From carbon nanostructures to new photoluminescence sources: an overview of new perspectives and emerging applications of low pressure PECVD

13 pages, 9 figures.Low-pressure, plasma-enhanced (PE)CVD is a powerful and versatile technique that has been used for thin-film deposition and surface treatment since the early 1960s. However, it is only recently that it has been used in applications other than the different stages of microelectronic circuit fabrication. Now, PECVD is being used in emerging applications due to new materials and process requirements in a wide variety of areas, such as biomedical applications, solar cells, fuel cell development, fusion research, or the synthesis of silicon nanocrystals showing efficient photoluminescence, useful for future solid-state light sources. These new scenarios have stimulated further development of novel PECVD diagnostic techniques, together with fundamental experimental and theoretical studies aimed at a better understanding of some of the basic processes underlying the plasma/surface interaction. This paper gives an overview of some new research areas where PECVD is finding promising applications.FJGV acknowledges partial financial support from CSIC-CAM (Project No. 200550M016 and 200650M016) and MEC (Projects No. MAT2006-13006-C02-01 and ENE2006-14577-C04-03), VJH and IT acknowledge funding from MEC (Projects No. FTN-2003-08228-C03-03, FIS2004-00456 and ENE2006-14577-C04-03).Peer reviewe

Energy distributions of neutrals and ions in H2 low temperature plasmas: a study of fast H atoms

In this work we study by visible emission spectroscopy, energy resolved ion mass spectrometry and electric probes, H2 plasmas generated in low pressure hollow cathode glow discharges. The study allows the determination of the energy distributions of the different plasma constituents (ions, electrons and neutrals), which span five orders of magnitude. The rotational H2 temperature, assumed to be close to the translational one, scarcely exceeds the room temperature (0.03 eV); free electrons, responsible of primary ionizations and dissociations, display mean kinetic energies of 3-6 eV; whereas ions (H+, H2+, H3+) and a part of H atoms reach the highest energies (300 eV). Our present study focuses on the line-shape analysis of the H Balmer series emitted by the plasma, whose spectral profiles evince its remarkable deviation from thermal equilibrium, and on the dependence of these profiles with H2 pressure. The aim is to get a deeper understanding of the processes responsible of this behaviour. Three different Doppler broadenings are found in the atomic lines: the narrow peak, the plateau, and the far wings, with FWHM of ~ 0.3, 6 and up to 80 eV, respectively [1]. Besides, a directional and asymmetric behaviour in the line shifts up to some 300 eV is observed when ions are directed preferentially towards the observation window through a grounded grid. The narrow line peak can be explained by direct electron impact excitation of the free H atoms and by Frank-Condon transitions to Rydberg levels. The plateau is also explained by electronic excitations, in this case to pre-dissociative levels and levels giving rise to dissociative ionization. Their effectiveness depends primarily on electron temperature. In contrast, the far wings are assumed to be mainly due to charge transfer processes and dissociative reactions of H2 with the H+, H2+ and H3+ ions accelerated in the plasma sheath. The latter processes depend strongly on collision frequency and increase appreciably with pressure. The ion energy distributions obtained by mass spectrometry [2, 3] and the electron temperatures measured with the electric probe support the validity of the proposed mechanisms.Peer Reviewe

Classical collision complexes in the D+H2(v=0, j=0)→HD(v′, j′)+H reaction

Detailed quasiclassical trajectory calculations of the D+H2(v=0, j=0)→HD(v′=0, j′)+H have been carried out in the range of collision energy from 0.35 to 1.25 eV. The calculated v′j′ state resolved differential cross sections and opacity functions show analogous structures to the ones obtained by accurate quantum mechanical results, that is, a peak along a line in the E−θ (or E−J) plane, that was attributed to broad resonances. Analysis of present results in terms of the duration of the collision indicates that those trajectories pertaining to these peaks proceed through the formation of short lived collision complexes with lifetimes of 15–35 fs. © American Institute of PhysicsPartly financed by the CICYT of Spain under Grant No. PB890041.Peer Reviewe

HDO infrared detection sensitivity and D/H isotopic exchange in amorphous and crystalline ICE

6 p. : gráf.The sensitivity of the OD stretching band as a probe to detect HDO in astrophysical ice is discussed based on IR laboratory spectra of HDO molecules embedded in H2O ice. This band is extremely broad and tends to disappear into the absorption continuum of H2O for low-temperature amorphous samples. Detectable HDO/H2O ratios with this technique may range from a few percent for amorphous samples to a few per thousand in crystalline ice. These relatively high upper limits and the appreciable dependence of the band shape on temperature, which would complicate the interpretation of data from many lines of sight, decisively limit the usefulness of the technique for HDO detection in astronomical observations. The process of isotopic H/D exchange in mixed ice of H2O/D2O is also studied through the evolution of the OD band in IR spectra. Isotopic exchange starts at ∼120 K and is greatly accelerated at 150 K, as crystallization proceeds in the ice. Annealed amorphous samples prove to be more favorable for isotope exchange than samples directly formed in crystalline phase. The annealing process seems to favor a polycrystalline ice morphology with a higher defect activity. These morphology differences can be of relevance for deuterium fractionation in astronomical . © 2011 The American Astronomical Society.Funded by the MCINN of Spain under grants FIS2007-61686 and FIS2010-16455; “Ramon y Cajal” programPeer reviewe

