2 (BL-MoS2) was investigated using an atomic-resolution annual dark-field scanning transmission electron microscope (ADF-STEM). It was revealed that the lithiation induced H → T' phase transition in BL-MoS2 strongly depended on the interlayer twist angle; i.e., the H → T' phase transition occurred in well-stacked H phase BL-MoS2 (with a twist angle of θt = 0°) but not for θt ≠ 0° BL-MoS2. The lithiated BL-MoS2 appeared in homophase stacking, either T'/T' or H/H (locally, no phase transformation) stacking, without any heterophase stacking such as H/T' or T'/H observed. This finding indicated the H → T' phase transition occurred via a domain-by-domain mode rather than layer-by-layer. Up to 15 types of stacking orders were experimentally identified locally in lithiated bilayer T'-MoS2, and the formation mechanism was attributed to the discrete interlayer translation with a unit step of (m/6a, n/6b) (m, n = 0, 1, 2, 3), where a and b were the primitive lattice vectors of T'-MoS2. Our experimental results were further corroborated by ab initio density functional theory (DFT) calculations, where the occurrence of different stacking orders can be quantitatively correlated with the variation of intercalated lithium contents into the BL-MoS2. The present study aids in the understanding of the phase transition mechanisms in atomically thin 2D transition metal dichalcogenides (TMDCs) and will also shed light on the precisely controlled phase engineering of 2D materials for memory applications.

Novel quinoxaline derivatives of 2, 3-diphenylquinoxaline-6-carbaldehyde and 4, 4′-(6-methylquinoxaline-2,3-diyl)bis(N,N-diphenylaniline): Synthesis, structural, DFT-computational, molecular docking, antibacterial, antioxidant, and anticancer studies

The quinoxaline derivatives of 2, 3-diphenylquinoxaline-6-carbaldehyde (DPQC) and 4, 4′-(6-methylquinoxaline-2,3-diyl)bis(N,N-diphenylaniline) (MDBD) were synthesized using direct condensation methods, and various spectral analysis were characterized by experimental and ab initio Density functional theory (DFT) theoretical methods at the B3LYP level with 6–311++G(d,p) basis sets were used for the different spectrum analysis, which included UV-Visible, Fourier-transform infrared spectroscopy (FTIR), and 1H and 13C nuclear magnetic resonance (NMR) chemical shifts. Furthermore, for these title molecules, Nonlinear Optical (NLO) was utilized to compute first-order hyperpolarizability (β) and Mulligan population analysis was performed. The ESI-Mass spectrum analysis was performed for these quinoxaline derivatives. In this study, title molecules of DPQC and MDBD were tested for in vitro antibacterial, antioxidant, and anticancer properties which exhibited enhanced biological activities. In particular, the title molecule MDBD gave enhanced anticancer activity at the lowest concentration of 125 µg/mL against human liver cancer (HepG2) cell lines, as well as potent bactericidal activity with a maximum of (19.5 ± 1.0 mm) at 2.5 µg/mL against Yersinia enterocolitica (MTCC 840). In an (H2O2) scavenging study, MDBD revealed potent antioxidant activity (64.21%). The DPPH radical scavenging antioxidant activity of MDBD was discovered at (67.48 %) concentration at 500 µg/mL. The best binding energy between anticancer target protein, specifically c-Met-kinase (hepatocyte growth factor; PDB ID: 3F66) and DPQC and MDBD compounds, were determined using in silico molecular docking. The auto dock software provided superior results for the title compound MDBD, which exhibited a higher ligand-receptor interaction energy value of -10.8 (kcal/mol) against 3F66 protein and a 0.01187 µM inhibition constant (ki) with (active site A) presented amino acids such as A: ARG1086, A: MET1211, A: VAL1092, A: ALA1226, A: ALA1108, A: MET1211. Further, studies are warranted to explore their promising anticancer and other pharmacological and biochemical properties.

Vibrational properties of graphene quantum dots: Effects of confinement, geometrical structure, and edge orientation

We perform ab initio density functional theory calculations to study the lattice vibrations of graphene quantum dots (GQDs) with triangular, quadrate, and hexagonal shapes in lateral confinement of 1 to 3 nm and terminated in both zigzag and armchair orientations. We see the vibrational properties of GQD transform from molecular type into bulk-like type with increasing dot size and the features of the vibrational density of state also highly depend on the symmetry of the GQD. We find that the out-of-plane vibrated standing waves induced by lateral confinement instead of the in-plane vibrations dominate the lattice vibrations of small GQDs. By projecting the vibrational eigenvectors of GQDs on those of single-layer graphene, we see the mixture of the out-of-plane and in-plane vibrational characters in GQDs as the result of the strong lateral confinement. We identify coherent acoustic phonon modes in GQDs and find that the size dependence of coherent acoustic phonon frequency increases with increasing the isotropy of nanostructures. Moreover, different confinement effects on lattice vibrations along zigzag and armchair edge orientations are identified from ab initio calculations. We describe the Raman intensity of GQDs and observe a blue shift of the G-mode position in GQDs comparing to that of the single-layer graphene. Combining the bond length distribution at the edge of GQDs with the positive Gr\"uneisen parameters, we link such blue shift to the surface effect of GQDs.

