Search

In the present work, we propose GaGeX 2 (X = N, P, As) monolayers and explore their structural, vibrational, piezoelectric, electronic, and transport characteristics for multifunctional applications based on first-principles simulations. Our analyses of cohesive energy, phonon dispersion spectra, and ab initio molecular dynamics simulations indicate that the three proposed structures have good energetic, dynamic, and thermodynamic stabilities. The GaGeX 2 are found as piezoelectric materials with high piezoelectric coefficient d 11 of -1.23 pm V -1 for the GaGeAs 2 monolayer. Furthermore, the results from electronic band structures show that the GaGeX 2 have semiconductor behaviours with moderate bandgap energies. At the Heyd-Scuseria-Ernzerhof level, the GaGeP 2 and GaGeAs 2 exhibit optimal bandgaps for photovoltaic applications of 1.75 and 1.15 eV, respectively. Moreover, to examine the transport features of the GaGeX 2 monolayers, we calculate their carrier mobility. All three investigated GaGeX 2 systems have anisotropic carrier mobility in the two in-plane directions for both electrons and holes. Among them, the GaGeAs 2 monolayer shows the highest electron mobilities of 2270.17 and 1788.59 cm 2 V -1 s -1 in the x and y directions, respectively. With high electron mobility, large piezoelectric coefficient, and moderate bandgap energy, the GaGeAs 2 material holds potential applicability for electronic, optoelectronic, piezoelectric, and photovoltaic applications. Thus, our findings not only predict stable GaGeX 2 structures but also provide promising materials to apply for multifunctional devices., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

In this study, using the tight-binding model and Green's function technique, we investigate potential electronic phase transitions in bilayer P 6 mmm borophene under the influence of external stimuli, including a perpendicular electric field, electron-hole coupling between sublayers (excitonic effects), and dopants. Our focus is on key electronic properties such as the band structure and density of states. Our findings reveal that the pristine lattice is metal with Dirac cones around the Fermi level, where their intersection forms a nodal line. The system undergoes transitions to a semiconducting state - elimination of nodal line - with a perpendicular electric field and a semimetallic state - transition from two Dirac cones to a single Dirac cone - with combined electric field and excitonic effects. Notably, with these, the system retains its massless Dirac-like bands characteristic at finite energy. However, introducing a dopant still leads to a metallic phase, but the Dirac-like bands become massive. Considering all these effects, the system ultimately reaches a semiconducting phase with massive Dirac-like bands. These results hold significance for optoelectronic applications.

The recently uncovered two-dimensional materials serve as versatile building blocks for electronic devices. In this study, we methodically investigate the impact of substrate-induced strain and exchange field effects on the electronic density of states (EDOS) and electronic heat capacity (EHC) of single-layer β 12 -borophene. Utilizing the Green's function approach, we compute these functions. The van Hove singularities in EDOS are observed to shift with strain, and depending on the direction and strength of the exchange field, the number of singularities increases. All these responses can be attributed to the renormalization of the velocity of electronic bands. Additionally, the inherent Schottky anomaly (an unusual peak at low temperatures) in the EHC undergoes a notable shift to higher and lower temperatures and variations in the intensity of the EHC due to substrate effects.

We theoretically study the role of adsorbed gas molecules on the electronic and optical properties of monolayer β 12 -borophene with {a,b,c,d,e} atoms in its unit cell. We focus our attention on molecules NH 3 , NO, NO 2 , and CO, which provide additional states permitted by the host electrons. Utilizing the six-band tight-binding model based on an inversion symmetry (between {a,e} and {b,d} atoms) and the Kubo formalism, we survey the anisotropic electronic dispersion and the optical multi-interband spectrum produced by molecule-boron coupling. We consider the highest possibilities for the position of molecules on the boron atoms. For molecules on {a,e} atoms, the inherent metallic phase of β 12 -borophene becomes electron-doped semiconducting, while for molecules on {b,d} and c atoms, the metallic phase remains unchanged. For molecules on {a,e} and {b,d} atoms, we observe a redshift (blueshift) optical spectrum for longitudinal/transverse (Hall) component, while for molecules on c atoms, we find a redshift (blueshift) optical spectrum for longitudinal (transverse/Hall) component. We expect that this study provides useful information for engineering field-effect transistor-based gas sensors.

