Plasmonic Silver Nanoparticles by Dewetting process: Applications in SERS and Thin Film Solar Cells

Abstract

The exploration of materials at the nanoscale, and their integration into optoelectronic devices, can be developed via new nanophotonic strategies based on plasmonic effects, which are nowadays regarded as the preferential solutions to overcome performance limitations in different types of applications. Those explored here concern the increase of efficiency of physically thin film silicon (Si) solar cells and of weak Raman signals for molecular detection (one scattered photon per million incident), employing metal nanoparticle (MNP) structures made of silver (Ag) which is the most effective material for plasmon-enhancement in solar cells and Raman Spectroscopy. The present thesis explored these effects employing thermal evaporation assisted by electron beam (e-beam) to deposit uniform thin layers of Ag, which then underwent a thermally-induced morphology transformation from a thin film (TF) to an array of NPs by a solid-state dewetting (SSD) mechanism. A novel procedure, involving a one-step methodology, without any post-deposition thermal procedures, is presented. This resulted in the direct arrangement of individual nanoparticles suitable for Raman amplification, with good control of their size and shape. The nanostructures that require a post-annealing process were essentially used for light trapping in solar cells. In this case a rapid thermal annealing (RTA) method was developed that yields highly reproducible and uniform plasmonic surfaces within a very fast (<10 min) annealing time when compared to other commonly employed annealing processes (>1 hour). The final results showed that microcrystalline silicon (μc-Si:H) solar cells deposited on improved ultra-fast plasmonics back reflectors (PBR), with Ag NPs with sizes of about 200 nm, exhibit an overall 11% improvement on device efficiency, corresponding to a photocurrent of 24.4 mA/cm2 and an efficiency of 6.78 %; against 21.79 mA/cm2 and 6.12 %, respectively, obtained on flat structures without NPs. For surface enhanced Raman spectroscopy (SERS) application, a remarkable 109 signal enhancement was obtained using rhodamine 6G (10-8 M) as the test analyte, and a new kind of costefficient SERS substrate (cardboard plates) was investigated for low-cost, flexible and disposable bio-detection devices. Besides such advantages, cardboard substrate proved to be a high-efficient, uniform and stable SERS substrate.