Grupo de Procesado por Láser del Instituto de Óptica “Daza de Valdés”
Pulsed Laser Deposited Silver Nanostructures for Molecular Sensing and Photovoltaic Applications

Author: Giorgio Baraldi
Fecha de defensa: Octubre de 2014
Director: Dr. Jose Gonzalo de los Reyes
Universidad Complutense de Madrid

The Laser Processing Group (LPG) of the Instituto de Óptica (IO) has a large and well established experience in fabricating nanostructured films containing nanoparticles using PLD. They successfully fabricated metal nanoparticles such as Au, Fe, Bi, Ag, Cu, Co generally supported or embedded in an amorphous aluminium oxide matrix (a-Al2 O3 ). In addition, they also explored the fabrication of nanostructures containing bi-metallic nanoparticles or made multi-layers containing nanoparticles of different metals. Finally, they also showed that a precise control of growth parameters allowed the fabrication of Ag nanocolumns of different aspect ratio oriented perpendicular to the substrate surface. Its research was mainly addressed to study the effect that the laser and experimental setup parameters (number of laser pulse, laser fluence, target and substrate geometrical configuration) have on the final morphology of nanoparticles and to investigate the relation between the nanoparticle morphology and their optical, vibrational and thermal response. On the one side, they have demonstrated that nanoparticle size increases with the number of laser pulses used to ablate a target, that too high laser fluences can lead to the formation of plasmas containing ions with very high kinetic energies (> 200 eV) that can promote material sputtering from or implantation in the substrate and that the homogeneity of the deposit can be improved by adjusting the position of the substrate with respect to that of the target.

On the other side, they have observed that the optical response of nanostructures depend on the nature, size, shape and spatial arrangement of nanoparticles. For example, non-spherical nanoparticles (prolate and oblate spheroids) show an optical response characterized by a longitudinal and transverse mode whose spectral positions depend on the aspect ratio of the nanoobject. Finally, the dependence of vibrational properties on the size and shape of nanoparticles was analyzed as well and, in addition, the interaction between surface plasmons and acoustic vibration was demonstrated. These findings support that a precise control over nanoparticle morphology is crucial for the fabrication of nanostructured films with specific optical response suitable for technological applications. In this thesis we consider the case of nanostructures containing Ag nanoparticles supported on or embedded in a-Al 2 O3 layers. In particular, we use PLD to fabricate films with different content of Ag and we explore the feasibility of thermal annealing and laser irradiation to improve the control over nanoparticle size, shape and orientation in the substrate plane. In addition, since solid substrates can guarantee more reliable and reproducible SERS spectra and the near-field enhancement and far-field scattering occurring at the SPR can improve the performance of thin film solar cells by promoting light trapping processes, we study the application of the fabricated nanostructures to SERS and as light trapping elements in photovoltaics. W chose Ag among other noble metals because its SPR is well separated from interband transitions and its larger scattering cross-section that can play a key role in applications such as photovoltaics,9 whereas we selected an a-Al 2 O3 matrix since it is a very robust and transparent dielectric that guarantees the proper deposition of metal nanoparticles as well as it has been commonly used as host matrix by the LPG, which eases the comparison of our results with those reported in previous works.

Since, in many applications such as SERS, Ag nanoparticles are required to be exposed to the external environment, we initially investigate the evolution of the morphology and optical response of nanoparticles deposited on a thin a-Al 2O3 layer with the amount of Ag in the films. In particular, we explore in detail three very different but representative cases of nanostructured films containing: small (~ 5 nm) and very dense Ag nanoparticles with almost circular in-plane projected shape; relatively large (~ 20 nm) and dense Ag nanoparticles with circular or elongated in-plane projected shapes; and an almost continuous Ag, i.e. percolated, film. The study of this type of nanostructure configuration allows us to observe that films deteriorate with time. We thus investigate the evolution of their optical response and chemical composition with time and we compare the obtained results with that of nanostructured films in which nanoparticles are covered with a thin layer of a-Al 2 O3 . We demonstrate that covered nanostructures show improved stability even for ultrathin covering layers (< 1 nm). Simultaneously, the comparison of TEM images of uncovered and covered films shows that the deposition of the a-Al2 O3 covering layer strongly affects the nanoparticle morphology. A similar effect was previously observed by Resta and co-workers for Au nanoparticles and associated to the species in the plasma generated upon ablation of ceramic Al 2 O3 target that arrive to the substrate with kinetic energy high enough to promote Au sputtering. We thus verified the observed result by measuring the Ag content in nanostructured films before and after deposition of the covering layer and by modelling sputtering using an approach based on SRIM 2008 software. In order to use in the model an ion kinetic energy distribution resembling that of the ions impinging on Ag nanoparticles, we study the dynamics of the plasma generated upon ablation of ceramic Al 2 O3 target using a Langmuir probe under similar experimental conditions to those used for nanostructured film fabrication. We then explore the effect of thermal annealing on percolated Ag films to verify its potential to improve our control over nanoparticle size. We performed annealing of percolated Ag films in different environments (air and vacuum) and we observe that, effectively, the dewetting process promoted by the temperature increase leads to the formation of Ag nanoparticles with size well above the one obtained spontaneously as a result of Ag nucleation and coalescence. In addition, we also demonstrate that large nanoparticles (> 100 nm) show an enhanced contribution of light scattering, contrary to relatively small nanoparticles (< 50 nm). Finally, due to Ag tarnishing, we deposit an a-Al2 O3 layer on annealed films and we observe that conformal growth is achieved. This result goes along with the reduction of Ag sputtering due to the increased nanoparticle size and reduced surface coverage.

In order to achieve a better control over nanoparticle shape and orientation, we performed laser irradiation experiments on covered nanostructured films containing coalesced nanoparticles with circular or elongated shapes, the long axis being randomly oriented. In particular, we investigate the effect that irradiations with nanosecond pulses at 800 nm and femtosecond pulses at 400 and 800 nm have on nanoparticle morphology and nanostructure optical response. We observe that nanoparticles are efficiently reshaped by both types of laser pulses with the difference that nanosecond pulses lead to the formation of nanoparticles with in-plane circular shape, while femtosecond pulses lead to in-plane elongated and aligned nanoparticles, which is attributed to the different mechanisms involved in morphology modification.

Finally, we report the results obtained using the fabricated nanostructures as SERS substrates and those related to the first attempts to incorporate our nanostructures in chalcopyrite thin film solar cells to improve their performance in terms of plasmonic light trapping processes. First, we describe and discussed the SERS activity as a function of nanoparticle morphology, molecule concentration and molecule-to-nanoparticle surface distance and we conclude the study by verifying if the fabricated films can act as support for creation of nanoparticles superstructures. Second, we report the result obtained by incorporating nanostructured films containing small and large nanoparticles in two different types of chalcopyrite solar cells based on CuInGaSe and CuInSe compounds. We demonstrate that the incorporation of nanostructured films allows the proper deposition of devices, but without any significant improvement of their characteristics. In this sense, it is important to underline that the performed experiments were the very first attempts and that the incorporation of nanoparticles in chalcopyrite based solar cell can be considered a pioneering work since most of reported works in literature deal with incorporation of nanoparticles in amorphous Si based solar cells.