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The electronic band-structure of the ternary sulfide Pb2GeS4 was investigated by combining experimental and theoretical methods. Binding energy (BE) values of core electrons of Pb2GeS4 are measured employing X-ray photoelectron spectroscopy (XPS) for as-synthesized and treated with Ar+-ions crystal surfaces. The XPS measurements indicate that Ar+-ion treatment does not change the BE values of the core-level electrons of atoms constituting the Pb2GeS4 single crystal as well as peculiarities of the XPS valence band (VB) spectrum. The treatment does not cause changes in the crystal surface stoichiometry. The band-structure calculations based on density functional theory (DFT) reveal total density of states and partial densities of states of Pb2GeS4 within different exchange-correlation approximations. The best fit with the experiment is derived when the DFT calculations of Pb2GeS4 employ modified Becke-Johnson potential with Hubbard-corrected functional and taking into account spin-orbit (SO) interaction. The calculations indicate that top and upper portion of the VB is composed mainly by S 3p states, its central portion is formed by Ge 4p and S 3p states, while contributions of Pb 6s states dominate at its bottom with slightly smaller contributions of Ge 4s states as well. Contributions of unoccupied Pb 6p states dominate at the conduction band (CB) bottom. Regarding the occupation of the VB by Ge 4p and S 3p states, the theoretical data are confirmed experimentally by matching the XPS VB spectrum on a common energy scale with the X-ray emission spectra representing the valence S p and Ge p states. The present calculations yield that the VB maximum is positioned at the Y point, while the CB minimum at the Г point; this fact indicates that Pb2GeS4 sulfide is an indirect-gap material. The principal optical constants are also elucidated using the ab initio DFT calculations. [ABSTRACT FROM AUTHOR]

We report on results of first-principles density functional theory (DFT) calculations of the total density of states (DOS) and the partial densities of states of a quaternary TlInGe2S6 compound, a promising nonlinear optical material. The calculations are made using the augmented plane wave + local orbitals method employing the WIEN2k package. The DOS curves are calculated in different approaches for exchange-correlation potentials, and it has been established that the best coincidence with the experiment is achieved when modified Becke-Johnson exchange potential is used with correction parameter U and an account of the spin-orbit splitting effect. The present DFT calculations present that the S 3p states are the primary contributors to the TlInGe2S6 valence band, making the main input to its top and upper portion. The primary contributors to the central portion of the valence band of TlInGe2S6 are the Tl 6s and Ge 4p states, while its bottom is generated due to contributions of mainly the In 5s states with slightly smaller contributions of the Ge 4p states as well. The bottom of the conduction band is dominated by the unoccupied Ge 4s states. The conduction band minimum and the valence band maximum are located at the L point of the first Brillouin zone resulting in a direct band gap of the TlInGe2S6 compound. The key optical constants are elucidated for TlInGe2S6 based on the present DFT calculations. [ABSTRACT FROM AUTHOR]

A nearly centimeter-size Tl 2 HgSnSe 4 crystal was successfully derived by Bridgman-Stockbarger growth technique and its electronic and optical properties are investigated experimentally and theoretically. The present XPS experiments imply very little hygroscopic behavior of the Tl 2 HgSnSe 4 crystal surface, a characteristic property to be very important for practical use of this crystal in devices working at ambient atmospheric conditions. The experimentally determined energy band gap, 1.26 eV, and high values of the photocurrent spectral dependence confirm that Tl 2 HgSnSe 4 crystal is a high-resistance photosensitive semiconductor possessing the p-type conductivity. The finest agreement with the experiment is detected when the theoretical calculation of the electronic structure of Tl 2 HgSnSe 4 is carried out using modified Becke-Johnson functional for the exchange-correlation potential and involving the Hubbard amendment parameter U and the spin-orbit coupling effect. The present experimental and theoretical data suggest good perspective of application of Tl 2 HgSnSe 4 in optoelectronics and as a suitable material of solar photovoltaics. [Display omitted] • Tl 2 HgSnSe 4 crystal was grown for the first time by Bridgman-Stockbarger method. • Tl 2 HgSnSe 4 is a p-type high-resistance photosensitive semiconductor with E g = 1.26 eV. • Electronic structure of the Tl 2 HgSnSe 4 crystal was investigated by XPS and XES. • Band-structure calculations within the mBJ+U+SO model are in agreement with experiments. • Principal optical properties of Tl2HgSnSe4 are explored in detail. [ABSTRACT FROM AUTHOR]

Major airports are often located within or close to large cities; their impacts on the deterioration of air quality at ground level are amply recognised. The international airport of Heathrow is a major source of nitrogen oxides in the Greater London area, but its contribution to the levels of submicrometre particles is unknown, and is the objective of this study. Two sampling campaigns were carried out during warm and cold seasons at a site close to the airfield (1.2 km). Size spectra were largely dominated by ultrafine particles: nucleation particles (< 30 nm) were found to be ~ 10 times higher than those commonly measured in urban background environments of London. A set of chemometric tools was used to discern the pollution arising from aircraft operations and those from other sources within the city or from the traffic generated by the airport. Five clusters and 6 factors were identified by applying k-means cluster analysis and positive matrix factorization (PMF) respectively to particle number size distributions; their interpretation was based on their modal structures, wind directionality, diurnal patterns, road and airport traffic volumes and on the relationship with weather and other air pollutants. Airport emissions, fresh and aged road traffic, urban accumulation mode and two secondary sources were then identified and apportioned. The comparison of cluster and PMF analyses allowed extraction of further information. The analysis of a strong regional nucleation event was also performed to detect its effect upon concentrations. The fingerprint of Heathrow has a characteristic modal structure peaking at < 20 nm and accounts for 30-35 % of total particles in both the seasons. Other main contributors are fresh (24-36 %) and aged (16-21 %) road traffic emissions and urban accumulation from London (around 10 %). Secondary sources accounted for less than 6 % in number concentrations but for more than 50 % in volume concentration. In 2016, the UK government provisionally approved the construction of a third runway; therefore the direct and indirect impact of Heathrow on local air quality is expected to increase unless mitigation strategies are applied successfully. [ABSTRACT FROM AUTHOR]

Exploring better Gamma, X-Ray and IR-Mid-R detectors, adequately sensitive to ambient temperatures and environmental conditions, is highly desirable. Therefore, following this path, we present detecting opportunities, based on data of experimental and theoretical investigation of the electronic and optical properties, photocurrent behavior at different bias voltages as well as the reflection for the He-Cd cw laser after irradiation by three different IR laser wavelengths, for recently grown TlPb 2 BrI 4 single crystal with strong anisotropic, two-dimensional (2D) layered structure. The X-ray photoelectron spectroscopy brings data on binding energy values of core levels of TlPb 2 BrI 4 for both as-grown and Ar+-ion-treated crystal surfaces as well as for elucidation of the energy distribution of the valence electronic states. In opposite to well-known IR-detector materials, TlPb 2 BrI 4 crystal surface shows high chemical stability concerning Ar+-ion bombardment. Further, we carry out ab initio band-structure calculations to gain curves of total and partial densities of states and to elucidate the principal optical constants of TlPb 2 BrI 4. The present studies indicate that, in spite of the fact that the TlPb 2 BrI 4 crystal contains two very hazardous/toxic chemical elements, thallium and lead; however, it is chemically stable and reveals very low hygroscopic behavior when being exposed to environmental conditions. [Display omitted] • Measurements of the time-resolved photocurrent are performed for TlPb 2 BrI 4 crystal. • Explanation of conversion rate of TlPb 2 BrI 4 crystal in the framework of the layered 2D structure is made. • IR photoinduced changes in reflectivity geometry of TlPb 2 BrI 4 are explored. • Crystal structure of novel layered TlPb 2 BrI 4 compound is refined. • Electronic structure and optical properties of TlPb 2 BrI 4 crystal are studied in detail. [ABSTRACT FROM AUTHOR]

