Honeycomb Boron Research Articles

Honeycomb Boron Allotropes with Dirac Cones: A True Analogue to Graphene.

We propose a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point, the same as graphene. In particular, the Dirac point of one honeycomb boron sheet locates precisely on the Fermi level, rendering it as a topologically equivalent material to graphene. Its Fermi velocity (vf) is 6.05 × 105 m/s, close to that of graphene. Although the freestanding honeycomb B allotropes are higher in energy than α-sheet, our calculations show that a metal substrate can greatly stabilize these new allotropes. They are actually more stable than α-sheet sheet on the Ag(111) surface. Furthermore, we find that the honeycomb borons form low-energy nanoribbons that may open gaps or exhibit strong ferromagnetism at the two edges in contrast to the antiferromagnetic coupling of the graphene nanoribbon edges.

A safe and cost-effective PMMA carbon source for MgB2

Carbon is proven to be very effective in pinning the magnetic vortices and improving the superconducting performance of <TEX>$MgB_2$</TEX> at high fields. In this work, we have used polymethyl methacrylate (PMMA) polymer as a safe and cost effective carbon source. The effects of molecular weight of PMMA on crystal structure, microstructure as well as on superconducting properties of <TEX>$MgB_2$</TEX> were studied. X-ray diffraction analysis revealed that there is a noticeable shift in (100) and (110) Bragg reflections towards higher angles, while no shift was observed in (002) reflections for <TEX>$MgB_2$</TEX> doped with different molecular weights of PMMA. This indicates that carbon could be substituted in the boron honeycomb layers without affecting the interlayer interactions. As compared to undoped <TEX>$MgB_2$</TEX>, substantial enhancement in <TEX>$J_c(H)$</TEX> properties was obtained for PMMA-doped <TEX>$MgB_2$</TEX> samples both at 5 K and 20 K. The enhancement could be attributed to the effective carbon substitution for boron and the refinement of crystallite size by PMMA doping.

Valley Pseudospin with a Widely Tunable Bandgap in Doped Honeycomb BN Monolayer.

Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra of the B0.92NC2.44 sample show a maximum valley-contrasting circular polarization of 40% and 70% at room temperature and 77 K, respectively. Our work demonstrates a class of valleytronic materials with a controllable bandgap.

Investigating Robust Honeycomb Borophenes Sandwiching Manganese Layers in Manganese Diboride.

We report a robust honeycomb boron layers sandwiching manganese layers compound, MnB2, synthesized by high pressure and high temperature. First-principle calculation combined with X-ray photoelectron spectrum unravel that the honeycomb boron structure was stabilized by filling the empty π-band via grabbing electrons from manganese layers. Honeycomb boron layers sandwiching manganese layers is an extraordinary prototype of this type of sandwiched structure exhibiting electronic conductivity and ferromagnetism. Hydrostatic compression of the crystal structure, thermal expansion, and the hardness testing reveal that the crystal structure is of strong anisotropy. The strong anisotropy and first-principle calculation suggests that B-B bonds in the honeycomb boron structure are a strong directional covalent feature, while the Mn-B bonds are soft ionic nature. Sandwiching honeycomb boron layers with manganese layers that combine p-block elements with magnetic transition metal elements could endow its novel physical and chemical properties.

Electrical and thermal conductivities of the graphene, boron nitride and silicon boron honeycomb monolayers

Density of states, electrical and thermal conductivities of electrons in graphene, boron nitride and silicon boron single sheets are studied within the tight-binding Hamiltonian model and Green's function formalism, based on the linear response theory. The results show that while boron nitride keeps significantly the lowest amounts overall with an interval of zero value in low temperatures, due to its insulating nature, graphene exhibits the most electrical and thermal conductivities, slightly higher than silicon boron except for low temperature region where the latter surpasses, owing to its metallic character. This work might make ideas for creating new electronic devices based on honeycomb nanostructures.

Dirac State in the FeB2 Monolayer with Graphene-Like Boron Sheet.

