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Layered MoTe2 has shown great promises for optoelectronics and energy-storage applications due to its exceptional optical and electrochemical properties. To date, considerable efforts have been devoted to fabricating layered MoTe2 with lateral orientation by means of mechanical/chemical exfoliation and chemical vapor deposition (CVD) methods. As compared to its horizontal counterparts, vertically aligned MoTe2 with higher density of active edge sites is expected to possess unique optoelectronic and electrochemical properties, while which has not been reported yet. In this work, we report a versatile and scalable CVD growth of vertically aligned MoTe2 with length of up to ∼ 7.5 µm on Mo foil. Remarkably, the dominant phase of the vertically aligned MoTe2 can be tuned from 2H to 1T’ by increasing the growth temperature from 630 to 780 °C. Owing to the weak interaction between the as-grown MoTe2 and Mo foil, the as-grown MoTe2 can be easily detached from the Mo foil. This in turn enabled economic reuse of the Mo foil for multiple growth. Moreover, the vertical growth of the MoTe2 is proposed to be caused by the internal strain generated during tellurization of Mo foil. Furthermore, the as-grown MoTe2 can also be directly dispersed in solvent to produce high-quality MoTe2 nanosheets. The versatility of this growth strategy was further demonstrated by fabricating other vertically aligned TMDs such as TaTe2 and MoSe2. Hence, this work paves the path towards achieving unique TMDs structures to enable high-performance optoelectronic and electrochemical devices.

Practical implementation of porous carbon-based composite phase-change materials (CPCMs) for heat dissipation in high-power-density electronics is usually limited by liquid leakage issues and unsatisfactory thermal conductivity resulting from their relatively low filler fraction and/or existence of interfacial thermal resistance between fillers. Therefore, development of shape-stable CPCMs with high thermal conductivity and large latent heat to avoid overheating of electronics remains challenging. Herein, graphene foams (GFs) with very high densities of up to 204 mg/cm3have been synthesized to act as interconnected porous networks of CPCMs. Notably, the obtained CPCM with a filler loading of 11.1 wt % preserves a high heat capacity (171.8 J/g) with a retention of 84.8% while showing a 22.6-fold enhancement in the thermal conductivity as compared to pure PCM (10.13 vs 0.43 W/m·K). A higher thermal conductivity of 14.29 W/m·K can be achieved by further increasing the filler loading to 17.7 wt %, which outperforms many of the previously reported CPCMs based on the interconnected porous carbon-based frameworks. Owing to the superior interconnected network structure of the dense GFs and the strong interconnection between them and PCM molecules, these CPCMs also exhibit leakage-proof shape stability and excellent thermal reliability (at least 100 cycles). Moreover, a state-of-the-art aluminum (Al) package based on the CPCM (filler loading: 11.1 wt %) possessing weight 60% less than its pure Al panel counterpart has been demonstrated to verify better heat transfer efficiency and more efficient phonon pathways of the CPCM composite than those of the pure PCM, which holds great promise for advanced thermal management of emerging applications in electronics. The authors would like to acknowledge the financial support from Thales.

The porous interconnected structure of three-dimensional graphene (3DC) combines the excellent thermal conductivity of graphene with an interconnected architecture, thereby creating a thermal network within composites infused with 3DC. In this study, improvements in thermal conductivity, latent heat of fusion (Hf) and shape-stability of paraffin were compared between paraffin phase change materials (PCM) infused with 3DC and with discrete graphene flakes (GP) at the same filler loading to quantify the advantage of the interconnected structure. Paraffin infused with a 3DC of higher bulk density (3DCH) was also compared to identify the effects of increasing filler density. Thermal conductivity of the PCM composites was measured using the hot-disk method and shape-stabilization was compared through thermal cycling in an environment chamber. We found that the interconnected architecture of 3DC improved the properties of the paraffin matrix in multiple ways. 3DC improved the solidification process for paraffin with heterogeneous nucleation, helped to retain the shape of the PCM composite over thermal cycling, reduced void formation within the PCM and induced a large increase in thermal conductivity at 7.4 times and 5.2 times that of neat paraffin for composites infused with 3DCH and regular 3DC respectively, with only a small trade-off in Hf. EDB (Economic Devt. Board, S’pore) Accepted version

