Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (V_N) or multiple (m-V_N with m=2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to V_N vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at ∼400 nm assigned to m-V_N vacancies; further irradiation breaks more B-N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B-N dissociation and preserves the bulk sample stoichiometry.     

Effects of defects on heat conduction of graphene/hexagonal boron nitride heterointerface

We investigate the effects of point defects on the Interface Thermal Resistance (ITR) of graphene/hexagonal boron nitride (G/h-BN) heterointerface with various stacking forms by ultrafast thermal pulse method. The results reveal that the ITR of different stacking forms presents a significant downward trend with the existence of point defects. This counterintuitive behavior is attributed to the defects increase the vibration intensity of out-of-plane phonons of graphene in low-frequency region, thus enhancing the phonons coupling between graphene and h-BN layer. ITR of G/h-BN is further reduced by 50% with the defect rate increases from 0% to 5% and that is reduced by 65% with the temperature rises from 200 K to 700 K. Besides, it is found that the defective G/h-BN has thermal rectification characteristic and that is positively related to temperature and defect rate. Our study provides a practical way for the application of defects in graphene and a new approach for the design of thermal rectifier devices.     

铜箔碳掺杂对化学气相沉积生长的六方氮化硼中含碳色心的高效制备及其调控

缺陷和掺杂对材料的力学、 电子和光学性能均具有重要影响. 六方氮化硼(Hexagonal Boron Nitride, hBN)由于具有a6 eV 的宽带隙, 其中的点缺陷可以在带隙内产生发射波长覆盖近紫外-近红外波段的室温稳定发光的深能级色心, 是量子光源等光学的理想载体. 近年来的实验和理论研究表明, 碳缺陷是h-BN 可见光波段单光子发射的主要可能来源之一; 然而, 当前对h-BN 中碳缺陷的有效制备和精确调控仍存在挑战. 本文报道了一种有效方法, 通过在化学气相沉积(Chemicalvapordeposition, CVD) 过程中预先控制用于催化生长h-BN 的铜箔衬底中的碳含量, 制备得到了不同密度的零声子发射(Zero-phononlines, ZPL)波长位于626±3 nm 的色心; 此外, 我们还观察到气氛退火对色心发光显著的调控作用. 该色心优良的单色性和可调控性有助于推动基于h-BN 的量子光源和光子学器件的潜在应用, 对理解h-BN 中碳缺陷的形成亦具有重要意义.     

含空位二维六角氮化硼电子和磁性质的密度泛函研究

基于密度泛函理论的第一性原理计算, 研究了含B原子空位(V_B), N原子空位(V_N), 以及含B–N键空位 (V_B+N)缺陷的二维氮化硼(h-BN)的电子和磁性质. 在微观结构上, V_B体系表现为在空位附近的N原子重构成等腰三角形, V_N体系靠近空穴的B 原子形成等边三角形, V_B+N体系靠近空穴处的B和N原子在h-BN平面上重构为梯形. 三种空位缺陷都使h-BN的带隙类型从直接带隙转变为间接带隙. V_B体系的总磁矩为1.0 μ_B, 磁矩全部由N原子贡献. 其中空穴周围的三个N原子磁矩方向不完全一致, 存在着铁磁性和反铁磁性两种耦合方式. 对于V_N 体系, 整个晶胞内的总磁矩也为1.0 μ_B, 磁矩在空穴周围区域呈现一定的分布. 关键词： 二维h-BN 空位 电子结构 磁性     

Permittivity and its temperature dependence in hexagonal structure BN dominated by the local electric field

This paper presents an analysis of the local electric field in hexagonal boron nitride (h-BN) by introducing a modified parameter. Based on the determination of the modified parameter of h-BN, the revised Lorenz equation is developed. Then the permittivity at high temperature and in the microwave frequency is investigated. In addition, this equation is derived for evaluating the temperature coefficient of the permittivity of h-BN. The analyses show that the permittivity increases with increasing temperature, which is mainly attributed to the positive temperature coefficient of the ionic polarizability.     

Magnetic-like field inducing negative Dirac mass in graphene on hexagonal boron nitride

The tight-binding electrons in graphene grown on top of hexagonal boron nitride (h-BN) substrate are studied. The two types of surfaces on the h-BN substrate give rise to Dirac fermions having positive and negative masses. The positive and negative masses of the Dirac fermions lead to the gapped graphene to behave as a “pseudo” ferromagnet. A very large (pseudo) tunneling magnetoresistance is predicted when the Fermi level approaches the gap region. The energy gap due to the breaking of sublattice symmetry in graphene on h-BN substrate is analogous to magnetic-induced energy gap on surface of topological insulators. We point out that positive and negative masses may correspond to signs of magnetic-like field perpendicular to graphene sheet acting on pseudo magnetic dipole moment of electrons, leading to pseudo-Larmor precession and Stern–Gerlach magnetic force.     

