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Two-dimensional (2D) piezoelectric nanomaterials have widely been studied recently due to their promise for various applications in technology. Investigation of vertical piezoelectricity will contribute to a deeper understanding of the intrinsic mechanism of piezoelectric effects in the 2D structures. In this paper, we report a first-principle study for the structural, electronic, piezoelectric, and transport properties of new-designed Janus WSi Z 3 H (Z = N, P, and As) monolayers. The structural stability of WSi Z 3 H is theoretically confirmed based on the energetic, phonon dispersion, and also elastic analyses. At the ground state, while WSiN 3 H is an indirect semiconductor, both WSiP 3 H and WSiAs 3 H are predicted to be direct semiconductors with smaller bandgaps. When the spin-orbit coupling effects are taken into account, a large valley spin splitting is observed at the K point of WSi Z 3 H materials. Interestingly, a giant Rashba spin splitting is found in WSiP 3 H and WSiAs 3 H with Rashba constant α R up to 770.91 meV Å. Additionally, our first-principles study indicates that Janus WSi Z 3 H monolayers are piezoelectric semiconductors with high out-of-plane piezoelectric coefficient | d 31 | , up to 0.15 pm/V, due to the broken mirror symmetry. Besides, with high electron mobilities and also possessing direct band gaps, WSiP 3 H and WSiAs 3 H monolayers are favorable for applications in optoelectronics. [ABSTRACT FROM AUTHOR]

In this work, magnetism engineering in the Gallium phosphide (GaP) monolayer is explored in order to create new two-dimensional (2D) spintronic materials. Pristine monolayer is a non-magnetic indirect gap semiconductor with band gap of 1.61(2.49) eV obtained from the PBE(HSE06)-based calculations. Both ionic and covalent characters form the Ga-P chemical bond, which are generated by the charge transfer and electronic hybridization, respectively. It is found that creating a single Ga vacancy magnetizes significantly GaP monolayer (V G a), where a large total magnetic moment of 2.88 μ B is obtained and magnetic properties are produced mainly by P atoms around the defect site. Meanwhile, GaP monolayer is metallized under effects of single P vacancy, where the non-magnetic nature is preserved. Significant magnetization is also achieved by doping with alkali (Li and Na - Li G a and Na G a) and alkaline earth (Be and Mg - Be G a and Mg G a) metals. In these cases, total magnetic moments of 2.00 and 1.00 μ B are obtained, respectively. Besides, doping with S (S P) and Se (Se P) atoms induces weaker magnetization, which is reflected in smaller total magnetic moments of 0.60 and 0.96 μ B , respectively. Interestingly, the half-metallicity is observed in V G a , Be G a , and Mg G a systems; while Li G a , Na G a , and Se P systems exhibit the diluted magnetic semiconductor behavior. In contrast, Cl (Cl P) and Br (Br P) impurities induces no magnetism in GaP monolayer. However both Cl P and Br P systems have energy gap of 0.76 eV, which corresponds to a reduction of the order of 52.80% from that of pristine monolayer. Results presented herein may introduce efficient methods to modify the GaP monolayer electronic and magnetic properties, such that new multifunctional 2D materials can be created for spintronic and optoelectronic applications. [ABSTRACT FROM AUTHOR]