Impact of visible light and humidity on the stability of high-power light emitting diode packaging material

There are many advantages of LEDs in energy and environmental conservation, but their short life in many outdoor applications prompt a necessity to have a detailed understanding of their degradations to prolong their lifetime, which can also conserve LED material and even expand their applications. Using ab initio density functional theory formulation, we identify the detail paths of the LED degradation in outdoor applications. We discovered that the main stressors are humidity and the light that is emitted from the LED chip itself. This is rather ironical. A mathematical model is developed based on the ab initio study, and excellent experimental agreements are found. With this model, we can predict the situations where no and slow degradations can be achieved, and these are verified experimentally here. We can also predict quantitatively the time to a specific degradation severity. Quality index of the housing material for LEDs can also be determined.

Magnetic-field tuning of the intraexcitonic absorption and gain in transition metal dichalcogenides

A systematic microscopic approach combining ab-initio density functional theory with the Dirac-Bloch equations is applied to investigate the intra-excitonic transitions of magneto-excitons in transition metal dichalcogenide monolayers. For the example of hBN-encapsulated $\mathrm{Mo}{\mathrm{S}}_{2}$, the linear optical response and mid-infrared spectra of the pre-excited system are numerically evaluated. It is shown that the transition probability between a subset of the magneto-excitons can be inverted under suitable conditions to display negative absorption, i.e., gain. With the help of an applied magnetic field, the absorption and gain spectra can be tuned over a wide spectral range. Evaluating the Zeeman shift of the excitonic states, effective $g$ factors are deduced that depend on the dielectric environment of the sample under consideration.

Stress and Strain Prediction of Zirconium Nitride under Oxygen Doping and Vacancy Introduction

Zirconium nitride (ZrN) is an important material for the mechanical industries due to its excellent properties such as excellent wear resistance, high hardness, etc. In practical applications, it is necessary to study how to regulate the mechanical properties of materials to meet the needs of different applications. To better understand the influence of vacancies and oxygen on the mechanical property of ZrN, we studied the tensile strength of the ZrN with oxygen atom doping and zirconium vacancy introduction by ab initio density functional theory. The mechanical property changes of modified ZrN in three crystallographic directions (&lt;001&gt;, &lt;110&gt;, and &lt;111&gt;) were calculated. The results show that the tensile strength of ZrN can be increased by oxygen doping at a certain concentration, while that of ZrN can be decreased by the introduction of zirconium vacancy.

Band gap Engineering of ZnO via transition metal Doping: An ab initio study

Ab initio density functional theory calculations are performed to calculate the electronic and optical properties of transition metal doped ZnO, in order to achieve optimum bandgap reduction required for a potential photocatalyst. Geometry optimization analysis reveals that Fe, Cr, Mn and Nb doped systems are the most stable structures among our studied materials, which are further investigated for their aforementioned properties. The analysis of the Cr doped system show the maximum decrease in the bandgap and a red shift toward the visible region due to the valence electrons, suggesting that Cr doped ZnO is substantially a promising photocatalytic material for dye degradation applications.

Fe-doped CuGaS2 (CuGa1-xFexS2) - Detailed analysis of the intermediate band optical response of chalcopyrite thin films based on first principle calculations and experimental studies

We have theoretically and experimentally studied the tailoring of intermediate bands in the CuGa1-xFexS2 chalcopyrite system. An ab initio density functional theory calculations within GGA + U approach have been performed to model pristine and Fe-doped CuGaS2 to understand their structural and electronic properties and thereby understand the origin of intermediate states produced with Fe doping. Thin films of the pristine and Fe-doped CuGaS2 of various compositions are deposited using chemical spray pyrolysis for the experimental studies. The crystal structure, morphology, and topography of the deposited thin films are examined using Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), respectively. The binding energy and chemical composition of CuGa0.95Fe0.05S2 thin films are determined using X-ray Photoelectron Spectroscopy (XPS). The direct and intermediate band optical responses have been probed through UV–Vis–NIR Spectroscopy. Pristine CuGaS2 exhibits a direct bandgap of 2.43 eV, with formation of intermediate bands in Fe-doped CuGaS2 at 1.82 eV, 1.75 eV, 1.54 eV and 1.49 eV with varying Fe atomic percentage (X = 0.025, 0.05, 0.075, and 0.1 at. %). The optimum Fe concentration is determined to be X = 0.05 (CuGa0.95Fe0.05S2) as it exhibits the highest photocurrent (1.46 mA), electrical conductivity, and intermediate band optical response. The IB formation is further supported by the calculated density of states of the Fe doped CuGaS2 structure.