Energy gaps and stark effect in boron nitride nanoribbons

Spin-polarized transport in hydrogen-passivated graphene and silicene nanoribbons with magnetic transition-metal substituents

We present a systematic theoretical study of the electronic transport in hydrogen passivated zigzag graphene and silicene nanoribbons with between zero and four neighboring H atoms on one edge replaced by magnetic transition metals (Fe, Co, and Ni). The calculations were performed using equilibrium transport and density-functional theory with the generalized gradient approximation to exchange and correlation. We considered the magnetic moments of the two edges aligned both ferromagnetically (Ferro-F form) and antiferromagnetically (Ferro-A form). The Ferro-A graphene-based ribbons were all semiconducting and would support moderate spin-polarized currents of either sign by applying positive or negative gate voltages. The Ferro-F graphene-based ribbons were all metallic; the most interesting for possible spintronic applications being that with a single Ni atom, in which strong spin-filtering at low bias resulted from a deep trough in the transmission of one spin component around the Fermi level. By contrast, in the Si-based analog this trough was split, partially eliminating the polarization of the current. This splitting was found to be related to the buckled structure of the Si-based nanoribbon, which has its origin in its preference for sp(3)-like hybridization.

Thermal exfoliation of stoichiometric single-layer silica from the stishovite phase: insight from first-principles calculations

Mechanical cleavage, chemical intercalation and chemical vapor deposition are the main methods that are currently used to synthesize nanosheets or monolayers. Here, we propose a new strategy, thermal exfoliation for the fabrication of silica monolayers. Using a variety of state-of-the-art theoretical calculations we show that a stoichiometric single-layer silica with a tetragonal lattice, T-silica, can be thermally exfoliated from the stishovite phase in a clean environment at room temperature. The resulting single-layer silica is dynamically, thermally, and mechanically stable with exceptional properties, including a large band gap of 7.2 eV, an unusual negative Poisson's ratio, a giant Stark effect, and a high breakdown voltage. Moreover, other analogous structures like single-layer GeO2 can also be obtained by thermal exfoliation of its bulk phase. Our findings are expected to motivate experimental efforts on developing new techniques for the synthesis of monolayer materials.

Tunable electronic and magnetic properties of two-dimensional materials and their one-dimensional derivatives

Low‐dimensional materials exhibit many exceptional properties and functionalities which can be efficiently tuned by externally applied force or fields. Here we review the current status of research on tuning the electronic and magnetic properties of low‐dimensional carbon, boron nitride, metal‐dichalcogenides, phosphorene nanomaterials by applied engineering strain, external electric field and interaction with substrates, etc, with particular focus on the progress of computational methods and studies. We highlight the similarities and differences of the property modulation among one‐ and two‐dimensional nanomaterials. Recent breakthroughs in experimental demonstration of the tunable functionalities in typical nanostructures are also presented. Finally, prospective and challenges for applying the tunable properties into functional devices are discussed. WIREs Comput Mol Sci 2016, 6:324–350. doi: 10.1002/wcms.1251

For further resources related to this article, please visit the WIREs website.

Conflict of interest: The authors have declared no conflicts of interest for this article.

Edge-Modified Phosphorene Nanoflake Heterojunctions as Highly Efficient Solar Cells

We propose to use edge-modified phosphorene nanoflakes (PNFs) as donor and acceptor materials for heterojunction solar cells. By using density functional theory based calculations, we show that heterojunctions consisting of hydrogen- and fluorine-passivated PNFs have a number of desired optoelectronic properties that are suitable for use in a solar cell. We explain why these properties hold for these types of heterojunctions. Our calculations also predict that the maximum energy conversion efficiency of these type of heterojunctions, which can be easily fabricated, can be as high as 20%, making them extremely competitive with other types of two-dimensional heterojunctions.

