SYNTHESIS AND PHOTOPHYSICS OF Q-PbS BASED COLLOIDAL NANOSTRUCTURES

Abstract

In recent years nanoscale colloidal semiconductors have attracted great interest because of their tunable electronic, optical and photophysical properties arising from the quantum confinement effects. Besides, in the quantized particles surface atoms are the large fraction of the total, which allows manipulating their surface conveniently using different chemical means. The passivation of the surface can be employed for the preparation of tailored particles with a narrow size distribution. Nature has utilized several biomolecules like proteins, aminoacids, nucleic acids etc. to synthesize complex nanostructures exhibiting unique chemical, optical, photophysical and mechanical properties. Biopolymers offer a great potential to synthesize functional nanomaterials, which could also serve as a building blockand can be exploited to produce a wide range of nanostructures. Creating synthetic materials with these exceptional properties has posed a great challenge to design such materials, which could allow manipulating their morphology and electronic properties for the development of devices. In this context progress in the processing of the colloids using wet chemistry offers an enormous advantage of synthesizing size and shape controlled nanomaterials in solution enabling to fine-tune their optical, electronic and photophysical properties with ease. Among various semiconductor systems, PbS has been investigated widely in recent years because of its extensive applications in the development of photoluminescent devices, solar cells, telecommunications, biological labeling fluorescence imaging and detection devices. PbS with a small band gap of 0.41 eV and large excitonic Bohr radius of 18 nm has been considered to exhibit enhanced size dependent electronic properties because of confinement of both the charge carriers. Synthesis of Q-PbS based core-shell nanostructures have been carried out by interfacing it with other semiconductor of higher band gap and sandwiching PbS between large band gap material (QDQW) in order to improve the photophysics and control of the charge dynamics in these system. Biological molecules-capped Q-PbS have been considered very promising for the restriction of the size of the particles through binding of these molecules with metal ion of the semiconductor for the control of their electronic and photophysical properties. The nature of surface capping agent has been found effective in controlling the photophysics and dynamics of the photogenerated charge carriers in these systems. The present thesis has been divided into five chapters. The chapter-wise details have been furnished below. The first chapter presents a brief overview of the progress of scientific research carried out on various nanomaterials during the last two decades. The synthesis and photophysical aspects of ceramics, metal and semiconductor nanoparticles, wherever available including PbS based nanosystems, have been presented. The influence of coupling of two semiconductors of different band gaps and capping of semiconductor nanoclusters by various inorganic, organic and biological capping agents on the change in optical and photophysical properties has been discussed. Relaxation kinetics study of PbS and related systems has been presented. It also specifies the objectives of the present investigation. The second chapter deals with the 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 in outlines of methods employed for the synthesis of different nanocolloids used in the present work. The third chapter incorporates synthesis and characterization of different Q-PbS based nanosystems consisting of HMP-capped ZnS/PbS, PbS/ZnS and ZnS/PbS/ZnS in aqueous basic medium. The surface of ZnS and PbS has been modified by interfacing PbS on ZnS, and ZnS on PbS and sandwiching of PbS between layers of ZnS nanoparticles to produce core-shell nanocomposites namely, ZnS/PbS, PbS/ZnS and ZnS/PbS/ZnS QDQW, respectively. In all the three structures PbS particles have been found to be present in cubic form with an average diameter of about 6 nm. The addition of Pb2+ in varied concentration range from 1.0 x 10"5 to 1.5 x 10"4 mol dm"3 to Q-ZnS (1.5xl0"4 mol dm"3) in the basic pH range produces the size quantized PbS particles at the interface ofZnS. At higher Pb2+ (> 2 x 10"4 mol dm"3) it formed fluorescent PbS coated by Zn(OH)2/Pb(OH)2. In these particles the relaxation kinetics ofcharge carriers has been followed by using picosecond single photon counting technique. At < 1.5 * 10 mol dm" Pb2+ an interfacial relaxation of charge from ZnS to PbS phase is observed to take place in nano- to sub-nanosecond time domain. An increase in [Pb2+] from 2x10" to 1 x 10" mol dm"3 enhanced the average emission lifetime of PbS from 9.4 to 19.4 ns. Composite PbS /ZnS nanoparticles are produced at relatively high [ZnS] (> 3x10"4 mol dm"3) only. These particles had emission lifetime in us time range. The dynamics of charge carriers has been monitored under various experimental conditions. ZnS/PbS/ZnS QDQW have been synthesized coating of ZnS/PbS precursor by different [ZnS]. These particles were found to fluoresce at different wavelengths upon varying the thickness ofouter ZnS layer and [Zn2+]. These systems exhibit lifetimes due IV to PbS core in u,s time range. Interestingly, the size of ZnS particles in theseQDQW were found to be 8 nm which is quite small as compared to ZnS nanoparticles (14 nm) prepared separately under similar conditions. The fourth chapter gives an account of synthesis, photophysics and charge dynamics of RNA templated Q-PbS nanocomposites. RNA mediated PbS nanoparticles have been synthesized in quantum-confined region in face centered cubic phase. Binding of RNA to the surface of PbS nanoparticles has been exploited to tailor its size besides improving the stability and electronic properties. Under optimum conditions these particles exhibit the onset of optical absorption in the visible range corresponding to a band gap of 1.86 eV with prominent excitonic bands at 350 nm and 580 nm. This system displayed relatively a strong narrow emission band (FWHM 70 nm) at 675 nm with a broad excitation range extending from 330 to 620 nm. In the presence of silver the fluorescence dueto PbS is quenched along with a simultaneous development of newband at 450 nm. An increase in molar ratio of Pb/S, a decrease in energy of excitation and aging of these colloidal systems enhanced the average lifetime due to PbS. On the contrary an increase in [RNA] reduced the lifetime significantly. Thus manipulation of experimental conditions could be utilized to control relaxation dynamics of charge carriers in illuminated PbS. The individual components of RNA viz. adenine monophosphate, guanosine monophosphate, cytosine monophosphate and uracil monophosphate as well as various combinations of their mixtures employed to synthesize PbS under identical conditions have been observed to show electronic features quite different to those observed with RNA capped PbS. The addition of Zn2+ to RNA capped PbS has been observed to modify its surface and results in the formation of nanostructures of different shape under varied [Zn ]. Growth of these particles upon aging produces nanowires, which improves the quantum efficiency of fluorescence significantly to 12%. The relaxation kinetics of the charge carriers demonstrate an increased charge separation in these systems enhancing the fluorescence lifetime to microsecond time domain under illumination by visible radiation. The fifth chapter includes a summary and discussion of the results presented in third and fourth chapters. Based oncharacterization using XRD, electron microscopy and different spectroscopic techniques, structures of Q-PbS based core-shell nanocomposites, RNA templated Q-PbS and Zn/PbS nanocomposites have been worked out. Photophysics ofthese systems have been understood by carrying out an analysis of charge dynamics in these systems under various experimental conditions. The observed physicochemical properties and electronic features of the studied nanostructures suggest them to be promising for applications in photonics, the fabrication of solar cells, fluorescence imaging, biological labeling, sensors and detection devices.