Nano Technology - Theses

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Browsing Nano Technology - Theses by Author "Dash, Raj Kishora"
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    Development of metal-assisted chemical etching (MACE) based silicon (Si) nano/microstructures and their applications in surface enhanced Raman spectroscopy (SERS) for chemical sensing
    (University of Hyderabad, 2018-04-01) Borgohain, Debabrot ; Dash, Raj Kishora
    Surface-enhanced Raman Scattering (SERS) has emerged as a key research area for its excellent detection capability and fingerprint identification of the various chemicals such as methylene blue (MB), malachite green (MG), melamine etc. Recently, it has drawn tremendous attention for potential application in the food sector, medical sector, security sector etc. Though some progresses have already been achieved in the molecular level detection of chemical species with SERS technique, still more challenges are there such as repeatability, scalability, reproducibility, shelf life. One of the key factors is the development of the SERS substrates for improvements in the SERS signal and detection limit as well. In this regard, significant progress has already been achieved in the fabrication of the SERS-active substrates by using both the top-down and bottom-up approaches. Bottom-up approaches, such chemical synthesis, self-assembly, electrodeposition, dip-coating processes are successfully employed to fabricate SERS-active substrates. On the other hand, the top-down approaches, such as lithography, oblique angle metal film deposition and metal-assisted chemical etching (MACE) and which are mainly used to fabricate various nanostructures, has shown very effective and reproducible SERS-active substrates preparation. However, still preparation of the SERS substrates with enhanced Raman signal and also with higher detection limit have been considered as the main challenges and need to be addressed. Though, several existing approaches such as conventional or unconventional nanofabrication methods are used for the preparation of Si-based SERS substrates by top-down approach, MACE process is emerging as a very simple and cost-effective process for fabrication of the nanostructures and hence, it has gained sufficient attention globally. Thus, the main objective of this present work is to fabricate Si nano/microstructure by using MACE process and then utilizing these Si substrates for detection of the methylene blue (MB).Different nano/microstructures were fabricated on Si by using noble metals catalyst such as gold (Au) and silver (Ag). The effect of different etching parameters such as thermal annealing of the metal catalyst, effect of the concentration of the oxidising agent, those are directly associated with the MACE process were investigated. The experimental results revealed that the thermal annealing process eliminated the pin-holes present in the metal catalyst when 50 nm thick Au acted as metal catalyst and successfully fabricated deeper micro-trenches on Si substrates than the un-annealed counterpart of it. Moreover, it also indicated that higher thickness of the metal catalyst micro-stripes help to fabricate flat bottom type deeper trench than the thinner counterpart. Furthermore, the mechanism for the obtained results are discussed in the thesis. The morphological effect of the metal micro-stripes was also examined and the results indicated that various nano/microstructures could be fabricated by controlling the morphology of the used metal catalyst. As it is very well known fact that the SERS detection limit or enhancement of Raman signal primarily depend on the fabricated nano/microstructures present on the surface of the Si substrate and incorporated with the noble metal (Ag) nanoparticles. The performance of the Si-based SERS substrates was tested for the chemical detection of MB with very low concentrations starting from nM to pM, and the result indicated that nanostructured Si substrate showed better SERS detection than its microstructured counterpart. The probable reasons for such behaviours are also discussed. More studies were also carried out to enhance the detection limit of the MB, since higher the detection limit is better for the usability of these substrates for molecular level detection of different chemical species. Thus, combination of discontinuous Au film and Ag nanoparticles (NPs) were deposited on the nanoporous Si substrates, those were obtained by MACE process and the result suggested that the detection limit of MB can be further improved by depositing discontinuous Au thin film with various thicknesses. It is also noticedfrom the experimental results that up to certain thickness of the Au discontinuous thin films, it helps to improve the limit. On the other hand, higher thickness of the Au discontinuous thin films, degrades the detection limit. Thus, there is a direct correlation between the detection limit and the thickness of the Au discontinuous film on the nanoporous Si substrate. Our experimental findings suggested that the best detection could be achieved when a combination of 30 nm Au discontinuous thin film and Ag NPs were used for preparation of the SERS-active substrates which can detect 10 pM MB. Furthermore, highest enhancement factor of the order of 108 is also achieved with this same combination of Au discontinuous thin film and Ag NPs. Thus, we believe that the prepared SERS-active substrate in this work can have the capability to detect MB at molecular level. We believe that the easy, simple and reusable SERS-active substrates will be more helpful to develop chemical sensors to detect molecular level chemical species other than MB with higher enhancement factor and can be utilized for the food safety, environmental monitoring, security and medical diagnosis in the near future.