Among the different growth mechanisms for two-dimensional (2D) hexagonal boron nitride (h-BN) synthesized using chemical vapor deposition, spiraling growth of h-BN has not been reported. Here we report the formation of intertwined double-spiral few-layer h-BN that is driven by screw dislocations located at the antiphase boundaries of monolayer domains. The microstructure and stacking configurations were studied using a combination of dark-field and atomic resolution transmission electron microscopy. Distinct from other 2D materials with single-spiral structures, the double-spiral structure enables the intertwined h-BN layers to preserve the most stable AA′ stacking configuration. We also found that the occurrence of shear strains at the boundaries of merged spiral islands is dependent on the propagation directions of encountering screw dislocations and presented the strained features by density functional theory calculations and atomic image simulations. This study unveils the double-spiral growth of 2D h-BN multilayers and the creation of a shear strain band at the coalescence boundary of two h-BN spiral clusters. National Research Foundation (NRF) Accepted version This work was supported by IBS-R019-D1, the National Research Foundation (NRF) grant funded by the Korea government (MSIT) (2018R1A2A2A05019598), and the NRF-ANR Joint (NRF2016-NRF-ANR001).

Construction of macroscopic three-dimensional (3D) architectures of graphene is crucial to harness the advantageous properties of planar 2D graphene and to enable integration to many conventional and novel applications. Ideally, the 3D structure of graphene should be free of defects, covalently interconnected and can be produced at large-scale. Among various assembly techniques, fabrication using chemical vapor deposition (CVD) enables the production of high-quality graphene where selection of template is key that determines its consequent crystalline quality and structural morphology. Herein, we present a new method to synthesize high-quality porous graphene film by incorporating an in-situ reduction-oxidation cycling treatment to generate micrometer-sized pores on commercial Ni foil using an all-CVD process route. Owing to the unique morphological features of the modified Ni template, the graphene film exhibits a holey surface with large-area exterior skin coverage of >94% and many interconnected ligaments within its porous interior. This extraordinary configuration give rise to superior in-plane electrical conductivity despite its low density. In comparison to state-of-the-art materials for electromagnetic interference shielding, this porous graphene film is among the best performing materials with a specific shielding effectiveness of >550 dB cm3 g-1 and absolute effectiveness of >220,000 dB cm2 g-1. Ministry of Education (MOE) Nanyang Technological University Submitted/Accepted version

Hexagonal boron nitride (h-BN) is an ideal substrate for two-dimensional (2D) materials because of its unique electrically insulating nature, atomic smoothness and low density of dangling bonds. Although mechanical exfoliation from bulk crystals produces the most pristine flakes, scalable fabrication of devices is still dependent on other more direct synthetic routes. To date, the most utilized method to synthesize large-area h-BN films is by chemical vapor deposition (CVD) using catalytic metal substrates. However, a major drawback for such synthetic films is the manifestation of thermally-induced wrinkles, which severely disrupt the smoothness of the h-BN films. Here, we provide a detailed characterization study of the microstructure of h-BN wrinkles and demonstrate an effective post-synthesis smoothening route by thermal annealing in air. The smoothened h-BN film showed an improved surface smoothness by up to 66% and resulted in a much cleaner surface due to the elimination of polymer residues with no substantial oxidative damage to the film. The unwrinkling effect is attributed to the hydroxylation of the h-BN film as well as the substrate surface, resulting in a reduction in adhesion energy at the interface. Dehydroxylation occurs over time under ambient conditions at room temperature and the smoothened film can be restored back with the intrinsic properties of h-BN. This work provides an efficient route to achieve smoother h-BN films, which are beneficial for high-performance 2D heterostructure devices. NRF (Natl Research Foundation, S’pore) Accepted version

Development of metal-free carbon-based electrocatalysts with high-efficiency and excellent durability towards both oxygen and hydrogen evolution reactions (OER and HER) in a single electrolyte system is crucial yet challenging for sustainable energy generation. In this work, we report a facile and scalable strategy for fabricating self-supporting boron carbon oxynitride nanofibrous (BCNONF) mats with controllable boron contents via electrospinning and subsequent thermal treatment. Notably, the optimal BCNONF mat affords outstanding OER performance in alkaline electrolyte with low overpotential of 403 mV at 10 mA·cm−2, small Tafel slop of 72.9 mV·dec−1, and high stability (88.1% current density retention after 10 h), outperforming the commercial Ir/C benchmark. Moreover, it can serve as a remarkable HER catalyst with better stability than that of the commercial Pt/C counterpart in the same electrolyte, indicating its bifunctional characteristics. When employed as both anode and cathode of an electrolyzer, the self-supporting BCNONF mats exhibit superior activities with a potential of only 1.79 V at 10 mA·cm−2 and high long-term durability (90.6% current density retention after 50 h) for overall water splitting. Furthermore, density functional theory (DFT) calculations reveal that the remarkable OER and HER bifunctional performance of the BCNONF catalyst are originated from the reduced adsorption strength of O atom and the stronger H* adsorption on the BCNO surface as compared to those on CNO surface, which in turn facilitate efficient interfacial charge transfer between the electrocatalytic intermediates and the BCNONF catalyst.

