SYNTHESIS, OPTICAL AND ELECTRONIC PROPERTIES OF RNA-MEDIATED COLLOIDAL CdS NANOSTRUCTURES

Abstract

In recent years nanoscale materials due to their unique size and shape dependent physicochemical properties have drawn considerable attention. For these reasons, over the last one and half decades nanotechnology has emerged as a frontier area for both fundamental research and developing new technologies. Nanoscience and nanotechnology deals with the matter having at least one of its dimensions of the order of a nanometer, and it allows the manipulation of material at the molecular or even at atomic level to design nanostructures of tunable size and shape. Because of the extensive change in the physicochemical properties of all class of materials viz. metals and their alloys, semiconductors, ceramics and carbon allotropes in nanorange these materials are being reinvestigated in nanodomain. Among different classes of nanomaterials, a large number of investigations have focused on analyzing the electronic properties of semiconductor quantum dots/nanostructures because of their extensive usage in the development of new solid state devices with precise control of various properties. Over these years a number of growth techniques comprising both physical and chemical methods have been developed for the synthesis of nanomaterials. Among different wet chemical methods, colloidal method using bottom up approach has proved to be an important tool for the preparation of nanoscale materials in highly controlled manners. These materials have large interface, it has tremendous potential to modify the surface of nanocrystals conveniently in solution to obtain the desired building blocks. Biomolecules having diverse functionality and highly specific inter- and intramolecular interactions provide a powerful tool to synthesize tunable self-assembled nanomaterials. These materials not only provide chemical functionality for integration with metal/semiconductor nanoparticles but also make it possible to modify and manipulate their structures conveniently with remarkably different physicochemical properties and varied morphologies under mild reaction conditions. The work embodied in the thesis has been divided into six chapters. The chapterwise details have been furnished below: The first chapter presents a brief overview of the work carried out during last one and half decades on various nanosystems. Synthesis and photophysical aspects of nanocaremics; nanosized metals and their hybrids, and semiconductors and their hybrids have been presented. It also includes the effect of coupling of two semiconductors of different band gaps and capping of semiconductor(s) by various inorganic, organic and biomolecules on the optical and photophysical behavior of these systems. This chapter also lists the objectives of the present work. The second chapter deals with experimental details of the used materials, equipment and techniques. It also gives a brief account of the methodology employed for carrying out various measurements using different techniques. A short description of methods used for the synthesis of different semiconductor nanocolloids has also been reported. The third chapter contains the synthesis, optical and photophysical behavior of RNA-CdS nanohybrids under varied experimental conditions in aqueous basic medium. RNA serves as an effective template for the synthesis of quantized CdS nanoparticles and mediates their growth to create novel assemblies. CdS nanostructure were produced in a hexagonal geometry. Unlike DNAstabilized particles in aqueous medium, these particles display fairly strong emission at 2.34 eV, which is further enhanced by more than 2.5 fold and blue shifts to higher energy (2.39 eV) upon aging. Chelation of Cd2+ with RNA in restricts the nucleation of CdS. A variation in the molar ratio of Cd/S from 2 to 6 produces different nanostructures with varied electronic properties. Aging of particles with molar ratio of Cd/S 2 enhanced the lifetime significantly from 33ns to 86 ns, but at higher molar ratios of CdS it did not exhibit any significant change. Unlike general colloidal systems aging of these nanoparticles produced smaller crystalline nanocrystals as evidenced by their blue shifted optical threshold and fluorescence maxima, and by AFM andTEManalysis. Different nanostructures grow upon aging to yield self-assembly of different shapes and morphology as a function of the change in the molar ratios of Cd/S. The fourth chapter gives an account of the synthesis, characterization and analysis of electronic properties of water soluble guanosine 5'-monophosphate (GMP) - mediated CdS quantum dots. The morphology, size and size distribution ofthese particles has been analyzed by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). These particles display the onset of absorption at 2.7 eV and emission at 2.2 eV. In comparison to other monophosphates of RNA (AMP, UMP and CMP), GMP-mediated CdS exhibit enhanced electronic properties. GMP, AMP and UMP-stabilized CdS particles were produced with an average size of 5 nm, 7 nm and 10 nm, respectively, in hexagonal phase. Upon aging of GMP-mediated CdS particles for three months these particles convert into nanorods. These nanorods exhibit a red shift in the absorption threshold and emission maximum compared to those of spherical particles. The participation of different functional groups of GMP in the stabilization of CdS nanoparticles has been analyzed by FTIR, 'H and 31P NMR spectroscopic techniques. Two types of binding sites involving phosphorous centres are IV indicated by IR and 31P NMR studies. The conversion of CdS Q-dots to nanorods has been monitored by using electron microscopy, steady-state optical and fluorescence measurements, fluorescence lifetime system coupled with anisotropy accessories. Thefifth chapter incorporates the synthesis, optical and photophysical behavior of RNA-templated CdS/ZnS co-colloids. Analysis of the morphology of these colloidal nanohybrids by AFM, FE-SEM and TEM measurements shows the formation of tubular structures. TEM and AFM measurements show the formation of tubular structures with an average dia. of 18 nm. EDAX analysis of these structures depicts the presence of Cd, Zn and S elements, which are homogeneously distributed along the entire length of nanotubes. SAED patterns of these nanotubes demonstrate the amorphous nature. XRD analysis of the nanohybrid depicts them to contain CdS, ZnS and Zn(OH)2 phases, each of which is present in the hexagonal, wurtzite and orthorhombic geometry. The amount of the added excess Zn +in these co-colloids strongly influences fluorescence intensity, charge carrier dynamics and morphology of these nanostructures. These tubular structures were found to be fairly stable and aging of these co-colloids for a period oftwo months did not depict any change in their optical properties. FTIR and ]HNMR studies shows that CdS/ZnS interact with different moieties/functional groups (purine, pyrimidine and 2'-OH) while excess Zn2+ ions mainly depict interaction with base moieties and P02\ The colloidal nanotubes depict a fairly high quantum efficiency of emission (~ 5.6%) at 509 nm upon excitation by the 400 nm light radiation. Surprisingly, the emission lifetime and quantum efficiency is significantly decreased when these cocolloids were excited by a higher energy radiation (340 nm). RNA-CdS with excessZn2+ possess relatively a higher value of anisotropy (r) and rotational correlation time (62) in comparison to that without excess Zn2+. The sixth chapter presents a summary and discussion of the results presented in the third, fourth and fifth chapters. The structures of CdS, and CdS/ZnS stabilized by RNA and its different monophosphate components have been analyzed based on experimental results obtained by using XRD, electron microscopy and different spectroscopic techniques. The interactions of these nanoparticles with the templates have been workedout using FTIR and NMR spectroscopic techniques. Electronic properties of these systems have been understood by carryingout by analyzing the dynamics of charge carriers in these systems under various experimental conditions. The observed controlled organization and electronic behavior of different nanostructures suggest their possible potential applications in nanoelectronics, fabrication of nanodevices, fluorescence imaging and sensing applications.