Results of computation of the electronic structure of low-temperature (LT) orthorhombic (space group P 2 1 2 1 2 1) and high-temperature (HT) tetragonal (space group P 4 1) polymorphous forms of Tl 3 PbBr 5 as well as of TlPb 2 Br 5 (monoclinic space group P 2 1 / c) compound as compared to the experimental XPS studies of these bromides are presented. Our results indicate that the best correspondence of the theoretical total density of states and the experimental XPS valence-band spectrum is detected when the computations are carried out employing the augmented plane wave + local orbitals method within a Density Functional Theory (DFT) formalism and using the modified Becke-Johnson (mBJ) functional as refined by Tran-Blaha and considering also the Hubbard correction parameter U for strongly correlated d electrons associated with Tl and Pb atoms and spin-orbit coupling (SOC) effect (TB-mBJ + U + SOC technique). Based on these findings, using the mBJ + U + SOC technique, we study in detail partial densities of states of LT - and HT -modifications of Tl 3 PbBr 5 and of TlPb 2 Br 5. Furthermore, for the above ternary bromides, for the first time, the main optical properties were calculated. Total DOS of (a) LT -Tl 3 PbBr 5 and (b) HT -Tl 3 PbBr 5 polymorphs calculated within the TB-mBJ + U + SOC model and adjusted on a general energy scale with their XPS valence-band spectra. [Display omitted] • ►Electronic structures of LT - and HT -modifications of Tl 3 PbBr 5 and of TlPb 2 Br 5 are studied. • Total and projected densities of states are computed within the TB-mBJ + U + SOC model. • Excellent agreement of the theoretical results with the XPS experiments is detected. • The main optical constants are calculated within TB-mBJ + U + SOC model. • TlPb 2 Br 5 and LT - and HT -modifications of Tl 3 PbBr 5 are prospective optoelectronic semiconductors. [ABSTRACT FROM AUTHOR]

Road traffic constitutes a major source of air pollutants in urban Beijing, which are responsible for substantial premature mortality. A series of policies and regulations has led to appreciable traffic emission reductions in recent decades. To shed light on long-term (2014–2020) roadside air pollution and assess the efficacy of traffic control measures and their effects on public health, this study quantitatively evaluated changes in the concentrations of six key air pollutants (PM 2.5 , PM 10 , NO 2 , SO 2 , CO and O 3) measured at 5 roadside and 12 urban background monitoring stations in Beijing. We found that the annual mean concentrations of these air pollutants were remarkably reduced by 47%–71% from 2014 to 2020, while the concurrent ozone concentration increased by 17.4%. In addition, we observed reductions in the roadside increments in PM 2.5 , NO 2 , SO 2 and CO of 54.8%, 29.8%, 20.6%, and 59.1%, respectively, indicating the high effectiveness of new vehicle standard (China V and VI) implementation in Beijing. The premature deaths due to traffic emissions were estimated to be 8379 and 1908 cases in 2014 and 2020, respectively. The impact of NO 2 from road traffic relative to PM 2.5 on premature mortality was comparable to that of traffic-related PM 2.5 emissions. The public health effect of SO 2 originating from traffic was markedly lower than that of PM 2.5. The results indicated that a reduction in traffic-related NO 2 could likely yield the greatest benefits for public health. • Emission control causes reduction of roadside air pollutants, but increases in O 3. • New standards and electric vehicle contributed to a decrease of air pollutants. • Control policies did not affect road increment of PM 2.5-10 due to non-exhaust sources. • Premature deaths were 8379 and 1908 cases, respectively, in 2014 and 2020. • Relative public impact of PM 2.5 comparable to that of NO 2 , higher than that of SO 2. [ABSTRACT FROM AUTHOR]

We report on successful synthesis of single-phase high-temperature (HT) orthorhombic modification of Cu 2 HgGeS 4 and studies of its electronic and optical properties from a viewpoint of experiment and theory. In particular, we have made measurements of the XPS core-level and valence-band spectra of the HT-Cu 2 HgGeS 4 modification and calculated density of states. The calculations are carried out within a framework of density functional theory and using varieties of techniques. We have realized that the best agreement of the theory and experiment is observed when we use in the calculations the modified Becke-Johnson (mBJ) functional as refined by Tran-Blaha and consider also the Hubbard correction parameter U and spin-orbit coupling (SOC). The calculations demonstrate that main contributions of sulfur 3p states occur near the topmost and upper part of the HT-Cu 2 HgGeS 4 valence band, copper 3d states dominate in its central part, the lower part is prevailed by contributions of mercury 6s and germanium 4p states, whereas near the valence band bottom the biggest contributions come from mercury 5d states. The theoretical predictions are verified experimentally. The calculations reveal that the HT-Cu 2 HgGeS 4 modification is a direct band gap semiconductor. The principal optical constants are elucidated following first-principles calculations of the quaternary sulfide under consideration. Calculated employing the TB-TB-mBJ ​+ ​U ​+ ​SOC approach the electronic band structure matched with total DOS of the constituting atoms of the HT-Cu 2 HgGeS 4 compound. [Display omitted] • Single-phase orthorhombic HT-Cu 2 HgGeS 4 modification was synthesized for the first time. • Electronic structure of HT-Cu 2 HgGeS 4 was studied by XPS. • Calculations within TB-mBJ ​+ ​U ​+ ​SOC approach are in agreement with XPS measurements. • The HT-Cu 2 HgGeS 4 modification is a direct band gap semiconductor. • Principal optical properties of HT-Cu 2 HgGeS 4 are studied in detail from first principles. [ABSTRACT FROM AUTHOR]

Given that many people typically spend the majority of their time at home, accurate measurement and modelling of the home environment is critical in estimating their exposure to air pollution. This study investigates the fate and impact on human exposure of outdoor and indoor pollutants in London homes, using a combination of sensor measurements, outdoor air pollution estimated from the CMAQ-urban model and indoor mass balance models. Averaged indoor concentrations of PM 2.5 , PM 10 and NO 2 were 14.6, 24.7 and 14.2 μg m−3 while the outdoor concentrations were 14.4, 22.6 and 21.4 μg m−3, respectively. Mean infiltration factors of particles (0.6–0.7) were higher than those of NO 2 (0.4). In contrast, higher loss rates were found for NO 2 (0.5–0.8 h−1) compared to those for particles (0.1–0.3 h−1). The average concentrations of PM 2.5 , PM 10 and NO 2 in kitchen environments were 22.0, 33.7 and 20.8 μg m−3, with highest hourly concentrations (437, 644 and 136 μg m−3, respectively) during cooking times (6–7 pm). Indoor sources increased the indoor concentrations of particles and NO 2 by an average of 26–37% in comparison to the indoor background level without indoor sources. Outdoor and indoor air exchange plays an important role in reducing air pollution indoors by 65–86% for particles and 42–65% for NO 2. • Mean Indoor/outdoor (I/O) ratios were 1.2 for PM whereas it was 0.84 for NO 2. • Higher infiltration factors were found for particles (0.6–0.7) compared to those for NO 2. • On average, indoor sources contributed 26–37% of indoor concentrations of particles and NO 2. • Outdoor and indoor air exchange plays a critical role in reducing indoor pollutants when indoor sources occur. [ABSTRACT FROM AUTHOR]

A centimeter size crystal of silver- and bismuth-bearing selenophosphate, AgBiP 2 Se 6 , was effectively grown for the first time by Bridgman technique and characterized via different experimental and theoretical techniques to explore its quality and some physicochemical properties. The present experimental and theoretical data reveal a well-defined layered structure of the AgBiP 2 Se 6 compound and its highly anisotropic optical constants. In particular, the AgBiP 2 Se 6 crystal was studied to estimate binding energies of core-level electrons of the atoms constituting the crystal and to explore the energy distribution of the valence electronic states for both as-grown and treated with middle-energy Ar+ ions surfaces. The XPS data reveal high hygroscopicity of the AgBiP 2 Se 6 crystal. The SEM/EDX results feature a composition of the crystal to be very close to a nominal stoichiometric content. In the present work we probe various models to gain the best correspondence of theoretical total density of states curve and the experimental XPS spectrum. We have established theoretically that contributions of Se p states prevail in the upper section of the valence band, its central section is prevailed by Ag d states, while the bottom is formed from contributions of P p states. Following the band-structure findings, we examine in detail the optical constants of the compound under study. In particular, the present calculations yield that AgBiP 2 Se 6 is a promising optoelectronic semiconductor with essential anisotropic behavior for two main non-zero diagonal constituents of the second order dielectric tensor in special energy regions for all the optical constants, whereas the energy-loss spectrum is isotropic. [Display omitted] • A centimeter size AgBiP 2 Se 6 crystal was successfully grown by Bridgman technique. • Electronic structure of AgBiP 2 Se 6 crystal is studied experimentally employing XPS and XES. • Total and partial DOS of AgBiP 2 Se 6 are calculated. • The TB-mBJ + U + SO calculating results correspond well to the XPS and XES data. • Optical properties are elucidated by DFT calculations. [ABSTRACT FROM AUTHOR]

