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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]

This work investigated the structural characteristics and SO2 adsorption-sensing properties of pristine HfS2 and TM-doped HfS2 monolayers (TM = Ni, Pd, or Pt) using DFT and NEGF-based calculations. The most stable doping site for the adatoms is found to be the hollow site between three adjacent sulfur atoms. Our results demonstrate weak physisorption of SO2 molecules on the pristine HfS2. The SO2 adsorption stability on the Ni-, Pd-, and Pt-doped HfS2 monolayers significantly improves compared with the pristine one. The calculated recovery times of Ni-doped, Pd-doped, and Pt-doped HfS2 monolayers after SO2 adsorption are 208.3 s, 92 s and 5.4 s, respectively. In addition, we designed SO2 sensor models containing Au-electrodes to investigate the influence of TM-doped HfS2 monolayers (TM = Ni, Pd, or Pt), as the material of the scattering region, on their I–V relationship and sensitivity. The sensitivities of TM-doped HfS2 monolayer (TM = Ni, Pd, or Pt)-based sensors are calculated to be 28%, 41%, and 44%, respectively. Our results suggest that Pd-doped and Pt-doped HfS2 monolayers are new promising 2D materials for designing SO2 sensors with high sensitivity and repeatability. [ABSTRACT FROM AUTHOR]

In this paper, we propose a series of two-dimensional asymmetric Janus γ-Sn2OX (X = S, Se, Te) monolayers and systematically calculate the electronic structure, piezoelectricity, and mobility of carriers by using the first-principles method. It is predicted that while γ-Sn2OS and γ-Sn2OSe monolayers are structurally stable, Janus γ-Sn2OTe is found to be unstable with the negative frequencies appearing in its phonon spectrum. Both γ-Sn2OS and γ-Sn2OSe monolayers are semiconductors with indirect bandgaps. Particularly, γ-Sn2OS has a camel's back-like structure in the top valence band and Rashba spin splitting with a Rashba energy of 3.50 meV is found in γ-Sn2OS when the spin-orbit coupling is taken into account. Our calculated results reveal that the Janus γ-Sn2OS and γ-Sn2OSe monolayers are piezoelectric with out-of-plane piezoelectric coefficient d31 predicted to be 0.18 and 0.26 pm V−1, respectively. It is also indicated that γ-Sn2OS and γ-Sn2OSe exhibit highly directionally anisotropic carrier mobility and their electron mobilities are high enough for applications in nanoelectronic devices. [ABSTRACT FROM AUTHOR]

This work is motivated by the recent fabrication of a new four-atom-thick hexagonal polymorph from group IV monochalcogenide, so-called γ -GeSe (Lee et al 2021 Nano Lett. 21 4305). In this paper, we propose and examine the structural characteristics, electronic properties, and carrier mobility of monolayers Janus γ -Ge2 XY ( X / Y = S, Se, or Te) based on comprehensive first-principles calculations. Monolayers γ -Ge2 XY are confirmed to be structurally stable. Our calculations reveal that γ -Ge2 XY monolayers are indirect semiconductors with Mexican-hat-like dispersions in the top valence band. While the effect of the electric field on the energy band dispersions of γ -Ge2 XY monolayers is weak, the energy band dispersions are changed drastically in the presence of strain, especially compressive strain. Interestingly, a structural phase transition from semiconductor to metal is observed in γ -Ge2 XY under compressive strain. γ -Ge2STe and γ -Ge2SeTe possess high electron mobility with values of 3.22 × 10 3 and 8.33 × 10 3 cm2 V−1 s−1, respectively. Our findings not only explore the fundamental physical properties of γ -Ge2 XY but also open up new opportunities in the design of high-performance electronic nanodevices based on layered nanomaterials with Mexican-hat-like dispersions. [ABSTRACT FROM AUTHOR]

In this Letter, we design Janus γ- Sn 2 X Y (X / Y = S, Se, Te) monolayers and predict their piezoelectricity and carrier mobility by using first-principles simulations. Janus γ- Sn 2 X Y are found to be indirect semiconducting characteristics with a camel's back-like dispersion in the top valence band. We discovered that Janus γ- Sn 2 X Y are piezoelectric with high out-of-plane piezoelectric coefficients. Our calculated results for the piezoelectricity demonstrate that the out-of-plane piezoelectric coefficient d31 of Janus γ-Sn2STe is calculated to be 1.02 pm/V, larger than that of other 2D structures. Moreover, our calculations for the transport features reveal that while the carrier mobility of γ-Sn2SSe is directionally isotropic, the electron mobility of both γ-Sn2STe and γ-Sn2SeTe exhibit high anisotropy along the two transport directions. The Janus γ- Sn 2 X Y monolayers have high electron mobility, especially the electron mobility of γ-Sn2STe exceeds 105 cm2 V–1 s–1, which is potential for nanoelectronic applications. [ABSTRACT FROM AUTHOR]

