Designing and searching for high lattice thermal conductivity materials in both bulk and nanoscale level is highly demanding for electronics cooling. Boron phosphide is a III-V compound semiconductor with superior structural and thermal properties. In this work, we studied the lattice thermal conductivity of hexagonal boron phosphide(h-BP) using first principles calculations. For pure h-BP, we found a high lattice thermal conductivity (at 300K) of 561.2 Wm-1K-1 and 427.4 Wm-1K-1 along a-axis and c-axis respectively. These values are almost equal to hexagonal silicon carbide(2H-SiC) and cubic boron phosphide(c-BP). We also computed the length dependence thermal conductivity for its applications in nanostructures. At nanoscale (L=100 nm), a high thermal conductivity of ~71.5 Wm-1K-1(56.2 Wm-1K-1) is observed along a-axis(c-axis). This result suggests that, h-BP will be a promising material for thermal management applications in micro/nano electronics.
Microstructure and mechanical properties of bulk polycristalline boron phosphides (cubic BP and rhombohedral B12P2) have been studied by scanning electron microscopy and micro- and nanoindentation. The obtained data on hardness, elastic properties and fracture toughness clearly indicate that both phosphides belong to a family of hard phases and can be considered as prospective binders for diamond and cubic boron nitride.
A new method of producing boron phosphide (BP) submicron powders by self-propagating high-temperature reaction between boron phosphate and magnesium in the presence of an inert diluent (sodium chloride) has been proposed. Bulk polycrystalline BP with microhardness of HV = 28(2) GPa has been prepared by sintering the above powders at 7.7 GPa and 2600 K.
Melting of cubic boron phosphide, BP has been studied at pressures to 9 GPa using synchrotron X-ray diffraction and electrical resistivity measurements. It has been found that above 2.6 GPa BP melts congruently, and the melting curve exhibits negative slope -60(7) K/GPa, which is indicative of a higher density of the melt as compared to the solid phase.
Recently, hydrogen generation by water-splitting photocatalysts is attracting attention as a sustainable and clean energy resource. Photocatalytic hydrogen-generation systems are much simpler, cheaper, and easier to scale up than the coupled systems of electrolysis and solar cells, wind-power generation, etc. However, photocatalytic hydrogen generation is currently inefficient. This paper proposes the monolayer of boron phosphide as a stable highly-efficient water-splitting photocatalyst by high-precision density-functional theory calculations using a HSE06 functional with a solvent effect. Th…
Boron phosphide has recently been identified as a potential high hole mobility transparent conducting material. This promise arises from its low hole effective masses. However, BP has a relatively small 2 eV indirect band gap which will affect its transparency. In this work, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a \emph{p}-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mo…
The 300 K equation of state of cubic (zinc-blende) boron phosphide BP has been studied by in situ single-crystal X-ray diffraction with synchrotron radiation up to 55 GPa. The measurements have been performed under quasi-hydrostatic conditions using a Ne pressure medium in a diamond anvil cell. A fit of the experimental p-V data to the Vinet equation of state yields the bulk modulus B0 of 179(1) GPa with its pressure derivative of 3.3(1). These values are in a good agreement with previous elastic measurements, as well as with semiempirical estimations.
Using machine learning (ML) approach, we unearthed a new III-V semiconducting material having an optimal bandgap for high efficient photovoltaics with the chemical composition of Gallium-Boron-Phosphide(GaBP$_2$, space group: Pna2$_1$). ML predictions are further validated by state of the art ab-initio density functional theory (DFT) simulations. The stoichiometric Heyd-Scuseria-Ernzerhof (HSE) bandgap of GaBP$_2$ is noted to 1.65 eV, a close ideal value (1.4-1.5 eV) to reach the theoretical Queisser-Shockley limit. The calculated electron mobility is similar to that of silicon. Unlike perovsk…
With an indirect band gap in the visible and a direct band gap at a much higher energy, boron phosphide (BP) holds promise as an unconventional p-type transparent conductor. Previous experimental reports deal almost exclusively with epitaxial, nominally undoped BP films by chemical vapor deposition. High hole concentrations were often observed, but it is unclear if native defects alone can be responsible for it. Besides, the feasibility of alternative deposition techniques has not been clarified and optical characterization is generally lacking. In this work, we demonstrate reactive sputtering…
Cubic boron phosphide BP has been studied in situ by X-ray diffraction and Raman scattering up to 55 GPa at 300 K in a diamond anvil cell. The bulk modulus of B0 = 174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state data, allowed us to estimate the Grüneisen parameters of the TO and LO modes of zinc-blende structure, γGTO = 1.16 and γGLO = 1.04, just like in the case of other AIIIBV diamond-like phases, for which γGTO > γGLO = 1. We also established that the pressure…
We performed a first-principles study of the electronic behavior of a 2D hexagonal boron phosphide monolayer (2D-h-BP). The system was deformed isotropically by applying a simultaneous tensile strain along the a and b crystal axes. We analyzed the band-gap evolution as function of the deformation percentage, ranging from 1% to 8%. Results show that the system behaves as a direct band-gap semiconductor, with the valence band maximum and conduction band minimum located at the K point (1/3, 1/3, 0) of the Brillouin zone. This behavior is unchanged despite the strain application. The band gap unde…
Monolayer of hexagonal boron nitride (h-BN), commonly known as "white graphene" is a promising wide bandgap semiconducting material for deep-ultaviolet optoelectronic devices. In this report, the light absorption of a single layer hexagonal boron nitride is calculated using a tight-binding Hamiltonian. The absorption is found to be monotonically decreasing function of photon energy compared to graphene where absorption coefficient is independent of photon energy and characterized by the effective fine-structure constant.
