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Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices. Here, using first principles calculations, we propose a GeC/C2N van der Waals heterostructure and investigate its electronic and optical properties. We demonstrate that the intrinsic electronic properties of both GeC and C2N monolayers are quite preserved in GeC/C2N HTS owing to the weak forces. At the equilibrium configuration, GeC/C2N HTS forms the type-II band alignment with an indirect band gap of 0.42 eV, which can be considered to improve the effective separation of electrons and holes. Besides, GeC/C2N vdW-HTS exhibits strong absorption in both visible and near ultra-violet regions with an intensity of 105 cm−1. The electronic properties of GeC/C2N HTS can be tuned by applying an electric field and vertical strains. The semiconductor to metal transition can be achieved in GeC/C2N HTS in the case when the positive electric field of +0.3 V A−1 or the tensile vertical strain of −0.9 A is applied. These findings demonstrate that GeC/C2N HTS can be used to design future high-performance multifunctional devices.

The fundamental investigation of topological crystalline insulator (TCI) thin films is essential for observing interesting phenomena. In practice, a promising pathway involves the application of electric and magnetic fields to tune the topological phases of TCI thin films. To achieve this, we applied a perpendicular electric field and an in-plane magnetic field to not only tune the Dirac gap of a SnTe(001) thin film and find the phase transition but also to directly connect them with their effects on the group velocity of both massless and massive surface Dirac fermions. The TCI thin film is an inherent insulator due to the hybridization between the front and back surfaces, and it transitions to a semimetal phase at a critical perpendicular electric field due to the Stark effect. Correspondingly, the anisotropic group velocity of the upper (lower) conduction (valence) band decreases (increases) with the electric field at certain momenta. We found that when one of the in-plane Zeeman field components becomes stronger than the intrinsic hybridization potential, the anisotropic Weyl cones with opposite chiralities retrieve at the critical momenta and the corresponding group velocities become zero. Further, the isotropic in-plane Zeeman field leads to rotation of the band structure, as expected, resulting in non-zero group velocities along all directions. Finally, for the sake of completeness, the combined Stark and Zeeman effects are tracked and the results show that the system is an insulator at all fields and the group velocities are altered more than when the individual Stark and Zeeman effects are applied. Our findings may provide interesting physical insights for practical applications in nanoelectronics and spintronics.

Motivated by the recent successful synthesis of the layered platinum mineral jacutingaite (Pt2HgSe3), we have studied the optoelectronic, mechanical, and thermoelectric properties of graphene hetero-multilayer on Pt2HgSe3 monolayer (PHS) heterostructures (LG/PHS) by using first-principles calculations. PHS is a topological insulator with a band gap of about 160 meV with fully relativistic calculations; when graphene layers are stacked on PHS, a narrow band gap of ∼10–15 meV opens. In the presence of gate-voltage and out-of plane strain, i.e. pressure, the electronic properties are modified; the Dirac-cone of graphene can be shifted upwards (downward) to a lower (higher) binding energy. The absorption spectrum shows two peaks, which are located around 216 nm (5.74 eV) and protracted to 490 nm (2.53 eV), indicating that PHS could absorb more visible light. Increasing the number of graphene layers on PHS has a positive impact on the UV-vis light absorption and gives a clear red-shift with enhanced absorption intensity. To investigate the electronic performance of the heterostructure, the electrical conductance and thermopower of a device composed of graphene layers and PHS is examined by a combination of DFT and Green function formalism. The number of graphene layers can significantly tune the thermopower and electrical conductance. This analysis reveals that the heterostructures not only significantly affect the electronic properties, but they can also be used as an efficient way to modulate the optic and thermoelectric properties.

Correction for ‘Theoretical prediction of the electronic structure, optical properties and photocatalytic performance of type-I SiS/GeC and type-II SiS/ZnO heterostructures’ by S. S. Sabir et al., RSC Adv., 2023, 13, 7436–7442, https://doi.org/10.1039/d3ra01061a.

Correction for ‘First principles study of electronic properties and optoelectronic performance of type-II SiS/BSe heterostructure’ by Shah Saleemullah Sabir et al., New J. Chem., 2023, 47, 4537–4542, https://doi.org/10.1039/D2NJ06198H.

The electrical contacts formed between the channel materials and the electrodes play a vital role in the design and fabrication of high-performance optoelectronic and nanoelectronic devices. In this work we propose combining metallic single-layer graphene (SLG) and a Janus SMoSiN2 semiconductor and investigate the electronic properties and contact types of the combined heterostructures (HTSs) using first-principles calculations. The effects of electric fields and interlayer coupling are also examined. The combined SLG/SMoSiN2 and SLG/N2SiMoS HTSs are both structurally and thermodynamically stable at equilibrium interlayer coupling. The combination between SLG and a Janus SMoSiN2 semiconductor generates a p-type or n-type Schottky contact, depending on the stacking configuration. The SLG/SMoSiN2 HTS generates a p-type Schottky contact while the SLG/N2SiMoS HTS forms an n-type one. Furthermore, applied electric field and strain can adjust the electronic features and contact types of the HTSs. An applied negative electric field and tensile strain lead to conversion from a p-type to an n-type Schottky contact in the SLG/SMoSiN2 stacking configuration, whereas a positive electric field and compressive strain give a transformation from an n-type to a p-type Schottky contact in the SLG/N2SiMoS stacking configuration. Our findings provide rational evidence for the fabrication and design of electrical and optical devices based on SLG/SMoSiN2 HTSs.

