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Motivated by the successful exfoliation of two-dimensional F-diamane-like C4F2 monolayer and the superior properties of graphene-based vdW heterostructures, in this work, we perform a first principles study to investigate the atomic structure, electronic properties and contact types of the graphene/F-diamane-like C4F2 heterostructure. The graphene/C4F2 vdW heterostructure is structurally stable at room temperature. In the ground state, the graphene/C4F2 heterostructure forms n-type Schottky contact with a Schottky barrier height of 0.46/1.03 eV given by PBE/HSE06. The formation of the graphene/C4F2 heterostructure tends to decrease in the band gap of the semiconducting C4F2 layer, suggesting that such a heterostructure may have strong optical absorption. Furthermore, the electronic properties and contact types of the graphene/C4F2 heterostructure can be adjusted by applying an external electric field, which leads to the change in the Schottky barrier height and the transformation from Schottky to ohmic contact. Our findings reveal the potential of the graphene/C4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.

In this work, by means of first-principles calculations, we investigate the structural and electronic properties of a two-dimensional ZnGeN2 monolayer as well as the effects of strain and an electric field. The ZnGeN2 monolayer is known to be a dynamically stable structure at room temperature. In the ground state, the ZnGeN2 monolayer possesses semiconducting characteristics with an indirect band gap of 1.73/2.96 eV obtained via PBE/HSE06 calculations. Furthermore, the electronic properties of the ZnGeN2 monolayer can be manipulated via strain engineering and an electric field. Both biaxial and uniaxial strain give rise to the change in the band gap and lead to the semiconductor-to-metal transition and from an indirect to a direct band gap, while the electric field leads to a decrease in the band gap and gives rise to the semiconductor-to-metal transition. Our findings suggest that the ZnGeN2 monolayer is a promising candidate for high-performance multifunctional nanodevices.

PurposeThis article aims to evaluate total quality management (TQM)-enabling factors' impact on corporate social responsibility (CSR) activities and business performance through evidence from Vietnamese coffee firms.Design/methodology/approachBased on collecting data via in-depth face-to-face interviews with employees, who are working in the Vietnamese coffee companies. The partial least squares structural equation modeling (PLS-SEM) approach has been employed to investigate the relationship among the TQM-enabling factors, corporate social performance (CSP) and business performance.FindingsA total of 13 TQM-enabling factors have been identified and divided into two categories, namely human and functional. The statistical results revealed a positive signal to remarkably enhance CSP and business performance by adopting those TQM-enabling factors into Vietnamese coffee firms.Research limitations/implicationsThe framework model of this research should be evaluated in different contexts worldwide or in another sector that can further identify the TQM-enabling factor and the correlation among these constructs.Practical implicationsThis article provides top managers of Vietnamese coffee firms with knowledge of TQM-enabling factors that may enable them to meet superior performance, including CSP, finance and reputation.Originality/valueThis is a unique study to employ the approach into the Vietnamese coffee industry context up-to-date, which is one of the essential sectors affecting Vietnam's sustainable development.

The effect due to doping by B, Si, N atoms on the magneto-transport properties of graphene is investigated using the generalized tight-binding model in conjunction with the Kubo formula. The crucial electronic and transport properties are greatly diversified by different types of dopant and doping concentrations. The effect of these guest atoms includes opening a band gap, thereby giving rise to rich Landau level energy spectra and consequently a unique quantum-Hall conductivity. The Fermi-energy dependent quantum-Hall effect appears as a step structure having both integer and half-integer plateaus. Doping with Si leads to an occurrence of a zero quantum-Hall conductivity, unlike the plateau sequence for pristine graphene. The predicted dopant- and concentration-enriched quantum-Hall effect for doped graphene can provide useful information for magneto-transport measurements, possible technological and even metrology applications.

