Two-dimensional Janus semiconductor BiTeCl and BiTeBr monolayers: a first-principles study on their tunable electronic properties via an electric field and mechanical strain.

Motivated by the recent successful synthesis of highly crystalline ultrathin BiTeCl and BiTeBr layered sheets [Debarati Hajra et al., ACS Nano, 2020, 14, 15626], herein for the first time, we carry out a comprehensive study on the structural and electronic properties of BiTeCl and BiTeBr Janus monolayers using density functional theory (DFT) calculations. Different structural and electronic parameters including the lattice constant, bond lengths, layer thickness in the z-direction, different interatomic angles, work function, charge density difference, cohesive energy and Rashba coefficients are determined to acquire a deep understanding of these monolayers. The calculations show good stability of the studied single layers. BiTeCl and BiTeBr monolayers are semiconductors with electronic bandgaps of 0.83 and 0.80 eV, respectively. The results also show that the semiconductor-metal transformation can be induced by increasing the number of layers. In addition, the engineering of the electronic structure is also studied by applying an electric field, and mechanical uniaxial and biaxial strain. The results show a significant change of the bandgaps and that an indirect-direct band-gap transition can be induced. This study highlights the positive prospect for the application of BiTeCl and BiTeBr layered sheets in novel electronic and energy conversion systems.

First-principles investigation of nonmetal doped single-layer BiOBr as a potential photocatalyst with a low recombination rate.

Nonmetal doping is an effective approach to modify the electronic band structure and enhance the photocatalytic performance of bismuth oxyhalides. Using density functional theory, we systematically examine the fundamental properties of single-layer BiOBr doped with boron (B) and phosphorus (P) atoms. The stability of the doped models is investigated based on the formation energies, where the substitutional doping is found to be energetically more stable under O-rich conditions than under Bi-rich ones. The results showed that substitutional doping of P atoms reduced the bandgap of pristine BiOBr to a greater extent than that of boron substitution. The calculation of the effective masses reveals that B doping can render the electrons and holes of pristine BiOBr lighter and heavier, respectively, resulting in a slower recombination rate of photoexcited electron-hole pairs. Based on the results of HOMO-LUMO calculations, the introduction of B atoms tends to increase the number of photocatalytically active sites. The top of the valence band and the conduction band bottom of the B doped BiOBr monolayer match well with the water redox potentials in an acidic environment. The absorption spectra propose that B(P) doping causes a red-shift. Overall, the results predict that nonmetal-doped BiOBr monolayers have a reduced bandgap, a slow recombination rate, more catalytically active sites, enhanced optical absorption edges, and reduced work functions, which will contribute to superior photocatalytic performance.

Strain, electric-field and functionalization induced widely tunable electronic properties in MoS2/BC 3, /C 3 N and /[Formula: see text] van der Waals heterostructures.

In this paper, the effect of BC 3, C 3 N and [Formula: see text] substrates on the atomic and electronic properties of MoS2 were systematically investigated using first-principles calculations. Our results show that the MoS2/BC 3 and MoS2/C 3 N4 heterostructures are direct semiconductors with band gaps of 0.4 and 1.74 eV, respectively, while MoS2/C 3 N is a metal. Furthermore, the influence of strain and electric field on the electronic structure of these van der Waals heterostructures is investigated. The MoS2/BC3 heterostructure, for strains larger than -4%, transforms it into a metal where the metallic character is maintained for strains larger than -6%. The band gap decreases with increasing strain to 0.35 eV (at +2%), while for strain (>+6%) a direct-indirect band gap transition is predicted to occur. For the MoS2/C3N heterostructure, the metallic character persists for all strains considered. On applying an electric field, the electronic properties of MoS2/C3N4 are modified and its band gap decreases as the electric field increases. Interestingly, the band gap reaches 30 meV at +0.8 V/Å, and with increase above +0.8 V/Å, a semiconductor-to-metal transition occurs. Furthermore, we investigated effects of semi- and full-hydrogenation of MoS2/C3N and we found that it leads to a metallic and semiconducting character, respectively.

Photochemical Etching of Carbonyl Groups from a Carbon Matrix: The (001) Diamond Surface.

