Byeongho Park

High-performance supercapacitor electrode based on a polyaniline nanofibers/3D graphene framework as an efficient charge transporter

The current paper describes chemically grown polyaniline (PANI) nanofibers on porous three dimensional graphene (PANI/3D graphene) as a supercapacitor electrode material with enhanced electrochemical performance. The chemical and structural properties of the electrode are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy with confirmation of a semi-crystalline nature. The homogeneous growth of PANI on the 3D graphene network is visualized by field emission scanning electron microscopy (FESEM) and shows a nanofibers-based morphology. The maximum specific capacitance of the PANI/3D graphene electrode is found to be ∼1024 F g−1 in 1 M H2SO4 within the potential window of −150 to 800 mV vs. Ag/AgCl at 10 mV s−1 scan rate (∼1002 F g−1 at 1 mA cm−2 discharge current density). The high surface area offered by the conducting, porous 3D graphene framework stimulates effective utilization of the deposited PANI and improves electrochemical charge transport and storage. This signifies that the 3D graphene framework is a proficient contender for high-performance capacitor electrodes in energy storage applications.

Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications

Graphene nanomeshes (GNMs) with nanoscale periodic or quasi-periodic nanoholes have attracted considerable interest because of unique features such as their open energy band gap, enlarged specific surface area, and high optical transmittance. These features are useful for applications in semiconducting devices, photocatalysis, sensors, and energy-related systems. Here, we report on the facile and scalable preparation of multifunctional micron-scale GNMs with high-density of nanoperforations by catalytic carbon gasification. The catalytic carbon gasification process induces selective decomposition on the graphene adjacent to the metal catalyst, thus forming nanoperforations. The pore size, pore density distribution, and neck size of the GNMs can be controlled by adjusting the size and fraction of the metal oxide on graphene. The fabricated GNM electrodes exhibit superior electrochemical properties for supercapacitor (ultracapacitor) applications, including exceptionally high capacitance (253 F g−1 at 1 A g−1) and high rate capability (212 F g−1 at 100 A g−1) with excellent cycle stability (91% of the initial capacitance after 50 000 charge/discharge cycles). Further, the edge-enriched structure of GNMs plays an important role in achieving edge-selected and high-level nitrogen doping.

Controlling the luminescence emission from palladium grafted graphene oxide thin films via reduction

The role of palladium (Pd) in the reduction of graphene oxide (GO) thin films was investigated using a Pd assisted grafting technique. The structural and optical characteristics of these thin films were obtained from various spectroscopic analyses, which confirmed increased C[double bond, length as m-dash]C-C aromatic ring vibration and oxidation of Pd with Ar annealing. In Pd free GO, annealing of films resulted in restoration of sp(2) clusters; however, Pd grafting with non-annealed film enhanced the possibility of restoration and further annealing dramatically increased the restoration rate with enhanced blue photoluminescence (PL) emission. The blue PL emission originates from sp(2) cluster sites and the yellow-green PL from defect trapped states. As reduction of GO increased, yellow-green emission decreased and blue PL became the prominent emission. These experimental findings open up a new feasible pathway for controlling the luminescence emission from graphene oxide that furthers the technological advancement of graphene based optoelectronic devices.

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.

A binder free synthesis of 1D PANI and 2D MoS2 nanostructured hybrid composite electrodes by the electrophoretic deposition (EPD) method for supercapacitor application

A facile, binder-free approach is applied, along with the electrophoretic deposition (EPD) method, to fabricate large-scale, hybrid 2D MoS2 nanosheets and 1D polyaniline (PANI) nanowires based electrodes on a conducting substrate for supercapacitor electrode material. The entire substrate surface is uniformly decorated by electrophoretically assembled MoS2 2D nanosheets and 1D nanowires of PANI, revealed by structural and morphological analysis. The electrochemical capacitive measurements of the MoS2/PANI hybrid electrode exhibit a specific capacitance of ∼485 F g−1 at a low charging–discharging current density (1 mA cm−2). The MoS2:PANI composition ratio was varied as 1:1, 1:2 and 1:3 to achieve high supercapacitive performance. The maximum supercapacitive performance (∼812 F g−1) was obtained for a 1:2 ratio of MoS2 and PANI, with high energy density (112 W h kg−1) and power density (0.6 kW kg−1). Synergistic interactions between conductive 1D PANI and 2D MoS2 nanosheets with high surface area lead to a high supercapacitive performance. A binder approach to the direct synthesis of hybrid electrode by the EPD method eradicates the drawbacks offered by conventional electrodes prepared by the general slurry coating technique with resistive binders.

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.

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.

Tunable near white light photoluminescence of lanthanide ion (Dy3+, Eu3+ and Tb3+) doped DNA lattices

For more than two decades, structural DNA nanotechnology has been investigated, yet researchers still have not clearly determined the functional changes and the applicability of DNA structures resulting from the introduction of a variety of ions. Lanthanide ions, such as Dy3+, Eu3+ and Tb3+, are interesting rare earth ions that have unique characteristics applicable to photonics. Here, we have constructed lanthanide ion doped double-crossover DNA lattices, a new class of functional DNA lattices, grown on a silica substrate. Deformation-free lattices were fabricated on a given substrate, and dopant ions were introduced to study their photoluminescence characteristics. The photoluminescence of the lanthanide ion-doped DNA lattices exhibited broad emission spectra in the visible region and a tendency of near white light emission composed of various colours. The intensity of the distinct spectral lines produced by the photoluminescence increased as the doping concentration of the ions reached the critical point, and the intensity then decreased with a further increase in the ions. Photoluminescence quenching was also observed when the excitation wavelength increased. These phenomena are the result of energy transfer between the DNA and the dopant ions. Finally, we make use of chromaticity diagrams to identify the colour coordinates of the luminescence produced by the lanthanide ion-doped DNA lattices, and this information may be useful to construct efficient bio-photonic devices or sensors in the future.

Surface roughness effects on the frequency tuning performance of a nanoelectromechanical resonator

Electrothermal heating is one of radio frequency tuning method in nanoelectromechanical resonators with magnetomotive transduction. This study confirmed that the surface roughness of the nanoresonator affects the electrothermal tuning performance under moderate conditions at room temperature. The effect of surface roughness on electrothermal tuning is complicated and involves interactions of mechanical and electrical properties. In addition, the electrothermal damping varied depending on the nanoscale molecular solid structure. These factors affect the signal-to-noise ratio, the effective stress of the beam, and the quality Q-factor of the nanoresonator.

Broadband supercontinuum generation using a hollow optical fiber filled with copper-ion-modified DNA

We experimentally demonstrated supercontinuum generation through a hollow core photonic bandgap fiber (HC-PBGF) filled with DNA nanocrystals modified by copper ions in a solution. Both double-crossover nano DNA structure and copper-ion-modified structure provided a sufficiently high optical nonlinearity within a short length of hollow optical fiber. Adding a higher concentration of copper ion into the DNA nanocrystals, the bandwidth of supercontinuum output was monotonically increased. Finally, we achieved the bandwidth expansion of about 1000 nm to be sufficient for broadband multi-spectrum applications.