A series of Pb(Zr1-xTix)O3 multilayer films alternatively stacked by Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.35Ti0.65)O3 layers have been deposited on corning glass by magnetron sputtering. The films demonstrate pure perovskite structure and good crystallinity. A large tetragonality (c/a) of ∼1.061 and a shift of ∼0.08 eV for optical bandgap were investigated at layer engineered films. In addition, these samples exhibited a wild tunable electro-optic behavior from tens to ∼250.2 pm/V, as well as fast switching time of down to a few microseconds. The giant EO coefficient was attribute the strain-polarization coupling effect and also comparable to that of epitaxial (001) single crystal PZT thin films. The combination of high transparency, large EO effect, fast switching time, and huge phase transition temperature in PZT-based thin films show the potential on electro-optics from laser to information telecommunication. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

In this work, we report for the first time a simple approach to fabricate 3D reduced graphene oxide/boron nitride (rGO/BN) sponges with no additional chemical cross-linkers. Encouragingly, such sponge possesses higher compressibility and better recoverability in both air and organic solvents as compared to the bare rGO sponge. The as-prepared rGO/BN sponge also exhibits excellent water resistance and high oil absorption capability, achieving up to 170 times its own weight toward a wide range of environmental contaminants. Especially, the absorbed oil can be easily removed by the absorption–squeezing process which is cost effective and environmental friendly for oil collection. Most importantly, the underlying reasons for the remarkable mechanical properties, excellent oil absorption capabilities have been analysed and revealed in depth. These exceptional characteristics together with the ease of scalable synthesis make the as-prepared 3D rGO/BN sponge show many promising applications ranging from tissue engineering, sensors, catalysis, energy storage and conversion to environmental remediation. MOE (Min. of Education, S’pore)

We demonstrate the potential of a novel hybrid nanostructure three-dimensional graphene (3D-C) coated with boron nitride (BN) as an electromagnetic interference (EMI) shield. BN, deposited by sputtering, encapsulates 3D-C to form a light-weight graphene based EM shield that has an electrically insulating exterior. The BN deposited on 3D-C was homogeneously distributed with even coverage on the struts of 3D-C. EMI shielding results have shown that the hybrid material has a total shielding effectiveness (SET) of 53dB and 58dB in the X band and Ku band respectively. EDB (Economic Devt. Board, S’pore) Accepted version

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.

Hexagonal boron nitride (hBN), which is a 2D layered dielectric material, sometimes referred as “white graphene” due to its structural similarity with graphene, has attracted much attention due to its fascinating physical properties. Here, for the first time the use of chemical vapor deposition -grown hBN films to fabricate ultrathin (≈3 nm) flexible hBN-based resistive switching memory device is reported, and the switching mechanism through conductive atomic force microscopy and ex situ transmission electron microscopy is studied. The hBN-based resistive memory exhibits reproducible switching endurance, long retention time, and the capability to operate under extreme bending conditions. Contrary to the conventional electrochemical metallization theory, the conductive filament is found to commence its growth from the anode to cathode. This work provides an important step for broadening and deepening the understanding on the switching mechanism in filament-based resistive memories and propels the 2D material application in the resistive memory in future computing systems.

Vertically aligned carbon nanotube (CNT) arrays have aroused considerable interest because of their remarkable mechanical properties. However, the mechanical behaviour of as-synthesized CNT arrays could vary drastically at a macro-scale depending on their morphologies, dimensions and array density, which are determined by the synthesis method. Here, we demonstrate a coaxial carbon@boron nitride nanotube (C@BNNT) array with enhanced compressive strength and shape recoverability. CNT arrays are grown using a commercially available thermal chemical vapor deposition (TCVD) technique and an outer BNNT with a wall thickness up to 1.37 nm is introduced by a post-growth TCVD treatment. Importantly, compared to the as-grown CNT arrays which deform almost plastically upon compression, the coaxial C@BNNT arrays exhibit an impressive ∼4-fold increase in compressive strength with nearly full recovery after the first compression cycle at a 50% strain (76% recovery maintained after 10 cycles), as well as a significantly high and persistent energy dissipation ratio (∼60% at a 50% strain after 100 cycles), attributed to the synergistic effect between the CNT and outer BNNT. Additionally, the as-prepared C@BNNT arrays show an improved structural stability in air at elevated temperatures, attributing to the outstanding thermal stability of the outer BNNT. This work provides new insights into tailoring the mechanical and thermal behaviours of arbitrary CNT arrays which enables a broader range of applications. MOE (Min. of Education, S’pore) Published version

