Large Scale Graphene/Hexagonal Boron Nitride Heterostructure for Tunable Plasmonics

Vertical integration of hexagonal boron nitride (h‐BN) and graphene for the fabrication of vertical field‐effect transistors or tunneling diodes has stimulated intense interest recently due to the enhanced performance offered by combining an ultrathin dielectric with a semi‐metallic system. Wafer scale fabrication and processing of these heterostructures is needed to make large scale integrated circuitry. In this work, by using remote discharged, radio‐frequency plasma chemical vapor deposition, wafer scale, high quality few layer h‐BN films are successfully grown. By using few layer h‐BN films as top gate dielectric material, the plasmon energy of graphene can be tuned by electrostatic doping. An array of graphene/h‐BN vertically stacked micrometer‐sized disks is fabricated by lithography and transfer techniques, and infrared spectroscopy is used to observe the modes of tunable graphene plasmonic absorption as a function of the repeating (G/h‐BN)_n units in the vertical stack. Interestingly, the plasmonic resonances can be tuned to higher frequencies with increasing layer thickness of the disks, showing that such vertical stacking provides a viable strategy to provide wide window tuning of the plasmons beyond the limitation of the monolayer.     

Organic Field Effect Transistors Based on Graphene and Hexagonal Boron Nitride Heterostructures

Enhancing the device performance of single crystal organic field effect transistors (OFETs) requires both optimized engineering of efficient injection of the carriers through the contact and improvement of the dielectric interface for reduction of traps and scattering centers. Since the accumulation and flow of charge carriers in operating organic FETs takes place in the first few layers of the semiconductor next to the dielectric, the mobility can be easily degraded by surface roughness, charge traps, and foreign molecules at the interface. Here, a novel structure for high‐performance rubrene OFETs is demonstrated that uses graphene and hexagonal boron nitride (hBN) as the contacting electrodes and gate dielectric layer, respectively. These hetero‐stacked OFETs are fabricated by lithography‐free dry‐transfer method that allows the transfer of graphene and hBN on top of an organic single crystal, forming atomically sharp interfaces and efficient charge carrier‐injection electrodes without damage or contamination. The resulting heterostructured OFETs exhibit both high mobility and low operating gate voltage, opening up new strategy to make high‐performance OFETs and great potential for flexible electronics.     

六方氮化硼忆阻器的研究进展

六方氮化硼是一种与石墨烯结构相似的材料,以六方氮化硼作为阻变介质层的忆阻器,具有良好的散热性能,不易发生介电击穿,能够实现小尺寸、低功耗和大的开关比;在计算机运算存储研究、人工神经网络和神经形态(即类脑)计算领域有极大的应用前景。文章主要介绍了忆阻器的分类,分析了六方氮化硼忆阻器的阻变机制,综述了六方氮化硼忆阻器的研究现状。最后,指出了六方氮化硼忆阻器当前面临的挑战,并展望了未来的发展方向。     

六方氮化硼中子探测器的研究进展

六方氮化硼中子探测器具有泄漏电流小、体积小、响应速度快、探测效率高、对γ射线不灵敏等优点,有望取代传统的³He气体探测器和微结构半导体中子探测器而得到广泛应用。文章介绍了六方氮化硼中子探测器的原理,从制备工艺、探测器结构、探测器性能等方面综述了六方氮化硼中子探测器近年来的研究进展。     

Surface Growth of Boron Nitride Nanotubes through Boron Source Design

Boron nitride nanotubes (BNNTs) are promising materials due to their unique physical and chemical properties. Fabrication technologies based on gas-phase reactions reduce the control and collection efficiency of BNNTs due to reactant and product dispersion within the reaction vessel. A surface growth method that allows for controllable growth of BNNTs in certain regions using a preburied boron source is introduced. This work leverages the high solubility of boron in metals to create a boronized layer on the surface which serves as the boron source to confine the growth of BNNTs. Dense and uniform BNNTs are obtained after loading catalysts onto the boronized substrate and annealing under ammonia. Confirmatory experiments demonstrate that the boride layer provides boron for BNNTs growth. Furthermore, the patterned growth of BNNTs is realized by patterning the boronizing region, demonstrating the controllability of this method. In addition, the Ni substrate with BNNTs growth exhibits better performance in corrosion resistance and thermal conductivity than pure Ni. This study introduces an alternative strategy for the surface growth of BNNTs based on boron source design, which offers new possibilities for the controllable preparation of BNNTs for various applications.     

