Unraveling the Catalytic Graphitization Mechanism of Ni-P Electroless Plated Cokes via In Situ Analytical Approaches.

We elucidate the catalytic graphitization mechanism using in situ analytical approaches. Catalytic graphitization is achieved through a Ni-P electroless plating process at a relatively low temperature of 1600 °C, which allows for a high crystallinity of coke. We also employ an ultrasonic treatment during the Ni-P electroless plating stage to effectively form metal layers on the surface. The impact of the ultrasonic treatment on the Ni-P electroless plating is confirmed by field emission scanning electron microscopy images of the cross-section and an elemental composition analysis using energy dispersive X-ray spectroscopy mapping. Structural analysis of the graphitized cokes via X-ray diffraction (XRD) and Raman spectroscopy shows that Ni-P electroless plating significantly accelerates the graphitization process. Furthermore, we illuminate the graphitization behavior through in situ transmission electron microscopy and XRD analysis. Nickel layers on the coke surface facilitate graphite formation by encouraging the dissolution and precipitation of amorphous carbons, thus resulting in efficient graphitization at a relatively low temperature.

Hexagonal Boron Nitride as an Intermediate Layer for Gallium Nitride Epitaxial Growth in Near-Ultraviolet Light-Emitting Diodes.

We introduce the development of gallium nitride (GaN) layers by employing graphene and hexagonal boron nitride (h-BN) as intermediary substrates. This study demonstrated the successful growth of GaN with a uniformly smooth surface morphology on h-BN. In order to evaluate the crystallinity of GaN grown on h-BN, a comparison was conducted with GaN grown on a sapphire substrate. Photoluminescence spectroscopy and X-ray diffraction confirmed that the crystallinity of GaN deposited on h-BN was inferior to that of GaN grown on conventional GaN. To validate the practical applicability of the GaN layer grown on h-BN, we subsequently grew an NUV-LED structure and fabricated a device that operated well in optoelectrical performance experiments. Our findings validate the potential usefulness of h-BN to be a substrate in the direct growth of a GaN layer.

A Graphene-Based Polymer-Dispersed Liquid Crystal Device Enabled through a Water-Induced Interface Cleaning Process.

We report the use of four-layer graphene (4LG) as a highly reliable transparent conductive electrode (TCE) for polymer-dispersed liquid crystal (PDLC)-based smart window devices. The adhesion between 4LG and the substrate was successfully improved through a water-induced interface-cleaning (WIIC) process. We compared the performance of a device with a WIIC-processed 4LG electrode with that of devices with a conventional indium tin oxide (ITO) electrode and a 4LG electrode without a WIIC. With the application of the WIIC process, the PDLC smart window with a 4LG electrode exhibited reduced turn-on voltage and haze compared to 4LG without the WIIC process and characteristics comparable to those of the ITO electrode. The WIIC-processed 4LG electrode demonstrated enhanced electrical properties and better optical performance, leading to improved device efficiency and reliability. Furthermore, our study revealed that the WIIC process not only improved the adhesion between 4LG and the substrate but also enhanced the compatibility and interfacial interactions, resulting in the superior performance of the smart window device. These findings suggest that 4LG with WIIC holds great promise as a transparent conductive electrode for flexible smart windows, offering a cost-effective and efficient alternative to conventional ITO electrodes.

Biorenewable Polymer-Based Light-Absorbing Porous Hydrogel for Efficient Solar Steam Desalination.

An efficient interfacial heating system composed of a light-absorbing material and a hydrophilic porous support is developed through eco-friendly and energy-effective fabrication processes. Lignin nanoparticles (NPs) and cellulose nanofibers (CNFs) are harnessed as biorenewable light absorbers and hydrophilic supports, respectively. Lignin NPs are prepared using a solvent exchange process of the fractionated lignin with organic solvents to improve its π-π stacking and light-absorbing property for efficient photothermal conversion. Then, the lignin NPs are mixed with CNFs and lyophilized to obtain a light-absorbing porous hydrogel (LAPH), and the resulting LAPHs are covalently cross-linked and hybridized with Au NPs through a seed-mediated growth to further enhance their mechanical stability, hydrophilicity, and photothermal conversion properties. The resulting LAPHs exhibit an outstanding and prolonged performance as a solar steam generator such as high salt and pH tolerance, evaporation rate (3.17 kg m-2 h-1), and solar steam generation efficiency (83.4%) under 1 sun irradiation.