Paper-based flexible supercapacitors (SCs) have attracted great attention as they enable the realization of next-generation bendable, light-weight, and environmentally-friendly portable electronics. However, conventional paper-based SCs adopt a sandwich-like structure suffering from poor rate performance, slow frequency response and difficulty in direct integration with other micro-devices. We report here for the first time paper-based all-solid-state flexible planar micro-supercapacitors (MSCs) using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-CNT/Ag as the electrode material by the inkjet printing technique. The as-fabricated paper-based all-solid-state flexible MSCs deliver the best rate capability among all reported paper-based MSCs/SCs (up to 10 000 mV s−1), fast frequency response (relaxation time constant τ0 = 8.5 ms), high volumetric specific capacitance (23.6 F cm−3) and long cycle stability (92% capacitance retention after 10 000 cycles), which shows a strong dependence on the film thickness and the interdigitated spacing between neighbouring fingers, respectively. Furthermore, the series and parallel connections reveal that the as-prepared paper-based MSCs obey the basic theorem of series and parallel connections of capacitors, respectively. The combination of the simple fabrication technology and excellent performances presented here not only make paper-based all-solid-state flexible MSCs an attractive candidate for powering future flexible portable electronics, but also provide important references for the design and fabrication of other high-performance flexible energy storage devices.

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 version

Vertically aligned carbon nanotube (CNT) arrays have been recognized as promising cushion materials because of their superior thermal stability, remarkable compressibility, and viscoelastic characteristics. However, most of the previously reported CNT arrays still suffer from permanent shape deformation at only moderate compressive strains, which considerably restricts their practical applications. Here, we demonstrate a facile strategy of fabricating supercompressible coaxial CNT@graphene (CNT@Gr) arrays by using a two-step route involving encapsulating polymer layers onto plastic CNT arrays and subsequent annealing processes. Notably, the resulting CNT@Gr arrays are able to almost completely recover from compression at a strain of up to 80% and retain ∼80% recovery even after 1000 compression cycles at a 60% strain, demonstrating their excellent compressibility. Furthermore, they possess outstanding strain- and frequency-dependent viscoelastic responses, with storage modulus and damping ratio of up to ∼6.5 MPa and ∼0.19, respectively, which are nearly constant over an exceptionally broad temperature range of -100 to 500 °C in ambient air. These supercompressibility and temperature-invariant viscoelasticity together with facile fabrication process of the CNT@Gr arrays enable their promising multifunctional applications such as energy absorbers, mechanical sensors, and heat exchangers, even in extreme environments.

Although two-dimensional boron (B) has attracted much attention in electronics and optoelectronics due to its unique physical and chemical properties, in-depth investigations and applications have been limited by the current synthesis techniques. Herein, we demonstrate that high-quality few-layer B sheets can be prepared in large quantities by sonication-assisted liquid-phase exfoliation. By simply varying the exfoliating solvent types and centrifugation speeds, the lateral size and thickness of the exfoliated B sheets can be controllably tuned. Additionally, the exfoliated few-layer B sheets exhibit excellent stability and outstanding dispersion in organic solvents without aggregates for more than 50 days under ambient conditions, owing to the presence of a solvent residue shell on the B sheet surface that provides excellent protection against air oxidation. Moreover, we also demonstrate the use of the exfoliated few-layer B sheets for high-performance supercapacitor electrode materials. This as-prepared device exhibits impressive electrochemical performance with a wide potential window of up to 3.0 V, excellent energy density as high as 46.1 Wh/kg at a power density of 478.5 W/kg, and excellent cycling stability with 88.7% retention of the initial specific capacitance after 6000 cycles. This current work not only demonstrates an effective strategy for the synthesis of the few-layer B sheets in a controlled manner but also makes the resulting materials promising for next-generation optoelectronics and energy storage applications. MOE (Min. of Education, S’pore) Accepted version

In conclusion, atomically thin 2D h-BN films are controllably grown on a variety of substrates, including Cu, quartz and amorphous SiO2 by CVD. Critical CVD parameters such as substrate position, growth temperature, growth time and precursor conditions affect the nucleation density and the lateral size of the h-BN domains. The growth of large h-BN domains is also particularly sensitive towards the surface morphology of the Cu substrate. A highly smoothened Cu surface would yield larger domains by suppressing nucleation. This thesis also reports on the discovery of the first hexagonal shaped h-BN domains which suggests that the formation of alternating N- and B-terminating edges or even armchair edges are stable. The triangles with truncated edges and hexagonal domains are rationalized with Wulff construction that has the minimum edge energy. This experimental evidence came as a breakthrough for many theoretical work which suggested that the single-crystal h-BN domains is only stable in the form of triangles.