First principles calculations were performed to investigate the photocatalytic behavior of 2D Janus monolayer HfSO at equilibrium and under the influence of strains and external electric fields. A thorough investigation of the structural, electrical, and optical properties of HfS2, HfO2, and HfSO was also provided. In 1T configuration, the three 2D Janus monolayers were found to be dynamically and thermally stable. The phonon dispersion of strained HfSO demonstrates the material's dynamical stability. The Young's modulus and Poisson's ratio of 1T HfSO reveal isotropic elastic characteristics on the x–y plane as well as HfSO's tendency to expand transversally under compressive strains. The three monolayers HfS2, HfO2, and HfSO have appropriate band gaps for redox reaction activation, and the corresponding band gaps are 2.48 eV, 6.50 eV, and 2.98 eV. Furthermore, the extremely anisotropic mobility of electrons and holes on the x–y plane benefits the electron–hole separation process. The equilibrium HfSO has a moderate absorption rate of 5 × 105 cm−1 in the visible range, with the intensity increasing by around 20% with a −6% biaxial strain. Consequently, the absorption range is moved to a lower energy level, covering the majority of visible regions. The 2D Janus HfSO's asymmetric surface electrical characteristic is also discovered. As a result, HfSO is suggested to be a very promising material for application in optoelectronic devices, as well as photovoltaic and water-splitting catalysts, which can be greatly enhanced by appropriate stresses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices. [ABSTRACT FROM AUTHOR]

In the present work, we consider systematically the electronic and optical properties of two-dimensional monolayer germanium monosulfide (GeS) under uniaxial strains along armchair (AC-strain) and zigzag (ZZ-strain) directions. Our calculations show that, at the equilibrium state, the monolayer GeS is a semiconductor with an indirect band gap of 1.82 eV. While monolayer GeS is still an indirect band gap semiconductor under ZZ-strain, an indirect–direct energy gap transition can be found in the monolayer GeS when the AC-strain is applied. The optical spectra of the monolayer GeS have strong anisotropy in the investigated energy range from 0 eV to 8 eV. Based on optical properties, we believe that the monolayer GeS is a potential candidate for applications in energy conversion and optoelectronic technologies. [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]

• 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]

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]

Successful synthesis by vertical solidification (Bridgman - Stockbarger) technique is reported for a centimetre size noncentrosymmetric TlSbP 2 Se 6 crystal (for the first time) with excellent transparency at 0.75–10 μm that is very useful for its practical application. The high homogeneity and stoichiometric composition have been confirmed by SEM-EDX. Next we determined the crystal structure, electronic properties by use of both experimental and theoretical methods. The layered structure of the crystal provides strong anisotropy of physical properties. Special attention was placed for an explanation of the source of the layer architecture and its effect on the strong anisotropy of properties. In particular, employing the XPS method, we have measured for the TlSbP 2 Se 6 crystal the binding energies of core-level electrons as well as energy distribution of the valence electrons for both pristine and treated with Ar+ ions surfaces and shown that the TlSbP 2 Se 6 crystal surface is highly hygroscopic. For interpretation and explanation of the measured XPS valence band spectrum, we examined different approaches for exchange correlation (XC) potential with a density functional theory (DFT) framework and found that the best agreement of the shapes of the XPS spectrum and the curve of total density of states is derived when the band structure calculations were made employing for XC potential modified Becke-Johnson (mBJ) functional in the form of Tran and Blaha. Based on the results of XPS measurements and calculations, we explained the contributions of individual ions to the band structure and convincingly demonstrated that ionic components of the chemical Sb–Se (P–Se) bonds in the TlSbP 2 Se 6 crystal are similar to those of its silver-bearing counterpart AgSbP 2 Se 6. The present DFT calculations within TB-mBJ + U + SO approach reveal that the TlSbP 2 Se 6 compound is a non-direct semiconductor with E g = 1.765 eV confirmed by UV-VIS measurements (1.75 eV). The main optical constants are explored in detail for TlSbP 2 Se 6 , as based on TB-mBJ + U + SO calculations. Finally, the result of the XPS investigations of the crystal surface after treatment with Ar+ ions supported by the calculations of the structure indicates its high chemical and mechanical resistance, which, despite the dangerous element contained in the composition, indicates its safe use in optoelectronic devices. • High quality TlSbP 2 S 6 single crystal with transparency at 0.75–10 μm has been grown for first time. • The appearance of the layered architecture of structure is explained. • TlSbP 2 Se 6 is a promising IR NLO material with band gap of 1.75 eV. • Electronic properties were probed employing the XPS and XES methods. • Excellent agreement with DFT approximation (TB-mBJ + U + SO) was achieved. [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]