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    Study of the influence of the different synthesis parameter on the structure, morphology, bonding and properties of the graphene oxide (GO), reduced graphene oxide and graphene/alumina nanocomposites
    (University of Hyderabad, 2017-03-01) Demudubabu, Dommisa ; Dash, Raj Kishora
    Graphene is an emerging material due to its unique properties such as electrical, mechanical, optical, chemical and thermal properties and hence it has a lot of potentials to use in several applications. The graphene oxide is generally defined as a single layer of graphene with oxygen functional groups present in basal planes and edges of the graphene structures. These functional groups are mostly hydroxyl, epoxide, ketone and carboxyl on the surface of graphene layer at basal plane and edges. Therefore, GO is easily dispersible in water and other organic solvents and hence, mostly applicable in different forms such as thin films, composites and hybrid materials. Composites are novel type materials and have unique properties than host matrix. Graphene can be used in the composite material as filler to develop novel composites which have novel and unique properties. Comparative other nanomaterial graphene is more suitable for making the nanocomposite because of easy synthesis process of graphene with large-scale production for industrial application. To prepare the graphene-based composite mostly used graphene derivatives, it has tunable electrical and optical properties, based on the composite properties the applicationsFrom the recent literature, we have concluded that graphene oxide can be synthesized by a chemical process which is a simple, cost- effective and more economical. Further, the obtained GO can be reduced by using suitable reducing agents to obtain reduced graphene oxide(RGO), which has a lot of potentials to be used in several applications. However, most of current studied mainly focus on the synthesis of the graphene oxide and the corresponding reduced graphene oxide by using one size graphite precursor material (~45μm). Therefore, the impacts of the different size graphite precursors ( very smaller to a larger size) as starting material for synthesis of GO and RGO and the details study of the oxidation process, structure, Morphology, bonding characteristics etc are not addressed to our best knowledge. Therefore, there is a lack of clear understanding of the influence of different sizes graphite precursor on the synthesis of GO and RGO by a chemical process. Thus, these issues and challenges need to be addressed. Further, the oxidation reaction temperature is a key factor for controlling the properties of the GO, and further, the quality of RGO obtain from the as-synthesized GO. Hence, the influence of the reaction temperature of the oxidation process need to be addressed and a more details investigation of its effects on the structure, morphology, bonding to be required. Moreover, the reduction of GO to RGO is generally carried out in a few solutions, so more work is still required to understanding the solution effects on the reduction of GO to RGO. In the second part of literature review part, we have thoroughly searched all the recent literature on the synthesis and characterization of the graphene/alumina nanocomposite for its potential to be utilized as potential materials for several applications such as energy, storage devices, capacitors, memory devices and so.on. Though, there are several existing syntheses processes has been reported by several groups and also mentioned that electrical and mechanical properties can be improved by 7 fabrication of the graphene alumina nanocomposite. It is observed that most of the reported work has chosen the α – phase alumina nanoparticles to synthesize the nanocomposites due to the stable phase of the alumina. Also, few reported work are existing, where they have prepared alumina by choosing the alumina precursor to synthesize the alumina by chemical processes such as sol-gel method, finally fabricating the graphene/alumina nanocomposites. Moreover, the most of the reported work mainly focuses on the fabrication of the bulk materials by SPS process and then to investigate the different properties of the nanocomposites. Therefore, to the best of our knowledge, still, there is a lacking of clearly understanding the graphene /alumina nanocomposite structure, morphology, and bonding characteristics, when γ – phase alumina is employed as matrix material insisted of α – phase alumina. Further, the as the obtained powder form of the graphene /alumina nanocomposite need to be studied in details, as these powder form can be used as starting material for thin films formation, for another hybrid material synthesis and also for utilizing as filler material for more complex hybrid material fabrication. Thus there are still more challenges are there to clearly understand the GO and RGO synthesis process by a chemical method and also for fabrication of graphene /alumina nanocomposites for effectively using these materials for mass production and also for several practical applications. In this