Two dimensional van der Waals crystals and their heterostructures provide an exciting alternative to bulk wide bandgap semiconductors as hosts of single photon emitters. Amongst different layered materials, bright and robust defect-based single photon emitters have been observed within hexagonal boron nitride, a layered wide-bandgap semiconductor. Despite research efforts to date, the identities of the deep defects responsible for quantum emissions in hexagonal boron nitride remain unknown. In this theoretical work, I demonstrate that the strain-induced changes in emission frequencies depend o…
Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of carbon phosphide (CP) monolayer consisted of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and \b{eta}-CP with puckered and buckled surfaces, respectively are semiconducting with highly anisotropic elect…
We study silicene on hexagonal boron nitride, using first principles calculations. Since hexagonal boron nitride is semiconducting, the interaction with silicene is weaker than for metallic substrates. It therefore is possible to open a 50 meV band gap in the silicene. We further address the effect of Mn decoration by determining the onsite Hubbard interaction parameter, which turns out to differ significantly for decoration at the top and hollow sites. The induced magnetism in the system is analyzed in detail.
Group theory and density functional theory methods are combined to obtain compact and accurate $k\cdot p$ Hamiltonians that describe the bandstructures around the $K$ and $Γ$ points for the 2D material hexagonal boron arsenide (h-BAs) predicted to be an important low-bandgap material for electric, thermoelectric, and piezoelectric properties that supplements the well-studied 2D material hexagonal boron nitride. Hexagonal boron arsenide is a direct bandgap material with band extrema at the $K$ point. The bandgap becomes indirect with a conduction-band minimum at the $Γ$ point subject to a stron…
An ab initio calculations have been carried out for examining the curvature effect of small diameter hexagonal boron nanotubes. The considered conformations of boron nanotubes are namely armchair (3,3), zigzag (5,0) and chiral (4,2), and consist of 12, 20, and 56 atoms, respectively. The strain energy is evaluated in order to examine the curvature effect. It is found that the strain energy of hexagonal BNT strongly depends upon the radius, whereas the strain energy of triangular BNTs depends on both radius and chirality.
We have carried out first-principles calculations on electronic properties of graphene quantum dots embedded in hexagonal boron nitride monolayer sheets. The calculations with density functional theory show that the band gaps of quantum dots are determined by the quantum confinement effects and the hybridization of π orbitals from B, N and C atoms. The energy states near the Fermi level are found to be strongly localized within and in the vicinity of the quantum dots.
Novel materials with tunable magnetic states play a significant role in the development of next-generation spintronic devices. In this paper, we examine the role of biaxial strain on the electronic properties of vacancy-decorated hexagonal boron nitride (h-BN) monolayers using density functional theory calculations. We found that the strain can lead to switching of the magnetic state for h-BN monolayers with boron vacancy or divacancy. Our findings promise a new route for the operation of low-dimensional spintronic devices.
The electrical conductivity of melts of boron and its carbide (B4C), nitride (BN), and phosphide (BP) has been studied at pressures to 7.7 GPa and temperatures to 3500 K. It has been shown that these melts are good conductors with specific electrical conductivity values comparable with that of iron melt at ambient pressure.
