Hybrid-DFT study of halide perovskites, an energy-efficient material under compressive pressure for piezoelectric applications.

Recent studies have reported that lead-halide perovskites are the most efficient energy-harvesting materials. Regardless of their high-output energy and structural stability, lead-based products have risk factors due to their toxicity. Therefore, lead-free perovskites that offer green energy are the expected alternatives. We have taken CsGeX3(X = Cl, Br, and I) as lead-free halide perovskites despite knowing the low power conversion rate. Herein, we have tried to study the mechanisms of enhancement of energy-harvesting capabilities involving an interplay between structure and electronic properties. A density functional theory simulation of these materials shows a decrease in the band gaps, lattice parameters, and volumes with increasing applied pressure. We report the high piezoelectric responses and high electro-mechanical conversion rates, which are intriguing for generating electricity through mechanical stress.

First-principles investigation of the electronics, optical, mechanical, thermodynamics and thermoelectric properties of Na based Quaternary Heusler alloys (QHAs) NaHfXGe (X = Co, Rh, Ir).

In this study, we explored the electronic and thermoelectric (TE) properties of the Na-based Quaternary Heusler Alloys (QHAs) NaHfXGe (X = Co, Rh, Ir) using density functional theory (DFT). We performed the spin-polarized DFT calculations at the general gradient approximation (GGA) level and confirmed the ground state non-magnetic configuration of NaHfXGe. The mechanical and thermodynamical stabilities are analyzed and discussed to validate the stability by calculating the elastic constant and phonon dispersion curve. A thorough investigation on the electronic properties are carried out by performing the GGA, GGA+U, and GGA+SOC formalism where we report the semi-conducting characteristic of NaHfCoGe and NaHfRhGe QHAs. However, NaHfIrGe is predicted to be a non-magnetic metal. From the calculated optical properties we found that the most active optical absorption occurs within the vis-UV region withα>105 cm-1, therefore the studied QHAs are proposed to be a promising optoelectronic materials. The results of the thermodynamic properties have shown that NaHfXGe follows Debye's low-temperature specific heat law and the classical thermodynamics of the Dulong-Petit law at high temperatures. The calculated TE efficiency using GGA+SOC formalism atT= 1200 K are ZT∼1.22 and 0.57 for NaHfCoGe and NaHfRhGe, suggesting that these materials are potential TE materials to operate at high temperature.

A theoretical investigation of the lead-free double perovskites halides Rb2 XCl6 (X = Se, Ti) for optoelectronic and thermoelectric applications.

In this study, structural, electronic, optical, thermoelectric, and thermodynamics properties of vacancy-ordered double perovskites Rb2 XCl6 (X = Se, Ti) were explored theoretically. The results revealed that Rb2SeCl6 and Rb2 TiCl6 are indirect band gap (Eg ) semiconductors with Eg values of 2.95 eV, and 2.84 eV respectively. The calculated properties (phonons, elastic constant, Poisson's ratio, and Pugh's ratio) revealed that both materials are dynamically and chemically stable and can exhibit brittle (Rb2 SeCl6 ) and ductile (Rb2 TiCl6 ) nature. From the analysis of optical parameters, it was noticed that the refractive index of the materials has a value of 1.5-2.0 where light absorption was found from the visible to the ultraviolet region. The thermoelectric properties determined by using the BoltzTrap code demonstrated that at room temperature, the Figure of merit (ZT) was found to be 0.74 and 0.76 for Rb2 SeCl6 and Rb2 TiCl6 , respectively. Despite a moderate value of ZT in such materials, further studies might explore effective methods for tuning the electronic band gap and improving the thermoelectric response of the material for practical energy production applications.

Electronic, magnetic, optical and thermoelectric properties of co-doped Sn1-2x Mn x A x O2 (A = Mo, Tc): a first principles insight.