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as opening opportunities for applications in solar energy conversion and nanoelectronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of a boron phosphide–SiC (BP–SiC) vdW heterostructure using first-principles calculations. The relaxed configuration is obtained from the binding energies, inter-layer distance, and thermal stability. We show that the BP–SiC vdW heterostructure has a direct band gap with type-II band alignment, which separates the free electrons and holes at the interface. Furthermore, the calculated absorption spectra demonstrate that the optical properties of the BP–SiC heterostructure are enhanced compared with those of the constituent monolayers. The intensity of optical absorption can reach up to about 105 cm−1. The band edges of the BP–SiC heterostructure are located at energetically favourable positions, indicating that the BP–SiC heterostructure is able to split water under working conditions of pH = 0–3. Our theoretical results provide not only a fascinating insight into the essential properties of the BP–SiC vdW heterostructure, but also helpful information for the experimental design of new vdW heterostructures.

Purpose The purpose of this paper is to identify the critical parameters of the Vietnam coffee industry and develop a comprehensive structural relationship to rank them for effective sustainable development. Design/methodology/approach The team of ten experts, having experience of over eight years in the field of various coffee industries in Vietnam, was created to give scores to multiple parameters. Technique for order of preference by similarity to ideal solution (TOPSIS) technique is used to develop the model for fifteen key parameters and then to rank them. Findings From the data analysis and results, marketing and brand innovation, product quality, strategic planning and implementation have emerged as top three key parameters while new technology development, supply chain and logistics management, and alliance-joint venture are identified as the bottom three parameters. The findings provide a rank of parameters that help to other coffee industries to identify their key parameters. Besides, the key parameters defined as necessary inputs show the firms more active and well-prepared comprised of ten companies. Practical implications The key parameters of the success of any organization are essential contributions. Aiming to improve organizational performance, metrics should be identified as the source of strength to achieve high productivity, profitability and sustainable business performance. Originality/value The emphasis of study on key parameters will help organizations to achieve competitive advantage and high productivity for high sustainability in an organizational context.

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.

Motivated by the successful exfoliation of two-dimensional F-diamane-like C

Although O is an element of chalcogen group, the study of two-dimensional (2D) O-based Janus dichalcogenides/monochalcogenides, especially their 1T-phase, has not been given sufficient attention. In this work, we systematically investigate the structural, electronic, and optical properties of 1T Janus GeSO monolayer by using the density functional theory. Via the analysis of phonon spectrum and evaluation of elastic constants, the GeSO monolayer is confirmed to be dynamically and mechanically stable. Calculated results for the elastic constants demonstrate that the Janus GeSO monolayer is much mechanically flexible than other 2D materials due to its small Young's modulus. At the ground state, while both GeS2 and GeO2 monolayers are indirect semiconductors, the Janus GeSO monolayer is found to be a direct band gap semiconductor. Further, effective masses of both electrons and holes are predicted to be directionally isotropic. The Janus GeSO monolayer has a broad absorption spectrum, which is activated from the visible light region and its absorption intensity is very high in the near-ultraviolet region. The calculated results not only systematically provide the fundamental physical properties of GeSO monolayer, but also stimulate scientists to further studying its importance both theoretically and experimentally.

In this paper, by introducing a generalized quantum-kinetic model which is coupled self-consistently with Maxwell and Boltzmann transport equations, we elucidate the significance of using input from first-principles band-structure computations for an accurate description of ultra-fast dephasing and scattering dynamics of electrons in graphene. In particular, we start with the tight-binding model (TBM) for calculating band structures of solid covalent crystals based on localized Wannier orbital functions, where the employed hopping integrals in TBM have been parameterized for various covalent bonds. After that, the general TBM formalism has been applied to graphene to obtain both band structures and wave functions of electrons beyond the regime of effective low-energy theory. As a specific example, these calculated eigenvalues and eigen vectors have been further utilized to compute the Bloch-function form factors and intrinsic Coulomb diagonal-dephasing rates for induced optical coherence of electron-hole pairs in spectral and polarization functions, as well as the energy-relaxation time from extrinsic impurity scattering of electrons for non-equilibrium occupation in band transport.