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]

Abstract XLa 2 S 4 (X = Ba; Ca) sulfides are widely used as materials for application in infrared region, but the theoretical studies on these materials are limited and incomplete. Using the augmented plane wave + local orbitals (APW + lo) method with different approximation functionals, we performed a systematic theoretical study on the structural, electronic, optical and elastic properties of XLa 2 S 4 (X = Ba; Ca). The calculated band gaps E g of BaLa 2 S 4 and CaLa 2 S 4 using the modified Becke-Johnson potential in the form proposed by Tran and Blaha (TB-mBJ approximation) are 2.63 eV and 2.80 eV, respectively, which agree well with the experimental data. The dependence of main optical properties such as the absorption coefficient, refractive index, extinction coefficient, electron energy-loss spectrum, and optical reflectivity of BaLa 2 S 4 and CaLa 2 S 4 on photon energy were calculated and discussed. The independent elastic constants and elastic moduli of BaLa 2 S 4 and CaLa 2 S 4 were determined by precise calculations. The degree of elastic anisotropy of XLa 2 S 4 (X = Ba; Ca) has been evaluated from 3D curved surface representations of Young's modulus. The results of the band structure and optical properties calculations suggest that XLa 2 S 4 (X = Ba; Ca) are potential materials for solar conversion applications. Highlights • The TB-mBJ band gaps of BaLa 2 S 4 and CaLa 2 S 4 are 2.63 eV and 2.80 eV, respectively, which agree well with the experimental data. • Optical properties of BaLa 2 S 4 and CaLa 2 S 4 studied in detail for a wide range of photon energy. • The independent elastic constants and elastic moduli of BaLa 2 S 4 and CaLa 2 S 4 were determined. • XLa 2 S 4 (X = Ba; Ca) are considered potential materials for solar energy conversion 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]

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]

Abstract In this work, we consider the effect of biaxial ɛ b and uniaxial ɛ ac ∕ zz strains on electronic properties and optical parameters of monolayer SnS using first-principles calculations. Our calculations show that the monolayer SnS is a semiconductor with an indirect energy gap of 1.63 eV at the equilibrium state. While an effect of tensile strains on bandgap is quite small, the bandgap of monolayer SnS depends strongly on the compressive strains, especially a semiconductor-metal phase transition is occurred due to the uniform compressive biaxial strain at −14 % elongation. The optical spectra of the monolayer are high anisotropic, and the absorption coefficient of monolayer SnS tends to increase in the presence of compression strains, while the tensile strains reduce the absorption coefficient of the monolayer SnS. We believe that the phase transition and extraordinary optical properties of the strained monolayer SnS will make it become a useful material in nanoelectromechanical devices and optoelectronic applications. Highlights • Monolayer SnS is a semiconductor with an indirect bandgap of 1.63 eV at the equilibrium state. • Compression biaxial strain can lead to the semiconductor-metal phase transition in monolayer SnS. • The optical spectra of SnS monolayer are highly anisotropy. • Absorption coefficient of monolayer SnS tends to increase in the presence of compression strains. [ABSTRACT FROM AUTHOR]