The surface of diamond is reported to undergo nonablative photochemical etching when exposed to ultraviolet (UV) radiation which allows controlled single and partial layer removal of lattice layers. Oxygen termination of surface dangling bonds is known to be crucial for the etching process; however, the exact mechanism of carbon ejection remains unclear. We investigate the interaction of UV laser pulses with oxygen-terminated diamond surfaces using atomic-scale surface characterization combined with first-principles time-dependent density functional theory calculations. We present evidence for laser-induced desorption (LID) from carbonyl functional groups at the diamond {001} surface. The doubly bonded carbonyl group is photoexcited into a triply bonded CO-like state, including scission of the underlying C─C bonds. The carbon removal process in LID is atom by atom; therefore, this mechanism provides a novel "top-down" approach for creating nanostructures on the surface of diamond and other carbon-containing semiconductors.

TDDFT Study of the Optical Excitation of Nucleic Acid Bases-C60 Complexes.

The potential of C60 as a nucleic acid base (NAB) optical sensor is theoretically explored. We investigate the adsorption of four NABs, namely, adenine, cytosine, guanine, and thymine, on C60 in the gas phase. For the optimal NAB@C60 adsorption configurations, obtained using a dispersion-corrected density functional, we calculate the vis-near-ultraviolet optical response using time-dependent density functional theory. While the isolated C60 and NAB molecules do not exhibit visible optical excitation, we find that C60/NAB conjugation gives rise to distinct spectral features in the visible range. These results suggest that C60 conjugation can be applied for photodetection of individual NABs.

Near-Perfect Spin Filtering and Negative Differential Resistance in an Fe(II)S Complex.

Density functional theory and nonequilibrium Green's function calculations have been used to explore spin-resolved transport through the high-spin state of an iron(II)sulfur single molecular magnet. Our results show that this molecule exhibits near-perfect spin filtering, where the spin-filtering efficiency is above 99%, as well as significant negative differential resistance centered at a low bias voltage. The rise in the spin-up conductivity up to the bias voltage of 0.4 V is dominated by a conductive lowest unoccupied molecular orbital, and this is accompanied by a slight increase in the magnetic moment of the Fe atom. The subsequent drop in the spin-up conductivity is because the conductive channel moves to the highest occupied molecular orbital, which has a lower conductance contribution. This is accompanied by a drop in the magnetic moment of the Fe atom. These two exceptional properties, and the fact that the onset of negative differential resistance occurs at low bias voltage, suggests the potential of the molecule in nanoelectronic and nanospintronic applications.

Acceptor doping in the proton conductor SrZrO3.

Perovskite zirconates such as SrZrO3 exhibit improved proton solubility and conductivity when doped with trivalent cations substituting at the Zr site. In this work, we present a detailed study of Sc and Y dopants in SrZrO3 based on first-principles, hybrid density-functional calculations. When substituting at the Zr site (ScZr, YZr), both dopants give rise to a single, deep acceptor level, where the neutral impurity forms a localized hole polaron state. The ε(0/-) charge transition levels are 0.60 eV and 0.58 eV above the valence-band maximum for ScZr and YZr, respectively. Under certain growth conditions, Sc and Y will form self-compensating donor species by substituting at the Sr site (ScSr, YSr), and this is detrimental to proton conductivity. Due to its larger ionic radius, Y exhibits a greater tendency than Sc to self-compensate at the Sr site. We also investigated the proton-dopant association. The binding energy of a proton to a negatively charged acceptor impurity is 0.41 eV for Sc and 0.31 eV for Y, indicating that proton transport is limited by trapping at impurity sites.

Endohedral metallofullerenes, M@C60 (M = Ca, Na, Sr): selective adsorption and sensing of open-shell NOx gases.

Based on density-functional theory and non-equilibrium Green's function calculations, we demonstrate that endohedral metallofullerenes (EMFs) are reactive to open-shell gases, and therefore have the potential application as selective open-shell gas sensors. The adsorption of eight gas species (CO, H2O, H2S, NO2, NO, SO2, O2 and NH3) on three EMFs (M@C60, M = Ca, Na and Sr) shows that the adsorption energies of the EMFs towards NO2 and NO are significantly higher than the closed-shell species. Moreover, the high selectivity appears relatively insensitive to the inserted metal atoms. The calculated current-voltage characteristics of gold-M@C60-gold structures (M = Ca, Na) show that the adsorption of NO2 leads to significant change in conductivity, suggesting a potential application as an EMF gas resistive sensing device.

