Development of Atmospheric PLD of Plasmonic Metal Nanoparticle Films for SERS Application

Abstract

For the fast uptake into industrial applications, the further development of fabrication methods for nanomaterials which are inexpensive and simultaneously technologically feasible is one of the major key factors. One technological approach, which has great potential for industrial scale application, is pulsed laser deposition (PLD) in a gas at atmospheric pressure, since it allows flexible environmental conditions for nanofabrication. This makes the newly introduced atmospheric PLD (APLD) a rapid and robust method, so that the further development of the method is of great scientific and technical interest. In the laser ablation at atmospheric gas pressure, there is strong collisional coupling of the ablation plume and the ambient gas. The expansion of the plume is restricted and confined close to the target surface and forms a nanoparticle (NP) aerosol by collisional condensation. For the fabrication of NP films at atmospheric pressure, the NP aerosols have to be effectively transported away from the target to the substrate. In this thesis, the fabrication of plasmonic silver NP films with APLD was developed and investigated. A variety of methods based on: normal gas flow, supersonic gas flow, atmospheric flowing DBD plasma and a laser-triggered-spark discharge, were devised and applied to develop this technique. In each method, the ablation of a silver target was done using a ns excimer laser (248 nm, 25 ns), ns fiber laser (1060 nm, 0.6 ?s), or fs laser (800 nm, 130 fs), in gas at atmospheric pressure. The NP aerosol produced by laser ablation was entrained in a gas flow or flowing atmospheric plasma and was transported to a substrate. In the flowing gas methods, a normal flowing gas jet, or a supersonic gas jet, was used to transport the NP aerosol. In the plasma assist methods, an atmospheric plasma jet of Ar or He from a dielectric-barrier-discharge (DBD) plasma source, or a hot plasma from a laser-triggered-spark discharge, was used to drive the NP aerosol towards the substrate. The DBD plasma assist method was used in the two different geometries, the open and closed geometry. In the open geometry, the aerosol was directly entrained with the flowing plasma jet, whereas in the closed geometry, the aerosol was first transported to the plasma active region by the gas flow and then in the downstream plasma jet to a substrate. Each method was successful in making plasmonic NP films at atmospheric gas pressure. The NP films produced by these methods were characterised to asses the efficacy of each approach. The DBD plasma significantly enhanced the deposition rate and altered the film morphology. The laser-triggered-spark method was more efficient than the DBD plasma assist method, but with a smaller deposition area. Supersonic APLD method was effective to produce a microparticulate, or NP film. The Ag NP films produced by these methods were also used in SERS for molecular detection of Rhodamnine 6G (R6G) to test their performance as SERS substrates, with the highest enhancement observed for the supersonic APLD Ag NP film.