• The electronic structure of Mg 2 Si 1-x Sn x compounds is studied in detail from first principles. • The transport parameters (S, σ, κ e) of Mg 2 Si 1-x Sn x were elucidated. • The energy band gaps decrease with increasing Sn content in Mg 2 Si 1-x Sn x. • Heavy conduction band, CB H , and light conduction band, CB L , are formed in Mg 2 Si 1-x Sn x. • n -type Mg 2 Si 1-x Sn x compounds are found to be excellent TE materials for practical use. Mg 2 X (X = Si, Sn, and Ge) based compounds have attracted great attention due to their numerous advantages in thermoelectric (TE) applications. In particular, to date, ternary Mg 2 X based alloys have been recognized the most extensively explored thermoelectric systems, because these compounds exhibit excellent TE properties. Using the first-principles methods, we accomplished mutual alloying of Mg 2 X (Si, Sn) compounds in order to develop their efficiency in expanding the range of TE applications. The electrical, structural, and thermoelectric properties of Mg 2 Si 1-x Sn x were studied. With increasing Sn content in Mg 2 Si 1-x Sn x compounds, our calculations reveal a linear increase of the unit-cell constant and a decrease of the bulk modulus. Besides, this Sn content increase also leads to a decrease in the energy gap values. Moreover, taking into account the carrier concentration impact on the TE properties, we can state that the n -type Mg 2 Si 1-x Sn x alloys reveal higher TE behavior resulting from the large effective mass m * (in terms of electron mass m 0) and higher DOS at the Fermi level. In particular, the supreme ZT value is about 1.4 for n -type Mg 2 Si 0.375 Sn 0.625 at 700 K with n = 4x1020 cm−3, where the combination of elevated power factor and small thermal conductivity values play a key role. The supreme value of ZT is 0.9 for p-type Mg 2 Si at 700 K. These results suggest that Mg 2 Si 1-x Sn x alloys could be considered exceptional TE materials for practical use due to their fair TE performance. [ABSTRACT FROM AUTHOR]

Surface modification has been widely employed to modify fundamental properties of two-dimensional (2D) materials and create new multifunctional candidates for practical applications. In this work, the effects of chemical functionalization on silicon germanide (SiGe) monolayer have been investigated using first-principles calculations. Pristine monolayer exhibits good dynamical stability and a Dirac cone at K point in the band structure. The nearly semimetallic nature and absence of magnetic properties represent great challenge for SiGe monolayer applications. Our results show that these deficiencies can be overcome with oxygenation and hydrogenation processes. Oxygen (O) and hydrogen (H) atoms prefer to be adsorbed on-top of bridge and Si atom, respectively. The non magnetic nature is preserved under oxygenation, however a significant band gap up to 1.35 eV can be introduced depending on the O concentration. Meanwhile, the ferromagnetic semiconducting is induced by hydrogenation, where magnetic properties are produced mainly by Ge atoms, regardless the H coverage. In fact, efficient methods have been introduced to make SiGe monolayer promising 2D material for optoelectronic and spintronic applications through adsorbing O and H atoms. [ABSTRACT FROM AUTHOR]

On the basis of first-principles projector augmented wave method, the effects of point defects and doping on the MgO monolayer electronic and magnetic properties are studied. Pristine monolayer is an insulator two-dimensional (2D) material with a band gap of 3.37 eV. Various defect types including single vacancy of Mg (V M g) and O (V O), Mg+O divacancy (V M g O), and antisites formed by replacing one Mg atom by one O atom (O M g), one O atom by one Mg atom (M g O ) and exchanging position of one Mg-O pair (M g ↔ O) have been considered. Material becomes ferromagnetic semiconductor upon creating a Mg single vacancy, whose magnetic properties are produced mainly by O atoms closest to the defect site. Meanwhile, the non-magnetic nature is preserved in other cases with a significant variation of the electronic band gap. Similarly, non-magnetic features are obtained when substituting one Mg atom by one Si or Ge atom (S i M g and G e M g ). In contrast, the feature-rich half-metallicity (semiconductor spin-up state and metallic spin-down state) is induced when doping at O site (S i O and G e O ). In these cases, magnetism is generated mainly by dopants with a small contribution from their neighbor O atoms. This work provides important insights on the MgO monolayer purposing efficient approaches to widen its working region for optoelectronic applications, as well as make it prospective to be applied as spin-filter and generate spin current in spintronic devices. [ABSTRACT FROM AUTHOR]