Large spin-filtering effect in Ti-doped defective zigzag graphene nanoribbon.

Through first-principles calculations using the nonequilibrium Green's function formalism together with density functional theory, we study the conductance of double-vacancy zigzag graphene nanoribbons doped with four transition metal atoms Ti, V, Cr and Fe. We show that Ti doping induces large spin-filtering with an efficiency in excess of 90% for bias voltages below 0.5 V, while the other metal adatoms do not induce large spin filtering. This is despite the fact that the Ti dopant possesses small spin moment, while large moments reside on V, Cr and Fe dopants. Our analysis shows that the suppression of transmission in the spin-down channel in the Ti-doped graphene nanoribbon, thus the large spin filtering efficiency, is due to transmission anti-resonance arising from destructive quantum interference. These findings suggest that the decoration of graphene with titanium, and possibly other transition metals, can act as effective spin filters for nanospintronic applications.

Communication: Electrical rectification of C59N: The role of anchoring and doping sites.

Based on the nonequilibrium Green's function formalism and density-functional theory, we investigate the onset of electrical rectification in a single C59N molecule in conjunction with gold electrodes. Our calculations reveal that rectification is dependent upon the anchoring of the Au atom on C59N; when the Au electrode is singly bonded to a C atom (labeled here as A), the system does not exhibit rectification, whereas when the electrode is connected to the C-C bridge site between two hexagonal rings (labeled here as B), transmission asymmetry is observed, where the rectification ratio reaches up to 2.62 at ±1 V depending on the N doping site relative to the anchoring site. Our analysis of the transmission mechanism shows that N doping of the B configuration causes rectification because more transmission channels are available for transmission in the B configuration than in the A configuration.

Enhanced oscillatory rectification and negative differential resistance in pentamantane diamondoid-cumulene systems.

We propose a new functionality for diamondoids in nanoelectronics. Based on the nonequilibrium Green's function formalism and density functional theory, we reveal that when attached to gold electrodes, the pentamantane-cumulene molecular junction exhibits large and oscillatory rectification and negative differential resistance (NDR) - depending on the number of carbon atoms in cumulene (Cn). When n is odd rectification is greatly enhanced where the rectification ratio can reach ∼180 and a large negative differential resistance peak current of ∼3 µA. This oscillatory behavior is well rationalised in terms of the occupancy of the carbon 2p states in Cn. Interestingly, different layers of C atoms in the pentamantane molecule have different contributions to transmission. The first and third layers of C atoms in pentamantane have a slight contribution to rectification, and the fifth and sixth layers have a stronger contribution to both rectification and NDR. Thus, our results suggest potential avenues for controlling their functions by chemically manipulating various parts of the diamondoid molecule, thus extending the applications of diamondoids in nanoscale integrated circuits.

High On/Off Conductance Switching Ratio via H-Tautomerization in Quinone.

Through first-principles electron transport simulations using the nonequilibrium Green's function formalism together with density functional theory, we show that, upon H-tautomerization, a simple derivative of quinone can act as a molecular switch with high ON/OFF ratio, up to 70 at low bias voltage. This switching behavior is explained by the quantum interference effect, where the positional change of hydrogen atoms causes the energies of the transmission channels to overlap. Our results suggest that this molecule could have potential applications as an effective switching device.

Bistable Magnetism and Potential for Voltage-Induced Spin Crossover in Dilute Magnetic Ferroelectrics.

A first-principles investigation into the magnetic ferroelectric PbTi(1-x)Co(x)O(3) has revealed a bi-stable magnetic system with strong spin-lattice coupling. The local distortions induced by the low-spin to high-spin crossover are ferroelectric in nature, and are characterized by the displacement of the dopant ion with respect to the surrounding O(6) octahedral cage. We demonstrate how this spin-lattice effect could mediate magnetoelectric coupling and possible electric field induced spin-crossover, indicating a promising route to voltage manipulation of isolated spins in a solid-state system.

Sensing sulfur-containing gases using titanium and tin decorated zigzag graphene nanoribbons from first-principles.