Motivated by industrialization and the need for manufacturability, a variety of synthesis techniques to achieve atomically thin h-BN films over large distances have been explored such as chemical vapor deposition (CVD) [1, 2, 3, 4, 5, 6], surface segregation method or solid source diffusion [7, 8, 9, 10], ion-beam sputtering deposition (IBSD) [11], pulsed-laser deposition (PLD) [12, 13], reactive magnetron sputtering [14] and molecular beam epitaxy (MBE) [15].

Heteroatom doping in graphene leads to bandgap opening and tunable electronic, magnetic and optical properties, which are important for graphene-based electronics applications. In recent years, scalable growth of nitrogen-doped graphene (NG) by chemical vapor deposition (CVD) has been extensively studied because of its potential for practical applications. A phenomenon that occurs exclusively for CVD-grown NG films is the segregation of doping concentration. However, most studies to date are conducted using highly polycrystalline NG films comprising small grain sizes. It is still unknown whether dopant segregation occurs in single crystalline NG domains. Here, we used hexamethylenetetramine ((CH2)6N4) as a single-source solid precursor to grow hexagonal-shaped monolayer NG single crystals of ∼20 µm on Cu substrates. The NG single crystals exhibit discrete concentric hexagonal rings comprising N depleted regions as determined by Raman spectroscopy. Supported by scanning tunneling microscopy experiments, we propose that the segregation of N dopants is caused by a competing N attachment mechanism to either zigzag or Klein edges during growth; where the former should result in higher N concentration and the latter with lower N concentration. This work provides critical insights into the growth mechanism of CVD-grown NG and enables new opportunities to engineer the properties of graphene by fabrication of lateral heterostructures. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Boron nitride (BN) is a synthetic material that exists in many phases isostructural to the polymorphs of carbon, such as amorphous (a), hexagonal (h), wurtzite (w), cubic (c) and turbostratic (t). In particular, hexagonal boron nitride (h-BN), which is also known as “white graphite”, is considered as the most stable among the BN polymorphs, consisting of alternating boron (B) and nitrogen (N) atoms, covalently bonded in a honeycomb structure with a lattice constant of 2.504 A Pease (Nature 165(4201):722–723, 1950) [1].

Although h-BN films can be grown directly on dielectric substrates [1, 2], their quality are degraded due to the lack of epitaxial relation and uncontrolled nucleation resulting in the formation of nano-sized crystals. To obtain large-area high quality h-BN single crystals, metal catalytic substrates such as Cu and Ni are still required. It is observed that the h-BN domains tend to nucleate in the forms of triangular [3, 4, 5, 6] or asymmetric diamond shape [4].

Achieving high quality large-area single-crystal h-BN monolayer and few-layer films are important to improve the electronic (Li et al. in Nano Lett 15(9):5804–5810, 2015, [1]), mechanical (Becton and Wang in Phys Chem Chem Phys 17(34), 21894–21901, 2015, [2]), and thermal (Mortazavi et al. in Sci Rep 5, 13228, 2015, [3]) properties of the heterostructure 2D devices. To date, centimeter-scale synthesis of polycrystalline ultrathin continuous h-BN films by CVD on various transition metals have been attempted.

Recently, high quality large-area mono- to few-layer 2D h-BN films were synthesized on a variety of transition metals such as Cu, Ni and Pt substrates by CVD using AB as the precursor (Song et al. in Nano Lett 10(8):3209–3215, 2010 [1], Shi et al. in Nano Lett 10(10):4134–4139, 2010 [2], Kim et al. in Nano Lett 13(4):1834–1839, 2013 [3], Kim et al. in Nano Lett 12(1), 161–166, 2012 [4]). This CVD approach thus opens up a potentially industry acceptable technique as it adopts a similar paradigm to large-area thin film deposition methodology.

