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.     

Charge distribution of a potassium-doped combined system of graphene and hexagonal boron nitride

By using first-principles density functional theory, we investigate the charge distribution of a potassium-doped layered combined system of graphene and hexagonal boron nitride. Two configurations of potassium-doped hexagonal boron nitride layers on graphenes and the reverse geometry of graphenes on hexagonal boron nitride layers are considered. We find that the charge distribution exhibits different features in these two situations. In the former case, the outmost hexagonal boron nitride layer cannot screen the external charges offered by potassium atom completely and most of the transferred charges reside on the two bounding layers. In contrary, the outmost graphene layer near the potassium atom can accept almost all of the transferred charges and only a few of them stay at interior layers in the latter case. A more amazing result is that the characteristics of charge transfer are independent of the number of hexagonal boron nitride layers and graphenes.     

六角氮化硼纳米带的几何和电子结构理论研究

本文采用密度泛函B3LYP方法在6-31G*水平上，对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加，纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性；锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡，且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布，呈现出向边界偏移的现象。     

六角氮化硼纳米带的几何和电子结构理论研究

本文采用密度泛函B3LYP方法在6-31G*水平上,对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加,纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性;锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡,且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布,呈现出向边界偏移的现象。     

Synthesis of boron nitride nanostructures from catalyst of iron compounds via thermal chemical vapor deposition technique

For the first time, patterned growth of boron nitride nanostructures (BNNs) is achieved by thermal chemical vapor deposition (TCVD) technique at 1150 °C using a mixture of FeS/Fe₂O₃ catalyst supported in alumina nanostructured, boron amorphous and ammonia (NH₃) as reagent gas. This innovative catalyst was synthesized in our laboratory and systematically characterized. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The X-ray diffraction profile of the synthesized catalyst indicates the coexistence of three different crystal structures showing the presence of a cubic structure of iron oxide and iron sulfide besides the gamma alumina (γ) phase. The results show that boron nitride bamboo-like nanotubes (BNNTs) and hexagonal boron nitride (h-BN) nanosheets were successfully synthesized. Furthermore, the important contribution of this work is the manufacture of BNNs from FeS/Fe₂O₃ mixture.     

无序双层六角氮化硼量子薄膜的电子性质

基于安德森紧束缚模型,本文研究了无序双层六角氮化硼量子薄膜的电子性质. 数值计算结果表明在双层都无序掺杂的情况下,六角氮化硼量子薄膜的电子是局域的, 其表现为绝缘体性质;而对于单层掺杂(无论是氮原子还是硼原子)的双层六角氮化硼量子薄膜, 在能谱的带尾出现了持续的迁移率边.这就说明在单层掺杂的双层六角氮化硼量子薄膜中产生了 金属绝缘体转变.这一结果证实了有序-无序分区掺杂的理论模型,为理解及调控双层六角氮化硼量子薄膜 的电子性质提供了有益的理论指导.     

高温高压下立方氮化硼和六方氮化硼的结构、力学、热力学、电学以及光学性质的第一性原理研究

本文采用基于密度泛函理论的第一性原理平面波赝势和局域密度近似方法,优化了立方和六方氮化硼的几何结构,系统地研究了零温高压下立方和六方氮化硼的几何结构、力学、电学以及光学性质.结构与力学性质研究表明:立方氮化硼的结构更加稳定,两种结构的氮化硼均表现出一定的脆性,而六方氮化硼的热稳定性则相对较差;电学性质研究表明:立方氮化硼和六方氮化硼均为间接带隙半导体,且立方氮化硼比六方氮化硼局域性更强;光学性质结果显示:立方氮化硼和六方氮化硼对入射光的通过性都很好,在高能区立方氮化硼对入射光的表现更加敏感.此外,还研究了高温高压下立方氮化硼的热力学性质,并得到其热膨胀系数、热容、德拜温度和格林艾森系数随温度和压力的变化关系.本文的理论研究阐述了高压下立方氮化硼和六方氮化硼的相关性质,为今后的实验研究提供了比较可靠的理论依据.     

Behavior of H2O molecule in carbon nanotube/boron nitride nanotube heterostructure

We perform total-energy electronic-structure calculations of a water molecule inside a (7, 7) carbon nanotube/boron nitride nanotube (CNT/BNNT) heterojunction. The van der Waals interaction is also considered in this study. We find that the equilibrium distance between the water molecule and the wall of the CNT (BNNT) is ≈ 3.3 Å, and the encapsulation energy is 0.22 eV (0.25 eV). The energy profile along the tube axis exhibits a dramatic change in the vicinity of the heterojunction. A speed change of water flow is expected to occur near the heterojunction. Such information would provide valuable insight in nanostructure design for nanofluidics.     