The rich and unique magnetic quantization phenomena of Si-doped graphene defect systems for various concentrations and configurations are fully explored by using the generalized tight-binding model. The non-uniform bond lengths, site energies and hopping integrals, as well as a uniform perpendicular magnetic field ( B z z ˆ ) are taken into account simultaneously. The quantized Landau levels (LLs) are classified into four different groups based on the probability distributions and oscillation modes. The main characteristics of the LLs are clearly reflected in the magneto-optical selection rules which cover the dominating Δ n = | n v − n c | = 0 , the coexistent Δ n = 0 and Δ n = 1 , along with the specific Δ n = 1 . These rules for inter-LL excitations are attributed to the non-equivalence or equivalence of the Ai and Bi sublattices in a supercell. The spectral intensity can be controlled by oscillator strength using a canonical momentum (vector potential) as well as by density of states using concentration and distribution of doped Si atoms.

We conduct a comprehensive investigation of the effect of an applied electric field on the optical and magneto-optical absorption spectra for AB-bt (bottom-top) bilayer silicene. The generalized tight-binding model in conjunction with the Kubo formula is efficiently employed in the numerical calculations. The electronic and optical properties are greatly diversified by the buckled lattice structure, stacking configuration, intralayer and interlayer hopping interactions, spin-orbital couplings, as well as the electric and magnetic fields ($${E}_{z}\hat{z}$$ E z z ˆ $$\& $$ & $${B}_{z}\hat{z}$$ B z z ˆ ). An electric field induces spin-split electronic states, a semiconductor-metal phase transitions and the Dirac cone formations in different valleys, leading to the special absorption features. The Ez-dependent low-lying Landau levels possess lower degeneracy, valley-created localization centers, peculiar distributions of quantum numbers, well-behaved and abnormal energy spectra in Bz-dependencies, and the absence of anti-crossing behavior. Consequently, the specific magneto-optical selection rules exist for diverse excitation categories under certain critical electric fields. The optical gaps are reduced as Ez is increased, but enhanced by Bz, in which the threshold channel might dramatically change in the former case. These characteristics are in sharp contrast with those for layered graphene.

The effect of edge modification of armchair graphene nanoribbons (AGNRs) on the collective excitations are theoretically investigated. The tight-binding method is employed in conjunction with the dielectric function. Unconventional plasmon modes and their association with the flat bands of the specially designed AGNRs are thoroughly studied. We demonstrate the robust relationship between the novel collective excitations and both the type and period of the edge modification. Additionally, we reveal that the main features displayed in the (momentum, frequency)-phase diagrams for both single-particle and collective excitations of AGNRs can be efficiently tuned by edge-extended defects. Our obtained plasmon modes are found to be analogous to magnetoplasmons associated with collective excitations of Landau-quantized electrons. This work provides a unique way to engineer discrete magnetoplasmon-like modes of AGNRs in the absence of magnetic field.

In this work, we perform first-principles calculations to examine the electronic, optical and photocatalytic properties of the BX–ZnO (X = As, P) heterostructures. The interlayer distance and binding energy of the most energetically favorable stacking configuration are 3.31 A and −0.30 eV for the BAs–ZnO heterostructure and 3.30 A and −0.25 eV for the BP–ZnO heterostructure. All the stacking patterns of the BX–ZnO heterostructures are proved to have thermal stability by performing AIMD simulations. The BAs–ZnO and BP–ZnO heterostructures are semiconductors with direct band gaps of 1.43 eV and 2.35 eV, respectively, and they exhibit type-I band alignment, which make them suitable for light emission applications with the ultra-fast recombination between electrons and holes. Both the BAs–ZnO and BP–ZnO heterostructures can exhibit a wider optical absorption range for visible-light owing to their reduced band gaps compared with the isolated BAs, BP and ZnO monolayers. The band alignment of both the BAs–ZnO and BP–ZnO heterostructures can straddle the water redox potential and they would have better performances owing to the direct band gap and the reduced band gap. All these findings demonstrate that the BX–ZnO heterostructures can be considered as potential photocatalysts for water splitting.

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.

The rich magneto-electronic properties of AA-bottom-top (bt) bilayer silicene are investigated using a generalized tight-binding model. The electronic structure exhibits two pairs of oscillatory energy bands for which the lowest conduction and highest valence states of the low-lying pair are shifted away from the K point. The quantized Landau levels (LLs) are classified into various separated groups by the localization behaviors of their spatial distributions. The LLs in the vicinity of the Fermi energy do not present simple wave function modes. This behavior is quite different from other two-dimensional systems. The geometry symmetry, intralayer and interlayer atomic interactions, and the effect of a perpendicular magnetic field are responsible for the peculiar LL energy spectra in AA-bt bilayer silicene. This work provides a better understanding of the diverse magnetic quantization phenomena in 2D condensed-matter materials.