The electronic, magnetic, optical and thermoelectric (TE) properties of Sn1-2x Mn x A x O2 (A = Mo/Tc) have been examined using density functional theory (DFT) based on the FP-LAPW approach. The results suggested that all the doped compounds show a half-metallic ferromagnet property with a 100% spin polarization at the Fermi level within GGA and mBJ. Moreover, doping SnO2 with double impurities reduces the bandgap. The reduced bandgaps are the result of impurity states which arise due to the Mn and Mo/Tc doping, leading to the shifts of the minima of the conduction band towards the Fermi energy caused by substantial hybridization between transition metals 3d-4d and O-2p states. Also, the (Mn, Mo) co-doped SnO2 system exhibits a ferromagnetic ground state which may be explained by the Zener double exchange mechanism. While the mechanism that controls the ferromagnetism in the (Mn, Tc) co-doped SnO2 system is p-d hybridization. Therefore, the role of this study is to illustrate the fact that half-metallic ferromagnet material is a good absorber of sunlight (visible range) and couples to give a combined effect of spintronics with optronics. Our analysis shows that Sn1-2x Mn x Mo x O2 and Sn1-2x Mn x Tc x O2 are more capable of absorbing sunlight in the visible range compared to pristine SnO2. In addition, we report a significant result for the thermoelectric efficiency ZT of ∼0.114 and ∼0.11 for Sn1-2x Mn x Mo x O2 and Sn1-2x Mn x Tc x O2, respectively. Thus, the coupling of these magnetic, optical, and thermoelectric properties in (Mn, A = Mo or Tc) co-doped SnO2 can predict that these materials are suitable for optoelectronic and thermoelectric systems.

Electronic, mechanical and piezoelectric properties of glass-like complex Na2Si1-x Ge x O3 (x = 0.0, 0.25, 0.50, 0.75, 1.0).

Motivated by our previous work on pristine Na2SiO3, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na2Si1-x Ge x O3 (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT). Interestingly, the optimized bond lengths and bond angles of Na2SiO3 and Na2GeO3 resemble each other with high similarity. On doping we report the negative formation energy and feasibility of transition of Na2SiO3 → Na2GeO3 while the structural symmetry is preserved. Analyzing the electronic profile, we have observed a reduced band gap on increasing x = Ge concentration at Si-sites. All the systems are indirect band gap (Z-Γ) semiconductors. The studied systems have shown mechanical stabilities by satisfying the Born criteria for mechanical stability. The calculated results have shown highly anisotropic behaviour and high melting temperature, which are a signature of glass materials. The piezoelectric tensor (both direct and converse) is computed. The results thus obtained predict that the systems under investigation are potential piezoelectric materials for energy harvesting.

Electronic, mechanical, optical and piezoelectric properties of glass-like sodium silicate (Na2SiO3) under compressive pressure.

The structural, mechanical, electronic, optical and piezoelectric properties of Na2SiO3 are studied under varying compressive unidirectional pressure (0-50 GPa with a difference of 10 GPa) using density functional theory (DFT). The calculated structural properties agree well with previously reported results. At 12 GPa, our calculation shows a structural phase transition from orthorhombic Cmc21 to triclinic P1. The mechanical profile of Na2SiO3 structures under different compressive unidirectional pressures are analysed by calculating the elastic moduli, Poisson's ratio and eigenvalues of stiffness matrix. Our study shows the mechanical stability of the system up to a pressure of 40 GPa. Herein, we have obtained an indirect band gap of 2.97 eV at 0 GPa. Between 0-50 GPa, the band gaps are within the range 2.62 to 3.46 eV. The system in our study possesses a wide band gap and high optical absorption in the UV-Vis range of electromagnetic radiation. The calculated static refractive indices η x,y,z (0) are close to unity suggesting its transparency. For piezoelectric properties, we have reported the total Cartesian polarization. Our calculations have revealed that Na2SiO3 is a promising candidate for optoelectronic devices while its application in ferroelectric and piezoelectric devices could be improved with further research.