Due to the broken vertical symmetry, the Janus material possesses many extraordinary physico-chemical and mechanical properties that cannot be found in original symmetric materials. In this paper, we study in detail the structural, electronic, and transport properties of 1T Janus PdXO monolayers (X = S, Se, Te) by means of density functional theory. PdXO monolayers are observed to be stable based on the analysis of the vibrational characteristics and molecular dynamics simulations. All three PdXO structures exhibit semiconducting characteristics with indirect bandgap based on evaluations with hybrid functional Heyd-Scuseria-Ernzerhof (HSE06). The influences of the spin-orbit coupling (SOC) on the band diagram of PdXO are strong. Particularly, when the SOC is included, PdTeO is calculated to be metallic by the HSE06+SOC approach. With high electron mobility, Janus PdXO structures have good potential for applications in future nanodevices., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit unique excitation peaks, and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discovered the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.

In this work, the perturbation-induced phase transitions in noncentrosymmetric quantum spin Hall insulators (QSHIs) are analytically addressed. In particular, the dilute charged impurity, the electric field, and the Zeeman splitting field are considered within the tight-binding Hamiltonian model, Green's function approach, and the Born approximation. Following the C 3 v symmetry breaking in the PbBiI compound as a representative QSHI, the band gap becomes larger via the electric field, while the system transits to the semimetallic phase via the dilute charged impurities and Zeeman field, modifying the degenerate states in the electronic density of states. While the coexistence of electric field and impurities demonstrate that the system backs to its initial semiconducting phase, the combined Zeeman field and impurities do not alter the robust semimetallic phase., (© 2021 IOP Publishing Ltd.)

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In 2 XY ( X / Y = S, Se, Te with X ≠ Y ) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In 2 XY are dynamically and thermally stable at room temperature. At equilibrium, both In 2 STe and In 2 SeTe are direct semiconductors while In 2 SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In 2 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 In 2 XY is negligible. Meanwhile, the optical absorbance intensity of the Janus In 2 XY monolayers is remarkably increased by compressive strain. Also, In 2 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., (© 2021 IOP Publishing Ltd.)

The strongly anisotropic properties of phosphorene makes it an attractive material for applications in deciding the specific direction for different purposes. Here we have particularly reported the competition between strain and electric field stimuli in evaluating the band gap and electron energy loss spectrum (EELS) of single-layer black phosphorus using the tight-binding method and the Kubo conductivity. We construct possible configurations for this competition and evaluate the interband optical excitations considering the corresponding band gap variations. The band gap increases with the individual electric field, while it increases (decreases) with tensile (compressive) uniaxial in-plane strain. Contrary to the in-plane strains, the uniaxial out-of-plane strain shows a critical strain at which the system suffers from a phase transition. Furthermore, the presence of these stimuli simultaneously results in an extraordinary band gap engineering. Based on the EELS response in the electromagnetic spectrum, the armchair (zigzag) direction is classified into the infrared and visible (ultraviolet) region. We report that the electric field gives rise to the blue shift in the interband optical transitions along the armchair direction, while the compressive/tensile (tensile/compressive) in-plane/out-of-plane strain provides a red (blue) shift. Moreover, we observe an inverse behavior of EELS response to the individual and combined effects of electric field and strains compared to the band gap behavior except at critical out-of-plane strain for which the physical theory of interband excitation is simply violated. Our results provide a new perspective on the applicability of phosphorene in stimulated optical applications.

Due to the coexistence of Dirac and triplet fermions, monolayer β12-borophene has recently attracted both experimental and theoretical researchers. In particular, various phase transitions have been recently reported in the structure, in the presence of dilute charged impurity and a perpendicular electric field, leading to interesting electronic heat capacity (HC). In this paper, we systematically examine the effects of charged impurity doping and electric field on the HC of monolayer β12-borophene. To do this, we utilize the five-band tight-binding Hamiltonian model, the Green's function, T-matrix, and the Born approximation for different models considering the substrate effects. Numerical analysis reveals that the inversion symmetric model is the proper model in the pristine and perturbed metallic β12-borophene, leading to a regular reduction of HC with both charged impurity and electric field. Moreover, the pristine and perturbed Schottky anomaly alterations are fully addressed. Unforeseeably, HC irregularly fluctuates with impurity in the homogeneous model. We believe that our results provide new physical insights into the thermal properties of monolayer β12-borophene.

