Spin filtering in graphene nanoribbons with Mn-doped boron nitride inclusions

We investigate the spin filtering effects in graphene nanoribbons, where inclusions of hexagonal boron nitride were introduced together with substitutional magnetic impurities. The embedded Mn-doped boron nitride regions serve as quasi-0D islands of diluted magnetic semiconductor in the otherwise metallic graphene nanoribbon. Our first principle approach based on non-equilibrium Green's functions gives the polarization of the spin current for structures with one or two Mn impurities as a function of the applied bias. For the two impurity case, ferromagnetic and antiferromagnetic spin configurations of the magnetic impurities are considered. The spin resolved current indicates that the analyzed structures are suitable for spin filter applications or for spin current switching devices.     

Strain-engineering of band gaps in piezoelectric boron nitride nanoribbons

Two-dimensional atomic sheets such as graphene and boron nitride monolayers represent a new class of nanostructured materials for a variety of applications. However, the intrinsic electronic structure of graphene and h-BN atomic sheets limits their direct application in electronic devices. By first-principles density functional theory calculations we demonstrate that band gap of zigzag BN nanoribbons can be significantly tuned under uniaxial tensile strain. The unexpected sensitivity of band gap results from reduced orbital hybridization upon elastic strain. Furthermore, sizable dipole moment and piezoelectric effect are found in these ribbons owing to structural asymmetry and hydrogen passivation. This will offer new opportunities to optimize two-dimensional nanoribbons for applications such as electronic, piezoelectric, photovoltaic, and opto-electronic devices.     

Energy gaps and stark effect in boron nitride nanoribbons

A first-principles investigation of the electronic properties of boron nitride nanoribbons (BNNRs) having either armchair or zigzag shaped edges passivated by hydrogen with widths up to 10 nm is presented. Band gaps of armchair BNNRs exhibit family dependent oscillations as the width increases and, for ribbons wider than 3 nm, converge to a constant value that is 0.02 eV smaller than the bulk band gap of a boron nitride sheet owing to the existence of very weak edge states. The band gap of zigzag BNNRs monotonically decreases and converges to a gap that is 0.7 eV smaller than the bulk gap due to the presence of strong edge states. When a transverse electric field is applied, the band gaps of armchair BNNRs decrease monotonically with the field strength. For the zigzag BNNRs, however, the band gaps and the carrier effective masses either increase or decrease depending on the direction and the strength of the field.     

Longitudinal splitting of boron nitride nanotubes for the facile synthesis of high quality boron nitride nanoribbons

Boron nitride nanoribbons (BNNRs), the boron nitride structural equivalent of graphene nanoribbons (GNRs), are predicted to possess unique electronic and magnetic properties. We report the synthesis of BNNRs through the potassium-intercalation-induced longitudinal splitting of boron nitride nanotubes (BNNTs). This facile, scalable synthesis results in narrow (down to 20 nm), few sheet (typically 2-10), high crystallinity BNNRs with very uniform widths. The BNNRs are at least 1 μm in length with minimal defects within the ribbon plane and along the ribbon edges.     

Stability and electronic properties of armchair boron nitride/carbon nanotubes

In this work are studied the electronic and structural properties of armchair boron nitride/carbon nanotubes using first principles calculations. The density functional within the generalized gradient approximation (HSEh1PBE-GGA) is used. For each composition, different bonding schemes for the construction of the hybrid systems were employed. Among them, structural stability with neutral charge was determined for the following compositions: T1: B₄₀N₃₅C₇₅H₂₀, T2: B₃₅N₄₀C₇₅H₂₀, T3: B₃₇N₃₈C₇₅H₂₀, T4 : B₃₇N₃₇C₇₆H₂₀, and T7: B₃₅N₃₅C₈₀H₂₀. All these hybrid nanotubes have high polarity; the T3, T4 and T7 are semiconductors: whereas T1 and T2 are conductor in character. The formers also have magnetic behavior. These properties together with a low-chemical potential suggest applications as nano-vehicle for drug delivery. These mixed nanotubes also have potential applications in the electronic devices based on the small work function.     