Chemical functionalization is one of the effective methods to tune the electronic properties of two-dimensional (2D) nanostructures. In this paper, we study the structural, electronic properties, and carrier mobilities of 2D SnAs monolayer functionalized by chlorine, bromine, and iodine, namely SnAsX (X = Cl, Br, and I), by means of density functional theory. The obtained results show that the 2D SnAsX are energetically, dynamically, and mechanically stable. While the 2D pristine SnAs monolayer exhibits metallic characteristics, SnAsX monolayers are direct semiconductors with direct band gaps. The influence of spin–orbit coupling on the electronic characteristics of SnAsX is significant, especially in the case of SnAsI monolayer. The direct–indirect gap transitions are found in all three SnAsX monolayers when the biaxial strain is applied. Meanwhile, the effects of external electric fields on the electronic characteristics of SnAsX are insignificant. Our calculated results indicate that SnAsX monolayers have very high electron mobility and their transport characteristics are directionally isotropic along the investigated transport directions.

We study the transport properties of silicene in a perpendicular magnetic field by evaluating the Hall and longitudinal conductivities and resistivities when the acoustic phonon correction to the Landau level (LL) energy is taken into account. The acoustic phonons are considered by three modes: the transverse (TA), longitudinal, and out-of-plane ones. Under the influence of the acoustic phonon correction, the quantum Hall effect plateaus occur at higher values of the magnetic field, where the TA phonon displays the strongest effect. The combined effects of the strong spin-orbit coupling in silicene, the external electric field, and the Zeeman field on the transport parameters are investigated. These combined effects lift the spin and valley degeneracy of the LLs, leading to the additional plateaus in the Hall conductivity with the sequence being found as ${\ensuremath{\sigma}}_{yx}=(4{e}^{2}/h)(n/4+1/2)$. The temperature has a significant effect on the width of the Hall conductivity plateaus. We also appraise the longitudinal conductivity ${\ensuremath{\sigma}}_{xx}$ and the Hall, ${\ensuremath{\rho}}_{xy}$, and longitudinal, ${\ensuremath{\rho}}_{xx}$, resistivities and show the difference between the present results and those for graphene as well as for silicene without the Zeeman field effect. The combined effects of the electric and Zeeman fields lead to the quadrupled peaks of ${\ensuremath{\rho}}_{xx}$ and the sequence of $(h/{e}^{2})/(n+2)$ in the height of the plateaus in ${\ensuremath{\rho}}_{xy}$.

This study was carried out under nethouse conditions in Chau Doc city with 6 types of black tomatoes to select the potential ones suitable for the local ecological conditions. The objective of this research was to evaluate growth parameters, fruit quality, and adaptability characteristics of black tomato varieties under nethouse conditions in Chau Doc city. The results showed that some promising cultivars had an early harvest ranging from 90 days to 103 days such as Indigo Rose, Viagra Socola, black Russian, Cherry Chocolate, Black Baron and Savior. All cultivars had a plant structure well adapted to the local nethouse conditions in Chau Doc city. Variety Cherry Chocolate gained the highest fruit setting with 203. Viagra Socola, black Russian, Cherry Chocolate gained higher at over 4.0 tons/1,000 m2. The lowest yield of the Black Baron was obtained at 2.83 tons/1,000 m2. The Savior was obtained at 7.05 tons/1,000 m2 with a relatively good fruit quality. Briefly, the promising cultivars are Viagra Socola, Cherry Chocolate, black Russian and further research is needed to evaluate their real field production.

In the present study, the effects of the strain engineering and electric field on electronic properties of the GaS monolayer are investigated by ab initio investigations. Our calculated results demonstrate that the GaS monolayer is a semi-conductor with a large indirect bandgap of 2.568 eV at the equilibrium. In the presence of a biaxial strain, the band structure of the GaS monolayer, especially the conduction band, changes significantly. However, while the effect of compressive strain on the energy gap of the GaS monolayer is quite weak, its energy gap depends strongly on the tensile strain. On the other hand, external electric fields can cause the semiconductor–metal transition in the monolayer. Being able to control electronic properties, especially the occurrence of the semiconductor–metal phase transition, makes the GaS monolayer a prospective material for nanoelectromechanical and nanospintronic applications.

In this study, we investigate systematically the effect of strain engineering and electric field on electronic properties of single-layer SnSe 2 using density functional theory . Our calculated results indicate that the single-layer SnSe2 is a semiconductor with a small band gap of 0.715 eV at the equilibrium state. The electronic states near the Fermi level are mainly contributed by Sn- d and Se-p orbitals, especially the contribution of the Se-p orbital to the valence band is dominant. Under biaxial strain, the band gap of the single-layer SnSe2 changes abnormally. While compressive biaxial strain reduces band gap rapidly, the band gap of the single-layer SnSe2 only increases slightly when increasing the tensile biaxial strain. In contrast to the strain-dependence case, the influence of the external electric field on the electronic properties of the single-layer SnSe2 is quite small and the energy gap of the single-layer SnSe2 does not depend on the direction of the perpendicular electric field. Our calculated results can provide more information for application possibility of the single-layer SnSe2 in nanoelectronic devices.

