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Very recently, the 2D form of poly-benzimidazobenzophenanthroline (BBL) structures has been successfully fabricated [Noh et al., Nat. Commun. 369, 670 (2020)]. Motivated by these exciting experimental results on 2D layered BBL structures, herein we perform density functional theory-based first-principles calculations in order to gain insight into the structural, electronic, and optical properties of the BBL monolayer and bilayer honeycomb crystal structures (planar and vertical). Our computational structural optimization reveals that the BBL monolayer crystallizes in a puckered, anisotropic hexagonal structure, while the BBL bilayer is composed of covalently bonded shifted one with respect to the other BBL layers. Two terminations with hydrogen and fluorine atoms are considered for the BBL bilayer, namely, H-BBL and F-BBL, respectively. The direct bandgaps of H-BBLs and F-BBLs are ∼ 1 eV and ∼ 1.2 eV. The top of the valence band and the bottom of the conduction band are flat due to the localized carbon states. The BBL monolayer and bilayer can absorb a wide range of visible light. The calculated refractive index of the BBL monolayer is ∼ 1, i.e., it is smaller than the refractive index of the common natural or synthetic polymers.

In this paper, the electronic, optical, and photocatalytic properties of GaSe/HfS2 heterostructure are studied via first-principles calculations. The stability of the vertically stacked heterobilayers is validated by the binding energy, phonon spectrum, and ab initio molecular dynamics simulation. The results reveal that the most stable GaSe/HfS2 heterobilayer retains a type-II alignment with an indirect bandgap 1.40 eV. As well, the results also show strong optical absorption intensity in the studied heterostructure (1.8 × 105 cm−1). The calculated hole mobility is 1376 cm2 V−1 s−1, while electron mobility reaches 911 cm2 V−1 s−1 along the armchair and zigzag directions. By applying an external electric field, the bandgap and band offset of the designed heterostructure can be effectively modified. Remarkably, a stronger external electric field can create nearly free electron states in the vicinity of the bottom of the conduction band, which induces indirect-to-direct bandgap transition as well as a semiconductor-to-metal transition. In contrast, the electronic properties of GaSe/HfS2 heterostructure are predicted to be insensitive to biaxial strain. The current work reveals that GaSe/HfS2 heterostructure is a promising candidate as a novel photocatalytic material for hydrogen generation in the visible range.

In present work, we study the electronic, optical and photocatalytic properties of strained MoSSe monolayer through first-principles study. A single layer Janus MoSSe possesses a semiconducting character with a direct band gap of 1.59/2.09 eV obtained by PBE/HSE06 method. The valence band maximum (VBM) of Janus MoSSe monolayer is mainly contributed by the S-p orbital, whereas the conduction band minimum (CBM) comes from the Mo- d x 2 . Furthermore, Janus MoSSe monolayer has been proved to be energetically stable with no imaginary frequency in its phonon spectrum. Interesting, both the tensile and compressive strains can transform Janus MoSSe monolayer from direct to indirect band gap nature as well as tune its band gap. The compressive strain tends to an increase in the band gap, whereas the tensile strain leads to decrease in the band gap. Optical absorption of Janus MoSSe monolayer demonstrates that the tensile strain gives rise to an existence of blue shift, while compressive strain is responsible for the formation of a red shift. Photocatalytic properties show that Janus MoSSe monolayer with 4% or 6% strained could be a catalyst for the H2O oxidation, making it suitable for water splitting applications.

Electronic structure, optical, and photocatalytic properties of SnS, SnSe and their van der Waals heterostructures are investigated by first-principle calculations. Thermal stability confirmed that SnS, SnSe and SnS-SnSe van der Waals heterostructure are thermodynamically stable. The calculated band structure shows that SnS, SnSe and SnS-SnSe van der Waals heterostructure are indirect band nature while the heterostructure are confirmed for type-II band alignment. Bader charge analysis shows that the charges are transfer from SnS layer to SnSe layer. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions are lies in visible region. The valence and conduction band edges straddle the standard redox potentials in SnS, SnSe and their van der Waals heterostructures van der Waals heterostructures, making them promising candidates for water splitting in the acidic solution.

The effect of biaxial strains e b and electric field E on the electronic structure and optical properties of Janus monolayer PtSSe was studied by Density Functional Theory (DFT). A reasonable band gap of PtSSe was found to be 1.547 eV. In the infrared region, both biaxial strains and electric fields result in noticeable enhancement of the electronic structure as well as optical properties of PtSSe. Especially, under biaxial strains, the change of PtSSe band gap obeys the form of an asymmetric concave down parabola. This result confirms the existence of a maximum PtSSe band gap under biaxial strains e b and the possibility of tuning PtSSe band gap to fit the requirement of the optoelectronic devices. The absorption rate in the visible light region of Janus monolayer PtSSe increases sharply and can be altered by strain engineering. Biaxial strain not only alters the absorption intensity but can also significantly shift the position of these absorption peaks. The present study provides additional information about the strain and electric field-induced electronic structure and optical properties of Janus monolayer PtSSe, which should be taken into account for better PtSSe-based devices.

