Molecular Carbon Species Research Articles

Characterizing Molecule-Metal Surface Chemistry with Ab Initio Simulation of X-ray Absorption and Photoemission Spectra.

X-ray photoemission and X-ray absorption spectroscopy are important techniques to characterize chemical bonding at surfaces and are often used to identify the strength and nature of adsorbate-substrate interactions. In this study, we judge the ability of X-ray spectroscopic techniques to identify different regimes of chemical bonding at metal-organic interfaces. To achieve this, we sample different interaction strength regimes in a comprehensive and systematic way by comparing two topological isomers, azulene and naphthalene, adsorbed on three metal substrates with varying reactivity, namely the (111) facets of Ag, Cu, and Pt. Using density functional theory, we simulate core-level binding energies and X-ray absorption spectra of the molecular carbon species. The simulated spectra reveal three distinct characteristics based on the molecule-specific spectral features which we attribute to types of surface chemical bonding with varying strength. We find that weak physisorption only leads to minor changes compared to the gas-phase spectra, weak chemisorption leads to charge transfer and significant spectral changes, and strong chemisorption leads to a loss of the molecule-specific features in the spectra. The classification we provide is aimed at assisting interpretation of experimental X-ray spectra for complex metal-organic interfaces.

Chain Formation during Hydrogen Loss and Reconstruction in Carbon Nanobelts

Using laser-induced vaporisation to evaporate and ionise a source of curved polyaromatic hydrocarbons (carbon nanobelts), we show collision impacts between species cause mass loss and the resultant ions are catalogued via mass-spectrometry. These data are interpreted via a series of “in-silico”-simulated systematic hydrogen-loss studies using density functional theory modelling, sequentially removing hydrogen atoms using thermodynamic stability as a selection for subsequent dehydrogenation. Initial hydrogen loss results in the formation of carbyne chains and pentagon-chains while the nanobelt rings are maintained, giving rise to new circular strained dehydrobenzoannulene species. The chains subsequently break, releasing CH and C2. Alternative routes towards the formation of closed-cages (fullerenes) are identified but shown to be less stable than chain formation, and are not observed experimentally. The results provide important information on collision degradation routes of curved molecular carbon species, and notably serve as a useful guide to high-energy impact conditions observed in some astrochemical environments.

Silicon damage control through the use of aromatic hydrocarbon ion implantation

The continuous shrinking of transistor devices calls for precise doping profile control. Generally, monomer carbon (C+) is implanted at doses too low to adequately form an amorphous layer in silicon. Previous experiments used a molecular carbon species combined with dedicated vaporizer-based hardware, but this experiment was carried out using a commercial implanter and source with a liquid precursor material. The use of higher mass molecular carbon ions, such as C7H7+, impart significantly more damage to the silicon than the monomer, and can easily result in a well-defined amorphous layer. There are many species of molecular carbon that can be implanted, but in order to utilize a hot cathode plasma (HCP) ion source some additional criteria must be met. The source feed material should fragment with high-yield of molecular carbon (Cx+, x = 2–8) ions, which could then be extracted at beam currents sufficient to meet production needs using standard ion implant techniques. In this work, we surveyed several candidates for forming the molecular carbon species that could meet the requirements outlined above, reported the performance in plasma formation, and investigated materials properties of the Si after implantation.

Emission features of femtosecond laser ablated carbon plasma in ambient helium

We investigated the optical emission features of plasmas produced by 800 nm, 40 fs ultrafast laser pulses on a carbon target in the presence of ambient helium or nitrogen gases at varied pressures. Fast photography employing intensified charge coupled device, optical emission spectroscopy, and temporally spatially resolved optical time of flight emission spectroscopy were used as diagnostic tools. Spatio-temporal contours of excited neutral, ionic, as well as molecular carbon species in the plume were obtained using time of flight emission spectroscopy. These contours provided detailed account of molecular species evolution and expansion dynamics and indicate that three-body recombination is a major mechanism for carbon dimers generation in ultrafast laser ablation plumes in the presence of ambient gas. A systematic comparison of the emission features from ns and fs laser ablation carbon plumes as well as their expansion in ambient helium is also given. C2 vibrational temperatures were estimated during carbon plasma expansion with lower values in ambient helium compared to nitrogen and showed decreasing values with respect to space and ambient gas pressure.

