Mn atomic layers under inert covers of graphene and hexagonal boron nitride prepared on Rh(111)

Intercalation of metal atoms into the interface of graphene and its supporting substrate has become an intriguing topic for the sake of weakening the interface coupling and constructing metal atomic layers under inert covers. However, this novel behavior has rarely been reported on the analogous hexagonal boron nitride (h-BN) synthesized on metal substrates. Here, we describe a comparative study of Mn intercalation into the interfaces of graphene/Rh(111) and h-BN/Rh(111), by using atomically-resolved scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The intercalation was performed by annealing as-deposited Mn clusters, and the starting temperature of Mn intercalation into h-BN/Rh(111) was found to be ～80 °C higher than that for graphene/Rh(111). Moreover, the intercalated islands of h-BN/Mn/Rh(111) usually possess more irregular shapes than those of graphene/Mn/Rh(111), as illustrated by temperature-dependent STM observations. All these experimental facts suggest a stronger interaction of Mn with h-BN/Rh(111) than that with graphene/Rh(111).      

Reversible phase transformation in boron nitride under pulsed mechanical action

It is established that hexagonal boron nitride (h-BN) exhibits transformation to high-pressure diamondlike phases (w-, c-BN) under conditions of treatment in a planetary ball mill. After a 12-h processing, the yield of these phases (?20%) no longer grows with further increasing the process duration, which is evidence for a reversible character of the transformation. This conclusion is confirmed by the w-BN → h-BN transformation observed under the same process conditions. In addition to h-, w-, and c-BN, we have also found several new modifications of boron nitride.     

Large scale synthesis of single-crystal and polycrystalline boron nitride nanosheets

Boron nitride nanosheets (BNNSs) have an identical crystal structure and similar lattice parameter to those of graphene sheets. However, growing quality BNNSs consisting of only several atomic layers remains a challenge. Here, we report on the synthesis of BNNSs at a temperature of 350 °C using a CO₂ pulsed laser plasma deposition (CO₂-PLD) technique by irradiating a pyrolytic hexagonal boron nitride (h-BN) target. The deposition was performed either in vacuum at a pressure of 0.2 Pa, for which we obtained polycrystalline BN, or in hydrogen (H₂) atmosphere at a pressure of 26 Pa for which we obtained single-crystal BNNSs. The presence of H₂ seems to minimize the side effects of sputtering and the material shows higher purity and better crystallinity. High resolution transmission electron microscopy (HRTEM) showed the sheets to be mostly defect-free and to have the characteristic honeycomb structure of six-membered B₃-N₃ hexagon. HRTEM, electron diffraction, X-ray diffraction, Raman scattering, and Fourier transform infrared spectroscopy clearly identified h-BN.     

Enhanced triethylamine sensing performance of superfine NiO nanoparticles decoration by two-dimensional hexagonal boron nitride

The novel two-dimensional hexagonal boron nitride (h-BN) decorated nickel oxide (NiO) heterojunction was successfully synthesized by a facile solvothermal precipitation method combining with heat treatment. SEM and TEM analysis were used to corroborate the average size (~8 nm) and overall distribution of superfine NiO nanoparticles on h-BN. XRD, FT-IR and XPS characterization confirmed the configuration of highly crystallinity and p-n heterojunction as well as the presence of surface oxygen vacancy defects. Gas sensing test results revealed that the decoration of h-BN could significantly enhanced triethylamine (TEA) sensing property of NiO. The main contribution of such remarkable results lies in NiO nanoparticles that are close to Debye length scale were embedded on vacancy defects of functionalized h-BN nanosheets, which can optimize sensitivity and selectivity by taming two-dimensional (2D) interfacial electronic effects that strongly affect nonmetal-support interaction between grain boundaries. Meanwhile, the formation of p-n Schottky nanoscale heterojunction between NiO and h-BN can significantly enlarge resistance variation and efficiently promoted the adsorbed triethylamine molecules to oxidize into NO₂, H₂O, and CO₂. Our work highlights the important role of coupling functionalized h-BN in gas sensors, which can also provide a valuable avenue in boosting the sensing performance.     

