Adsorption to carbon nanotubes

We have probed the adsorption property of single-wall carbon nanotube (SWNT) bundles using the temperature-programmed desorption technique. The SWNT sample cleanliness effect on the 4He adsorption was investigated. Room air contacting significantly decreased the 4He adsorption capacity. The 4He adsorption vs. pump-out temperature on SWNT samples and on charcoal was obtained. A two-state binding site model did not fit well to the SWNT data, while it fit well to the charcoal data indicating the 4He binding energy on charcoal to be 400 ± 32 K which agreed with other group\u27s value. Using the desorption rate isotherm analysis technique, we obtained coverage dependant 4He binding energies on SWNT bundles. Our values agreed with other group\u27s results at near 400 K where the coverages overlapped, and our energy value increased to a much higher value at near 900 K at lower coverages beyond the lowest coverage of other group. The 4He addition temperature was changed from 273 K to lower values in the 8–40 K range for three SWNT samples and a charcoal sample. While the 4He adsorption was not sensitive on the addition temperature on charcoal, it was different on SWNT samples. Some sites were not accessible for 4He atoms at low temperatures. The 4He access to these sites increased as the gas addition temperature increased, and at 35 K and above a full 4He access to a 273 K dosed level was observed. An activated diffusion model fit to the 4He amount, vs. gas addition temperature data yielded the activation energy for diffusion to be 28 ± 14 K and 47 ± 6 K on two samples. One sample showed more restricted 4He access for 4He at 15 K. This sample had more impurities. Codesorption measurements were done on SWNT samples. Xe in smaller quantity (6% level) than 4He and H2, suppressed the adsorption of other gases to the background level. H2 suppressed 4 He to the background level, when added in equal amount at 273 K. However when 4He was added at 273 K and H2 was added later at 19 K, H2 did not suppress the 4He adsorption. Equal mixture doses of 4He and 3He at 273 K yielded 8.4 times more 4He binding than 3He. This strong isotope selectivity agreed with the predicted quantum sieving effect

Optical observation of single layer graphene on silicon nitride substrate

Optical observation of graphene on substrates is an important aspect of graphene research. However, while graphene visibility on SiO2/Si substrates has already been thoroughly investigated, that on Si3N4/Si substrates, which excludes background oxygen, requires further investigation. Here, we present theoretical and experimental results of graphene visibility on Si3N4/Si substrates. On these substrates, the graphene contrast that diverges and changes sign over narrow ranges, which makes it more difficult to observe graphene on them, was predicted and experimentally verified. Under appropriate conditions, it was possible to observe single layer graphene on these substrates, as demonstrated by our experimentally observed results. Our findings will be useful for enabling the use of Si3N4/Si substrates in characterizing functionalized graphenes

Silicon Carbide Micro-devices for Combustion Gas Sensing under Harsh Conditions

ABSTRACT A sensor based on the wide bandgap semiconductor, silicon carbide (SiC), has been developed for the detection of combustion products in power plant environments. The sensor is a catalytic gate field effect device that can detect hydrogen-containing species in chemically reactive, high temperature environments. For fast and stable sensor response measurements, a gate activation process is required. Activation of all sensors took place by switching back and forth between oxidizing (1.0 % oxygen in nitrogen) and reducing (10 % hydrogen in nitrogen) gases for several hours at a sensor temperature ≥620 °C. All 52 devices on the sensor chip were activated simultaneously by flooding the entire chip with gas. The effects of activation on surface morphology and structure of Pt gates before and after activation were investigated. The optical images obtained from Pt gates demonstrated a clear transition from a smooth and shiny surface to a grainy and cloudy surface morphology. XRD scans collected from Pt gates suggest the presence of an amorphous layer and species other than Pt (111) after activation. The reliability of the gate insulator of our metal-oxide-SiC sensors for long-term device operation at 630 °C was studied. We find that the dielectric is stable against breakdown due to electron injection from the substrate with gate leakage current densities as low at 5nA/cm 2 at 630 °C. We also designed and constructed a new nano-reactor capable of high gas flow rates at elevated pressure. Our reactor, which is a miniature version of an industrial reactor, is designed to heat the flowing gas up to 700 °C. Measurements in ultrahigh vacuum demonstrated that hydrogen sulfide readily deposits sulfur on the gate surface, even at the very high hydrogen/hydrogen sulfide ratios (10 3 -10 5 ) expected in applications. Once deposited, the sulfur adversely affects sensor response, and could not be removed by exposure to hydrogen at the temperatures and pressures accessible in the ultrahigh vacuum experiments. Oxygen exposures, however, were very effective at removing sulfur, and the device performance after sulfur removal was indistinguishable from performance before exposure to H 2 S.

