One-dimensional hexagonal boron nitride conducting channel

Hexagonal boron nitride (hBN) is an insulating two-dimensional (2D) material with a large bandgap. Although known for its interfacing with other 2D materials and structural similarities to graphene, the potential use of hBN in 2D electronics is limited by its insulating nature. Here, we report atomically sharp twin boundaries at AA???/AB stacking boundaries in chemical vapor deposition???synthesized few-layer hBN. We find that the twin boundary is composed of a 6???6??? configuration, showing conducting feature with a zero bandgap. Furthermore, the formation mechanism of the atomically sharp twin boundaries is suggested by an analogy with stacking combinations of AA???/AB based on the observations of extended Klein edges at the layer boundaries of ABstacked hBN. The atomically sharp AA???/AB stacking boundary is promising as an ultimate 1D electron channel embedded in insulating pristine hBN. This study will provide insights into the fabrication of single-hBN electronic devices

Phonon Polaritons in Monolayers of Hexagonal Boron Nitride.

Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antenna-based near-field microscopy, propagating surface phonon polaritons in mono- and bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the free-space photons. Both the polariton dispersion and their wavelength-thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer-thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements

Chemical vapor deposition growth and characterization of two-dimensional hexagonal boron nitride

Atomically thin hexagonal boron nitride (h-BN) film is a highly attractive dielectric and a crucial material for next-generation high performance two-dimensional (2D) heterostructure devices. In this thesis, controllable growth of 2D h-BN films on various substrates using chemical vapor deposition (CVD) is demonstrated. An atmospheric pressure CVD system is developed for h-BN growth using ammonia borane (AB) as precursor and the effects of various critical parameters are systematically investigated. The h-BN film grown on Cu substrate is substrate-position dependent due to gas flow dynamics, which result in an increase in nucleation density as well as domain size downstream along the quartz tube. The effects of other CVD parameters such as growth temperature, growth time and various precursor conditions are further examined. Importantly, a slight increase in the growth temperature of 50 °C (from 1000 to 1050 °C) resulted in a significant increase in average domain size of ~17-fold. This parametric study thus highlights the impact of the crucial parameters to control the growth of h-BN films in terms of domain sizes, film coverage and thickness of the films. The growth of h-BN films are further investigated using a metal-catalyst-free approach directly on amorphous SiO2/Si and quartz substrates. The as-grown films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited surface. Varying thickness of ~2 to 25 nm can be obtained through process control. The crystallite sizes are small of ~25 nm, as determined by Raman spectroscopy, due to the random and uncontrolled nucleation. The absence of transfer process eliminates additional degradation to the film which is detrimental to device performance. In order to increase the size of the single-crystal h-BN domains, highly smoothened electropolished Cu foils are utilized to suppress the amount of nucleation and to enhance lateral growth of the 2D crystals. Large domains with size of up to ~35 µm2 which are hexagonal in shape are observed for the first time. This discovery is in contradiction to many theoretical works which revealed that 2D h-BN domains are mostly stable in the form of triangles because of their asymmetric N- and B-edge energies. Therefore, these hexagonal shaped h-BN domains are extensively characterized to prove its validity. This work verifies that h-BN domains are stable in the form of hexagons and open up new avenues for further theoretical exploration. A “multi-nucleation” approach to obtain mosaic single crystalline h-BN films is further explored. Aligned h-BN domains for over centimeter distances are achieved by using resolidfied Cu substrate with (110) surface orientation and the edge interactions between coalesced domains are investigated. Due to the strict epitaxial relationship between h-BN and Cu lattices, well-defined symmetric multifaceted shapes such as “butterfly” and “6-apex star” are formed by convergence between adjacent triangular or hexagonal shaped domains. Defect lines are generated along the grain boundaries of mirroring h-BN domains due to the two different polarities (BN and NB) and edges with the same termination. The triangular domains with truncated edges and alternatively hexagonal domains are rationalized with Wulff shapes that have minimum edge energy. This work establishes a complete study and reveals essential insights to the various issues on the in-plane coalescence of 2D materials with binary configuration. Lastly, a new single-source precursor, trimethylamine borane (TMAB), is successfully used for the first time to grow monolayer h-BN single crystals as well as few-layer C-doped h-BN (h-BCN) films. As compared to AB, TMAB is a much cheaper alternative making it highly attractive in a manufacturing perspective. Importantly, pristine 2D h-BN films with a wide band gap of ~6.1 eV can be achieved by limiting the sublimation temperature of TMAB at 40 °C, while C dopants are introduced to the h-BN films when the sublimation temperature is further increased. The h-BCN thin films displayed band gap narrowing effects due to substitutional C doping. The chemical structure of the h-BCN films can be perceived as the B atoms are partially substituted by C atoms in an h-BN matrix. This study thus provides new insights into the design and fabrication of large-area atomically thin h-BN/h-BCN films toward practical applications and suggests that other anime borane complexes can be potentially used to synthesize such films as well.Doctor of Philosoph