High pressure Raman experiments on Boron Nitride multi-walled nanotubes show that the intensity of the vibrational mode at ~ 1367 cm-1 vanishes at ~ 12 GPa and it does not recover under decompression. In comparison, the high pressure Raman experiments on hexagonal Boron Nitride show a clear signature of a phase transition from hexagonal to wurtzite at ~ 13 GPa which is reversible on decompression. These results are contrasted with the pressure behavior of carbon nanotubes and graphite.
Quantum emitters in layered materials are promising candidates for applications in nanophotonics. Here we present a technique based on charge transfer to graphene for measuring the charge transition levels ($\rm E_t$) of fluorescent defects in a wide bandgap 2D material, and apply it to quantum emitters in hexagonal boron nitride (hBN). Our results will aid in identifying the atomic structures of quantum emitters in hBN, as well as practical applications since $\rm E_t$ determines defect charge states and plays a key role in photodynamics.
We propose monolayer epitaxial graphene and hexagonal boron nitride (h-BN) as ultimate thickness covalent spacers for magnetoresistive junctions. Using a first-principles approach, we investigate the structural, magnetic and spin transport properties of such junctions based on structurally well defined interfaces with (111) fcc or (0001) hcp ferromagnetic transition metals. We find low resistance area products, strong exchange couplings across the interface, and magnetoresistance ratios exceeding 100% for certain chemical compositions. These properties can be fine tuned, making the proposed ju…
We demonstrate single crystal growth of wafer-scale hexagonal boron nitride (hBN), an insulating atomic thin monolayer, on high-symmetry index surface plane Cu(111). The unidirectional epitaxial growth is guaranteed by large binding energy difference, ~0.23 eV, between A- and B-steps edges on Cu(111) docking with B6N7 clusters, confirmed by density functional theory calculations.
Imaging and spectroscopy performed in a low-voltage scanning transmission electron microscope (LV-STEM) are used to characterize the structure and chemical properties of boron-terminated tetravacancies in hexagonal boron nitride (h-BN). We confirm earlier theoretical predictions about the structure of these defects and identify new features in the electron energy-loss spectra (EELS) of B atoms using high resolution chemical maps, highlighting differences between these areas and pristine sample regions. We correlate our experimental data with calculations which help explain our observations.
We show that carbon-doped hexagonal boron nitride (h-BN) has extraordinary properties with many possible applications. We demonstrate that the substitution-induced impurity states, associated with carbon atoms, and their interactions dictate the electronic structure and properties of C-doped h-BN. Furthermore, we show that stacking of localized impurity states in small C clusters embedded in h-BN forms a set of discrete energy levels in the wide gap of h-BN. The electronic structures of these C clusters have a plethora of applications in optics, magneto-optics, and opto-electronics.
Two-dimensional materials are characterised by a number of unique physical properties which can potentially make them useful to a wide diversity of applications. In particular, the large thermal conductivity of graphene and hexagonal boron nitride has already been acknowledged and these materials have been suggested as novel core materials for thermal management in electronics. However, it was not clear if mass produced flakes of hexagonal boron nitride would allow one to achieve an industrially-relevant value of thermal conductivity. Here we demonstrate that laminates of hexagonal boron nitri…
We investigate tunneling in metal-insulator-metal junctions employing few atomic layers of hexagonal boron nitride (hBN) as the insulating barrier. While the low-bias tunnel resistance increases nearly exponentially with barrier thickness, subtle features are seen in the current-voltage curves, indicating marked influence of the intrinsic defects present in the hBN insulator on the tunneling transport. In particular, single electron charging events are observed, which are more evident in thicker-barrier devices where direct tunneling is substantially low. Furthermore, we find that annealing th…
The hybrid graphene - hexagonal boron nitride (G-hBN) systems offer new routes in the design of nanoscale electronic devices. Using {\it ab initio} density functional theory calculations we investigate the dynamics of zig-zag nanoribbons a few interatomic distances wide. Several structures are analyzed, namely pristine graphene, hBN and G-hBN systems. By passivating the nanoribbon edges with hydrogen and different halogen atoms, one may tune the electronic and mechanical properties, like the band gap energies and the natural frequencies of vibration.
The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices, as it is insulating, atomically fl…
Few-layer flakes of hexagonal boron nitride were prepared by ultrasonication of bulk crystals and agglomerated to form thin films. The transmission and reflection spectra of the thin films were measured. The spectral dependences of the linear and circular polarization revealed a hidden anisotropy of the films over the whole sample area which could not be explained by the anisotropy of the chaotically-oriented individual particles. Statistical analysis of optical microscopy images showed a macroscopic particle density distribution with ordering corresponding to the optical axis observed in the …
The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Green's functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly con…
Devising ways of opening a band gap in graphene to make charge-carrier masses finite is essential for many applications. Recent experiments with graphene on hexagonal boron nitride (h-BN) offer tantalizing hints that the weak interaction with the substrate is sufficient to open a gap, in contradiction of earlier findings. Using many-body perturbation theory, we find that the small observed gap is what remains after a much larger underlying quasiparticle gap is suppressed by incommensurability. The sensitivity of this suppression to a small modulation of the distance separating graphene from th…
A new interlayer force-field for layered hexagonal boron nitride (h-BN) based structures is presented. The force-field contains three terms representing the interlayer attraction due to dispersive interactions, repulsion due to anisotropic overlaps of electron clouds, and monopolar electrostatic interactions. With appropriate parameterization, the potential is able to simultaneously capture well the binding and lateral sliding energies of planar h-BN based dimer systems as well as the interlayer telescoping and rotation of double walled boron-nitride nanotubes of different crystallographic ori…
Optical reflectivity contrast provides a simple, fast and noninvasive method for characterization of few monolayer samples of two-dimensional materials. Here we apply this technique to measure the thickness of thin flakes of hexagonal Boron Nitride (hBN), which is a material of increasing interest in nanodevice fabrication. The optical contrast shows a strong negative peak at short wavelengths and zero contrast at a thickness dependent wavelength. The optical contrast varies linearly for 1-80 layers of hBN, which permits easy calibration of thickness. We demonstrate the applicability of this q…
The ability to locally activate or generate quantum emitters in two-dimensional materials is of major interest for the realization of integrated quantum photonic devices. In particular, hexagonal boron nitride (hBN) has recently been shown to allow a variety of techniques for obtaining quantum emitters at desired locations. Here, we use cathodoluminescence (CL) to monitor in situ the local activation of color centers by an electron beam in hBN. We observe that the CL signal saturates at a given surface dose, independently of the electron current density. Based on photoluminescence and photon c…
We determine the electronic structure of a graphene sheet on top of a lattice-matched hexagonal boron nitride (h-BN) substrate using ab initio density functional calculations. The most stable configuration has one carbon atom on top of a boron atom, the other centered above a BN ring. The resulting inequivalence of the two carbon sites leads to the opening of a gap of 53 meV at the Dirac points of graphene and to finite masses for the Dirac fermions. Alternative orientations of the graphene sheet on the BN substrate generate similar band gaps and masses. The band gap induced by the BN surface …
Many quantum emitters have been measured close or near the grain boundaries of the two-dimensional hexagonal boron nitride where various Stone-Wales defects appear. We show by means of first principles density functional theory calculations that the pentagon-heptagon Stone-Wales defect is an ultraviolet emitter and its optical properties closely follow the characteristics of a 4.08-eV quantum emitter often observed in polycrystalline hexagonal boron nitride. We also show that the square-octagon Stone-Wales line defects are optically active in the ultraviolet region with varying gaps depending …
Periodically corrugated epitaxial graphene and hexagonal boron nitride (h-BN) on metallic substrates are considered as perspective templates for the self-assembly of nanoparticles arrays. By using first-principles calculations, we determine binding energies and diffusion activation barriers of metal adatoms on graphene and h-BN. The observed chemical trends can be understood in terms of the interplay between charge transfer and covalent bonding involving the adatom d electrons. We further investigate the electronic effects of the metallic substrate and find that periodically corrugated templat…
Despite a recent flurry of experimental and simulation studies, an accurate estimate of the interaction strength of water molecules with hexagonal boron nitride is lacking. Here we report quantum Monte Carlo results for the adsorption of a water monomer on a periodic hexagonal boron nitride sheet, which yield a water monomer interaction energy of -84 +/- 5 meV. We use the results to evaluate the performance of several widely used density functional theory (DFT) exchange correlation functionals, and find that they all deviate substantially. Differences in interaction energies between different …
From monolayers composed of different atoms, we can build structures with spontaneous vertical polarization by conveniently stacking multiple layers. We have studied, using first-principles methods and based on modern polarization theory, a system composed of three layers of hexagonal Boron-Nitride(h-BN) in all possible stackings. We obtain for each of the configurations how charge transfers between the layers and how it impacts on the polarization of the system. In addition, we studied a system of three layers, one of them rotated, and we found that not only did the magnitude of the polarizat…
