Wonhee Lee

Chemical Attachment of Organic Functional Groups to Single-walled Carbon Nanotube Material

We have subjected single-walled carbon nanotube materials (SWNTMs) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTMs, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNTs and reaction with the amorphous carbon.

XPS study of carbon fiber surfaces treated by thermal oxidation in a gas mixture of O2/(O2 + N2)

Abstract X-ray photoelectron spectroscopy (XPS) has been used to investigate the surface characteristics of two carbon fibers and determine the role of surface treatment in improving the properties of a carbon fiber composite. Carbon and oxygen were the major elements observed on the surface of the carbon fiber samples. Small amounts of lightly oxidized nitrogen amounting to about 3xa0at.% were detected on the surface of thermally oxidized carbon fiber in a gas mixture of O 2 /(N 2 +O 2 ). Graphitic carbon was the major carbon functional component on the surface of carbon fiber samples but other functional forms of carbon were also present such as C–O, Cue605O, and O–Cue605O. The biggest differences in the surface chemistry between the carbon fibers with and without surface treatment was in the carbonyl group (Cue605O) content. Thermal oxidation processing in a gas mixture of O 2 /(N 2 +O 2 ) can be employed to increase the carbonyl functional group which is the main contributor in improving the interlaminar shear strengths of carbon fiber composite materials.

Vapor adsorption on coal- and wood-based chemically activated carbons: (I) Surface oxidation states and adsorption of H2O

Abstract X-ray photoelectron spectroscopy (XPS) was employed to evaluate the surface element distribution/concentration and surface chemical structure of coal- and wood-based chemically activated carbons and to investigate the effect of these features on the adsorption of water vapor at low relative pressure and room temperature. It was found that high surface area carbon samples generally show low concentrations of surface oxygen and low surface area carbon samples generally show high concentrations of surface oxygen. The concentrations of carbon surface oxygen groups such as C–O and Cue605O generally decrease with increasing heat treatment temperature. Increased water vapor adsorption on the lower surface area activated carbons can be correlated with higher concentrations of surface oxygen functional groups.

Diffusion barrier and electrical characteristics of a self-aligned MgO layer obtained from a Cu(Mg) alloy film

Diffusion barrier characteristics and electrical properties of self-aligned MgO layers obtained from a Cu(Mg) alloy film have been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing a Cu(Mg) film in an oxygen ambient and prevented interdiffusion of Cu in SiO2 up to 700u200a°C. The thermal stability of a pure Cu/TiN/Si multilayer system has been significantly enhanced up to 800u200a°C by the MgO layers by forming a MgO/Cu/MgO/TiN/Si multilayer system. A combined structure of Si3N4(500u200aA)/MgO(100u200aA) increased the breakdown voltage up to 20 V from 15 V and reduced the leakage current density down to 3×10−9u200aA/cm2 from 1×10−8u200aA/cm2 compared to a pure copper system. Consequently, the deposition of Cu(Mg) alloy followed by annealing in an oxygen ambient gives rise to the formation of a self-aligned MgO layer with excellent diffusion barrier and electrical characteristics and the film can be applied as a gate electrode in thin-film transistor/liquid-crystal displays, resulting in a r...

Factors Affecting Passivation of Cu(Mg) Alloy Films

Variables affecting the passivation capability of Cu(Mg) alloy films, which were sputter deposited from a Cu (4.5 atom %) target, have been investigated. As-deposited Cu(Mg)/SiO 2 /Si multilayer samples were annealed for 30 min in various oxygen ambients at pressures ranging from 10 mTorr to 30 Torr and at various temperatures in the 200-800°C range. The results show that the passivation capability of a Cu(Mg) alloy film is a function of annealing temperature, O 2 pressure, and Mg content in the film. Increasing the annealing temperature favors formation of a dense MgO layer on the surface. Decreasing the O 2 pressure enhances the preferential oxidation of Mg over Cu. Furthermore, increasing the Mg content in the Cu(Mg) film promotes formation of a dense MgO layer. Vacuum preannealing before taking the as-deposited samples to O 2 annealings was found to be very effective in segregating Mg to the surface, facilitating the passivation capability of the Cu(Mg) alloy film even when the Mg content is low. In the current study, self-aligned MgO layers with low resistivity and an effective passivation capability over the Cu surface have been obtained by manipulating these factors when Cu(Mg) thin films are annealed.