六边形BN片掺杂的ZGNR的磁电子学特性

利用基于密度泛函理论的第一性原理,研究了六边形BN片掺杂的锯齿型石墨烯纳米带(ZGNR)的磁电子学特性。研究表明,当处于无磁(NM)态时,随着掺杂浓度的增大,可以实现金属-准金属的相变。当处于铁磁(FM)态时,随着掺杂浓度的增大,可以实现自旋金属-自旋半导体的相变,且为双极化自旋半导体。当处于反铁磁(AFM)态时,不同浓度掺杂的ZGNR都是自旋半导体,随着掺杂浓度的增大,下旋带隙逐渐减小,而上旋带隙是先减小后增大。在不同浓度掺杂情况下,AFM态都是基态。随着掺杂浓度的增大,结合能逐渐增大,故掺杂浓度最小的ZGNR最稳定。该研究对于发展基于石墨烯的纳米电子器件很有意义。     

Dielectric Dispersion and High Field Response of Multilayer Hexagonal Boron Nitride

The dielectric dispersion of a material holds significant importance for the understanding of basic material characteristics and the design parameters of a functional device. Here, the dielectric dispersion characteristics of multilayer hexagonal boron nitride (hBN) using time domain reflectometry under an extended device operating frequency range up to 100 MHz are studied. Contrary to what is previously reported, the capacitance, hence the effective dielectric constant, of hBN decreases with the increase of frequency above the MHz range, indicating heat dissipation in lossy hBN dielectric. Furthermore, hBN shows stubborn dielectric characteristics with temperature changes that confirm its thermal stability in extreme operating conditions. The charge carriers in hBN are transported by Fowler–Nordhiem tunneling with increasing the electrical field. Lastly, hBN endures electrical field of 7.8 MV cm^?1 that implies its potential use as a promising dielectric material. These results will benefit the research and development of hBN supported high‐speed electronics operated at high‐frequency conditions for energy‐efficient device applications.     

Origin and Control of Polyacrylonitrile Alignments on Carbon Nanotubes and Graphene Nanoribbons

While one of the most promising applications of carbon nanotubes (CNTs) is to enhance polymer orientation and crystallization to achieve advanced carbon fibers, the successful realization of this goal has been hindered by the insufficient atomistic understanding of polymer–CNT interfaces. Herein, polyacrylonitrile (PAN)‐CNT hybrid structures are theoretically studied as a representative example of polymer–CNT composites. Based on density functional theory calculations, it is first found that the relative orientation of polar PAN nitrile groups with respect to the CNT surface is the key factor that determines the PAN–CNT interface energetics and the lying‐down PAN configurations are much more preferable than their standing‐up counterparts. The CNT curvature is identified as another important factor, giving the largest binding energy in the zero‐curvature graphene limit. Charge transfer analysis explains the unique tendency of linear PAN alignments on the CNT surface and the possibility of ordered PAN–PAN assembly. Next, performing large‐scale molecular dynamics simulations, it is shown that the desirable linear PAN–CNT alignment can be achieved even for relatively large initial misorientations and further demonstrate that graphene nanoribbons are a promising carbon nano‐reinforcement candidate. The microscopic understanding accumulated in this study will provide design guidelines for the development of next‐generation carbon nanofibers.     

Hexagonal Boron Nitride and Graphite Oxide Reinforced Multifunctional Porous Cement Composites

The synthesis and characterization of multifunctional cement and concrete composites filled with hexagonal boron nitride (h‐BN) and graphite oxide (GO), is reported and their superior mechanical strength and oil adsorption properties compared to composites devoid of fillers are illustrated. GO is utilized to bridge the cement surfaces while h‐BN is used to mechanically reinforce the composites and adsorb the oil. Introduction of these fillers even at low filler weight fractions increases the compressive strength and toughness properties of pristine cement and of porous concrete significantly, while the porous composite concrete illustrates excellent ability for water separation and crude oil adsorption. Experimental results along with theoretical calculations show that such nanoengineered forms of cement based composites would enable the development of novel forms of multifunctional structural materials with a range of environmental applications.     

六方氮化硼在二维晶体微电子器件中的应用与进展

六方氮化硼(h-BN)因其优异的性能和潜在的应用前景而受到广泛关注.着眼于h-BN在微电子器件领域中的发展与应用,总结了近年来国内外通过化学气相沉积(CVD)方法实现h-BN的高质量、大规模可控制备及图形化的代表性工作.围绕h-BN的高介电常数、原子级平滑表面、高导热性和高稳定性,重点介绍了h-BN在二维晶体介电衬底、...     