Hexagonal boron nitride hollow capsules with collapsed surfaces:Chemical vapor deposition with single-source precursor ammonium fluoroborate

SBA-15(mesoporous SiO_2) is used to stabilize and transfer F-in the NH_4BF_4 CVD reaction for the first time, and a large-scale crystalline h-BN phase can be prepared. We successfully fabricate hollow h-BN capsules with collapsed surfaces in our designed NH_4BF_4 CVD system. Optimum temperature conditions are obtained, and a detailed formation mechanism is further proposed. The successful SBA-15-assisted NH_4BF_4 CVD route is of importance and enriches the engineering technology in the h-BN single-source CVD reaction.     

立方氮化硼薄膜生长过程中的界面控制

在立方氮化硼薄膜气相生长过程中生成的无定形初期层和乱层结构氮化硼中间层，一直是阻碍立方氮化硼薄膜外延生长的主要原因.系统地分析了硅衬底预处理对立方氮化硼薄膜中无定形初期层成分的影响，发现在等离子体化学气相生长法制备薄膜时，硅衬底上形成无定形初期层的可能原因有氧的存在、离子轰击以及高温下硅的氮化物的形成.在H₂气氛中1200K热处理硅衬底可以有效地减少真空室中残留氧浓度，除去硅表面的自然氧化层，保持硅衬底表面晶体结构.控制衬底温度不超过900 K，就能防止硅的氮化物的形成，成功地除去无 关键词： 立方氮化硼薄膜 等离子体化学气相生长 界面 电子显微镜     

Van der Waals heterostructure of phosphorene and hexagonal boron nitride: First-principles modeling

We have studied the structural and electronic properties of a hybrid hexagonal boron nitride with phosphorene nanocomposite using ab initio density functional calculations. It is found that the interaction between the hexagonal boron nitride and phosphorene is dominated by the weak van der Waals interaction, with their own intrinsic electronic properties preserved. Furthermore, the band gap of the nanocomposite is dependent on the interfacial distance. Our results could shed light on the design of new devices based on van der Waals heterostructure.     

Effect of triangle vacancy on thermal transport in boron nitride nanoribbons

Recently, triangle vacancy in hexagonal boron nitride is observed experimentally. Using nonequilibrium Green’s function method, we investigate thermal transport properties of boron nitride nanoribbons (BNNRs) with a triangle vacancy. The effect of triangle vacancy on the phonon transmission of zigzag-edged BNNRs (Z-BNNRs) is different from that of armchair-edged BNNRs (A-BNNRs). The triangle vacancy induces antiresonant dips in the spectrum of Z-BNNRs. Moreover, the boron-terminated triangle vacancy causes antiresonant zero-transmission dip and the number of the zero-transmission dip increases with the geometrical size of triangle vacancy. For the A-BNNRs with triangle vacancy, except some antiresonant dips, a resonant peak is found in the transmission. The antiresonant and resonant phenomena are explained by analyzing local density of states and local thermal currents. Although the antiresonant dip and the resonant peak are both originated from quasibound states, their distributions of local thermal currents are distinct, which leads to the transport discrepancy. In addition, the thermal conductance of BNNRs decreases linearly with increasing the vacancy size.     

Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin films were studied in this paper. The films were characterized by Fourier transform infrared spectroscopy, UV--visible transmittance and reflection spectra. h-BN thin films with a wide optical band gap E_g (5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency (RF) bias magnetron sputtering and post-deposition annealing at 970~K. The optical absorption behaviour of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting is made by the Urbach tail model. The annealed film shows satisfactory structure stability. However, high temperature still has a significant effect on the optical absorption properties, refractive index n, and optical conductivity σ of h-BN thin films. The blue-shift of the optical absorption edge and the increase of E_g probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive index clearly exhibits different trends in the visible and ultraviolet regions. Previous calculational results of optical conductivity of h-BN films are confirmed in our experimental results.     

轴压和扭转复合载荷作用下氮化硼纳米管屈曲行为的分子动力学模拟

采用分子动力学方法模拟了氮化硼纳米管在轴压和扭转复合荷载作用下的屈曲和后屈曲行为.在各加载比例下,给出了初始线性变形阶段和后屈曲阶段原子间相互作用力的变化,确定了屈曲临界荷载关系.通过对屈曲模态的细致研究,从微观变形机理上分析了纳米管对不同外荷载力学响应的差异.研究结果表明,扶手型和锯齿型纳米管均呈现出非线性的屈曲临界荷载关系,复合加载下的屈曲行为具有强烈的尺寸依赖性.温度升高将导致屈曲临界荷载的下降,且温度的影响随加载比例的变化而变化.无论在简单加载或复合加载中,同尺寸的碳纳米管均比氮化硼纳米管具有更强地抵抗屈曲荷载的能力.     