Ultrahigh sensitive and selective triethylamine sensor based on h-BN modified MoO3 nanowires

In this work, a novel structure of 1D MoO₃ nanowires wrapped by 2D hexagonal boron nitride (h-BN) was synthesized via a simple solvothermal method with subsequent annealing process for triethylamine (TEA) detection. The samples were characterized by XPS, SEM, HRTEM and N₂ adsorption-desorption. Gas sensing performance test results illuminate that the typical 2 wt% h-BN/MoO₃ sensor possesses an ultrahigh response (8616) toward 500 ppm TEA. The promoted sensing performance of TEA may be caused by the forming of heterojunction between h-BN and MoO₃, the increased specific surface area of h-BN modification, providing a highly active sites for the adsorption of TEA gas, which greatly enhance the response of the sensor. The adsorption energy of a single oxygen molecule on MoO₃ (0 1 0) surface was calculated by DFT, indicating the most stable site is the terminal oxygen position (Top O-1), with an adsorption energy of ?2.075 eV. This work provides an inspiration to design highly efficient TEA gas sensor on basis of h-BN/MoO₃ nanocomposites.     

Bio-inspired structured boron carbide-boron nitride composite by reactive spark plasma sintering

Nature creates composite materials with complex hierarchical structures that possess impressive mechanical properties enhancement capabilities. An approach to improve mechanical properties of conventional composites is to mimic the biological material structured ‘hard’ core and ‘soft’ matrix system. This would allow the efficient transfer of load stress, dissipation of energy and resistance to cracking in the composite. In the current study, reactive spark plasma sintering (SPS) of boron carbide B₄C was carried out in a nitrogen N₂ gas environment. The process created a unique core-shell structured material with the potential to form a high impact-resistant composite. Transmission electron microscopy observation of nitrided-B₄C revealed the encapsulation of B₄C grains by nano-layers of hexagonal-boron nitride (h-BN). The effect of the h-BN contents on hardness were measured using micro- and nano-indentation. Commercially available h-BN was also mechanically mixed and sintered with B₄C to compare the effectiveness of nitrided B₄C. Results have shown that nitrided B₄C has a higher hardness value and the optimum content of h-BN from nitridation was 0.4%wt with the highest nano-indentation hardness of 56.7 GPa. The high hardness was attributed to the h-BN matrix situated between the B₄C grain boundaries which provided a transitional region for effective redistribution of the stress in the material.     

A Robust n-n Heterojunction: Cu?N and B?N Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with Cu N and B N dual active sites is synthesized via in situ growth of a conductive metal–organic framework (MOF) [Cu₃(HITP)₂] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu₃(HITP)₂@h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu₃(HITP)₂@h-BN shows the outstanding eNRR performance with the NH₃ production of 146.2 µg h⁻¹ mg_cat⁻¹ and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and Cu N/B N dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH₃ production catalyzed by the Cu₃(HITP)₂@h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.     

Hybrid porous nanotube crystal networks for nanostructured device applications

A set of new porous materials, namely zeolite nanocage schwarzite-like crystals with the elements of both nanotubes and fullerenes in the structure is proposed as a result of ab initio and density-functional theory calculations. Twelve new Extradiamond phases of boron nitride, carbon, silicon and silicon carbide are calculated as three different hybridized crystals. The details of recently synthesized Explosion-BN (E-BN) phase are highlighted for the first time with electronic structure and vibrational frequency analysis. E-BN is supposed to be sp ²/sp ³-hybridized FAU-zeolite structure with calculated unit cell of 12.177 Å and a band gap of 3.2 eV. Calculated IR bands for E-BN₁₂₀ cluster and observed experimentally E-BN absorption spectrum are well-correlated with appropriate IR spectra of FAU-zeolite. Armchair and zig-zag nanotubes are classified as (n,n,k) and (n,0,k), respectively, where k is the number of hexagons along the nanotube axis. Novel materials are proposed as (n,m,k)-FTC, where FTC stands for framework type code. We also indicate the possibility of creation of filled hybrid networks of different segment lengths, radii and compositions for thermoelectric and novel device applications.     

Thin film growth of boron nitride on α-Al2O3 (0 0 1) substrates by reactive sputtering

Boron nitride thin films were grown on α-Al₂O₃ (0 0 1) substrates by reactive magnetron sputtering. Infrared attenuated total reflection (ATR) spectra of the films gave an intense signal associated with in-plane B-N stretching TO mode of short range ordered structure of BN hexagonal sheets. X-ray diffraction for the film prepared at a low working pressure (ca. 1 × 10⁻³ Torr) gave a diffraction peak at slightly lower angle than that corresponding to crystal plane h-BN (0 0 2). It is notable that crystal thickness calculated from X-ray peak linewidth (45 nm) was close to film thickness (53 nm), revealing well developed sheet stacking along the direction perpendicular to the substrate surface. When the substrates of MgO (0 0 1) and Si (0 0 1) were used, the short-range ordered structure of h-BN sheet was formed but the films gave no X-ray diffraction. The film showed optical band gap of 5.9 eV, being close to that for bulk crystalline h-BN.     

