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1.
Nanotechnology ; 30(47): 475603, 2019 Nov 22.
Article in English | MEDLINE | ID: mdl-31426050

ABSTRACT

We experimentally analyze different growth regimes of Ti thin films associated to the existence of kinetic energy-induced relaxation mechanisms in the material's network when operating at oblique geometries. For this purpose, we have deposited different films by evaporation and magnetron sputtering under similar geometrical arrangements and at low temperatures. With the help of a well-established growth model we have found three different growth regimes: (i) low energy deposition, exemplified by the evaporation technique, carried out by species with typical energies in the thermal range, where the morphology and density of the film can be explained by solely considering surface shadowing processes, (ii) magnetron sputtering under weak plasma conditions, where the film growth is mediated by surface shadowing mechanisms and kinetic-energy-induced relaxation processes, and (iii) magnetron sputtering under intense plasma conditions, where the film growth is highly influenced by the plasma, and whose morphology is defined by nanocolumns with similar tilt than evaporated films, but with much higher density. The existence of these three regimes explains the variety of morphologies of nanocolumnar Ti thin films grown at oblique angles under similar conditions in the literature.

2.
Nanotechnology ; 28(46): 465605, 2017 11 17.
Article in English | MEDLINE | ID: mdl-29063864

ABSTRACT

In this work we analyze a phenomenon that takes place when growing magnetron sputtered porous/compact multilayer systems by alternating the oblique angle and the classical configuration geometries. We show that the compact layers develop numerous fissures rooted in the porous structures of the film below, in a phenomenon that amplifies when increasing the number of stacked layers. We demonstrate that these fissures emerge during growth due to the high roughness of the porous layers and the coarsening of a discontinuous interfacial region. To minimize this phenomenon, we have grown thin interlayers between porous and compact films under the impingement of energetic plasma ions, responsible for smoothing out the interfaces and inhibiting the formation of structural fissures. This method has been tested in practical situations for compact TiO2/porous SiO2 multilayer systems, although it can be extrapolated to other materials and conditions.

3.
Sci Rep ; 7(1): 5924, 2017 07 19.
Article in English | MEDLINE | ID: mdl-28725039

ABSTRACT

Plasma treatment is recognized as a suitable technology to improve germination efficiency of numerous seeds. In this work Quinoa seeds have been subjected to air plasma treatments both at atmospheric and low pressure and improvements found in germination rate and percentage of success. Seed water uptake by exposure to water vapor, although slightly greater for plasma treated seeds, did not justify the observed germination improvement. To identify other possible factors contributing to germination, the chemical changes experienced by outer parts of the seed upon plasma exposure have been investigated by X-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM-EDX). XPS revealed that the outer layers of the Quinoa plasma treated seeds were highly oxidized and appeared enriched in potassium ions and adsorbed nitrate species. Simultaneously, SEM-EDX showed that the enrichment in potassium and other mineral elements extended to the seed pericarp and closer zones. The disappearance from the surface of both potassium ions and nitrate species upon exposure of the plasma treated seeds to water vapor is proposed as a factor favoring germination. The use of XPS to study chemical changes at seed surfaces induced by plasma treatments is deemed very important to unravel the mechanisms contributing to germination improvement.


Subject(s)
Chenopodium quinoa/chemistry , Germination/drug effects , Plasma Gases/pharmacology , Seeds/chemistry , Photoelectron Spectroscopy , Seeds/ultrastructure , Surface Properties , Water/chemistry
4.
Nanotechnology ; 24(4): 045604, 2013 Feb 01.
Article in English | MEDLINE | ID: mdl-23299349

ABSTRACT

Growth regimes of gold thin films deposited by magnetron sputtering at oblique angles and low temperatures are studied from both theoretical and experimental points of view. Thin films were deposited in a broad range of experimental conditions by varying the substrate tilt angle and background pressure, and were analyzed by field emission scanning electron microscopy and grazing-incidence small-angle x-ray scattering techniques. Results indicate that the morphological features of the films strongly depend on the experimental conditions, but can be categorized within four generic microstructures, each of them defined by a different bulk geometrical pattern, pore percolation depth and connectivity. With the help of a growth model, a microstructure phase diagram has been constructed where the main features of the films are depicted as a function of experimentally controllable quantities, finding a good agreement with the experimental results in all the studied cases.


Subject(s)
Gold/chemistry , Membranes, Artificial , Metal Nanoparticles/chemistry , Metal Nanoparticles/ultrastructure , Models, Chemical , Models, Molecular , Computer Simulation , Crystallization/methods , Hot Temperature , Macromolecular Substances/chemistry , Magnetic Fields , Materials Testing , Molecular Conformation , Particle Size , Porosity , Surface Properties
5.
Phys Rev Lett ; 96(23): 236101, 2006 Jun 16.
Article in English | MEDLINE | ID: mdl-16803386

ABSTRACT

The microstructure and the scaling properties of films grown by plasma enhanced chemical vapor deposition are reproduced with a discrete model that takes into account the angular distribution function of the particles and the lateral growth of the films. Both the experimental and simulated surfaces exhibit a granular microstructure and an anomalous scaling behavior characterized by values of the growth exponent beta that vary with the scale of measurement. Depending on the angular distribution function used in the model, values of beta ranging from 0.86 to 0.2 are obtained.