• Janus Al 2 SSe monolayer is dynamically and thermally stable at room temperature. • Al 2 SSe monolayer is an indirect semiconductor with band gap of 2.079 eV. • Electronic properties of Al2SSe monolayer depend strongly on biaxial strain. • Al 2 SSe monolayers can strongly absorb the near-ultraviolet light. • Optical absorbance intensity of Al2SSe can be enhanced by strain. Motivated by the successful experimental fabrication of Janus structures recently, a systematic study of the structural, electronic, and optical properties of the Janus Al 2 SSe monolayer has been done through first-principles approach. Through phonon spectrum and ab initio molecular dynamics analysis, Al 2 SSe was confirmed to be dynamically and thermally stable. The Janus Al 2 SSe monolayer exhibits a semiconducting characteristics with indirect band gap of 2.079 eV at equilibrium. While biaxial strain can significantly alter band gap, the influence of electric field on electronic as well as optical properties is negligible. The optical absorbance of Al 2 SSe is activated in the visible light region and its intensity can be enhanced by strain. Our findings only highlight the physical properties but also provide an effective way for tuning the electronic and optical properties of Al 2 SSe monolayer by strain engineering. [ABSTRACT FROM AUTHOR]

[Display omitted] • Optoelectronic properties of XAgSe 2 (X = Eu and Er) are explored. • The electronic properties reveals that the compounds are indirect band gap semiconductors. • The optical properties and their related constants are studied for the first time. Here in we have reported on band-structure calculations, within First principles density functional theory (DFT) based calculations are performed for the investigation of the electronic and optical properties of silver lanthnide dichalcogenides XAgSe 2 (X = Eu and Er). The current calculations (DFT) possess an admirable correspondence of the total density of states with the optical properties using the generalized gradient approximation. Additionally, the spin–orbit coupling (SOC) effect and Hubbard correction parameter U (GGA + SOC + U), (GGA + U) that is useful for the strongly correlated electron system The basic optical constants are calculated within the GGA + SOC + U technique and they expose a big viewpoint of using the XAgSe 2 (X = Eu and Er) compound in optoelectronic appliances. [ABSTRACT FROM AUTHOR]

In present work, the strain effect on the electronic, optical and thermoelectric properties of the Boron phosphide (BP) monolayer have been systematically investigated using first-principles calculations. Simulations show that the single layer at hand is a direct semiconductor possessing a K − K band gap of 0.910 eV. This parameter increases when switching the strain nature from compressive to tensile. Results indicate that the optical absorption in the visible and near ultraviolet regimes may be enhanced by the lattice compression. Finally, the thermoelectric properties are considered by determining the Seebeck coefficient, electrical conductivity, electronic thermal conductivity and dimensionless figure of merit. Compressive strains lead to the improvement of the thermoelectric performance with the figure of merit up to 0.665 at room temperature. However, chemical potential variations may favor the figure of merit increasing up to values close to unity. The findings may provide important information to estimate the prospective applicability of the BP monolayer in the optoelectronic and thermoelectric technologies. • The BP monolayer is a direct semiconductor with good stability. • Electronic band gap shows sensitivity to the external strain, which increases by switching from compression to tension. • Compressive strains enhance the absorption from visible to near ultraviolet regions. • Figure of merit increases with the compressive strains. • Figure of merit can reach values close to unity by the chemical potential variation. [ABSTRACT FROM AUTHOR]

We present a comprehensive first-principles study on the physical properties of the lead-free double perovskites halides, Cs 2 LiYX 6 (X = Br, I). Our calculated results of lattice constants (a 0) of both compounds are in nice agreement with the reported experimental and theoretical investigations. The predicted band structures of both compounds show that both compounds are wide and direct bandgap materials at T-point. Similarly, our computed results of elastic analysis predict that the investigated materials are elastically anisotropic, mechanically stable, and of ductile nature. The results of optical parameters such as absorption coefficients, refractive index, optical conductivity, optical reflectivity, electron energy loss, and extinction coefficients for an energy range of 0–14 eV are calculated and analyzed as well. The analysis of obtained results of wide and direct band gaps as well as optical parameters particularly absorption coefficients reflect the suitability of both compounds for ultraviolet high-frequency device applications. • Cs 2 LiYBr 6 , and Cs 2 LiYI 6 compounds are wide and direct bandgap materials at T-point. • The investigated materials are elastically anisotropic, mechanically stable, and of ductile nature. • Cs 2 LiYBr 6 , and Cs 2 LiYI 6 compounds are dominated with ionic bonding. • Both compounds can be utilized for ultraviolet high-frequency device applications. [ABSTRACT FROM AUTHOR]

