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Summary Physical unclonable function (PUF) is a cornerstone of anticounterfeiting. However, conventional PUF key-based secure tags encounter several bottlenecks, such as complicated manufacturing, specialized and tedious readout, long authentication time, and insufficient stability. Here, we utilize various two-dimensional materials (2DMs), including Ti3C2Tx MXene and graphene oxide, to construct multigenerational microstructures as PUF patterns. Two intermediate treatments, cation intercalation and moisture-induced lubrication, are introduced in between sequential contractions to engineer the multiscale patterns in a transfer-free and scalable fashion. A deep learning (DL)-facilitated software is developed to pre-categorize the hierarchical topographies with classifiable features. Thereafter, the search-and-compare is conducted within a smaller database to shorten the overall authentication time. The synergy between 2DM tags and DL-facilitated software enables a reliable and environmentally stable anticounterfeiting technology, DeepKey, showing superior encoding capacity (>10144,494) and short authentication time (∼3.5 min). Our 2DM anticounterfeiting tag is finally integrated with QR codes to provide two-layer information security.

In the past decade, a lot of research has been conducted on the potential of carbon nanostructured materials to emit sound via thermoacoustics through both simulations and experiments. However, experimental validation of simulations for three-dimensional graphene (3D-C), which has a complicated 3D structure, has yet to be achieved. In this paper, 3D-C is synthesized via thermal chemical vapor deposition and its microstructure and quality tested using Scanning Electron Microscopy and Raman spectroscopy respectively. Then, a two temperature model is used to predict the effects of numerous parameters: frequency, input power, sample size, connection area, connection path, pores per inch, thickness, compression as well as the addition of a backing on the acoustic performance and temperature of the sample. The experimental results presented in this paper validate the predictions of the adopted two temperature model. The efficiency of 3D-C is then compared with results presented in other studies to understand how the presented 3D-C fared against ones from the literature as well as other carbon nanostructured materials. Accepted version This study is supported in part by the Economic Development Board of Singapore under the Industrial Postgraduate Program (EDB-IPP) with Thales Solutions Asia Pte Ltd and Temasek Laboratories@Nanyang Technological University seed funding. It is also supported by the French DGA (Direction Générale de l'Armement) under the Conventions Industrielles de Formation par la REcherche Program (CIFRE) with Thales Underwater Systems. This study is developed in collaboration with CINTRA, UMI 3288 CNRS/NTU/THALES (Singapore).

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.

Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra-thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide-angle range. Here, a tunable perfect absorption from normal incidence to a wide-angle range (0° to 70°) by utilizing a two-layer stack consisting of a high refractive index low-loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low-loss phase change material such as stibnite (Sb2 S3 ) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb2 S3 from amorphous to crystalline. Agency for Science, Technology and Research (A*STAR) P.P. and R.S. acknowledge the funding support from Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) by Agency for Science, Technology, and Research (A*STAR). K.V.S. and J.T. acknowledge A*STAR for funding support in Grant Nos. H19H6a0025, A20E5c0084, A2083c0058 and CRF SC25/21-110318.

High performance tuneable dielectrics at millimetre-wave frequencies are crucial constituents for emerging adaptive and reconfigurable electronic applications in the automotive, artificial intelligence, and telecommunication industries. Hexagonal boron nitride (h-BN), an ideal candidate for gate-insulating dielectrics, is attractive for integrated circuits and photonic devices. However, advanced application to electronic and optoelectronic devices has often been limited by synthesis techniques and flake size, as well as dielectric reliability. Herein, we have studied the isotope engineering of h-BN thin films directly grown on wafer-scale Si and GaN substrates with pure boron isotopes (B10 and B11) in comparison with controlled isotopic compositions. The dielectric characteristics of isotope-enriched h-BN films at frequencies ranging up to 107 Hz were investigated, exhibiting a broad dielectric dispersion with a low dielectric loss, below 1.3%. Furthermore, their optical band gap energies indicate a strong dependence on isotopic composition, ranging from 5.54 to 5.79 eV. Thermal conductivity of pure B10N and B11N over a broad temperature range is superior to those of other compositions, with an enhancement of around 231%. Therefore, the great thermal response combined with excellent dielectric properties and a wide band gap make h-BN a promising dielectric material for heat self-dissipating GaN and AlGaN /GaN transistors. Hall mobility, sheet resistivity and sheet concentration of GaN with B10N films were analyzed, ascertaining that h-BN does function well as both a dielectric layer and a passivating layer on electronic devices. Our findings could lead to microelectronics thermal management and integrated optoelectronic applications at these frequencies. National Research Foundation (NRF) Accepted version The authors would like to acknowledge the funding support from National Research Foundation, Singapore under its NRF-ANR programme (Grant award number: NRF2016-NRF-ANR001).

