A new family of NaTMGe (TM = 3d transition metals) half-Heusler compounds: the role of TM modification.

Exploring Heusler based materials for different practical applications has drawn more and more attention. In this work, the structural, electronic, magnetic, and mechanical properties of NaTMGe (TM = all 3d transition metals) half-Heusler compounds have been systematically investigated using first-principles calculations. The TM modification plays a determinant role in the fundamental properties. Except NaNiGe and NaCuGe, the studied materials exhibit good dynamical stability. Calculations reveal the non-magnetic semiconductor of NaScGe with a direct energy gap of 1.21 eV. Prospective spintronic applications of NaVGe and NaCrGe-NaMnGe are also suggested by their magnetic semiconductor and half-metallic behavior, respectively, where their magnetic properties follow the Slater-Pauling rule. Nevertheless, the remaining materials are either magnetic or non-magnetic metallic. For the magnetic systems, the magnetism is induced mainly by the TM constituents with either spin-up (V, Cr, Mn, and Fe) or spin-down (Co) 3d states. Calculated elastic constants indicate that all compounds are mechanically stable. Furthermore, they exhibit significant elastic anisotropy, where NaScGe and NaZnGe are the least and most anisotropic materials, respectively. Also, modifying the TM elements influences the materials' ductile and brittle behaviors. Our work unravels clearly the effects of TM modification on the fundamental properties of NaTMGe compounds. NaTMGe materials show excellent versatility with promising properties for optoelectronic and spintronic applications.

Ferromagnetic half-metallicity of the cubic NaMgO3 perovskite: from bulk to (001) surfaces.

Exploration of new half-metallic materials for spintronic applications has drawn great attention from researchers. In this work, we investigate the structural, electronic, and magnetic properties of the NaMgO3 perovskite in the bulk and (001) surface conformations. The results show the half-metallic nature of bulk NaMgO3 generated by insulator spin-up channels with a large band gap of 6.08 eV and metallic spin-down channels. A total magnetic moment of 3 (µB) is obtained, which is produced mainly by O atoms with a local magnetic moment of 0.94 (µB). Once the bulk is cleaved along the (001) direction, atomic relaxation takes place to reach an equilibrium, where all constituent atoms exhibit an inward movement. Interestingly, the half-metallicity is retained from the bulk to the (001) surface conformation. The effects of slab termination and thickness on the surface energy, stability, band edges, spin-up energy gaps, and magnetic anisotropy will be also analyzed in detail. The results presented herein introduce the NaMgO3 perovskite as a promising half-metallic material to generate spin current in spintronic devices.

Nitrogen doping and oxygen vacancy effects on the fundamental properties of BeO monolayer: a DFT study.

In practice, modifying the fundamental properties of low-dimensional materials should be realized before incorporating them into nanoscale devices. In this paper, we systematically investigate the nitrogen (N) doping and oxygen vacancy (OV) effects on the electronic and magnetic properties of the beryllium oxide (BeO) monolayer using first-principles calculations. Pristine BeO single layer is a non-magnetic insulator with an indirectK-Γ gap of 5.300 eV. N doping induces a magnetic semiconductor nature, where the spin-up and spin-down band gaps depend on the dopant concentration and N-N separation. Creating one OV leads to the energy gap reduction of 31.06% with no spin-polarization, which is due to the abundant 2p electrons of the Be atoms nearest the OV site. The further increase to two OVs and varying the OV-OV distance affect the band gap values, however the spin independence is retained. The magnetic semiconducting behavior is also obtained by the simultaneous N doping and OV presence. Calculations reveal significant magnetization of the BeO@1N, BeO@2N-n, BeO@NOV-nsystems, which is produced mainly by the spin-up N-2p state. Except for the BeO@NOV-1 and BeO@NOV-2, whose magnetic properties are created by the spin-up 2p state of the Be atoms closest to the OV site. The variation of the N-N and N-OV distances keeps the ferromagnetic ordering in the BeO@2N and BeO@NOV layers. Results presented herein may propose efficient methods to artificially modify the physical properties of BeO monolayer, leading to the formation of novel two-dimensional (2D) materials for optoelectronic and spintronic applications.

Encapsulation of Pollutant Gaseous Molecules by Adsorption on Boron Nitride Nanotubes: A Quantum Chemistry Study.