Synthesis of the uniform hollow spherical Sr2SiO4:Eu2+ phosphors via an h-BN protective method

The uniform hollow spherical Sr₂SiO₄:Eu²⁺ green emitting phosphors have been successfully synthesized using hollow silica spheres as templates by an h-BN protective method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results directly confirmed the existence of the hollow spherical structure with a narrow size distribution and a shell thickness of 15–25 nm. The h-BN protective film, observed by high resolution TEM, plays an important role in the formation of the hollow spherical morphology and the improvement of photoluminescence properties. Comparing with the Sr₂SiO₄:Eu²⁺ micron-phosphor prepared by the traditional solid state reaction method, the hollow spherical phosphor with nano-sized grains exhibits stronger green emission under ultraviolet–blue light excitation. This could be attributed to the elimination of surface defects by the h-BN coating. This research gives an economic and convenient way to synthesize uniform spherical phosphors with high quantum efficiency.     

Nearly stoichiometric BN fiber with low dielectric constant derived from poly[(alkylamino)borazine]

Nearly stoichiometric boron nitride (BN) fiber with low dielectric constant was prepared by curing, pyrolysis of a novel poly[(alkylamino)borazine] (PAAB) under NH₃ up to 1200 °C in a ceramic yield of 60 wt.%. Based on IR, XRD, XPS and elemental analysis (EA), the fiber with a composition of BN_1.09 shows a characteristic of turbostratic BN (t-BN). The tensile strength is approximately 0.6 GPa with 13 μm in diameter. Moreover, the fiber possesses very low real part (ε′) of 2.66 and loss tangent (tan δ) of 0.0012 at 10 GHz, respectively, making it possible to be used in microwave-transparent material.     

Photoluminescence of cubic BN doped with europium and codoped with europium and chromium

We have prepared cubic boron nitride samples doped with europium and codoped with europium and chromium. Their red photoluminescence is due to Eu³⁺ and Eu³⁺-Cr³⁺ electronic transitions involving Eu³⁺ in a noncentrosymmetric position in a field of cubic symmetry. Chromium is shown to have a positive effect on the incorporation of Eu³⁺ into lattice sites of c-BN. The photoluminescence spectrum of polycrystalline c-BN codoped with Eu³⁺ and Cr³⁺ is a combination of the spectra of Eu³⁺ in fields of cubic and monoclinic symmetries, the latter spectrum being blue-shifted relative to the spectrum of Eu³⁺-doped microcrystalline powder. The c-BN materials prepared in this study can be used as red phosphors and light emitters possessing high thermal stability, radiation hardness, and chemical resistance.     

Chemical Vapor Deposition of Boron Nitride in the B–N–H–He–O System

Thermodynamic analysis of boron nitride (cubic, hexagonal, and wurtzite forms) chemical vapor deposition in the B–N–H–He–O system was carried out for temperatures from 300 to 2100 K, a total pressure of 1.33 Pa, residual pressures from 1.33 × 10^–5 to 0.133 Pa, and a wide range of He : B₃N₃H₆ ratios. The conditions for the deposition of c-BN, h-BN, or mixtures of BN and B₂O₃ (solid or liquid) were established. Oxygen impurities are shown to have a significant effect on the temperature stability limits of the condensed phases involved.     

Chemical Composition of Boron Carbonitride Films Grown by Plasma-Enhanced Chemical Vapor Deposition from Trimethylamineborane

Boron carbonitride and boron nitride films were grown by plasma-enhanced chemical vapor deposition using trimethylamineborane and its mixtures with ammonia, hydrogen, or helium. The effects of the starting-mixture composition and substrate temperature on the chemical composition of the deposits was studied by ellipsometry, scanning microscopy, IR spectroscopy, Raman scattering, and x-ray photoelectron spectroscopy. The results indicate that the initial composition of the gas mixture, the nature of the activation gas, and substrate temperature play a key role in determining the deposition kinetics and the physicochemical properties of the deposits. Depending on these process parameters, one can obtain h-BN, h-BN + B₄C, or BC _x N _y films.     