Tin sulfide (SnS) is known for its effective gas-detecting ability at low temperatures. However, the development of a portable and flexible SnS sensor is hindered by its high resistance, low response, and long recovery time. Like other chalcogenides, the electronic and gas-sensing properties of SnS strongly depend on its surface defects. Therefore, understanding the effects of its surface defects on its electronic and gas-sensing properties is a key factor in developing low-temperature SnS gas sensors. Herein, using thin SnS films annealed at different temperatures, we demonstrate that SnS exhibits n-type semiconducting behavior upon the appearance of S vacancies. Furthermore, the presence of S vacancies imparts the n-type SnS sensor with better sensing performance under UV illumination at room temperature (25 &deg, C) than that of a p-type SnS sensor. These results are thoroughly investigated using various experimental analysis techniques and theoretical calculations using density functional theory. In addition, n-type SnS deposited on a polyimide substrate can be used to fabricate high-stability flexible sensors, which can be further developed for real applications.

In this work, we construct the BlueP/ZrSSe heterostructure and explore systematically its electronic characteristics and interface features in the framework of first principles calculations. The stacking and electric field effects on the interface characters of BlueP/ZrSSe heterostructure are also considered. We find that the BlueP layer interacts with Janus ZrSSe layer via the weak van der Waals forces, which keeps the BlueP/ZrSSe heterostructure feasible. Both the BlueP/SZrSe and BlueP/SeZrS heterostructures possess indirect semiconductor and exhibits type-I band alignment. Furthermore, electric field can tune the band alignment and switch the BlueP/ZrSSe heterostructure from semiconductor to metal. These findings could provide a helpful guidance for using BlueP/ZrSSe heterostructure in practical applications of nanoelectronics and optoelectronics.

In this work, detailed investigations of the electronic and optical properties of a Janus Ga2STe 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 Ga2STe monolayer are verified. Our obtained results showed that the Janus Ga2STe 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 Ga2STe. At equilibrium, the optical absorbance of the Janus Ga2STe 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 Ga2STe 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 Ga2STe monolayer has many promising applications in opto-electronic devices but also motivates experimental works on Janus structures in near future.

In this paper, the optical absorption coefficients (OACs) and the refractive index changes (RICs) due to both intra- and inter-band transitions in monolayer WS 2 are theoretically investigated in the presence of a magnetic field. The results showed that both OACs and RICs display the blue-shift behavior with the increase of the magnetic field. The Zeeman fields have no effect on the peak positions but reduce slightly peak intensities. Besides, the strong spin–orbit coupling in monolayer WS 2 causes the significant difference of the peak due to spin up and down. The OACs and RICs due to intra-band transition display only one peak in the THz range, while the inter-band spectra show a series of peaks in the near-infrared to the visible optical range, making monolayer WS 2 to be a promising candidate for novel optoelectronic applications.

In this paper, we study the effect of uniaxial and biaxial strain on the structural and electronic properties of MoS monolayers by first-principle calculations based on density functional theory. Our calculations show that the bond length between Mo and S atoms depends linearly on the strain. At the equilibrium state, MoS has a direct band gap of 1.72 eV opening at the K-point. However, an indirect–direct band gap transition has been found in MoS monolayer when the strain is introduced. MoS becomes a semiconductor with an indirect band gap when the uniaxial strain or the biaxial strain . Under biaxial strain, a metal–semiconductor transition occurs at of elongation. The indirect character and phase transition will largely constrain application of MoS monolayer to electronic and optical devices.

In this work, transport properties of armchair graphene nanoribbons (AGNRs) under uniaxial strain are considered using density functional theory. We found that carrier mobility of AGNRs depends strongly on uniaxial strain. The electron mobility of 5-AGNR is up to 38.5 × 10 4 cm 2 / V s at an elongation of 6%. However, the dependence of the effective mass of electrons and holes of AGNR on uniaxial strain can almost be described by the same function and their effective masses coincide at an elongation of 10%. The sensitivity to strain of the transport properties of AGNRs opens many ways for applications in nanoelectromechanical devices.