Bayesian Optimization of Wet-Impregnated Co-Mo/Al2O3 Catalyst for Maximizing the Yield of Carbon Nanotube Synthesis.

Multimetallic catalysts have demonstrated their high potential for the controlled synthesis of carbon nanotubes (CNTs), but their development requires a more complicated optimization than that of monometallic catalysts. Here, we employed Bayesian optimization (BO) to optimize the preparation of Co-Mo/Al2O3 catalyst using wet impregnation, with the goal of maximizing carbon yield in the chemical vapor deposition (CVD) synthesis of CNTs. In the catalyst preparation process, we selected four parameters to optimize: the weight percentage of metal, the ratio of Co to Mo in the catalyst, the drying temperature, and the calcination temperature. We ran two parallel BO processes to compare the performance of two types of acquisitions: expected improvement (EI), which does not consider noise, and one-shot knowledge gradient (OKG), which takes noise into account. As a result, both acquisition functions successfully optimized the carbon yield with similar performance. The result suggests that the use of EI, which has a lower computational load, is acceptable if the system has sufficient robustness. The investigation of the contour plots showed that the addition of Mo has a negative effect on carbon yield.

Defect-selective-etched porous GaN as a buffer layer for high efficiency InGaN/GaN light-emitting diodes.

Substrate-induced biaxial compressive stress and threading dislocations (TDs) have been recognized to severely impair the performance, stability, and reliability of InGaN/GaN light-emitting diodes (LEDs) for quite some time. In this study, a defect-selective-etched (DSE) porous GaN layer is fabricated employing electro-chemical etching and applied as a buffer layer for the development of InGaN/GaN LEDs with high quantum efficiency. Based on the analysis of photoluminescence and micro-Raman spectra, it has been revealed that the overgrown GaN epilayer on the DSE porous GaN has a relatively low TDs and relaxation of compressive stress in comparison to the conventional GaN epilayer. The remarkable improvement in the internal quantum efficiency of the InGaN/GaN LEDs is directly attributable to the strong radiative recombination in InGaN/GaN multi-quantum-wells caused by stress relaxation and TDs annihilation. Our findings indicate that the use of DSE porous GaN as a buffer layer may be a viable approach for producing crystalline GaN epilayers and high-performance LEDs.

Investigation of the Ligand Exchange Process on Gold Nanorods by Using Laser Desorption/Ionization Time-of-Flight Mass Spectrometry.

The ligand exchange process on gold nanorods (Au NRs) was explored by using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). Cetyltrimethylammonium bromide (CTAB) adsorbed on Au NRs was replaced with alkanethiol derivatives presenting different functional groups. The ligand exchange process was investigated under various conditions, such as in the presence of different functional groups in the ligands and with different concentrations of CTAB. The ligand-exchanged Au NRs were characterized by using a combination of UV-Vis spectroscopy and LDI-TOF-MS. Based on the results, it was revealed that LDI-TOF-MS analysis can provide crucial and distinct information about the degree of ligand exchange on Au NRs.

Two-Dimensional Stacked Composites of Self-Assembled Alkane Layers and Graphene for Transparent Gas Barrier Films with Low Permeability.

Self-assembled alkane layers are introduced between graphene layers to physically block nanometer size defects in graphene and lateral gas pathways between graphene layers. A well-defined hexatriacontane (HTC) monolayer on graphene could cover nanometer-size defects because of the flexible nature and strong intermolecular van der Waals interactions of alkane, despite the roughness of graphene. In addition, HTC multilayers between graphene layers greatly improve their adhesion. This indicates that HTC multilayers between graphene layers can effectively block the lateral pathway between graphene layers by filling open space with close-packed self-assembled alkanes. By these mechanisms, alternately stacked composites of graphene and self-assembled alkane layers greatly increase the gas-barrier property to a water vapor transmission rate (WVTR) as low as 1.2 × 10-3 g/(m2 day), whereas stacked graphene layers generally show a WVTR < 0.5 g/(m2 day). Furthermore, the self-assembled alkane layers have superior crystallinity and wide bandgap, so they have little effect on the transmittance.

Nanoscale layer of a minimized defect area of graphene and hexagonal boron nitride on copper for excellent anti-corrosion activity.

In this work, we synthesized a monolayer of graphene and hexagonal boron nitride (hBN) using chemical vapor deposition. The physicochemical and electrochemical properties of the materials were evaluated to determine their morphology. High-purity materials and their atomic-scale coating on copper (Cu) foil were employed to prevent fast degradation rate. The hexagonal two-dimensional (2D) atomic structures of the as-prepared materials were assessed to derive their best anti-corrosion behavior. The material prepared under optimized conditions included edge-defect-free graphene nanosheets (∼0.0034µm2) and hBN (∼0.0038µm2) per unit area of 1µm2. The coating of each material on the Cu surface significantly reduced the corrosion rate, which was âˆ¼2.44 × 10-2/year and 6.57 × 10-3/year for graphene/Cu and hBN/Cu, respectively. Importantly, the corrosion rate of Cu was approximately 3-fold lower after coating with hBN relative to that of graphene/Cu. This approach suggests that the surface coating of Cu using cost-effective, eco-friendly, and the most abundant materials in nature is of interest for developing marine anti-corrosion micro-electronic devices and achieving surface modification of pure metals in industrial applications.

Quantifying Carbon Edge Sites on Depressing Hydrogen Evolution Reaction Activity.

To understand the effect of microstructural characteristics of carbon materials on their electrochemical or electrocatalytic performance, an in-depth study of the edges in carbon materials should be carried out. In this study, catalytically grown platelet-type carbon nanofibers (CNFs) with fully exposed edges were physically and chemically passivated to clarify the relationship between the edge density and the hydrogen evolution reaction (HER) activity. Due to the aligned structure along the fiber axis, the edges on the outer surface of the CNFs were easily modified without using a complex process. The edges on the surface of the CNFs were inactivated by sequentially forming single, double, and multiple loops as the heat treatment temperatures increased. The number of edges within the CNFs was quantitatively measured using temperature-programmed desorption (TPD) up to 1800 °C. The surviving edges on the surface of thermally treated CNFs were identified by chemical functionalization via an amination reaction. We identified a close relationship between the HER activity and the edge density. When evaluating the electrochemical and electrocatalytic activity of carbon materials, it is important to know the portion of the edge surface area with respect to the total surface area and edge ratio.

Effect of Polymeric In Situ Stabilizers on Dispersion Homogeneity of Nanofillers and Thermal Conductivity Enhancement of Composites.

Boron nitride (BN) nanofiller-based polymer composites have been considered promising candidates for efficient heat-dissipating packaging materials because of their superior thermal conductivity, mechanical strength, and chemical resistance. However, strong aggregation of the BN nanofillers in the composite matrix as well as the difficulty in the modification of the chemically inert surface prevents their effective use in polymer composites. Herein, we report an effective method by using in situ stabilizers to achieve homogeneous dispersion of boron nitride (BN) nanofillers in an epoxy-based polymeric matrix and demonstrate their use as efficient heat-dissipating materials. Poly(4-vinylpyridine) (P4VP) is designed and added into the epoxy resin to produce in situ stabilizers during preparation of hexagonal BNs (h-BNs) and BN nanotubes (BNNTs) dispersion. In-depth experimental and theoretical studies indicated that the homogeneous distribution of BN nanofillers in epoxy composites achieved by using the in situ stabilizer enhanced the thermal conductivity of the composite by ∼27% at the same concentration of the BN nanofillers. In addition, the thermal conductivity of the h-BN/epoxy composite (∼3.3 W/mK) was dramatically improved by ∼48% (4.9 W/mK) when the homogeneously dispersed BNNTs (∼1.8 vol %) were added. The concept of the proposed in situ stabilizer can be further utilized to prepare the epoxy composites with the homogeneous distribution of BN nanofillers, which is critical for reproducible and position-independent composite properties.

Boron Nitride as a Passivation Capping Layer for AlGaN/GaN High Electron Mobility Transistors.

We report on the electrical characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) with hexagonal boron nitride (h-BN) as a passivation capping layer. The HEMTs with h-BN layers showed an increase in current drainage and 103-times reduction in the gate-leakage current compared with those of conventional unpassivated HEMTs. Moreover, the extrinsic transconductance and the pulse responses were improved due to the reduced charge-trapping effect at the surface of HEMTs. From our observations, the h-BN can be used as a passivation capping layer for high-power electronic devices.