Structural and electronic properties of zigzag InP nanoribbons with Stone-Wales type defects

By means of density-functional-theoretic calculations, we investigate the structural and electronic properties of a hexagonal InP sheet and of hydrogen-passivated zigzag InP nanoribbons (ZInPNRs) with Stone-Wales (SW)-type defects. Our results show that the influence of this kind of defect is not limited to the defected region but it leads to the formation of ripples that extend across the systems, in keeping with the results obtained recently for graphene and silicene sheets. The presence of SW defects in ZInPNRs causes an appreciable broadening of the band gap and transforms the indirect-bandgap perfect ZInPNR into a direct-bandgap semiconductor. An external transverse electric field, regardless of its direction, reduces the gap in both the perfect and defective ZInPNRs.

Variations in Crystalline Structures and Electrical Properties of Single Crystalline Boron Nitride Nanosheets

We report the studies of (1) the basic mechanism underlying the formation of defect-free, single crystalline boron nitride nanosheets (BNNSs) synthesized using pulsed laser plasma deposition (PLPD) technique, (2) the variation in the crystalline structure at the edges of the hexagonal boron nitride (h-BN) nanosheets, and (3) the basic electrical properties related to the BNNSs tunneling effect and electrical breakdown voltage. The nanoscale morphologies of BNNSs are characterized using scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results show that each sample consisted of a number of transparent BNNSs that partially overlapped one another. Varying the deposition duration yielded different thicknesses of sample but did not affect the morphology, structure, and thickness of individual BNNSs pieces. Analysis of the SEM and HRTEM data revealed changes in the spatial period of the B₃–N₃ hexagonal structures and the interlayer distance at the edge of the BNNSs, which occurred due to the limited number of atomic layers and was confirmed further by x-ray diffraction (XRD) study. The experimental results clearly indicate that the values of the electrical conductivities of the super-thin BNNSs and the effect of temperature relied strongly on the direction of observation.

Effects of stacking order, layer number and external electric field on electronic structures of few-layer C2N-h2D

Recently, a new type of two-dimensional layered material, i.e. a nitrogenated holey two-dimensional structure C2N-h2D, has been synthesized using a simple wet-chemical reaction and used to fabricate a field-effect transistor device (Nat. Commun., 2015, 6, 6486). Here we have performed a first-principles study of the electronic properties of few-layer C2N-h2D with different stacking orders and layer numbers. Because of the interlayer coupling mainly in terms of the orbital interaction, band structure of this system, especially splitting of the bands and band gap, depends on its stacking order between the layers, and the band gap exhibits monotonically decreasing behavior as the layer number increases. All the few-layer C2N-h2D materials have characteristics of direct band gap, irrespective of the stacking order and layer number examined in our calculations. And bulk C2N-h2D has an indirect or direct band gap, depending on the stacking order. Besides, when we apply an out-of-plane electric field on few-layer C2N-h2D, its band gap will decrease as the electric field increases due to a giant Stark effect except for the monolayer case, and even a semiconductor-to-metal transition may occur for few-layer C2N-h2D with more layers under an appropriate electric field. Owing to their tunable band gaps in a wide range, the layered C2N-h2D materials will have tremendous opportunities to be applied in nanoscale electronic and optoelectronic devices.

Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous

Two-dimensional (2D) materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, Ec. Increasing E ext beyond Ec can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semimetal transition.

Metal-free ferromagnetic metal and intrinsic spin semiconductor: two different kinds of SWCNT functionalized BN nanoribbons