Field emission and strain engineering of electronic properties in boron nitride nanotubes

The electrical properties of boron nitride (BN) nanostructures, particularly BN nanotubes (NTs), have been studied less in comparison to the counterpart carbon nanotubes. The present work investigates the field emission (FE) behavior of BNNTs under multiple cycles of FE experiments and demonstrates a strain-engineering pathway to tune the electronic properties of BNNTs. The electrical probing of individual BNNTs were conducted inside a transmission electron microscope (TEM) using an in?situ electrical holder capable of applying a bias voltage of up to 110?V. Our results indicate that in the first cycle a single BNNT can exhibit the current density of ～1?mA?cm(-2) at 110?V and the turn-on voltage of 325?V?μm(-1). However, field emission properties reduced considerably in subsequent cycles. Real-time imaging revealed the structural degradation of individual BNNTs during FE experiments. The electromechanical measurements show that the conductivity of BNNTs can be tuned by means of mechanical straining. The resistance of individual BNNTs reduced from 2000 to 769?MΩ and the carrier concentration increased from 0.35?×?10(17) to 1.1?×?10(17)?cm(-3) by straining the samples up to 2.5%.     

Nitrogen doped n-type CdS nanoribbons with tunable electrical and photoelectrical properties

Single-crystal nitrogen doped CdS nanoribbons (NRs) with wurtzite structure were synthesized in ammonia atmosphere via a thermal evaporation deposition route. X-ray diffraction patterns reveal a significant contraction of the lattice constants due to the incorporation of nitrogen. Temperature-varied photoluminescence spectra of CdS:N NRs exhibit spectral features near the band edge, which can be ascribed to free excition and neutral acceptor-bound excition emissions. Electrical and photoelectrical properties of the CdS:N NRs were systemically studied by constructing the field-effect transistors based on individual NRs. The conductivity of the NRs can be tuned by two orders of magnitude by controlling the N doping concentration. Moreover, by post-annealing, the device performance is remarkably improved, in particular, the mobility of the CdS:N NRs is increased by nearly three orders of magnitude from approximately 10(-1) to hundreds of cm2/Vs. I(on)I(off) ratio of the annealed device reaches over 10(4). Photoconductive properties of the CdS:N NRs were also studied. The doped NRs show high sensitivity to the light with energy larger than band-gap and the response amplitude and speed depend on the doping concentration.     

Photoluminescence properties of pyrolytic boron nitride

We report on spectroscopic study of pyrolytic hBN (pBN) by means of time- and energy-resolved photoluminescence methods. A high purity pBN samples (though low crystallinity) allow complementary information about excited states involved into the luminescence process. We affirm our recent conclusions about the impurity-related nature of most of fluorescence bands in microcrystalline hBN. In addition, a broad band centred at 3.7 eV previously not considered because of its superposition with an intense structured impurity emission is attributed to the radiative recombination of deep DAPs.     

Edge stability of boron nitride nanoribbons and its application in designing hybrid BNC structures

First-principles investigations of the edge energies and edge stresses of single-layer hexagonal boron nitride (BN) are presented. The armchair edges of BN nanoribbons (BNNRs) are more stable in energy than zigzag ones. Armchair BNNRs are under compressive edge stress while zigzag BNNRs are under tensile edge stress, due to the edge reconstruction effect and edge coulomb repulsion effect. The intrinsic spin-polarization and edge saturation play important roles in modulating the edge stability of BNNRs. The edge energy difference between BN and graphene can be used to guide the design of specific hybrid BNC structures as the hybrid BNC systems prefer the low-energy edge configurations: In an armchair BNC nanoribbon (BNCNR), BN domains are expected to grow outside of C domains, while the opposite occurs in a zigzag BNCNR. More importantly, armchair BNCNRs can reproduce unique electronic properties of armchair graphene nanoribbons (GNRs), which are expected to be robust against edge functionalization or disorder. Within a certain range of C/BN ratios, zigzag BNCNRs may exhibit intrinsic half-metallicity without any external constraints. These diverse electronic properties of BNCNRs may offer unique opportunities to develop nanoscale electronics and spintronics beyond individual graphene and BN. More generally, these principles for designing BNC can also be extended to other hybrid nanostructures.      