Coarse-grained modeling methods allow simulations at larger scales than molecular dynamics, making it feasible to simulate multifluid systems. It is, however, critical to use model parameters that represent the fluid properties with fidelity under both equilibrium and dynamic conditions. In this work, dissipative particle dynamics (DPD) methods were used to simulate the flow of oil and water in a narrow slit under Poiseuille and Couette flow conditions. Large surfactant molecules were also included in the computations. A systematic methodology is presented to determine the DPD parameters necessary for ensuring that the boundary conditions were obeyed, that the oil and water viscosities were represented correctly, and that the velocity profile for the multifluid system agreed with the theoretical expectations. Surfactant molecules were introduced at the oil–water interface (sodium dodecylsulfate and octaethylene glycol monododecyl ether) to determine the effects of surface-active molecules on the two-phase flow. A critical shear rate was found for Poiseuille flow, beyond which the surfactants desorbed to form the interface forming micelles and destabilize the interface, and the surfactant-covered interface remained stable under Couette flow even at high shear rates. [ABSTRACT FROM AUTHOR]

We report on successful growth by Bridgman method of an optical quality K 0.4 Rb 0.6 Pb 2 Cl 5 crystal and determination of its crystal structure and electronic and optical properties. In particular, the present results indicate that the K 0.4 Rb 0.6 Pb 2 Cl 5 crystal crystallizes in monoclinic space group P 2 1 / c , with the unit-cell parameters as follows: a = 8.9484(2) Å, b = 7.9802(2) Å, c = 12.5359 Å, and β = 90.1220(10)°. For the K 0.4 Rb 0.6 Pb 2 Cl 5 crystal, we use X-ray photoelectron spectroscopy (XPS) to measure binding energies of the core-level electrons for constituting atoms and to reveal the energy distribution of the valence electronic states. Our XPS measurements indicate very low hygroscopicity of the K 0.4 Rb 0.6 Pb 2 Cl 5 crystal surface and partial alteration from Pb2+ ions to Pb0 when using bombardment with 3 kV Ar + ions. To find peculiarities of filling the valence band and the conduction band of the K 0.4 Rb 0.6 Pb 2 Cl 5 crystal by partial densities of electronic states associated with the composing atoms, we apply different approaches for exchange-correlation potential using model K 0.5 Rb 0.5 Pb 2 Cl 5 solid solution. We have found that the finest agreement of the experimental and theoretical data is derived when in the calculating procedure we use Tran-Blaha modified Becke-Johnson (TB-mBJ) potential involving spin-orbit coupling and the Hubbard parameter U (TB-mBJ + U + SOC approach). The present theoretical TB-mBJ + U + SOC results predict that K 0.5 Rb 0.5 Pb 2 Cl 5 is a non-direct material with energy band gap of 4.167 eV. The optical properties of K 0.5 Rb 0.5 Pb 2 Cl 5 are elucidated theoretically in detail based on first-principles calculations within the TB-mBJ + U + SOC model. [Display omitted] • Optical quality K 0.4 Rb 0.6 Pb 2 Cl 5 crystal was grown by Bridgman method. • Electronic structure of the K 0.4 Rb 0.6 Pb 2 Cl 5 crystal was studied by XPS. • Total and projected densities of states of K 0.5 Rb 0.5 Pb 2 Cl 5 were calculated. • The calculations indicate that K 0.5 Rb 0.5 Pb 2 Cl 5 is an indirect gap material. • The main optical constants of K 0.5 Rb 0.5 Pb 2 Cl 5 were calculated. [ABSTRACT FROM AUTHOR]

• Two different stacking configurations ZnO/SePtS and ZnO/SPtSe are formed with very small lattice mismatch. • Both ZnO/SePtS and ZnO/SPtSe heterostructures are indirect semiconductors. • Band gap of heterostructures depend strongly on the interlayer distance and electric field. In the present study, we report the electronic properties of ZnO/PtSSe van der Waals heterostructure by using the density functional theory. Two different stacking configurations ZnO/SePtS and ZnO/SPtSe are formed with very small lattice mismatch and they are confirmed to be stable through ab initio molecular dynamic simulations. Obtained results demonstrate that both ZnO/SePtS and ZnO/SPtSe stacking configurations are indirect semiconductors. At the ground state, band gaps of ZnO/SePtS and ZnO/SPtSe configurations are respectively 0.895 and 0.448 eV, that quite smaller than that of both ZnO and PtSSe monolayers. The band gap of heterostructure depends strongly on the interlayer distance and electric field. Our findings not only give insight into the physical properties of heterostructures but also open up possibilities for their application in electronic devices. [ABSTRACT FROM AUTHOR]

We report on a coupled experimental and theoretical study of electronic and optical properties of quaternary Tl 2 CdGeSe 4 selenide. The experimental results reveal p-type electrical conductivity, the band gap energy of E g ​= ​1.52 ​eV for this compound and its non-direct nature. The latter experimental findings are confirmed theoretically by first-principles calculations based on density functional theory (DFT). The DFT calculations present that Se 4p states give principal contributions to the top of the valence band of Tl 2 CdGeSe 4 , whereas the central portion of the band is composed from Se 4p, Ge 4p and Cd 5s states, its lower portion is due to the input of Tl 6s states and the band bottom is dominated by Ge 4s states with smaller Se 4p states contributions. To verify the DFT calculations the XPS spectrum of valence electrons was measured for Tl 2 CdGeSe 4 crystal and XPS core-level spectra were studied to evaluate charge states of the composing atoms and features of the chemical bonding. The present theoretical data of calculations of the principal optical constants reveal that Tl 2 CdGeSe 4 selenide is a very promising semiconductor for optoelectronics. The Tl 2 CdGeSe 4 crystal was found to be rather stable with respect to treatment its surface with middle-energy Ar ​+ ​ions. The crystal surface of Tl 2 CdGeSe 4 demonstrates comparatively low hygroscopicity. Band dispersion along particular paths defined by high-symmetry k points of the first Brillouin zone for Tl 2 CdGeSe 4 adjusted with total densities of states of atoms constituting this compound. [Display omitted] • Electronic structure of Tl 2 CdGeSe 4 crystal was studied by XPS. • Tl 2 CdGeSe 4 possesses p-type of electrical conductivity. • Energy band gap of Tl 2 CdGeSe 4 is equal to 1.52 ​eV at room temperature. • Calculations within TB-mBJ ​+ ​SOC ​+ ​U model are in agreement with XPS measurements. • Main optical properties of Tl 2 CdGeSe 4 are calculated from first principles. [ABSTRACT FROM AUTHOR]

An ab − initio density functional theory (DFT) method has been used to study the elastic and thermodynamical properties of Fe 2 CoAl under hydrostatic pressure. In calculating mechanical and dynamical properties we have studied the elastic constants, anisotropy parameters, compressibility, ductility, plasticity, Debye temperature, vibrational energy, specific heat etc., under different applied pressures. While applying isotropic compressive pressure we have observed that elastic stiffness constants and hardness of the system increases almost linearly. However, the Pugh's ratio, B / G > 1.75 for all applied pressures except for 20 GPa ≤ P ≤ 40 GPa. The value of Poisson's ratio (σ) fluctuates around 0.25. Our calculations have revealed an admixture of covalent and ionic bond in Fe 2 CoAl. The compound under investigation is ductile in nature at all pressures considered. • Elastic constants and Hardness factor increases with the increase in the isotropic compressive pressure. • Under all applied pressures Fe2CoAl exhibits mechanical stability, elastic anisotropy and ductility. • Mixed nature of bonding with the presence of covalent and ionic is confirm from Poisson's ratio. • B/G ratio increases with increase in compressive pressures. However at 20–40 GPa it exhibit unusual trend. • Debye temperature, Specific heat, Vibrational energy & melting temperature are reported. [ABSTRACT FROM AUTHOR]