We study the electronic heat capacity (EHC) and the Pauli spin paramagnetic susceptibility (PSPS) of topological crystalline insulator SnTe (001) thin film in the presence of dilute charged impurities in an effective Hamiltonian model for the low-energy regime such as impurity concentration and impurity scattering potential effects. To calculate the EHC and PSPS using the Boltzmann method, we first calculate the electronic density of states by means of the Green's function approach. Also, the impurity effects are considered with the aid of the T-matrix approximation. We demonstrate that the hybridization potential between the front and back surfaces in SnTe (001) thin films leads to the band gap opening and to the zero PSPS at low temperatures obeying the Fermi liquid theory. In particular, we demonstrate two scenarios including the possibilities of the same and different impurity doping. It is shown that in both cases the midgap states emerge, the cation-anion symmetry breaks down and the Fermi liquid theory loses its validity. Moreover, the critical scattering potential with respect to the hybridization potential is found for the validity limit of the Fermi liquid theory. Finally, the Schottky anomaly and the crossover in EHC and PSPS, respectively, are discussed. Our results have strong implications for applications based on SnTe (001) thin films.

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.

The unique physical and chemical properties of β 12 -borophene stem from the coexistence of the Dirac and triplet fermions. The metallic phase of β 12 -borophene transitions to the semiconducting one when it is subjected to a perpendicular electric field or bias voltage. In this work, with the aid of a five-band tight-binding Hamiltonian, the Green's function approach and the Kubo-Greenwood formalism, the electronic thermal conductivity (ETC) of the semiconducting phase of β 12 -borophene is studied. Two homogeneous (H) and inversion symmetric (IS) models are considered depending on the interaction of the substrate and boron atoms. In addition, due to the anisotropic structure of β 12 -borophene, the swapping effect of bias poles is addressed. First of all, we find the pristine ETC IS < ETC H independent of the temperature. Furthermore, a decrease of 74.45% (80.62%) is observed for ETC H (ETC IS ) when strong positive bias voltages are applied, while this is 25.2% (47.48%) when applying strong negative bias voltages. Moreover, the shoulder temperature of both models increases (fluctuates) with the positive (negative) bias voltage. Our numerical results pave the way for setting up future experimental thermoelectric devices in order to achieve the highest performance.

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/C 2 N van der Waals heterostructure and investigate its electronic and optical properties. We demonstrate that the intrinsic electronic properties of both GeC and C 2 N monolayers are quite preserved in GeC/C 2 N HTS owing to the weak forces. At the equilibrium configuration, GeC/C 2 N 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/C 2 N vdW-HTS exhibits strong absorption in both visible and near ultra-violet regions with an intensity of 10 5 cm -1 . The electronic properties of GeC/C 2 N HTS can be tuned by applying an electric field and vertical strains. The semiconductor to metal transition can be achieved in GeC/C 2 N HTS in the case when the positive electric field of +0.3 V Å -1 or the tensile vertical strain of -0.9 Å is applied. These findings demonstrate that GeC/C 2 N HTS can be used to design future high-performance multifunctional devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

β12-Borophene is a perfect planar nanolattice comprising of massless Dirac fermions and massless/massive triplet fermions considering the inversion symmetry lattice model. In this paper, a detailed study of the electric field and the effects of low concentrations of impurities on the electronic phase and the electrical conductivity of β12-borophene is presented. As a direct manner to judge the electronic features of pristine and perturbed monolayer β12-borophene, the five-band tight-binding Hamiltonian model, the T-matrix theory, the linear response theory, and the Green's function approach are investigated. Our investigation reveals that the massless Dirac and triplet fermions become massive when an electric field is applied. Also, we found out that the electric current and eventually the electrical conductivity are not the same along different directions and an enhancement of around 18.53% (15.38%) for the x-direction (in-plane) component is observed at a certain thermal energy. Furthermore, the metal-to-semiconductor electronic phase transition in the presence of different impurity atoms results in a 197.16% (198.23%) enhancement in x- (in-plane-) component of electrical conductivity. The results provide a basis of designing novel electronic devices based on β12-borophene.