Two different kinds of SWCNT functionalized zigzag edge BN nanoribbons with n chains (n-ZBNNRs), namely, (a) B-edge functionalized by (m,m)SWCNT and N-edge modified with H (nZBNNR-B-(m,m)SWCNTs); and (b) the B-edge modified with H and the N-edge functionalized by (m,m)SWCNT (nZBNNR-N-(m,m)SWCNTs), have been predicted. Amazingly, we find that unlike the semiconducting and nonmagnetic H-modified n-ZBNNRs, the nZBNNR-B-(m,m)SWCNTs are intrinsic ferromagnetic metals, regardless of ribbon widths n and tube diameters (m,m). At a given (m,m), their local magnetic moments, at first, exhibit oscillation with increasing n, whereas when n is larger than 5, they are independent of n. In contrast, unlike the metallic and nonmagnetic (m,m)SWCNTs, the nZBNNR-N-(m,m)SWCNTs are ferromagnetic intrinsic spin-semiconductors with direct band gaps, regardless of n and (m,m). Their local magnetic moments and band gaps are independent of n and (m,m). The DFT calculations reveal that the process of SWCNT functionalization of the n-ZBNNRs does not need any activation energy. Moreover, the formation energies of the SWCNT functionalized n-ZBNNRs are always less than zero. Therefore, the SWCNT functionalized n-ZBNNRs are not only stable, but can also be spontaneously formed. Furthermore, compared with n-ZBNNRs, the SWCNT functionalized n-ZBNNRs show significant improvements in their thermal and mechanical stabilities. Thus, (m,m)SWCNT functionalization of n-ZBNNRs may open new routes toward practical nanoelectronic and optoelectronic as well as spintronic devices based on BNC-based materials.

Ab Initio Study of the Dielectric and Electronic Properties of Multilayer GaS Films

The dielectric properties of multilayer GaS films have been investigated using a Berry phase method and a density functional perturbation theory approach. A linear relationship has been observed between the number of GaS layers and slab polarizability, which can be easily converged at a small supercell size and has a weak correlation with different stacking orders. Moreover, the intercoupling effect of the stacking pattern and applied vertical field on the electronic properties of GaS bilayers has been discussed. The band gaps of different stacking orders show various downward trends with the increasing field, which is interpreted as giant Stark effect. Our study demonstrates that the slab polarizability as the substitution of conventional dielectric constant can act as an independent and reliable parameter to elucidate the dielectric properties of low-dimensional systems and that the applied electric field is an effective method to modulate the electric properties of nanostructures.

Structures, electronic properties and charge carrier mobility of graphdiyne-like BN nanoribbons

The structures, stabilities, electronic properties and charge carrier mobility of graphdiyne-like BN nanoribbons are investigated using the SCF-CO method.

Boron nitride zigzag nanoribbons: optimal thermoelectric systems

Conventional and spin related thermoelectric effects in zigzag boron nitride nanoribbons are studied theoretically within the Density Functional Theory (DFT) approach.

Hybrid structures of a BN nanoribbon/single-walled carbon nanotube: ab initio study

Hybrid structures of a zigzag edge BN nanoribbon/single-walled carbon nanotube, have been studied via standard spin-polarized density functional theory (DFT) calculations as well as ab initio molecular dynamics (MD) simulations.

High-yield synthesis of boron nitride nanoribbons via longitudinal splitting of boron nitride nanotubes by potassium vapor

Boron nitride nanoribbons (BNNRs) are theorized to have interesting electronic and magnetic properties, but their high-yield synthesis remains challenging. Here we demonstrate that potassium-induced splitting of BN nanotubes (BNNTs) is an effective high-yield method to obtain bulk quantities of high-quality BNNRs if a proper precursor material is chosen. The resulting BNNRs are crystalline; many of them have a high aspect ratio and straight parallel edges. We have observed numerous few-layer and monolayer BNNRs; the multilayered ribbons predominantly have an AA' stacking. We present a detailed microscopy study of BNNRs that provides important insights into the mechanism of the formation of BNNRs from BNNTs. We also demonstrate that the BNNTs prepared by different synthetic approaches could exhibit dramatically different reactivities in the potassium splitting reaction, which highlights the need for future comparison studies of BN nanomaterials prepared using different methods to better understand their preparation-dependent physical and chemical properties.

Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field

Density functional theory computations with dispersion corrections (DFT-D) were performed to investigate the dihalogen interactions and their effect on the electronic band structures of halogenated (fluorinated and chlorinated) BN bilayers and aligned halogen-passivated zigzag BN nanoribbons (BNNRs). Our results reveal the presence of considerable homo-halogen (FF and ClCl) interactions in bilayer fluoro (chloro)-BN sheets and the aligned F (Cl)-ZBNNRs, as well as substantial hetero-halogen (FCl) interactions in hybrid fluoro-BN/chloro-BN bilayer and F-Cl-ZBNNRs. The existence of interfacial dihalogen interactions leads to significant band-gap modifications for the studied BN nanosystems. Compared with the individual fluoro (chloro)-BN monolayers or pristine BNNRs, the gap reduction in bilayer fluoro-BN (B-FF-N array), hybrid fluoro-BN/chloro-BN bilayer (N-FCl-N array), aligned Cl-ZBNNRs (B-ClCl-N alignment), and hybrid F-Cl-ZBNNRs (B-FCl-N alignment) is mainly due to interfacial polarizations, while the gap narrowing in bilayer chloro-BN (N-ClCl-N array) is ascribed to the interfacial nearly-free-electron states. Moreover, the binding strengths and electronic properties of the interactive BN nanosheets and nanoribbons can be controlled by applying an external electric field, and extensive modulation from large-gap to medium-gap semiconductors, or even metals can be realized by adjusting the direction and strength of the applied electric field. This interesting strategy for band gap control based on weak interactions offers unique opportunities for developing BN nanoscale electronic devices.

Electronic and magnetic properties of boron nitride nanoribbons with square-octagon (4 | 8) line defects

The electronic and magnetic properties of boron nitride nanoribbons (BNNRs) with square-octagon (4 | 8) line defects (LD-(4 | 8)), parallel to their edges, have been investigated by first-principles calculations. It is found that the zigzag type LD-(4 | 8) BNNR with boron terminated edges is an antiferromagnetic (AFM' + - + -') insulator with an indirect bandgap, but that with nitrogen terminated edges has two degenerate ground states with the same energy, among which one is ferromagnetic (FM ' + + + +') half-metallic and the other is AFM ' + + - -' metallic. In contrast, it is more interesting to find that the bare and fully-hydrogen terminated armchair edge BNNRs with unsymmetric (4 | 8) line defects have an indirect and direct gap, respectively, both of which show a characteristic three family oscillation behavior, depending only on the ribbon width in its narrower part, but not the whole BNNR's width. Finally, the stabilities of a two-dimensional h-BN sheet with a zigzag or armchair type LD-(4 | 8) in it are further confirmed, among which the one with the armchair type LD-(4 | 8) is much more stable than that with the zigzag type counterpart.

Porous Boron Nitride with Tunable Pore Size

On the basis of a global structural search and first-principles calculations, we predict two types of porous boron-nitride (BN) networks that can be built up with zigzag BN nanoribbons (BNNRs). The BNNRs are either directly connected with puckered B (N) atoms at the edge (type I) or connected with sp(3)-bonded BN chains (type II). Besides mechanical stability, these materials are predicted to be thermally stable at 1000 K. The porous BN materials entail large surface areas, ranging from 2800 to 4800 m(2)/g. In particular, type-II BN material with relatively large pores is highly favorable for hydrogen storage because the computed hydrogen adsorption energy (-0.18 eV) is very close to the optimal adsorption energy (-0.15 eV) suggested for reversible hydrogen storage at room temperature. Moreover, the type-II materials are semiconductors with width-dependent direct bandgaps, rendering the type-II BN materials promising not only for hydrogen storage but also for optoelectronic and photonic applications.

Tuning the electron transport properties of boron-nitride nanoribbons with electron and hole doping

C chain doped BN nanoribbons can be either metallic or semiconducting, depending on how the C chains are located.