We used ab initio calculations to systematically investigate the adsorption of atomic oxygen on non-stoichiometric polar TiC(111) and Ti x C y (111) with Ti/C vacancies surface simulating its potential tructions with laser radiation. Local atomic structures of O/Ti x C y (111) polar surfaces were studied in the selected models as well as their thermodynamic and electronic properties based on the density functional theory. The bond length and adsorption energy for various reconstructions of the O/Ti x C y (111) surface atomic structure were established. We also have examined the effects of oxygen adsorption upon the band and electron spectra of TiC(111) surface in its various reconstructions. We have established a correlation between the energy level of flat bands (− 5.1 eV and − 5.7 eV) responsible for the doublet of singular peaks corresponding to partial densities of oxygen 2 p electrons and the energy of oxygen adsorption in non-stoichiometric O/TiC y (111) systems. Effective charges of the oxygen atom and the titanium and carbon atoms nearest to it were identified in the examined adsorption models. We have established charge transfer from titanium atom to oxygen and carbon atoms determined by the reconstruction of local atomic and electronic structures. Charge transfer correlates with the electronegativity values of titanium, carbon, and oxygen atoms, and chemisorption processes. Calculated values of structural parameters in the studied models of ultrathin O/TiC(111) and O/Ti x C y (111) films correlate well with experimental findings and other theoretical results.

In this work, we investigate band-gap tuning in bilayer MoS2 by an external electric field and by applied biaxial strain. Our calculations show that the band gaps of bilayer MoS2 can be tuned by the perpendicular electric field or biaxial strain. The band gaps of bilayer MoS2 decrease with increasing applied electric field or biaxial strain. When the electric field was introduced, electronic levels are split due to the separation of the valence sub-band and the conduction sub-band states. Our calculations also show that the change in the band gap of bilayer MoS2 is due to the separation of electronic levels by electric field via the Stark effect. At the electric field $$E_{\mathrm{Field}} = 5.5$$ V/nm or biaxial strain $$\varepsilon = 15\%,$$ bilayer MoS2 becomes metallic. The semiconductor–metal phase transition in bilayer MoS2 plays an important role in its application for nanodevices.

In this work, making use of density functional theory (DFT) computations, we systematically investigate the effect of biaxial strain engineering and external electric field applied perpendicular to the layers on the band gaps and electronic properties of monolayer MoS 2 . The direct-to-indirect band gaps and semiconductor-to-metal transition are observed in monolayer MoS 2 when strain and electric field are applied in our calculation. We show that when the biaxial strain and external electric field are introduced, the electronic properties including band gaps of monolayer MoS 2 can be reduced to zero. Our results provide many useful insights for the wide applications of monolayer MoS 2 in electronics and optoelectronics.

Herein, using first-principles calculations the structural and electronic properties of the Ti(2)CO(2)MXene monolayer with and without oxygen vacancies are systematically investigated with different defect concentrations and patterns, including partial, linear, local, and hexagonal types. The Ti(2)CO(2)monolayer is found to be a semiconductor with a bandgap of 0.35 eV. The introduction of oxygen vacancies tends to increase the bandgap and leads to electronic phase transitions from nonmagnetic semiconductors to half-metals. Moreover, the semiconducting characteristic of O-vacancy Ti(2)CO(2)can be adjusted via electric fields, strain, and F-atom substitution. In particular, an electric field can be used to alter the nonmagnetic semiconductor of O-vacancy Ti(2)CO(2)into a magnetic one or into a half-metal, whereas the electronic phase transition from a semiconductor to metal can be achieved by applying strain and F-atom substitution. The results provide a useful guide for practical applications of O-vacancy Ti(2)CO(2)monolayers in nanoelectronic and spinstronic nanodevices. National Research Foundation of Korea (NRF) - Korean government (MSIT) [NRF-2017R1A2B2011989] This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2017R1A2B2011989).

Hydrogen generation via photocatalytic water splitting is considered as a source of clean energy and an alternative approach to deal with energy crises and environmental problems. By using first-principles computations, we determine that boron phosphide-blue phosphorene van der Waals heterostructure is a potential candidate for overall water splitting at low pH values (0–3). The heterostructure is a semiconductor with a direct band gap of 1.47 eV and possesses type-II band alignment. More importantly, this heterostructure contents the water reduction and oxidation levels for water splitting, with enhanced optical absorption in infrared, visible and ultraviolet regions. Particularly, tensile strain can enhance the optical absorption in the visible range and increase the solar energy conversion efficiency. Our study widens the application of boron phosphide-blue phosphorene heterostructures, and helps design more heterostructured photocatalysts.

