[Show abstract][Hide abstract] ABSTRACT: Ag and Au discontinuous films were exposed to single nanosecond pulses of a homogenized beam of an excimer laser operating at 193 nm. For low fluences, the films convert into big, almost spherical and isolated nanoparticles (NPs) due to laser-induced dewetting. Their optical response exhibits a sharp surface plasmon resonance (SPR) consistent with that of spherical and non-interacting NPs. For higher fluences, the formation of many small NPs and almost no big NPs is observed instead. The SPR features change and the plasmonic response becomes influenced by multipolar interactions among neighbouring NPs. Low and high fluence regimes are respectively related to melting and boiling threshold of the metal, and additionally, craters appear in the latter regime.
[Show abstract][Hide abstract] ABSTRACT: This work studies the changes in the optical response and morphological features of 6±1 nm diameter Au nanoparticles (NPs) when covered by a layer of a-Al2O3 by pulsed laser deposition (PLD). The laser fluence used for ablating the Al2O3 target is varied in order to modify the kinetic energy (KE) of the species bombarding the NPs during their coverage. When the ion KE < 200 eV, the structural features and optical properties of the NPs are close to those of uncovered ones. Otherwise, a shift to the blue and a strong damping of the surface plasmon resonance is observed as fluence is increased. There are two processes responsible for these changes both related to aluminum ions arriving to the substrate during the coverage process, i.e. sputtering of the metal and implantation of aluminum species in the metal. Both processes have been simulated using standard models for ion bombardment, the calculated effective implanted depths allow explaining the observed changes in the optical response, and the use of a size-dependent sputtering coefficient for the Au NPs predicts the experimental sputtering fractions. In spite of the work is based on PLD, the concepts investigated and conclusions can straightforwardly be extrapolated to other physical vapor deposition techniques or processes involving ion bombardment of metal NPs by ions having KE > 200 eV.
Journal of Applied Physics 03/2014; 115(12):124303. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work reports on the fabrication of 1D fringed patterns on nanostructured porous silicon (nanoPS) layers (563, 372, and 290 nm thick). The patterns are fabricated by phase-mask laser interference using single pulses of an UV excimer laser (193 nm, 20 ns pulse duration). The method is a single-step and flexible approach to produce a large variety of patterns formed by alternate regions of almost untransformed nanoPS and regions where its surface has melted and transformed into Si nanoparticles (NPs). The role of laser fluence (5–80 mJ cm−2), and pattern period (6.3–16 μm) on pattern features and surface structuring are discussed. The results show that the diameter of Si NPs increases with fluence up to a saturation value of 75 nm for a fluence ≈40 mJ cm−2. In addition, the percentage of transformed to non-transformed region normalized to the pattern period follows similar fluence dependence regardless the period and thus becomes an excellent control parameter. This dependence is fitted within a thermal model that allows for predicting the in-depth profile of the pattern. The model assumes that transformation occurs whenever the laser-induced temperature increase reaches the melting temperature of nanoPS that has been found to be 0.7 of that of crystalline silicon for a porosity of around 79%. The role of thermal gradients across the pattern is discussed in the light of the experimental results and the calculated temperature profiles, and shows that the contribution of lateral thermal flow to melting is not significant for pattern periods ≥6.3 μm.
Journal of Applied Physics 01/2014; 115(18):184902-184902-8. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Metal nanoparticles (NPs) are often used as building blocks for nanostructured thin film systems, the properties of the system depending on the morphology and dimensions of the NPs. When using thin film technologies, the surface where the NPs nucleate on plays an important role on the growth process and eventually on the NPs features. The aim of this work is to produce metal NPs on substrates with different shapes.
Gold NPs are produced by pulsed laser deposition on amorphous carbon, glass and MgO substrates at room temperature. The average dimensions of the NPs are controlled through the deposition time to have dimensions < 10 nm. The morphology of the NPs is studied by high resolution electron transmission microscopy (HRTEM) and the optical properties are studied by absorption spectroscopy.
The results show that Au NPs are crystalline in all cases and for large NPs (> 5 nm), they are elongated due to coalescence and coarsening. The main difference when using amorphous or single crystalline substrate is that the many NPs become faceted and textured in the latter case. The optical spectra show however no significant differences. The results will be discussed in terms of the random orientation of the NPs across the substrate plane.
[Show abstract][Hide abstract] ABSTRACT: Pairs of samples containing Ag nanoparticles (NPs) of different dimensions have been produced under the same conditions but on different substrates, namely standard glass slides and a thin layer of amorphous aluminum oxide (a-Al2O3) on-glass. Upon storage in ambient conditions (air and room temperature) the color of samples changed and a blue-shift and damping of the surface plasmon resonance was observed. The changes are weaker for the samples on-glass and tend to saturate after 12 months. In contrast, the changes for the samples on a-Al2O3 appear to be still progressing after 25 months. While x-ray photoelectron spectroscopy shows a slight sulfurization and negligible oxidation of the Ag for the on-glass samples upon 25 months aging, it shows that Ag is strongly oxidized for the on a-Al2O3 samples and sulfurization is negligible. Both optical and chemical results are consistent with the production of a shell at the expense of a reduction of the metal core dimensions, the latter being responsible for the blue-shift and related to the small (<10 nm initial diameter) of the NPs. The enhanced reactivity of the Ag NPs on the a-Al2O3 supports goes along with specific morphological changes of the Ag NPs and the observation of nitrogen.