Based on density functional theory (DFT) and the semiempirical method PM7, we analyze the encapsulation process of polluting gases and/or their adsorption on different sites, viz., on the inner wall, the outer wall, and on the boron nitride (BN) nanotube ends, with chirality (7,7) armchair. DFT calculations are performed using the Perdew-Burke-Ernzerhof (PBE) functional and the M06-2X method through the 6-31G(d) divided valence orbitals as an atomic basis. Various geometrical configurations were optimized by minimizing the total energy for all analyzed systems, including the calculation of vibrational frequencies, which were assumed to be of a nonmagnetic nature, and where the total charge was kept neutral. Results are interpreted in terms of adsorption energy and electronic force, as well as on the analysis of quantum molecular descriptors for all systems considered. The study of six molecules, namely, CCl4, CS2, CO2, CH4, C4H10, and C6H12, in gas phase is addressed. Our results show that C4H10, C6H12, and CCl4 are chemisorbed on the inner surfaces (encapsulation) and on the nanotube ends. In contrast, the other molecules CS2, CO2, and CH4 show weak interaction with the nanotube surface, leading thereby to physisorption. Our findings thus suggest that this kind of polluting gases can be transported within nanotubes by encapsulation.

Modeling the aluminum-doped and single vacancy blue phosphorene interactions with molecules: a density functional theory study.

Structural, electronic, binding energies and magnetic properties of aluminum-doped and single vacancy blue phosphorene interacting with pollutant molecules are investigated using the density functional theory (DFT) with periodic boundary conditions. Acetylene, ozone, sulfur trioxide, hydrogen selenide, and sulfur dichloride molecules are considered to show the efficiency and enhancement of the sensing properties in comparison with the pristine blue phosphorene. Acetylene, sulfur trioxide, hydrogen selenide, and sulfur dichloride show chemisorption (> 0.5 eV/molecule) when interacting with the aluminum-doped system, but the ozone molecule dissociates in all configurations and symmetry sites. On the other hand, the acetylene, ozone, and sulfur trioxide with the single vacancy blue phosphorene exhibit chemisorption, the hydrogen selenide molecule exhibit a weak interaction energy, and the sulfur dichloride dissociates in all configurations and symmetry sites. In all the cases, the enhancement in the interaction energy was achieved when compared to other results for the same molecules. Finally, the single vacancy blue phosphorene shows a magnetic moment of ~1 µB/supercell, as induced by the vacancy.

Tuning MoSO monolayer properties for optoelectronic and spintronic applications: effect of external strain, vacancies and doping.

Since the successful synthesis of the MoSSe monolayer, two-dimensional (2D) Janus materials have attracted huge attention from researchers. In this work, the MoSO monolayer with tunable electronic and magnetic properties is comprehensively investigated using first-principles calculations based on density functional theory (DFT). The pristine MoSO single layer is an indirect gap semiconductor with energy gap of 1.02(1.64) eV as predicted by the PBE(HSE06) functional. This gap feature can be efficiently modified by applying external strain presenting a decrease in its value upon switching the strain from compressive to tensile. In addition, the effects of vacancies and doping at Mo, S, and O sites on the electronic structure and magnetic properties are examined. Results reveal that Mo vacancies, and Al and Ga doping yield magnetic semiconductor 2D materials, where both spin states are semiconductors with significant spin-polarization at the vicinity of the Fermi level. In contrast, single S and O vacancies induce a considerable gap reduction of 52.89% and 58.78%, respectively. Doping the MoSO single layer with F and Cl at both S and O sites will form half-metallic 2D materials, whose band structures are generated by a metallic spin-up state and direct gap semiconductor spin-down state. Consequently, MoV, MoAl, MoGa, SF, SCl, OF, and OCl are magnetic systems, and the magnetism is produced mainly by the Mo transition metal that exhibits either ferromagnetic or antiferromagnetic coupling. Our work may suggest the MoSO Janus monolayer as a prospective candidate for optoelectronic applications, as well as proposing an efficient approach to functionalize it to be employed in optoelectronic and spintronic devices.

New equiatomic quaternary Heusler compounds without transition metals KCaBX (X = S and Se): Robust half-metallicity and optical properties.

It is known that high spin-polarization and magnetism can be found even in materials with neither transition metals nor rare earths. In this paper, we report results of the structural design, electronic structure, magnetic and optical properties of new equiatomic quaternary Heusler (EQH) KCaBX (X = S and Se) compounds. Electron exchangecorrelation interactions are described by the Wu-Cohen (WC) functional and Tran-Blaha modified Becke-Johnson exchange (mBJ) potential. Ferromagnetic ordering is stable for the cubic structure of space group F43 m in which the K, Ca, B and X atoms are located at 4c, 4d, 4a and 4b Wyckoff positions, respectively. Quaternaries at hand exhibit a perfect spin-polarization around the Fermi level, which is a result of the half-metallicity with metallic spin-up channel and semiconductor spin-dn channel. The ferromagnetic half-metallic and spin-flip band gaps are 2.648(2.470) and 0.673(0.526), respectively, for KCaBS(KCaBSe). Both studied compounds have a total magnetic moment of 2.000 µB. Additionally, the strain effect on the electronic and magnetic properties is also examined. Finally, the optical properties of the KCaBX alloys are investigated for energies up to 25 eV. Optical spectra show the metallic behavior at extremely low energies and semiconductor nature at higher energies. Interestingly, KCaBS and KCaBSe exhibit prospective absorption properties with a quite large absorption coefficient in the ultraviolet regime.