The rich optical properties of AA-bottom-top (bt) bilayer silicene (BS) in a uniform perpendicular electric field (E) are investigated through the use of the tight-binding model. The distinctive multivalley band structure presents a semimetallic behavior but with a sizeable intraband gap. The main features of the energy dispersion appear in the optical absorption spectrum through transitions between band-edge states obeying specific selection rules. The E field clearly modifies the energy dispersion, especially opening a band gap, leading to a substantial influence on the optical properties. This field creates additional excitation channels and increases the frequency and intensity of the spectral structures. The inclusion of electron-hole interaction gives rise to a slight difference in the spectral intensity and strongly affects the fluctuation in the threshold intensity as the field strength is varied. The interplay between the special lattice structure, atomic interactions, electron-hole interaction, and a E field robustly diversifies the absorption spectra.

We investigate the peculiar electronic and optical properties of zigzag graphene nanoribbons (GNRs) under the effects of uniform and modulated electric fields. The hybridization between the four atomic orbitals ( s , p x , p y , p z ) is considered by means of the tight-binding model calculations. Both the π bands originating from the p z orbital and σ bands arising from the ( s , p x , p y ) orbitals play critical roles in the electronic structure and thus the absorption spectra. They determine the Fermi level and the available excitation channels between the occupied and unoccupied states. We show that an in-plane electric field in the form of modulated potential can significantly modify the electronic and optical properties of zigzag GNRs. The field-induced modification in energy dispersion, band-edge states, and band gap gives rise to rich absorption spectra with diverse structures. Our theoretical prediction could be verified by angle-resolved photoemission spectroscopies and optical spectroscopies.

Using a tight-binding Hamiltonian for phosphorene, we have calculated the real part of the polarizability and the corresponding dielectric function, Re$[\epsilon(\textbf{q},\omega)]$, at zero temperature (T = 0) with free carrier density $10^{13}$/ $cm^2$. We present results showing the real part of dielectric function in different directions of the transferred momentum $\bf{q}$. When $q$ is larger than a particular value which is twice the Fermi momentum $k_F$, Re$[\epsilon(\textbf{q},\omega)]$ becomes strongly dependent on the direction of $\bf{q}$. We also discuss the case at room temperature (T = 300K). These results which are similar to those previously reported by other authors are then employed to determine the static shielding of an impurity in the vicinity of phosphorene.

The Hall conductivity ${\ensuremath{\sigma}}_{xy}$ of many condensed matter systems presents a step structure when a uniform perpendicular magnetic field is applied. We report the quantum Hall effect in buckled AB-bottom-top bilayer silicene and its robust dependence on the electronic valley and spin-orbit coupling. With the unique multivalley electronic structure and the lack of spin degeneracy, the quantization of the Hall conductivity in this system is unlike the conventional sequence as reported for graphene. Furthermore, the conductivity plateaus take different step values for conduction ($2{e}^{2}/h$) and valence ($6{e}^{2}/h$) bands since their originating valleys present inequivalent degeneracy. We also report the emergence of fractions under significant effect of a uniform external electric field on the quantum Hall step structure by the separation of orbital distributions and the mixing of Landau levels from distinct valleys. The valley- and spin-dependent quantum Hall conductivity arises from the interplay of lattice geometry, atomic interaction, spin-orbit coupling, and external magnetic and electric fields.

We comprehensively study the electronic and optical properties of InTe monolayer in the presence of biaxial strain and electric field using density functional theory. Via analysis phonon spectrum, InTe monolayer is confirmed to be dynamically stable. InTe monolayer has an indirect semiconducting characteristic with a band gap of 1.25 eV at equilibrium and can be controlled by strain or electric field. The semiconductor–metal phase transition has been found in InTe monolayer at a large electric field of E = 5 V/nm. The optical attributes of InTe monolayer depend strongly on the polarization direction of the incident light. The optical absorption coefficient is activated in the infrared region and increases rapidly in the visible light region. While optical properties are independent of the electric field, strain engineering alters not only the intensity of optical peaks but also their position.