Uniaxial tensile properties of hexagonal boron nitride nanoribbons and dependence of these properties on temperature, strain rate, and the inclusion of vacancy defects have been explored with molecular dynamics simulations using Tersoff potential. The ultimate tensile strength of pristine hexagonal boron nitride nanoribbon of 26 nm x 5 nm with armchair chirality is found to be 100.5 GPa. The ultimate tensile strength and strain have been found decreasing with increasing the temperature while an opposite trend has been observed for increasing the strain rate. Furthermore, the vacancy defects re…
Defects in hexagonal boron nitride (hBN) exhibit single-photon emission (SPE) and are thus attracting broad interest as platforms for quantum information and spintronic applications. However, the atomic structure and the specific impact of the local environment on the defect physical properties remain elusive. Here we articulate X-ray photoelectron spectroscopy (XPS) and first-principles calculations to discern the experimentally-observed point defects responsible for the quantum emission observed in hBN. XPS measurements show a broad band, which was deconvolved and then assigned to $N_{B}V_{N…
Metal halide perovskites are known to suffer from rapid degradation, limiting their direct applicability. Here, the degradation of phenethylammonium lead iodide (PEA2PbI4) two-dimensional perovskites under ambient conditions is studied using fluorescence, absorbance and fluorescence lifetime measurements. It is demonstrated that a long-term stability of two-dimensional perovskites can be achieved through the encapsulation with hexagonal boron nitride. While un-encapsulated perovskite flakes degrade within hours, the encapsulated perovskites are stable for at least three months. In addition, en…
Nanoscale optical thermometry is a promising non-contact route for measuring local temperature with both high sensitivity and spatial resolution. In this work, we present a deterministic optical thermometry technique based on quantum emitters in nanoscale hexagonal boron-nitride. We show that these nanothermometers exhibit better performance than that of homologous, all-optical nanothermometers both in sensitivity and range of working temperature. We demonstrate their effectiveness as nanothermometers by monitoring the local temperature at specific locations in a variety of custom-built micro-…
Graphene nanoribbons (GNRs) are ultra-narrow strips of graphene that have the potential to be used in high-performance graphene-based semiconductor electronics. However, controlled growth of GNRs on dielectric substrates remains a challenge. Here, we report the successful growth of GNRs directly on hexagonal boron nitride substrates with smooth edges and controllable widths using chemical vapour deposition. The approach is based on a type of template growth that allows for the in-plane epitaxy of mono-layered GNRs in nano-trenches on hexagonal boron nitride with edges following a zigzag direct…
We perform both lattice dynamics analysis and molecular dynamics simulations to demonstrate the existence of topologically protected phonon modes in a two-dimensional, monolayer hexagonal boron nitride sheet. The topological phonon modes are found to be localized at an in-plane interface that divides the system into two regions of distinct valley Chern numbers. The dispersion of this topological phonon mode crosses over the frequency gap [1123, 1278] cm^{-1}, which is opened through analogy with the quantum valley Hall effect by breaking inversion symmetry of the boron and nitride atoms in the…
Optically addressable spins in materials are important platforms for quantum technologies, such as repeaters and sensors. Identification of such systems in two-dimensional (2d) layered materials offers advantages over their bulk counterparts, as their reduced dimensionality enables more feasible on-chip integration into devices. Here, we report optically detected magnetic resonance (ODMR) from previously identified carbon-related defects in 2d hexagonal boron nitride (hBN). We show that single-defect ODMR contrast can be as strong as 6% and displays a magnetic-field dependence with both positi…
We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B$_{4}$ tetrahedron and possesses the same space group $P6_{3}/mmc$ as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point f…
Strongly anisotropic media where the principal components of the dielectric tensor have opposite signs are called hyperbolic. Such materials length exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces the size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material, hexagonal boron nitride i…
The defect-mediated hexagonal boron nitride (hBN) supercell display the visible optical spectra and electronic characteristics. The defects in the hBN supercell include the atomic vacancy, antisite, antisite vacancy, and substitution of a foreign atom for boron or nitrogen. The hBN supercell with VB, CB, and NB-VN has the high electron density of states across the Fermi level which indicates the high conductive electronic characteristics. The hBN with defects including atomic vacancy, antisite vacancy, and substitution of a foreign atom for boron or nitride exhibit the distinct spin-resolved o…
Hexagonal boron nitride (hBN) is a wide bandgap van der Waals material that has recently emerged as promising platform for quantum photonics experiments. In this work we study the formation and localization of narrowband quantum emitters in large flakes (up to tens of microns wide) of hBN. The emitters can be activated in as-grown hBN by electron irradiation or high temperature annealing, and the emitter formation probability can be increased by ion implantation or focused laser irradiation of the as-grown material. Interestingly, we show that the emitters are always localized at edges of the …