Synthesis of hexagonal boron nitride films by dual temperature zone low-pressure chemical vapor deposition

Hexagonal boron nitride (h-BN) films are synthesized by dual temperature zone low-pressure chemical vapor deposition (LPCVD) through using a single ammonia borane precursor on non-catalytic c-plane Al₂O₃ substrates. The grown films are confirmed to be h-BN films by various characterization methods. Meanwhile, the growth rates and crystal quality of h-BN films at different positions in the dual temperature zone are studied. It is found that the growth rates and crystal quality of the h-BN films at different positions on the substrate are significantly different. The growth rates of the h-BN thin films show their decreasing trends with the rearward position, while the crystal quality is improved. This work provides an experimental basis for the preparation of large area wafer thick h-BN films by LPCVD.     

六方氮化硼的结晶度对合成立方氮化硼的影响

 将3种不同初始结晶度的六方氮化硼（h-BN）分别与Li₃N按一定比例混合,在低于立方氮化硼（c-BN）的合成温度和压力条件下,采用高温、高压预处理方式,使3种h-BN的结晶度发生改变。对不同预处理条件下得到的h-BN进行X射线衍射分析,确定了结晶度的变化程度。用不同结晶度的h-BN作为初始原料合成c-BN,观察并分析了h-BN向c-BN转化时微观结构的变化以及h-BN的结晶度对合成c-BN的影响。实验结果表明,结晶度低的h-BN更容易合成c-BN。     

Optical transitions in single-wall boron nitride nanotubes

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.     

Fabrication of particular structures of hexagonal boron nitride and boron–carbon–nitrogen layers by anisotropic etching

Anisotropic etching of hexagonal boron nitride (h-BN) and boron–carbon–nitrogen (BCN) basal plane can be an exciting platform to develop well-defined structures with interesting properties. Here, we developed an etching process of atomically thin h-BN and BCN layers to fabricate nanoribbons (NRs) and other distinct structures by annealing in H₂ and Ar gas mixture. BCN and h-BN films are grown on Cu foil by chemical vapor deposition (CVD) using solid camphor and ammonia borane as carbon, nitrogen and boron source, respectively. Formation of micron size well-defined etched holes and NRs are obtained in both h-BN and BCN layers by the post growth annealing process. The etching process of h-BN and BCN basal plane to fabricate NRs and other structures with pronounced edges can open up new possibilities in 2D hybrid materials.     

STRESS ANALYSIS IN CUBIC BORON NITRIDE FILMS BY X-RAY DIFFRACTION USING sin2 ψ METHOD

The boron nitride (BN) films containing cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN) were prepared by radio frequency a ssisted thermal filament chemical vapor deposition. The stress and strain in BN films were investigated by X-ray diffraction analysis using the sin² ψ method. The results showed that both c-BN and h-BN in the same film have similar values of elastic strain, however, the compressive stress in c-BN is much greater than that in h-BN for the same film. Both stress and strain gradually decre ased with the increase of substrate temperature (T_s). The effective stress in the films calculated by the effective stress model increased with the increase of T_s. Furthermore, the dependence of effective stress in the films on T_s was also investigated.     

Theoretical study of vacancies and adatoms in white graphene

The stability of the B and N atomic vacancies and divacancies in an h-BN monolayer deformed by 2 and 4% along one of the axes has been investigated. It has been established that the N atomic vacancies are most stable; their concentration is insignificant and does not affect the properties of white graphene. The number of vacancies depends on the mobility of N and B atoms on the layer surface; therefore, the probability of recombination with the vacancies has been estimated. It has been revealed that the energy barrier for the migration of the B and N adatoms is about 0.23 and 1.23 eV, respectively. In view of such a low barrier for the B adatom, this type of adatoms will quite rapidly move over the surface and recombine with vacancies, in contrast to the N adatoms. Therefore, only nitrogen atom vacancies can exist in the h-BN monolayer grown by the methods, where the adatoms could possibly appear on the surface.     

Bilayer graphene on h-BN substrate: investigating the breakdown voltage and tuning the bandgap by electric field

By performing density functional theory calculations we show that it is possible to make the electronic bandgap in bilayer graphene supported on hexagonal boron nitride (h-BN) substrates tunable. We also show that, under applied electric fields, it is possible to insert states from h-BN into the bandgap, which generate a conduction channel through the substrate making the system metallic. In addition, we verify that the breakdown voltage strongly depends on the number of h-BN layers. We also show that both the breakdown voltage and the bandgap tuning are independent of the h-BN stacking order.     