We theoretically study the combined effect of Al-concentration, hydrostatic pressure, and temperature on the magneto-optical absorption properties of a semi-parabolic quantum well (SPQW) by investigating the magneto-optical absorption coefficient (MOAC) and the full-width at half-maximum (FWHM). The expression of MOAC is expressed by the second-order golden rule approximation where the electron–longitudinal-optical (LO) phonon interaction is taken into account while the FWHM is obtained from the profile of the curves. The numerical calculations are made for a typical GaAs/GaAlAs quantum well. Our results showed that the Al-concentration, hydrostatic pressure, and temperature affect the magneto-optical absorption properties strongly. Threshold energy and the resonant peaks are non-monotonic functions of the Al-concentration. Moreover, it has been revealed that the magneto-optical absorption properties of SPQW can be controlled by changing these parameters.

The vertical stacking and influence of external electric field are good techniques to tune the electronic properties of two-dimensional materials for potential nanoelectronics and optoelectronic devices. The structural and electronic properties of P-ZnO and SiC-ZnO van der Waals (vdW) heterostructures are investigated by first principles calculations. P-ZnO (SiC-ZnO) heterostructure exhibits an indirect type-I (direct type-II) semiconducting band character. The effect of perpendicular applied external electric field on the electronic properties of the most stable vdW heterostructure has also been systematically discussed. Remarkable variations in the band gap nature are induced by increasing field strength from 0.1 to 1.0 V/A (for P-ZnO) and 0.1 to 0.8 V/A (for SiC-ZnO). The intrinsic indirect type-I is modulated to type-II semiconducting band gap in P-ZnO by applying electric field strength 0.1 V/A. More interestingly, reduction in the size with transition from an indirect-to-direct type-II semiconductor band gap nature is noted at 0.5 V/A. However, a direct type-II semiconductor to metallic character is found at 1.0 V/A (for P-ZnO) and 0.8 V/A (for SiC-ZnO). Our results suggests that these vdW heterostructures are promising candidates for electronic and optoelectronic device applications.

This work is based on a recent theoretical study of how the hydrostatic pressure and core/shell sizes affect the optical properties associated with the transition from the ground state to first excited state (1s–1p), of an exciton confined in spherical core/shell quantum dots (SCSQDs). We have computed under an effective mass framework, linear, third-order nonlinear, and total absorption coefficients (AC) and refractive index (RI) as functions of photon energy for different sizes of SCSQDs with varying hydrostatic pressure. Our results show that the optical absorption is deeply dependent on the incident light intensity. Both AC and RI significantly influenced by the confinement and pressure effects.

In this letter, we systematically investigate the electronic properties of the PtS2/InSe heterostructure using first-principle calculations. At the equilibrium interlayer distance D = 3.23 A, the PtS2/InSe heterostructure displays a semiconducting character with an indirect band gap. Moreover, it forms a type-II band alignment, making the PtS2/InSe heterostructure a potential material for efficient separation of photogenerated electron-hole pairs. More interestingly, by applying vertical strain and electric field, the electronic properties of the PtS2/InSe heterostructure can be effectively controlled, and a semiconductor-to-metal transition even emerges. These findings suggest attractive potential application for PtS2/InSe heterostructure as a novel optolectronic nanodevices, along with a potential pholocatalyst.

We investigate the combined effects of hydrostatic pressure, Al-concentration, temperature, and well-width parameter on the magneto-optical absorption properties (MOAPs) of a hyperbolic-type quantum well (HTQW). The results covered all possible processes: both phonon absorption and emission as well as both one- and two-photon. Our results show that the MOAPs of the HTQW significantly depend on the pressure, the Al-concentration, the temperature, and the well-width. We also suggest two new expressions for the dependence of the full-width at half-maximum on the pressure and concentration which need an experimental study to evaluate their validity. Our study provides a systematic results of the combined effects of pressure, Al-concentration, temperature, electron-phonon scattering, and the two-photon absorption on the nonlinear optical properties of such two-dimensional semiconductors, which would be useful for the applications in photonic devices.

Constructing van der Waals heterostructures (vdWHs) based on different two-dimensional materials could afford many interesting properties, which may not hold for single-layered materials. In this study, we design a novel vdWH-GaSe/MoSe2 and investigate its electronic properties using first-principles calculations. It has a type-II band alignment with an indirect bandgap. Moreover, we found that the band alignment transformation of the GaSe/MoSe2 vdWH from type-II to type-I can be realized by decreasing the interlayer distance or by applying a positive electric field. Our findings could provide fundamental insights into the GaSe/MoSe2 vdWH for designing high-performance optoelectronic nanodevices.

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 e ac and zigzag e 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 e b at 12% compression to 12% tension. We observe an enhancement of real e 1 ( ω ) and imaginary e 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 e 1 ( ω ) and e 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.

