Photo- and cathodoluminescence of cubic boron nitride micropowders activated by Tm,Tb, and Eu rare-earth ions

This work is devoted to the production of cubic boron nitride (c-BN) micropowders that are activated by ions of rare-earth elements, such as europium (Eu), terbium (Tb), and thulium (Tm), as well as to the study of the structural properties and photo- and cathodoluminescence of these micropowders. The micropowders have been synthesized from a hexagonal boron nitride powder in the presence of a catalyst under pressures of 4–6 GPa at temperatures of 1800–1900 K. The activation of the micropowders by the rareearth elements (REEs) has been carried out by introducing the corresponding REE compounds into synthesis precursor. The efficiency of the introduction of an impurity into the c-BN lattice is ~5%. The composition and structure of the samples have been examined using X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectrometry. The results obtained during studying c-BN, c-BN:Tm, c-BN:Tb, and c-BN:Eu micropowders using color cathodoluminescence clearly demonstrate their ability to emit light in the wide spectral range, which is of interest for developing new light-emitting devices that are intended for operation in corrosive ambient. An analysis of the photoluminescence spectra of c-BN:REE micropowders has made it possible to find that the observed spectral bands belong to the corresponding transitions between the energy levels of the REEs, as well as to determine the probable positions of Tb³⁺ and Eu³⁺ ions in the cubic boron nitride lattice.     

Electron-beam assisted growth of hexagonal boron-nitride layer

It has been known that a good quality h-BN layer can only be grown within a narrow temperature window of 1020–1100 K on a copper substrate. We found that the growth temperature window on Cu(111) surface could be lowered up to 100 K by ionizing and/or exciting borazine precursor gas with an electron-beam. The structures of a hexagonal boron nitride (h-BN) layers grown at various substrate temperatures on a Cu(111) were examined using scanning tunneling microscopy. We found that the grown h-BN film exhibits highly inert behavior with wide bandgap semiconductor characteristics.     

Luminescence spectra of hexagonal forms of silicon carbide in mosaic films grown by solid-state epitaxy

The luminescence spectra of silicon carbide films grown on silicon by solid-state epitaxy have been studied. It has been shown that, depending on the growth conditions, one can obtain films of different SiC polytypes, including the cubic and hexagonal ones. In many cases, the films thus grown display a mixture of various polytypes, but it is possible to prepare films of predominantly hexagonal symmetry (the coexistence of the 4H and 2H hexagonal phases, which are close in properties, is also possible). It thus has been demonstrated that the silicon carbide films grown on silicon by solid-state epitaxy are promising for application as damping layers in fabrication of wide-band-gap hexagonal semiconductors on silicon substrates.     

Eigensolutions of cyclopolyacene graphs

A graph of {X, Y}-cyclopolyacene with n of hexagonal rings has been presented that contains four orbits, of which orbits 1 and 4 are occupied by the X-type of vertex and orbits 2 and 4 are occupied by the Y-type, or vice versa. Eigensolutions for such a graph have been derived in analytical form through the use of rotational symmetry followed by a plane of symmetry. Varying X ( = C, N, B, …) and Y ( = C, N, B, …) several types of cyclopolyacene graph may be obtained. Eigenvalue-expressions for such systems containing C, N and B have been shown in analytical form and their total π-electron energies with 2–6 hexagonal rings have been calculated with the help of the expressions developed.     

High-order harmonic generation from wurtzitic and hexagonal BN

In this letter, the high-order harmonic generation (HHG) of wurtzitic and hexagonal boron nitride (h-BN) under ultrafast intense laser field was studied by solving an extended multiband semiconductor Bloch equations (SBEs). The results showed that for both the wurtzitic and hexagonal structures, the cutoff energy of the HHG was extended linearly by increasing the field strength, and the efficiency was suppressed as the wavelength of laser increased. It was worth noting that for hexagonal structure, the efficiency of HHG was 1–2 orders of magnitude higher than that of the wurtzitic structure. At the same time, the hexagonal structure significantly improved the cutoff energy of HHG from BN.