We present data of an experimental and theoretical study of the electronic band structure of ternary thallium lead iodide, Tl 4 PbI 6. The XPS studies yield high quality of the Tl 4 PbI 6 crystal, its low hygroscopicity, high stability regarding Ar+-ion-irradiation and comparatively strong degrees of covalency of the chemical Tl–I and Pb–I bonds. The latter peculiarity of the crystal under study is confirmed by first-principles calculations that are made within density functional theory (DFT). In the present DFT calculations we consider different models for exchange-correlation potential, and found that the best agreement between experiment and theory is observed when we use modified Becke-Johnson potential as elaborated by Tran-Blaha and involve also the Hubbard correcting parameter U and spin-orbit coupling (SOC) effect (the so-called TB-mBJ + U + SOC model). Employing the TB-mBJ + U + SOC calculations, we examine in detail partial and projected densities of states as well as band dispersions. The TB-mBJ + U + SOC calculations indicate that Tl 4 PbI 6 is a direct gap semiconductor with the energy band gap value of 2.347 eV. This theoretical value is in excellent agreement with that evidenced from experimental measurements of spectral allocation of the absorption coefficient in the range of the fundamental absorption edge yielding E g = 2.35 eV at room temperature. The present measurements of the second harmonic generation intensity of the Tl 4 PbI 6 crystal feature high prospective of its application in NLO devices for the studied range of the fundamental beam, approximately 1 μm. We have established that the ignoring of the SOC effect in the calculations predicts a nondirect semiconductor nature in Tl 4 PbI 6 being in conflict with the experiment. Based on the TB-mBJ + U + SOC model, we have calculated in detail the main optical constants of Tl 4 PbI 6 allowing to draw a conclusion that it is a good semiconductor for application in optoelectronic devices. [Display omitted] • Electronic structure of Tl 4 PbI 6 crystal is studied by XPS and DFT calculations. • Total and projected densities of states are computed within different approaches. • TB-mBJ + U + SOC calculations indicate that Tl 4 PbI 6 is a direct gap semiconductor. • The second harmonic generation intensity of the Tl 4 PbI 6 crystal has been measured. • The main optical constants are calculated within TB-mBJ + U + SOC model. [ABSTRACT FROM AUTHOR]

We report on experimental and theoretical studies of the electronic structure and optical properties of cesium lead chloride, CsPbCl 3. We employ X-ray photoelectron spectroscopy (XPS) to determine binding energies of core-level electrons of as-grown surface of CsPbCl 3 crystal and to measure the energy distribution of electronic states in the valence band region. To achieve the best agreement between the shape and energy positions of peculiarities of the XPS valence-band spectrum and the curve of total density of states, we used various approaches treating exchange-correlation potential. In particular, we have realized that the fair agreement is derived when performing calculations within a density functional theory framework using modified Becke-Johnson potential and treating the Hubbard parameter U and spin-orbit coupling. Using possibilities of this technique, we study in detail curves of partial density of states, energy band dispersion, and principal optical constants of CsPbCl 3. • Total and projected DOS of CsPbCl 3 are computed within different approaches. • Electronic structure of CsPbCl 3 crystal is studied experimentally by XPS and XES. • The mBJ + U + SO calculations fit well data of XPS and XES measurements. • Main optical constants of CsPbCl 3 are clarified by mBJ + U + SO calculations. [ABSTRACT FROM AUTHOR]

A single crystal of the non-centrosymmetric low-temperature modification of Tl 2 CdSnSe 4 (space group I 4 ‾ 2m) was grown, for the first time, and its electronic and optical properties were studied from both experimental and theoretical viewpoints. The Tl 2 CdSnSe 4 crystal was grown from the solution-melt by vertical Bridgman-Stockbarger method. Centimeter dimensions of the crystal allow its application in practice. The bandgap energy, E g = 1.32 eV, estimated from the optical absorption coefficient agrees well with the value of E g = 1.29 eV calculated from the photosensitivity measurements. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure and charge states of atoms composing the crystal under consideration. To detect the best agreement of the curve of total densities of states (DOS) with the experimental XPS spectrum of valence electrons of Tl 2 CdSnSe 4 , we performed theoretical calculations within a density functional theory (DFT) framework treating different models for exchange correlation (XC) potential. Our findings yield that the best agreement of the experimental and theoretical distributions of the valence electronic states is derived when using in the computation procedure for XC potential the modified Becke-Johnson potential in the form of Tran-Blaha (TB-mBJ), which involves also the Hubbard parameter U for strongly correlated d electrons and spin-orbit coupling (SOC) effect (TB-mBJ + U + SOC model). Based on this approach, we calculated partial densities of states, energy band dispersion, and main optical constants of Tl 2 CdSnSe 4. Importantly, the TB-mBJ + U + SOC model reveals the E g value which is close to those determined experimentally; therefore, scissors correction adjustment is not required when performing DFT calculations of the optical constants in such a case. The present experimental XPS measurements of the treated with middle-energy Ar+-ions Tl 2 CdSnSe 4 crystal surface and the theoretical DFT calculations data indicate, in spite of its rather hazardous chemical elements, thallium and cadmium, the crystal surface is rather stable and, coupled with suitable energy band gap, makes quaternary selenide under discussion a very promising material for using in thin film solar cells and optoelectronics as well as in highly efficient photocatalytic devices. Image 1 • For the first time, a crystal of non-centrosymmetric Tl 2 CdSnSe 4 (space group I 4 ‾ 2m) was grown. • Electronic structure of Tl 2 CdSnSe 4 crystal was studied by XPS. • Total and partial densities of states are calculated withinDFT framework. • Calculations within TB-mBJ + SOC + U model are in agreement with XPS measurements. • Principal optical constants of Tl 2 CdSnSe 4 are explored in detail. [ABSTRACT FROM AUTHOR]

The effect of biaxial strains ε b and electric field E on the electronic structure and optical properties of Janus monolayer PtSSe was studied by Density Functional Theory (DFT). A reasonable band gap of PtSSe was found to be 1.547 eV. In the infrared region, both biaxial strains and electric fields result in noticeable enhancement of the electronic structure as well as optical properties of PtSSe. Especially, under biaxial strains, the change of PtSSe band gap obeys the form of an asymmetric concave down parabola. This result confirms the existence of a maximum PtSSe band gap under biaxial strains ε b and the possibility of tuning PtSSe band gap to fit the requirement of the optoelectronic devices. The absorption rate in the visible light region of Janus monolayer PtSSe increases sharply and can be altered by strain engineering. Biaxial strain not only alters the absorption intensity but can also significantly shift the position of these absorption peaks. The present study provides additional information about the strain and electric field-induced electronic structure and optical properties of Janus monolayer PtSSe, which should be taken into account for better PtSSe-based devices. • Spin-orbital coupling effect lowers PBE band gap of 1.547 eV–1.388 eV for equilibrium Janus monolayer PtSSe. • Under biaxial strains, band gap of PtSSe has the form of asymmetric concave down parabola with one maximum. • The ability of biaxial strains to tune PtSSe band gap to some values suitable for optoelectronic materials. • Good enhancement of absorption and reflectivity by biaxial strains and electric fields in the infrared and visible regions. [ABSTRACT FROM AUTHOR]

In optoelectronics, the perovskite structures in layered form, also called pyrochlore have been demonstrated to be very largely favorable to the insertion of lanthanides ions, which led them to create new phosphor emitting in a broad spectral range going from visible to near infrared. The pyrochlore oxides such as La 2 Sn 2 O 7 have attained much interest due to their electronic and optical properties. In the present work, we used first-principle calculations to compute the electronic band structure and related optical properties for pure La 2 Sn 2 O 7 and La 2 Sn 2 O 7 doped with europium. Spin orbit coupling (SOC) and electron-electron interaction, both within the generalized gradient approximation plus Hubbard parameter U (SOC ​+ ​GGA ​+ ​U) approximations have been used within the framework of density functional theory (DFT). It was found that La 2 Sn 2 O 7 compound exhibits a direct band gap semiconductor with 2.7 ​eV. Doping of europium further decreases the band gap of pyrochlore of material up to 1.8 ​eV. Optical properties revealed that both compounds are potential materials for optoelectronic device fabrication. Image 1 • For computations of band structure and optoelectronic properties of La 2 Sn 2 O 7 and La 2 Sn 2 O 7 : Eu, GGA ​+ ​U ​+ ​SOC approximation has been used. • La 2 Sn 2 O 7 compound is direct bandgap semiconductor having gap 2.7 ​eV. • Doping of Europium further decreases the bandgap of pyrochlore of material up to 1.8 ​eV. • The results of optoelectronic properties revealed the investigated compounds can be used in optoelectronic device fabrication. [ABSTRACT FROM AUTHOR]