Effect of molecular monomer structure on the composition and properties of BN via the preceramic polymer route

Boron nitride (BN) ceramics were prepared via the preceramic polymer route from two (alkylamino)borazine (AAB) monomers. The effect of monomer structure on the chemical composition, microstructure, oxidation resistance and high-temperature stability of BN was investigated. The two ceramics were characterized by elemental analysis (EA), IR, XPS, XRD, HRTEM and TGA. With similar composition, BN derived from symmetric monomer had a higher crystallinity and exhibited better oxidation resistance/high temperature stability. This is due to that the structure of symmetric molecular derived polymeric precursor is close to that of hexagonal BN (h-BN).     

Deposition and characterization of BN/Si(0 0 1) using tris(dimethylamino)borane

Boron nitride thin films could be deposited on Si(0 0 1) by chemical vapor deposition (CVD) at atmospheric pressure using a single source precursor. IR absorption spectra of films deposited between 750 and 1000°C using B[N(CH₃)₂]₃ (tris(dimethylamino)borane, TDMAB) as the boron and nitrogen source showed a peak absorption at ∼1360 cm⁻¹ characteristic of the in-plane vibrational mode seen in h-BN. It was noted that the mode at 800 cm⁻¹ is very weak. The observed growth rate varied exponentially with temperature in the range 850-900°C. Ellipsometry measurements were used to investigate the thickness and optical constant of the films. The refractive index, slightly lower than the bulk material, is close to 1.65-1.7 depending on the surface morphology of the films. The surface morphology of thin layers has been observed by atomic force microscopy with an increase of the surface roughness from 0.3 to 3.5 nm as the growth temperature increases from 800 to 950°C.     

Diode-Like Selective Enhancement of Carrier Transport through Metal–Semiconductor Interface Decorated by Monolayer Boron Nitride

2D semiconductors such as monolayer molybdenum disulfide (MoS₂) are promising material candidates for next-generation nanoelectronics. However, there are fundamental challenges related to their metal–semiconductor (MS) contacts, which limit the performance potential for practical device applications. In this work, 2D monolayer hexagonal boron nitride (h-BN) is exploited as an ultrathin decorating layer to form a metal–insulator–semiconductor (MIS) contact, and an innovative device architecture is designed as a platform to reveal a novel diode-like selective enhancement of the carrier transport through the MIS contact. The contact resistance is significantly reduced when the electrons are transported from the semiconductor to the metal, but is barely affected when the electrons are transported oppositely. A concept of carrier collection barrier is proposed to interpret this intriguing phenomenon as well as a negative Schottky barrier height obtained from temperature-dependent measurements, and the critical role of the collection barrier at the drain end is shown for the overall transistor performance.     

Structural evolution of boron nitride films grown on diamond buffer-layers

Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B₂H₆ and NH₃ in a mixture of H₂ and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp²-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films.     

Synthesis of Composite Nanosheets of Graphene and Boron Nitride and Their Lubrication Application in Oil

Composite nanosheets of graphene and boron nitride have been produced in large quantities for the first time using high‐energy ball milling in ammonia gas as an exfoliation agent. The anti‐wear properties of the composite nanosheets as a lubricant additive are investigated via a four‐ball method. The results show that the composite nanosheets are exfoliated from the commercial graphite and h‐BN powders and combined into graphene/BN composite nanosheets during the ball milling process. The composite nanosheets formed have diameters larger than 200 nm and consist of heterostructures of approximately 10 monolayers of graphene and BN. The composite nanosheets exhibit better wear resistance and friction reduction properties than the homogeneous nanosheets because of the stronger interaction between graphene and BN nanosheets, which can effectively improve the anti‐wear properties of mineral base oil as a lubricant additive.

     

Crystallization Kinetics of Cubic Boron Nitride in the B-N-Mg-H-F System

The kinetics of the h-BN → c-BN transformation in a mixture of composition 78% h-BN + 5% NH₄F + 2% B + 15% Mg are studied at 5 GPa and temperatures from 1670 to 1910 K. The activation energy of c-BN formation, temperature-dependent reaction rate constant, strength of the forming c-BN particles, and density of paramagnetic defects in c-BN are evaluated.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 816–818.Original Russian Text Copyright © 2005 by Shipilo, Ignatenko, Anichenko, Azarko.