Electromagnetic wave absorbers are essential devices in imaging, wireless communication and energy harvesting systems. In this paper, we propose and experimentally demonstrate a flexible and ultra-broadband terahertz (THz) wave absorber based on graphene–vertically aligned carbon nanotube (G-VACNT) hybrids. The THz wave absorber consists of Cu/PDMS/graphene/VACNT functional layers on a PET substrate. The measured results show a 100% qualified operating bandwidth (i.e., absorptance >0.9) and an average power absorptance of 0.986 within the 0.2–3.0 THz range. The absorber also exhibits good absorption performances for a wide range of incident angles up to 60°, and can function normally in different bending states due to its excellent flexibility. The incoming energy of THz waves absorbed by the G-VACNT hybrids results in a temperature increase whose spatial distribution is corresponding to the profile of the incident THz beam, providing an efficient and low cost approach for THz beam profiling, collimation and focusing. This work paves the way for the development of large-scale and broadband THz wave absorbers.

Materials with high and broadband absorption characteristics in the terahertz (THz) range are desirable for many applications. In this paper, we propose, fabricate and experimentally demonstrated a wafer-scale vertically aligned carbon nanotube (VACNT) array for broadband THz wave absorption. The effects of VACNT parameters on the absorption performance are investigated within the THz and infrared spectra using the Maxwell-Garnett theory, revealing that the absorption in the THz range can be greatly enhanced by suitable selections of the length, volume fraction and vertical alignment factor of CNTs. A VACNT array with an average CNT length of ∼600 μm is fabricated on a 4-inch silicon substrate. Experimental results measured by a THz time-domain spectroscopic system show an average power absorptance of ∼98% from 0.3 to 2.5 THz, and agree well with the numerical modelling. This device can be used as a cost-effective near-perfect absorber across the THz and infrared regions for thermal emission and imaging, electromagnetic interference shielding, stealth and energy harvesting applications. This work was partially supported by the National Key R&D Program of China (Grant No. 2018YFC0603301), the National Natural Science Foundation of China (Grant No. 61801185), and HUST Key Innovation Team Foundation for Interdisciplinary Promotion (Grant No. 2016JCTD102). We thank Kejia Wang and Yue Song at the Wuhan National Laboratory for Optoelectronics at HUST for their assistance in THz-TDS measurements.

Ultra-wideband absorbers have been extensively used in wireless communications, energy harvesting, and stealth applications. Herein, with the combination of experimental and theoretical analyses, we develop a flexible ultra-wideband terahertz absorber based on vertically aligned carbon nanotubes (VACNTs). Measured results show that the proposed absorber is able to work efficiently within the entire THz region (e.g., 0.1–3.0 THz), with an average power absorptance of >98% at normal incidence. The absorption performance remains at a similar level over a wide range of incident angle up to 60°. More importantly, our devices can function normally, even after being bent up to 90° or after 300 bending cycles. The total thickness of the device is about 360 μm, which is only 1/8 of the wavelength for the lowest evaluated frequency of 0.1 THz. The new insight into the VACNT materials paves the way for applications such as radar cross-section reduction, electromagnetic interference shielding, and flexible sensing because of the simplicity, flexibility, ultra-wideband operation, and large-scale fabrication of the device. Accepted version This work was partially supported by the National Key R&D Program of China (grant no. 2018YFC0603301), and the National Natural Science Foundation of China (grant nos. 61801185, 51902112).