• We have considered two MoS 2 termed as 1T-MoS 2 and with a void is 1H-MoS 2. • BLYP and HSE has given the enhanced band gap of 1.83 eV and 2.35 eV, respectively. • The direct band gap in UV–vis range is highly desirable for optoelectronics. • Qualitatively our result from hybrid functional agrees well with the experiment. First, we have considered the two structures of MoS 2 : one with the hole as no atom is present in the center of the hexagonal cage termed as 1H-MoS 2 and the second one with the presence of an atom at the center of the hexagonal cage is 1T-MoS 2. We started the calculation employing generalized gradient approximation (GGA) and modified Becke Johnson (mBJ) within a framework of density functional theory (DFT). Interestingly, both GGA and mBJ have given the same low value of band gaps. Therefore, further calculation has been proceeded with the computationally more expensive hybrid functionals like Heyd-Scuseria-Ernzerhof (HSE) and Becke-Lee-Yang-Par (BLYP) within the LCAO-DFT approach. The calculated value of band gap from HSE is found to be 2.35 eV, compared with the previously reported band gap from GW method. However, the direct band gap calculated by BLYP method is 1.83 eV which is consistent with some of the experimental results. The presence of the direct band gap along K - K symmetry in UV–vis range predicts that 1H-MoS 2 is a potential candidate for the optoelectronic applications. [ABSTRACT FROM AUTHOR]

• Layered selenophosphate AgSbP 2 Se 6 single crystal was grown for the fist time in practical size. • Bridgeman method was used to synthesize high-quality AgSbP 2 Se 6 single crystal. • XPS core-level and valence-band spectra were recorded for pristine and Ar+ ion-irradiated surfaces. • Ab-initio DFT calculations of the structural, electronic and optical properties were done and compared with XPS data. • High anisotropy of the optical constants of the AgSbP 2 Se 6 crystal was shown theoretically. The Bridgman method was firstly applied for growing high quality layered selenophosphate AgSbP 2 Se 6 single crystals in practical size. The comparatively high hygroscopicity of AgSbP 2 Se 6 was observed by analyzing the XPS spectra of core and valence bands for both the pristine surfaces and the surfaces irradiated by Ar+ ions. The TB-mBJ + U calculations show that the valence band (VB) maximum of AgSbP 2 Se 6 is mainly contributed by Ag-4 d , Ag- s , and Sb/P/Se- p states. Meanwhile, the lower parts of the VB are mostly formed by the s -states of Sb, P, and Se atoms. The conduction band (CB) is dominated by Sb-4 p states, and minor contribution of Se-4 p , P-3 s , and P-3 p states. The absorption coefficient α (ω) is in the order of magnitude of 106 cm−1 in the energy range of 5–18 eV. Highly anisotropic optical properties of the AgSbP 2 Se 6 single crystal allow suggesting its application in optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this work, we investigate the electronic and optical properties of two configurations of haft-hydrogenated indium nitride monolayers H–InN and 2H–InN using first-principles calculations. Both H–InN and 2H–InN monolayers are semiconductors with indirect bandgap quite larger than that of pure InN monolayer. When the spin–orbit coupling was included, their bandgap is significantly reduced. Upon the hydrogenation, charge transfers from InN plane to functionalized species H in H–InN while a small amount of charge has been transferred from H atoms to InN plane in 2H–InN monolayer. Haft-hydrogenated InN monolayers can strongly absorb light in a wide range from visible light to infrared which opens possibilities for their application in optoelectronic devices. • Haft-hydrogenated InN monolayers are indirect semiconductors at equilibrium. • Band gaps of H–InN and 2H–InN monolayers are larger than that of pure InN monolayer. • There is a difference in the mechanism of charge transferring between H–InN and 2H–InN. • Haft-hydrogenated InN monolayers can strongly absorb both the visible and infrared lights. [ABSTRACT FROM AUTHOR]

We report on a complex study employing both theoretical and experimental methods with the aim of detailed elucidating the electronic and optical properties of potassium lead chloride, KPb 2 Cl 5. In particular, we employ possibilities of X-ray photoelectron spectroscopy (XPS) to measure for an optical quality KPb 2 Cl 5 crystal the binding energies of the core-level electrons as well as to elucidate the peculiarity of the energy distribution of the valence electrons associated with the atoms composing the compound under study. The XPS data reveal low hygroscopicity of the KPb 2 Cl 5 crystal and minor transformation of the Pb 2 + ions to Pb0 under treatment of the crystal surface with middle-energy Ar + bombardment. Furthermore, in the present work we apply different approaches for exchange–correlation potential to find how different factors affect the theoretical curve of total density of states in comparison with the valence-band XPS spectrum. The Tran–Blaha modified Becke–Johnson (TB-mBJ) potential, including spin–orbital effect and the Coulomb repulsion (Hubbard parameter U), is found to give the most reliable theoretical electronic structure of KPb 2 Cl 5. Based on these findings, detailed studies on the occupation of the valence and conduction bands by electronic states associated with the atoms composing the KPb 2 Cl 5 compound as well as on the optical properties are performed in the present work. • TB-mBJ+U+SO DOS is in excellent agreement with XPS data. • KPb 2 Cl 5 crystal possesses low hygroscopicity. • KPb 2 Cl 5 is transparent for photon energies smaller than 4 eV. • The absorption rate α (ω) of KPb 2 Cl 5 is high at energies of 6–8 eV. • The energy loss L (ω) is caused by thermalization at energies 5–10 eV. [ABSTRACT FROM AUTHOR]

• Fully hydrogenated InN monolayer is a semiconductor with indirect band gap of 2.591 eV. • Indirect-direct gap transition can occur in presence of biaxial strain and electric field. • Photocatalytic activity in the H–InN–H monolayer can be enhanced by strain or electric field. • Optical characteristics of the H–InN–H monolayer depends strongly on the biaxial strain. We investigate the electronic, photocatalytic and optical properties of a fully hydrogenated indium nitride H–InN–H monolayer under biaxial strain ε b and external electric field E using density functional theory. Our findings demonstrate that the H–InN–H monolayer is a semiconductor with an indirect energy gap of 2.591 eV. Under a biaxial strain or electric field, the indirect-direct band gap transition can occur and its band gap depends dramatically on the ε b and E. Our analysis of band edge alignment shows that the H–InN–H monolayer can possess photocatalytic activity for water splitting when an electric field or biaxial strain is applied. The optical characteristics of the H–InN–H monolayer depends greatly on the strain. The first optical gap of the H–InN–H monolayer is at the incident energy light of 3.320 eV and the tensile strain causes the first optical gap to shift towards the visible light region. [ABSTRACT FROM AUTHOR]