Chemical Sharpening,Shortening, and Unzipping of Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs), the one‐dimensional member of the boron nitride nanostructure family, are generally accepted to be highly inert to oxidative treatments and can only be covalently modified by highly reactive species. Conversely, it is discovered that the BNNTs can be chemically dispersed and their morphology modified by a relatively mild method: simply sonicating the nanotubes in aqueous ammonia solution. The dispersed nanotubes are significantly corroded, with end‐caps removed, tips sharpened, and walls thinned. The sonication treatment in aqueous ammonia solution also removes amorphous BN impurities and shortened BNNTs, resembling various oxidative treatments of carbon nanotubes. Importantly, the majority of BNNTs are at least partially longitudinally cut, or “unzipped”. Entangled and freestanding BN nanoribbons (BNNRs), resulting from the unzipping, are found to be ～5–20 nm in width and up to a few hundred nanometers in length. This is the first chemical method to obtain BNNRs from BNNT unzipping. This method is not derived from known carbon nanotube unzipping strategies, but is unique to BNNTs because the use of aqueous ammonia solutions specifically targets the B‐N bond network. This study may pave the way for convenient processing of BNNTs, previously thought to be highly inert, toward controlling their dispersion, purity, lengths, and electronic properties.     

聚苯乙烯/六方氮化硼微波复合基板的制备与性能研究

针对高功率器件、高密度封装等微波通信领域对高性能微波复合基板的迫切需求,该文提出了一种将双螺杆造粒和热压成型结合的新技术,制备了以高抗冲聚苯乙烯(HIPS)为基体、六方氮化硼(h-BN)陶瓷为填料的高导热微波复合基板,并对基板的显微结构、热学性能和介电性能进行了全面表征。结果表明,采用大粒径(?25μm)的h-BN(h-BN₂₅)比小粒径(?5μm)的h-BN(h-BN₅)填充后更有利于提高复合基板的热导率(λ),降低其介电损耗(tan δ)。随着h-BN₂₅质量分数(w(h-BN₂₅))从0增加至70%,HIPS/h-BN₂₅微波复合基板的λ从0.13 W·m^-1·K^-1提高到7.43 W·m^-1·K^-1(面内)和2.55 W·m^-1·K^-1(面间),分别是纯HIPS的57倍和20倍,表明采用以上制备技术能实现h-BN在HIPS基体中的定向排列,...     

Monolayer Boron Nitride Substrate Interactions with Graphene Under In-Plane and Perpendicular Strains: A First-Principles Study

Effects of strain on the electronic and optical properties of graphene on monolayer boron nitride (BN) substrate are investigated using first-principle calculations based on density functional theory. Strain-free graphene/BN has a small band gap of 97 meV at the K point. The magnitude of band gap increases with in-plane biaxial strain while it decreases with the perpendicular uniaxial strain. The \( \varepsilon_{2} (\omega ) \) spectrum of graphene/BN bilayer for parallel polarization shows red and blue shifts by applying the in-plane tensile and compressive strains, respectively. Also the positions of peaks in the \( \varepsilon_{2} (\omega ) \) spectrum are not significantly changed under perpendicular strain. The calculated results indicate that graphene on the BN substrate has great potential in microelectronic and optoelectronic applications.     

Highly Thermally Conductive Dielectric Nanocomposites with Synergistic Alignments of Graphene and Boron Nitride Nanosheets

Electrically insulating polymer dielectrics with high energy densities and excellent thermal conductivities are showing tremendous potential for dielectric energy storage. However, the practical application of polymer dielectrics often requires mutually exclusive multifunctional properties such as high dielectric constants, high breakdown strengths, and high thermal conductivities. The rational assembly of 2D nanofillers of boron nitride nanosheets (BNNS) and reduced graphene oxide (rGO) into a well‐aligned micro‐sandwich structure in polyimide (PI) composites is reported. The alternating stacking of rGO and BNNS synergistically exploits the large difference in their electrical conductivities to yield a high dielectric constant with a moderate breakdown strength. Moreover, the distinctively separated rGO and BNNS layers give rise to higher thermal conductivities of composites than those containing mixed fillers because of reduced phonon scattering at the interfaces between two identical fillers, as verified by molecular dynamics simulations. Consequently, the micro‐sandwich nanocomposite prevails over the PI film with a simultaneously high dielectric constant of ≈579, a high energy density (43‐fold higher than PI) and an excellent thermal conductivity (11‐fold higher than PI) at a low hybrid filler content of only 2.5 vol%. The multifunctional nanocomposites developed in this work are promising for flexible dielectrics with excellent heat dissipation.     