International audience; Phase Change Material (PCM) have been widely used for thermal energy storage due to their high latent heat of fusion. Several types of PCM are available and can be selected to meet the required melting temperature and latent heat value. However their low thermal conductivity drastically reduces their performances. Without thermal conductivity enhancement, melting would mainly occur at the interface between the heated surface and the PCM, and would slowly spread in the bulk of the PCM. In that sens the use of fillers is necessary. We present here a study on the use of metallic lattice structure as PCM enhancers. In this paper we illustrate the use of a Effective Thermal Conductivity (ETC) model for four different lattice structures to optimize a PCM based device for the thermal management of power electronics. The optimization is done in terms of device overall dimensions (especially its height) but also in term of composite (or enhanced PCM) enclosure. Actually the composite needs to be enclosed to avoid leaking but the enclosure also serves a heat spreading role which is studied here.

International audience

International audience; Electroacoustic transducers along with piezoelectric devices are the most widely used methods for acoustic sound generation in gas and liquids. A mechanical movement of a membrane induces fluid vibration thus creating an acoustic wave. The thermoacoustic process on the other hand uses fast paces temperature variations in a sample to excite the fluid (generally air). The rapidly changing temperature generate a compression expansion of the air and thus creates an acoustical wave. Such materials are called thermophones. They were discovered in the same time period as traditional electroacoustic transducers but their limited efficiency coupled with the technological limits of fabrication prevented scientific craze at the time. In 1999 a new thermophone was presented with a significant improvement compared to the samples used a century prior. This article coupled with the newly found ease of access to complex fabrication process of nanomaterials rekindle the interest in thermoacoustic for audio purposes. In this work a thorough literature review is presented and a novel multilayer model for thermoacoustic sound generation is derived. This model was solved for plane wave, cylindrical wave and spherical wave generation. Another model based on a two temperatures hypothesis for plane wave generation is also solved to represent more accurately the generation of thick porous thermophones. An extensive analysis of those models allowed for a detailed understanding of the thermoacoustic sound generation: its strengths, weaknesses and differences with traditional speakers. Lastly, experimental investigations of porous carbon foams in partnership with CINTRA Singapore are presented. Validation of the models and insights about the handling of such flexible and lightweighted but fragile samples are presented as well at their potential applications for scientific or commercial purposes as broad band sensors.

Layered van der Waals (vdW) materials, consisting of atomically thin layers, are of paramount importance in physics, chemistry, and materials science owing to their unique properties and various promising applications. However, their fast and large‐scale growth via a general approach is still a big challenge, severely limiting their practical implementations. Here, we report a universal method for rapid (~60 min) and large‐scale (gram scale) growth of phase‐pure, high‐crystalline layered vdW materials from their elementary powders via microwave plasma heating in sealed ampoules. This method can be used for growth of 30 compounds with different components (binary, ternary, and quaternary) and properties. The ferroelectric and transport properties of mechanically exfoliated flakes validate the high crystal quality of the grown materials. Our study provides a general strategy for the fast and large‐scale growth of layered vdW materials with appealing physiochemical properties, which could be used for various promising applications.

All inorganic cesium lead halide perovskite semiconductors exhibit great potential for nanolasers, light-emitting diodes, and solar cells, because of their unique properties including low threshold, high quantum efficiency and low cost. However, the high material refractive index of perovskite semiconductors hinders light extraction efficiency for photonic and illumination applications. In this paper, we demonstrate high light extraction efficiency achieved in CsPbBr2.75I0.25 two-dimensional photonic crystals. The perovskite photonic crystals exhibit both emission rate inhibition and light energy redistribution simultaneously. We observed a 7.9-fold reduction of spontaneous emission rate with a slower decay in CsPbBr2.75I0.25 photonic crystals, because of the photonic bandgap effect (PBG). We also observed a 23.5-fold PL emission enhancement, as a result of light energy redistribution from 2D guided modes to vertical direction in perovskite photonic crystals thin films, indicating a high intrinsic light e...

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).