A self-passivated Cu(Mg) gate electrode for an amorphous silicon thin-film transistor

The feasibility of using Cu(Mg) alloy film as a gate electrode for thin-film transistor (TFT) liquid crystal displays has been investigated. When pure Cu was used as a gate electrode, severe interdiffusion occurred between Cu and the gases SiH4, NH3, and CF4 during plasma-enhanced chemical vapor deposition of a gate dielectric, SiNx, and dry etching of the SiNx. On the other hand, the deposition of a Cu(Mg) alloy film gives rise to the formation of a MgO/Cu bilayer structure with low Cu resistivity, good adhesion to SiO2, higher leakage current density, and excellent passivation capability. A hydrogenated amorphous silicon TFT with a MgO encapsulated Cu gate exhibited a gate voltage swing of 0.91 V/dec. and a threshold voltage of 6.8 V, resulting in a reduction of process steps and better performance.

Studies of cobalt thin films deposited by sputtering and MOCVD

Abstract Cobalt films were deposited on a Si wafer at substrate temperatures ranging from 50 to 200xa0°C by metal-organic chemical vapor deposition (MOCVD) using Co2(CO)8 as a precursor. As-deposited MOCVD Co films contained low impurity contents and were obtained as a microcrystalline structure compared to sputtered Co films. After annealing at 400xa0°C, the resistivity of a 100xa0nm thick MOCVD Co film decreased to about 6xa0μΩxa0cm. This was similar to the resistivity of bulk Co and lower than that of a sputtered Co film. The decrease in resistivity coincides with grain growth in the Co film. In addition, annealing at 300xa0°C produced a stronger (0xa00xa02) fiber texture in the MOCVD Co film compared to a sputtered Co film. Grain growth appears to be coupled to the strong (0xa00xa02) texture evolution, possibly indicating that surface energy minimization can be a driving force for grain growth in MOCVD Co films. Formation of Co oxide inhibited grain growth and led to a high resistivity of the sputtered Co films. CoSi2 was formed in MOCVD Co films whereas Co2Si was formed in the sputtered Co films after annealing at 700xa0°C.

Vapor adsorption on coal- and wood-based chemically activated carbons: (III) NH3 and H2S adsorption in the low relative pressure range

Abstract NH 3 and H 2 S adsorption studies on coal- and wood-based chemically activated carbons were carried out at room temperature using a gravimetric adsorption technique. The adsorption capacity of NH 3 and H 2 S in the very low relative pressure range was independent of the surface area development. Activated carbons with lower surface area generally adsorbed more of these vapors than those with higher surface area. This can be attributed to the presence of smaller average micropore sizes and a greater number of active adsorption sites on the low surface area chemically activated carbons.

Surface analysis of carbon black waste materials from tire residues

Abstract X-ray photoelectron spectroscopy (XPS) has been used to obtain surface chemical state information on two carbon black waste materials in terms of the surface element distribution/concentration and chemical structure. Small amounts of sulfur in the form of CS2 were detected on the surface (less than 1.7 mass %). C–H/C–C was the major carbon functional component on the surface of carbon black samples but other functional forms of carbon were also present such as Cue605O and C–O. The surface of the carbon black obtained from a hydropyrolysis process was highly oxidized primarily in the form of carbon based oxygen groups. On the other hand, surface oxygen atoms on the surface of the carbon black obtained from a pyrolysis process in the absence of H2 were in the form of both metal oxides and carbon based oxygen groups.

Vapor adsorption on coal- and wood-based chemically activated carbons (II) adsorption of organic vapors

The present investigation was undertaken to determine the adsorption properties and evaluate the adsorption capacities of several coal- and wood-based chemically activated carbons using the Dubinin–Radushkevich (DR) characteristic adsorption analysis. Limiting micropore volumes (W0), determined from CCl4 and acetone adsorption isotherms at room temperature, were generally in good agreement with the development of surface area. An effect of adsorbate polarity on the adsorption capacity was found in the case of lower surface area activated carbons rather than higher surface area carbons. This can be attributed to higher density of surface oxygen and carbon surface oxygen functional groups on the lower surface area carbon samples. Characteristic adsorption energy (E0) values obtained from coal-based KOH activated carbons were generally higher than those obtained from wood-based H3PO4 activated carbons. These results indicate that the coal-based KOH activated carbons have narrower micropores and uniform micropore size distributions. The average micropore sizes were mostly affected by the degree of surface area development which depended on the heat treatment temperature during the synthesis process.