We study the magneto-optical properties of a Poschl–Teller-like quantum well made of different materials such as GaAs, GaSb, InAs, and InSb. The energy difference becomes smaller with the wider well-width and becomes bigger with the deeper well-depth. The energy difference appears smallest in GaAs, followed by that in GaSb, while InSb shows the largest. The magneto-optical absorption coefficient (MOAC) shifts to the lower (higher) energy side when the well-width (well-depth and magnetic field) increases, but does not change the position with the change of temperature. The full-width at half-maximum (FWHM) increases with the increase of the well-depth, the temperature, and the magnetic field but decreases with the well-width. The phonon-emission process always happens stronger than the absorption one. The MOAC intensity in GaAs is the highest. The temperature-dependent-FWHM is supported by experimental results in GaAs quantum well.

Motivated by some earlier experimental observations on the magneto-optical absorption and cyclotron resonance in graphene, we carry out systematically a theoretical investigation of the magneto-optical absorption in graphene monolayer situated on a substrate. The effect of electron-impurity scattering is considered at low temperatures. The magneto-optical absorption coefficient (AC) is calculated using the perturbation theory taking account of both one- and two-photon absorption processes. Numerical results are obtained for the SiO2, SiC, and h-BN substrates to compare and show effects of the substrate. There appear the cyclotron-impurity resonance peaks in the AC which show a blue-shift with increasing magnetic field strength. In general, the absorption intensity, the resonance energy and the FWHM depend strongly on the substrate. Also, the FWHM increases with increasing the impurity density by the law FWHM = β n i , where β is a constant whose value is different in different substrates and n i the impurity density. In particular, the resonance energy versus magnetic field strength and the full width at half maximum (FWHM) for the SiO2 substrate are in good agreement with those obtained by earlier experiments.

The combination of two-dimensional materials in the form of van der Waals heterostructures has been proved to be an effective approach for designing electronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of vdW heterostructures based on BlueP, SiC, ZnO, and g-GaN using density functional theory. We find that all the g-GaN based vdW heterostructures are energetically and thermally stable at room temperature. The g-GaN–BlueP and g-GaN–SiC heterostructures show indirect bandgaps with the type-II and type-I band alignments, respectively, whereas the g-GaN–ZnO heterostructure shows a direct bandgap with type-II band alignment. Furthermore, the absorption coefficient is also calculated to understand the optical behavior of these hetrostructures. Our results demonstrate that the lowest energy transitions are dominated by excitons, and the blue shift is also observed in these hetrostructures. The g-GaN–BlueP, g-GaN–SiC, and g-GaN–ZnO vdW heterostructures possess outstanding optical absorption in the visible light. The g-GaN–P shows the highest absorption intensity of 10 5 cm − 1, which is larger than that of g-GaN–SiC and g-GaN–ZnO vdW heterostructures by three times. These findings demonstrate that these vdW heterostructures are promising candidates for water splitting in the visible light region. Moreover, the heterostructures also show good response to the photocatalytic properties at pH = 0 and pH = 7.

In this work, we extensively investigate the structural and electronic properties of van der Waals heterostructures (HTs) constructed by MoS${_2}$/$BC_{3}$, MoS${_2}$/$C_{3}N$, MoS${_2}$/$C_{3}N_{4}$, MoS${_2}$/$C_{4}N_{3}$ and those using Janus MoSSe instead of MoS$_2$ by performing density functional theory calculations. The electronic band structure calculations and the corresponding partial density of states reveal that the significant changes are driven by quite strong layer-layer interaction between the constitutive layers. Our results show that although all monolayers are semiconductors as free-standing layers, the MoS${_2}$/$C_{3}N$ and MoS${_2}$/$C_{4}N_{3}$ bilayer HTs display metallic behavior as a consequence of transfer of charge carriers between two constituent layers. In addition, it is found that in MoSSe/$C_{3}N$ bilayer HT, the degree of metallicity is affected by the interface chalcogen atom type when Se atoms are facing to $C_{3}N$ layer, the overlap of the bands around the Fermi level is smaller. Moreover, the half-metallic magnetic $C_{4}N_{3}$ is shown to form magnetic half-metallic trilayer HT with MoS$_2$ independent of the stacking sequence, i.e. whether it is sandwiched or two $C_{4}N_{3}$ layer encapsulate MoS$_2$ layer. We further analyze the trilayer HTs in which MoS$_2$ is encapsulated by two different monolayers and it is revealed that at least with one magnetic monolayer, it is possible to construct a magnetic trilayer. While the trilayer of $C_{4}N_{3}$/MoS${_2}$/$BC_{3}$ and $C_{4}N_{3}$/MoS${_2}$/$C_{3}N_{4}$ exhibit half-metallic characteristics, $C_{4}N_{3}$/MoS${_2}$/$C_{3}$N possesses a magnetic metallic ground state. Overall, our results reveal that holly structures of BCN crystals are suitable for heterostructure formation even over van der Waals type interaction which significantly changes electronic nature of the constituent layers.

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.

In this work, we construct van der Waals heterostructure based on graphene and InN monolayer and analyze the interface, electronic and optical properties of formed heterostructure using first-principles calculations. Our results prove that the graphene/InN heterostructure is mainly contributed by the weak vdW interactions, which keep it feasible and thus it can be realized easily in recent experiment. Moreover, the intrinsic superior properties of both graphene and InN monolayers are well preserved in the graphene/InN heterostructure. In the graphene/InN heterostructure, InN layer becomes the electron-rich layer, while the graphene layer becomes the hole-rich layer, leading to the transportation of electrons from graphene to InN monolayer. The heterostructure possesses high performance absorption in the visible and near-infrared regions, revealing it is useful for acquiring efficient photocatalysts. The graphene/InN heterostructure at the equilibrium state form the n-type Schottky contact with a small of Schottky barrier height of 0.246 eV. More interestingly, our results predict that external electric fields can tune the graphene/InN heterostructure from n-type to p-type Schottky contact and to n-type Ohmic contact, demonstrating that such heterostructure is appreciate material for highly integrated devices, such as Schottky devices and Schottky diodes.

Combining different two-dimensional materials into layered van der Waals heterostructures are recently considered as an effective route to enhance the electronic properties of the constituent materials and to extend the application range in next-generation nanodevices. Here, the GeC/GaSe heterostructure and its electronic properties controlled by electric field have been constructed and systematically investigated using first-principles calculations. Five different stacking patterns of the GeC/GaSe heterostructure are constructed to consider the stacking effects on the electronic properties. We find that the GeC/GaSe heterostructure is mainly characterized by the weak van der Waals forces, dominating between GeC and GaSe layers, preserving their intrinsic properties in GeC/GaSe heterostructure. The GeC/GaSe heterostructure exhibits the type-II band alignment, where the electron–hole pairs are separated, making it suitable for fabricating next-generation optoelectronic nanodevices. Moreover, the stacking configurations have little affect the structural and electronic properties of the GeC/GaSe heterostructures. The pattern-I stacking configuration has the lowest binding energy and shortest interlayer distance as compared with other stacking patterns of the GeC/GaSe heterostructure. Furthermore, our results demonstrate that the electric field is considered as an effective route to modulate the electronic properties of GeC/GaSe heterostructure from semiconductor to metal. This finding makes the GeC/GaSe heterostructure promising material for optoelectronic nanodevices.

In this work, we investigate the low-energy bands of monolayer WS2 under external electric and magnetic fields using an effective low-energy Hamiltonian. Our calculations indicate that band gap of monolayer WS2 increases linearly with an external perpendicular electric field and when the electric field energy Δ z is equal to the size of spin splitting, the band gap including spin–orbit interaction of the monolayer increases to its band gap as the case of without spin–orbit interaction. In the presence of the magnetic field, spin splitting energy in the conduction and the valence bands at the valleys depends strongly on the magnetic field. In this work, the optical properties of monolayer WS2 are also calculated by using the Ehrenreich–Cohen formula. Our estimation for the square of the interband matrix element for transitions near the K points is in good agreement with previous calculations.

Constructing van der Waals heterostructures can be considered as an effective strategy for generating new physical phenomena that merit for high-efficiency nanodevices. Here, we construct the PbI 2 / SnSe 2 heterostructure using first-principles calculations and explore its electronic structure and optical performance as well as the effects of electric fields. The PbI 2 / SnSe 2 heterostructure owns an indirect band gap and exhibits type-I band alignment, making it promising candidate for light emission applications. Furthermore, the optical absorption of the PbI 2 / SnSe 2 heterostructure in both the visible light and ultra-violet regions is enhanced as compared with that of the PbI 2 and SnSe 2 monolayers. Furthermore, in the PbI 2 / SnSe 2 heterostructure, the electric fields can convert the type-I to type-II band alignment and tune the transition from semiconductor to metal. Our results provide useful guidance for practical application in high-efficiency nanodevices.

We investigate the electronic, photocatalytic and optical properties of a fully hydrogenated indium nitride H–InN–H monolayer under biaxial strain e 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 e 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.

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.