Barrier-assisted vapor phase CVD of large-area MoS2 monolayers with high spatial homogeneity.

Atomically thin molybdenum disulphide (MoS2) is a direct band gap semiconductor with negatively charged trions and stable excitons in striking contrast to the wonder material graphene. While large-area growth of MoS2 can be readily achieved by gas-phase chemical vapor deposition (CVD), growth of continuous MoS2 atomic layers with good homogeneity is indeed one of the major challenges in vapor-phase CVD involving all-solid precursors. In this study, we demonstrate the growth of large-area continuous single crystal MoS2 monolayers on c-plane sapphire by carefully positioning the substrate using a facile staircase-like barrier. The barrier offered great control in mitigating the secondary and intermediate phases as well as second layer nucleation, and eventually a continuous monolayer with high surface homogeneity is realized. Both micro-Raman and high-resolution transmission electron microscopy (HRTEM) results confirmed the high structural quality of the grown MoS2 layers. Using low temperature photoluminescence spectroscopy, additional pieces of information are provided for the strong band-edge emission in the light of vacancy compensation and formation of Mo-O bonding. The monolayer MoS2 transferred to SiO2/Si exhibited a room temperature field-effect mobility of ∼1.2 cm2 V-1 s-1 in a back-gated two-terminal configuration.

The Effect of Oxidative Debris on the Laser Desorption/Ionization Efficiency of Graphene Oxide Derivatives for Mass Spectrometric Analysis of Small Molecules and Synthetic Polymers.

The influence of oxidative debris (OD) on laser desorption/ionization time of flight mass spectrometry (LDI-TOF-MS) analysis using graphene oxide (GO) derivatives was systematically investigated. Specifically, the effects on LDI-TOF-MS analysis of small molecules and synthetic polymers were highlighted. The analytical efficiency of GO was significantly enhanced by removing OD from its surface and the working molecular weight range of GO was considerably extended for analysis of synthetic polymers and small molecules.

Neuronal differentiation of human mesenchymal stem cells in response to the domain size of graphene substrates.

Graphene is a noncytotoxic monolayer platform with unique physical, chemical, and biological properties. It has been demonstrated that graphene substrate may provide a promising biocompatible scaffold for stem cell therapy. Because chemical vapor deposited graphene has a two dimensional polycrystalline structure, it is important to control the individual domain size to obtain desirable properties for nano-material. However, the biological effects mediated by differences in domain size of graphene have not yet been reported. On the basis of the control of graphene domain achieved by one-step growth (1step-G, small domain) and two-step growth (2step-G, large domain) process, we found that the neuronal differentiation of bone marrow-derived human mesenchymal stem cells (hMSCs) highly depended on the graphene domain size. The defects at the domain boundaries in 1step-G graphene was higher (×8.5) and had a relatively low (13% lower) contact angle of water droplet than 2step-G graphene, leading to enhanced cell-substrate adhesion and upregulated neuronal differentiation of hMSCs. We confirmed that the strong interactions between cells and defects at the domain boundaries in 1step-G graphene can be obtained due to their relatively high surface energy, which is stronger than interactions between cells and graphene surfaces. Our results may provide valuable information on the development of graphene-based scaffold by understanding which properties of graphene domain influence cell adhesion efficacy and stem cell differentiation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 43-51, 2018.

Graphene-Carbon-Metal Composite Film for a Flexible Heat Sink.

The heat generated from electronic devices such as light emitting diodes (LEDs), batteries, and highly integrated transistors is one of the major causes obstructing the improvement of their performance and reliability. Herein, we report a comprehensive method to dissipate the generated heat to a vast area by using the new type of graphene-carbon-metal composite film as a heat sink. The unique porous graphene-carbon-metal composite film that consists of an electrospun carbon nanofiber with arc-graphene (Arc-G) fillers and an electrochemically deposited copper (Cu) layer showed not only high electrical and thermal conductivity but also high mechanical stability. Accordingly, superior thermal management of LED devices to that of conventional Cu plates and excellent resistance stability during the repeated 10 000 bending cycles has been achieved. The heat dissipation of LEDs has been enhanced by the high heat conduction in the composite film, heat convection in the air flow, and thermal radiation at low temperature in the porous carbon structure. This result reveals that the graphene-carbon-metal composite film is one of the most promising materials for a heat sink of electronic devices in modern electronics.