In this paper, we design and investigate the stability, electronic characteristics, and transport parameters of a series of 12 two-dimensional materials, including six ternary Al M X 3 and six quaternary Janus Al 2 M 2 X 3 Y 3 (M = Ge, Sn; X / Y = S, Se, and Te; X ≠ Y) monolayers by using first-principles calculations based on the density functional theory. The phonon spectra and Born elastic stability criteria are used to consider the stability of the studied materials. Our examinations indicate that, except for Janus Al 2 Sn 2 S 3 Te 3 monolayer, the other structures are structurally stable. While the ternary Al M X 3 monolayers are found to be indirect semiconductors, quaternary Janus Al 2 M 2 X 3 Y 3 structures can be either direct semiconductors or metallic (Al 2 Ge 2 S 3 Te 3). Interestingly, Mexican hat-like dispersion can be found in the topmost valence band of ternary Al M X 3 monolayers, which are not observed in their corresponding Janus structures. All studied semiconducting materials exhibit a high electron mobility, up to 1. 51 × 1 0 3 cm 2 V−1s−1 for Al 2 Sn 2 Se 3 Te 3 monolayer. The electron mobilities of the studied structures are high directional isotropic along the x and y axes and much higher than those of the hole mobility. The unique electronic and transport features of ternary Al M X 3 and quaternary Janus Al 2 M 2 X 3 Y 3 monolayers can be suitable for applications in electronic nanodevices. [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]

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]

In this work, we study the adsorption properties of N2, H2, O2, NO, NO2, CO, CO2, and SO2 on a Sc2CO2 monolayer based on first-principles calculations from density functional theory. The structural properties of the Sc2CO2 monolayer after adsorption of various gases have been elucidated. The preferred adsorption sites for each gas are determined. It has been established that the CO2, CO, H2 and N2 molecules are physically adsorbed on the Sc2CO2 monolayer, while the NO, SO2, NO2, and O2 molecules are chemically adsorbed on the Sc2CO2 monolayer. The chemical adsorption of O2 molecules, which are a fundamental component of air, significantly changes the electronic properties of the Sc2CO2 monolayer. This fact limits the prospect of application of the Sc2CO2 monolayer in gas sensors or capture of toxic gases such as SO2 and NO. [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]

Highlights • Electronic and optical responds of monolayer SnSe are anisotropic to strains. • Direct-indirect and semiconductor-metal transitions of monolayer SnSe under both biaxial and uniaxial strains. • The dielectric function under biaxial strain is enhanced by magnitude, rising of new peaks and shifting closer to infrared region. • Light absorption of monolayer SnSe is enhanced to the order of 105 cm−1 and is widen to infrared and violet region. • Strain-engineering monolayer SnSe can give better performance in photovoltaic and solar energy-conversion application. Abstract Strain effects on the electronic and optical properties of monolayer SnSe is studied by APW + lo method in DFT framework. The applied strains cause direct-indirect transition of SnSe band gap which is mainly constructed by s / p hybridization. The armchair ε ac and zigzag ε zz reduce the unstrained band gap of 1.05 eV down to 0 eV at 12% compression, but at 12% tension, the band gap decreases to 0.726–0.804 eV. The band gap always increases under biaxial strain ε b at 12% compression to 12% tension. We observe an enhancement of real ε 1 (ω) and imaginary ε 2 (ω) parts of dielectric function by 14%–30% of magnitude, wider peak distribution to infrared and ultra-violet regions, and appearance of new peaks in the ε 1 (ω) and ε 2 (ω) spectrums. As a consequence, the light absorption α (ω) is significantly enhanced in the ultra-violet region and the absorption even starts at lower energy at infrared region. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we study the difference in electronic and optical properties of bulk and monolayer zirconium sulfide by applying the APW + lo method in the framework of density functional theory. All calculation is performed at the energy level of visual light and higher ranging from 0 eV to 15 eV. Our results demonstrates that except for the underestimated band gap like other GGA calculation, the remain properties like dielectric function, the reflectivity, absorption and loss energy are close to experiment. The valence band is constructed by mainly sulfur s / p -states and the lower portion of zirconium s / p / d -states. The conduction band is mostly donated by zirconium d -state. In contrast with bulk structure, the valence band maximum in monolayer has the triple peak at Γ point, making its monolayer be more sensitive to light absorption. The dielectric function has the highest peak at about 1.5–2.5 eV with remarkable anisotropy, beyond this level to the ultraviolet region the anisotropy decreases and almost disappears at energy larger than 10 eV. The absorption is at 106 x 10 4 cm -1 for energy range 5–10 eV, while the reflectivity is at its highest value of 30 %–50 % in the energy range from 0 to 8 eV. The energy loss of monolayer is higher than those of bulk. For optical and electronic properties, the monolayer show sharper peaks and their clear separation indicate the progressive application of monolayer zirconium sulfide. Highlight • Monolayer ZrS 2 has new interband transitions at energy levels of 4-7 eV and 7-10 eV. • For energy ranging from -14 eV to 6 eV, the dielectric constants, energy loss, and reflectance are close to experimental values. • ZrS 2 is highly anisotropic to the parallel and perpendicular components of electric field at energy lower than 10 eV. • The energy loss L (ω) in monolayer is higher than in bulk. • Our results give an explanation on perspective optical properties of monolayer ZrS 2 , which can be helpful for further design and application of this material. [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]