     

Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D⁺ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D⁺ desorption yield η(hν) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials.     

Adsorption configurations of carbon monoxide on gold monolayer supported by graphene or monolayer hexagonal boron nitride: a first-principles study

Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small charge from graphene and monolayer h-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/h-BN with Au-C distance 2.17?2.50 Å and Au-C-O angle of 123.9°–139.6°. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules.     

Formation,structure, and properties of “welded” <Emphasis Type="Italic">h</Emphasis>-BN/graphene compounds

Structures of h-BN/graphene with holes where atoms at the edges are bonded to each other by sp² hybridized C–B and C–N bonds and form continuous junctions from layer to layer with topological defects inside holes have been considered. Their formation, as well as the moiré-type stable atomic structure of such compounds (with different rotation angles of graphene with respect to the hexagonal boron nitride monolayer) with closed hexagonal holes in the AA centers of packing of the moiré superlattice, has been studied. The stability, as well as the electronic and mechanical properties, of such bilayer BN/graphene nanomeshes has been analyzed within electron density functional theory. It has been shown that they have semiconducting properties. Their electronic band structures and mechanical characteristics differ from the respective properties of separate monolayer nanomeshes with the same geometry and arrangement of holes.     

Theoretical study on Si-doped hexagonal boron nitride (h-BN) sheet: Electronic,magnetic properties,and reactivity

The properties and reactivity of Si-doped hexagonal boron nitride (h-BN) sheets were studied using density functional theory (DFT) methods. We find that Si impurity is more likely to substitute the boron site (Si_B) due to the low formation energy. Si-doping severely deforms h-BN sheet, resulting in the local curvature changes of h-BN sheet. Moreover, Si-doping introduces two spin localized states within the band gap of h-BN sheet, thus rendering the two doped systems exhibit acceptor properties. The band gap of h  -BN sheet is reduced from ∼4.70 eV$\sim 4.70\text{eV}$ to 1.24 (for Si_B) and 0.84 eV (for Si_N), respectively. In addition, Si-doped one exhibits higher activity than pristine one, endowing them wider application potential.     

The spin density distribution in some phenyl substituted imidazyl and pyrryl radicals

ENDOR spectroscopy has been used to interpret the E.S.R. spectra of the 2,4,5-triphenylimidazyl, tetraphenylpyrryl, tetrakis (p-tolyl) pyrryl, tetrakis (p-anisyl) pyrryl radicals. Using the hyperfine coupling constants thus obtained in conjunction with a McLachlan-type HMO calculation the spin density distribution in these radicals is calculated. The configuration of the phenyl rings and the deviation of their bond lengths from hexagonal symmetry is inferred from the MO parameters required to fit the experimental couplings.     

Physical properties of Ima2-BN under pressure: First principles calculations

A fully orthorhombic boron nitride (BN) polymorph with an orthorhombic symmetry (Ima2-BN, space group: Ima2) was investigated by first-principles calculations. The Ima2-BN under 30 GPa is both mechanically and dynamically stable via elastic constants and phonon spectra. The anisotropic and electronic properties of Ima2-BN under different pressure are investigated in this work. The anisotropic properties calculations show that the Young's modulus of Ima2-BN in (001) plane exhibits the greatest anisotropy under ambient pressure, while in (111) plane it is the greatest when P > 20 GPa, while the (010) plane has always exhibited the minimal anisotropy whether under ambient pressure or high pressure. Ima2-BN is an indirect wider band gap semiconductor material under ambient pressure, and the band gap of Ima2-BN decreases with the increasing pressure. The minimum thermal conductivities κ_min of Ima2-BN is 1.85 W/(cmK), it is slightly higher than of B₄N₄-I and c-BN.     