thesis research work, the influence of the synthesis parameters such as oxidation reaction temperature (low and high) on the structure, morphology, bonding and properties of the graphene oxide(GO) and the corresponding chemically and thermally reduced graphene oxide(RGO) was studied by using the standard Modified Hummers synthesis method. In the second part of the thesis work, the influence of the different sizes source graphite on the synthesis of graphene oxide (GO) and corresponding reduced graphene oxide (RGO) was investigated by choosing the optimum synthesis parameters as obtained from the first part of the research work. The impacts of host graphite powder sizes on the structure, morphology, disorder, bonding and properties were also investigated by using the different analytical instruments. In the final part of the thesis work, a set of six GO/RGO-Al2O3 nanocomposite was synthesized by choosing -phase Al2O3 using the colloidal mixing process and the effects of the different additives such as HCl, Hydrazine hydrate and without any additives were investigated. The corresponding nanocomposites structure, morphology, bonding, interface and thermal stability were investigated by using different analytical instruments such as XRD, FESEM, TEM, FT IR, Raman spectroscopy and TGA/DTA for understanding the nanocomposite formation. Graphene oxide was synthesized by varying the reaction temperature (low and high) by using the equal ratios of graphite, H2SO4, NaNO3 and KMnO4. All the samples were analyzed by using the XRD, FT IR, Raman, TEM, FESEM, UV and TGA/DTA for structural, molecular & bonding characteristics, morphological and thermal analysis. Experimental results indicated that oxidation reaction temperature has an effective role in the synthesis of GO. It was observed that low- temperature reaction showed the good quality of graphene oxide(GO) with better structural, less disorder and 2-3 layers of GO as compared to the higher reaction temperature. Further, the 8 corresponding reduced graphene oxide (RGO) was obtained by using hydrazine hydrate and thermal reduction process, and experimental results confirmed that the RGO which was obtained by using the lower reaction temperature GO showed less disorder, more graphical structure, flat type and 2-3 layers as compared to the GO obtained at higher temperature. Three different sizes graphite precursor (2-15 μm, <45 μm, and 170-840μm,) were oxidized by MHM method and then corresponding RGO were synthesized by the hydrazine hydrate reduction process. The experimental results indicated that the smaller size precursor graphite was fully oxidized in comparison to the large size graphite as source material. Hence, oxidation of graphite to graphene oxide is size depended on the source graphite and also showed better quality of reduced graphene oxide than other sizes. Further, from the TGA and DTA analysis, it is observed that different weight loss and different exothermic peak position in the graphite oxide which suggest that weight loss also depends on the size graphite due to the oxidation rate. UV spectroscopy analysis shows different abortion peaks which indicated that all the sizes graphite oxidized differently. Hence, the precursor graphite size has a key role in the synthesis of GO and corresponding RGO. A set of six GO/RGO-Alumina nanocomposites were prepared by colloidal mixing process by varying the reaction temperature (at RT and 800c) and different additives such as HCL and hydrazine hydrate. From all the analytical analysis results such as XRD, FESEM, FTIR, Raman, TEM is TGA and DTA, it was concluded that only when hydrazine hydrate was used as an additive GO was reduced during the synthesis process and RGO – Al2O3 nanocomposite was formed. All other cases GO – Al2O3 nanocomposites formed. Hence, RGO-Alumina nanocomposite can be synthesized by using hydrazine hydrate as an additive during fabrication process by using a smaller size -phase alumina nanoparticles. Different GO/RGO- Al2O3 nanocomposites were synthesized by using the optimum process parameters as obtained by varying the wt% GO (0.5, 1, 3, 5, 10 and 20 wt %) and all the samples were analyzed by using XRD, FT IR, Raman and TEM. Experimental results indicated that as the GO content increases more disorder and less agglomeration of the alumina nanoparticles are observed. Further, it was seen that as the GO content increases the oxygen-related function groups are removed because of more interaction between the GO and alumina nanoparticles. In this thesis work, GO/RGO- alumina nanocomposites were synthesized and formation of the graphene-alumina nanocomposite was confirmed based on the structural, bonding, morphological and TGA/DTA analysis, however still to use such nanocomposites in practical application such as energy storage, memory devices and other applications, properties such as mechanical, catalytic and electrical are needed to be addressed. Therefore, the future scope of this thesis works to investigate the electrical, mechanical properties and the percolation threshold in order to utilize this nanocomposite for practical memory devices and energy applications.