Three-dimensional (3D) graphene architectures with well-controlled structure and excellent physiochemical properties have attracted considerable interest due to their potential applications in flexible electronic devices. However, the majority of the existing 3D graphene still encounters several drawbacks such as brittleness, non-uniform building units, and limited scale (millimeter or even micrometer), which severely limits its practical applications. Herein, we demonstrate a new scalable technique for the preparation of thin-layer graphite foam (GF) with controllable densities (27.2-69.2 mg cm-3) by carbonization of polyacrylonitrile using a template-directed thermal annealing approach. By integrating the GF with poly(dimethylsiloxane) (PDMS), macroscopic porous GF@PDMS with variable thin-layer GF contents ranging from 15.9 to 31.7% was further fabricated. Owing to the robust interconnected porous network of the GF and the synergistic effect between GF and PDMS, GF@PDMS with a 15.9% thin-layer GF content exhibited an impressive 254% increase in compressive strength over the bare GF. In addition, such 15.9% GF@PDMS can totally recover after the first compression cycle at a 95% strain and maintain ∼88% recovery even after 1000 compression cycles at an 80% strain, demonstrating its superior compressibility. Moreover, all of the as-prepared GF@PDMS samples possessed high electrical conductivity (up to 34.3 S m-1), relatively low thermal conductivity (0.062-0.076 W m-1 K-1), and excellent electromagnetic interference shielding effectiveness (up to 36.1 dB) over a broad frequency range of 8.2-18 GHz, indicating their great potential as promising candidates for high-performance electromagnetic wave absorption in flexible electronic devices. Accepted version

Spiral growth of various nanomaterials including some two-dimensional (2D) transition metal dichalcogenides had recently been experimentally realized using chemical vapor deposition (CVD). However, such growth that is driven by screw dislocation remained elusive for graphene and is rarely discussed because of the use of metal catalysts. In this work, we show that formation of few-layer graphene (FLG) with a spiral structure driven by screw dislocation can be obtained alongside FLG having a concentric layered structure formed by interfacial nucleation (nucleation at the graphene/Cu interface) using Cu-catalyzed ambient pressure CVD. Unlike commonly reported FLG grown by interfacial nucleation where the second layer is grown independently beneath the first, the growth of a spiral structure adopts a top growth mechanism where the top layers are an extension from the initial monolayer which spirals around an axial dislocation in self-perpetuating steps. Since the same atomic orientation is preserved, the subsequent spiraling layers are stacked in an oriented AB-stacked configuration. This contrasts with FLG formed by interfacial nucleation where turbostratic stacking of the entire adlayer may exist. In both growth scenarios, the second layer (either top or bottom) can grow across the grain boundaries of the initial monolayer domains, forming partial regions with turbostratic stacking configuration due to weak interlayer van der Waals interactions. The unique interlayer coupling of FLG spirals, which enable superior conductivity along the normal of the 2D crystal with spiraling trajectories, are expected to have new and interesting nanoscale applications. NRF (Natl Research Foundation, S’pore) Accepted version

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.

We engineered the localized color centers in two-dimensional hexagonal boron nitrides (hBN) monolayers using laser writing. A 4 nm narrow linewidth, well polarized and single photon emission with 3–4 ns short lifetime was observed in the vicinity of the irradiated region.

We demonstrate the thermal conductivity enhancement of the vertically aligned carbon nanotube (CNT) arrays (from ∼15.5 to 29.5 W/mK, ∼90% increase) by encapsulating outer boron nitride nanotube (BNNT, 0.97 nm-thick with ∼3–4 walls). The heat transfer enhancement mechanism of the coaxial C@BNNT was further revealed by molecular dynamics simulations. Because of their highly coherent lattice structures, the outer BNNT serves as additional heat conducting path without impairing the thermal conductance of inner CNT. This work provides deep insights into tailoring the heat transfer of arbitrary CNT arrays and will enable their broader applications as thermal interface material. MOE (Min. of Education, S’pore)

Poly(vinyl alcohol) (PVA) hydrogels with tissue-like viscoelasticity, excellent biocompatibility, and high hydrophilicity have been considered as promising cartilage replacement materials. However, lack of sufficient mechanical properties is a critical barrier to their use as load-bearing cartilage substitutes. Herein, we report hydroxylated boron nitride nanosheets (OH-BNNS)/PVA interpenetrating hydrogels by cyclically freezing/thawing the aqueous mixture of PVA and highly hydrophilic OH-BNNS (up to 0.6 mg/mL, two times the highest reported so far). Encouragingly, the resulting OH-BNNS/PVA hydrogels exhibit controllable reinforcements in both mechanical and thermal responses by simply varying the OH-BNNS contents. Impressive 45, 43, and 63% increases in compressive, tensile strengths and Young’s modulus, respectively, can be obtained even with only 0.12 wt% (OH-BNNS:PVA) OH-BNNS addition. Meanwhile, exciting improvements in the thermal diffusivity (15%) and conductivity (5%) can also be successfully achieved. These enhancements are attributed to the synergistic effect of intrinsic superior properties of the as-prepared OH-BNNS and strong hydrogen bonding interactions between the OH-BNNS and PVA chains. In addition, excellent cytocompatibility of the composite hydrogels was verified by cell proliferation and live/dead viability assays. These biocompatible OH-BNNS/PVA hydrogels are promising in addressing the mechanical failure and locally overheating issues as cartilage substitutes and may also have broad utility for biomedical applications, such as drug delivery, tissue engineering, biosensors, and actuators. MOE (Min. of Education, S’pore)