Corrugated single layer templates for molecules: From h-BN nanomesh to graphene based quantum dot arrays

Functional nano-templates enable self-assembly of otherwise impossible arrangements of molecules. A particular class of such templates is that of sp ² hybridized single layers of hexagonal boron nitride or carbon (graphene) on metal supports. If the substrate and the single layer have a lattice mismatch, superstructures are formed. On substrates like rhodium or ruthenium these superstructures have unit cells with ∼3-nm lattice constant. They are corrugated and contain sub-units, which behave like traps for molecules or quantum dots, which are small enough to become operational at room temperature. For graphene on Rh(111) we emphasize a new structural element of small extra hills within the corrugation landscape. For the case of molecules like water it is shown that new phases assemble on such templates, and that they can be used as “nano-laboratories” where many individual processes are studied in parallel. Furthermore, it is shown that the h-BN/Rh(111) nanomesh displays a strong scanning tunneling microscopy-induced luminescence contrast within the 3 nm unit cell which is a way to address trapped molecules and/or quantum dots.     

h-BN/Ru(0 0 0 1) nanomesh: A 14-on-13 superstructure with 3.5 nm periodicity

The structure of epitaxially grown hexagonal boron nitride (h-BN) on the surface of a Ru(0 0 0 1) single crystal was investigated using surface X-ray diffraction, which showed the system to form a commensurate 14-on-13 superstructure. This result disagrees with previous reports on superstructures of the same system and arguments based on simple thermal expansion coefficient calculations. We argue that the larger observed superstructure forms because of the strong bonding of h-BN to Ru. In comparison to h-BN/Rh(1 1 1) it can accommodate more induced lateral in-plane strain- or lock-in energy over larger regions (referred to as the holes) within the superstructure, which itself can consequently become larger.     

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

In Dy³⁺ and Li⁺ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A₁ and E₁ symmetry also show a noticeable red shift from E₁ symmetry (in ab plane) to A₁ symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.     

h-BN on Rh(1 1 1): Persistence of a commensurate 13-on-12 superstructure up to high temperatures

We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(1 1 1). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830 °C. Surface X-ray diffraction measurements up to 830 °C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.     

Inclusions and inhomogeneities in electroelastic media with hexagonal symmetry

For a long time, the absence of explicit Green's functions (fundamental solutions) for electroelastic media has hindered progress in the modelling of the properties of piezoelectric materials. Michelitsch's recently derived explicit electroelastic Green's function for the infinite medium with hexagonal symmetry (transversely isotropic medium) [4] is used here to obtain compact closed-form expressions for the electroelastic analogue of the Eshelby tensor for spheroidal inclusions. This represents a key quantity for the material properties of piezoelectric solids and analysis of the related electroelastic fields in inclusions. For the limiting case of continuous fibers our results coincide with Levin's expressions [8]. The derived method is useful for an extension to non-spheroidal inclusions or inhomogeneities having an axis of rotational symmetry parallel to the hexagonal c-axis. Received 14 September 1999     

Quasi elasto-electromagnetic surface waves on a piezo-plasma-like layer

Considering a piezo-plasma-like layer with finite thickness and hexagonal symmetry whose main symmetry axis is parallel to the z axis and approximating it by an isotropic medium, we study the coupling of the elastic wave with plasma properties of the medium with and without spatial dispersion and collisions. In this case we investigate the coupled surface quasi elasto-electromagnetic wave propagating on the interface of piezoelectric layer with vacuum. Furthermore, the coupling of elasticity and ion-acoustic waves is investigated.     

Zero temperature solutions of the Edwards-Anderson model in random Husimi lattices

We solve the Edwards-Anderson model (EA) in different Husimi lattices using the cavity method at replica symmetric (RS) and 1-step of replica symmetry breaking (1RSB) levels. We show that, at T = 0, the structure of the solution space depends on the parity of the loop sizes. Husimi lattices with odd loop sizes may have a trivial paramagnetic solution thermodynamically relevant for highly frustrated systems while, in Husimi lattices with even loop sizes, this solution is absent. The range of stability under 1RSB perturbations of this and other RS solutions is computed analytically (when possible) or numerically. We also study the transition from 1RSB solutions to paramagnetic and ferromagnetic RS solutions. Finally we compare the solutions of the EA model in Husimi lattices with that on the (short loops free) Bethe lattices, showing that already for loop sizes of order 8 both models behave similarly.