Sun Jun Kim

Phosphorus-Doped Graphene Oxide Layer as a Highly Efficient Flame Retardant

A simple and easy process has been developed to efficiently dope phosphorus into a graphene oxide surface. Phosphorus-doped graphene oxide (PGO) is prepared by the treatment of polyphosphoric acid with phosphoric acid followed by addition of a graphene oxide solution while maintaining a pH of around 5 by addition of NaOH solution. The resulting materials are characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The as-made PGO solution-coated cloth exhibits excellent flame retardation properties. The PGO-coated cloth emits some smoke at the beginning without catching fire for more than 120 s and maintains its initial shape with little shrinkage. In contrast, the pristine cloth catches fire within 5 s and is completely burned within 25 s, leaving trace amounts of black residue. The simple technique of direct introduction of phosphorus into the graphene oxide surface to produce phosphorus-doped oxidized carbon nanoplatelets may be a general approach towards the low-cost mass production of PGO for many practical applications, including flame retardation.

Efficient Direct Reduction of Graphene Oxide by Silicon Substrate

Graphene has been studied for various applications due to its excellent properties. Graphene film fabrication from solutions of graphene oxide (GO) have attracted considerable attention because these procedures are suitable for mass production. GO, however, is an insulator, and therefore a reduction process is required to make the GO film conductive. These reduction procedures require chemical reducing agents or high temperature annealing. Herein, we report a novel direct and simple reduction procedure of GO by silicon, which is the most widely used material in the electronics industry. In this study, we also used silicon nanosheets (SiNSs) as reducing agents for GO. The reducing effect of silicon was confirmed by various characterization methods. Furthermore, the silicon wafer was also used as a reducing template to create a reduced GO (rGO) film on a silicon substrate. By this process, a pure rGO film can be formed without the impurities that normally come from chemical reducing agents. This is an easy and environmentally friendly method to prepare large scale graphene films on Si substrates.

Layer dependence and gas molecule absorption property in MoS2 Schottky diode with asymmetric metal contacts.

Surface potential measurement on atomically thin MoS2 flakes revealed the thickness dependence in Schottky barriers formed between high work function metal electrodes and MoS2 thin flakes. Schottky diode devices using mono- and multi- layer MoS2 channels were demonstrated by employing Ti and Pt contacts to form ohmic and Schottky junctions respectively. Characterization results indicated n-type behavior of the MoS2 thin flakes and the devices showed clear rectifying performance. We also observed the layer dependence in device characteristics and asymmetrically enhanced responses to NH3 and NO2 gases based on the metal work function and the Schottky barrier height change.

High-concentration dispersions of exfoliated MoS2 sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS2 sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS2 nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS2 nanosheets using freeze-dried silk fibroin powders. Compared to MoS2 dispersion in the absence of silk fibroin powder, sonicated MoS2 dispersions with silk fibroin powder (MoS2/Silk dispersion) show noticeably higher exfoliated MoS2 nanosheet yields, with suspended MoS2 concentrations in MoS2/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL–1, respectively. The MoS2 concentration in the MoS2/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS2 nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS2 nanosheets spin-cast from MoS2 /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS2 crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS2 nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS2 nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS2 nanosheets contents.

Tunable wide blue photoluminescence with europium decorated graphene

The current paper describes europium decorated graphene (EuG) which provides high and wide blue emission at 400 nm and 458 nm. The chemical and structural properties of the products are characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy. Fourier transform infrared (FT-IR) and UV–Vis spectrometery are employed to analyze the optical properties. The photoluminescence features are investigated using the excitation/emission spectra and fluorescence microscopy images. The photoluminescence intensity of EuG with the bright fluorescent nature of europium is higher than that of reduced graphene oxide. The transition of trivalent europium (Eu3+) that leads to the radiation of light with a 590 nm wavelength can be turned into a 4f–4f transition of divalent (Eu2+) europium upon heating in the presence of the graphene sheet, which assists the reduction of the europium ion. The enhancement of the blue emission at 458 nm with quenching in the red at 590 nm is affected by the modification of properties (by → via) the europium–graphene composite concentration and external thermal energy. The result suggests a new possibility for the fluorescence characteristics of the lanthanide–graphene nanocomposite that can be applied to the display, optoelectronic devices, and bio-imaging fields. The temperature-tunable photoluminescence characteristics can be used as a non-contact thermal sensor.

All-solid-state flexible asymmetric micro supercapacitors based on cobalt hydroxide and reduced graphene oxide electrodes

In-plane micro supercapacitors (micro-SC) have attracted interest due to their high areal and volumetric capacitance that is dependent upon their electrode structure. This study proposes simply-fabricated micro-SCs based on cobalt hydroxide and electrochemically-reduced graphene oxide (erGO). The Au electron collector was prepared via photolithography, and Co(OH)2 and erGO were deposited on the Au surface via electrodeposition. Using facile two-step fabrication method and cost-effective materials, the prepared micro-SCs exhibit good electrochemical performance in PVA–KOH–KI solid electrolyte. The in-plane interdigitated electrode maximizes the areal capacitance by increasing the facing area between the anode and cathode. The micro-SC presents good power density (100.38 μW cm−2) and a wide potential window due to the electric double-layer properties of erGO and pseudocapacitive performance of Co(OH)2. These fabricated micro-SCs are flexible and can be used in various wearable and small-scale energy storage devices.