Nano-solenoid: helicoid carbon-boron nitride hetero-nanotube

As a fundamental element of a nanoscale passive circuit, a nano-inductor is proposed based on a hetero-nanotube consisting of a spiral carbon strip and a spiral boron nitride strip. It is shown by density functional theory associated with nonequilibrium Green function calculations that the nanotube exhibits attractive transport properties tunable by tube chirality, diameter, component proportion and connection manner between the two strips, with excellent 'OFF' state performance and high current on the order of 10-100 μA. All the hetero-nanotubes show negative differential resistance. The transmission peaks of current are absolutely derived from the helicoid carbon strips or C-BN boundaries, giving rise to a spiral current analogous with an energized nano-solenoid. According to Ampere's Law, the energized nano-solenoid can generate a uniform and tremendous magnetic field of more than 1 tesla, closing to that generated by the main magnet of medical nuclear magnetic resonance. Moreover, the magnitude of magnetic field can be easily modulated by bias voltage, providing great promise for a nano-inductor to realize electromagnetic conversion at the nanoscale.

Helium separation via porous silicene based ultimate membrane

Helium purification has become more important for increasing demands in scientific and industrial applications. In this work, we demonstrated that the porous silicene can be used as an effective ultimate membrane for helium purification on the basis of first-principles calculations. Prinstine silicene monolayer is impermeable to helium gas with a high penetration energy barrier (1.66 eV). However, porous silicene with either Stone-Wales (SW) or divacancy (555,777 or 585) defect presents a surmountable barrier for helium (0.33 to 0.78 eV) but formidable for Ne, Ar, and other gas molecules. In particular, the porous silicene with divacancy defects shows high selectivity for He/Ne and He/Ar, superior to graphene, polyphenylene, and traditional membranes.

Chemical unzipping of WS2 nanotubes

WS2 nanoribbons have been synthesized by chemical unzipping of WS2 nanotubes. Lithium atoms are intercalated in WS2 nanotubes by a solvothermal reaction with n-butyllithium in hexane. The lithiated WS2 nanotubes are then reacted with various solvents--water, ethanol, and long chain thiols. While the tubes break into pieces when treated with water and ethanol, they unzip through longitudinal cutting along the axes to yield nanoribbons when treated with long chain thiols, 1-octanethiol and 1-dodecanethiol. The slow diffusion of the long chain thiols reduces the aggression of the reaction, leading to controlled opening of the tubes.

Self-modulated band gap in boron nitride nanoribbons and hydrogenated sheets

Using hybrid density functional theory calculations with van der Waals correction, we show that polar boron nitride (BN) nanoribbons can be favorably aligned via substantial hydrogen bonding at the interfaces, which induces significant interface polarizations and sharply reduces the band gap of insulating ribbons well below the silicon range. The interface polarization can strongly couple with carrier doping or applied electric fields, yielding not only enhanced stability but also widely tunable band gap for the aligned ribbons. Furthermore, similar layer-by-layer alignment also effectively reduces the band gap of a 2D hydrogenated BN sheet and even turns it into metal. This novel strategy for band gap control appears to be general in semiconducting composite nanostructures with polar nonbonding interfaces and thus offers unique opportunities for developing nanoscale electronic and optical devices.

Two-Dimensional Hexagonal Beryllium Sulfide Crystal

We report a new two-dimensional hexagonal beryllium sulfide (h-BeS) sheet with exceptional properties by extensive first-principles calculations. The h-BeS sheet presents an indirect energy gap of 4.26 eV and an outstanding thermodynamic stability up to 1000 K. Armchair-edged nanoribbons of h-BeS are wide-energy-gap semiconductors with a giant Stark effect, while the zigzag-edged ones are metals with spin glass state. Especially, the ferromagnetic zigzag nanoribbons exhibit a net magnetic moment of nearly 1.15 μB. These interesting electronic and magnetic properties suggest the promise of the h-BeS crystal for potential applications and should inspire experimental enthusiasm.