• Electronic properties of graphene and WSeTe are well preserved in heterostructure. • Optical absorption of heterostructure is enhanced in both visible and UV lights. • Graphene/WSeTe heterostructure tends to own high carrier mobility. • Depending on stacking configurations, heterostructure can form Ohmic or Schottky contact. • Electric field and vertical strain can tune p-type Schottky contact to n-type one or p-type Ohmic contact. The formation of the graphene-based van der Waals (vdW) heterostructures has shown great potential for designing novel electronic and optoelectronic nanodevices. Here, we construct the Graphene/WSeTe heterostructure and investigate its structural, electronic, optical and transport properties through first-principles calculations. We find that the electronic properties of both graphene and Janus WSeTe are well preserved in Graphene/WSeTe heterostructure because of the weak vdW interactions. The optical absorption of the Graphene/WSeTe heterostructure is enhanced in both regions of the visible and UV lights in comparison with that of the graphene and Janus WSeTe monolayers. The absorption coefficient of the Graphene/WSeTe heterostructure for the visible light can reach 5 × 104 cm−1, which is as twice as that of Janus WSeTe monolayer. Whereas, for the UV light, the absorption coefficient of such heterostructure can reach up to 105 cm−1. Moreover, the Graphene/WSeTe heterostructure tends to own a high carrier mobility for both electrons and holes as compared with single-layered Graphene. Especially, a band gap of about 10 meV at the Dirac cone of graphene in such heterostructures can be opened. Depending on the stacking configurations, the Graphene/WSeTe heterostructure can form the p -type Ohmic contact or p -type Schottky contact with a small Schottky barrier of 0.35 eV. Furthermore, our results demonstrate that the electric fields and vertical strains can be effectively used to tune both the contact types and the Schottky barrier height of Graphene/WSeTe heterostructure from the p -type Schottky contact to n -type one or to p -type Ohmic contact. Our results could provide a significant guidance for understanding the physical properties of the Graphene/WSeTe heterostructure towards nanoelectronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this work, we study the electronic, optical, and photocatalytic properties of fully hydrogenated GeC monolayer under strain engineering and external electric field using first-principles investigations. Our calculations demonstrate that at the equilibrium state, fully hydrogenated GeC monolayer is a indirect-semiconductor with band gap of 3.493 eV and it possesses photocatalytic characteristics for water splitting and in particular, photocatalytic activities can be enhanced by a negative electric field under ultraviolet light. We can control the band gap of fully hydrogenated GeC monolayer by biaxial strain or external electric field and semiconductor–metal phase transition happens at certain elongation of biaxial strain. Compared to pure monolayer GeC, the fully hydrogenation causes optical absorption peaks of GeC shifting to a higher energy region. While the optical spectra of the fully hydrogenated GeC monolayer are strongly dependent on the strain, the effect of the electric field on them is negligible. Our findings can provide useful information for the applicability of fully hydrogenated GeC monolayer in nanoelectronic devices and photocatalytic water splitting. • Fully hydrogenated GeC monolayer is a semiconductor with indirect bandgap of 3.493 eV. • Semiconductor–metal phase transition happens in H–GeC–H at certain elongation of biaxial strain. • Full hydrogenation causes optical absorption peaks of GeC shifting to a higher energy region. • H–GeC–H possesses photocatalytic characteristics for water splitting at equilibrium. • Photocatalytic activities in H–GeC–H can be enhanced by a negative electric field under ultraviolet light. [ABSTRACT FROM AUTHOR]

We report data of simulation within a density functional theory (DFT) framework, employing different approaches for exchange correlation (XC) potential, of the electronic structure and experimental verification of these band-structure calculations using X-ray photoelectron spectroscopy (XPS) of Cu 2 HgSnSe 4 crystal. The present studies indicate that fair good agreement of total density of states (DOS) with the valence band XPS spectrum of the Cu 2 HgSnSe 4 crystal is derived when DFT simulation is performed within modified Becke-Johnson (mBJ) potential and taking into consideration Hubbard correction parameter U and spin-orbit (SO) coupling (mBJ–U–SO approach). The mBJ–U–SO calculation reveals the best fit of total DOS to the experimental valence band XPS spectrum as well as a fairly good energy gap value for this compound. The present theoretical data indicate that the principal contributors to the Cu 2 HgSnSe 4 valence band are Se 4p states with their main input in upper and central portions of the band, while Sn 5 s and Hg 6 s states make prevailed contributions in its lower portion and the bottom is formed by Hg 5d states. The quaternary Cu 2 HgSnSe 4 selenide is a direct gap semiconductor. Based on the present mBJ–U–SO band-structure data, the main optical constants are calculated indicating that the compound under consideration is a very promising optoelectronic material. [ABSTRACT FROM AUTHOR]

• Monolayer GeSe is a semiconductor with a moderate indirect gap of 1.13 eV at equilibrium. • Semiconductor-metal phase transition occurs in monolayer GeSe at large compression biaxial strain. • Optical spectrum of monoalyer GeSe is highly anisotropy. • Monolayer GeSe possesses photocatalytic properties for water splitting at certain elongation of strain. In this work, we investigate the electronic and optical properties of monolayer GeSe and the possibility of enhancement the photocatalytic activities for the water splitting of monolayer GeSe through strain engineering using first-principles calculations. Our calculations indicate that monolayer GeSe is a semiconductor with a moderate indirect gap of 1.13 eV at equilibrium and we can control its band gap by biaxial strain. In the presence of biaxial strain ε b , the semiconductor-metal phase transition happens at large compressive strain of - 10 % and the indirect-direct gap transition occurs at ε b = 4 %. The optical spectrum of monolayer GeSe are highly anisotropic and biaxial strain can increase the absorption coefficient of monolayer GeSe up to about 6 × 10 5 cm−1. Our calculations demonstrate that monolayer GeSe possesses photocatalytic properties for water splitting at ε b = 5 % and we can enhance its photocatalytic activity by strain. [ABSTRACT FROM AUTHOR]

We report the effect of different approaches based on density functional theory (DFT) on first principles calculations of mechanical, electronic, and optical properties of ternary thallium tantalum sulfide, Tl 3 TaS 4 , a prospective semiconductor for application in surface acoustic wave devices. The present findings indicate that the best fit with experimental data is achieved when the DFT band-structure calculations are performed employing modified Becke–Johnson (mBJ) functional and including also in the consideration the Hubbard parameter U and spin–orbital effect (so-called mBJ+U+SO method). In particular, the application of the mBJ+U+SO method allows achieving an indirect band gap of 2.842 eV, which is close to that measured experimentally for Tl 3 TaS 4. The calculations indicate that the valence band maximum of Tl 3 TaS 4 is dominated by hybridized S-3p and Tl-6s states, while unoccupied hybridized S-3p and Ta-5d states yield the principal contributions to the conduction band minimum. With respect to the occupation of the valence and conduction bands by S-3p states, the present theoretical results are found to be in excellent agreement with experimental X-ray emission and absorption spectroscopy measurements of Tl 3 TaS 4. The main optical constants have been calculated within mBJ+U+SO method revealing big perspective of application of the of Tl 3 TaS 4 compound in optoelectronic devices. • Tl 3 TaS 4 is a ductile semiconductor with both covalent and ionic bond nature. • The mBJ+U+SO method reproduces reliable indirect Tl 3 TaS 4 band gap of 2.842 eV. • The loss energy L (ω) is rather high for energy ranging from 20 eV to 28 eV. • Tl 3 TaS 4 has high R (ω) in three energy ranges: 2–9 eV, 10–17 eV, and 18–24 eV. • Electromagnetic wave with energy higher 21 eV can propagate easier with k (ω) < 1. [ABSTRACT FROM AUTHOR]

In this work, we systematically investigate the electronic and optical properties of surface-functionalized GeC monolayer with F and Cl using density functional theory. Our calculations indicate that the surface functionalization of the GeC with F and C atoms leads to the disruption of the planar structure of the GeC monolayer and the surface-functionalized GeC monolayer has a low-bucking structure. At equilibrium, all four configurations of surface-functionalized GeC monolayer with F and Cl, i.e., F–GeC–F, F–GeC–Cl, Cl–GeC–F, and Cl–GeC–Cl, are direct semiconductors. Their band gaps vary from 2.839 eV to 3.175 eV which are calculated using Heyd–Scuseria–Ernzerhof (HSE) hybrid functional. Compared to the other configurations, the formation energy of F–GeC–F is the smallest, − 9. 097 eV , which implies that this configuration is the most likely to occur. We also used the Mulliken population analysis to estimate the internal charge distribution and transferred charge in the systems. The functionalization of the surface leads to the shifting the first optical gap of the material. The fully chlorination of GeC causes its absorption coefficient to increase significantly, up to 14. 912 × 1 0 4 cm−1 at the incident light energy of 13.173 eV. Besides, surface-functionalized GeC monolayer with F and Cl strongly absorbs light in the near ultraviolet region. Our calculation results provide detailed information on how to change the electronic and optical properties of monolayer GeC by surface functionalization, which has promising applications in opto-electronic devices. • All four configurations of surface-functionalized GeC monolayer with F and Cl are direct semiconductors. • Fully fluorinated F–GeC–F is the most likely to occur due to smallest formation energy. • The functionalization of GeC monolayer leads to the shifting the first optical gap of the material. • Surface-functionalized GeC monolayer with F and Cl strongly absorbs light in the near ultraviolet region. [ABSTRACT FROM AUTHOR]

Strain and external electric field effect on electronic structure of GeI 2 monolayer has been investigated using first principles calculations. The obtained results indicate that GeI 2 monolayer is an indirect semiconductor with band gap value of 2.188 eV. With biaxial strain, the band gap of considered material increases slightly with compression up to −6% and then it decreases and shows abrupt drop for strain −9% to −12%, whereas it just shows decreasing trend with tensile strain. In case of uniaxial strain, the band gap value increases nearly linearly under the effect of considered strain range. The weak external electric field has no significant effect on the band gap of GeI 2 monolayer, while with E = ± 0.6 (eV/Å/e), the band gap decreases considerably as I- 6s state in conduction band moves to the lower energy levels. [ABSTRACT FROM AUTHOR]

