Investigation of Soot Deposition and Composition in a Radio Frequency Plasma of Benzene

A radio frequency (rf) discharge has been utilized to study the decomposition of benzene. SEM inspection has shown that the size of soot particles ranged from 0.5 to several μm. The soot deposited on silicon wafers was analyzed by gas chromatography/mass spectrometry (GC/MS). It was shown that the main components of the soot are polyphenyls (biphenyl and terphenyls) and a trace amount of polycyclic aromatic hydrocarbons (PAHs). Acetylene and hydrogen have been detected by plasma diagnostics techniques using Fourier transform infrared (FTIR) and optical emission spectroscopic techniques. However, GC/MS analysis has shown that the relative yields of PAHs are much less than those of polyphenyls, which indicates that the conventional hydrogen abstraction-acetylene addition (HACA) model for soot formation is not applicable to the benzene plasma due to the reason of temperature. The rf power, the carrier gas flow-rate, the relative yields of polyphenyls, and plasma temperatures were correlated. The reaction pathways of benzene elimination and soot formation in plasma are discussed. This study has provided a new route to control the contamination due to PAHs.     

Raman investigation on carbonization process of metal–organic frameworks

Under inert gas flow and high temperature, carbonization of aluminum‐based metal–organic frameworks (MOFs) was carried out. The formation rate of carbonized MOFs (CMOFs) was monitored by the variation of the Raman D band to G band intensity ratio with heat treatment duration. Powder x‐ray diffraction (PXRD) and scanning electron microscope (SEM) techniques were used to confirm the formation of CMOFs. The activation energy was extracted from the temperature‐dependent rate constants using the Arrhenius equation and correlated with the structural properties of precursor MOFs such as pore size and the number of carbon atoms per ligand. A reaction mechanism is proposed and discussed for the formation of CMOFs based on Raman observation. Copyright © 2016 John Wiley & Sons, Ltd.     

Direct Spectroscopic Evidence of the Influence of Chamber Wall Condition on Oxide Etch Rate

The drift of TEOS etch rate has been observed during MERIE oxide etch for the damascene process. The etch rate typically fluctuates between 5300 Å/min and 6000 Å/min. Studies using fluorocarbon-based chemistry show a normal TEOS etch rate when the chamber wall is heavily coated with polymer deposition. On the other hand, a lower etch rate appears when the chamber has less deposition. Hysteresis behavior has been observed during the etch rate of TEOS, as well as emission intensity trends of F, CF _x (x=1~3), and SiF. From the observed emission intensity variation of F, CF _x , and SiF, a model is proposed to explain the impact of chamber wall polymer deposition on the etch rate of TEOS. This model includes a mechanism of etch rate enhancement by embedding oxygen in the chamber wall polymer. From the correlation between etch rate and emission intensity, it clearly shows that F is directly responsible for the etch of TEOS. Compared to F, CF _x plasma chemistry has a closer link to chamber wall polymer formation, but contributes less in the etch of TEOS.     

Effect of plasma treatment on electrical conductivity and Raman spectra of carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were treated with a radio-frequency discharge. We found that MWCNTs showed opposite trends in electrical conductivity when treated with oxygen and hydrogen plasmas. MWCNTs showed enhanced electrical conductivity when placed at cathode with oxygen plasma treatment, whereas MWCNTs treated at positive column did not show such a trend. In contrast, the conductivity of MWCNTs dropped sharply with hydrogen plasma treatment. The measured conductivity trends of MWCNTs are correlated with observed Raman spectral shift. The possible mechanisms of the change in electrical conductivity in plasma-treated MWCNTs are discussed.     

Observation of the diameter-dependent Raman dispersion effect in chemically oxidized multiwalled carbon nanotubes

Chemical oxidation of multiwalled carbon nanotubes (MWCNTs) using H₂SO₄/HNO₃ solution has been monitored by micro-Raman spectroscopy and X-ray absorption spectroscopy. The diameter distribution variation in MWCNTs due to chemical oxidation has been measured by scanning electron microscopy and transmission electron microscopy. The Raman dispersion behaviors of the intensity ratio and the band positions of the D, G, and G′ bands were found to be correlated with the MWCNT diameter distribution. It was also found that, during the nanotube unzipping process, defect formation complicates the observation of the diameter-dependent Raman dispersion effect. The curvature effect plays an important role in the intensity ratio trend. On the other hand, defect formation dominates the band position trend.     

