Joon-Suk Oh

Graphene oxide porous paper from amine-functionalized poly(glycidyl methacrylate)/graphene oxide core-shell microspheres

Various functional materials, such as metal nanoparticles, carbon nanotubes and conducting polymers were coated on polymer microspheres finding various uses in electronic and biomedical applications. Herein, we demonstrate that single graphene oxide (GO) sheets could be easily wrapped on amine-functionalized poly(glycidyl methacrylate) (PGMA-ed) microspheres (∼2.5 μm in diameter) to form a PGMA-ed/GO core-shell structure with a thickness of ca. 50 nm. Subsequently, the synthesized GO-skin microspheres were consolidated by heating them to 60 °C to form a robust self-standing paper through the formation of electrostatic and van der Waals attractive forces between the very large surfaces of the GO, together with the formation of hydrogen bonding during the dewatering and drying processes, where the GO skins are interconnected to provide an electrical path and mechanical skeletal structure. When a stabilized GO dispersion was added to the PGMA-ed microspheres, the GO sheets were uniformly self-assembled on the PGMA-ed microsphere surfaces, seemingly through dipole–dipole interactions and amine–epoxide chemical reactions. This approach provides a simple route for the large volume production of PGMA-ed/GO core-shell microspheres and large-sized self-standing paper that may find various uses in optoelectronic device materials and porous membrane applications.

Electromagnetic Interference (EMI) Transparent Shielding of Reduced Graphene Oxide (RGO) Interleaved Structure Fabricated by Electrophoretic Deposition

Here we introduce the electromagnetic shielding effectiveness (SE) of reduced graphene oxide (RGO) sheets interleaved between polyetherimide (PEI) films fabricated by electrophoretic deposition (EPD). Incorporating only 0.66 vol % of RGO, the developed PEI/RGO composite films exhibited an electromagnetic interference shielding effectiveness (EMI SE) at 6.37 dB corresponding to ∼50% shielding of incident waves. Excellent flexibility and optical transparency up to 62% of visible light was demonstrated. It was achieved by placing the RGO sheets in the localized area as a thin film (ca. 20 nm in thickness) between the PEI films (ca. 2 μm) to be an interleaved and alternating structure. This unique interleaved structure without any delamination areas was fabricated by a successive application of cathodic and anodic EPD of both RGO and PEI layers. The EPD fabrication process was ensured by an alternating deposition of the quarternized-PEI drops and RGO, each taking positive and negative charges, respectively, in the water medium. We believe that the developed facile fabrication method of RGO interleaved structure with such low volume fraction has great potential to be used as a transparent EMI shielding material.

An interleaved porous laminate composed of reduced graphene oxide sheets and carbon black spacers by in situ electrophoretic deposition

Although the graphene-based materials have a great potential to be used for various energy storage devices, the expected performance of graphene has not been achieved yet seemingly due to the lack of interconnected porosity and actively-exposed surface area that should be developed in the re-stacked graphene electrodes. Herein we used an electrophoretic deposition (EPD) method to fabricate a binder-free porous supercapacitor electrode composed of reduced graphene oxide (RGO) sheets and conductive carbon black (CB) particles. Applying EPD for an electrostatically-stabilized aqueous mixture of RGO and CB nanoparticles, the electrophoretic squeezing force in EPD induced the RGO sheets to align in the in-plane direction along with the CB particles placed in the interlayers of RGO. The developed ladder-like interleaved composite structure allowed a desirable porosity network and conductive path for a facile movement of ions and electrons. Controlling the ratios of concentrations (Cs,RGO/Cs,CB) and/or zeta potentials (ξRGO/ξCB) of the RGO and CB nanoparticles in aqueous mixtures, different nanostructures of the interleaved RGO/CB laminates could be fabricated. Thoroughly tested as a supercapacitor electrode in an organic electrolyte (TEA BF4), the developed RGO/CB electrodes provided excellent performance of the specific capacitance of 218 F g−1 at a scan rate of 1 mV s−1 (133.3 F g−1 at a current density of 2 A g−1), energy density of 43.6 W h kg−1 and power density of 71.3 kW kg−1. It is believed that an ideal performance of intrinsic graphene properties could be exerted by the unique nanostructure of binder-free interleaved graphene laminates as developed by the scalable in situ EPD process for large-volume production.

Transparent actuator made with few layer graphene electrode and dielectric elastomer, for variable focus lens

A transparent dielectric elastomer actuator driven by few-layer-graphene (FLG) electrode was experimentally investigated. The electrodes were made of graphene, which was dispersed in N-methyl-pyrrolidone. The transparent actuator was fabricated from developed FLG electrodes. The FLG electrode with its sheet resistance of 0.45 kΩ/sq (80 nm thick) was implemented to mask silicone elastomer. The developed FLG-driven actuator exhibited an optical transparency of over 57% at a wavenumber of 600 nm and produced bending displacement performance ranging from 29 to 946 μm as functions of frequency and voltage. The focus variation was clearly demonstrated under actuation to study its application-feasibility in variable focus lens and various opto-electro-mechanical devices.