In this work, we have investigated the electronic, magnetic and optical properties of hexagonal monolayer ZnSe compound via atomic vacancy defects by using DFT-1/2 approximation. Our calculations revealed that monolayer ZnSe is a direct band semiconductor with the transition along Γ−Γ symmetry. The measured value of electronic band gap is ~2.68 eV in very good agreement with the experimental results. The optical spectra of hexagonal ZnSe is measured along the x- and z-axis as a function of wave length (λ). The absorption coefficient is highly anisotropic with majority of optical response lies along x-axis. The maximum absorption intensity is ~6.5 × 104cm−1 in violet-blue range of UV–Vis spectra. Our results of optical spectra comprehended the experimental results. Image 1 • Electronic, Magnetic and Optical properties of hexagonal ZnSe has been calculated from LCAO method. • DFT-1/2 incorporating the half-occupation state of Slater improve the band gap. • The defect at Zn sites induced ferromagnetism. • The maximum absorption intensity falls within the violet-blue range of UV–Vis spectra. [ABSTRACT FROM AUTHOR]

Transparent conducting oxides (TCOs) of semiconductor family gained significant attention due to increasing trends in the optoelectronic and thermo-physical applications. In current work, we reported electronic, optical, transport and thermodynamical properties of spinel oxides ZnGaO 2 , [ZnGaO 2 :Mn3+ and [ZnGaO 2 :Rh3+ compounds. Based on DFT, we employed first-principles calculations implemented in Wien 2k using the modified-Becke-Johnson (mBJ) on parent spinel and generalized-gradient-approximation plus Hubbard potential U (GGA + U) on doped materials, respectively. The calculated band structure shows insulating nature of parent compound, while doped material observed semiconducting nature contains direct band gap for both spin channels with band gaps of [ZnGaO 2 :Mn3+ (0.59 up, 2.4 eV dn) and [ZnGaO 2 :Rh3+ (2.1 eV up/dn) respectively. The electronic and optical results reveal that hybridization occurred mainly due to O-p/Zn, Mn-d, Rh-d and Ga-s orbitals. It is analyzed that Mn-doped material shows good absorption in the visible region while other are good in UV region. The effective masses of spinel oxides are also computed at high symmetry directions hence varied nonlinearly with the doping. The stability of materials is checked by calculating formation energies which indicate Mn-doped spinel oxide is most stable as that of others. The thermoelectric properties of spinel oxides were carried out by Post-DFT (Boltztrap) calculations. Large values of Seebeck coefficient and power factor of Mn-doped spinel oxide indicate that this material can be used for thermoelectric devices. The thermodynamical properties are calculated by quasi-harmonic Debye model implemented in GIBBS 2 code. Moreover, the pressure and temperature dependence of all (TD) parameters of investigated spinel oxides are analyzed using quasi-harmonic Debye model. Image 1 • We investigated structral, optoelectronic and tranport properties of spinel oxides Fe 3 BO 6 and Rh 0.08 Fe 0.92 BO 6 compounds. • The approximation used for these calculations was generalized gradient plus Hubbard potential (GGA+U). • The band structure of parent compound for spin up shows insulating nature while other channel is semiconducting nature (4.181 eV Up/2.41 eV Dn). • The optical dispersion exhibits strong absorption in the UV region, for both materials. • The transport parameters are computed by Post-DFT approximations based on rigid band approach in Boltzmann code. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials design for nanoscience and nanotechnology has attracted continuously the researchers' attention because of the important role in the next generation of optoelectronics. In this paper, we report results concerning the prediction of a new 2D material, Lithium Chloride LiCl, using first-principles calculations. This material is proposed for the detection and absorption of ultraviolet (UV) radiation. Our calculations assert that the pristine LiCl monolayer, which crystallizes in the D 3 h symmetry (space group P 6 ‾ 2 m) is dynamically stable. The monolayer at hand is an indirect gap insulator with a band gap of 6.048(7.212) eV predicted by WC(HSE06) functional. The external strains effect on the electronic structure of LiCl monolayer is also investigated. Optical absorption calculations show a wide absorption band in the UV region and high absorption coefficients. Results suggest that the LiCl monolayer may be a promising candidate for optoelectronic applications in the UV-detection and absorption. We believe that results presented in this work should pave a solid way for the applications of alkaline halides monolayers with a wide electronic band gap in optoelectronic devices. • h-LiCl monolayer is dynamically stable. • The monolayer has an indirect band gap of 6.048 eV. • External strain effect on the electronic structure is examined. • The single layer has wide absorption and high absorption coefficient in the UV spectrum. • h-LiCl monolayer may be promising candidate for UV detection and absorption. [ABSTRACT FROM AUTHOR]