Shape memory polymers (SMPs) are a polymeric smart material that can register two or more temporary shapes and transform to one another through an external stimulus. Despite their compactness and customizability, SMPs haven't been able to be adopted for mainstream applications. Since the majority of SMPs are triggered by heat, and SMPs have a very poor thermal conductivity, large thermal gradients within the polymer appear which cause slow response, inhomogeneous heat distribution and thus non-uniform transformation of shapes and cracks. Many have attempted to improve their thermal performance through the incorporation of filler-based nanomaterials. However, the outcome is ineffective as the spatial dispersion of fillers within the SMP is inhomogeneous and leads to performance loss. Contrastingly, the herein presented new class of nanocomposite-SMP, composed by 3D-foam fillers, showcase a much more efficient SMP adaptable to larger area with faster transformation speed and without any performance loss. Furthermore, the improved thermal properties lead to a decrease in required input energy, as well as render the SMP a self-heating capability which can be further designed into timed multi-step SMP behavior.

With the current trend and need of devices to thrive toward higher performance and faster operational speeds, while shrinking in size and weight, harsh and stringent conditions are also expected from their thermal-management capabilities. An important role in maintaining these devices at a safe operational temperature is fulfilled by thermal interface materials (TIMs). However, even state-of-the-art TIMs are reaching their limits in terms of operational temperature range and reliability, with the tendency to “pump-out” as well as “dry-out” during temperature cycling, which tremendously decreases its thermal performance and, thus, leads to failures in thermal management of the device overall. Worse, with the demand of operation at higher and higher temperatures, these failures often occur sooner and more frequently, leading to failure of the TIM before the end of the lifetime of the device. In order to deliver a higher performance TIM, capable of withstanding higher temperatures, we have recently presented a new class of nano-TIM, “3-D-foams.” These foam-like structures are composed of multilayer domains of 2-D-graphene and h-BN; they have a very high thermal conductivity, high surface conformity, and are tailorable for different electrical conduction needs. In this paper, we further studied these nano-TIMs and assessed their reliability under thermal cycling, according to JEDEC Standard JESD22-A104D, and high temperature/humidity environment, following JEDEC Standard JESD22-4118, to evaluate whether their performance can be translated into practical operating conditions of electronic devices. Results show that these 3-D-foams are able to maintain their high performance throughout all strenuous conditions they were exposed to, without any loss in chemical and physical composition, as well as thermal properties. A shelf life of over 400 years under normal office conditions, and more than ten years for extreme humid conditions was extrapolated, as well as an operational lifetime of over ten years in consumer electronics, which far exceeds normal lifetimes of current electronic appliances. These results demonstrate that this new class of nano-TIM is capable of not only outperforming state-of-the-art TIMs but is also a viable candidate to withstand current and future needs of operational temperatures and lifetimes of new electronics.

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

Ferroelectric materials for precise control of light from lasers to optical communications have sparked great interest owing to their large electro-optic (EO) coefficients, low propagation loss, and fast switching time. Here, we report the deposition of highly oriented Ni-doped lead zirconate titanate (PZT) thin films on glass substrates as a novel way to seamlessly connect the electrical, optical, and magnetic domain. Small dielectric dispersion, low dielectric loss, and a large dielectric constant ranging from 102 Hz to 106 Hz were observed at a Ni content of 0.5 mol%. These films show well-saturated ferroelectric hysteresis with a large spontaneous polarization (>30 μC cm−2) and a high Curie temperature (>350 °C). In addition, optical measurements indicate a large refractive index (∼2.43), a low propagation loss (∼4.14 dB cm−1), a fast response time (4.02 μs), and an effective EO coefficient (167.7 pm V−1), which are five times larger than those of the current standard material for EO devices (LiNbO3). More importantly, such films can work well up to 250 °C and retain above 80% of the EO performance at 104 Hz. Finally, the substitution of Ni2+ at the Ti4+ site shows distinct magnetic behaviors. The integration of EO active films could pave the way for future power-efficient, ultrafast switches, and compact integrated nanophotonic and magneto-optic devices. 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