By introducing the commonly utilized Fe atoms into a two-dimensional (2D) honeycomb boron network, we theoretically designed a new Dirac material of FeB2 monolayer with a Fermi velocity in the same order of graphene. The electron transfer from Fe atoms to B networks not only effectively stabilizes the FeB2 networks but also leads to the strong interaction between the Fe and B atoms. The Dirac state in FeB2 system primarily arises from the Fe d orbitals and hybridized orbital from Fe-d and B-p states. The newly predicted FeB2 monolayer has excellent dynamic and thermal stabilities and is also the global minimum of 2D FeB2 system, implying its experimental feasibility. Our results are beneficial to further uncovering the mechanism of the Dirac cones and providing a feasible strategy for Dirac materials design.

Aqueous dispersions of few-layer-thick chemically modified magnesium diboride nanosheets by ultrasonication assisted exfoliation.

The discovery of graphene has led to a rising interest in seeking quasi two-dimensional allotropes of several elements and inorganic compounds. Boron, carbon’s neighbour in the periodic table, presents a curious case in its ability to be structured as graphene. Although it cannot independently constitute a honeycomb planar structure, it forms a graphenic arrangement in association with electron-donor elements. This is exemplified in magnesium diboride (MgB2): an inorganic layered compound comprising boron honeycomb planes alternated by Mg atoms. Till date, MgB2 has been primarily researched for its superconducting properties; it hasn’t been explored for the possibility of its exfoliation. Here we show that ultrasonication of MgB2 in water results in its exfoliation to yield few-layer-thick Mg-deficient hydroxyl-functionalized nanosheets. The hydroxyl groups enable an electrostatically stabilized aqueous dispersion and create a heterogeneity leading to an excitation wavelength dependent photoluminescence. These chemically modified MgB2 nanosheets exhibit an extremely small absorption coefficient of 2.9 ml mg−1 cm−1 compared to graphene and its analogs. This ability to exfoliate MgB2 to yield nanosheets with a chemically modified lattice and properties distinct from the parent material presents a fundamentally new perspective to the science of MgB2 and forms a first foundational step towards exfoliating metal borides.

First-principles calculations of a robust two-dimensional boron honeycomb sandwiching a triangular molybdenum layer

A graphenelike two-dimensional boron honeycomb is inherently prohibited due to its empty \ensuremath{\pi} valence band. Based on chemical intuition and first-principles calculations, we design a two-dimensional crystal ${\mathrm{MoB}}_{4}$ with two graphenelike boron honeycombs sandwiching a triangular molybdenum layer. It has the attractive electronic structure of double Dirac cones near Fermi level with high Fermi velocity, which are contributed by the coupling of Mo $d$ orbitals and B ${p}_{z}$ orbitals. Such a metal stabilized boron honeycomb system could even have both superconductivity and ferromagnetism through appropriate selection of the metal layer, such as manganese. The unique electronic properties of these two-dimensional systems inspire broad interest in nanoelectronics.

Crystal Structure of W1−x B3 and Phase Equilibria in the Boron-Rich Part of the Systems Mo-Rh-B and W-{Ru,Os,Rh,Ir,Ni,Pd,Pt}-B

The crystal structure of W1−x B3 has been reinvestigated by x-ray single crystal diffraction and revealed isotypism with the Mo1−x B3 structure type (space group P63/mmc; a = 0.52012(1), c = 0.63315(3) nm; R F = 0.040). As a characteristic feature of the structure, planar hexagonal metal layers (1/3 of atoms removed from ordered positions) sandwich planar boron honeycomb layers. One of the two W-sites shows a random defect of about 73%. Strong metal boron and boron-boron bonds are responsible for high mechanical stability. Although W1−x B3 at about 80 at.% B is the metal boride richest in boron, it contains no directly linked three-dimensional boron framework. The solubility of Rh, Ir, Ni, Pd and Pt in W1−x B3 as well as of Rh in Mo1−x B3 has been investigated in as cast state and after annealing. Furthermore, phase equilibria in the boron rich part of the corresponding isothermal sections W-TM-B (TM = Rh, Ir at 1100 °C, TM = Ni, Pd at 900 °C and TM = Pt at 800 °C) and Mo-Rh-B (at 1100 °C) have been established. A ternary compound only forms in the system W-Ir-B: τ1-W1−x Ir x B2 with ReB2 structure type (space group P63/mmc; a = 0.2900, c = 0.7475 nm). The type of formation and crystal structure of diborides W1−x TM x B2 (TM = Ru, Os, Ir) isotypic with ReB2 were studied by x-ray powder diffraction and electron probe microanalysis in as cast state and after annealing at 1500 °C. Accordingly, W0.5Os0.5B2 (a = 0.29127(1), c = 0.7562(1) nm) forms directly from the melt, whereas W0.4Ru0.6B2 (a = 0.29027(1), c = 0.74673(2) nm) and W0.6Ir0.4B2 (a = 0.29263(1), c = 0.75404(8) nm) are incongruently melting. Annealing at 1500 °C leads in case of the iridium compound to an almost single-phase product but the same procedure does not increase the amount of the ruthenium diboride.