Time- and space-resolved spectroscopic characterization of a laser carbon plasma plume in an argon background

We present time- and space-resolved spectroscopic observations of a laser-produced carbon plasma, in an argon background. An Nd : YAG laser pulse, 370 mJ, 3.5 ns, at 1.06 µm, with a fluence of 6.9 J cm−2, is used to produce a plasma from a solid graphite target in a 0.5 to 415 mTorr argon background. The spectral emission in the visible is recorded with 15 ns time resolution. We use 20 ns time resolution plasma imaging, filtered at characteristic carbon species emission wavelengths, to study the dynamics of the expanding plasma. The carbon plasma emission is found to evolve from the characteristic of single ionized carbon, to a more complex one, where C2 and C3 molecular bands dominate. Several plasma fronts, with either ionic or molecular composition, are seen to detach from the laser target plasma. The temporal and spatial features of the molecular carbon species evolution are found to be dependent on the actual argon background pressure.

Room-temperature deposition of carbon nanomaterials by excimer laser ablation

Numerous investigators have reported on pulsed laser deposition of carbon nanotubes, mostly using the Nd:YAG laser for ablation. In all cases the depositions have been conducted at high-temperatures and high pressures. Here we report on the deposition of carbon nanostructures at room temperature using a 248 nm excimer laser nm to ablate mixed graphite-nickel/cobalt targets. We find that the formation of the carbon nanomaterials is dependent on the particular ambient gas employed. In O 2 gas, carbon nanotubes and nano-onions are produced. The nanotubes have notably large channel diameters of 100–200 nm and the nano-onion structures are 100–200 nm in diameter, also much larger than previously observed. High-resolution, in-situ, time-resolved emission spectroscopy has been used to follow the production of molecular carbon species such as C 2 and C 3, as well as metals such as Ni or Co in the different ambients employed. Spectral modeling reveals significant differences in the vibrational–rotational temperatures of C 2 spectra in O 2 versus Ar. Mechanistic details of the formation of carbon nanotubes and nano-onions, and in-situ optical emission spectroscopy are described.

Sputtering and formation of C 1 and C 2 in the regenerative sooting discharge

Photoemission spectroscopy of the regenerative soot in neon's glow discharge plasma reveals the contributions from the sputtered atomic and molecular carbon species. We present the pattern of sputtering and the formation of monoatomic C 1 and diatomic C 2 as a function of the discharge current, the support gas pressure and the number densities of the excited and ionized neon as the active constituents of the carbonaceous plasma.

Species analysis of automotive carbon particles: Application of XPS for integral analysis of filter samples

In the discussion of possible toxic effects of carbon particle emissions from automotive sources the question about the origin of the material is of relevance. Besides the emissions of Diesel soot (DS) a second source of debris from tire abrasion (TD) has the potential to contribute significantly to the carbon particle emissions. The discrimination between these chemically different species is complicated by the unavoidable presence of molecular carbon species from general sources, from fuel components and from polycyclic aromatic hydrocarbons (PAH) on the filter surface deposits from imission tests. Based on the different surface reactivity of DS and TD, a quantitative data analysis procedure using the X-ray photoelectron spectroscopy (XPS) carbon 1s profile is suggested, which gives an estimate of the fraction of DS in the total carbon present. The difference to 100% is the sum of TD and molecular carbon which constitutes its dominant contribution. The method agrees semi-quantitatively with traditional chemical analysis of the elemental carbon fraction on the filters. Several independent chemical tests are provided for the internal consistency of the analysis method which is based upon the use of reference spectra from pure TD. The application of a simplified analytical equipment for large numbers of samples is discussed.

Depth profiling free carbon in silicon carbide

A fingerprint for free carbon was identified from the negative ion mass spectra of various carbon and silicon carbide standards. It was found that high‐mass carbon molecular species can be used to distinguish free carbon in silicon carbide. Secondary ion mass spectrometry (SIMS) depth profiling (12 keV Cs+ primary beam, negative SIMS mode) was used to calculate the useful yield of negatively charged molecular carbon ions from carbon (in various forms) and silicon carbide. The useful yield ratio of C−n molecules from carbon to SiC increased with increasing n, meaning higher mass molecular carbon species are more likely to originate from free carbon than silicon carbide. 72C−6 and 96C−8 species were used to depth profile free carbon in chemically vapor deposited SiC/C coatings on SCS‐0 and SCS‐6 silicon carbide fibers.

Molecular carbon vapor source for flowing systems

A furnace for the production of molecular carbon species is described. The furnace generated equilibrium concentrations of carbon species (predominantly C3, C, and C2) by sublimation from a resistively heated graphite tube. Inert gas, admitted at the base of the furnace, provides a carrier for the carbon vapor. Furnace operating parameters and stability are also discussed.