Here, we have studied the effect of loading fraction of 3D-graphene (3D-C) on the properties of epoxy-based shape memory polymer (SMP). 3D-C has already been proven to be an effective filler to improve the thermal conductivity, resulting in increased shape transformation efficiency of the SMP. We prepared a series of 3D-C/SMP composite with different volume fractions of 3D-C and compared their thermal, mechanical, and shape memory properties. We observed that as the 3D-C content increases, the thermal conductivity of the composite increases along with shape transformation performances. However, interestingly, in conflict with typical nanofillers, we observed that the mechanical strength decreases at higher loading fraction due to the obstruction of polymer chains by interconnected branches. Hence, to leverage the extraordinary improvement of thermal conductivity by 3D-C, a careful design based on the amount of loading is essential to obtain a balance between thermal conductivity, mechanical strength and shape memory performance. Submitted/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)

Dye-sensitized solar cells (DSSCs) offer an optimal trade-off between conversion-efficiency and low-cost fabrication. However, since all its electrodes need to fulfill stringent work-function requirements, its materials have remained unchanged since DSSC’s first report early-90s. Here we describe a new material, oxidized-three-dimensional-graphene (o-3D-C), with a band gap of 0.2 eV and suitable electronic band-structure as alternative metal-free material for DSSCs-anodes. o-3D-C/dye-complex has a strong chemical bonding via carboxylic-group chemisorption with full saturation after 12 sec at capacity of ∼450 mg/g (600x faster and 7x higher than optimized metal surfaces). Furthermore, fluorescence quenching of life-time by 28–35% was measured demonstrating charge-transfer from dye to o-3D-C.

Orientation controlled hexagonal boron nitride (h-BN) films exhibit excellent mechanical and thermal properties, making them attractive for diverse applications. However, wafer-scale synthesis of vertically oriented h-BN films is still a significant challenge. Herein, utilizing high power impulse magnetron sputtering (HIPIMS) of up to around 500 W, a series of h-BN thin films have been directly deposited on silicon wafers in various nitrogen environments. Our analyses reveal that the degree of microstructure alignment of the as-grown films strongly depends on the nitrogen gas flow ratio. Between 20% to 40% gas flow ratios, defined to be the ratio of the nitrogen gas flow rate to the total gas flow rate of nitrogen and argon, the film attained its maximum alignment as measured by the R value. Correspondingly, the maximum thermal conductivity of the films occurs in the same region and the value ranges from 0.5 to 1.5 W m−1 K−1. Interestingly, such a BN-encapsulated InSe transistor shows a typical semiconductor characteristic and works well even after 2 months or longer. Additionally, the InSe transistor exhibits electro-dominated transport with high mobility (71.0 cm2 V−1 s−1) and performs well up to 200 °C. Our study suggests that alignment engineering in h-BN samples is plausible for thermal performance enhancement, which can broaden the thermal management applications in electrical and optoelectronic fields. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Bulk 1T-TaSe2 as a charge-density-wave (CDW) conductor is of special interest for CDW-based nanodevice applications because of its high CDW transition temperature. Reduced dimensionality of the strongly correlated material is expected to result in significantly different collective properties. However, the growth of atomically thin 1T-TaSe2 crystals remains elusive, thus hampering studies of dimensionality effects on the CDW of the material. Herein, chemical vapor deposition (CVD) of atomically thin TaSe2 crystals is reported with controlled 1T phase. Scanning transmission electron microscopy suggests the high crystallinity and the formation of CDW superlattice in the ultrathin 1T-TaSe2 crystals. The commensurate–incommensurate CDW transition temperature of the grown 1T-TaSe2 increases with decreasing film thickness and reaches a value of 570 K in a 3 nm thick layer, which is 97 K higher than that of previously reported bulk 1T-TaSe2. This work enables the exploration of collective phenomena of 1T-TaSe2 in the 2D limit, as well as offers the possibility of utilizing the high-temperature CDW films in ultrathin phase-change devices. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version H.W., Y.C. and C.Z. contribute equally. Z.L. acknowledges support from the Singapore National Research Foundation (NRF-RF2013-08) and the Singapore Ministry of Education (MOE2015-T2-2-043, MOE2017-T2-2-136, Tier 3 MOE2018-T3-1-002). Y.C. and T.Y. acknowledge support from the Singapore Ministry of Education MOE2019-T2-1-044 (S).