• 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]

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]

• 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]

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]

In this paper, we theoretically study the optical properties of both bulk and monolayer MoS 2 using first-principles calculations. The optical characters such as: dielectric function, optical reflectivity, and electron energy-loss spectrum of MoS 2 are observed in the energy region from 0 to 15 eV. At equilibrium state the dielectric constant in the parallel E ∥ x and perpendicular E ∥ z directions are of 15.01 and 8.92 for bulk while they are 4.95 and 2.92 for monolayer MoS 2 , respectively. In the case of bulk MoS 2 , the obtained computational results for both real and imaginary parts of the dielectric constant are in good agreement with the previous experimental data. In the energy range from 0 to 6 eV, the dielectric functions have highly anisotropic, whereas they become isotropic when the energy is larger than 7 eV. For the adsorption spectra and optical reflectivity, both the collective plasmon resonance and ( π + σ ) electron plasmon peaks are observed, in which the transition in E ∥ x direction is accordant with the experiment data more than the transition in E ∥ z direction is. The refractive index, extinction index, and electron energy-loss spectrum are also investigated. The observed prominent peak at 23.1 eV in the energy-loss spectra is in good agreement with experiment value. Our results may provide a useful potential application for the MoS 2 structures in electronic and optoelectronic devices. [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]

We investigate systematically the electronic properties of fully hydrogenated GaN (HGaNH) monolayer under effects of the biaxial strain and external electric field. Results assert that the pristine GaN monolayer is an indirect semiconductor with a band gap of 1.933 eV. Band structures indicate that the N-p and Ga-p states are the main contributors to the valence and conduction bands near Fermi level, respectively. The full hydrogenation can induce an increase of this parameter to 3.019 eV, with H-s state being present in both valence and conduction bands. Interestingly, the HGaNH monolayer transforms from indirect to direct gap as a consequence of GaN hydrogenation. Based on results, the electronic band gap of HGaNH monolayer shows sensitivity to the biaxial strain and external electric field. Therefore, these factors can be used to effectively tune the electronic properties of HGaNH monolayer in order to make it more suitable for optoelectronic applications. • GaN monolayer has a planar graphene-like structure and the hydrogenation generates a buckling height of 0.705 Å. • GaN monolayer is a semiconductor with a band gap of 1.933 eV and the hydrogenated has a band gap of 3.019 eV. • Biaxial strain and external electric field effect on the electronic properties of the hydrogenated GaN monolayer is studied in details. • The hydrogenated GaN monolayer band gap is sensitive to biaxial strain and external electric field. [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]

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]

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]

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]

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]

The characteristics of defective ZrS 2 monolayers containing S-vacancies adsorbed various gases (NO, NO 2 , CO, SO 2 , N 2 , H 2 , O 2 , CO 2) were studied using a theoretical approach based on the density functional theory calculations. A favorable configuration for each gas adsorption was also found. The structural properties of defected ZrS 2 monolayers with S-vacancies adsorbed the given gas contents were elucidated. The obtained results show that the defected ZrS 2 monolayers with S-vacancies enhance the capacity of the gas adsorption in respect of the perfect ZrS 2 structures. Among those gases, the CO 2 , CO and N 2 molecules are physically adsorbed on S-vacancies of the defected ZrS 2 monolayers, whilst the O 2 , NO, SO 2 molecules are chemically adsorbed in the areas closed to S-vacancies of the ZrS 2 monolayers. Moreover, a NO 2 molecule can be separated by a S-vacancy of the defected ZrS 2 monolayer. The results of this work suggest that defective ZrS 2 monolayers with S-vacancies can be used as an adsorbent using in gas sensors to detect CO 2 , CO and N 2 gas contents. Therein, the highest sensitivity in detecting N 2 molecules is observed. • The S-vacancy on the ZrS 2 monolayer enhances the gas adsorption properties compared to the perfect ZrS 2 monolayer. • The molecules of CO 2 , CO and N 2 are physically adsorbed on the S-vacancies of defected ZrS 2 monolayers. • The molecules of O 2 , NO and SO 2 are chemically adsorbed on the areas closed to the S-vacancies of the defected ZrS 2 monolayers. • NO 2 molecule can be separated by a S-vacancy on a ZrS 2 monolayer. • The defected ZrS 2 monolayers can be used in gas sensors to detect CO 2 , CO and N 2 contents. [ABSTRACT FROM AUTHOR]