Computational prediction of the spin-polarized semiconductor equiatomic quaternary Heusler compound MnVZrP as a spin-filter.

In this work, a new equiatomic quaternary Heusler (EQH) compound, MnVZrP, is predicted using first principles calculations. Simulations show the good stability of the material, suggesting experimental realization. Results show that MnVZrP is a magnetic semiconductor material, exhibiting semiconductor characteristics in both spin channels, however, with strong spin-polarization. Electronic band gaps of 0.97 and 0.47 eV are obtained in the spin-up and spin-dn states, respectively. Mainly the d-d coupling regulates the electronic band structure around the Fermi level. Strain effects on the electronic properties of the proposed compound are also investigated. Simulations give the total magnetic moment of 3 µ B satisfying the Slate-Pauling rule. The main magnetic contributions are given by the Mn and V constituents. The results presented here suggest the promising applicability of EQH MnVZrP as a spin-filter. Additionally, the elastic property calculations indicate the mechanical stability and elastic anisotropy. The work may be useful in the magnetic Heusler alloys field, introducing a new member to the small group of magnetic semiconductor EQH compounds for spin-filter applications.

Structural, electronic and optical properties of pristine and functionalized MgO monolayers: a first principles study.

In this paper, we present a detailed investigation of the structural, electronic, and optical properties of pristine, nitrogenated, and fluorinated MgO monolayers using ab initio calculations. The two dimensional (2D) material stability is confirmed by the phonon dispersion curves and binding energies. Full functionalization causes notable changes in the monolayer structure and slightly reduces the chemical stability. The simulations predict that the MgO single layer is an indirect semiconductor with an energy gap of 3.481 (4.693) eV as determined by the GGA-PBE (HSE06) functional. The electronic structure of the MgO monolayer exhibits high sensitivity to chemical functionalization. Specifically, nitrogenation induces metallization of the MgO monolayer, while an indirect-direct band gap transition and band gap reduction of 81.34 (59.96)% are achieved by means of fluorination. Consequently, the functionalized single layers display strong optical absorption in the infrared and visible regimes. The results suggest that full nitrogenation and fluorination may be a quite effective approach to enhance the optoelectronic properties of the MgO monolayer for application in nano-devices.

Ultranarrow heterojunctions of armchair-graphene nanoribbons as resonant-tunnelling devices.

A systematic investigation is performed on the electronic transport properties of armchair-graphene nanoribbon (AGNR) heterojunctions using spin-polarized density functional theory calculations in combination with the non-equilibrium Green's function formalism. 9-AGNR and 5-AGNR structures are used to form a single-well configuration by sandwiching a 5-AGNR between two 9-AGNRs. At the same time, these 9-AGNRs are matched at the left and right to electrodes, 9 and 5 being the number of carbon dimers as width. This heterojunction mimics an electronic device with two potential barriers (9-AGNR) and one quantum well (5-AGNR) where quasi-bound states are confined. First, we study the ground state properties, and then we calculate the electron transport properties of this device as a function of the well width. We show the presence of electronic tunnelling resonances between the barriers by delocalized electron density inside the well's structure. This is corroborated by transmission curves, localized densities of states (LDOS), current-vs.-bias voltage results, and the trend of the resonances as a function of the well width. This work shows that carbon AGNRs may be used as resonant-tunnelling devices for applications in nanoelectronics.

Acetylene chain reaction on hydrogenated boron nitride monolayers: a density functional theory study.