Analyses of benzophenones by capillary electrochromatography using methacrylate ester-based monolithic columns

In this study, eight benzophenones, which are commonly used as UV filters in various cosmetics and plastics, were analyzed by capillary electrochromatography with a methacrylate ester-based monolithic column. The effects of the composition and pH of mobile phase, porogenic solvent ratio, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) content on benzophenone separations were examined. For all benzophenones, separation performances were markedly improved in monolithic columns with larger 1-propanol ratio and higher AMPS content. Furthermore, a twofold increase in AMPS content almost reduced the separation time in half when a monolithic column had an adequately high surface area, i.e. monolithic column was produced in a higher ratio of 1-propanol. As well, the retention behaviors of these analytes in the monolithic column were strongly influenced by the level of acetonitrile in the mobile phase, and the pH of the mobile phase also had an apparent influence on separation resolution.     

Atmospheric Pressure Plasma Degradation of Azo Dyes in Water: pH and Structural Effects

The degradation of aqueous solutions containing azo dyes (ortho-, meta-, and para-methyl red) was carried out by means of atmospheric pressure plasma treatment. As evidenced by optical emission spectroscopy, the metastable argon in the discharge is responsible for initiating reactions in dye solutions. The bleaching of aqueous solutions is attributed to the destruction of dye molecules as observed in the UV–visible absorption spectra. We found that the degradation pathways of methyl red critically depend on the pH values in aqueous solutions as well as isomeric structures. The reaction pathways are entirely different in basic (pH = 11), near-neutral (pH = 6), and acidic conditions (pH = 2). Kinetic analysis shows that acidic condition gives the fastest degradation rates of methyl red isomers with removal rate: ortho > meta > para among all conditions. At basic condition, the degradation rates are equally slow for all methyl red isomers.     

Sample stacking for the analysis of catechins by microemulsion EKC

In this study, an on-line concentration method, ASEI (anion-selective exhaustive injection)-sweeping technology which was coupled with microemulsion EKC (MEEKC), was used to analyze and detect six catechins ((-)-epicatechin, (+)-catechin, (-)-epigallocatechin gallate, (-)-epicatechin gallate, (-)-epigallocatechin, and (-)-gallocatechin). In addition to the effects of the buffer pH and electrolyte concentration on stacking, the compositions of microemulsion (types of oil phase, and types and levels of cosurfactant) also dominated the stacking effect of catechins. In MEEKC, the effect of the type of oil in microemulsion on separation mechanism is often unclear. This study had demonstrated that the oil type in microemulsion indeed altered the affinity of oil droplets with analytes. Finally, this proposed ASEI-sweeping MEEKC method was able to detect trace level of catechins in food products that was not previously possible by a normal MEEKC method.     

Metal–organic frameworks: a novel SERS substrate

We report the direct observation of surface‐enhanced Raman scattering (SERS) effect using metal–organic frameworks (MOFs) as substrates. Without the aid of any metal colloids or enhancing agents, the SERS signals of methyl orange (MO) adsorbed in MOFs were observed and even remained active if the organic linkers in MOFs were completely removed by high temperature and O₂ plasma treatments. It implies that the SERS active site is at the metal oxide clusters. The ultraviolet‐visible spectra of MO, MOFs, and MO–MOF complexes show that absorption peaks are far from laser excitation line. Thus, conventional resonance enhancement effect should be ruled out, and charge‐transfer mechanism is the most likely scenario responsible for the observed SERS effect. Density functional theory (DFT) was used to interpret the chemical enhancement mechanism and the adsorption orientation‐dependent SERS spectra in our observation. The preferred adsorption orientations calculated by DFT method are consistent with the observed SERS results. Copyright © 2013 John Wiley & Sons, Ltd.     

OES and GC/MS Study of RF Plasma of Xylenes

The radio-frequency discharge of xylene isomers was monitored with optical emission spectroscopy (OES). It was found that the meta isomer showed relatively stronger excimer to monomer intensity ratio than the other two isomers. OES also indicates the formation of xylyl (methylbenzyl) radicals. The reaction products of low pressure xylene plasmas were analyzed by gas-chromatography mass spectrometry (GC/MS). It showed that the main composition of the reaction products was 1,2-di-p-tolylethane (DPTE), regardless the types of xylene isomers used. It is known that o- and m-xylyl radicals can undergo rearrangement and convert to p-xylyl radicals. Similar to the cases in benzene and toluene plasmas, the recombination reaction between two p-xylyl radicals is believed to be responsible for the formation of DPTE. Density functional theory calculations suggest that the direct conversion of xylene excimers to DPTE is unlikely.