In this work, the doping effects of rare-earth La and Ce atoms on electronic and optical properties of Janus MoSSe monolayer are investigated by means of first principles calculations. Our results imply that when one La and Ce doped to one S or Se side of Janus MoSSe monoalayer, it leads to a decrease in the band gap and results in the transition from direct to indirect. With increasing the La and Ce doping concentration, the Janus MoSSe monolayer switched from semiconductor to metal. Moreover, we find that effective masses of all the La and Ce doped Janus MoSSe systems are decreased as compared to pristine state, rendering their high carrier mobility. Furthermore, all the La and Ce doped MoSSe systems have red shift and possess high absorption ability in the visible and infrared regions. These findings suggest that rare-earth La and Ce doped MoSSe monolayer are potential candidate for spintronics, nanoelectronics and optoelectronics.

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.

We developed the generalized tight-binding model to study the magneto-electronic properties of AAB-stacked trilayer graphene. Three groups of Landau levels (LLs) are characterized by the dominating subenvelope function on distinct sublattices. Each LL group could be further divided into two sub-groups in which the wavefunctions are, respectively, localized at 2/6 (5/6) and 4/6 (1/6) of the total length of the enlarged unit cell. The unoccupied conduction and the occupied valence LLs in each sub-group behave similarly. For the first group, there exist certain important differences between the two sub-groups, including the LL energy spacings, quantum numbers, spatial distributions of the LL wavefunctions, and the field-dependent energy spectra. The LL crossings and anticrossings occur frequently in each sub-group during the variation of field strengths, which thus leads to the very complex energy spectra and the seriously distorted wavefunctions. Also, the density of states (DOS) exhibits rich symmetric peak structures. The predicted results could be directly examined by experimental measurements. The magnetic quantization is quite different among the AAB-, AAA-, ABA-, and ABC-stacked configurations.

The diverse electronic and optical properties of doped graphene with chosen densities of Si, B, and N guest atoms are investigated with the use of the tight-binding model. The main characteristics of the energy dispersion are substantially modified by doping. This includes the opening of a band gap, a change in the number of energy subbands, and a shift in Fermi levels. The presence of different dopants in graphene greatly diversifies the optical-absorption spectra through changes in the spectral frequency, intensity, and absorption structures. Doping gives rise to more excitation channels, leading to unique spectral structures as well as fluctuation in the absorption frequency and intensity. Our theoretical predictions provide a clear understanding of the electronic and optical properties of graphene-related systems, which could be helpful in the designing of novel electronic, photonic and optoelectronic devices based on these materials.

Excited conduction electrons, conduction holes, and valence holes in monolayer electron-doped germanene exhibit unusual Coulomb decay rates. The deexcitation processes are studied using the screened exchange energy. They might utilize the intraband single-particle excitations (SPEs), the interband SPEs, and the plasmon modes, depending on the quasiparticle states and the Fermi energies. The low-lying valence holes can decay through the undamped acoustic plasmon, so that they present very fast Coulomb deexcitations, nonmonotonous energy dependence, and anisotropic behavior. However, the low-energy conduction electrons and holes are similar to those in a two-dimensional electron gas. The higher-energy conduction states and the deeper-energy valence ones behave similarly in the available deexcitation channels and have a similar dependence of decay rate on the wave vector $\mathbf{k}$.

The magneto-optical properties of bilayer phosphorene is investigated by the generalized tight-binding model and the gradient approximation. The vertical inter-Landau-level transitions, being sensitive to the polarization directions, are mainly determined by the spatial symmetries of sub-envelope functions on the distinct sublattices. The anisotropic excitations strongly depend on the electric and magnetic fields. A uniform perpendicular electric field could greatly diversify the selection rule, frequency, intensity, number and form of symmetric absorption peaks. Specifically, the unusual magneto-optical properties appear beyond the critical field as a result of two subgroups of Landau levels with the main and side modes. The rich and unique magnetoabsorption spectra arise from the very close relations among the geometric structures, multiple intralayer and interlayer hopping integrals and composite external fields.