Graphene based two-dimensional layered materials are attracting wide attention both experimentally and theoretically and show many superior properties that individual layers may not hold. In this work, we study theoretically the electronic properties of the graphene/WSe2 van der Waals heterobilayer using the first-principle calculations. Our results demonstrate that the intrinsic electronic properties of graphene and WSe2 monolayer are quite well preserved due to the weak van der Waals interactions. We find that the graphene/WSe2 heterobilayer forms a p-type Schottky contact with the Schottky barrier height of 0.60 eV and shows a good thermoelectric material with high Seebeck coefficient at room temperature. Moreover, the p-type Schottky contact of the graphene/WSe2 heterobilayer can be tailored by inserting WSe2 monolayers to form graphene/WSe2/WSe2 and WSe2/graphene/WSe2 heterotrilayers or by applying electric field perpendicular to the heterobilayer. The p-type Schottky barrier decreases with the insertion of the WSe2 layers, whereas it can be transformed to the n-type one when the negative electric field of −1.5 V/nm is applied. The results reveal the physical nature of the van der Waals heterostructures based on graphene and other two-dimensional transition metal dichalcogenides, which are helpful in providing a route to design graphene-based high-performance optoelectronic nanodevices, such as Schottky diodes and interlayer tunneling field-effect transistors.

In this paper, van der Waals heterostructures consisting of MX2 (M = Zr, Hf and X = S, Se) monolayers are modeled. The favorable stacking and stability of the modeled monolayer heterostructures are confirmed through binding energy and phonon dispersion calculations. After confirming stability, the electronic and thermoelectric properties of these compounds are explored using the first-principles calculations combined with semiclassical Boltzmann transport theory. It is found that type-II band alignment in ZrS2 HfSe2 facilitates charge separation for optoelectronics and solar energy conversion. All studied heterostructures show remarkably higher electrical conductivity than corresponding monolayers, responsible for large power factor values, especially at 1200 K. These findings indicate that the creation of van der Waals heterostructures from MX2 may be promising for efficient optoelectronic and thermoelectric devices.

In the present work, we consider electronic properties of WX2 (X = S, Se) monolayers under a biaxial strain e b using the first principles study. Our calculations indicate that, at equilibrium, the WS2 and WSe2 monolayers are semiconductors with a direct band gap of respectively 1.800 eV and 1.566 eV while their bulk structures are indirect semiconductors. The electronic properties of the WX2 monolayers are very sensitive with the biaxial strain, especially compression strain. The biaxial strain e b is the cause of the band gap of the WX2 monolayers and especially the semiconductor-metal phase transition has occurred in the WS2 monolayer at e b = - 10 % . In addition, the direct-indirect band gap transition was observed in both WS2 and WSe2 monolayers at a certain elongation of biaxial strain e b . The phase transitions in these monolayers can be very useful for their applications in nanoelectromechanical devices.

Transition-metal dichalcogenide monolayers recently have attracted much attention motivated by their exotic physical properties. In this paper, we theoretically investigate the optical absorption in the monolayer MoS2 which is subjected to a uniform static magnetic field and an electromagnetic wave. The magneto-optical absorption power (MOAP) is calculated using the projection operator technique in the linear response scheme, taking account of the electron–optical phonon interaction at high temperature. Both phonon emission and phonon absorption processes are included. The cyclotron–phonon resonance (CPR) is observed in the photon energy dependence of the MOAP. Numerical analyses show that the photon energy satisfying CPR condition depends linearly on the strength of magnetic field, which is similar to that in conventional low-dimensional semiconductors but different from that in graphene. The increase of the full width at half maximum (FWHM) of CPR peaks with increasing magnetic field shows a similar behaviour to that in graphene. In addition, FWHM increases slightly with temperature, which is different from that in graphene where the FWHM is temperature independent. Our investigation provides basic information about the magneto-optical properties of the monolayer MoS2 that are useful for further experiments and applications.

In this work, we construct the C2N/InSe heterostructure and investigate its electronic properties as well as the effect of strain and electric field. Our results demonstrate that the weak van der Waals interactions are dominated in such heterostructure. It forms the type-II band alignment and implies the spatial separation of photogenerated electron-hole pairs. The type-II band alignment can be switched to type-I one and an indirect to direct band gap transition can be achieved by applying the electric field or vertical strain. Our findings demonstrate that the C2N/InSe heterostructure can be considered to be a good candidate for optoelectronic and nanoelectronic devices.

In this work, the structure, electronic properties, and the Schottky barrier of the van der Waals heterostructure (vdWH) based on graphene and gallium sulfide (GaS) have been theoretically considered using density functional theory. We found that the graphene/GaS vdWH keeps the extraordinary intrinsic properties of both the graphene and GaS monolayer. Moreover, an n-type Schottky contact with a small Schottky barrier of 0.51 eV was formed in the ground state of the heterostructure. Especially, our results demonstrated that applying an electric gating can tune effectively the Schottky barrier and contact types. The transformations from the n-type Schottky contact to the p-type one and from the Schottky to the Ohmic contacts were observed in the vdWH under electric gating. These results propose a great potential for the van der Waals heterostructure in future nanoelectronic and optoelectronic devices.