Growth and characterization of PECVD growth CNTs and CNT networks

Aligned carbon nanotubes (ACNTs) were prepared by Plasma Enhance Chemical Vapor Deposition (PECVD). Acetylene (C2H2) was used as the carbon feedstock gas. The effects of temperature, pressure, processing gas flow rate and growth time on the growth of ACNTs using Nickel and Gold/Nickel as the catalyst on N++, P++, SiO2 and undoped substrates were systematically studied. For the catalysts of ACNTs growth, Nickel with 30nm in thickness and Gold with 50nm on top of Nickel with 30nm thickness were deposited onto the silicon substrates. Growth of ACNTs were monitored using SEM and its quality by Raman spectroscopy. The ACNTs were then used to grow networks which were prepared by PECVD. The samples were inverted and placed onto the stage. The effects of temperature, pressure, processing gas flow rate and growth time on the density of the networks were examined using SEM and analysis of its quality were done by Raman spectroscopy. Surface wettability characterization was done using goniometer to measure the contact angle of the DI water droplet. SEM images and Raman spectra results were use to observe the surface morphology and the quality of the CNTs. Bachelor of Engineerin

High-Performance Microsupercapacitors Based on Two-Dimensional Graphene/Manganese Dioxide/Silver Nanowire Ternary Hybrid Film

Microsupercapacitors (MSCs), as one type of significant power source or energy storage unit in microelectronic devices, have attracted more and more attention. However, how to reasonably design electrode structures and exploit the active materials to endow the MSCs with excellent performances in a limited surface area still remains a challenge. Here, a reduced graphene oxide (RGO)/manganese dioxide (MnO₂)/silver nanowire (AgNW) ternary hybrid film (RGMA ternary hybrid film) is successfully fabricated by a facile vacuum filtration and subsequent thermal reduction, and is used directly as a binder-free electrode for MSCs. Additionally, a flexible, transparent, all-solid state RMGA-MSC is also built, and its electrochemical performance in an ionic liquid gel electrolyte are investigated in depth. Notably, the RGMA-MSCs display superior electrochemical properties, including exceptionally high rate capability (up to 50000 mV·s^–1), high frequency response (very short corresponding time constant τ₀ = 0.14 ms), and excellent cycle stability (90.3% of the initial capacitance after 6000 cycles in ionic liquid gel electrolyte). Importantly, the electrochemical performance of RGMA-MSCs shows a strong dependence on the geometric parameters including the interspace between adjacent fingers and the width of the finger of MSCs. These encouraging results may not only provide important references for the design and fabrication of high-performance MSCs, but also make the RGMA ternary hybrid film promising for the next generation film lithium ion batteries and other energy storage devices

Controllable synthesis of highly luminescent boron nitride quantum dots

Boron nitride quantum dots (BNQDs), as a new member of heavy metal‐free quantum dots, have aroused great interest in fundamental research and practical application due to their unique physical/chemical properties. However, it is still a challenge to controllably synthesize high‐quality BNQDs with high quantum yield (QY), uniform size and strong fluorescent. In this work, BNQDs have been successfully fabricated by the liquid exfoliation and the subsequent solvothermal process with respect to its facileness and easy large scale up. Importantly, BNQDs with high‐quality can be controllably obtained by adjusting the synthetic parameters involved in the solvothermal process including filling factor, synthesis temperature, and duration time. Encouragingly, the as‐prepared BNQDs possess strong blue luminescence with QY as high as 19.5%, which can be attributed to the synergetic effect of size, surface chemistry and edge defects. In addition, this strategy presented here provides a new reference for the controllable synthesis of other heavy metal‐free QDs. Furthermore, the as‐prepared BNQDs are non‐toxic to cells and exhibit nanosecond‐scaled lifetimes, suggesting they have great potential biological and optoelectronic applications.MOE (Min. of Education, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore

Reduced graphene oxide/boron nitride composite film as a novel binder-free anode for lithium ion batteries with enhanced performances

Reduced graphene oxide (rGO)/boron nitride (BN) composite films were successfully fabricated by facile vacuum filtration and subsequent thermal treatment. Their morphology, structure and electrochemical performance were systematically characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. Importantly, the as-prepared rGO/BN composite film with a 2 wt.% BN content as binder-free anode material for lithium ion batteries (LIBs) exhibited a high reversible capacity of 278 mAh·g−1 at a high current density of 100 mA·g−1, high rate capability, and high capacity retention over the first 200 cycles. The enhanced electrochemical performances of rGO/BN composite film are attributed to the unique structure and the synergistic effects between layered BN and graphene, which favored electrolyte penetration and buffered the volume expansion during the lithiation and delithiation process. In addition, this work not only provides a versatile strategy for fabrication of other graphene-based films, but also shows the potential promise of rGO/BN composite film for other energy storage devices.Accepted versio

Probing the Atomic Structures of Synthetic Monolayer and Bilayer Hexagonal Boron Nitride Using Electron Microscopy

Monolayer hexagonal boron nitride (h-BN) is a phenomenal two-dimensional material; most of its physical properties rival those of graphene because of their structural similarities. This intriguing material has thus spurred scientists and researchers to develop novel synthetic methods to attain scalability for enabling its practical utilization. When probing the growth behaviors and structural characteristics of h-BN, the use of appropriate characterization techniques is important. In this review, we detail the use of scanning and transmission electron microscopies to investigate the atomic configurations of monolayer and bilayer h-BN grown via chemical vapor deposition. These advanced microscopy techniques have been demonstrated to provide intimate insights to the atomic structures of h-BN, which can be interpreted directly or indirectly using known growth mechanisms and existing theoretical calculations. This review provides a collective understanding of the structural characteristics and defects of synthetic h-BN films and facilitates a better perspective toward the development of new and improved synthesis techniques.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Published versio

Configurable three-dimensional boron nitride-carbon architecture and its tunable electronic behavior with stable thermal performances

Recent developments of 3D-graphene and 3D-boron-nitride have become of great interest owing to their potential for ultra-light flexible electronics. Here we demonstrate the first synthesis of novel 3D-BNC hybrids. By specifically controlling the compositions of C and BN, new fascinating properties are observed, such as highly tunable electrical conductivity, controllable EMI shielding properties, and stable thermal conductivity. This ultra-light hybrid opens up many new applications such as for electronic packaging and thermal interface materials (TIMs)

A wafer-scale graphene and ferroelectric multilayer for flexible and fast-switched modulation applications

Here we report a wafer-scale graphene/P(VDF-TrFE)/graphene multilayer for light-weight, flexible and fast-switched broadband modulation applications. The P(VDF-TrFE) film not only significantly reduces the sheet resistance of graphene throughout heavy doping of ∼0.8 × 1013 cm−2 by nonvolatile ferroelectric dipoles, but also acts as an efficient electro-optic (EO) layer. Such multilayered structural integration with remarkable ferroelectric polarization, high transparency (>90%), low sheet resistance (∼302 Ω □−1), and excellent mechanic flexibility shows the potential of a flexible modulation application over a broad range of wavelengths. Moreover, the derived device also exhibits strong field-induced EO modulation even under bending and one large Pockels coefficient (∼54.3 pm V−1) is obtained. Finally, the graphene and ferroelectric hybrid demonstrates a fast switching time (∼2 μs) and works well below low sheet resistance level over a long time. This work gives insights into the potential of graphene and ferroelectric hybrid structures, enabling future exploration on next-generation high-performance, flexible transparent electronics and photonics.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio