135 research outputs found
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
Structure and infrared spectra of hydrocarbon interstellar dust analogs
9 págs.; 6 figs.; 5 tabs.A theoretical study of the structure and mid infrared (IR) spectra of interstellar hydrocarbon dust analogs
is presented, based on DFT calculations of amorphous solids. The basic molecular structures for these
solids are taken from two competing literature models. The first model considers small aromatic units
linked by aliphatic chains. The second one assumes a polyaromatic core with hydrogen and methyl
substituents at the edges. The calculated spectra are in reasonably good agreement with those of
aliphatic-rich and graphitic-rich samples of hydrogenated amorphous carbon (HAC) generated in our
laboratory. The theoretical analysis allows the assignment of the main vibrations in the HAC spectra and
shows that there is a large degree of mode mixing. The calculated spectra show a marked dependence
on the density of the model solids, which evinces the strong influence of the environment on the
strengths of the vibrational modes. The present results indicate that the current procedure of estimating
the hydrogen and graphitic content of HAC samples through the decomposition of IR features into
vibrational modes of individual functional groups is problematic owing to the mentioned mode mixing
and to the difficulty of assigning reliable and unique band strengths to the various molecular vibrations.
Current band strengths from the literature might overestimate polyaromatic structures. Comparison with
astronomical observations suggests that the average structure of carbonaceous dust in the diffuse
interstellar medium lies probably in between those of the two models considered, though closer to the
more aliphatic structure. ©the Owner Societies 2017This work has been funded by the MINECO of Spain under
grant FIS2013-48087-C2-1P, and AYA2015-71975-REDT, by the
MICINN of Spain under grant CDS2009-00038, and by the
European project ERC-2013-SyG, Grant Agreement 610256
‘‘NANOCOSMOS’’. G. M acknowledges MINECO PhD grant
BES-2014-069355.Peer reviewe
Plasma generation and processing of interstellar carbonaceous dust analogs
ICPIG (2017) ; Presentación de 32 diapositivas ; Estoril, Lisbon, 9 to 14 July 2017 ; Invited Lecture to XXXIII ICPIG ; http://icpig2017.tecnico.ulisboa.pt/Different samples of plasma deposited amorphous hydrogenated carbon (HAC or a-C:H) are used
as analogs of carbonaceous dust in the diffuse interstellar (IS) medium. Comparison of measured
and theoretically calculated spectra suggests that IS dust grains are likely made of small aromatic
islands linked by aliphatic chains. Irradiation of the HAC deposits with 5 keV electrons shows that
the effects of cosmic rays on the aliphatic dust component, characterized by an absorption band at
3.4 m, are small and cannot explain the disappearance of this band in dense interstellar clouds.N
Inverse Spin Hall Effect in nanometer-thick YIG/Pt system
High quality nanometer-thick (20 nm, 7 nm and 4 nm) epitaxial YIG films have
been grown on GGG substrates using pulsed laser deposition. The Gilbert damping
coefficient for the 20 nm thick films is 2.3 x 10-4 which is the lowest value
reported for sub-micrometric thick films. We demonstrate Inverse spin Hall
effect (ISHE) detection of propagating spin waves using Pt. The amplitude and
the lineshape of the ISHE voltage correlate well to the increase of the Gilbert
damping when decreasing thickness of YIG. Spin Hall effect based
loss-compensation experiments have been conducted but no change in the
magnetization dynamics could be detected
Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies
Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (108–109 W cm−2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed
Electronic control of the spin-wave damping in a magnetic insulator
It is demonstrated that the decay time of spin-wave modes existing in a
magnetic insulator can be reduced or enhanced by injecting an in-plane dc
current, , in an adjacent normal metal with strong spin-orbit
interaction. The demonstration rests upon the measurement of the ferromagnetic
resonance linewidth as a function of in a 5~m diameter
YIG(20nm){\textbar}Pt(7nm) disk using a magnetic resonance force microscope
(MRFM). Complete compensation of the damping of the fundamental mode is
obtained for a current density of , in
agreement with theoretical predictions. At this critical threshold the MRFM
detects a small change of static magnetization, a behavior consistent with the
onset of an auto-oscillation regime.Comment: 6 pages 4 figure
Optimization of laser-firing processes for silicon-heterojunction solar-cell back contacts
One of the key steps to achieve high efficiencies in amorphous/crystalline silicon photovoltaic structures is to design low-ohmic-resistance backcontacts with good passivation in the rear part of the cell. A well known approach to achieve this goal is to use laser-fired contact (LFC) processes in which a metal layer is fired through the dielectric to define good contacts with the semiconductor. However, and despite the fact that this approach has demonstrated to be extremely successful, there is still enough room for process improvement with an appropriate optimization. In this paper, a study focused on the optimal adjustment of the irradiation parameters to produce laser-fired contacts in a-Si:H/c-Si heterojunctionsolarcells is presented. We used samples consisting of crystalline-silicon (c-Si) wafers together with a passivation layer of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) deposited by plasma-enhanced chemical deposition (PECVD). Then, an aluminum layer was evaporated on both sides, the thickness of this layer varied from 0.2 to 1 μm in order to identify the optimal amount of Al required to create an appropriate contact. A q-switched Nd:YVO4laser source, λ = 532 nm, was used to locally fire the aluminum through the thin a-Si:H(i)-layers to form the LFC. The effects of laser fluences were analyzed using a comprehensive morphological and electrical characterization
Optimization of laser-firing processes for silicon-heterojunction solar-cell back contacts
One of the key steps to achieve high efficiencies in amorphous/crystalline silicon photovoltaic structures is to design low-ohmic-resistance backcontacts with good passivation in the rear part of the cell. A well known approach to achieve this goal is to use laser-fired contact (LFC) processes in which a metal layer is fired through the dielectric to define good contacts with the semiconductor. However, and despite the fact that this approach has demonstrated to be extremely successful, there is still enough room for process improvement with an appropriate optimization. In this paper, a study focused on the optimal adjustment of the irradiation parameters to produce laser-fired contacts in a-Si:H/c-Si heterojunctionsolarcells is presented. We used samples consisting of crystalline-silicon (c-Si) wafers together with a passivation layer of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) deposited by plasma-enhanced chemical deposition (PECVD). Then, an aluminum layer was evaporated on both sides, the thickness of this layer varied from 0.2 to 1 μm in order to identify the optimal amount of Al required to create an appropriate contact. A q-switched Nd:YVO4laser source, λ = 532 nm, was used to locally fire the aluminum through the thin a-Si:H(i)-layers to form the LFC. The effects of laser fluences were analyzed using a comprehensive morphological and electrical characterization
Study of a-Si crystallization dependence on power and irradiation time using a CW green laser
An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. Laser energy is used to heat the a-Si thin film to change the microstructure to poly-Si. Thin film samples of a-Si were irradiated with a CW-green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. These parameters are identified as key variables in the crystallization process. The power threshold for crystallization is reduced as the irradiation time is increased. When this threshold is reached the crystalline fraction increases lineally with power for each irradiation time. The experimental results are analysed with the aid of a numerical thermal model and the presence of two crystallization mechanisms are observed: one due to melting and the other due to solid phase transformation
New strategies in laser processing of TCOs for light management improvement in thin-film silicon solar cells
Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells
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