SERS and optical investigations on ordered and disordered metallic nanostructures

Abstract

This thesis presents a comprehensive experimental investigation of SERS and optical response from various fabricated metal nanostructures using a controlled recipe. It focuses on the extensive understanding of governing the SERS behaviour via surface topographical parameters of associated templates, thickness of the overlayers as well as putting a special emphasis on the suitable choice of Au-Ag alloy as a plasmon-active material. The major content of the thesis demonstrates the fabrication of potential SERS substrates by simultaneously targeting the existing challenges of cost-effectivity, ease of fabrication, time savior, uniformity and reproducibility of SERS signals. Later on, we extend our study by introducing low energy ion beam irradiation to fabricate controlled metal nanostructures, which can be utilized for future SERS applications. Beginning with a short introduction on the fundamental concepts of interaction of light with metal nanostructures, the basis of SERS phenomenon and optical behaviour is presented. A literature survey is done and the major shortcomings as well as challenges for utilizing this phenomenon for potential applications are described thereafter. First, we prove an Au-Ag alloy as a suitable material of choice for SERS in contrast to bare individual elements Au and Ag respectively. We utilized this Au-Ag alloy as an overlayer over pyramidal Si-array for cost-effective SERS substrates. Moreover, these Si-pyramidal arrays are generated by employing wet chemical etch method which is a considerably simpler and cost-effective method over conventionally used lithographic techniques. SERS response, by probing the Rh6G as an analyte molecule, exhibits a higher and environmentally stable Raman behavior rather than traditionally used bare elements Au and Ag respectively. Consequent to the above study of a superior SERS response from an Au-Ag alloy, we subsequently explore the dependence of the SERS behaviour on the pyramidal surface pretemplates with varying surface characteristics. We achieved this by fabricating different pyramidal surfaces by varying the etching time. A detailed statistical analysis was carried out to quantify these texture parameters which fluctuate both spatially and temporally and subsequently explore their correlation with SERS behaviour. Our results show that a template consisting of small, dense and uniform pyramids are responsible for higher SERS activity owing to a strong coupling of electromagnetic field between two consecutive pyramids. Further, we have tried to find out the role of overlayer thickness over these pre-templates for an enhanced SERS signal. We have varied three different thicknesses viz., 30, 50 and 70 nm over three selective different pyramidal templates. A thickness of 50 nm is found to contribute towards a strong SERS count. Next, we focus on the reproducibility and uniformity aspects of SERS signals. We briefly discuss how these important aspects of SERS are correlated with surface height fluctuations. Here, we systematically investigate and compare the need for 3D vis-‘a-vis 2D surface morphologies of SERS templates. Apparently, the surface with 3D morphology shows a slightly higher response than the surface possessed with a few nm of surface roughness (2D). But from signal uniformity and reproducibility considerations, the 2D morphology gives the lowest value of relative standard deviation (RSD) of 5.56% in contrast to a high value of 27.13% for the 3D morphology. Further, its universal applicability and ultra-sensitivity is testified by detection of methylene blue and methyl parathion up to nM concentrations. Based on the knowledge acquired, we expand our investigations toward cheaper flexible SERS substrates for on-field food safety applications. We start by reviewing day-routine materials directly as SERS templates utilizing their naturally endowed surface structuring. Day-routine material papers, adhesive tapes and aluminum foil are explored for flexible SERS templates. We find an excellent SERS response exhibited by crumbled Al foil. This template is further investigated for pesticides detection which proves its potential applicability in agriculture based SERS applications. Finally, we present the low energy ion beam irradiation method as an alternative route for fabricating nanoripple like surface structures which is also a successful technique for scale-up fabrication. Here, we show that nanoripple like morphology evolves on Cu thin film at a particular angle of ion incidence. The surface parameters of the films evolve with increasing ion fluence. An optical response measured from these surfaces exhibit specular as well as diffuse spectral features. Our results establish that these responses have a strong correlation with surface ordering parameters like correlation length and local surface slope. This technique can further be utilized for future industrial SERS applications. Our findings underscore the benefits of Au-Ag alloy with a combination of wet chemical etch method over cost-intensive lithographic techniques for effective SERS substrates. In addition to this, we are able to present an optimized texture recipe of surface feature with suitable thickness for an excellent and stable SERS response. Furthermore, considering the uniformity and reproducibility aspect of SERS signal, the requirement of a traditionally employed 3D morphology of SERS template is discussed. It is observed that even a 2D morphology with small scale topographical fluctuations can also serve the purpose. Finally, the optical behaviour of metallic nanostructures fabricated using ion irradiation is studied with respect to their surface correlation parameters.