In this work, we consider systematically the influence of uniaxial strain and external electric field E on electronic properties of a monolayer InSe using ab initio approach based on density functional theory. Our calculations indicate that the monolayer InSe has a medium indirect energy bandgap of 1.38 eV at equilibrium. The calculated results also demonstrate that we can adjust the bandgap of the monolayer InSe by strain engineering or electric field. The bandgap of the monolayer InSe changes dramatically when the uniaxial strain is applied. Especially, under the compressed uniaxial strain, an indirect–direct bandgap transition has been observed at certain elongations. Within the electric field magnitude E range from 0 to 5 V/nm, the calculated results show that the negative electric field changes the bandgap of the monolayer InSe up to 23% while the positive electric field effect on its bandgap is negligible.

van der Waals heterostructures (vdWHs), obtained by vertically stacking different two-dimensional (2D) layered materials are being considered intensively as potential materials for nanoelectronic and optoelectronic devices because they can show the most potential advantages of individual 2D materials. Here, we construct the SnSe2/MoS2 vdWH and investigate its electronic and optical properties using first-principles calculations. We find that the band structures of both MoS2 and SnSe2 monolayers are well kept in the SnSe2/MoS2 vdWH because of their weakly interacting features via vdW interaction. The SnSe2/MoS2 vdWH forms a type-I band alignment and exhibits an indirect semiconductor band gap of 0.45 eV. The type-I band alignment makes the SnSe2/MoS2 vdWH a promising material for optoelectronic nanodevices, such as light emitting diodes because of ultra-fast recombination of electrons and holes. Moreover, the band gap and band alignment of the SnSe2/MoS2 vdWH can be tailored by the electric field and the insertion of a graphene sheet. After applying an electric field, type-I to type-II and semiconductor to metal transitions can be achieved in the SnSe2/MoS2 vdWH. Besides, when a graphene sheet is inserted into the SnSe2/MoS2 vdWH to form three stacking types of G/SnSe2/MoS2, SnSe2/G/MoS2 and SnSe2/MoS2/G, the p-type semiconductor of the SnSe2/MoS2 vdWH is converted to an n-type Ohmic contact. These findings provide theoretical guidance for designing future nanoelectronic and optoelectronic devices based on the SnSe2/MoS2 vdWH.

van der Waals heterostructures can be effectively used to enhance the electronic and optical properties and extend the application range of two-dimensional materials. Here, we construct for the first time MoSeTe/X(OH)2 (X = Ca, Mg) heterostructures and investigate their electronic and optical properties as well as the relative orientation of these layers with respect to each other and the effects of an electric field. Our results show that in the MoSeTe/X(OH)2 heterostructures, the Janus MoSeTe monolayer is bonded to the X(OH)2 layer via weak van der Waals forces. Owing to different kinds of chalcogen Se and Te atoms in both sides of Janus MoSeTe, there exist two main stacking types of the MoSeTe/X(OH)2 heterostructures, that are MoSeTe-Se/X(OH)2 and MoSeTe-Te/X(OH)2 heterostructures. Interestingly, the Se- and Te-interface can induce straddling type-II and type-I band alignments. The MoSeTe-Se/X(OH)2 heterostructure exhibits a type-II band alignment, thus endowing it with a potential ability to separate photogenerated electrons and holes. Whereas, the MoSeTe-Te/Ca(OH)2 heterostructure displays a type-I band alignment, which may result in an ultrafast recombination between electrons and holes, making the MoSeTe-Te/Ca(OH)2 heterostructure a suitable material for optoelectronic applications. The MoSeTe/X(OH)2 heterostructures show an isotropic behavior in the low energy region while an anisotropic behaviour in the high photon energy region. The dielectric function of the MoSeTe-Te/Ca(OH)2 heterostructure is high at low photon energy relative to other heterostructures verifying it to have a good optical absorption. Furthermore, the band gap values and band alignment of the MoSeTe/X(OH)2 heterostructures can be modulated by applying an electric field, which induces semiconductor-to-metal and type-I(II) to type-II(I) band alignment. These results demonstrate that the MoSeTe/X(OH)2 heterostructures are promising candidates for optoelectronic and photovoltaic nanodevices.

Janus monolayers and their van der Waals heterostuctures are investigated by hybrid density functional theory calculations. MoSSe, WSSe, MoSeTe and WSeTe are found to be direct band gap semiconductors. External electric fields are used to transform indirect MoSTe and WSTe to direct band gap semiconductors. MoSSe-WSSe, MoSeTe-WSeTe and MoSTe-WSTe vdW heterostructures are also indirect band gap semiconductors with type-II band alignment. Similar to the corresponding monolayers, in some of the above mentioned vdW heterostructures an external electric field and tensile strain can transform indirect to direct band gaps, while sustaining type-II band alignment. Janus monolayers have lower values of the work function (φ) than their vdW heterostructure counterparts. Furthermore, absorption spectra, absorption efficiency, and valence(conduction) band edge potentials are calculated to understand the optical and photocatalytic behavior of these systems. Red and blue shifts are observed in the position of excitonic peaks due to the induced strain in Janus monolayers. Strong device absorption efficiencies (80-90%) are observed for the WSeTe, MoSTe and WSTe monolayers in the visible, infra-red and ultraviolet regions. Energetically favourable band edge positions in Janus monolayers make them suitable for water splitting at zero pH. We find that the MoSSe-WSSe heterostructure and the MoSTe monolayer are promising candidates for water splitting with conduction and valence band edges positioned just outside of the redox interval.

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.