Neutral and ion kinetics in glow discharges of H2/O2 mixtures. Diagnostics and modelling

A combined diagnostics and modelling of low pressure H2/O2 plasmas at different pressures and mixture ratios, generated in a low pressure hollow cathode DC reactor, is presented. Neutral and ion distributions are measured by mass spectrometry. Langmuir probes provide charge densities and electron temperatures. As expected, apart from the precursors, H2O is detected in considerable amounts and H3O+ is a major ion for H-rich mixtures. A zero order kinetic model is used to explain the experimental results. H2O is produced via plasma-surface interactions in a multistep process. The ion distributions are determined in each case by a balance between the relative weights of electron impact processes and proton transfer chemistry.N

Neutral and ion kinetics in glow discharges of H2 / O2 mixtures. Diagnostics and modelling

London, United Kingdom, 14–17 April 2014; http://plasma14.iopconfs.org/homeLow pressure plasmas in electrical discharges with H2 and O2 are of interest in a variety of fields. In astrochemistry, the formation of H2O and H3O+ is of great relevance as they can be detected in interstellar environments. In fusion research, discharge cleaning is used to eliminate the residual molecules in a vacuum vessel, of which oxygen and water are major components. In this work, we present a study of the chemistry of neutral and ionic species in H2/O2 plasmas based on the experimental diagnostics and kinetic modelling of hollow cathode discharges at different pressures and mixture proportions. Neutral and ion distributions are measured by mass spectrometry, and Langmuir probes provide charge densities and electron temperatures. As expected, apart from the precursors, H2O is detected in considerable amounts. Concerning the charged species, pure hydrogen and oxygen ions are detected together with mixed ones. With increasing pressure, the ion distributions are dominated by H3O+ for mixtures with H2 concentrations higher than ~ 30%, in contrast, the protonated species O2H+ is hardly formed. A zero order kinetic model is used to explain the experimental results. H2O is produced via plasma-surface interactions in a multistep process. The ion distributions are determined in each case by a balance between the relative weights of electron impact processes and proton transfer chemistry. The relevance of negative ions in the plasma chemistry is also studied.Peer Reviewe

Chemistry in glow discharges of H2/O2 mixtures. Diagnostics and modelling

Peer Reviewe

The D+H2(v=1,j)→HD(v′,j′)+H reaction. A detailed quasiclassical trajectory study

Thorough quasiclassical trajectory (QCT) calculations have been carried out for the D+H2(v =1,j) exchange reaction. These calculations include integral and differential cross sections, rate constants, reaction probabilities as a function of total energy, opacity functions, and distributions of internal states of the HD product in the range of collision energies from the reaction threshold to 1.5 eV and initial j values from 0 to 12. An overall good agreement with some discrepancies is found between the present QCT results and those from experiments and accurate quantum-mechanical calculations. © 1994 American Institute of Physics.German-Spanish scientific exchange program >Acciones Integradas HispanoAlemanas,> under Project No. HA-063. Financial support by the DGICYT under Project No. PB92-0219-C03.Peer Reviewe

Influence of the reactants rotational excitation on the H + D2(v = 0, j) reactivity

10 págs.; 10 figs.; 1 tab.; Special Issue: Dynamics of Molecular Collisions XXV: Fifty Years of Chemical Reaction DynamicsWe have analyzed the influence of the rotational excitation on the H + D(v = 0, j) reaction through quantum mechanical (QM) and quasiclassical trajectories (QCT) calculations at a wide range of total energies. The agreement between both types of calculations is excellent. We have found that the rotational excitation largely increases the reactivity at large values of the total energy. Such an increase cannot be attributed to a stereodynamical effect but to the existence of recrossing trajectories that become reactive as the target molecule gets rotationally excited. At low total energies, however, recrossing is not significant and the reactivity evolution is dominated by changes in the collision energy; the reactivity decreases with the collision energy as it shrinks the acceptance cone. When state-to-state results are considered, rotational excitation leads to cold products rovibrational distributions, so that most of the energy is released as recoil energy.The authors acknowledge funding by the Spanish Ministry of Science and Innovation (grant Consolider Ingenio 2010 CSD2009-00038). J.A., F.J.A. and P.G.J. acknowledge also funding by the Spanish Ministry of Economy and Competitiveness (grant CTQ2012-37404-C02), and V.J.H. acknowledges additional funding by the Spanish Ministry of Science and Innovation (FIS2013-48087-C2-1P) and by the European Research Council (ERC-2013-Syg-610256).Peer Reviewe

High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 μm absorption band

9 págs; 5 figs.; 6 tabs.The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH3 and CH2 in carbonaceous dust. It is widely observed in the diffuse interstellar medium, but disappears in dense clouds. Destruction of CH3 and CH2 by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, cules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron the 3.4 μm band carriers lie in the 108 yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3×107 yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.This work has been funded by the MINECO of Spain under grant FIS2013-48087-C2-1P, by the MICINN of Spain under grant CDS2009-00038, and by the European project ERC-2013-Syg 610256. G.M. acknowledges MINECO PhD grant BES-2014-069355.Peer reviewe