Quantum Emitters in Hexagonal Boron Nitride Have Spectrally Tunable Quantum Efficiency

Understanding the properties of novel solid‐state quantum emitters is pivotal for a variety of applications in research fields ranging from quantum optics to biology. Recently discovered defects in hexagonal boron nitride are especially interesting, as they offer much desired characteristics such as narrow emission lines and photostability. Here, the dependence of the emission on the excitation wavelength is studied. It is found that, in order to achieve bright single‐photon emission with high quantum efficiency, the excitation wavelength has to be matched to the emitter. This is a strong indication that the emitters possess a complex level scheme and cannot be described by a simple two or three‐level system. Using this excitation dependence of the emission, further insight to the internal level scheme is gained and it is demonstrated how to distinguish different emitters both spatially as well as in terms of their photon correlations.     

Piezoelectricity in Monolayer Hexagonal Boron Nitride

2D hexagonal boron nitride (hBN) is a wide-bandgap van der Waals crystal with a unique combination of properties, including exceptional strength, large oxidation resistance at high temperatures, and optical functionalities. Furthermore, in recent years hBN crystals have become the material of choice for encapsulating other 2D crystals in a variety of technological applications, from optoelectronic and tunneling devices to composites. Monolayer hBN, which has no center of symmetry, is predicted to exhibit piezoelectric properties, yet experimental evidence is lacking. Here, by using electrostatic force microscopy, this effect is observed as a strain-induced change in the local electric field around bubbles and creases, in agreement with theoretical calculations. No piezoelectricity is found in bilayer and bulk hBN, where the center of symmetry is restored. These results add piezoelectricity to the known properties of monolayer hBN, which makes it a desirable candidate for novel electromechanical and stretchable optoelectronic devices, and pave a way to control the local electric field and carrier concentration in van der Waals heterostructures via strain. The experimental approach used here also shows a way to investigate the piezoelectric properties of other materials on the nanoscale by using electrostatic scanning probe techniques.     

Polymer Brushes on Hexagonal Boron Nitride

Direct covalent functionalization of large‐area single‐layer hexagonal boron nitride (hBN) with various polymer brushes under mild conditions is presented. The photopolymerization of vinyl monomers results in the formation of thick and homogeneous (micropatterned, gradient) polymer brushes covalently bound to hBN. The brush layer mechanically and chemically stabilizes the material and allows facile handling as well as long‐term use in water splitting hydrogen evolution reactions.     

Doping Nanoscale Graphene Domains Improves Magnetism in Hexagonal Boron Nitride

Carbon doping can induce unique and interesting physical properties in hexagonal boron nitride (h‐BN). Typically, isolated carbon atoms are doped into h‐BN. Herein, however, the insertion of nanometer‐scale graphene quantum dots (GQDs) is demonstrated as whole units into h‐BN sheets to form h‐CBN. The h‐CBN is prepared by using GQDs as seed nucleations for the epitaxial growth of h‐BN along the edges of GQDs without the assistance of metal catalysts. The resulting h‐CBN sheets possess a uniform distrubution of GQDs in plane and a high porosity macroscopically. The h‐CBN tends to form in small triangular sheets which suggests an enhanced crystallinity compared to the h‐BN synthesized under the same conditions without GQDs. An enhanced ferromagnetism in the h‐CBN emerges due to the spin polarization and charge asymmetry resulting from the high density of C? N and C? B bonds at the boundary between the GQDs and the h‐BN domains. The saturation magnetic moment of h‐CBN reaches 0.033 emu g^?1 at 300 K, which is three times that of as‐prepared single carbon‐doped h‐BN.     

六方氮化硼颗粒制备方法研究进展

概述了六方氮化硼颗粒制备方法及研究现状。介绍了先驱体法、化学气相沉积法、传统高温法、水(溶剂)热法等不同的制备方法,对合成六方氮化硼颗粒的工艺条件对颗粒形成的影响做了深入的综述。经过分析进一步得出高温精制及在焙烧时加入适量的助剂将会提高产物的结晶度和纯度,指出了各种制备方法的优劣及如何避免合成中存在的问题,并进一步展望了各种制备方法的前景。     

Manipulation and Steering of Hyperbolic Surface Polaritons in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume‐confined hyperbolic polaritons and sidewall‐confined hyperbolic surface polaritons (HSPs). In this work, efficient excitation, control, and steering of HSPs are demonstrated in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared nanoimaging and numerical simulations, the reflection, transmission, and scattering of HSPs are investigated at the hBN corners with various apex angles. It is also shown that the sidewall‐confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.     

前驱体法制备六方氮化硼颗粒及高温精制

以三聚氰胺和硼酸为原料,通过共沉淀法合成了二硼酸-三聚氰胺,在最佳工艺条件下以二硼酸-三聚氰胺为前驱体在氨气气氛中烧结制得六方氮化硼颗粒。采用IR、XRD、TG、SEM等测试方法对中间物和产物进行了表征,确定了中间物及产物的组成、物相、粒度及形貌。研究结果表明:以二硼酸-三聚氰胺为前驱体在1200℃烧结4h,可制得高纯度、晶型好的六方氮化硼。经过高温精制后的六方氮化硼颗粒具有较高的结晶度和致密度,其粒径可以达到20μm以上,纯度为98.5%。     

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E_2g vibration mode (12 cm⁻¹) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS₂/h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.     

Dielectric Environment Sensitivity of Carbon Centers in Hexagonal Boron Nitride

A key advantage of utilizing van-der-Waals (vdW) materials as defect-hosting platforms for quantum applications is the controllable proximity of the defect to the surface or the substrate allowing for improved light extraction, enhanced coupling with photonic elements, or more sensitive metrology. However, this aspect results in a significant challenge for defect identification and characterization, as the defect's properties depend on the the atomic environment. This study explores how the environment can influence the properties of carbon impurity centers in hexagonal boron nitride (hBN). It compares the optical and electronic properties of such defects between bulk-like and few-layer films, showing alteration of the zero-phonon line energies and their phonon sidebands, and enhancements of inhomogeneous broadenings. To disentangle the mechanisms responsible for these changes, including the atomic structure, electronic wavefunctions, and dielectric screening, it combines ab initio calculations with a quantum-embedding approach. By studying various carbon-based defects embedded in monolayer and bulk hBN, it demonstrates that the dominant effect of the change in the environment is the screening of density–density Coulomb interactions between the defect orbitals. The comparative analysis of experimental and theoretical findings paves the way for improved identification of defects in low-dimensional materials and the development of atomic scale sensors for dielectric environments.     

Effect of Nano Hexagonal Boron Nitride Lubricant Additives on the Friction and Wear Properties of AISI 4140 Steel

The aim of this study was to investigate the effects of nano hexagonal boron nitride(hBN) particles on the friction and wear properties of AISI 4140 steel material when the hBN particles are used as an oil additive. Nano hexagonal boron nitride powders, which were produced using a special process, were dispersed in engine oil (SAE10 W) to enhance lubrication. The amount of nano hexagonal boron nitride in the engine oil was varied from 0 to 10% by volume, and four different lubricant samples were prepared. Wear tests were conducted using ball-on-disc geometry. The worn surfaces of substrates were analyzed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). The experiments showed that the nano hexagonal boron nitride particles that were used as an oil additive affected the friction and wear behavior. A 14.4% improvement in the friction coefficient and a 65% decrease in the wear rate were achieved through the use of the nano hBN as an oil additive.