Spin filtering and magneto-resistive effect at the graphene/h-BN ribbon interface

Advances in the realization of hybrid graphene/h-BN materials open new ways to control the electronic properties of graphene nanostructures. In this paper, the structural, electronic, and transport properties of heterojunctions made of bare zigzag-shaped h-BN and graphene ribbons are investigated using first-principles techniques. Our results highlight the potential of graphene/h-BN junctions for applications in spintronic devices. At first, density functional theory is used to detail the role played by the edge states and dangling bonds in the electronic and magnetic behavior of h-BN and graphene ribbons. Then, the electronic conductance of the junction is computed in the framework of Green's function-based scattering theory. In its high-spin configuration, the junction reveals a full spin polarization of the propagating carriers around the Fermi energy, and the magnitude of the transmission probability is predicted to be strongly dependent on the relative orientation of magnetic momenta in the leads.

Electronic properties of pure and p-type doped hexagonal sheets and zigzag nanoribbons of InP

Unlike graphene, a hexagonal InP sheet (HInPS) cannot be obtained by mechanical exfoliation from the native bulk InP, which crystallizes in the zinc blende structure under ambient conditions. However, by ab initio density functional theory calculations we found that a slightly buckled HInPS is stable both in pristine form and when doped with Zn atoms; the same occurred for hydrogen-passivated zigzag InP nanoribbons (ZInPNRs), quasi-one-dimensional versions of the quasi-two-dimensional material. We investigated the electronic properties of both nanostructures, in the latter case also in the presence of an external transverse electric field, and the results are compared with those of hypothetical planar HInPS and ZInPNRs. The band gaps of planar ZInPNRs were found to be tunable by the choice of strength of this field, and to show an asymmetric behavior under weak electric fields, by which the gap can either be increased or decreased depending on their direction; however, this effect is absent from slightly buckled ZInPNRs. The binding energies of the acceptor impurity states of Zn-doped HInPS and ZInPNRs were found to be similar and much larger than that of Zn-doped bulk InP. These latter findings show that the reduction of the dimensionality of these materials limits the presence of free carriers.

Advances in 2D boron nitride nanostructures: nanosheets, nanoribbons, nanomeshes, and hybrids with graphene

The recent surge in graphene research has stimulated interest in the investigation of various 2-dimensional (2D) nanomaterials. Among these materials, the 2D boron nitride (BN) nanostructures are in a unique position. This is because they are the isoelectric analogs to graphene structures and share very similar structural characteristics and many physical properties except for the large band gap. The main forms of the 2D BN nanostructures include nanosheets (BNNSs), nanoribbons (BNNRs), and nanomeshes (BNNMs). BNNRs are essentially BNNSs with narrow widths in which the edge effects become significant; BNNMs are also variations of BNNSs, which are supported on certain metal substrates where strong interactions and the lattice mismatch between the substrate and the nanosheet result in periodic shallow regions on the nanosheet surface. Recently, the hybrids of 2D BN nanostructures with graphene, in the form of either in-plane hybrids or inter-plane heterolayers, have also drawn much attention. In particular, the BNNS-graphene heterolayer architectures are finding important electronic applications as BNNSs may serve as excellent dielectric substrates or separation layers for graphene electronic devices. In this article, we first discuss the structural basics, spectroscopic signatures, and physical properties of the 2D BN nanostructures. Then, various top-down and bottom-up preparation methodologies are reviewed in detail. Several sections are dedicated to the preparation of BNNRs, BNNMs, and BNNS-graphene hybrids, respectively. Following some more discussions on the applications of these unique materials, the article is concluded with a summary and perspectives of this exciting new field.