In this work, the electronic and optical properties of monolayer GaTe under biaxial strain and external electric field were investigated by density functional theory. Our calculated results indicate that the monolayer GaTe is an indirect-semiconductor with an energy gap of 1.41eV at equilibrium. Electronic properties of monolayer GaTe, especially the energy gap, depend greatly on the biaxial strain and external electric field. While the compressive strain slightly increases the energy gap, the tensile strain reduces quite rapidly the energy gap of the monolayer GaTe. In particular, semiconductor–metal phase transition can be observed when the external electric field was introduced. The GaTe monolayer strongly absorbs light in the ultraviolet region and the biaxial strain greatly changes its optical characteristics, especially the compression strain significantly increasing the absorption coefficient of the monolayer. Our results can provide more useful information for the prospect of application of the GaTe monolayer in next-generation optoelectronic devices. • Monolayer GaTe is an indirect-semiconductor with a moderate gap of 1.41eV at equilibrium. • Band gap of monolayer GaTe depends strongly on the biaxial strain. • Semiconductor–metal phase transition can be occurred in monolayer GaTe due to electric field. • Monolayer GaTe strongly absorbs light in the ultraviolet region. • Biaxial strain causes a significant shift in the first optical peaks of the GaTe monolayer. [ABSTRACT FROM AUTHOR]

The transition metals (TM = V, Cr and Mn) doping effects on the structural, electronic and magnetic properties of boron arsenide (BAs) monolayer have been comprehensively investigated using pseudopotential calculations within the framework of density functional theory (DFT). Calculations show that the pristine BAs monolayer has a graphene-like structure and its chemical bond is a mixture of covalent and ionic nature. Electronic properties results indicate that the BAs monolayer is a paramagnetic semiconductor with a direct band gap of 0.682 eV. The possibility of V-, Cr- and Mn-doping is determined by positive binding energy of 1.592, 1.668 and 1.876 eV, respectively. The incorporation of V and Cr generates the half-metallicity in the BAs monolayer with a spin-down band gap of 0.873 and 0.897 eV, respectively, while the one doped with Mn is nearly half-metallic. The doping also induces the magnetism and it is produced mainly from the 3 d orbital of the TMs with a small contribution from the nearest neighbor As atoms. Results presented in this work may be useful for the design of the magnetic nano-materials. • BAs monolayer adopts a honeycomb planar arrangement. • Transition metals doping induces local structural distortion. • Pristine BAs monolayer is a direct semiconductor. • V- and Cr-doped BAs monolayer show half-metallicity, while Mn-doped system is nearly half-metallic. • Magnetism is produced mainly from transition metal spin-up and small contribution from As spin-down. [ABSTRACT FROM AUTHOR]

Boron nitride (BN) monolayer (ML) has been the subject of many investigations, however, its large electronic band gap has limited so much its applications. Therefore, the band gap engineering is a necessary step at the time of designing practical uses of the BN ML to extend its applicability to nano-optoelectronic devices. In this work, we investigate systematically the impact of P- and As-doping as well as an external electric field on the electronic structure of BN ML using first-principles calculations based on the pseudopotential density functional theory. Results indicate that the pristine BN ML has an indirect K − Γ band gap of 4.589 eV. The N-substitution by P and As induces a band gap reduction of the order of 19.50% and 22.29%, respectively. Additionally, the external electric fields with strength in the range of −0.5 to 0.5 (eV/Å/e) are applied along the z-axis perpendicular to the MLs for both upward and downward directions. Under a weak electric field ± 0.1 (eV/Å/e), the electronic band gap of all three considered monolayers does not show a significant change. However, a stronger electric field will decrease considerably this parameter. Our results confirm that the coexistence of doping and external electric field can be a quite efficient approach to fine-tune the electronic structure of BN ML. • BN monolayer has a graphene-like honeycomb planar structure. • P- and As-doping induces a local structural distortion. • Pristine BN monolayer is an indirect semiconductor with band gap of 4.589 eV. • The electronic band gap decreases considerably under both doping and external electric field presence effect. [ABSTRACT FROM AUTHOR]