Electronics with multifunctionalities such as transparency, portability, and flexibility are anticipated for future circuitry development. Flexible memory is one of the indispensable elements in a hybrid electronic integrated circuit as the information storage device. Herein, we demonstrate a transparent, flexible, and transferable hexagonal boron nitride (hBN)-based resistive switching memory with indium tin oxide (ITO) and graphene electrodes on soft polydimethylsiloxane (PDMS) substrate. The ITO/hBN/graphene/PDMS memory device not only exhibits excellent performance in terms of optical transmittance (∼85% in the visible wavelength), ON/OFF ratio (∼480), retention time (∼5 × 104 s) but also shows robust flexibility under bending conditions and stable operation on arbitrary substrates. More importantly, direct observation of indium filaments in an ITO/hBN/graphene device is found via ex situ transmission electron microscopy, which provides critical insight on the complex resistive switching mechanisms.

Controlling the composition and structure of boron carbonitride nanotubes (BCNNTs) is the critical factor for tuning their electrical and optical properties, which in turn allows for the broadening of their applications. However, most of the known methods for synthesizing BCNNTs employ toxic precursors at high temperatures (up to 2000 °C) and the achieved BCNNTs usually encounter phase-separation of BN and C. Herein, a facile large-scale synthesis of ternary BCNNTs with controllable composition by a chemical vapor deposition process at a relatively low temperature of 900 °C is reported. By simply adjusting the synthetic parameters, BCNNTs with two different atomic ratios of B : C : N can be successfully synthesized. Their morphologies and ternary structure as well as optical properties are further investigated. Notably, the as-prepared BNCNTs-50 are stable up to 900 °C in air and exhibit an optical band gap of ∼4.36 eV. The results demonstrated in this study will open new avenues for a variety of potential applications of BCNNTs in electronics, photonics, sensors and high temperature lubricants. MOE (Min. of Education, S’pore)

Wafer‐scale vertically aligned carbon nanotube/poly(vinyl alcohol) (VACNT/PVA) hydrogels were fabricated by infiltration of PVA among VACNTs and subsequent in situ hydrogelation. Owing to the homogeneous PVA infiltration, uniform distribution and orientation of the CNTs are well retained, which in turn provide large CNT‐PVA interfaces throughout the resulting hydrogels. Compared to bare PVA hydrogels, such composite hydrogels perform significantly enhanced longitudinal compressive responses upon both static and dynamic loadings, achieving improvements of up to 25.2‐ and 9.0‐fold in strength and loss modulus, respectively. The novel scalable fabrication of VACNT/PVA hydrogels and their attractive performances hold great potential for a wide range of applications. MOE (Min. of Education, S’pore) Accepted version

Due to their exceptional chemical and thermal stabilities as well as electrically insulating property, atomically thin hexagonal boron nitride (h-BN) films have been identified as a promising class of dielectric substrate and encapsulation material for high-performance two-dimensional (2D) heterostructure devices. Herein, we report a facile chemical vapor deposition synthesis of large-area atomically thin h-BN including monolayer single crystals and C-doped h-BN (h-BCN) films utilizing a relatively low-cost, commercially available trimethylamine borane (TMAB) as a single-source precursor. 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 as identified by an additional shoulder at 205 nm observed in their absorbance spectra. Presence of N–C bonds in the h-BCN structures with a doping concentration of ∼2 to 5% is confirmed by X-ray photoelectron spectroscopy. The inclusion of low C doping in the h-BN films is expected to result in constructive enhancement to its mechanical properties without significant alteration to its electrically insulating nature. This study provides new insights into the design and fabrication of large-area atomically thin h-BN/h-BCN films toward practical applications and suggests that the range of precursors can be potentially extended to other anime borane complexes as well. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore)

