PLASMONIC PROPERTIES OF CLOSELY SPACED METALLIC NANOISLANDS THIN FILMS

Abstract

Interaction of light with a metal leads to the oscillations of the free electrons. Quantization of the collective oscillations of electrons are called Plasmons. Different phenomenon related to plasmons is popularly known as Plasmonics. The frequency of the incident light when coincides with the characteristic frequency of the metal then resonance occurs. In case of metal nanoparticles or nanoislands thin films this will lead to Localized Surface Plasmon Resonance (LSPR). It becomes great attraction for researchers for their wide applications for enhancing the efficiency of optoelectronic devices like solar cells, LEDs and photodiodes and also for biosensors. Various theoretical approaches has been given and used successfully to understand the optical behavior of the assembly and array of nanoparticles. Due to the rapid advancement of the field there is a strong requirement to develop a general analytical approach to estimate the plasmonic behavior both qualitatively and quantitatively. Other widely used theoretical approaches like discrete dipole approximation and transfer matrix method are based upon point dipole approximation which works better when nanoislands are separated by each other at a larger distance in comparison to their sizes. In recent times, various experimental techniques have been developed which enable to fabricate nanoparticle array at sub nanometer scale gap between the particles. Spectroscopic ellipsometric characterization has improved the understanding of plasmonic behavior of closely spaced nanoparticles. Therefore, effective medium theory which works well for non-interacting particles and the point dipole approximation methods are no more valid for close spaced array, which is highly sensitive to interparticle gap. Yamaguchi’s model is widely known to understand the splitting of optical spectra of a particle when supported by substrate. However, Modified Yamguchi Model (MYM) is applicable to a distribution of nanoparticles of high filling fraction and high aspect ratio. This method is based upon exact calculation of electrostatic potential outside an ellipsoids in terms of its shape parameters and position coordinates of neighboring particles. This provides a solution to the problem of calculation of in-plane and out of plane optical spectra of closely spaced nanoparticles. However, particle’ shape and size dependent damping is required to be included in MYM to calculate effective dielectric constants. The main objectives of the present work are (i) to develop of an analytical approach to calculate the effective dielectric constants for in-plane and out of plane plasmons of close spaced ii array of various shapes of nanoparticles (spherical, oblate, prolate and supported by rippled substrate) nanoislands following the approach used in MYM, (ii) to investigate of the behavior of in-plane and out of plane plasmons with increase in filling fraction and to find the limitations of effective medium theories with applied field direction and particle’s shape, (iii) to fabricate the metal nanoislands thin films using Pulsed Laser Deposition technique and characterize them using spectroscopic ellipsometer and to correlate the results with analytical model, (iv) to verify of the model by calculating the plasmon induced SERS enhancement using morphological parameters of previously reported samples and compare those results with experimental observations. A chapter-wise summary of the thesis is given as follows: Chapter 1 gives an overview of history and development in the field of plasmonics and present state of the art. Different analytical approaches like Discrete Dipole Approximation (DDA), Transfer Matrix Method are discussed to evaluate LSPR peak. Different nanoparticle distribution schemes such as randomly distributed nanoparticle, one- and two -dimensional array of nanoparticles and the tunability of LSPR peak has been depicted. Motivation and objectives of the thesis have been discussed. Chapter II describes the analytical modelling to calculate the effective optical constants of distribution of ellipsoidal nanoparticles which has been developed using the approach of Modified-Yamguchi model. Effect of broadening of plasmon linewidth depending upon particle’s shape, size and embedding environment has been included. Following the similar formulation extended calculations has been done to calculate effective dielectric constants of spherical, oblate, prolate and ortho-prolate array for nanoparticles. Using this formulation volume filling fraction limitation beyond which effective medium theories are not applicable has been verified. Validity of the model has been checked to calculate plasmonic response of previously published results for ripple supported spherical and elongated nanoparticles. A code has been developed to generate random distribution of nanoparticles then application of MYM provides an insight to fit ellipsometric data with suitable oscillators. Few sets of silver metal island films are grown using Pulsed Laser Deposition method and their dielectric coefficients are calculated using spectroscopic ellipsometer. Dielectric constants of the films obtained from spectroscopic ellipsometer have been simulated with the use of Modified-Yamaguchi’s model using aspect ratio, filling fraction and depolarization factor as fitting parameters. Limitations of iii MYM has been tested for pulsed laser ablation films have been tested by following present formulation. The fabrication of thin films in present thesis has been carried out using Nd: YAG pulsed laser deposition technique. A brief literature survey on fabrication of metal nanoislands thin films using pulsed laser technique has been included in the Chapter III. Further the surface morphology and microstructure of thin films were studied using Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM). A detailed discussion on the measurement of ellipsometric parameters and extraction of real dielectric constants from them has been depicted in this chapter. Chapter IV includes the analysis of fabrication and characterization of pulsed laser deposited silver nanoislands thin films using spectroscopic ellipsometer. Effective dielectric constants of the films are obtained by applying suitable set of oscillators to fit ellipsometric data. Nucleation and growth of the films have been investigated using atomic force microscopy (AFM). A comparison has been made between the growth and dielectric constants of the films deposited using the wavelength 355 nm (Third harmonics) and 1064 nm (Fundamental) of Q switched Nd:YAG laser. This chapter includes the measured and fitted psi and delta parameters for a wavelength range 300-900 nm for all fabricated samples. The fitting reports have been generated form software CompleteEase. These reports contain the information about mean square error, average film thickness, roughness, type of oscillators applied along various directions with associated peak energy, amplitude and broadening. A detailed discussion on the observation of in- plane and out of plane LSPR in P-polarized and S-polarized reflection spectra for three samples has been presented. Annealing induced change in plasmonic properties of silver nanoislands film has also been investigated in a similar manner. In Chapter V, present model has been used to calculate the near-field response of various silver nanoparticle array supported on ion beam induced ripple template substrates. Hot spot intensity between close spaced nanoparticles have been calculated using simplified expression of near field decay. Simulated results agree well with the experimental results observed from Surface Enhanced Raman Spectroscopy (SERS). The calculated and measured optical response unambiguously reveal the importance of interparticle gap where a high intensity Raman signal is obtained. iv Chapter VI presents the summary and conclusion of the entire work presented in the thesis and also proposes the future scope of work in which direction these studies can be extended.