BCx layers with honeycomb lattices on an NbB2(0001) surface

At elevated temperatures of 1000–1500 K, carbon (C) atoms that segregated to a surface and mixed with the boron (B) honeycomb lattice resulted in the formation of three different BCx layers as the topmost layers of NbB2(0001). Two of the layers were commensurate lattices: and structures; the third was incommensurate. The characteristic features of the lattice with a honeycomb structure are discussed on the basis of the experimental data.

An unexpected softening from WB3 to WB4

We report a drastic reduction of hardness of about 61% from WB3 to WB4. The three-dimensional covalent network consisting of boron honeycomb planes interconnected with strong zigzag W-B bonds underlies the high hardness of WB3. Despite the strong intralayer and interstitial B-B bonds, the interlayer B-B nonbonding and the considerably weak zigzag W-B bonding allow the layers of WB4 to cleave readily, which results in the anomalous softening of WB4. The results provide robust evidence that the highest boride of tungsten, which is characterized experimentally by an inexpensive superhard material, should be stoichiometric WB3, not WB4.

Tunable and sizable band gap of single-layer graphene sandwiched between hexagonal boron nitride

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs. Jing Lu and co-workers have revealed how to open up a tunable band gap in single-layer graphene, the one-atom-thick honeycomb carbon layer that has sparked much interest both in fundamental physics and in practical applications. Although graphene's excellent mechanical, thermal and electrical properties are very attractive, one major drawback is its lack of ‘band gap’ — the energy gap in the electronic structure of a material that enables to switch its conductivity on and off. Previous attempts to create such a gap in single-layer graphene have typically damaged its structure or conductivity. Through extensive calculations, the researchers have now examined the properties of single-layer graphene when sandwiched between two honeycomb boron nitride (BN) layers. They revealed that for a specific positioning of the layers, a sizable band gap can be opened and further tuned by applying an electric field without causing damage, making the sandwich structure a promising component for field-effect transistors. An electric field-enhanced transport gap is well established in a dual-gated field effect transistor (FET) based on the h-BN/single-layer graphene/h-BN sandwich structure, and the on/off current ratio is increased by a factor of 8.0 compared with pure single-layer graphene FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single-layer graphene FETs.

Resonant and crossover phenomena in a multiband superconductor: Tuning the chemical potential near a band edge

Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second mini-band generated by quantum confinement effects. The shape resonances at T=0 in the superconducting gaps (belonging to the class of Feshbach-like resonances) is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second mini-band. The case of a superlattice of quantum wells is considered and the amplification of the superperconducting gaps at the 3D-2D Fermi surface topological transition is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.

Absence of superconductivity in the high-pressure polymorph ofMgB2

We report a high-pressure orthorhombic ${\text{KHg}}_{2}$-type polymorph (space group Imma, 4 f.u./cell) of ${\text{MgB}}_{2}$ stable above 190 GPa predicted through ab-initio evolutionary simulations. The formation of this new phase results from the strong out-of-plane distortions of the two-dimensional honeycomb boron sublattice of the low-pressure ${\text{AlB}}_{2}$-type structure creating a peculiar tetrahedrally bonded three-dimensional boron network. This high-pressure phase is a weak metal and not superconducting, re-highlighting the key role of the planar boron sublattice in forming the superconducting state and clear structure-property relations that can enable design of new superconductors.