The magneto-optical transport properties in monolayer silicene subjected simultaneously to a perpendicular magnetic field and an electromagnetic wave (optical field) are theoretically studied. The nonlinear absorption coefficient is calculated using perturbation theory taking account of the electron–impurity scattering. The cyclotron–impurity resonance (CIR) is observed through the absorption spectrum. The photon energy at resonances is found to be proportional to the square root of magnetic field. This behaviour is similar to that in graphene but different from that in conventional low-dimensional semiconductors. The full width at half maximum (FWHM) of CIR peaks increases with increasing magnetic field by the laws FWHM [meV] ≈ 0.432 B [ T ] and FWHM [meV] ≈ 0.215 B [ T ] for one- and two-photon absorption, respectively. The obtained FWHM is about one order of magnitude smaller than it is in graphene monolayers. Moreover, the temperature dependence of the FWHM is similar to that in graphene but different from that in conventional low-dimensional semiconductors.

Two-dimensional MoSi

Graphene-based van der Waals heterostructures have received tremendous interest from both fundamental and experimental studies because they can enhance the properties and expand the possibility of applications of both graphene and two-dimensional materials. Motivated by the successful synthesis of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure [Chang et al., Adv. Funct. Mater. 28, 1800179 (2018)]., here, we systematically investigate the electronic structure and interfacial characteristics of this material using first-principles simulations. We find that the structure of the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure is mainly characterized by weak van der Waals interactions, which keeps the heterostructure feasible. In the ground state, the graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ heterostructure forms the $n$-type Schottky contact with a barrier of 0.53 eV. The barriers of the Schottky contact can be adjusted by various factors, including interlayer coupling and electric gating. Both the interlayer coupling and electric gating lead to the transformation from the $n$-type Schottky contact to the $p$-type one or to the $n$-type Ohmic contact. These findings demonstrate that graphene/$\mathrm{Bi}{\mathrm{I}}_{3}$ can be considered a promising building block for high-performance photoresponsive optoelectronic devices.

The breaking of the vertical symmetry in Janus monochalcogenides gave rise to many properties that were not present in the original monochalcogenide monolayers. However, recent papers have often focused only on Janus monochalcogenides containing S, Se, and Te elements despite that O is also one of the group VI chalcogen elements. In this paper, we systematically investigate the electronic, transport, optical, and thermoelectric properties of Janus monolayers ${\mathrm{In}}_{2}X\mathrm{O}$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) using first-principles calculations. Based on phonon spectrum analysis and ab initio molecular dynamics simulations at room temperature, ${\mathrm{In}}_{2}X\mathrm{O}$ monolayers were reported to be stable. Our calculations reveal that, while ${\mathrm{In}}_{2}\mathrm{SO}$ is an indirect semiconductor, ${\mathrm{In}}_{2}\mathrm{SeO}$ exhibits a direct semiconducting characteristic, and biaxial strain can lead to the semiconductor-metal phase transition in ${\mathrm{In}}_{2}\mathrm{SeO}$. Monolayer ${\mathrm{In}}_{2}\mathrm{TeO}$ is metal at equilibrium, and its metallic characteristics are prevented under biaxial strains. Calculations for transport properties show that the carrier mobilities of ${\mathrm{In}}_{2}\mathrm{SO}$ and ${\mathrm{In}}_{2}\mathrm{SeO}$ monolayers are highly anisotropic, and electron mobility of ${\mathrm{In}}_{2}\mathrm{SO}$ exceeds $3\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$. In this paper, the optical and thermoelectric properties of ${\mathrm{In}}_{2}\mathrm{SO}$ and ${\mathrm{In}}_{2}\mathrm{SeO}$ monolayers are also investigated and discussed in detail. Finally, the electronic properties of all four possible stacking configurations of the Janus bilayers are briefly calculated. Our findings not only contribute to a more general view of the physical properties of the Janus group III monochalcogenides but also recommend them as potential nanomaterials for applications in optoelectronic and thermal devices.

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In2 XY (X/Y = S, Se, Te with X ≠ Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In2 XY are dynamically and thermally stable at room temperature. At equilibrium, both In2STe and In2SeTe are direct semiconductors while In2SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In2 XY and their photocatalytic activity. Besides, the indirect–direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In2 XY is negligible. Meanwhile, the optical absorbance intensity of the Janus In2 XY monolayers is remarkably increased by compressive strain. Also, In2 XY monolayers exhibit very low lattice thermal conductivities resulting in a high figure of merit ZT, which makes them potential candidates for room-temperature thermoelectric materials.

The influences of an external electric field with uniform or modulated potential on the electronic and optical properties of armchair graphene nanoribbons (GNRs) are explored using the multi-orbital tight-binding Hamiltonian. The interplay between an electric field and interaction between (s, p x , p y , p z ) orbitals remarkably enriches the main features of band structures and absorption spectra. The applied electric field can notably alter the energy dispersions of π and σ bands, leading to the deformation of band-edge states, open and close of a band gap, and modification of the Fermi energy. The vertical optical excitations happen among the π bands, while their available channels depend on the Fermi level which is controlled by the σ-edge bands and a finite potential. With the rich and unique properties, GNRs are suitable candidates for applications in the fields of photodetectors, nanoelectronics, and spintronics. The calculated results are expected to be examined by the angle-resolved photoemission spectroscopies and optical spectroscopies.