Boron nitride nanotubes as a heat sinking and stress-relaxation layer for high performance light-emitting diodes.

High-density threading dislocations, the presence of biaxial compressive strain, and heat generation are the major limitations obstructing the performance and reliability of light emitting diodes (LEDs). Herein, we demonstrate a facile epitaxial lateral overgrowth (ELOG) method by incorporating boron nitride nanotubes (BNNTs) on a sapphire substrate by spray coating to resolve the above issues. Atomic force microscopy, X-ray diffraction, micro-Raman, and photoluminescence measurements confirmed the growth of a high quality GaN epilayer on the BNNT-coated sapphire substrate with reduced threading dislocations and compressive strain owing to the ELOG process. GaN LEDs fabricated using this approach showed a significant enhancement in the internal quantum efficiency and electroluminescence intensity compared to conventional LEDs grown on sapphire. Moreover, reduced efficiency droop and surface temperature at high injection currents were achieved due to the excellent thermal stability and conductivity of BNNTs. Based on our findings we infer that the BNNTs would be a promising material for high power devices vulnerable to self-heating problems.

Facile Synthesis of Highly Crystalline and Large Areal Hexagonal Boron Nitride from Borazine Oligomers.

A novel and facile synthetic method for h-BN films from borazine oligomer (B3N3H4)x precursors has been developed. This method only includes spin-coating of borazine oligomer onto nickel catalysts and a subsequent annealing step. Large areal and highly crystalline h-BN films were obtained. The stoichiometric B/N ratio of borazine oligomer precursor was preserved in the final h-BN product such that it was close to 1 as revealed by XPS. Catalytic effect of nickel for h-BN formation was clearly demonstrated by lowering crystallization temperature compared to the growth condition in the absence of catalyst. The graphene field effect transistor (GFET) characterization has proved the high quality synthesis of h-BN films, showing the shift of neutrality point and the increase of the mobility. This method can also provide functional h-BN coating on various surfaces by annealing Ni-coated borazine oligomer films and subsequent removal of Ni catalyst.

The role of graphene formed on silver nanowire transparent conductive electrode in ultra-violet light emitting diodes.

This paper reports a highly reliable transparent conductive electrode (TCE) that integrates silver nanowires (AgNWs) and high-quality graphene as a protecting layer. Graphene with minimized defects and large graphene domains has been successfully obtained through a facile two-step growth approach. Ultraviolet light emitting diodes (UV-LEDs) were fabricated with AgNWs or hybrid electrodes where AgNWs were combined with two-step grown graphene (A-2GE) or conventional one-step grown graphene (A-1GE). The device performance and reliability of the UV-LEDs with three different electrodes were compared. The A-2GE offered high figure of merit owing to the excellent UV transmittance and reduced sheet resistance. As a consequence, the UV-LEDs made with A-2GE demonstrated reduced forward voltage, enhanced electroluminescence (EL) intensity, and alleviated efficiency droop. The effects of joule heating and UV light illumination on the electrode stability were also studied. The present findings prove superior performance of the A-2GE under high current injection and continuous operation of UV LED, compared to other electrodes. From our observation, the A-2GE would be a reliable TCE for high power UV-LEDs.

Carbon-nanotube-assisted nanoepitaxy of Si-doped GaN for improved performance of InGaN/GaN light-emitting diodes.

Using single-walled carbon nanotubes (SWCNTs) as nanomasks on an undoped GaN template, a significant biaxial stress relaxation was achieved in the subsequently-grown Si-doped n-GaN layer. Enhanced near band edge (NBE) emission intensity, similar free carrier concentrations, and the reduced peak width of the asymmetric (102) crystallographic plane all confirmed the suppression of threading dislocations due to the nanoepitaxial growth process. Temperature-dependent photoluminescence (PL) revealed improved internal quantum efficiency (IQE) of InGaN/GaN multi-quantum wells (MQWs) grown on this n-GaN layer. Furthermore, enhanced light output power and a remarkable reduction in efficiency droop were observed for the blue light-emitting diodes (LEDs), especially at higher injection currents. Our results emphasize the strong potential for SWCNTs as nanomasks in the heteroepitaxy of GaN-based devices without the exploitation of complicated lithography or etching processes.