Atom implantation in graphene or graphene nanoribbons offers a rich opportunity to tune the material structure and functional properties. In this study, zigzag graphene nanoribbons with Ti or Sn adatoms stabilised on a double carbon vacancy site are theoretically studied to investigate their sensitivity to sulfur-containing gases (H2S and SO2). Due to the abundance of oxygen in the atmosphere, we also consider the sensitivity of the structures in the presence of oxygen. Density functional theory calculations are performed to determine the adsorption geometry and energetics, and nonequilibrium Green's function method is employed to compute the current-voltage characteristics of the considered systems. Our results demonstrate the sensitivity of both Ti- and Sn-doped systems to H2S, and the mild sensitivity of Ti-doped sensor systems to SO2. The Ti-doped sensor structure exhibits sensitivity to H2S with or without oxidation, while oxidation of the Sn-doped sensor structure reduces its ability to adsorb H2S and SO2 molecules. Interestingly, oxygen dissociates on the Ti-doped sensor structure, but it does not affect the sensor's response to the H2S gas species. Oxidation prevents the dissociation of the H-S bond when H2S adsorbs on the Ti-doped structure, thus enhancing its reusability for this gas species. Our study suggests the potential of Ti- and Sn-doped graphene in selective gas sensing, irrespective of the sensing performance of the bulk oxides.

Density-functional prediction of a surface magnetic phase in SrTiO(3)/LaAlO(3) heterostructures induced by Al vacancies.

Based on first-principles density functional calculations we propose a novel Al vacancy induced ferromagnetism occurring at the LaAlO(3) surface of SrTiO(3)/LaAlO(3) bilayers. Magnetism at cation vacancies away from the surface is quenched due to charge compensation. Magnetic surface Al vacancies are stabilized due to the built-in electric field inside the LaAlO(3) region that raises the energy of the defect level, making charge compensation unfavorable. Surface Al vacancies prefer to form clusters and exhibit two-dimensional ferromagnetic alignment mediated by a long-range magnetic interaction. These results are discussed in light of recent experimental observations.

Mitigation of CO poisoning on functionalized Pt-TiN surfaces.

It has been previously reported that the system of single Pt atoms embedded in N-vacancy (V(N)) sites on the TiN(100) surface (Pt-TiN) could be a promising catalyst for proton exchange membrane fuel cells (PEM FCs). The adsorption of molecules on Pt-TiN is an important step, when it is incorporated as the anode or cathode of PEM FCs. Utilizing first principles calculations based on density functional theory, systematic investigations are performed on the adsorption of several atomic and molecular species on the Pt-TiN system, as well as the co-adsorption of them. The favorable binding sites and adsorption energies of several molecular species, namely carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), hydrogen (H2), hydroxyl (OH), an oxygen atom (O), and a hydrogen atom (H), are explored. For each, the adsorption energy and preferred binding site are identified and the vibrational frequencies calculated. It is found that CO2, CO and H prefer the Pt top site while OH and O favorably adsorb on the Ti top site. When CO and OH are co-adsorbed on the Pt-TiN(100) surface, OH weakens the adsorption of CO. The weakening effect is enhanced by increasing the coverage of OH. A similar behavior occurs for H and OH co-adsorption on the Pt-TiN(100) surface. Because co-adsorption with OH and H species weakens the adsorption of CO on Pt-TiN, it is expected that the acid and base conditions in PEM FCs could mitigate CO poisoning on functionalized Pt-TiN surfaces.

Graphene based dots and antidots: a comparative study from first principles.

Graphene based quantum dots and antidots are two nanostructures of primary importance for their fundamental physics and technological applications, particularly in the emerging field of graphene-based nanoelectronics and nanospintronics. Herein, based on first principles density functional theory calculations, we report a comparative study on the electronic structure of these two structurally complementary entities, where the bandgap opening, edge magnetism and the role of hydrogenation are investigated. Our results show the diversity of electronic structures of various dots and antidots, whose properties are sensitive to the edge detailed geometry (including size and shape and edge type). Hydrogen passivation plays an essential roal in affecting the related properties, in particular, it leads to larger bandgap values and suppress the edge magnetism. The frontier orbital analysis is employed to rationalize and compare the complicated nature of dots and antidots. Based on the specific geometrical consideration and the total energy competition of the ground antiferromagnetic and the ferromagnetic states, some magnetic structures (the unpassivated 42-atom-antidot and 54-atom-dot) are proposed to be useful as magnetic switches.