等离子体增强化学气相沉积法制备立方氮化硼薄膜过程中的表面生长机理

利用感应耦合等离子体增强化学气相沉积法以Ar，He，N₂和B₂H₆为反应气体制备了高纯立方氮化硼薄膜.用四极质谱仪对等离子体状况进行了系统的分析，发现B₂H₆完全被电离而N₂只是部分被电离.H₂和过量的N₂在等离子体中生成大量中性的H原子和活化的N^*₂，它们与表面的相互作用严重地阻碍了立方 关键词： 立方氮化硼薄膜 等离子体 质谱     

Deposition of hexagonal boron nitride thin films on silver nanoparticle substrates and surface enhanced infrared absorption

Silver nanoparticle thin films with different average particle diameters are grown on silicon substrates. Boron nitride thin films are then deposited on the silver nanoparticle interlayers by radio frequency （RF） magnetron sputtering. The boron nitride thin films are characterized by Fourier transform infrared spectra. The average particle diameters of silver nanoparticle thin films are 126.6, 78.4, and 178.8 nm. The results show that the sizes of the silver nanoparticles have effects on the intensities of infrared spectra of boron nitride thin films. An enhanced infrared absorption is detected for boron nitride thin film grown on silver nanoparticle thin film. This result is helpful to study the growth mechanism of boron nitride thin film.     

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.     

石墨烯和六方氮化硼纳米片的几何结构、电子结构、磁性及边缘特性研究

采用基于密度泛函理论(DFT)的第一性原理计算方法,计算了多种尺寸的石墨烯和六方氮化硼纳米片的性质,系统研究了其中的量子尺寸效应.研究的最大尺寸纳米片的直径约为5.5 nm,包含816个原子.对纳米片及其边缘的几何结构、电子结构、磁性性质以及电子分布进行了深入探讨,发现石墨烯和六方氮化硼纳米片最外层原子有由锯齿形向圆形变化的趋势,使得纳米片最外层更加平滑.随着纳米片尺寸的增加,能级由分立逐渐变得连续,纳米片由孤立分子态逐渐变得接近无限的晶体;禁带宽度总体有下降的趋势,符合量子尺寸效应.纳米片存在明显的磁性,磁矩主要集中在最外层原子上,且在相对平滑的地方容易出现磁性,相对弯曲的地方不易出现磁性.当增加体系的电子数时,增加的电子主要分布在最外层,使得纳米片整体磁性呈递减的趋势;当减少体系的电子数时,减少的电子的分布逐渐由最外层向内收缩,体系的总磁矩略有增加.研究结果对石墨烯和六方氮化硼纳米片的应用有参考价值.     

Preparation and characterization of boron nitride coatings on carbon fibers from borazine by chemical vapor deposition

Boron nitride (BN) coatings were deposited on carbon fibers by chemical vapor deposition (CVD) using borazine as single source precursor. The deposited coatings were characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The effect of temperatures on growth kinetics, morphology, composition and structure of the coatings was investigated. In the low temperature range of 900 °C-1000 °C, the growth rate increased with increasing temperature complying with Arrhenius law, and an apparent active energy of 72 kJ/mol was calculated. The coating surface was smooth and compact, and the coatings uniformly deposited on individual fibers of carbon fiber bundles. The growth was controlled by surface reaction. At 1000 °C, the deposition rate reached a maximum (2.5 μm/h). At the same time, the limiting step of the growth translated to be mass-transportation. Above 1100 °C, the growth rate decreased drastically due to the occurrence of gas-phase nucleation. Moreover, the coating surface became loose and rough. Composition and structure examinations revealed that stoichiometric BN coatings with turbostratic structure were obtained below 1000 °C, while hexagonal BN coatings were deposited above 1100 °C. A penetration of carbon element from the fibers to the coatings was observed.     

X-ray photoelectron spectroscopy study of cubic boron nitride single crystals grown under high pressure and high temperature

The defects, impurities and their bonding states of unintentionally doped cubic boron nitride (cBN) single crystals were investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that nitrogen vacancy (V_N) is the main native defect of the cBN crystals since the atomic ratio of B:N is always larger than 1 before Ar ion sputtering. After sputter cleaning, around 6 at% carbon, which probably comes from the growth chamber, remains in the samples as the main impurity. Carbon can substitute nitrogen lattice site and form the bonding states of CBN or CB, which can be verified by the XPS spectra of C1s, B1s and N1s. The C impurity (acceptor) and N vacancy (donor) can compose the donor-acceptor complex to affect the electrical and optical properties of cBN crystals.