Surface Reactions Of Radicals Research Articles

Propyl-cyanide isomer formation on interstellar ices from radical association: a quantum theoretical study

ABSTRACT Both linear and branched isomers of propyl cyanide (PrCN; C3H7CN) are ubiquitous in interstellar space. To date, PrCN is one of the most complex molecules found in the interstellar medium. Furthermore, it is the only one observed species to share the branched atomic backbone of amino acids, some of the building blocks of life. Radical–radical chemical reactions are examined in detail using density functional theory, ab initio methods, and the energy resolved master equation formalism to compute rate constants at a low pressure value prevalent in the ISM. Quantum chemical studies are reported for both isomers considering two possibilities: the gas phase association and the surface reactions of radicals on a 34-water amorphous ice model. The reaction mechanism involves the following radicals association: CH3CHCH3 + CN, CH3 + CH3CHCN and CH3CH2 + CH2CN, CH3 + CH2CH2CN, CN + CH3CH2CH2 for iso-PrCN and n-PrCN formation, respectively. Two DFT methods: M062X and ωB97XD with the 6-311++G(d,p) basis set were tested for reactions in gas phase and on the ice mantle. In the gas phase, MP2/aug-cc-pVTZ level of theory is also used, and the energetics of the five reactions are calculated using explicitly correlated coupled cluster (CCSD(T)-F12) method. All reaction paths are exoergic and barrierless in the gas phase and on the ice-model, suggesting that the formation of iso-PrCN and n-PrCN is efficient on the ice model adopted in this paper. The gas phase rate constants of formation of both isomers can be eventually used as a high limit for the solid state reactions.

Effects of intentionally introduced nitrogen and substrate temperature on growth of diamond bulk single crystals

Aiming at stable growth of bulk single-crystal diamond, multiple effects of intentional nitrogen introduction and substrate temperature on the growth were studied. The intensity of fluorescence of the nitrogen-vacancy (NV0) center was qualitatively correlated with the concentrations of nitrogen in the grown layers. Growth rates and morphologies varied with nitrogen concentration in the gas phase and substrate temperature. It was shown that appropriate control of substrate temperature allows continuous growth, which makes the substrate thicker. The mechanism underlying the effect of nitrogen on growth rate is discussed on the basis of the obtained results, which suggest also the importance of substrate temperature in controlling migration and the surface reactions of radicals.

Development of New Hydrogenated Amorphous Silicon Thin Films with Quantum Wires

A new type of hydrogenated amorphous silicon (a-Si:H) thin film with nanowires was formed by applying a bias electric field along the growth surface of an a-Si:H thin film during a glow discharge in monosilane gas. Analyses of structural, electrical and optical properties revealed that nanoparticles with a radius of approximately 20 nm were formed and grew into nanowires with a length of 1 µm on the a-Si:H thin film surface as the bias electric field increased. In comparison with the carrier mobility of a conventional a-Si:H thin film, the carrier mobility of the nanowire estimated from the anisotropic electric conductivity was inferred to increase by up to three orders of magnitude or more. The dependence of the electric field on the optical band gap implies that quantum wires exist in the nanowire. These results suggest that quantum wires were formed in nanowires because of the effect of the bias electric field on the surface reaction of radicals along the growth surface during glow discharge.

CF and CF2 Radical Densities in 13.56-MHz CHF3/Ar Inductively Coupled Plasma

Radial and axial distributions of CF and CF2 radical densities in CHF3/Ar inductively coupled plasma (ICP) generated by a radio frequency of 13.56 MHz were evaluated using laser-induced fluorescence (LIF). The radical densities in the bulk plasma were estimated by combining the density distributions measured by LIF with the absolute radical density measured by appearance mass spectrometry (AMS) near the reactor wall. Axial and radial distributions of CF and CF2 radical densities were extremely hollow. The radial distribution was strongly correlated with the electron density distribution and was significantly influenced by the dissociation process resulting from the electron excitation in the reactor space. The axial distribution, on the other hand, was mainly determined by the surface reaction of radicals on the wafer.

Kinetics of deposition and electrical properties of silicon nitride films obtained by 185 nm photolysis of SiH4-NH3 mixtures

The photolysis of NH3 and SiH4 by the 185 nm radiation of a low-pressure Hg lamp is used to deposit silicon nitride films at temperatures ranging from 50 up to 420 degrees C. The deposition kinetics are analysed by considering the gas phase and surface reactions of radicals (H, NH2, SiH3, SiH2) involved in the photochemical process. Radical densities and flux to the walls in the deposition chamber are computed from numerical solutions of one-dimensional spatially resolved balance equations for creation, reaction and diffusion of each species. The film composition is analysed by X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS). The thickness and refractive index are obtained by ellipsometry. Finally the electrical properties are derived from current-voltage I(V) and capacitance-voltage C(V) characteristics. The optimum dielectric properties are obtained for deposition temperatures between 250 and 350 degrees C and for SiH4 concentration in NH3 of about 1%.

Surface reactions of radicals during deposition of amorphous semiconductors by photochemical decomposition of molecular hydrides

Surface reactions of SiH 3 and GeH 3 radicals during deposition of a-Si:H and a-Ge:H by Hg-photosensitization of SiH 4 and GeH 4 are studied between room temperature and 350°C. Surface reaction probabilities are determined and formation of XH 4 and X 2H 6 (X=Si,Ge) molecules at the surface is demonstrated. A drastic change in GeH 3 reaction mechanisms is observed above 200°C at the onset of GeH 4 thermal CVD.