6.
Phys Rev E Stat Nonlin Soft Matter Phys ; 72(1 Pt 2): 016401, 2005 Jul.
Article in English | MEDLINE | ID: mdl-16090093

ABSTRACT

In this work the influence of the excited states on the electron-energy distribution function has been determined for an argon microwave discharge at low pressure. A collisional-radiative model of argon has been developed taking into account the most recent experimental and theoretical values of argon-electron-impact excitation cross sections. The model has been solved along with the electron Boltzmann equation in order to study the influence of the inelastic collisions from the argon excited states on the electron-energy distribution function. Results show that under certain conditions the excited states can play an important role in determining the shape of the distribution function and the mean kinetic energy of the electrons, deplecting the high-energy tail due to inelastic processes from the excited states, especially from the 4s excited configuration. It has been found that from the populations of the excited states an excitation temperature can be defined. This excitation temperature, which can be experimentally determined by optical emission spectroscopy, is lower than the electron kinetic temperature obtained from the electron-energy distribution function.

7.
J Phys Chem A ; 109(22): 4930-8, 2005 Jun 09.
Article in English | MEDLINE | ID: mdl-16833840

ABSTRACT

The plasma chemistry of NO has been investigated in gas mixtures with oxygen and/or hydrocarbon and Ar as carrier gas. Surface wave discharges operating at microwave frequencies have been used for this study. The different plasma reactions have been analyzed for a pressure range between 30 and 75 Torr. Differences in product concentration and/or reaction yields smaller than 10% were found as a function of this parameter. The following gas mixtures have been considered for investigation: Ar/NO, Ar/NO/O2, Ar/NO/CH4, Ar/CH4/O2, Ar/NO/CH4/O2. It is found that NO decomposes into N2 and O2, whereas other products such as CO, H2, and H2O are also formed when CH4 and O2 are present in the reaction mixture. Depending on the working conditions, other minority products such as HCN, CO2, and C2 or higher hydrocarbons have been also detected. The reaction of an Ar/NO plasma with deposits of solid carbon has also been studied. The experiments have provided useful information with respect to the possible removal of soot particles by this type of plasma. It has been shown that carbon deposits are progressively burned off by interaction with the plasma, and practically 100% decomposition of NO was found. Plasma intermediate species have been studied by optical emission spectroscopy (OES). Bands and/or peaks due to N2*, NO*, OH*, C2*, CN*, CH*, or H* were detected with different relative intensities depending on the gas mixture. From the analysis of both the reaction products and efficiency and the type of intermediate species detected by OES, different plasma reactions and processes are proposed to describe the plasma chemistry of NO in each particular mixture of gases. The results obtained provide interesting insights about the plasma removal of NO in real gas exhausts.

8.
Phys Rev E Stat Nonlin Soft Matter Phys ; 66(6 Pt 2): 066401, 2002 Dec.
Article in English | MEDLINE | ID: mdl-12513406

ABSTRACT

Both an energy balance equation and a collisional-radiative model were developed in order to discover which process is responsible for gas heating in a low-pressure argon discharge. In this way, for a wide range of plasma conditions, the space-charge field contribution to gas heating was found to be negligible compared to that resulting from elastic collisional processes, although the value of the former is higher than the latter when calculating the absorbed power per electron. This is due to (1) the heating associated with the space-charge field only being effective in the plasma sheath, which is very close to the vessel inner wall. (2) The vessel temperature value at the external wall is taken as a boundary condition, as a result of which the space-charge field influence on gas heating is indirectly imposed on the model. The results of the collisional-radiative model take into account the influence of gas heating on the electron temperature and on the argon low-lying excited levels. Two different zones have been found. The first corresponds to low electron densities, in which the gas temperature remains constant, whereas in the second (high electron densities) the heating of the gas takes on great importance. These results compare well with experimental data.

9.
Talanta ; 39(4): 341-7, 1992 Apr.
Article in English | MEDLINE | ID: mdl-18965385

ABSTRACT

A simple continuous flow generation of volatile bromine is described for the determination of low concentrations of the elements by atmospheric-pressure argon microwave induced plasma (MIP) surfatron. Bromine is continuously generated by mixing the bromide with sulphuric acid and hypochlorite solutions. The bromine vapor is separated from the aqueous phase by a gas-liquid separator and is desiccated by passing it through concentrated sulphuric acid. The detection limit attained was 2 microg/l. and the precision was +/-0.7% (at the 80 microg/l. level). The proposed determination is very selective if oxidizing/reducing agents are absent. The procedure has been tested for bromide determination in two drug preparations. Good agreement between the experimental results and the certified values has been obtained.

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