Field emission properties of carbon nanotubes coated with boron nitride

Field emission properties of carbon nanotubes coated with a single layer of boron nitride are calculated using the first-principles pseudopotential method. At lower bias voltage, the emission current of the coated nanotube is comparable to that of the bare carbon nanotube and is dominated by the contribution from localized states at the tip of the tube. At higher voltage, newly generated hybridized states between the carbon nanotube tip and the even-membered boron nitride rings contribute significantly to the emission current because they experience a low tunneling barrier compared with the bare carbon nanotube case. Our results suggest that the insulator coating can, besides protecting the nanotube tip from the attack of gas molecules, substantially enhance the field emission current.     

Synthesis of cubic boron nitride with boron nitride powder formed from triammoniadecaborane

Cubic BN was synthesized under high temperature and pressure conditions from BN powder formed by reaction of triammoniadecaborane (TAD) with ammonia. The BN powder formed from TAD and ammonia had a low degree of ordering. The crystal lattice of the BN powder increased in regularity with increasing synthesis temperature and time for the reaction of TAD with ammonia. The conversion yield of cubic BN at 1300 °C and 6.5 GPa in the presence of AIN increased with decreasing of reaction temperature of TAD and ammonia from 1000–700 °C. Cubic BN decreased in yield with increasing reaction time of TAD and ammonia at 800 °C. BN powder pre-heat treated at 1550 °C had a crystallite size,L _c, of 22 nm, and was converted to cubic BN in a 43% yield at 1300 °C and 6.5 GPa for 10 min. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 97 kJ mol^–1, when the starting BN was synthesized at 800 °C. The conversion yield of cubic BN from the disordered BN-20 mol% AIN was 100% after heat treatment at 1300 °C and 6.5 GPa for 20 min.     

Stability,electronic and magnetic properties of the Octagraphene-like boron nitride Nanosheets: In silico studies

The characterization of the structural, electronic, and magnetic properties of octagraphene-like boron-nitride: BN (B₃₂N₃₂H₂₄, B₃₇N₂₇H₂₄, and B₂₇N₃₇H₂₄), nanosheets were performed by means of density functional theory all-electron calculations at the HSEh1PBE/GGA level. Orbital 6–311G(d,p) basis sets were used. Non-stoichiometric B₃₇N₂₇H₂₄ and B₂₇N₃₇H₂₄ compositions: rich in boron or nitrogen atoms, forming homonuclear B or N bonds, respectively, were chosen. The obtained results reveal that these BN nanosheets reach structural stability in the anionic form, where semiconductor behaviors are promoted. Indeed, the HOMO-LUMO gap values are: 5.09, 1.48 and 2.53 eV, for stoichiometric and non-stoichiometric models, respectively. Whereas the magnetic moments are coming from the boron atoms in all cases (1.0 bohrs magneton). The rich in boron and nitrogen nanosheets presents high-polarity; either in the gas phase or embedded in aqueous mediums like water. Also appears low chemical reactivity, signifying potential applicability as nanovehicles of pharmaceutical species. The nanosheets rich in boron atoms are promising candidates for the design of nanosensors, because they possess low-work functions, mainly arising from the homonuclear boron bond formation.     

Radial mechanical properties of single-walled boron nitride nanotubes

The radial mechanical properties of single-walled boron nitride nanotubes (SW-BNNTs) are investigated by atomic force microscopy. Nanomechanical measurements reveal the radial deformation of individual SW-BNNTs in both elastic and plastic regimes. The measured effective radial elastic moduli of SW-BNNTs are found to follow a decreasing trend with an increase in tube diameter, ranging from 40.78 to 1.85 GPa for tube diameters of 0.58 to 2.38 nm. The results show that SW-BNNTs have relatively lower effective radial elastic moduli than single-walled carbon nanotubes (SWCNTs). The axially strong, but radially supple characteristics suggest that SW-BNNTs may be superior to SWCNTs as reinforcing additives for nanocomposite applications.     

Effective mechanical properties of hexagonal boron nitride nanosheets

We propose an analytical formulation to extract from energy equivalence principles the equivalent thickness and in-plane mechanical properties (tensile and shear rigidity, and Poisson's ratio) of hexagonal boron nitride (h-BN) nanosheets. The model developed provides not only very good agreement with existing data available in the open literature from experimental, density functional theory (DFT) and molecular dynamics (MD) simulations, but also highlights the specific deformation mechanisms existing in boron nitride sheets, and their difference with carbon-based graphitic systems.     