This thesis reports on the fabrication and evaluation of carbon composites whose constituents are based solely on carbon materials. In particular, three types of carbon composites term multilayer amorphous carbon (a-C) films, gradient a-C films and carbon nanomattress (CNM) are discussed. DOCTOR OF PHILOSOPHY (EEE)

International audience; Phase Change Materials (pcm) have been widely used for thermal energy storage due to their high latent heat of fusion. However, PCMs suffer from their very low thermal conductivity which limits heat spreading around the heat source. Without proper thermal conductivity enhancement, melting would occur mainly around the heat source and heat would be conducted too slowly for the device to be efficient. It is especially true when dealing with high power densities. Metallic foams are usually used as thermal conductivity enhancer, yet recent progress in additive manufacturing have allowed architected structures to be used and optimized. We present here an analytical investigation of the Effective Thermal Conductivity (etc) of porous architected structures and emphasize is made on the effect of thermal constriction at the interface with a heat spreader in contact with the heat source. To confirm the efficiency of the model, it is compared to simulation data as well as experimental data obtained using flash laser method. Flash laser method data processing had to be modified to adapt to the porous media being characterized. For that purpose, a 1D finite difference model has been developed to solve the heat equation under flash laser conditions and derive the porous material effective properties. Using this model, architected structure were proven to have an ETC up to 75% higher than the one of foam for similar porosity in particular direction of space. The validity of the above mentioned model where proven through simulation, giving an almost perfect match and experiments detailed in this paper,which showed a maximum deviation of 11%.

High anisotropic thermal materials, which allow heat to dissipate in a preferential direction, are of interest as a prospective material for electronics as an effective thermal management solution for hot spots. However, due to their preferential heat propagation in the in-plane direction, the heat spreads laterally instead of vertically. This limitation makes these materials ineffective as the density of hot spots increases. Here, we produce a new dielectric thin film material at room temperature, named vertically ordered nanocrystalline h-BN (voBN). It is produced such that its preferential thermally conductive direction is aligned in the vertical axis, which facilitates direct thermal extraction, thereby addressing the increasing challenge of thermal crosstalk. The uniqueness of voBN comes from its h-BN nanocrystals where all their basal planes are aligned in the direction normal to the substrate plane. Using the 3ω method, we show that voBN exhibits high anisotropic thermal conductivity (TC) with a 16-fold difference between through-film TC and in-plane TC (respectively 4.26 and 0.26 W·m–1·K–1). Molecular dynamics simulations also concurred with the experimental data, showing that the origin of this anisotropic behavior is due to the nature of voBN’s plane ordering. While the consistent vertical ordering provides an uninterrupted and preferred propagation path for phonons in the through-film direction, discontinuity in the lateral direction leads to a reduced in-plane TC. In addition, we also use COMSOL to simulate how the dielectric and thermal properties of voBN enable an increase in hot spot density up to 295% compared with SiO2, without any temperature increase. MOE (Min. of Education, S’pore) Accepted version

Compression studies on three-dimensional foam-like graphene and h-BN (3D-C and 3D-BN) revealed their high cross-plane thermal conductivity (62–86 W m–1 K–1) and excellent surface conformity, characteristics essential for thermal management needs. Comparative studies to state-of-the-art materials and other materials currently under research for heat dissipation revealed 3D-foam’s improved performance (20–30% improved cooling, temperature decrease by ΔT of 44–24 °C). MOE (Min. of Education, S’pore) Accepted version

Thermal management in harsh environment electronics (i.e. automotive, avionics, oil and gas industry) has become a latent problem. Due to the more stringent requirements of the materials, such as higher operating temperatures, ability to operate under exposure to vibration and shock, and over larger gaps with complex paths, and increased demands in terms of reliability, the solutions adopted for consumer electronics cannot be directly translated into these applications and the current materials and approaches used are reaching their limits. Herein we present a new polymer/3D-foam composite that can fill large gaps and retain a thermal conductivity of 62–86 W m−1 K−1, while providing strong mechanical support with a strong restorative force (i.e. can withstand vibration). At the same time, it is demonstrated to have a superior surface conformity to usual gap fillers and is able to remain stable up to temperatures as high as 330 °C, which is ∼180 °C higher than current conventional packaging materials.