Spin-polarized first-principles total-energy calculations have been performed to investigate the possible chain reaction of acetylene molecules mediated by hydrogen abstraction on hydrogenated hexagonal boron nitride monolayers. Calculations have been done within the periodic density functional theory (DFT), employing the PBE exchange correlation potential, with van der Waals corrections (vdW-DF). Reactions at two different sites have been considered: hydrogen vacancies on top of boron and on top of nitrogen atoms. As previously calculated, at the intermediate state of the reaction, when the acetylene molecule is attached to the surface, the adsorption energy is of the order of -0.82 eV and -0.20 eV (measured with respect to the energy of the non interacting molecule-substrate system) for adsorption on top of boron and nitrogen atoms, respectively. After the hydrogen abstraction takes place, the system gains additional energy, resulting in adsorption energies of -1.52 eV and -1.30 eV, respectively. These results suggest that the chain reaction is energetically favorable. The calculated minimum energy path (MEP) for hydrogen abstraction shows very small energy barriers of the order of 5 meV and 22 meV for the reaction on top of boron and nitrogen atoms, respectively. Finally, the density of states (DOS) evolution study helps to understand the chain reaction mechanism. Graphical abstract Acetylene chain reaction on hydrogenated boron nitride monolayers.

Thymine adsorption on two-dimensional boron nitride structures: first-principles studies.

First-principles total-energy calculations were performed to investigate the structural and electronic properties of thymine (T) adsorption on pristine and Al-doped two-dimensional hexagonal boron nitride (2D-hBN) surfaces. Periodic density functional theory, as developed in the PWscf code of the quantum espresso package, was applied. The pseudopotential theory was used to deal with electron-ion interactions. The generalized gradient approximation was applied to treat the exchange-correlation energies. Van der Waals interactions were incorporated in the calculations. Considering T as an elongated molecule and the interactions through one oxygen atom of the molecule ring, two geometries were explored in pristine and Al-doped systems: in (1) the ring side O interacts with B, and (2) the O at the molecule end interacting with the B. The pristine case yields (4 × 4-a), (5 × 5-b) and (6 × 6-b) as the ground states, , while the doped system shows (4 × 4-a), (5 × 5-a) and (6 × 6-a) as the ground states. Calculations of the adsorption energies indicate chemisorption. Doping enhances the surface reactivity, inducing larger binding energies. The total density of states (DOS) was calculated and interpreted with the aid of the projected DOS. Below the Fermi energy, the DOS graphs indicate that p orbitals make the largest contributions. Above the Fermi level, the DOS is formed mainly by -s and H-s orbitals. The DOS graphs indicate that the structures have non-semiconductor behavior.

Reactivity of phosphorene with a 3d element trioxide (CrO3) considering van der Waals molecular interactions: a DFT-D2 study.

First-principle calculations are performed to investigate the interaction between clean black phosphorene and the CrO3 molecule which is known to be a powerful oxidizer and a suspected carcinogen. Van der Waals forces are included in all calculations through empirical corrections. Energetics studies are first done to determine the structural stability. Then charge density, Löwdin population analysis and electronic states are evaluated. Results show that the CrO3 molecule, with an acceptor electron character, is chemisorbed on the phosphorene surface inducing minimal geometrical distortions, however, after adsorption, a partial charge gradient is produced between the P atoms located at the phosphorene upper and lower planes. Furthermore, variations on the CrO3 concentration causes different interaction strengths. At high concentrations of adsorbed CrO3 molecules, the interaction with the surface becomes stronger due to an increased steric effect between neighboring molecules. Nevertheless, this effect along with the geometrical distortions produced on the phosphorene structure, due to the large number of molecules adsorbed, leads to a decrement on the adsorption energy. It is expected that the reported results may render phosphorene as a promising material for application as a gas sensor.

Two-dimensional boron nitride structures functionalization: first principles studies.

Density functional theory calculations have been performed to investigate two-dimensional hexagonal boron nitride (2D hBN) structures functionalization with organic molecules. 2x2, 4x4 and 6x6 periodic 2D hBN layers have been considered to interact with acetylene. To deal with the exchange-correlation energy the generalized gradient approximation (GGA) is invoked. The electron-ion interaction is treated with the pseudopotential method. The GGA with the Perdew-Burke-Ernzerhoff (PBE) functionals together with van der Waals interactions are considered to deal with the composed systems. To investigate the functionalization two main configurations have been explored; in one case the molecule interacts with the boron atom and in the other with the nitrogen atom. Results of the adsorption energies indicate chemisorption in both cases. The total density of states (DOS) displays an energy gap in both cases. The projected DOS indicate that the B-p and N-p orbitals are those that make the most important contribution in the valence band and the H-s and C-p orbitals provide an important contribution in the conduction band to the DOS. Provided that the interactions of the acetylene with the 2D layer modify the structural and electronic properties of the hBN the possibility of structural functionalization using organic molecules may be concluded.

First principles calculations of phenol adsorption on pristine and group III (B, Al, Ga) doped graphene layers.