Tuning Magnetism and Electronic Phase Transitions by Strain and Electric Field in Zigzag MoS2 Nanoribbons

Effective modulation of physical properties via external control may open various potential nanoelectronic applications of single-layer MoS2 nanoribbons (MoS2NRs). We show by first-principles calculations that the magnetic and electronic properties of zigzag MoS2NRs exhibit sensitive response to applied strain and electric field. Tensile strain in the zigzag direction produces reversible modulation of magnetic moments and electronic phase transitions among metallic, half-metallic, and semiconducting states, which stem from the energy-level shifts induced by an internal electric polarization and the competing covalent/ionic interactions. A simultaneously applied electric field further enhances or suppresses the strain-induced modulations depending on the direction of the electric field relative to the internal polarization. These findings suggest a robust and efficient approach to modulating the properties of MoS2NRs by a combination of strain engineering and electric field tuning.

Synthesis, properties and applications of nanoscale nitrides, borides and carbides

Nanoscale nitrides, borides and carbides are a fascinating type of materials, which have aroused tremendous and continuous research interest for decades owing to their special mechanical, electrical, optical, photoelectronic, catalytic properties and widespread uses. In this feature article, recent developments and breakthroughs in the synthesis, properties and applications of nanometre scale nitrides (BN, Si(3)N(4), GaN, noble nitrides), borides (LnB(6), LnB(2), Fe(3)BO(5), LiMBO(3)) and carbides (carbon, SiC, TiC, NbC, WC) were briefly reviewed in sequence of their different dimensions (1D, 2D and 3D). In particular, our latest advances in the "autoclave route" fabrication of nanoscale nitrides, borides, and carbides were highlighted. The challenges, issues and perspectives of the synthetic methodologies and potential applications concerning the above-mentioned materials were also briefly discussed.

Spin-spin and spin-orbit interactions in nanographene fragments: a quantum chemistry approach

The relativistic behavior of graphene structures, starting from the fundamental building blocks--the poly-aromatic hydrocarbons (PAHs) along with other PAH nanographenes--is studied to quantify any associated intrinsic magnetism in the triplet (T) state and subsequently in the ground singlet (S) state with account of possible S-T mixture induced by spin-orbit coupling (SOC). We employ a first principle quantum chemical-based approach and density functional theory (DFT) for a systematic treatment of the spin-Hamiltonian by considering both the spin-orbit and spin-spin interactions as dependent on different numbers of benzene rings. We assess these relativistic spin-coupling phenomena in terms of splitting parameters which cause magnetic anisotropy in absence of external perturbations. Possible routes for changes in the couplings in terms of doping and defects are also simulated and discussed. Accounting for the artificial character of the broken-symmetry solutions for strong spin polarization of the so-called "singlet open-shell" ground state in zigzag graphene nanoribbons predicted by spin-unrestricted DFT approaches, we interpolate results from more sophisticated methods for the S-T gaps and spin-orbit coupling (SOC) integrals and find that these spin interactions become weak as function of size and increasing decoupling of electrons at the edges. This leads to reduced electron spin-spin interaction and hence almost negligible intrinsic magnetism in the carbon-based PAHs and carbon nanographene fragments. Our results are in agreement with the fact that direct experimental evidence of edge magnetism in pristine graphene has been reported so far. We support the notion that magnetism in graphene only can be ascribed to structural defects or impurities.

Intrinsic metallic and semiconducting cubic boron nitride nanofilms

We show by density functional theory calculations with both hybrid and semilocal functionals that cubic boron nitride (111) nanofilms are intrinsically metallic and even turn into semiconductors once the thickness is less than 0.69 nm, which is in sharp contrast to the known insulating nature of boron nitride materials. The exceptional metallic or semiconducting band gap is due to a combined effect of thickness-dependent inbuilt electric polarization and labile near-gap states unique in the polar nanofilms. The band gap and dipole moment of the nanofilms can be further significantly tuned by applying an in-plane strain. These distinguished features of the boron nitride nanofilms are robust to surface passivation and can be enhanced by hybridizing with diamond films, thereby opening an exciting prospect of using the versatile cubic nanofilms in future electronic and piezoelectric devices.