N-doped direction-dependent electronic and mechanical properties of single-walled carbon nanotube (SWCNT) from a first-principles density functional theory (DFT) and MD-simulation.

In this paper, the electronic and mechanical properties of Nitrogen (N) doped (6,1) single walled carbon nanotube (SWCNT) is analysed based on the first principles density functional theory (DFT) and Molecular dynamic (MD) calculation. A systematic N-doping on SWCNT was performed along zigzag (zz) and armchair (ac) direction, where the armchair doping is parallel to tube axis while zigzag is along the cross-section perpendicular to tube axis. The zz and ac doping resulted in variations in the electronic properties of the even and odd number of N-dopant atoms. To evaluate the mechanical properties, ab - initio MD-simulations was carried out. We found a dependence of the tensile response of the tube on the dopant concentration and doping pattern. Single N-doped system show enhanced tensile stress by 55% as compared to the pristine SWCNT with marginal changes in the young's modulus for all N-doped systems.

Electronic and optical properties of bulk and surface of CsPbBr3 inorganic halide perovskite a first principles DFT 1/2 approach.

This work aims to test the effectiveness of newly developed DFT-1/2 functional in calculating the electronic and optical properties of inorganic lead halide perovskites CsPbBr3. Herein, from DFT-1/2 we have obtained the direct band gap of 2.36 eV and 3.82 eV for orthorhombic bulk and 001-surface, respectively. The calculated energy band gap is in qualitative agreement with the experimental findings. The bandgap of ultra-thin film of CsPbBr3 is found to be 3.82 eV, which is more than the expected range 1.23-3.10 eV. However, we have found that the bandgap can be reduced by increasing the surface thickness. Thus, the system under investigation looks promising for optoelectronic and photocatalysis applications, due to the bandgap matching and high optical absorption in UV-Vis (Ultra violet and visible spectrum) range of electro-magnetic(em) radiation.

Sn3C2monolayer with transition metal adatom for gas sensing: a density functional theory studies.

The gas sensing properties of pristine Sn3C2monolayer and different transition metal adatom (TM-Sn3C2, where TM = Fe, Co, Ni, Cu, Ru, Rh, Pd and Ag) was investigated using van der Waals corrected density functional theory. The understanding and potential of use of Sn3C2monolayers as sensors or adsorbent for CO, CO2, NO, NO2and SO2gaseous molecules is evaluated by calculating the adsorption and desorption energetics. From the calculated adsorption energies, we found that the pristine Sn3C2monolayer and 3dTM has desirable properties for removal of the considered molecules based on their high adsorption energy, however the 4dTM is applicable as recoverable sensors. We applied an Arrhenius-type equation to evaluate the recovery time for the desorption of the molecules on the pristine and TM adatom on Sn3C2monolayer. We found that the negative adsorption energies from -1 to -2 eV of the molecules resulted in easier recovery of the adsorbed gases at reasonable temperatures compared to adsorption energies in between 0 and -1 eV (weakly physiosorbed) and below -2 eV (strongly chemisorbed). Hence, we obtained that the Rh-Sn3C2, Ru-Sn3C2, Pd-Sn3C2, Pd-Sn3C2, and Rh-Sn3C2monolayers are good recoverable scavengers for the CO, CO2, NO, NO2, and SO2molecules. The current theoretical calculations provide new insight on the effect of TM adatoms on the structural, electronic, and magnetic properties of the Sn3C2monolayer and different transition metal adatom as well as shed light on their application as gas sensors/scavengers.

Electronic, magnetic, vibrational, and X-ray spectroscopy of inverse full-Heusler Fe2IrSi alloy.