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is to demonstrate that single-photon photoionization processes make a significant difference in the expansion and temperature of the plasma produced by laser ablation of ceramic Al2O3 in vacuum as well as to show their consequences in the kinetic energy distribution of the species that eventually will impact on the film properties produced by pulsed laser deposition. This work compares results obtained by mass spectrometry and optical spectroscopy on the composition and features of the plasma produced by laser ablation at 193 nm and 248 nm, i.e., photon energies that are, respectively, above and below the ionization potential of Al, and for fluences between threshold for visible plasma and up to ≈2 times higher. The results show that the ionic composition and excitation of the plasma as well as the ion kinetic energies are much higher at 193 nm than at 248 nm and, in the latter case, the population of excited ions is even negligible. The comparison of Maxwell-Boltzmann temperature, electron temperatures, and densities of the plasmas produced with the two laser wavelengths suggests that the expansion of the plasma produced at 248 nm is dominated by a single population. Instead, the one produced at 193 nm is consistent with the existence of two populations of cold and hot species, the latter associated to Al+ ions that travel at the forefront and produced by single photon ionization as well as Al neutrals and double ionized ions produced by electron-ion impact. The results also show that the most energetic Al neutrals in the plasma produced at the two studied wavelengths are in the ground state.
Journal of Applied Physics 06/2013; 113(22). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is to produce 2D plasmonic and diffractive structures in Ag films with sharp features for which both a deeper understanding of laser induced transformation upon modulated laser intensity and a correlation between structural and optical properties are required. We compare results obtained by exposing silver films to an excimer laser operating at 193 nm whose intensity is either modulated or homogeneous. In all cases, one laser exposure is enough to break the film into nanoparticles (NPs). The use of the modulated beam intensity leads to diffractive 2D patterns that are formed by rectangular regions of untransformed material surrounded by transformed regions covered by NPs. The former have sharp edges that are consistent with the absence of significant mass transport that is discussed in terms of the thermal gradient induced. The latter contain NPs whose diameter increases as the initial film effective thickness increases. The surface plasmons associated with the NPs in the transformed regions dominate the reflectivity spectrum and the 2D array formed by the untransformed regions is responsible for the diffractive properties. Evidence for spinodal dewetting is only observed in our case for the steep gradient conditions achieved at the border of the homogeneously irradiated regions.
[Show abstract][Hide abstract] ABSTRACT: Discontinuous Au films are prepared on glass substrates by pulsed laser deposition with two different metal coverages that lead to a film being formed by irregular coalesced nanoparticles (NPs) and to another film close to the percolation limit. The films are exposed to three interfering beams at different intensities produced by the fourth harmonic of a Nd:YAG laser (266nm, 10ns). Scanning electron microscopy and extinction spectra are used respectively to study the structural and optical properties before and after the laser structuring. Round metal NPs appear in the laser transformed areas due to melting followed by rapid solidification that is reflected in the extinction spectra by the appearance of a surface plasmon resonance around 530–540nm. The areas with NPs are surrounded by non-transformed areas forming a periodic pattern that evolves from a 2D array to parallel lines when local laser intensity increases to cover the whole sample at high intensity. The accumulation of several pulses at low fluence can also transform the metal film almost completely by creating alternating areas having different NP dimensions. The accumulation of metal in some areas of the pattern is consistent with mass transport towards the lower temperature regions.
[Show abstract][Hide abstract] ABSTRACT: This work reports on the optical and structural properties of nanostructured films formed by Ag nano-objects embedded in amorphous aluminium oxide (a-Al(2)O(3)) prepared by alternate pulsed laser deposition (PLD). The aim is to understand the importance of the energetic species involved in the PLD process for nanostructuring, i.e. for organizing nanoparticles (NPs) in layers or for self-assembling them into nanocolumns (NCls), all oriented perpendicular to the substrate. In order to change the kinetic energy of the species arriving at the substrate, we use a background gas during the deposition of the embedding a-Al(2)O(3) host. It was produced either in vacuum or in a gas pressure (helium and argon) while the metal NPs were always produced in vacuum. The formation of NPs or NCls is easily identified through the features of the surface plasmon resonances (SPR) in the extinction spectra and confirmed by electron microscopy. The results show that both the layer organization and self-assembling of the metal are prevented when the host is produced in a gas pressure. This result is discussed in terms of the deceleration of species arriving at the substrate in gas that reduces the metal sputtering by host species (by ≈58%) as well as the density of the host material (by ≥19%). These reductions promote the formation of large voids along which the metal easily diffuses, thus preventing organization and self-organisation, as well as an enhancement of the amount of metal that is deposited.
