Role Of Surface And Inter-particle Spacing On Optical Properties Of Single And Hybrid Nanoparticle Assemblies

Abstract

Optical properties of nanoscopic materials have been intensively perused over last couple of decades due to their tunable optical properties. Recent interests in this field have been mainly focused on the preparation of ordered arrays of nanoscopic materials and study of their optical properties. These interests have been motivated by the usability of such systems for nano photonic devices. Theoretical predictions from such systems reveal complex absorption and emission properties, different from individual ones mainly because of energy transfer between them. These properties can be controlled further by preparing hybrid arrays of nanostructures, including nano crystals of different types. Hybrid arrays with semiconducting quantum dots and metallic nanoparticles are an example of such system. Optical properties of such a system can be tuned by controlling the interaction between excitons and plasmons. This the-sis presents the experimental studies on optical properties of polymer capped polymer nanoparticles, quantum dot arrays and hybrid arrays with semiconducting quantum dot and metal nanoparticles. A brief summary of the experi-mental methods and results have been highlighted below. First chapter deals with the theoretical aspects of confined nanoscopic materials, especially describing the physics of zero dimensional systems and its optical properties. The discussions are mostly focused on two types of nano materials cadmium selenide (CdSe) quantum dot (QDs) and gold nano particles (Au NPs), used for the experimental study. Variation of energy levels of CdSe QDs and its absorption and emission properties under strong confinement regime has been discussed with respect to effective mass approximation (EMA) model. This is followed by the discussion on optical properties of Au NPs, describing absorption properties, based on Mie theory. Size dependent variation of absorption spectra of Au NPs and the modifications based on different models has been discussed. Second part of the chapter describes the physics of QD arrays and theory of exciton plasmon interactions based on the recent literatures. Energy transfer mechanism between semiconducting QDs and metal nanoparticles has been discussed based on numerical method and dipole approximation. Second chapter deals with the discussion on experimental techniques used for the study. Chapter 2 starts with the discussion on the synthesis method for CdSe QDs and Au NPs with different capping ligands. Preparation of QD ar-rays and hybrid arrays using self assembly technique has been discussed in this chapter. Preparation CdSe QD arrays and hybrid arrays with CdSe QDs and Au NPs using block copolymer (BCP) template and Langmuir Blodgett (LB) technique has been the main focus in the discussion. This is followed by the discussion on optical microscopy techniques, confocal, near field scanning microscopy (NSOM), Brewster angle microscopy and electron microscopy techniques, transmission electron microscopy and scanning electron microscopy. Studies on variation of band structure of small polymer capped Au NPs, with respect to the size and grafting density of the capping polymer is discussed in chapter 3. Polymer capped Au NPs with sizes 2-5 nm was used for the study. Dielectric constants of Au NPs were extracted from the absorption spectra by fitting the data using modified Mie theory. Dielectric constants of Au NPs were reproduced using an analytical expression, describing the contribution from different transitions in the optical regions. Results indicate systematic variations of the band structure with respect to the particle size and grafting density. The observations have been interpreted in terms of variation of co ordination number and chemical interaction of capping polymer with the surface atoms. Our new method analysis points to the importance of both quantum and surface effects in determining optical and electronic properties of polymer capped gold nanoparticles. Chapter 4 describes the study on morphology of the CdSe QD arrays prepared using different BCP templates and its correlation with optical properties. Spatially resolved spectra from the thin films of QD arrays were collected in near field and the compared with the spectra collected in far field. Spectra collected in near field mode shows sharp features in the emission spectra, possibly indicating the interaction of optical near field with QD excitation. It has been suggested that such fine structure could be induced by coupling between optical near filed and excitons and this coupling seems to be determined by local heterogeneity in QD density and disorder. Variation of exciton life time with respect to QD density and absorption spectra from the QD -BCP system is also described in chapter 4. Chapter 5 and 6 deals with the experimental studies on exciton -plasmon interaction in hybrid arrays of CdSe QDs and Au NPs. Emission properties hybrid arrays prepared using BCP templates has been the focus of chapter 5. Photoluminescence (PL) and lifetime measurements were performed on hybrid arrays and their variation with respect to the density and dispersion of Au NPs has been described. Optical measurements were performed on two sets of films using two different sizes of CdSe QDs, with the smaller QD emission overlapping with the plasmon resonance of Au NPs, while a red shifted emission peak for larger QDs. PL emission from hybrid arrays with smaller QDs shows en-hancement/quenching with respect to the dispersion of Au NPs, also showing systematic reduction of life time of CdSe QDs with Au NP density. Even though enhancement/quenching of emission properties of hybrid film with large QD shows similar behavior, PL decay measurements from such films shows non monotonic variation of exciton life time with respect to Au NP density. The enhancement/quenching behavior of the PL emission has been explained in terms of two competing mechanism, electromagnetic field enhancement and non radiative energy transfer. However to explain the energy transfer mechanism in hybrid arrays requires more systematic calculations. Chapter 6 describes the optical properties of highly compact hybrid arrays prepared using LB techniques. Hybrid arrays prepared at the air water inter-face were transferred to a glass substrates. The main focus on chapter 6 is to study the emission properties of highly compact hybrid arrays with respect to the spectral overlap between exciton energy of CdSe QDs and plasmon band of Au NPs with respect to their surface density (inter particle distance). Hybrid arrays were prepared with three types of QDs, with smaller QDs emission peak overlapping with plasmon band of Au NPs and clearly separated exciton and plasmon band for largest QDs. The PL emission from hybrid arrays with smaller QDs shows quenching, compared to strong enhancement in the emission from hybrid films with larger QDs. The disagreement of the observed results with respect to the theoretical calculations based on dipole approximation has been highlighted in the chapter. Chapter 7 includes the summary of the experimental results and the future works to be carried out as a continuation of the work presented in this thesis.