SYNTHESIS OF Ag/CdS NANOCOMPOSITES - AN ANALYSIS OF THEIR OPTICAL AND PHOTOPHYSICAL BEHAVIOR

Abstract

During last two decades nanoscience and nanotechnology have made a vast global impact on the society because of their multifold scientific and technological applications. Over this period a number of physical and chemical methods have been developed for the preparation of nanomaterials, which have opened several interesting opportunities in nanoscience, enabling one to fine tune their optical, photophysical, and electronic properties at ease. Among different wet methods, progress in the processing of colloids offer the advantage of synthesizing size- and shape-dependent nanoparticles in solution. The current interest in colloid chemistry is the fabrication of superlattices of size and shape controlled nanosystems. Metal-semiconductor assemblies comprising colloidal particles are interesting because of a weak van der Waals interaction between the two phases and are expected to have several beneficial effects in terms of their organization, increased interface, improved charge carrier dynamics, reactivity and selectivity. Studies on these systems have reported new methodology for their synthesis and control of their optical and electronic properties, but it lacks a complete analysis of interaction between its components. A better understanding of these nanocomposites, however, requires characterizing these systems in terms of their size, shape, structure, and the interaction between metal and semiconductor components. It also requires to analyze their optical and electronic properties, and to study the dynamics of charge carriers in the irradiated system(s). The present thesis has been divided into six chapters. The chapterwise details have been furnished below: The first chapter presents a brief overview of the work done on various nanosystems for about last one and a half decades. Synthesis and photophysical properties of certain nanoceramics; nanosized metals and their hybrids; semiconductors and their hybrids, and metal-semiconductor nanocomposite/nanohybrids have been presented. Various core-shell nanocompsites/nanohybrids have been discussed in which either of metal(s) or semiconductor(s) used as a shell and vice-versa. This chapter also lists the objectives of the present work. The second chapter deals with experimental details of the used materials, equipment and techniques. A brief account of the methodology used for carrying out various measurements on different techniques has been included. It also gives a brief description of methods employed for the synthesis of metal and semiconductror nanocolloids used in the present work. The third chapter presents the synthesis, optical and photophysical behavior of Ag/CdS-HMP nanocomposites. The content of Ag modifies the nature of surface interaction between the two components as revealed by TEM, electronic and fluorescence measurements. Increasing contents of silver in the composite reduces the interparticle separation. At low concentrations of silver, Ag particles are surrounded by CdS and remain fairly separated to each other. At mild concentrations of Ag the formation of clusters starts, whereas at its higher concentrations CdS particles are produced at the interface of Ag particles, and these particles grew to form a chain like system. In the nanocomposite both the Ag and CdS are produced in the hexagonal phase(s). The addition of silver blue shifts the absorption maxima, and influences the emission behavior and charge dynamics in a complex manner. At low molar ratio of Ag/ u CdS (0.25) about five-fold enhancement in fluorescence is observed which is attributed to the excited state charge transfer interaction between the two components. Onthe contrary at high Ag the fluorescence intensity is reduced significantly to become smaller than that of pure CdS nanoparticles. Time resolved fluorescence lifetime measurements have been carried out to study the dynamics of charge carrier in the irradiated systems. For the nanocomposite containing Ag/CdS of 0.25, the average fluorescence lifetime (<x>= 17.2 ns) is increased by more than 4 times to that of bare CdS particles prepared under identical conditions. Aging of these particle further enhance the lifetime to 29.4 ns. This study provides an insight into the mechanism of interparticle interaction and dynamics of their charge carriers. The fourth chapter gives an account of the synthesis, optical and photophysical behavior of adenine-templated Ag/CdS nanohybrids. These nanohybrids have been synthesized and characterized by transmission electron microscopy, selected area electron diffraction, x-ray diffraction, optical, fluorescence and time resolved emission spectroscopy. Adenine serves as an effective matrix for the stabilization of Ag/CdS through interaction of N(l), N(3) and -NH2 with Ag. The amount of Ag in the nanohybrid is observed to influence the organization of the Ag and CdS phase in the composite and also modifies the nature of electronic transition in CdS. For the nanohybrid containing molar ratio of 0.1 of Ag/CdS, CdS nanoparticles (2.5 nm) surrounds the Ag (6.5 nm) as core. In this system, both the Ag and CdS have also been found to be present in the hexagonal phase. The excitation of these particles by 340 nm light, where the absorption due to Ag phase in the nanohybrid is negligibly small, results in the enhancement of fluorescence by in a factor of 7 compared to that of bare CdS. Whereas for the particles containing molar ratio of Ag/CdS of unity, bigger clusters (14 nm) are produced and it causes the quenching of emission of CdS. In time resolved emission spectroscopy the spectral shift from 415 nm (3.0 eV) to 550 nm (2.26 eV) monitored over a period of 1 ns to 220 ns, which is understood by the relaxation of charge within the surface states of varied energy from 180 eV to 370 eV. The fifth chapter presents the synthesis, optical and photophysical behavior of GMP-templated Ag/CdS nanohybrids. The optimum molar ratio of Ag/CdS is determined to be 0.1. Analysis of the morphology of these nanohybrids by AFM, FE-SEM and TEM measurements shows that the fresh sample of the nanohybrid having molar ratio of Ag/CdS of 0.1 contains the chains of aggregates consisting of particles, which transformed into nanowire upon aging. EDAX analysis of nanohybrid shows the presence of Ag, Cd and S elements, which are homogeneously distributed in the particles/nanowires. Both SAED and XRD analysis of the nanohybrid depicts them to contain Ag, CdS and Cd(OH)2 phases, each of which is present in the hexagonal geometry. The amount of the Ag in the nanohybrid influences the organization of Ag and CdS. TEM analysis of the fresh sample of nanohybrid depict it to contain spherical particles arranged in an organized pattern with an average dia of 5 nm. Aging of these particles results in the formation of chain of aggregates with relatively bigger size particles. The excitation of these particles by 340 nm light results in the enhancement of fluorescence intensity by a factor of 7.5 with a blue shift in the emission maximum compared to that of the bare GMP-CdS. The fluorescence intensity at 573 nm IV corresponded to the quantum yield of about 3.9%. But at higher molar ratio of Ag/CdS (> 0.1), the quenching of emission of CdS was observed with a red shift in emission maximum. An analysis of the fluorescence lifetime data reveals that for the sample having the molar ratio of Ag/CdS of 0.1 the <x> is determined to be 160 ns, which is reduced to 38 ns for the sample having molar ratio of unity. In the time resolved emission spectroscopy a spectral shift is observed from 405 nm (3.07 eV) to 545 nm (2.28 eV) corresponding to the formation of different intermediates over a period of 1 ns to 192 ns time delay. These changes are understood to arise by the relaxation of charge within the surface states of varied energy from 220 eV to 367 eV. An examination of anisotropy measurements of these nanohyrids shows that the sample having molar ratio of Ag/CdS of 0.1, possess the highest value of anisotropy (r), which is further increased upon aging from 0.28 to 0.38. A corresponding change in the rotational correlation time (02) is noted from 42.9 ns to 167 ns. The sixth chapter presents a summary and discussion of the results presented in the third, fourth and fifth chapters. The structures of Ag/CdS nanocomposites stabilized by various matrix have been analysed based on data obtained by using XRD, electron microscopy and different spectroscopic techniques. Photophysics of these systems have been understood by carrying out an analysis of charge dynamics in these systems under various experimental conditions. The observed physicochemical characteristics and electronic behavior of different nanostructures suggest themto be the promising materials for optoelectronic, photonics, fluorescence imaging, molecular recognition and sensing applications.