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 energyregion. 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. Refereed/Peer-reviewed

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

In the present work, the effect of biaxial and uniaxial strains on the electronic and optical properties of heterostructure MoS2/WS2 has been studied by density functional theory. Our numerical calculations demonstrate that the heterostructure MoS2/WS2 is an indirect semiconductor with a type-II band alignment and its energy gap is 1.45 eV. Besides, the semiconductor–metal transition can be found at large biaxial strain of 14% in the van der Waals heterostructure MoS2/WS2. Influence of the strain engineering on the optical spectra of the heterostructure MoS2/WS2 are also considered in this work. Our calculations indicate that, while the effect of uniaxial strain on the optical properties is quite weak, the optical parameters of the heterostructure MoS2/WS2 depend strongly on the biaxial strain. Our predicted results may be a reference for future experimental and theoretical studies of the heterostructures.

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 e b , the semiconductor-metal phase transition happens at large compressive strain of - 10 % and the indirect-direct gap transition occurs at e 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 e b = 5 % and we can enhance its photocatalytic activity by strain.

The stacking of monolayers in the form of van der Waals heterostructures is a useful strategy for band gap engineering and the control of dynamics of excitons for potential nano-electronic devices. We performed first-principles calculations to investigate the structural, electronic, optical and photocatalytic properties of the SiC-MX

In this work, the phonon-assisted cyclotron resonance in a special symmetric quantum well is studied in detail by evaluating the magneto-optical absorption coefficient (MOAC) and the full width at half maximum (FWHM). The results include the two-photon absorption process and the electron–phonon interaction as well. The influence of the magnetic field, the quantum well parameter, and the temperature on the MOAC and the FWHM is illustrated plainly. Our results are in good agreement with those accessible to the theoretical and experimental works of literature. In addition, we found a new rule of the temperature-dependent FWHM for the phonon absorption process, which should be examined by an experiment work in the future.

Hot electrons cooling by phonons in GaAs/AlAs cylindrical quantum wire (CQW), under the influence of an intense electromagnetic wave (EMW), is studied theoretically. Analytic expression for the electron cooling power (CP) is derived from the quantum transport equation for phonons, using the Hamiltonian of interacting electron–optical phonon system. Both photon absorption and emission processes are considered. Numerical results show that the CP reaches maximum when the energy difference between electronic subbands equals the energy of an optical phonon plus the photon energy. Under the influence of the EMW, the negative CP is observed showing that electrons gain energy from phonon and photon instead of losing their energy. Also, the CP increases with increasing the EMW amplitude. Our results theoretically clarify the mechanism of the electron cooling process by phonons in the GaAs/AlAs CQW under the EMW, which is of significance for designing and fabricating high-speed nanoelectronic devices based on this material.

In this work, we investigate the structural and electronic properties of the combined InSe/Ca(OH)2 heterostructure through density functional theory. It suggests that a combination of InSe and Ca(OH)2 tends to a significant decrease in the band gap of the heterostructure, which may result from the vacuum energy difference of the monolayers. The InSe/Ca(OH)2 heterostructure mediates by the weak vdW interactions and possesses a type-II semiconductor with a direct band gap of 0.55 eV, which can also be engineered by applying electric field or vertical strains. The semiconductor-to-metal and direct-to-indirect transitions can also emerge, which make InSe/Ca(OH)2 heterostructure promising material for electronic nanodevices.

First principles calculations are performed to investigate the structural and electronic properties of MX2 (M = Nb, Pt; X = S, Se) monolayers and their van der Waals (vdW) heterostructures. The dynamical stability of monolayers and vdW heterostructures is confirmed by binding energy and phonon spectra. An indirect band gap nature is found for PtS2 and PtSe2 monolayers while NbS2, NbSe2 and all vdW heterostructures are metals. The intrinsic electronic properties of both NbX2 and PtX2 are well preserved due to weak vdW contact. It is demonstrated that a p-type Schottky contact with a small barrier height is formed at NbX2-PtX2 interface. The zero tunnel barrier and higher potential drop across the interface in these contacts imply large transfer of charge carriers across the interface, making them potential candidates in nanoelectronic device applications.

In this work, using density functional theory we investigated systematically the electronic properties and Schottky barrier modulation in a multilayer graphene/bilayer-GaSe heterostructure by varying the interlayer spacing and by applying an external electric field. At the equilibrium state, the graphene is bound to bilayer-GaSe by a weak van der Waals interaction with the interlayer distance d of 3.40 A with the binding energy per carbon atom of -37.71 meV. The projected band structure of the graphene/bilayer-GaSe heterostructure appears as a combination of each band structure of graphene and bilayer-GaSe. Moreover, a tiny band gap of about 10 meV is opened at the Dirac point in the graphene/bilayer-GaSe heterostructure due to the sublattice symmetry breaking. The band gap opening in graphene makes it suitable for potential applications in nanoelectronic and optoelectronic devices. The graphene/bilayer-GaSe heterostructure forms an n-type Schottky contact with the Schottky barrier height of 0.72 eV at the equilibrium interlayer spacing. Furthermore, a transformation from the n-type to p-type Schottky contact could be performed by decreasing the interlayer distance or by applying an electric field. This transformation is observed when the interlayer distance is smaller than 3.30 A, or when the applied positive external electric field is larger than 0.0125 V A-1. These results are very important for designing new electronic Schottky devices based on graphene and other 2D semiconductors such as a graphene/bilayer-GaSe heterostructure.