Two-dimensional graphene-based van der Waals heterostructures have received considerable interest because of their intriguing characteristics compared with the constituent single-layer two-dimensional materials. Here, we investigate the interfacial characteristics, Schottky contact, and optical performance of $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ van der Waals (vdW) heterostructure using first-principles calculations. The effects of stacking patterns, electric gating, and interlayer coupling on the interfacial properties of $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures are also examined. Our results demonstrate that the Dirac cone of graphene is well preserved at the $\mathrm{\ensuremath{\Gamma}}$ point in all stacking patterns due to the weak vdW interactions, which keep the heterostructures feasible such that they can be obtained in further experiments. Moreover, depending on the stacking patterns, a small band gap of about 13--17 meV opens in graphene and has a high carrier mobility, indicating that the $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures are potential candidates for future high-speed nanoelectronic applications. In the ground state, the $\mathrm{graphene}/{\mathrm{Ga}}_{2}\mathrm{SSe}$ heterostructures form an $n$-type Schottky contact. The transformation from an $n$-type to a $p$-type Schottky contact or to an Ohmic contact can be forced by electric gating or by varying the interlayer coupling. Our findings could provide physical guidance for designing controllable Schottky nanodevices with high electronic and optical performances.

Van der Waals heterostructures, created by putting graphene on other two-dimensional semiconducting materials, have become an effective strategy to enhance the physical properties and extend the possible applications of two-dimensional (2D) materials. Motivated by the successful synthesis of a graphene/${\mathrm{PbI}}_{2}$ heterostructure in a recent experiment [Nat. Commun. 11, 823 (2020)], here we use first-principles calculations to construct and investigate the electronic properties and interface characteristics of graphene/${\mathrm{PbI}}_{2}$ heterostructure. We find that the weak forces occurring at the interface keep heterostructures stable and maintain the intrinsic properties of the constituent graphene and ${\mathrm{PbI}}_{2}$ monolayers. At the equilibrium interlayer distance of 3.48 \AA{}, the graphene/${\mathrm{PbI}}_{2}$ heterostructure forms an $n$-type Schottky contact. More interestingly, the Schottky barrier height and contact types in the graphene/${\mathrm{PbI}}_{2}$ heterostructure can be adjusted by electric field and interlayer coupling. The graphene/${\mathrm{PbI}}_{2}$ heterostructure can transform from a $n$-type Schottky contact to a $p$-type one or to Ohmic contact by applying electric field or by adjusting interlayer distance. The controllable electronic properties and contact types in graphene/${\mathrm{PbI}}_{2}$ heterostructure make it a promising candidate for designing and improving the performance of high-efficiency Schottky nanodevices.

In this work, by performing first-principles calculations, we explore the effects of various atom impurities on the electronic and magnetic properties of single layers of C

In this paper, detailed investigations of the electronic and optical properties of a Janus SnSSe monolayer under a biaxial strain and electric field using ab initio methods are presented. Our calculations indicate that the Janus SnSSe monolayer is a semiconductor with an indirect band gap larger/lower than that of the SnSe2/SnS2 monolayer. To obtain accurate estimates of the band gap, both Perdew–Burke–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE06) hybrid functionals have been used and the effect of spin–orbit coupling has also been included. While the influence of the electric field on the electronic and optical properties of the Janus SnSSe monolayer is quite weak, biaxial strain plays a key role in controlling these properties. The Janus SnSSe monolayer has a wide absorption spectrum, from visible light to the ultraviolet region. At equilibrium, the maximum absorption coefficient of the monolayer is up to 11.152 × 104 cm−1 in the ultraviolet region and it can be increased by strain engineering. With high absorption intensity in the visible light area and being able to tune the absorbance by strain, the Janus SnSSe monolayer becomes a promising material for applications in optoelectronic devices.

In this work, we systematically studied the electronic structure and optical characteristics of van der Waals (vdW) heterostructure composed of a single layer of GaN and GeC using first principles calculations. The GaN–GeC vdW heterostructure exhibits indirect band gap semiconductor properties and possesses type-II energy band arrangement, which will help the separation of photogenerated carriers and extend their lifetime. In addition, the band edge positions of the GaN–GeC heterostructure meet both the requirements of water oxidation and reduction energy, indicating that the photocatalysts have the potential for water decomposition. The GaN–GeC heterostructure shows obvious absorption peaks in the visible region, leading to the efficient use of solar energy. Tensile and compressive strains of up to 10% are also proposed. Tensile strain leads to an increase in the blue shift of optical absorption, whereas a red shift is observed in the case of the compressive strain. These fascinating characteristics make the GaN–GeC vdW heterostructure a highly effective photocatalyst for water splitting.

We study the optical absorption properties of silicene and germanene in the presence of the perpendicular magnetic and electric fields. Their low-energy Landau level (LL) spectra are controllable by an external electric field, where the spin- and valley-degeneracy of the LLs are strongly influenced by the electric and Zeeman fields. The electric field has removed spin-degeneracy at a given valley. Analytical expressions for the magneto-optical absorption coefficient (MOAC) are expressed in the presence of the interaction between carriers and random impurities and the intrinsic phonons including the spin and valley effects. The results evaluated for the topological insulator and valley-spin-polarized metal phases showed that when the electric field is included, the MOAC peaks are separated for opposite spin cases where the splitting in germanene is stronger than that in silicene. The peak's intensity caused by the carrier-photon-impurity scattering is the highest, the next is the carrier-photon scattering, while the carrier-photon-phonon scattering shows the lowest in both materials. Among the different phonon modes, the out-of-plane (ZA) mode in silicene dominates the others, being attributed to its buckled atomic structure, which does not exist in graphene. When the ZA mode is taken into account, the estimated resultant mobility from the full-width at half-maximum is significantly supported by the experimental result in silicene.

Vertical stacking of two-dimensional materials into layered van der Waals heterostructures has recently been considered as a promising candidate for photocatalytic and optoelectronic devices because it can combine the advantages of the individual 2D materials. Janus transition metal dichalcogenides (JTMDCs) have emerged as an appealing photocatalytic material due to the desirable electronic properties. Hence, in this work, we systematically investigate the geometric features, electronic properties, charge density difference, work function, band alignment and photocatalytic properties of ZnO–JTMDC heterostructures using first-principles calculations. Due to the different kinds of chalcogen atoms on both sides of JTMDC monolayers, two different possible stacking patterns of ZnO–JTMDC heterostructures have been constructed and considered. We find that all these stacking patterns of ZnO–JTMDC heterostructures are dynamically and energetically feasible. Moreover, both ZnO–MoSSe and ZnO–WSSe heterostructures are indirect band gap semiconductors and present type-I and type-II band alignments for model-I and model-II, respectively. The Rashba spin polarization of the ZnO–WSSe heterostructure for model-I is greater than that in the others. Furthermore, valence (conduction) band edge potentials are calculated to understand the photocatalytic behavior of these systems. Energetically favorable band edge positions in ZnO–Janus heterostructures make them suitable for water splitting at zero pH. We found that the ZnO–Janus heterostructures are promising candidates for water splitting with conduction and valence band edges positioned just outside of the redox interval.

Constructing vertical heterostructures by placing graphene (Gr) on two-dimensional materials has recently emerged as an effective way to enhance the performance of nanoelectronic and optoelectronic devices. In this work, first principles calculations are employed to explore the structural and electronic properties of Gr/GeC and Gr/functionalized-GeC by H/F/Cl surface functionalization. Our results imply that the electronic properties of the Gr, GeC and all functionalized-GeC monolayers are well preserved in Gr/GeC and Gr/functionalized-GeC heterostructures, and the Gr/GeC heterostructure forms a p-type Schottky contact. Interestingly, we find that the p-type Schottky contact in Gr/GeC can be converted into the n-type one and into an n-type ohmic contact by H/F/Cl surface functionalization to form Gr/functionalized-GeC heterostructures. Furthermore, we find that electric fields and strain engineering can change both the Schottky barrier heights and the contact types of the Gr/functionalized-GeC vdWHs. These findings suggest that Gr/functionalized-GeC heterostructures can be considered as a promising candidate for designing high-performance optoelectronic and nanoelectronic devices.

In this work, we consider the electronic and optical properties of chemically functionalized InN monolayers with F and Cl atoms (i.e., F–InN–F, F–InN–Cl, Cl–InN–F, Cl–InN–Cl monolayers) using first-principles calculations. The adsorption of the F and Cl atoms on the InN monolayer is determined to be chemically stable and the F–InN–F monolayer is most likely to occur. Our calculations show that the chemical functionalization with Cl and F atoms not only breaks the planar structure of InN monolayer but also increases its band gap. By using both Perdew, Burke, and Ernzerhof (PBE) and the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functionals, all four models of chemically functionalized InN monolayers are found to be semiconductors with direct energy gaps and these gaps depend on the constituent species. When the spin–orbit coupling (SOC) was included, the energy gap of these monolayers was reduced and an energy splitting was found at the Γ-point in the valence band. Chemically functionalized InN monolayers can absorb light in a wide region, especially the F–InN–F and Cl–InN–F monolayers have a strong ability to absorb the visible light. Our findings reveal that the chemically functionalized InN monolayers have potential applications in next-generation optoelectronic devices.

We study the optical transport properties of the monolayer transition metal dichalcogenides (TMDCs) such as ${\mathrm{MoS}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$ in the presence of a magnetic field. The TMDCs band structures are obtained and discussed by using the effective massive Dirac model, in which the spin and valley Zeeman effects as well as an external electric field are included. The magneto-optical absorption coefficient (MOAC) is derived as a function of absorbed photon energy when the carriers are scattered by random impurities combined with the intrinsic acoustic and optical phonons in TMDCs and the surface optical (SO) phonons of substrates. Our result shows that the spin-splitting feature appeared in all four TMDC materials. The combination of strong spin-orbit coupling (SOC) and Zeeman fields has doubled the Landau levels but has not changed the energy gap of the TMDCs monolayer, which can be controlled by the electric field. Because of their strong SOC effect, the absorption spectrum in monolayer TMDCs is separated into two separate peaks caused by spin up and down. At the low temperature, the MOAC intensity via impurity scattering is the biggest followed by that of the SO phonons while the intrinsic acoustic and optical phonon scatterings display the smallest. For the monolayer TMDCs on substrates, ${\mathrm{SiO}}_{2}$ always shows its superiority in comparison with the others. Among the four TMDC materials, ${\mathrm{MoSe}}_{2}$ shows the biggest MOAC intensity, while ${\mathrm{WS}}_{2}$ has the biggest value of the absorbed photon energy. The full-width at half-maximum (FWHM) via impurity scattering achieves its highest value in ${\mathrm{WS}}_{2}$, while this occurs in ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ via intrinsic acoustic and optical phonon scatterings, respectively. Our estimation of mobility from FWHM gives good agreement with the experimental results in ${\mathrm{WS}}_{2}$.