Both ternary sulfides, BaLa 2 S 4 and CaLa 2 S 4 , are found to possess a suitable band gap for absorbing visual light; moreover, the E C B and E V B are at right positions allowing the photo-generated electron/hole pairs to split water. The valence band of BaLa 2 S 4 (CaLa 2 S 4) are characterized by high peaks at −1 eV and −2 eV, and strong hybridization of S/La- p and Ba- s (Ca- s / p) states. These features help to enhance the separation of electron/hole pairs. Meanwhile, the conduction band has high peak at 3 eV, which is mostly constructed by hybridization of La- f , La- d and Ba- s (Ca- s / p) states. The conduction band's smooth dispersion is favorable for inter-band transition, thus it enhances the absorption rate of two considered compounds. The Ba/Ca/La–S bonds are predominantly covalent with electronegativity difference of about 31–38%. The large difference in effective mass between electron and hole enhances the separation of electron/hole pairs. CaLa 2 S 4 and BaLa 2 S 4 are p-type semiconductors with positive thermopower S of about 229.8 μ V / K and 234.8 μ V / K , respectively, at Fermi level (μ = 0) and temperature 300 K. Both materials show good thermoelectric performance for hole concentration being in the range 1 × 1019–1x1021 cm-3. The electronic figure of merit Z T e marks the highest value of about 0.95 and 0.97 at 2 × 1019 cm-3 for CaLa 2 S 4 and BaLa 2 S 4 , respectively. Image 1 • Band structure of Ba/CaLa 2 S 4 is suitable for light absorption and water splitting. • Electronic structure is favorable for electron/hole separation and charge mobility. • BaLa 2 S 4 and CaLa 2 S 4 are p-type semiconductors. • Figure of merit is 0.95 (CaLa 2 S 4) and 0.97 (BaLa 2 S 4) at 2x1019 cm-3. [ABSTRACT FROM AUTHOR]

The orthorhombic CaM 2 S 4 (M = Sc, Y) structures were investigated in the present work by first-principles calculations. CaM 2 S 4 (M = Sc, Y) compounds are found to be mechanically stable with the anisotropic elastic constants that make them to be stable with respect to uniaxial deformation (Young's moduli are 73.724 and 57.441 GPa for CaSc 2 S 4 and CaY 2 S 4 , respectively) but easy to be undergone to shear deformation (Poisson's ratios are 0.352 and 0.371 for CaSc 2 S 4 and CaY 2 S 4 , respectively). Due to their relatively small direct energy band gaps (1.89 eV for CaSc 2 S 4 and 2.26 eV for CaY 2 S 4), the two compounds are intriguing candidates for long-wave transparency applications. The high hybridization degree of Ca-d, Y/Sc-d and S-p orbitals results in a wide energy range between the valence highest band and the conduction lowest band enabling these compounds to interact actively with visible and ultra-violet waves. The high absorption rate of 12–18 × 105 cm−1 and good hole-electron separation make CaM 2 S 4 (M = Sc, Y) compounds to consider them good candidates for optoelectronic materials. Moreover, CaM 2 S 4 (M = Sc, Y) compounds show good ability in the process of photo-driven photocatalyst water splitting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mo-Zn@SiO 2 catalyst was simply prepared using a facile impregnation. • The Mo addition facilitates the formation of new catalytic active site of ZnMoO 4. • Adsorption energy is optimized over ZnMoO 4 surface to boost the PET depolymerization. • 5Mo-10Zn@SiO 2 exhibited 100% PET conversion and 90.4 % BHET at mild conditions. The unprecedented rise of polyethylene terephthalate (PET) waste demands sustainable solutions like catalytic glycolysis, a promising recycling technology that mitigates environmental harm. Herein, we synthesized a series of novel Mo-Zn@SiO 2 catalysts for PET glycolysis, derived from readily available rice husk ash via the wetness impregnation method. The optimum Mo-Zn@SiO 2 exhibits exceptional activity, achieving complete PET conversion and a 90.4 % yield of bis(2-hydroxyethyl) terephthalate (BHET) – one of the highest performances reported for heterogeneous catalysts. Density functional theory (DFT) calculations suggest that the formation of the ZnMoO 4 phase optimizes reactant and product adsorption energies, enhancing Lewis acidity and facilitating depolymerization. Kinetic studies reveal a low apparent activation energy (139.27 kJ/mol) for BHET production at 175–190 °C. Additionally, the catalyst demonstrates excellent recyclability, maintaining activity after five cycles. This work presents a viable and efficient strategy for PET waste valorization using a low-cost, readily available, and easily synthesized catalyst. [ABSTRACT FROM AUTHOR]

Tetra-strontium di-pnictide oxides Sr 4 Pn 2 O (Pn = Sb, Bi) belong to the oxypnictide-type materials attracting a lot of attention due their unique properties which are very advantageous for photovoltaic and thermoelectric applications. In spite of the fact that structural features of Sr 4 Pn 2 O (Pn = Sb, Bi) are known, however, their ability for applying in renewable energy has not been discovered yet. Therefore, in this study, the electronic structure of the two compounds was studied in detail that provides a key knowledge on understanding their optical and thermoelectric properties. The present data indicate that Sr 4 Pn 2 O (Pn = Sb, Bi) oxides have suitable band structure for photovoltaic applications with high absorption rates of 105–106 cm−1. The two compounds are also promising p-type thermoelectric materials with the figure of merit of about 0.45–0.55. Sr 4 Pn 2 O materials have strong nonlinear characteristics, making them highly promising for optical switching devices. Our research provides further insights into the features of Sr 4 Sb 2 O and Sr 4 Bi 2 O compounds besides current experimental data, demonstrating that both compounds have promising characteristics as optoelectronic materials. [ABSTRACT FROM AUTHOR]

In this paper, the structural, mechanical and optoelectronic properties of X 3 P 2 (X= Mg, Ca) have been investigated by using the first-principles calculation. The obtained results from the structural and mechanical properties reveal that our X 3 P 2 compounds are thermodynamically and mechanically stable. Moreover, the elastic constants and bulk modulus result imply that Mg 3 P 2 is ductile and Ca 3 P 2 is brittle. The calculated band structure reveals that our two compounds have a direct bandgap (Γ - Γ ) of 0.523 eV and 0.446 eV for Mg 3 P 2 and Ca 3 P 2 , respectively, from GGA. However, on using HSE06 hybrid functional the bandgap has been enhanced to 1.282 eV for Mg 3 P 2 and 1.092 eV for Ca 3 P 2 . Both compounds exhibit a high optical absorption in the visible region (≥ 10 5 cm - 1 ), making them potential candidate for photovoltaic applications. [ABSTRACT FROM AUTHOR]

We have investigated the electronic and optical properties of bilayer AA′ stacked hexagonal Boron Nitride (h-BN) with B and N vacancy defects by using density functional theory (DFT). The two single layers of h-BN are stacked in layers to form the h-BN bilayer. The inter-layer interaction between the two layers of h-BN bilayer is governed by the introduction of van der Waals potential (vdW). The calculated energy band gap for the pristine h-BN bilayer is found to be ~4.56 eV. The density of states (DOS) and electronic band structure showed that both Boron and Nitrogen vacancies in bilayer h-BN results in magnetic ground state. Considering the presence of 1,3,4-Boron vacancies, half-metallic character is observed. However, the 2 Boron vacancy resulted in metallic character. The bilayer with 1,2,3,4-Nitrogen vacancies preserved the semiconducting band gaps of different width in both the spin channels which are significantly less than the pristine band gap. Also, B and N vacancy induces ferromagnetism in the h-BN bilayer. The maximum total magnetic moment for the Boron vacant system is 6.583 μ B in case of 4-Boron vacancy defects. In case of Nitrogen vacancy system it is found to be 3.926 μ B for 4-Nitrogen vacancy defects. The optical response of the system is presented in terms of the absorption coefficient, refractive index and dielectric constant for pristine as well as for the defective configurations. Negative value of dielectric constant for Boron vacant system has been observed in the energy range 0.9–1.4 eV and for Nitrogen vacant system the energy range 0.5–0.8 eV. These results open an opportunity for it's utilization in the negative index optical materials. The current study shows that B and N vacancies in bilayer h-BN could have potential applications in nano-structure based electronics, optoelectronics and spintronic devices. • We have analyzed the electronic and optical properties of bilayer h-BN on defects at B/N sites. • The interactions between the layers are treated with van der Waals interactions within GGA. • The vacancy defects changed the profile of electronic structure and induced ferromagnetism. • The total magnetic moment at B vacant system is high as compared to N vacant system. • The B vacant system exhibit half-metallic behaviour, promising for spintronic applications. [ABSTRACT FROM AUTHOR]

