Sooman Lim

Electrochemical Properties of High-Power Supercapacitors Using Single-Walled Carbon Nanotube Electrodes**

We have investigated the key factors determining the performance of supercapacitors constructed using single-walled carbon nanotube (SWNT) electrodes. Several parameters, such as composition of the binder, annealing temperature, type of current collector, charging time, and discharging current density have been optimized for the best performance of the supercapacitor with respect to energy density and power density. We find a maximum specific capacitance of 180 F/g and a measured power density of 20 kW/kg at energy densities in the range from 7 to 6.5 Wh/kg at 0.9 V in a solution of 7.5 N KOH (the currently available supercapacitors have energy densities in the range 6‐7 Wh/kg and power density in the range 0.2‐5 kW/kg at 2.3 V in non-aqueous solvents).

TAILORING THE CHARACTERISTICS OF GRAPHITE OXIDES BY DIFFERENT OXIDATION TIMES

Graphite oxide was synthesized using various oxidation times and characterized by its physical and chemical properties. The degree of oxidation of the graphite oxide was systematically controlled via oxidation time up to 24h. Three phases of interlayer distances were identified by x-ray diffraction: pristine graphite (3.4A), intermediate (4A) and fully expanded graphite oxide (6A) phases. These phases were distinguished by an atomic ratio of O/C, which occurred from the different compositions of epoxide, carboxyl and hydroxyl groups. The band gap of the graphite oxides was also tuned via the oxidation time, resulting in direct band gap engineering from 1.7 to 2.4eV and strong correlation with the atomic ratio of O/C. (Some figures in this article are in colour only in the electronic version)

Field-emission properties of vertically aligned carbon-nanotube array dependent on gas exposures and growth conditions

We grew vertically aligned carbon nanotubes (CNTs) using microwave plasma-enhanced (MPE) and thermal chemical-vapor deposition (CVD) and characterized their field emission properties. We observe that the flickering and instability in the field emission are due to the metal particles present on the field-emission array (FEA) surface, particularly from the MPECVD-grown samples. The existence of metal particles is an obstacle to obtaining reliable emission properties. The emission properties of the CNT–FEA are studied as a function of gas-exposure time with hydrogen, nitrogen, and oxygen gases. Gas exposures affected turn-on voltage, hysteresis, and the slope of Fowler–Nordheim plots. We observe that the saturation of emission currents is attributed to gas adsorbates present on the surface of the FEA. Oxygen exposures induce more severe degradation on the field-emission properties than nitrogen, whereas emission properties are improved by hydrogen gas exposures that clean the surface of emitters. In addition...

The rapid growth of vertically aligned carbon nanotubes using laser heating

Growth of densely packed vertically aligned carbon nanotubes (VA-CNTs) using laser-induced chemical vapor deposition with visible laser (lambda = 532 nm) irradiation at room temperature is reported. Using a multiple-catalyst layer (Fe/Al/Cr) on quartz as the substrate and an acetylene-hydrogen mixture as the precursor gas, VA-CNT pillars with 60 microm height and 4 microm diameter were grown at a high rate of around 1 microm s(-1) with good reproducibility. It is demonstrated that the fabrication of uniform pillar arrays of VA-CNTs can be achieved with a single irradiation for each pillar using LCVD with no annealing or preprocessing of the substrate. Here, laser fast heating is considered the primary mechanism facilitating the growth of VA-CNT pillars. Field emission characteristics of an array of VA-CNT pillars were then examined to investigate their potential application in vacuum electronic devices.

Electrohydrodynamic printing of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) electrodes with ratio-optimized surfactant

An electrohydrodynamic (EHD) printing process was optimized for the printing of a (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) conductive polymer by manipulating the surface tension of a PEDOT:PSS solution. A stable cone-jet mode was confirmed by adjusting the process parameters of the EHD printing process, such as applied voltage, flow rate, and working distance. The addition of a nonionic surfactant, Triton X-100, enabled both printing of PEDOT:PSS conductive lines with widths ranging from 335 μm to 90 μm in a low-power operation (0.5 kV), as well as a 100-fold increase in conductivity of the PEDOT:PSS film compared with that of the pristine one. To utilize printed PEDOT:PSS lines for high functional applications, a multi-deposition technique was carried out, which results in a decrease in line resistance from 1.3 × 104 Ω mm−1 to 0.2 × 103 Ω mm−1.

Direct patterning of conductive carbon nanotube/polystyrene sulfonate composites via electrohydrodynamic jet printing for use in organic field-effect transistors

Carbon-based conductive lines were directly patterned using an electrohydrodynamic (EHD) jet printing technique. We successfully dispersed multiwalled carbon nanotubes (MWCNTs) in a polar solvent through a size-sorting and blending process involving polystyrene sulfonate (PSS), and we fabricated a homogeneous MWCNT/PSS nano-composite ink for EHD printing. We demonstrated that EHD printing was affected by the type of solvent, which adjusted the surface tension of the MWCNT/PSS ink. Ethanol, with a lower surface tension compared to water, provided four different jetting modes, depending on the applied voltage and working distance. EHD-printed MWCNT/PSS conductive lines were used as source/drain (S/D) electrodes in organic field-effect transistors (OFETs). The resulting OFETs showed reliable performance with negligible hysteresis on chlorosilane-terminated polystyrene (PS-brush)-modified substrates.

Electrohydrodynamic printing for scalable MoS2 flake coating: application to gas sensing device

Scalable sub-micrometer molybdenum disulfide ([Formula: see text]) flake films with highly uniform coverage were created using a systematic approach. An electrohydrodynamic (EHD) printing process realized a remarkably uniform distribution of exfoliated [Formula: see text] flakes on desired substrates. In combination with a fast evaporating dispersion medium and an optimal choice of operating parameters, the EHD printing can produce a film rapidly on a substrate without excessive agglomeration or cluster formation, which can be problems in previously reported liquid-based continuous film methods. The printing of exfoliated [Formula: see text] flakes enabled the fabrication of a gas sensor with high performance and reproducibility for [Formula: see text] and [Formula: see text].