We report the electronic, magnetic, structural, vibrational, and X-ray absorption spectroscopy of the inverse full-Heusler Fe2IrSi alloy. We employed state-of-the-art first-principles computational techniques. Our ab initio calculations revealed a ferromagnetic half-metallicity with a magnetic moment of ∼5.01 µB, which follows the Slater Pauling rule. We show rich magnetic behavior due to spin-orbit coupling through the entanglement of the Fe-3d/Ir-5d orbitals. The large extension of the Ir-5d orbital and the itinerant Fe-3d states enhanced spin-orbit and electron-electron interactions, respectively. The analyses of our results reveal that electron-electron interactions are essential for the proper description of the electronic properties while spin-orbit coupling effects are vital to accurately characterize the X-ray absorption and X-ray magnetic circular dichroism spectra. We estimate the strength of the spin-orbit coupling by comparing the intensity of the white-line features at the L3 and L2 absorption edges. This led to a branching ratio that deviates strongly from the statistical ratio of 2, indicative of strong spin-orbit coupling effects in the inverse full-Heusler Fe2IrSi alloy.

Modulation of optical absorption in m-Fe1-xRuxS2 and exploring stability in new m-RuS2.

A first-principle computational method has been used to investigate the effects of Ru dopants on the electronic and optical absorption properties of marcasite FeS2. In addition, we have also revealed a new marcasite phase in RuS2, unlike most studied pyrite structures. The new phase has fulfilled all the necessary criteria of structural stability and its practical existence. The transition pressure of 8 GPa drives the structural change from pyrite to orthorhombic phase in RuS2. From the thermodynamical calculation, we have reported the stability of new-phase under various ranges of applied pressure and temperature. Further, from the results of phonon dispersion calculated at Zero Point Energy, pyrite structure exhibits ground state stability and the marcasite phase has all modes of frequencies positive. The newly proposed phase is a semiconductor with a band gap comparable to its pyrite counterpart but vary in optical absorption by around 106 cm-1. The various Ru doped structures have also shown similar optical absorption spectra in the same order of magnitude. We have used crystal field theory to explain high optical absorption which is due to the involvement of different electronic states in formation of electronic and optical band gaps. LÓ§wdin charge analysis is used over the customarily Mulliken charges to predict 89% of covalence in the compound. Our results indicate the importance of new phase to enhance the efficiency of photovoltaic materials for practical applications.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides.

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In2XY(X/Y= S, Se, Te withX≠Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In2XYare dynamically and thermally stable at room temperature. At equilibrium, both In2STe and In2SeTe are direct semiconductors while In2SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In2XYand their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In2XYis negligible. Meanwhile, the optical absorbance intensity of the Janus In2XYmonolayers is remarkably increased by compressive strain. Also, In2XYmonolayers exhibit very low lattice thermal conductivities resulting in a high figure of meritZT, which makes them potential candidates for room-temperature thermoelectric materials.

Outstanding elastic, electronic, transport and optical properties of a novel layered material C4F2: first-principles study.

Motivated by very recent successful experimental transformation of AB-stacking bilayer graphene into fluorinated single-layer diamond (namely fluorinated diamane C4F2) [P. V. Bakharev, M. Huang, M. Saxena, S. W. Lee, S. H. Joo, S. O. Park, J. Dong, D. C. Camacho-Mojica, S. Jin, Y. Kwon, M. Biswal, F. Ding, S. K. Kwak, Z. Lee and R. S. Ruoff, Nat. Nanotechnol., 2020, 15, 59-66], we systematically investigate the structural, elastic, electronic, transport, and optical properties of fluorinated diamane C4F2 by using density functional theory. Our obtained results demonstrate that at the ground state, the lattice constant of C4F2 is 2.56 Å with chemical bonding between the C-C interlayer and intralayer bond lengths of about 1.5 Å which are close to the C-C bonding in the bulk diamond. Based on calculations for the phonon spectrum and ab initio molecular dynamics simulations, the structure of C4F2 is confirmed to be dynamically and thermally stable. C4F2 exhibits superior mechanical properties with a very high Young's modulus of 493.19 N m-1. Upon fluorination, the formation of C-C bonding between graphene layers has resulted in a comprehensive alteration of electronic properties of C4F2. C4F2 is a direct semiconductor with a large band gap and phase transitions are found when a biaxial strain or external electric field is applied. Interestingly, C4F2 has very high electron mobility, up to 3.03 × 103 cm2 V-1 s-1, much higher than other semiconductor compounds. Our findings not only provide a comprehensive insight into the physical properties of C4F2 but also open up its applicability in nanoelectromechanical and optoelectronic devices.