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 version

There is an increasing amount of research interest in synthesizing boron nitride nanotubes (BNNTs) as well as BN coatings to be used for various applications due to their outstanding mechanical, electrical, and thermal properties. However, vertically aligned (VA) BNNTs are difficult to synthesize and the longest VA-BNNTs achieved to date are up to several tens of microns. Here, we report the synthesis of over millimeters long multiwalled BN coated carbon nanotubes (BN/CNT) and BNNT forests via a facile and effective two-step route involving template-assisted chemical vapor deposition at a relatively low temperature of 900 °C and a subsequent annealing process. The as-prepared BN/CNTs and BNNTs retain the highly ordered vertically aligned structures of the CNT templates as identified by scanning electron microscopy. The structure and composition of the resulting products were studied using transmission electron microscopy, electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. This versatile BN coating technique and the synthesis of millimeter-scale BN/CNTs and BNNTs pave the way for new applications especially where the aligned geometry of the NTs is essential such as for field-emission, interconnects, and thermal management. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore)

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)

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material for photonics device due to its large bandgap and flexibility in nanophotonic circuits. Here, we report bright and localized luminescent centres can be engineered in hBN monolayers and flakes using laser irradiation. The transition from hBN to cBN emerges in laser irradiated hBN large monolayers while is absent in processed hBN flakes. Remarkably, the colour centres in hBN flakes exhibit room temperature cleaner single photon emissions with g 2(0) ranging from 0.20 to 0.42, a narrower line width of 1.4 nm and higher brightness compared with monolayers. Our results pave the way to engineering deterministic defects in hBN induced by laser pulse and show great prospect for application of defects in hBN used as nano-size light source in photonics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

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Tuning of band gap of hexagonal boron nitride (h-BN) has been a challenging problem due to its inherent chemical stability and inertness. In this work, we report the changes in band gaps in a few layers of chemical vapor deposition processed as-grown h-BN using a simple oxygen plasma treatment. Optical absorption spectra show a trend of band gap narrowing monotonically from 6 eV of pristine h-BN to 4.31 eV when exposed to oxygen plasma for 12 s. The narrowing of band gap causes the reduction in electrical resistance by ∼100 fold. The x-ray photoelectron spectroscopy results of plasma treated hexagonal boron nitride surface show the predominant doping of oxygen for the nitrogen vacancy. Energy sub-band formations inside the band gap of h-BN, due to the incorporation of oxygen dopants, cause a red shift in absorption edge corresponding to the band gap narrowing. Published version

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).

Atomically thin hexagonal-boron nitride (h-BN) films are primarily synthesized through chemical vapor deposition (CVD) on various catalytic transition metal substrates. In this work, a single-step metal-catalyst-free approach to obtain few- to multi-layer nanocrystalline h-BN (NCBN) directly on amorphous SiO2/Si and quartz substrates is demonstrated. The as-grown thin films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited substrate as inspected using optical and atomic force microscopy. The starting layers of NCBN orient itself parallel to the substrate, initiating the growth of the textured thin film. Formation of NCBN is due to the random and uncontrolled nucleation of h-BN on the dielectric substrate surface with no epitaxial relation, unlike on metal surfaces. The crystallite size is ∼25 nm as determined by Raman spectroscopy. Transmission electron microscopy shows that the NCBN formed sheets of multi-stacked layers with controllable thickness from ∼2 to 25 nm. The absence of transfer process in this technique avoids any additional degradation, such as wrinkles, tears or folding and residues on the film which are detrimental to device performance. This work provides a wider perspective of CVD-grown h-BN and presents a viable route towards large-scale manufacturing of h-BN substrates and for coating applications. Published version

The growth of hexagonal boron nitride (h-BN) is of much interest owing to its outstanding properties and for scalable two dimensional (2D) electronics applications. Here, we report the controllable growth of h-BN on a copper substrate using the atmospheric pressure chemical vapor deposition (APCVD) method using ammonia borane as the precursor. The advantages of using APCVD include its ease of setup utilizing fewer resources, low cost and fast growth, all of which are essential for full film coverage and the mass production of 2D h-BN. In this study, we observed a substrate-position dependent evolution of h-BN domains at various stages of growth as the density and size of the domains increased downstream along the quartz tube. Other critical parameters such as growth temperature, deposition time, temperature and mass of precursor were also systemically investigated in order to understand the factors influencing the growth of the h-BN film. Importantly, with a slight increase in the growth temperature of 50 °C, we observe a significant (∼17-fold) increase in the average domain size, and its further expansion for a longer duration of growth. Likewise, our parametric study highlights the impact of other crucial parameters on domain size, coverage, and thickness of the h-BN film.

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 version