In this paper, the potential of engineering and manipulating the electronic heat capacity and Pauli susceptibility of pristine and perturbed hydrogenated AA-stacked graphene, SiC (silicon carbide), and h-BN (hexagonal boron nitride) bilayers is studied using a designed transverse Zeeman magnetic field and the dilute charged impurity. The tight-binding Hamiltonian model, the Born approximation and the Green's function method describe the carrier dynamics up to a certain degree. The unperturbed results show that the heat capacity and susceptibility of all bilayers increase with different hydrogenation doping configurations. We also found that the maximum heat capacity and susceptibility relates to the chair-like and table-like configurations. Also, the graphene possesses the highest activity compared to SiC and h-BN lattices due to its zero on-site energies. For the Zeeman magnetic field-induced Schottky anomaly and the Néel temperature corresponding to the maximum electronic heat capacity, EHC Max. , and Pauli spin paramagnetic susceptibility, PSPS Max. , respectively, the pristine EHC Max. (PSPS Max. ) decreases (increases) with the Zeeman field. On the other hand, the corresponding results for reduced table-like and reduced chair-like lattices illustrate that both EHC Max. and PSPS Max. decrease with the Zeeman field, on average. However, under the influence of the dilute charged impurity, the pristine EHC Max. of graphene (SiC and h-BN) decreases (increases) with impurity concentration for all configurations while the corresponding PSPS Max. fluctuates (decreases) for the pristine (reduced table-like and reduced chair-like) case. These findings introduce hydrogenated AA-stacked bilayers as versatile candidates for real applications., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We theoretically address the electronic structure of mono- and simple bi-layer armchair graphene nanoribbons (AGNRs) when they are infected by extrinsic charged dilute impurity. This is done with the aid of the modified tight-binding method considering the edge effects and the Green's function approach. Also, the interplay of host and guest electrons are studied within the full self-consistent Born approximation. Given that the main basic electronic features can be captured from the electronic density of states (DOS), we focus on the perturbed DOS of lattices corresponding to the different widths. The modified model says that there is no metallic phase due to the edge states. We found that the impurity effects lead to the emergence of midgap states in DOS of both systems so that a semiconductor-to-semimetal phase transition occurs at strong enough impurity concentrations and/or impurity scattering potentials. The intensity of semiconductor-to-semimetal phase transition in monolayer (bilayer) ultra-narrow (realistic) ribbons is sharper than bilayers (monolayers). In both lattices, electron-hole symmetry breaks down as a result of induced-impurity states. The findings of this research would provide a base for future experimental studies and improve the applications of AGNRs in logic semiconductor devices in industry.

Optical activity is one of the most fascinating fields in current physics. The strong anisotropic feature in monolayer phosphorene leads to the emergence of non-trivial optoelectronic physics. This paper is devoted to a detailed analysis of strain effects on the optical activity of phosphorene ranging from low-optical-field to high-optical-field. To do so, a numerical study of the two-band tight-binding model is accomplished using the Harrison rule and the linear response theory. Although the transparency of phosphorene confirms at all frequencies independent of the strain modulus and direction, on average, from low- to high-optical-field limit, the polarization of the reflected wave at critical strains becomes circular and the ellipse axis tends to a rotation of 180°. It is found that the maximum absorption takes place at high-energy transitions, which quantitatively depends strongly on the strain modulus and direction. Furthermore, a detailed investigation of compressive and tensile strains results in the dominant contribution of the in-plane compressive and out-of-plane tensile strains to the reflected/transmitted light for low- and intermediate-optical-field ranges, whilst both contribute for the high-optical-field limit. However, overall, in-plane compressive and out-of-plane tensile strains come in to play a role in the absorption spectra. Thereby, the quality of the determined reflection, transmission and absorption waves depends on the regarded regime of the optical field, strain modulus, and strain orientation. These findings if sufficient can be performed and/or tuned experimentally, and a vast number of phosphorene-based optoelectronic devices can be achieved., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Combining van der Waals heterostructures by stacking different two-dimensional materials on top of each other layer-by-layer can enhance their desired properties and greatly extend the applications of the parent materials. In this work, by means of first principles calculations, we investigate systematically the structural and electronic properties of six different stacking configurations of a Si/GaSe heterostructure. The effect of biaxial strain and electric field on the electronic properties of the most energetically stable configuration of the Si/GaSe heterostructure has also been discussed. At the equilibrium state, the electronic properties of the Si/GaSe heterostructure in all its stacking configurations are well kept as compared with that of single layers owing to their weak van der Waals interactions. Interestingly, we find that a sizable band gap is opened at the Dirac K point of silicene in the Si/GaSe heterostructure, which could be further controlled by biaxial strain or electric field. These findings open up a possibility for designing silicene-based electronic devices, which exhibit a controllable band gap. Furthermore, the Si/GaSe heterostructure forms an n-type Schottky contact with a small Schottky barrier height of 0.23 eV. A transformation from the n-type Schottky contact to a p-type one, or from the Schottky contact to an ohmic contact may occur in the Si/GaSe heterostructure when strain or an electric field is applied.

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 Å 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 Å, or when the applied positive external electric field is larger than 0.0125 V Å-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.