Phase stability and optoelectronic characteristics of Ba1-xBexS: a DFT-based simulation.

The structural stability and optoelectronic properties of the ternary Ba1-xBexS alloys along with the pure binary compounds BaS and BeS in the rock-salt (B1) and zinc-blende (B3) phases were investigated by the density functional theory (DFT) within the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the Wien2k package. The generalized gradient approximation of Wu and Cohen (WC-GGA) was used for the exchange-correlation potential (Vxc) to compute the equilibrium structural parameters, lattice constant (a), and bulk modulus (B). In addition to the GGA approach, the modified Becke-Johnson potential of Tran and Blaha (TB-mBJ) scheme coupled with the spin-orbit interaction was used to calculate the band gap energies. Results reveal that BaS, Ba0.75Be0.25S, and Ba0.5Be0.5S compounds are stable in the rock-salt phase, while Ba0.25Be0.75S and BeS are found to be stable in the zinc-blende phase. The computed results for the band structures and optical constants are compared with other available theoretical calculations and experimental measurements.

Controlling the electronic and optical properties of HfS2 mono-layers via lanthanide substitutional doping: a DFT+U study.

Two dimensional HfS2 is a material with potential applications in the field of photo-catalysis and advanced solid state devices. Density functional theory with the Hubbard U parameter (DFT+U) calculations were carried out to investigate the structural, electronic and optical properties of lanthanide dopant atoms (LN = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in the HfS2 mono-layer. The calculated electronic band gap for a pristine HfS2 mono-layer is 1.30 eV with a non-magnetic ground state. The dopant substitutional energies under both Hf-rich and S-rich conditions were evaluated, with the S-rich condition for the dopant atoms being negative. This implies that the incorporation of these LN dopant atoms in the HfS2 is feasible and experimental realization possible. The introduction of LN dopant atoms in the HfS2 mono-layer resulted in a significant change of the material properties. We found that the presence of LN dopant atoms in the HfS2 mono-layer significantly alters its electronic ground states by introducing defect states as well as changes in the overall density of states profile resulting in a metallic ground state for the doped mono-layers. The doped mono-layers are all magnetic with the exception of La and Lu dopant atoms. We found that LN dopant atoms in the HfS2 mono-layer influence the absorption and reflectivity spectra with the introduction of states in the lower frequency range (<1.30 eV). Furthermore, we showed that the applicability of doped HfS2 mono-layers as photo-catalysts is very different compared with the pristine HfS2 mono-layer.

Spin-orbit coupling effect on electronic, optical, and thermoelectric properties of Janus Ga2SSe.

In this paper, we investigate the electronic, optical, and thermoelectric properties of Ga2SSe monolayer by using density functional theory. Via analysis of the phonon spectrum and ab initio molecular dynamics simulations, Ga2SSe is confirmed to be stable at room temperature. Our calculations demonstrate that Ga2SSe exhibits indirect semiconductor characteristics and the spin-orbit coupling (SOC) effect has slightly reduced its band gap. Besides, the band gap of Ga2SSe depends tightly on the biaxial strain. When the SOC effect is included, small spin-orbit splitting energy of 90 meV has been found in the valence band. However, the spin-orbit splitting energy dramatically changes in the presence of biaxial strain. Ga2SSe exhibits high optical absorption intensity in the near-ultraviolet region, up to 8.444 × 104 cm-1, which is needed for applications in optoelectronic devices. By using the Boltzmann transport equations, the electronic transport coefficients of Ga2SSe are comprehensively investigated. Our calculations reveal that Ga2SSe exhibits a very low lattice thermal conductivity and high figure of merit ZT and we can enhance its ZT by temperature. Our findings provide further insight into the physical properties of Ga2SSe as well as point to prospects for its application in next-generation high-performance devices.