Comparison of hydrogen and deuterium adsorption on Pd(100).

Low energy ion recoil spectroscopy is a powerful technique for the determination of adsorbate position on metal surfaces. In this study, this technique is employed to compare the adsorption sites of hydrogen and deuterium on Pd(100) by detection of either H or D recoil ions produced by Ne(+) bombardment. Comparisons of experimental and Kalypso simulated azimuthal yield distributions show that, at room temperature, both hydrogen isotopes are adsorbed in the fourfold hollow site of Pd(100), however, at different heights above the surface (H-0.20 A and D-0.25 A). The adsorbates remain in the hollow site at all temperatures up to 383 K even though they move up to 0.40-0.45 A above the surface. Density functional theory calculations show a similar coverage dependent adsorption height for both H and D and confirm a real difference between the H and D adsorption heights based on zero point energies.

Oxygen adsorption on the (1×1) and (2×1) reconstructed C(111) surfaces: a density functional theory study.

The structural and electronic properties, stability, optimum coverage and workfunction of oxygen atoms at different sites on the (1×1) unreconstructed and the (2×1) reconstructed C(111) surfaces have been investigated using density functional theory. Oxygen atoms prefer on-top sites on the C(111)-(1×1) surface, with an optimum coverage of 1/3 monolayers (ML), while on the (2×1) reconstructed surface, bridge sites are preferred with a coverage of 1/2 ML. With increasing oxygen coverage greater than 1/3 ML on the (1×1) surface, a repulsive interaction develops between the oxygen atoms, while for the (2×1) surface such a repulsive interaction occurs for coverages greater than 0.5 ML. For both surfaces, the workfunction initially increases with oxygen adsorption relative to that of each of the clean surfaces, reaching about ∼6 eV and then decreasing slightly at full monolayer coverage. Minimal buckling of the upper π-bonded chains and no dimerization of the clean (2×1) reconstructed surface was observed. An average valence band width of ∼21 eV occurs, and a surface state associated with the (2×1) surface reconstruction was established at ∼-2.5 eV. In addition, O 2s states were established at around -21 eV for the (1×1) surface and at ∼-22.5 eV on the (2×1) surface. These corresponded to similarly located C 2s states at -21.25 eV for both (1×1) and (2×1) surfaces. O 2p states were observed at the Fermi level, ∼-1.25, -2.5, -4.0, and ∼-7.5 eV on the (1×1) surface, and at ∼-2.5 eV, between -4 and -5 eV and at ∼-7.5 eV on the (2×1) surface.

Interaction of diamond (111)-(1 × 1) and (2 × 1) surfaces with OH: a first principles study.

The properties of hydroxyl groups on C(111)-(1 × 1) and reconstructed (2 × 1) surfaces at different sites and for various coverages are investigated using density functional theory. Out of the adsorption sites considered, i.e. face centred cubic, hexagonal close packed, on-top and bridge sites, the on-top site is the most stable for OH on the C(111)-(1 × 1) surface for all coverages. On the reconstructed (2 × 1) surface the on-top site is the preferred configuration. Adsorption of OH was not stable however at any site on the reconstructed C(111)-(2 × 1) relative to the (1 × 1) surface; thus adsorption of OH leads to the de-reconstruction of the former surface. Both the 0.5 and 1 monolayer (ML) coverages were able to lift the (2 × 1) surface reconstruction. Repulsion between the OH adsorbates on the (1 × 1) surface sets in for coverages greater than 0.5 ML. A general decrease in the work function with increasing OH coverage was observed on both the (1 × 1) and (2 × 1) surfaces relative to the values of their respective clean surfaces. Regarding the electronic structure, O 2p states on the reconstructed (2 × 1) surface are observed at around - 21, - 8.75 , - 5 and - 2.5 eV, while O 2s states are present at - 22.5 eV. On the (1 × 1) surface (for 0.33 ML in the on-top site), O 2p states occurred between - 8 and - 9 eV, - 5 and - 4 eV and at around - 2.5 eV. O 2s states are established between - 22.5 and - 21 eV. The valence band width is 21 eV, and a hybrid 2s/2p state that is characteristic of diamond is located at about 12.5 eV below the valence band minimum.