Strong Relationship between Irreversibility Field and Crystallinity Discovered in Undoped and Carbon Substituted MgB2 Bulks

The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including SiC or B4C doped samples was studied. Among the XRD peaks of MgB2, the FWHM of (110) reflection corresponding to the in-plane disorder was strongly dependent on the samples and Hirr was found to be systematically increased with an increase of the FWHM of MgB2 (110) peak. Enhanced intra-band scattering and strengthened grain boundary flux pinning are suggested to contribute to the excellent Hirr characteristics. On the other hand, weak correlation between Hirr and FWHM of (002) peak was confirmed. These mean that introduction of disorders into the ab-plane, i.e., distortion in honeycomb boron sheet, is essentially effective to enhance Hirr.

Pinning enhancement in nano-SiC and Si 3N 4 doped MgB 2 tapes: A comparative study

The effect of nanometer SiC and Si 3N 4 particles addition on the crystal structure and superconducting properties of MgB 2 tapes was systemically studied. The a-axis of MgB 2 lattice was shortened by nano-SiC addition due to the carbon substitution induced lattice distortion of the honeycomb boron sheet. By contrast, the a-axis parameter was slightly increased in nano-Si 3N 4 doped tapes, possibly because of a slight Si substitution for B site. The amount of Mg 2Si phase was lower in nano-SiC doped tapes than in nano-Si 3N 4 doped ones with the same doping concentration of Si, suggesting that more Si might have entered the MgB 2 matrix in the former. This can be treated as an indirect evidence for the co-substitution of Si and C for B in nano-SiC doped samples. Our comparative study reveals that the enhanced flux pinning in both nano-SiC and Si 3N 4 doped tapes should mainly originate from the in-plane lattice distortion due to the element-substitution on boron sheet.

Feshbach shape resonances in nanoscale multiband systems for high Tcsuperconductivity

We show that electron doped MgB 2 , made of a superlattice of boron honeycomb layers, is a multiband superconductor in the clean limit that provides the simplest high T c superconducting system (HTcS) called also the atomic hydrogen for HTcS. The chemical potential has been tuned by atomic substitution keeping the superconductivity in the clean limit. This is a two component electronic system made of a first Fermi gas in the n subband that coexists with a second low density electron fluid in the o subband close the localization limit 5 < r s < 30. The Feshbach shape resonance of the interband exchange-like pairing term centered at the 2D to 3D Lifshitz electronic topological transition (2D/3D ETT) is shown to be the key term controlling the critical temperature in the range from 0.3 K to 40 K.

Novel macroscopic BC 3 honeycomb sheet

We have successfully grown a uniform BC 3 honeycomb sheet with high crystalline quality over the entire macroscopic area of the NbB 2 (0001) surface by carbon-substituted technique in a boron honeycomb. This is the first macroscopic uniform sheet of single-crystal BC 3 with a monolayer thickness. So far, only BC 3 micro-crystals with radii of a few nanometres were produced. The properties of the sheet have been investigated by using XPS, AES, LEED, STM, ARUPS and HREELS. All the experimental data and the theoretical calculations, which are concerning with atomic structure, chemical concentration, and phonon structure, indicated that the uniform BC 3 honeycomb sheet was grown in an epitaxial manner to the substrate lattice.

Universal relationship between crystallinity and irreversibility field of MgB2

The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including carbon substituted samples was studied. The Hirr was found to increase with an increase of full width at half maximum (FWHM) of MgB2 (110) peak, which corresponds to distortion of honeycomb boron sheet, and their universal correlation was discovered even including carbon substituted samples. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr can explain the large variation of Hirr for MgB2 bulks, tapes, single crystals, and thin films.

Structural and critical current properties in Al-doped MgB 2

A series of Al-doped Mg 1− x Al x B 2 samples have been fabricated and systematic study on structure and superconducting properties have been carried out for the samples. In addition to a structural transition observed by XRD, TEM micrographs showed the existence of a superstructure of double c-axis lattice constant along the direction perpendicular to the boron honeycomb sheet. In order to investigate the effect of Al doping on flux pinning and critical current properties in MgB 2, measurements on the superconducting transition temperature T c, irreversible field B irr and critical current density J c were performed too, for the samples with the doping levels lower than 0.15 in particular. These experimental observations were discussed in terms of Al doping induced changes in carrier concentration.