We studied the doping effects on the electronic and structural properties of graphene upon interaction with phenol. Calculations were performed within the periodic density functional theory as implemented in PWscf code of the Quantum Espresso package. Graphene layers were modeled using 3 × 3 and 4 × 4 periodic supercells. Doping was explored considering boron (B), aluminum (Al) and gallium (Ga) atoms. The results showed that pristine graphene and graphene doped with B atoms interacting with phenol display similar structural and electronic properties, exhibiting weak physical interactions. However, when the doping is with Al or Ga , the results are quite different. Al and Ga doping induces a stronger interaction between the phenol molecule and the doped layer, yielding chemical adsorption. In all cases, the zero gap energy characteristic is unchanged. The Dirac lineal dispersion relation is preserved in both pristine graphene and B-doped graphene.

Armchair BN nanotubes--levothyroxine interactions: a molecular study.

The density functional theory has been applied to investigate the structural and electronic properties of single-wall boron nitride nanotubes (SW-BNNT) of (5,5) chirality, with surface and ends functionalized by the drug levothyroxine (C15H11NI4O4). The exchange-correlation energies have been modeled according to the Hamprecht-Cohen-Tozer-Handy functional within the generalized gradient approximation (HCTH-GGA) and a base function with double polarization has been used. The (5,5) BNNT-Levothyroxine structural optimization has been done considering the minimum energy criterion in nine possible atomic structures. Simulation results indicate that the preferential adsorption site (chemical adsorption) of the levothyroxine fragment is at the nanotube ends. The BNNT-Levothyroxine system polarity increases which indicates the possible dispersion and solubility both non-solvated and solvated in water. The BNNT-Levothyroxine solvated in water modifies its chemical reactivity which may allow the drug delivery within the biological systems. On the other hand, the decrease in the work function is important for the optoelectronic device design, which also makes these materials suitable to improve the field emission properties.

Density functional theory studies of the adsorption of hydrogen sulfide on aluminum doped silicane.

First principles total energy calculations have been performed to study the hydrogen sulfide (H2S) adsorption on silicane, an unusual one monolayer of Si(111) surface hydrogenated on both sides. The H2S adsorption may take place in dissociative or non-dissociative forms. Silicane has been considered as: (A) non-doped with a hydrogen vacancy, and doped in two main configurations; (B) with an aluminum replacing a hydrogen atom and (C-n; n = 1, 2, 3) with an aluminum replacing a silicon atom at a lattice site. In addition, three supercells; 4x4, 3x3 and 2x2 have been explored for both non-doped and doped silicane. The non-dissociative adsorption takes place in geometries (A), (C-1), (C-2) and (C-3) while the dissociative in (B). Adsorption energies of the dissociative case are larger than those corresponding to the non-dissociated cases. In the dissociative adsorption, the molecule is fragmented in a HS structure and a H atom which are bonded to the aluminum to form a H-S-Al-H structure. The presence of the doping produces some electronic changes as the periodicity varies. Calculations of the total density of states (DOS) indicate that in most cases the energy gap decreases as the periodicity changes from 4x4 to 2x2. The features of the total DOS are explained in terms of the partial DOS. The reported charge density plots explain quite well the chemisorptions and physisorptions of the molecule on silicane in agreement with adsorption energies.

First-principles simulations of the chemical functionalization of (5,5) boron nitride nanotubes.

We perform density functional theory studies to investigate structural and electronic properties of the (5,5) boron nitride nanotubes (BNNTs) with surfaces and ends functionalized by thiol (SH) and hydroxyl (OH) groups. The exchange-correlation energies are treated according to the functional of Hamprecht-Cohen-Tozer-Handy within the generalized gradient approximation (HCTH-GGA). We use the base function with double polarization DNP. To determine the (5,5) BNNT-SH and (5,5) BNNT-OH relaxed structures the minimum energy criterion is applied considering six different geometries depending upon the SH and OH functional groups orientation: (C1) The adsorbed functional group is oriented toward the N atom, (C2) the functional group is oriented toward the B atom, (C3) the functional group is at the central hexagon of the BNNT surface. The (C4) fourth and (C5) fifth configurations are formed by allowing bonds (of S or O) with B or N atoms at one end of the nanotube. (C6) The sixth geometry is obtained by placing the functional group at the center of one end of the BNNT. The (5,5) BNNT-SH system, in vacuum, suffers a semiconductor to metal transition while the (5,5) BNNT-OH system retains the semiconductor behavior. When structures are solvated in water these systems behave as semiconductors. The polarity increases as a consequence of the functional group-nanotube interactions no matter if they are in vacuum or in solvation situation, which indicates the possible solubility and dispersion. According to the work function the best option to construct a device is with the BNNT-OH system.