Light‐Assisted Charge Propagation in Networks of Organic Semiconductor Crystallites on Hexagonal Boron Nitride

Introducing organic semiconductors as additional building blocks into heterostructures of 2D materials widens the horizon of their applications. Organic molecules can form self‐assembled and self‐aligned crystalline nanostructures on 2D materials, resulting in well‐defined interfaces that preserve the intrinsic properties of both constituents. Thus, organic molecules add unique capabilities to van der Waals heterostructures that have no analogues in inorganic matter. This study explores light‐assisted charge propagation in organic semiconductor networks of quasi‐1D needle‐like crystallites, epitaxially grown on insulating hexagonal boron nitride. Electrostatic force microscopy is employed to demonstrate that upon external illumination it is possible to change the conductivity of organic crystallites by more than two orders of magnitude. Furthermore, by exploiting the highly anisotropic optical properties of the organic nanoneedles, a selective charge propagation along the crystallites is triggered that matches the orientation of the molecular backbones with the incident light's polarization direction. These results demonstrate the possibility to use a “light‐gate” to switch on the conductivity of organic nanostructures and even to guide the charge propagation along desired directions in self‐assembled crystallite networks.     

Formation of a Quasi‐Free‐Standing Single Layer of Graphene and Hexagonal Boron Nitride on Pt(111) by a Single Molecular Precursor

It is shown that on Pt(111) it is possible to prepare hexagonal boron nitride (h‐BN) and graphene (G) in‐plane heterojunctions from a single molecular precursor, by thermal decomposition of dimethylamine borane (DMAB). Photoemission, near‐edge X‐ray absorption spectroscopy, low energy electron microscopy, and temperature programmed desorption measurements indicate that the layer fully covers the Pt(111) surface. Evidence of in‐plane layer continuity and weak interaction with Pt substrate has been established. The findings demonstrate that dehydrogenation and pyrolitic decomposition of DMAB is an efficient and easy method for obtaining a continuous almost freestanding layer mostly made of G, h‐BN with only a low percentage (<3%) of impurities (B and N‐doped G domains or C‐doped h‐BN or boron carbonitride, BCN at the boundaries) in the same 2D sheet on a metal substrate, such as Pt(111), paving the way for the advancement of next‐generation G‐like‐based electronics and novel spintronic devices.     

Growth of Magnetic Yard‐Glass Shaped Boron Nitride Nanotubes with Periodic Iron Nanoparticles

Novel yard‐glass shaped boron nitride nanotubes (YG‐BNNTs) periodically filled with Fe nanoparticles were synthesized by a catalytic reaction process of ammonia with boron precursors at 1300 °C. Such novel structures were extensively characterized using X‐ray diffraction and advanced electron microscopy. The Fe‐filled boron nitride nanotubes show excellent ferromagnetic properties at room temperature with superior chemical stability. A growth model is proposed for the formation of such novel BN nanostructures.     

Coexistence of Grain‐Boundaries‐Assisted Bipolar and Threshold Resistive Switching in Multilayer Hexagonal Boron Nitride

The use of 2D materials to improve the capabilities of electronic devices is a promising strategy that has recently gained much interest in both academia and industry. However, while the research in 2D metallic and semiconducting materials is well established, detailed knowledge and applications of 2D insulators are still scarce. In this paper, the presence of resistive switching (RS) in multilayer hexagonal boron nitride (h‐BN) is studied using different electrode materials, and a family of h‐BN‐based resistive random access memories with tunable capabilities is engineered. The devices show the coexistence of forming free bipolar and threshold‐type RS with low operation voltages down to 0.4 V, high current on/off ratio up to 10⁶, and long retention times above 10 h, as well as low variability. The RS is driven by the grain boundaries (GBs) in the polycrystalline h‐BN stack, which allow the penetration of metallic ions from adjacent electrodes. This reaction can be boosted by the generation of B vacancies, which are more abundant at the GBs. To the best of our knowledge, h‐BN is the first 2D material showing the coexistence of bipolar and threshold RS, which may open the door to additional functionalities and applications.