Pressure dependent half-metallic ferromagnetism in inverse Heusler alloy Fe2CoAl: a DFT+U calculations.

We report the electronic and magnetic properties along with the Curie temperature (T C) of the inverse full Heusler alloy (HA) Fe2CoAl obtained by using the first-principles computational method. Our calculations suggests that Fe2CoAl is a magnetic metal when treated within PBE-GGA under the applied compressive pressures. However, the implementation of electron-electron (U) (i.e., GGA+U) with varying compressive pressure (P) drastically changes the profile of the electronic structure. The application of GGA+U along with pressure induces ferromagnetic half-metallicity with an integer value of total magnetic moment ∼4.0 µ B per unit cell. The integer value is in accordance with the Slater-Pauling's rule. Here, we demonstrate the variation of semiconducting gap in the spin down channel. The band gap increases from 0.0 eV to 0.72 eV when increasing the pressure from 0 to 30 GPa. Beyond 30 GPa, the electronic band gap decreases, and it is completely diminished at 60 GPa, exhibiting metallic behaviour. The analysis of the computed results shows that the treatment of electron-electron interactions within GGA+U and the application of compressive pressure in Fe2CoAl enables d-d orbital hybridization giving rise to a half-metal ferromagnet. The T C calculated from mean field approximation (MFA) decreases up to 30 GPa and then increases linearly up to 60 GPa.

Electronic, magnetic, optical and thermoelectric properties of Ca2Cr1-x Ni x OsO6 double perovskites.

With the help of density functional theory calculations, we explored the recently synthesized double perovskite material Ca2CrOsO6 and found it to be a ferrimagnetic insulator with a band gap of ∼0.6 eV. Its effective magnetic moment is found to be ∼0.23 µ B per unit cell. The proposed behavior arises from the cooperative effect of spin-orbit coupling and Coulomb correlation of Cr-3d and Os-5d electrons along with the crystal field. Within the ferrimagnetic configuration, doping with 50% Ni in the Cr-sites resulted in a half-metallic state with a total moment of nearly zero, a characteristic of spintronic materials. Meanwhile, the optical study reveals that both ε 1 xx and ε 1 zz decrease first and then increase rapidly with increasing photon energy up to 1.055 eV. We also found optical anisotropy up to ∼14 eV, where the material becomes almost optically isotropic. This material has a plateau like region in the σ xx and σ zz parts of the optical conductivity due to a strong 3d-5d interband transition between Cr and Os. In addition, we performed thermoelectric calculations whose results predict that the material might not be good as a thermoelectric device due to its small power factor.

Electronic properties and low lattice thermal conductivity (κ l) of mono-layer (ML) MoS2: FP-LAPW incorporated with spin-orbit coupling (SOC).

This paper focuses on the electronic and thermoelectric properties of monolayer MoS2. Here, we have examined the structure of MoS2, in which the hole in the center of the hexagonal cage is considered as a void atom, termed 1H-MoS2. Density functional theory (DFT) employing the generalized gradient approximation (GGA) and spin-orbit coupling (SOC) has been used for all calculations. Incorporation of SOC resulted in a significant change in the profile of the band energy, specifically the splitting of the valence band maximum (VBM) into two sub-bands. The "split-off" energy is found to be ∼20.6 meV. The reduction of the band gap with SOC is a prominent feature at the K-K location in the Brillouin zone. The band gap calculated with the GGA is ∼1.75 eV. However, on implementation of SOC, the GGA band gap was reduced to ∼1.68 eV. The frequency-dependent phonon dispersion curve was obtained to analyse the thermodynamical stability. 1H-MoS2 is found to be thermodynamically stable with no imaginary frequency. We report a low value of lattice thermal conductivity (κ l) and low electron effective masses, which are desirable for potential applications in thermoelectric devices.