Silicon Carbide Micro-devices for Combustion Gas Sensing under Harsh Conditions

A sensor based on the wide bandgap semiconductor, silicon carbide (SiC), has been developed for the detection of combustion products in power plant environments. The sensor is a catalytic gate field effect device that can detect hydrogen-containing species in chemically reactive, high temperature environments. For fast and stable sensor response measurements, a gate activation process is required. Activation of all sensors took place by switching back and forth between oxidizing (1.0% oxygen in nitrogen) and reducing (10% hydrogen in nitrogen) gases for several hours at a sensor temperature {ge}620 C. All 52 devices on the sensor chip were activated simultaneously by flooding the entire chip with gas. The effects of activation on surface morphology and structure of Pt gates before and after activation were investigated. The optical images obtained from Pt gates demonstrated a clear transition from a smooth and shiny surface to a grainy and cloudy surface morphology. XRD scans collected from Pt gates suggest the presence of an amorphous layer and species other than Pt (111) after activation. The reliability of the gate insulator of our metal-oxide-SiC sensors for long-term device operation at 630 C was studied. We find that the dielectric is stable against breakdown due to electron injection from the substrate with gate leakage current densities as low at 5nA/cm{sup 2} at 630 C. We also designed and constructed a new nano-reactor capable of high gas flow rates at elevated pressure. Our reactor, which is a miniature version of an industrial reactor, is designed to heat the flowing gas up to 700 C. Measurements in ultrahigh vacuum demonstrated that hydrogen sulfide readily deposits sulfur on the gate surface, even at the very high hydrogen/hydrogen sulfide ratios (10{sup 3}-10{sup 5}) expected in applications. Once deposited, the sulfur adversely affects sensor response, and could not be removed by exposure to hydrogen at the temperatures and pressures accessible in the ultrahigh vacuum experiments. Oxygen exposures, however, were very effective at removing sulfur, and the device performance after sulfur removal was indistinguishable from performance before exposure to H{sub 2}S

Optical endpoint detection for plasma reduction of graphene oxide

The plasma reduction process for the production of reduced graphene oxide (rGO) requires precise process control in order to avoid the degradation of electrical characteristics. We report that the reduction status of the graphene oxides could be determined by monitoring the optical emission intensity at 844.6 nm. Properties of the rGO samples processed with various plasma exposure times were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, and 4-point probe measurements. Optimum electrical performance and surface morphology were obtained from the sample for which the reduction process was stopped when the emission intensity at 844.6 nm began to decrease

Fast and low-temperature reduction of graphene oxide films using ammonia plasma

Reduced graphene oxide (rGO) has been produced using an ammonia (NH3) plasma reduction method. Simultaneous nitrogen doping during the reduction process enabled a rapid and low-temperature restoration of the electrical properties of the rGO. The chemical, structural, and electrical properties of the rGO films were analyzed using x-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, and conductivity measurements. The oxygen functional groups were efficiently removed, and simultaneous nitrogen doping (6%) was carried out. In addition, the surface of the rGO film was flattened. Consequently, the rGO films exhibited electrical properties comparable to those prepared via other reduction methods

Optimization of graphene oxide synthesis parameters for improving their after-reduction material performance in functional electrodes

Optimization of the parameters of a modified Hummers' method for graphene oxide (GO) synthesis is conducted in this study. Focusing specifically on their applications for transparent electrodes and supercapacitor electrodes, the properties of the thin layers and electrodes prepared from GO and thermally reduced GO(RGO) were investigated using UV-visible spectroscopy, Hall measurements, atomic force microscopy, x-ray photoelectron spectroscopy, and cyclic voltammetry. The obtained results reveal that promoting the oxidation of graphite, either by increasing the acid reaction time or the oxidant dosage, improves the morphological and optoelectrical properties of the resulting graphene thin layers for transparent electrode applications. On the other hand, improving the synthesis parameters was not necessary for obtaining high-quality supercapacitor electrodes. Adopting thermal reduction conditions (involving thermal shocking) was as effective as using optimized synthetic conditions in increasing the gravimetric specific capacitance for the supercapacitor electrodes prepared using RGO.1152sciescopu

Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes

This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of similar to 620 Omega/square with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices