Spectral and nonlinear optical characteristics of nanocomposites of ZnO-CdS

In this article, we present the spectral and nonlinear optical properties of ZnO–CdS nanocomposites prepared by colloidal chemical synthesis. The optical band gap (Eg) of the material is tunable between 2.62 and 3.84 eV. The emission peaks of ZnO–CdS nanocomposites change from 385 to 520 nm almost in proportion to changes in Eg. It is possible to obtain a desired luminescence color from UV to green by simply adjusting the composition. The nonlinear optical response of these samples is studied by using nanosecond laser pulses from a tunable laser at the excitonic resonance and off-resonance wavelengths. The nonlinear response is wavelength dependent, and switching from saturable absorption (SA) to reverse SA (RSA) has been observed for samples as the excitation wavelength changes from the excitonic resonance to off-resonance wavelengths. Such a changeover in the sign of the nonlinearity of ZnO–CdS nanocomposites is related to the interplay of exciton bleach and optical limiting mechanisms. The ZnO–CdS nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior at off-resonant wavelengths. The nonlinear refractive index and the nonlinear absorption increase with increasing CdS volume fraction at 532 nm. The observed nonlinear absorption is attributed to two photon absorption followed by weak free carrier absorption. The enhancement of the third-order nonlinearity in the composites can be attributed to the concentration of exciton oscillator strength. This study is important in identifying the spectral range and composition over which the nonlinear material acts as a RSA based optical limiter. ZnO–CdS is a potential nanocomposite material for the tunable light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Defect analysis of semiconductor thin films for photovoltaic applications using photo-luminescence and photo-conductivity

The present thesis can be divided into three areas:1) the fabrication of a low temperature photo-luminescence and photoconductivity measuring unit 2) photo-luminescence in the chalcopyrite CulnSez and CulnS2 system for defect and composition analysis and 3) photo-luminescence and photo-conductivity of In:JS3. This thesis shows that photo-luminescence is one of most essential semiconductor characterization tool for a scientific group working on photovoltaics. Tools which can be robust, non-destructive, requiring minimal sample preparation for analysis and most informative of the device applications are sought after by industries and this thesis is towards establishing photo-luminescence as "THE" tool for semiconductor characterization. The possible application of photo-luminescence as a tool for compositional and quality analysis of semiconductor thin films has been worked upon by this thesis. Photo-conductivity complement photo-luminescence and together they provide all the information required for the fabrication of an opto-electronic device.

Enhanced luminescence and nonlinear optical properties of nanocomposites of ZnO–Cu

In this article, we present the spectral and nonlinear optical properties of ZnOCu nanocomposites prepared by colloidal chemical synthesis. The emission consisted of two peaks. The 385-nm ultraviolet (UV) peak is attributed to ZnO and the 550-nm visible peak is attributed to Cu nanocolloids. Obvious enhancement of UV and visible emission of the samples is observed and the strongest UV emission of a typical ZnOCu nanocomposite is over three times stronger than that of pure ZnO. Cu acts as a sensitizer and the enhancement of UV emission are caused by excitons formed at the interface between Cu and ZnO. As the volume fraction of Cu increases beyond a particular value, the intensity of the UV peak decreases while the intensity of the visible peak increases, and the strongest visible emission of a typical ZnOCu nanocomposite is over ten times stronger than that of pure Cu. The emission mechanism is discussed. Nonlinear optical response of these samples is studied using nanosecond laser pulses from a tunable laser in the wavelength range of 450650 nm, which includes the surface plasmon absorption (SPA) band. The nonlinear response is wavelength dependent and switching from reverse saturable absorption (RSA) to saturable absorption (SA) has been observed for Cu nanocolloids as the excitation wavelength changes from the low absorption window region to higher absorption regime near the SPA band. However, ZnO colloids and ZnOCu nanocomposites exhibit induced absorption at this wavelength. Such a changeover in the sign of the nonlinearity of ZnOCu nanocomposites, with respect to Cu nanocolloids, is related to the interplay of plasmon band bleach and optical limiting mechanisms. The SA again changes back to RSA when we move over to the infrared region. The ZnOCu nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior. The nonlinear refractive index and the nonlinear absorption increases with increasing Cu volume fraction at 532 nm. The observed nonlinear absorption is explained through two-photon absorption followed by weak free-carrier absorption and interband absorption mechanisms. This study is important in identifying the spectral range and composition over which the nonlinear material acts as a RSA-based optical limiter. ZnOCu is a potential nanocomposite material for the light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Studies on Surface Properties and Catalytic Activity of Some Chromites and Related Spinels

The prime intension of the present work was a synthetic investigation of the preparation, surface properties and catalytic activity of some transition metal substituted copper chromite catalysts. Homogeneous co-precipitation method is employed for the preparation of catalysts. Since the knowledge about the structure and composition of the surface is critical in explaining the reactivity and selectivity of a solid catalyst. a systematic investigation of the physico-chemical properties of the prepared systems was carried out. The catalytic activity of these systems has also been measured in several oxidation reactions of industrial as well as environmental relevance. The thesis is dedicated to several aspects of chromite spinels giving emphasis to its preparation, characterization and catalytic performance towards oxidation reactions.

Purification and characterisation of Vibriophage isolated from Mangalavanam mangrove

The temperate, filamentous phage ФMV -5 isolated from Mangalavanam mangrove of Kochi, using the environmental strain of Vibrio sp. MV-5 shares many similar properties with other marine phage isolates, while also remaining unique. The study has revealed that the interaction of temperate phages and the microbial population in the marine environment may contribute significantly to microbial genetic diversity and composition by conversion and transduction and which requires greater study.Prophages contribute a substantial share of the mobile DNA of their bacterial hosts and seem to influence the short-term evolution of pathogenic bacteria. Automated methods for systematic investigation of prophages and other mobile DNA elements in the available bacterial genome sequences will be necessary to understand their role in bacterial genome evolution. In the past, phages were mainly investigated as the simplest model systems in molecular biology. Now it is increasingly realized that phage research will be instrumental in the understanding of bacterial abundance in the environment. One can predict that phage research will impact diverse areas such as geochemistry and medicine. Success will largely depend on integrative multidisciplinary approaches in this field. Clearly, further studies are required to understand how vibriophages interact with Vibrios to promote this organism's acquisition of the critical genes which alter its virulence or adaptation to its environmental niche.It is evident from this study and comparison with those reports cited above that vibriophage ФMV-5 is a previously unreported bacteriophage. It is recommended that the minimum requirement for reporting a new phage should be novel morphological markers and a description of host range, both of which have been achieved in this study.

Algal Blooms and Associated Bacteria along the Southwest Coast of India

It is essential to understand how the distribution and composition of microalgae as well as dynamics of HABs in economically important shelf seas relate to the particular physico-chemical and biological properties of the water column in which they live. In view of the importance of southwest coast of India, which is considered as one of the most biologically productive areas in the world, regular monitoring of distribution and abundance of microalgae is important. The present work is concentrated on the estuarine and coastal open sea stations along the southwest coast of India. In order to get further insights into the abiotic factors governing bloom dynamics, the physicochemical parameters that regulated three particular bloom events during this period were studied. Bearing in mind the role of bacterial fauna associated with algal blooms as a biological factor in regulating its dynamics, isolation of bacteria associated with the algal blooms, their identification, enumeration, and ability to produce extracellular enzymes have been duly incorporated into this study