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. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant Number 103.01-2016.07 and the Belarusian Scientific Program ”Convergence”.

Modulation of semiconductor–halfmetal–metal transition in the antiferromagnetic (AF) ordering of bilayer zigzag graphene nanoribbons (BZGNRs) on hexagonal boron nitride (h-BN) heterostructure using the external force field F ext and transverse electric fields E ext (in the presence of interaction with the substrate) was performed within the framework of the density functional theory (DFT). We established critical values of E ext and interlayer distance in the bilayer providing for semiconductor–halfmetal–metal transition in one of electron spin configurations. Our calculations also show that the energy gap E g in the AF-BZGNR/h-BN(0001) heterostructure can be controlled in a wide range of the F ext and E ext . This makes the AF-8-ZGNR/h-BN(0001) heterostructure being potentially promising for application in spintronic devices.

Band structure investigation results of two-dimensional (2D) graphene (SLG) on Al 2 O 3 (0001) using the density functional theory (DFT) method as a possible element base for spintronics are presented. Regularities of a band structure change in the order three-dimensional (3D) Al 2 O 3 → 2D Al 2 O 3 (0001) → 2D SLG/Al 2 O 3 (0001) as well as features of a chemical bond between SLG and sapphire on the basis of DFT calculations have been studied. Analysis of the band structure and interatomic spacing in the interface for both models allows speaking about physical SLG adsorption on the (0001)-surface sapphire constrained by aluminum atoms. Energy distribution features of surface states in 2D SLG/Al 2 O 3 (0001) interface are discussed. Analysis of effective atomic charge in the interface revealed surface charge fluctuations on the substrate in the presence of SLG, which can be explained by a decrease of the energy of occupied subsurface Al 2 O 3 states relatively to the Fermi level.

The effect of an external electric field on the electronic and magnetic properties of the heterostructure of zigzag graphene nanoribbons (ZGNRs) placed on an aluminium nitride nanosheet (AlNNS) is studied using density functional theory (DFT). DFT calculations show that the local magnetic moments and total magnetization of the edge carbon atoms in the 4-ZGNR/AlNNS with a spin up electron subsystem are strongly dependent on a transverse electric field. We can control the band gap of the 4-ZGNR/AlNNS by using an external electric field. We established the critical values of the transverse electric field providing for semiconductor–metal phase transition in spin down electron configuration, which opens up potential opportunities for applications in spintronics devices. The effect of an external electric field (both amplitude and direction) on the effective charge and formation of the interface state energy is also studied and discussed.

The phonon-assisted cyclotron-resonance (PACR) effect due to the one-photon (1P) and two-photon (2P) absorption processes in a quantum well (QW) with the Poschl-Teller (PT) potential is studied. The magneto-optical absorption coefficient as functions of photon energy for different QW-parameters, magnetic fields, and temperatures is presented. The calculations are done for four different possible cases: both 1P- and 2P-processes as well as both phonon emission and absorption processes. The numerical results for a GaAs, InAs, InSb, and GaSb PT-QW show that the PACR peaks are blueshifted with κ and magnetic field, redshifted with QW-length, but remained unchanged with temperature. Peak intensities are found to increase with κ and magnetic field and decrease with QW-length but have a complex dependency on the temperature. The full-width at half-maximum is found to increase with κ, magnetic field, and temperature but found to decrease with QW-length. The phonon emission process is always dominant in comparison with absorption. The 2P-process is comparable with the 1P-process and should be taken into account when studying the PACR in QWs.The phonon-assisted cyclotron-resonance (PACR) effect due to the one-photon (1P) and two-photon (2P) absorption processes in a quantum well (QW) with the Poschl-Teller (PT) potential is studied. The magneto-optical absorption coefficient as functions of photon energy for different QW-parameters, magnetic fields, and temperatures is presented. The calculations are done for four different possible cases: both 1P- and 2P-processes as well as both phonon emission and absorption processes. The numerical results for a GaAs, InAs, InSb, and GaSb PT-QW show that the PACR peaks are blueshifted with κ and magnetic field, redshifted with QW-length, but remained unchanged with temperature. Peak intensities are found to increase with κ and magnetic field and decrease with QW-length but have a complex dependency on the temperature. The full-width at half-maximum is found to increase with κ, magnetic field, and temperature but found to decrease with QW-length. The phonon emission process is always dominant in comp...