Boron nitride nanoribbons become metallic

Standard spin-polarized density functional theory calculations have been conducted to study the electronic structures and magnetic properties of O and S functionalized zigzag boron nitride nanoribbons (zBNNRs). Unlike the semiconducting and nonmagnetic H edge-terminated zBNNRs, the O edge-terminated zBNNRs have two energetically degenerate magnetic ground states with a ferrimagnetic character on the B edge, both of which are metallic. In contrast, the S edge-terminated zBNNRs are nonmagnetic albeit still metallic. An intriguing coexistence of two different Peierls-like distortions is observed for S edge-termination that manifests as a strong S dimerization at the B zigzag edge and a weak S trimerization at the N zigzag edge, dictated by the band fillings at the vicinity of the Fermi level. Nevertheless, metallicity is retained along the S wire on the N edge due to the partial filling of the band derived from the p(z) orbital of S. A second type of functionalization with O or S atoms embedded in the center of zBNNRs yields semiconducting features. Detailed examination of both types of functionalized zBNNRs reveals that the p orbitals on O or S play a crucial role in mediating the electronic structures of the ribbons. We suggest that O and S functionalization of zBNNRs may open new routes toward practical electronic devices based on boron nitride materials.     

Purification of single-walled boron nitride nanotubes and boron nitride cages

Continuous laser vaporization of a BN target under N2 atmosphere is up to now the unique route to single-walled boron nitride nanotubes (BN-SWNTs). Although grams of product can be obtained by this technique, the raw material contains in addition to the BN-SWNTs, different by-products made of boron and nitrogen. Since these materials are undesirable for the studying of the intrinsic properties of the nanotubes, we have undertaken a purification process using chemical and physical methods to separate the different components. We show here that most impurities can be removed by successive cycles of washing, sonication, and centrifugation. Furthermore, the two different types of boron nitride nanostructures i.e., BN-SWNTs and BN-cages can be isolated. Efficiency of the separation was monitored by transmission electron microscopy (TEM) at the different steps of the process. Finally, we envisage the further purification of the nanotubes-enriched fraction by functionalizing the nanotubes in a non covalent manner by specific polymers as for carbon nanotubes and BN multi-walled nanotubes.     

Transport properties of graphene nanoroads in boron nitride sheets

We demonstrate that the one-dimensional (1D) transport channels that appear in the gap when graphene nanoroads are embedded in boron nitride (BN) sheets are more robust when they are inserted at AB/BA grain boundaries. Our conclusions are based on ab initio electronic structure calculations for a variety of different crystal orientations and bonding arrangements at the BN/C interfaces. This property is related to the valley Hall conductivity present in the BN band structure and to the topologically protected kink states that appear in continuum Dirac models with position-dependent masses.     

Structural and optical properties of plasma-deposited boron nitride films

The plasma-enhanced chemical vapor deposition of boron nitride films in a low pressure, parallel plate reactor incorporating an electromagnet was investigated. Films were deposited from gas mixtures of diborane, hydrogen and ammonia. The ratio of boron to nitrogen was approximately 1.7 when an ammonia-to-diborane ratio of 4 was used. The films had the following optical properties: a band gap in the range 5.6–5.8 eV, an absorption coefficient (at 6.0 eV) of about 1×10⁵ cm⁻¹ and an index of refraction of 1.7. In general the optical properties were identical, with or without the application of a low intensity magnetic field.     

Static and dynamic properties of single-walled boron nitride nanotubes

This paper reports the study of the static and dynamic properties of single-walled boron nitride nanotubes by using the molecular structural mechanics approach. The stiffness parameters of the boron-nitride bond are based on the DREIDING force field. The effects of tube diameter and chirality are investigated. The computational results show that the Young's modulus and the shear modulus of boron nitride nanotubes increase monotonically with nanotube diameter and reach up to 0.9 TPa and 0.5 TPa, respectively, at large tube diameter. The nanotube chirality has only slight effects on the moduli only when tube diameters are small. The fundamental frequencies of boron nitride nanotubes are found to be dependent not only on the nanotube aspect ratio but also on the tube diameter.