Graphane/fluorographene bilayer: considerable C-H···F-C hydrogen bonding and effective band structure engineering

Systematic density functional theory (DFT) computations revealed the existence of considerable C-H···F-C bonding between the experimentally realized graphane and fluorographene layers. The unique C-H···F-C bonds define the conformation of graphane/fluorographene (G/FG) bilayer and contribute to its stability. Interestingly, G/FG bilayer has an energy gap (0.5 eV) much lower than those of individual graphane and fluorographene. The binding strength of G/FG bilayer can be significantly enhanced by applying appropriate external electric field (E-field). Especially, changing the direction and strength of E-field can effectively modulate the energy gap of G/FG bilayer, and correspondingly causes a semiconductor-metal transition. These findings open new opportunities in fabricating new electronics and opto-electronics devices based on G/FG bilayer, and call for more efforts in using weak interactions for band structure engineering.

Electric field effects on armchair MoS2 nanoribbons

Ab initio density functional theory calculations are performed to investigate the electronic structure of MoS(2) armchair nanoribbons in the presence of an external static electric field. Such nanoribbons, which are nonmagnetic and semiconducting, exhibit a set of weakly interacting edge states whose energy position determines the band gap of the system. We show that, by applying an external transverse electric field, E(ext), the nanoribbon band gap can be significantly reduced, leading to a metal-insulator transition beyond a certain critical value. Moreover, the presence of a sufficiently high density of states at the Fermi level in the vicinity of the metal-insulator transition leads to the onset of Stoner ferromagnetism that can be modulated, and even extinguished, by E(ext). In the case of bilayer nanoribbons we further show that the band gap can be changed from indirect to direct by applying a transverse field, an effect that might be of significance for opto-electronics applications.

Band-gap engineering via tailored line defects in boron-nitride nanoribbons, sheets, and nanotubes

We perform a comprehensive study of the effects of line defects on electronic and magnetic properties of monolayer boron-nitride (BN) sheets, nanoribbons, and single-walled BN nanotubes using first-principles calculations and Born-Oppenheimer quantum molecular dynamic simulation. Although line defects divide the BN sheet (or nanotube) into domains, we show that certain line defects can lead to tailor-made edges on BN sheets (or imperfect nanotube) that can significantly reduce the band gap of the BN sheet or nanotube. In particular, we find that the line-defect-embedded zigzag BN nanoribbons (LD-zBNNRs) with chemically homogeneous edges such as B- or N-terminated edges can be realized by introducing a B(2), N(2), or C(2) pentagon-octagon-pentagon (5-8-5) line defect or through the creation of the antisite line defect. The LD-zBNNRs with only B-terminated edges are predicted to be antiferromagnetic semiconductors at the ground state, whereas the LD-zBNNRs with only N-terminated edges are metallic with degenerated antiferromagnetic and ferromagnetic states. In addition, we find that the hydrogen-passivated LD-zBNNRs as well as line-defect-embedded BN sheets (and nanotubes) are nonmagnetic semiconductors with markedly reduced band gap. The band gap reduction is attributed to the line-defect-induced impurity states. Potential applications of line-defect-embedded BN nanomaterials include nanoelectronic and spintronic devices.

Modulating the bandgaps of graphdiyne nanoribbons by transverse electric fields

The effect of external transverse electric fields on the bandgaps of graphdiyne nanoribbons is investigated from first-principles calculations. The giant Stark effect is observed in the ribbons. When the field is applied, the valence and conduction band edge states are found to be strongly localized at low and high potential edges of the ribbon, respectively. Due to the wavefunction localization, the bandgap decreases with increasing field strength, and a semiconductor-metal transition occurs below a threshold field value. It is also shown that the bandgap decreasing rate depends linearly on the ribbon width. The tunable bandgap of a graphdiyne nanoribbon under an electric field would be helpful for practical applications.