Crack-Release Transfer Method of Wafer-Scale Grown Graphene Onto Large-Area Substrates

We developed a crack-release graphene transfer technique for opening up possibilities for the fabrication of graphene-based devices. Graphene film grown on metal catalysts/SiO2/Si wafer should be scathelessly peeled for sequent transferring to a target substrate. However, when the graphene is grown on the metal catalyst on a silicon substrate, there is a large tensile stress resulting from the difference of the coefficient of thermal expansion in the catalyst and silicon. The conventional methods of detaching graphene from metal catalysts were found to induce considerable mechanical damage on graphene films during separation processes including metal wet etching. Here we report a new technique wherein bubbles generated by electrolysis reaction separate thin metal catalysts from the SiO2/Si wafer. The dry attachment of graphene to the target wafer was processed utilizing a wafer to wafer bonding technique in a vacuum. We measured the microscopic image, Raman spectra, and electrical properties of the transferred graphene. The optical and electrical properties of the graphene transferred by the bubbles/dry method are better than those of the graphene obtained by mechanical/wet transfer.

Surface plasmon enhancement of photoluminescence in photo-chemically synthesized graphene quantum dot and Au nanosphere

Graphene quantum dots (GQDs) are promising candidates for potential applications such as novel optoelectronic devices and bio-imaging. However, insufficient light absorption to exhibit their intriguing characteristics. The strong confinement of light caused by the Au nanoparticles as an antenna can considerably boost the light absorption. With the assistance of ultraviolet irradiation, we prepared bluish-green luminescent nanospheres by the hybridization of GQD and Au nanoparticles (GQD/Au). These nanospheres showed a photoluminescence quantum yield of up to 26.9%. The GQD/Au nanospheres were synthesized using a solution of GQDs and HAuCl4 by a photochemical method with the reduction of GQDs and the formation of metallic Au. The GQDs and Au nanoparticles self-assembled and aggregated into nanospheres via aurophilicity and hydrogen bonding interactions. The average size of the GQD/Au nanospheres was found to be in the range of 150–170 nm, which is much larger than that of the pristine GQDs (4–7 nm). The GQD/Au nanospheres exhibited an absorption band at 541 nm, which indicates the presence of Au in the nanospheres. The typical absorbance features of GQDs were observed near 236 and 303 nm. The photoluminescence characteristics were investigated using the excitation and emission spectra. The GQD/Au nanospheres exhibited two emission peaks at 468 and 529 nm in the visible range. The green fluorescent peak located at 529 nm was newly generated by the hybridization. The GQD/Au nanospheres showed an emission efficiency which was two times more than that of the intrinsic GQDs. The reason for this increase was the surface plasmon resonance from the Au particles, which improved the fluorescence property of the resulting nanospheres. These nanospheres can be perceived as outstanding candidates for applications such as displays, optoelectronic devices, and imaging of the biological samples with high emission intensity.

Carrier Transport Properties of MoS 2 Asymmetric Gas Sensor Under Charge Transfer-Based Barrier Modulation

Over the past few years, two-dimensional materials have gained immense attention for next-generation electric sensing devices because of their unique properties. Here, we report the carrier transport properties of MoS2 Schottky diodes under ambient as well as gas exposure conditions. MoS2 field-effect transistors (FETs) were fabricated using Pt and Al electrodes. The work function of Pt is higher than that of MoS2, while that of Al is lower than that of MoS2. The MoS2 device with Al contacts showed much higher current than that with Pt contacts because of its lower Schottky barrier height (SBH). The electrical characteristics and gas responses of the MoS2 Schottky diodes with Al and Pt contacts were measured electrically and were simulated by density functional theory calculations. The theoretically calculated SBH of the diode (under gas absorption) showed that NOx molecules had strong interaction with the diode and induced a negative charge transfer. However, an opposite trend was observed in the case of NH3 molecules. We also investigated the effect of metal contacts on the gas sensing performance of MoS2 FETs both experimentally and theoretically.

Frontispiece: Phosphorus‐Doped Graphene Oxide Layer as a Highly Efficient Flame Retardant

Flame Retardant Materials A simple and easy process developed to efficiently dope phosphorus into a graphene oxide surface is reported by S. Some, S. C. Jun et al. in their Communication on page 15480 ff. Phosphorus-doped graphene oxide (PGO) is prepared by the treatment of polyphosphoric acid with phosphoric acid followed by addition of a graphene oxide solution while maintaining a pH of around 5 by addition of NaOH solution. The as-made PGO solution-coated cloth exhibits excellent flame-retardation properties. This simple technique for direct introduction of phosphorus into the graphene oxide surface to produce phosphorus-doped oxidized carbon nanoplatelets may be a general approach towards the low-cost mass production of PGO for many practical applications, including flame retardation.