[Show abstract][Hide abstract] ABSTRACT: The aim of this work is to study the significance of negative ions in the plasma produced by ablation of a simple oxide ceramic target (Al2O3) at distances and fluences typically used in pulsed-laser deposition processes. The results show that negative ions are indeed produced, the majority of which (>82%) being O− that are predominantly produced by neutralization of O+ followed by electron attachment. They represent one third of the O+ population at low fluences for a distance of 4 cm from the target at which most deposition experiments are performed. AlO− represents up to 15% of the negative ions and their amount increases at the expense of O− as fluence is increased. The most abundant as well as the fastest species in the plasma is by far Al+ that represent >80% of ions having kinetic energy
Journal of Physics D Applied Physics 01/2012; 45(28). · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Phase switching in GeTe thin films (grown using a modified metal organic chemical vapor deposition system) upon pulsed femtosecond and nanosecond laser irradiation has been studied. Two in situ methods, i.e., optical microscopy and real-time reflectivity measurements, have been used in order to compare the optical response before and after phase change and to follow the phase change dynamics with a time resolution close to 400 ps. The results show that cycling is possible under irradiation with both fs and ns pulses using single pulses for amorphization and multiple pulses for crystallization. The use of ns pulses favors the crystalline-to-amorphous phase transformation, with a characteristic transformation time of ∼15 ns. The presence of the liquid phase was identified and temporally resolved, featuring a well-defined transient reflectivity state, in between those of the crystalline and amorphous phases. We have also studied the role of material configuration in the phase change dynamics and the mechanisms involved in the re-crystallization process.
Journal of Applied Physics 06/2011; 109(12):123102-123102-7. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effects induced during the covering/embedding of metal nanoparticles (NPs) produced by pulsed laser deposition (PLD) and their impact on the structural and optical properties have been studied by producing pairs of samples containing Au NPs that are either uncovered (i.e., at the surface) or covered (i.e., embedded in an amorphous a-Al 2 O 3 host). The main result is that covering species can sputter up to 100% of the Au atoms, the smaller the NPs the higher the sputtered fraction. This fraction has been simulated using standard models for ion bombardment and taking into account the kinetic energy distribution of arriving species and the cohesive energy dependence on NPs dimensions. Although all models well predict the order of magnitude of the sputtering yield, the calculated values are generally smaller than the experimental ones and do not account for the experimental dependence on NPs dimensions. This disagreement is discussed in terms of the limitations of standard models that do not take into account the lower adhesion of small NPs to the substrate, the high flux of species involved in PLD and, possibly to lesser extent, the use of some bulk material parameters. The metal sputtering during the coverage regulates the NPs morphology, through a reduction of dimensions and dimension dispersion. Most changes of structural features and optical spectra when covering the NPs are directly related to the variation in the amount of metal with the exception of a strong blueshift of the surface plasmon resonance when NPs are covered. This shift could be consistent with mixing of covering layer species and metal at the surface of the NPs.
Journal of Applied Physics 06/2011; · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work reports the study of ion dynamics produced by ablation of Al, Cu, Ag, Au, and Bi targets using nanosecond laser pulses at 193 nm as a function of the laser fluence from threshold up to 15 J cm−2. An electrical (Langmuir) probe has been used for determining the ion yield as well as kinetic energy distributions. The results clearly evidence that ablation of Al shows unique features when compared to other metals. The ion yield both at threshold (except for Al, which shows a two-threshold-like behavior) and for a fixed fluence above threshold scale approximately with melting temperature of the metal. Comparison of the magnitude of the yield reported in literature using other wavelengths allows us to conclude its dependence with wavelength is not significant. The evolution of the ion yield with fluence becomes slower for fluences above 4–5 J cm−2 with no indication of saturation suggesting that ionization processes in the plasma are still active up to 15 J cm−2 and production of multiple-charged ions are promoted. This dependence is mirrored in the proportion of ions with kinetic energies higher than 200 eV. This proportion is not significant around threshold fluence for all metals except for Al, which is already 20%. The unique features of Al are discussed in terms of the energy of laser photons (6.4 eV) that is enough to induce direct photoionization from the ground state only in the case of this metal.
Journal of Applied Physics 02/2011; 109(4). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the dynamics of ions produced upon ablation of Al and ceramic Al2O3 targets using nanosecond laser pulses at 193 nm (6.4 eV) as a function of the laser fluence from threshold up to 12 J cm−2. An electrical (Langmuir) probe located at 40 mm from the target surface has been used for determining the ion yield and calculating the kinetic energy distributions. The results for both targets show the existence of a significant amount of ions having kinetic energies >200 eV (≈20% around threshold fluence), and kinetic energies are up to >1.5 keV. The results are related with the existence of direct photonionization processes caused by the photon energy of the laser being higher than the ionization potential of Al (5.98 eV). Comparison of the ion yield when ablating the two types of targets for fluences above threshold to data reported in the literature suggests that the magnitude of the yield and its threshold are parameters depending on the thermal properties of the target rather than on the laser wavelength. Around threshold, the different behavior of ion yield when ablating Al and Al2O3 targets suggests that the threshold for neutral aluminium and ion species in the case of ablation of the Al2O3 target must be similar.
Applied Physics A 01/2011; 105(1). · 1.69 Impact Factor