In this work, we propose an ultrathin graphene/Ca(OH) 2 van der Waals heterostructure (vdWH) and investigate its structural stability, electronic structures and Schottky contact types modulation by ab initio calculations. Our results show the preservation of graphene and Ca(OH) 2 intrinsic electronic properties in graphene/Ca(OH) 2 vdWH, which is mainly characterized by the physicoadsorption interaction with the binding energy of -33.37 meV per carbon atom. Ca(OH) 2 monolayer stacking on graphene to form the vdWH forms the p -type Schottky contact and opens a valuable graphene’s band gap of 9.7 meV, suggesting its promising application in high speed nanoelectronic devices. Furthermore, electric field and vertical strain can be used to modulate the Schottky contact from the p -type to the n -type one and to Ohmic contact. These predictions demonstrate the potential candidate of the G/Ca(OH) 2 vdWH for future nanoelectronic applications.

We present electric field effects on the optical field-dependent dielectric function of phosphorene. We use a two-band model and the Kubo formula to obtain refractive index and absorption coefficient. From our findings, the refraction (absorption) along the armchair direction is smaller (larger) than the zigzag direction for the optical energies close to and larger than the band gap. Moreover, we find that for frequencies beneath the band gap, dynamical dielectric function decreases with the electric field independent of the orientation, whereas there is a strong anisotropic treatment above the band gap.

In this work, we propose ultrathin trilayered heterostructures (TL-HTSs) of graphene (G), gallium selenide (GaSe), and molybdenum selenide ( MoSe 2) monolayers and investigate their structural and electronic properties in the framework of first-principles calculations. By calculating the binding energies and interlayer distances and comparing them with those of the typical vdW HTSs, we find that the systems we consider are energetically stable and are characterized by weak vdW interactions. The formation of G, GaSe, and MoSe 2 monolayers to form G / GaSe / MoSe 2, GaSe / G / MoSe 2, and G / MoSe 2 / GaSe HTSs leads to the opening of a sizable bandgap in graphene at the Dirac point and shows the p-type Schottky contact. Among these kinds of TL-HTSs, the G / GaSe / MoSe 2 has many more advantages than the others due to the lowest binding energy of − 29.47 meV /A2, the biggest bandgap opening in G of 84.7 meV, and the smallest Schottky barrier height of 0.63 eV. Furthermore, we find that the p-type Schottky contact of G / GaSe / MoSe 2 HTS can be turned into an n-type one or into an Ohmic contact when vertical strain or electric field is applied. These results show a potential candidate of the combined HTSs of G, GaSe, and MoSe 2 monolayers for developing high speed nanoelectronic and optoelectronic devices.

In this work, we investigate theoretically the structural and electronic properties of the monolayer Ti2CO2 MXene using first-principles calculations. At the ground state, the monolayer Ti2CO2 MXene structure is dynamically stable and is a semiconductor with an indirect band gap of 0.75 eV, which can be controlled by strain engineering and electric field. We find that the indirect band gap of the monolayer Ti2CO2 MXene could be transformed into a direct one. Whereas, a semiconductor–metal transition can be achieved by applying the compressive biaxial strain of −10%. Our results demonstrate that the monolayer Ti2CO2 MXene can be a potential material for designing optical nanodevices with the strain engineering and electric field tunable of the band gap.

Electronic structure, optical, and photocatalytic properties of P, BSe, and SiC monolayers and their van der Waals heterostructures are investigated by (hybrid) first-principle calculations. The stability of the heterostructures and their corresponding induced-strain/unstrain monolayers are confirmed by the phonon spectra calculations. Similar to the corresponding parent monolayers, P-BSe (BSe-SiC) heterostructures are indirect type-II (type-I) bandgap semiconductors. A tensile strain of 10% (2%) transforms P-BSe (BSe-SiC) to type-I (type-II) direct bandgap nature. Interestingly, irrespective of the corresponding monolayers, the P-SiC heterostructure is a direct bandgap (type-II) semiconductor. The calculated electron and hole carrier mobilities of these heterostructures are in the range of 1.2 × 10 4 cm 2 / Vs to 68.56 × 10 4 cm 2 / Vs. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions are dominated by excitons. The valence and conduction band edges straddle the standard redox potentials in P-BSe, BSe-SiC, and P-SiC (strained) heterostructures, making them promising candidates for water splitting in the acidic solution. An induced compressive strain of 3.5% makes P suitable for water splitting at pH = 0.