立方氮化硼薄膜沉积过程的相变研究

从能量和结构两个角度分析了BN四种相的转变过程，以及杂质和缺陷对立方氮化硼(c-BN)薄膜制备的影响.研究了从六角氮化硼(h-BN)到c-BN转变的一个可能的过程，即h-BN→菱形氮化硼(r-BN)→c-BN过程.对纯的h-BN到r-BN的转变需要克服一个很高的能量势垒，在实验室条件下很难能够提供能量来越过这个势垒.而从r-BN到c-BN的转变只需要克服一个很低的能量势垒.这个能量势垒要低于从h-BN到纤锌矿氮化硼(w-BN)转变所需要克服的能量势垒.c-BN薄膜的制备过程中，薄膜在高能粒子轰击下，会产生大量的缺陷，这些缺陷对立方相的形成起到了重要的作用，缺陷和杂质的存在大大降低了从h-BN到r-BN转变的能量势垒.根据这个理论模型，在两步法制备c-BN薄膜的基础上，调整实验参数，形成三步法制备高质量c-BN薄膜.主要研究了三步法中第一步的时间和衬底负偏压对c-BN薄膜制备的影响，找到合适的沉积时间和衬底负偏压分别为5min和-180V.采用三步法制备薄膜，可以重复得到高立方相体积分数(立方相体积分数超过80%)的BN薄膜，并且实验重复性达到70%以上. 关键词： 立方氮化硼 能量势垒 缺陷 衬底偏压     

Extinction mechanisms of hyperbolic h-BN nanodisk

We applied the finite element method to calculate the extinction spectrum of single hyperbolic hexagonal boron nitride(h-BN) nanodisk. We show that the hyperbolic h-BN nanodisk exhibits two extinction mechanisms in the mid-infrared region. The volume confined phonon polaritons resonances of the nanodisk give rise to a series of weak extinction peaks.The localized surface phonon polaritons lead to a robust dipolar extinction, and the extinction peak position is tunable by varying the size of the h-BN nanodisk. These findings reveal the mechanisms of the interaction between light and resonant h-BN nanodisk, which are essential for h-BN related opto-electromagnetic applications.     

Study of boron nitride by electron energy-loss spectroscopy

An interband π plasmon and π* edge have been observed in the spectrum of hexagonal boron nitride (h-BN). This result is in good agreement with that     

First-principles study of intrinsic defect properties in hexagonal BN bilayer and monolayer

The formation energies and transition energy levels of intrinsic defects in hexagonal BN (h-BN) bilayer and monolayer have been studied by first-principles calculations based on density functional theory. Our calculated results predict that excellent intrinsic p-type and n-type conductivities are very difficult to be realized in h-BN bilayer and monolayer. This is because of the high formation energies of acceptor-like defects (≥4.6 eV ) and the rather deep transition energy levels of donor-like defects (≥2.0 eV ). In order to obtain h-BN layers with more efficient p-type and n-type conductivity, extrinsic doping using foreign impurities is necessary.     

Vacancy growth and migration dynamics in atomically thin hexagonal boron nitride under electron beam irradiation

Atomically thin hexagonal boron nitride (h‐BN) is investigated using aberration‐corrected ultra‐high resolution electron microscopy under 80 kV electron beam. This study focuses on the in situ formation, growth and migration of vacancies in h‐BN. This study also reveals interaction dynamics of edges and vacancies with adatoms and molecules under the electron beam. According to this investigation, boron monovacancies migrate through their second neighbor to reduce the surface energy of the membrane. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Universal symmetry property of point defects in crystals

Point defects (vacancies, solute atoms, and disorder) are ubiquitous in crystalline solids. With in situ transmission electron microscopy we find clear evidence for the existence of a universal symmetry property of point defects; i.e., the symmetry of short-range order of point defects follows the crystal symmetry when in equilibrium. We further show that this symmetry-conforming property can lead to various interesting effects including "aging-induced microstructure memory" and the associated "time-dependent two-way shape memory."     

Precursor Based on Polyborylborazine for BN Fiber: Preparation and Rheology Properties

A polyborylborazine precursor for hexagonal boron nitride (h-BN) was obtained by reaction of boron trichloride with methylamine and its structure was characterized by ¹¹BNMR, ¹³CNMR, ¹HNMR, and FTIR. The results show that the molecular precursor consists of borazine rings connected via a cross-linked network. The results of shear rheological tests indicated that the polymer is capable of being melt spun at moderate temperature, which implies that the structure of the molecular chains of the precursor polymer is branched. The precursor polymer was spun into a continuous polymer fiber in the melt state and then subsequently heat-treated under NH₃ up to 1000°C for conversion into BN fibers. Its surface morphology was observed by scanning electron microscopy (SEM); the fiber was free of defects and cavities.