Polyimides (PIs) are a class of polymers used in diverse applications due to their excellent thermal stability, outstanding chemical resistance, and superb mechanical properties. One of the most widely used PIs is Pyromellitic Dianhydride-Oxydianiline (PMDA-ODA) PI (Kapton®), which can be purchased as thin films or tubes, thus limiting its use to specific applications. In this work, a novel thermoforming process is proposed which facilitates forming of planar PI and novel three-dimensional graphene PI (3D-C/PI) shapes into 3D contours. Our thermoforming process allows the formation of various shapes, such as helical, angular, or cylindrical profiles, thus expanding PIs' usage to new applications. The effect of the thermoforming process on the PI's chemical composition, mechanical properties, and chemical resistance to oxidizing plasma environment are studied. Also investigated is the creep resistance of the thermoformed PI compared to pristine PI at various temperatures; an analytical creep model has been fitted to the experimental data. Our results show that accelerated creep tests of thermoformed and pristine PI result in the same deformation and that the thermoforming process has no influence on the PI's chemical composition or chemical resistance. We also found that 3D-C/PI presents excellent thermoformability and that thermoformed PI exhibits higher elasticity compared to pristine PI.

Hexagonal boron nitride (hBN) has become a prominent material for nanophotonic and quantum technology studies. Its wide bandgap can accommodate room temperature radiative optical transitions originating from defect states in different atomic structures. Here, we report the engineering of visible defects in chemical vapor deposited monolayer hBN irradiated by femtosecond pulses (0.1–0.5 W) at room temperature. Photoluminescence and transient photoluminescence measurements reveal the presence of a sharp emission at 630 nm with a time constant of approximately 3 ns randomly distributed around the irradiated region. We imaged the distribution of the photogenerated electrons by time-resolved photoemission electron microscopy in the picosecond and nanosecond timescales with 100 nm spatial resolution. We determined the precise location of the defects within the region of interest corresponding to an optical transition between 1.95 eV and 3.90 eV above the valence band maximum of hBN, ascribed to NBVN color centers.

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

3D transition metal nitrides are well recognized for their good electrical conductivity, superior mechanical properties, and high chemical stability. Recently, 2D transition metal nitrides have been successfully prepared in the form of nanosheets and show potential application in energy storage. However, the synthesis of highly crystalline and well-shaped 2D nitrides layers is still in demand for the investigation of their intrinsic physical properties. The present paper reports the growth of ultrathin tungsten nitride crystals on SiO2/Si substrates by a salt-assisted chemical vapor deposition method. High-resolution transmission microscopy confirms the as-grown samples are highly crystalline WN. The stiffness of ultrathin WN is investigated by atomic force microscopy–based nanoindentation with the film suspended on circular holes. The 3D Young's modulus of few-layer (4.5 nm thick or more) WN is determined to be 3.9 × 102 ± 1.6 × 102 GPa, which is comparable with the best experimental reported values in the 2D family except graphene and hexagonal boron nitride. The synthesis approach presented in this paper offers the possibilities of producing and utilizing other highly crystalline 2D transition-metal nitride crystals. Accepted version

Journal of Physics: Materials 1(1), 01LT02 (2018). doi:10.1088/2515-7639/aadd8c

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.

Three different configurations of GaN are analysed to show robust and tunable light emitters from localised point defects induced in GaN crystal by a femtosecond laser (fs-laser). Localised irradiations of GaN are achieved using a fs-laser. The laser-induced damage threshold is found at a fluence of 130 ± 10 mJ/cm2. Raman spectroscopy allows for the characterization of irradiated GaN crystal while quasi-resonant photoluminescence mapping reveals a defect-related visible emission corresponding to the fs-laser irradiated area. From three different configurations of GaN, emission peaks vary from 620 to 680 nm-wavelengths for thin film, MBE intrinsically doped and Mg-doped NWs of GaN, respectively. The red emission in GaN is localized thanks to the new laser-induced fabrication and the engineering of the defect emission paves the way to further lighting applications.