Electronic structure, optical and thermoelectric properties of the HfSSe Janus monolayer have been comprehensively investigated using first-principles calculations based on the full-potential linearized augmented plane-wave (FP-LAPW) method. Results show that the HfSSe Janus monolayer is dynamically stable. Electronic properties features indicate that the two-dimensional (2D) material at hand is an indirect gap semiconductor with a band gap of 0.756 eV calculated by the Wu-Cohen scheme within generalized gradient approximation (GGA-WC). The spin-orbit coupling (SOC) inclusion in the calculations breaks the degeneracy in both valence and conduction bands, and gives a smaller band gap of 0.653 eV. Additionally, the external strain effect on the electronic structure is also examined. The metallization can be achieved when a compressive strain of −8% is applied. Optical properties results suggest that the studied 2D monolayer may be a promising candidate to be applied in nano-optoelectronic devices provided that it displays a wide absorption band from visible to ultraviolet with high absorption coefficient of the order of 103/cm. Finally, the thermoelectric properties including electrical conductivity, Seebeck coefficient, electronic thermal conductivity and power factor are calculated using the semi-classical Boltzmann transport theory. Results indicate that the electron doping may be favorable for the thermoelectric performance of the HfSSe Janus monolayer due to large power factor values as compared with those of the hole doping. • HfSSe monolayer is dynamically stable. • HfSSe is an indirect gap semiconductor with a band gap of 0.756 eV. • Spin-orbit coupling removes the energy degeneracy and decreases slightly the band gap. • HfSSe shows good optical absorption from visible to ultraviolet regime. • HfSSe possesses high thermopower and power factor, and the hole doping is favorable. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials play key role in designing and fabricating diminutive optoelectronic devices with high efficiency. In this paper, we report the results of a comprehensive first-principles study on the structural and electronic properties of the pristine and hydroxyl group OH-functionalized (OH-AlN-OH) AlN monolayer. GGA-PBE and hybrid HSE06 functionals are employed to describe the exchange-correlation potential. According to our calculations, the pristine AlN monolayer has a wide indirect band gap of 2.954(4.000) eV determined by PBE(HSE06) level of theory. Indirect-direct gap transition is obtained through the chemical functionalization and the band gap reduces to 0.775(2.125) eV. Results shows that the OH-AlN-OH monolayer is more suitable for optoelectronic applications. Finally, the strain is proven to be efficient factor to tune the electronic properties of the studied monolayers. • AlN monolayer is dynamically stable. • AlN monolayer has wide indirect gap of 2.954(4.000) eV predicted by PBE(HSE06) functional. • OH-functionalization induces the indirect-direct gap transition and considerable band gap reduction to 0.775(2.125) eV. • Electronic band gaps show significant strain-dependence. [ABSTRACT FROM AUTHOR]