Hanoi, the capital of Vietnam, frequently experiences heavy air pollution episodes in the winter, causing health concerns for the 7.5 million people living there. Spatial-temporal variations in PM 2.5 levels can provide useful information about the sources and transportation of PM 2.5. However, the published spatial-temporal data in the area are limited. In this research, PM 2.5 concentrations at two sites in Hanoi and a site in Thai Nguyen (60 km north of Hanoi) were observed from October 2017 to April 2018, using newly available low-cost sensors. Hourly concentrations of PM 2.5 at the three sites were similar on average (57.5, 54.9, and 53.6 μg m−3) and clearly co-varied, suggesting remarkable large-scale effects. The contribution of long-range transport and meteorological factors on PM 2.5 levels were investigated with a machine learning technique based on a random forest (RF) algorithm and concentration weight trajectory (CWT). The results showed that the contribution of long-range transport from the north and northeast to local PM 2.5 levels was significant. Moreover, weather normalized PM 2.5 concentrations and partial plots of meteorological factors on the levels of PM 2.5 showed that meteorological conditions play a significant role in the formation of winter haze events. Image 1 • Observation of PM 2.5 levels at three sites using sensors. • Moderate to good correlation factors among PM 2.5 at sites showed regional effects. • Partial effects of meteorological and temporal factors on PM 2.5 were determined by a machine learning approach. • Contribution of meteorological factors on haze revealed by weather normalized PM 2.5 • PM 2.5 long-range transport investigated by CWT and partial trajectory correlation. [ABSTRACT FROM AUTHOR]

In this paper, the HfS 2 monolayer electronic structure and optical properties under vertical strains are theoretically explored using density functional theory (DFT) calculations. The HfS 2 single layer dynamical stability is examined by calculating the phonon dispersion curves. Based on our simulation, the studied two-dimensional (2D) material is an indirect gap semiconductor with band gap value of 1.545 eV. The band gap engineering can be effectively realized by applying the vertical strains. In this regard, the indirect-direct gap transition in the monolayer at hand may be induced by compressive strains with strength from − 9%. Consequently, significant changes of the optical properties may be obtained, in particular when visible to middle ultraviolet regime radiation is incident on the sample. The HfS 2 monolayer displays promising optoelectronic applicability with a high absorption coefficient reaching to 49.600 (10 4 /cm) and 88.122 (10 4 /cm) in the visible and ultraviolet regime, respectively. Results may suggest an effective approach to modify the optoelectronic properties of the HfS 2 single layer at the time of designing its practical applications. [ABSTRACT FROM AUTHOR]

In this work, detailed investigations of the electronic and optical properties of a Janus Ga 2 STe monolayer under a biaxial strain and electric field have been performed using density functional theory. Via the phonon spectrum and ab-initio molecular dynamics simulations, the dynamical and thermal stabilities of the Janus Ga 2 STe monolayer are verified. Our obtained results showed that the Janus Ga 2 STe exhibits a direct semiconducting characteristic and its band gap depends greatly on the biaxial strain. While both the electronic and optical properties are very weakly dependent on the electric field, strain engineering can cause a direct–indirect band gap transitions in the Janus Ga 2 STe. At equilibrium, the optical absorbance of the Janus Ga 2 STe monolayer is activated in the infrared light region of about 0.9 eV, which is close to its band gap value. The main peak of the optical absorbance spectrum is located in the ultraviolet light region with an absorbance intensity of 11.914 × 104 cm−1 may be increased by compression strain. In particular, the absorbance intensity of the Janus Ga 2 STe monolayer increases rapidly in the visible light region, reaching 4.810 × 104 cm−1 and can be altered by strain. Our results not only show that the Janus Ga 2 STe monolayer has many promising applications in opto-electronic devices but also motivates experimental works on Janus structures in near future. • Janus Ga 2 STe monolayer is dynamically and thermal stable at room temperature. • Janus Ga 2 STe is a direct semiconductor and its band gap can be strongly altered by strain. • The direct-indirect gap transitions were found in strained Janus Ga 2 STe monolayer. • Electronic and optical properties of Ga 2 STe depend weakly on external electric field. • Ga 2 STe has wide absorption spectrum, stretching from infrared to ultraviolet light region. [ABSTRACT FROM AUTHOR]

It is well known that the optoelectronic properties of two-dimensional (2D) materials may vary considerably by changing the layers number as well as the stacking order. In this work, the structural, electronic and optical properties of Cadmium oxide (CdO) monolayer and bilayers with different stacking orders are explored using first principles calculations. Simulations assert that the CdO single layer is a direct gap semiconductor with energy gap of 0.811 (1.883) eV as predicted by PBE(HSE06) functional. The bilayer formation leads to the considerable reduction of this parameter, however the direct nature is retained. Results show that the bilayers band gap depends strongly on the stacking order. The charge density maps are indicative of either chemical or physical interlayers interaction of the CdO bilayers. Our optical properties calculations show that the bilayers are more sensitive to the incident light than the monolayer, where the in-plane polarized light can excite more effectively the electronic transitions from the valence band to the conduction band. Additionally, the prospective optoelectronic applicability of the CdO 2D materials studied here is also demonstrated through the capability of absorbing a wide range of electromagnetic radiation with large absorption coefficient. Results reported here are useful covering the lack of knowledge on the optoelectronic properties of the CdO nanomaterials, and may be good guidance for the design of practical applications. • CdO monolayer shows graphene-like planar structure, a buckling height appears in the bilayers. • CdO single is a direct semiconductor material with a band gap of 0.811 (1.883) eV as predicted by PBE(HSE06) functional. • Bilayer formation leads to the considerable reduction of the energy gap, retaining the direct gap feature. • Electronic transitions probability is increased with increasing the layer number. • CdO monolayer and bilayers show good absorption properties of the sunlight. [ABSTRACT FROM AUTHOR]

• Electronic structure of Cu 2 HgGeSe 4 crystal is studied by using XES and XPS techniques. • Total and partial densities of states are computed within TB-mBJ + SOC + U approach. • Calculations within TB-mBJ + SOC + U approach are in fair agreement with XES and XPS. • Main optical constants of Cu 2 HgSnSe 4 are explored in detail. We report on band-structure calculations, within a density functional theory (DFT), of total and projected densities of states as well as experimental investigation of the electronic structure of Cu 2 HgGeSe 4 using X-ray photoelectron spectroscopy (XPS) method. The present DFT calculations bring an excellent correspondence of the total density of states curve to the experimental XPS spectrum of valence electrons employing in the calculations modified Becke-Johnson (mBJ) functional in the type of Tran and Blaha (TB-mBJ) for exchange–correlation (XC) potential and, in addition, the spin–orbit coupling (SOC) effect and Hubbard correction parameter U (TB-mBJ + SOC + U technique). The basic optical constants are calculated within the TB-mBJ + SOC + U technique and they reveal a big perspective of using the Cu 2 HgGeSe 4 compound in optoelectronic appliances. [ABSTRACT FROM AUTHOR]

In this paper, we report the results of theoretical calculation on the structural, electronic, optical and thermoelectric properties of 2T-Al 2 MgC 2 compound. These properties are investigated through the calculations based on the density functional theory and Boltzmann transport theory. Our calculated structural parameters are in very good agreement with other experimental and theoretical values available in the literature. Studied material has an indirect band gap K Γ − L of 2.579 eV calculated with the Tran-Blaha modified Becke-Johnson exchange (mBJ) potential, which is higher than those obtained with the standard functionals. 2T-Al 2 MgC 2 compound has a wide absorption band from the high energy zone of visible to ultraviolet region. While it shows the transparent properties when the photon energy is lower than 2.579 eV and beyond 25 eV. Considered ternary is a good thermoelectric material with high thermopower and the figure of merit close to unity. At the hole (electron)-doping level of 1 × 10 17 (cm−3), the calculated figure of merit is 0.98 (0.97). Image 1 [ABSTRACT FROM AUTHOR]