Polyimide Surface Treatment by Atmospheric Pressure Plasma for Metal Adhesion

The surface of polyimide PI films before/after plasma surface treatment using a remote-type modified dielectric barrier discharge was investigated to improve the adhesion between the PI substrate and the metal thin film. Among the plasma treatments of the PI substrate surface using various gas mixtures, the surface treated with the N2/He/SF6/O2 plasma showed the lowest contact angle value due to the high CvO bondings formed on the PI surface, while that treated with N2/He/SF6 showed the highest contact angle value due to the high C–Fx chemical bondings on the PI surface. Specifically, when the O2 gas flow was varied from 0 to 2.0 slm in the N2 40 slm /He 1 slm /SF6 1.2 slm /O2 x slm gas composition, the lowest contact angle value of about 9.3° was obtained at an O2 gas flow of 0.9 slm. And it was due to the high content of oxygen radicals in the plasma, which leads to the formation of the highest CvO bondings on the PI surface. When the interfacial adhesion strength between the Ag film and PI substrate was measured after the treatment with N2 40 slm /He 1 slm /SF6 1.2 slm /O2 0.9 slm followed by the deposition of Ag, a peel strength of 111 gf/mm was observed, which is close to the adhesion strength between a metal and the PI treated by a low pressure plasma.

Supercapacitor characteristics of pressurized RuO2/carbon powder as binder-free electrodes

RuO2/carbon powder electrodes have been designed to be enclosed in a supercapacitor cell and compressed under a constant pressure (4.84 kgf cm−2), which could overcome binder failure under repeated volumetric changes. In this binder-free powder electrode system, the resistance and ESR are substantially decreased (0.39 Ω at 1 kHz) and the capacitive retention is also greatly improved (7% decrement after 4000 cycles) with a maximum specific capacitance of 835 F (g of total electrode weight)−1, corresponding to 1391.7 F (g of RuO2 weight)−1, which is likely close to the intrinsic energy-storage capability of RuO2 materials. Comparing the specific capacitances at different discharge rates of 5 mV s−1 and 100 mV s−1, a decrement of only 24% occurs in specific capacitance, which is considered to be excellent in RuO2 systems. Furthermore, in the Ragone plot analysis, the developed binder-free system shows an excellent energy and power density of 24.9 W h kg−1 and 16.1 kW kg−1, respectively, at 50 mV s−1, which are ca. 63.2% and 44.7% higher, respectively, than those of a typical binder-containing system (11.1 W h kg−1 and 7.2 kW kg−1).

In situ fabrication of platinum/graphene composite shell on polymer microspheres through reactive self-assembly and in situ reduction

We present a simple method to fabricate a uniform-sized graphene–metal–polymer composite microsphere of core–shell structure. On the surface of amine-functionalized polymer microsphere, graphene oxide (GO) sheets were affixed to give a core–shell structure by self-assembly process followed by the immobilization of platinum (Pt) ions to the assembled GO shell. Subsequently, they were chemically reduced in situ converting both GO and Pt ions to reduced GO (RGO) and Pt nanoparticles (NPs), respectively. As a result, a robust RGO-Pt composite shell, composed of RGO sheets and well-distributed Pt NPs, was fabricated on the microsphere surface. Meanwhile, the insulative GO shell was converted to the conductive RGO-Pt shell giving 24.0 S m−1 of electrical conductivity. We demonstrated that the electrical property of the shell was significantly improved by the incorporation of Pt NPs.

Characteristics of SiO x Thin Film Deposited by Atmospheric Pressure Plasma-Enhanced Chemical Vapor Deposition Using PDMS ∕ O2 ∕ He

Silicon oxide thin films were deposited using a modified, pin-to-plate, dielectric barrier discharge system with polydimethylsiloxane (PDMS), bubbled by He/O 2 gas mixtures at atmospheric pressure and a temperature of less than 50°C. Increasing PDMS flow rate in the gas mixture increased the deposition rate, but also increased the surface roughness due to the formation of particles in the gas phase as a result of increased PDMS and silanol groups, leading to incomplete decomposition or oxidation of PDMS. The increase in the ratio of oxygen flow rate to PDMS flow rate decreased the surface roughness with increasing deposition rate due to the efficient oxidation of PDMS. However, when the oxygen flow rate was raised above 1 slm, due to the increased oxidation of PDMS in the gas phase and the decreased PDMS dissociation by the decreased plasma density, the surface roughness was again increased with decreasing deposition rate. At the gas mixture of 9 slm PDMS/He and 1 slm oxygen, a smooth, SiO 2 -like thin film was obtained at a deposition rate of 12 nm/min.

X-ray photoelectron spectroscopic study of Ge2Sb2Te5 etched by fluorocarbon inductively coupled plasmas

X-ray photoelectron spectroscopy was used to determine the level of surface fluorination damage of Ge2Sb2Te5 (GST) etched by fluorocarbon gases at different F/C ratios. When blank GST was etched, the gas with a higher F/C ratio produced a thinner C–F polymer on the etched surface but fluorinated Ge, Sb, and Te compounds were observed in the remaining GST. When the sidewall of the etched GST features was investigated, a thicker fluorinated layer was observed on the GST sidewall etched by the higher F/C ratio gas, indicating more fluorination due to the difficulty in preventing F diffusion into the GST through the thinner C–F layer.

An aggregation-mediated assembly of graphene oxide on amine-functionalized poly(glycidyl methacrylate) microspheres for core–shell structures with controlled electrical conductivity

We present a novel method for regulating the shell thickness of core–shell microspheres consisting of a polymer core and graphene oxide (GO) or a reduced graphene oxide (RGO) shell. A strategy of pH-induced instability of GO and its gradual assembly with the core particles is employed. Interestingly, the thickness of the shell gradually increases from around 20 to 70 nm as the pH of the suspension decreases from 10 to 3 during the assembly process, which significantly changes the electrical properties of the core–shell microspheres. We fabricated core–shell microspheres with a broad range of electrical conductivities from 1.79 to 31.43 S m−1.