Investigation of 1D nanostructures and heterostructures using electron microscopy

Abstract

Inorganic colloidal nanocrystals are extensively used in various fields such as electronics, sensing, photonics, thermoelectrics and catalysis. The intriguing properties of these colloidal nanocrystals can be attributed to their surface and interfaces. Thus, engineering the interface leads to the formation of nanoscale heterostructures with controlled properties. The properties exhibited by nanoscale heterostructures are often superior to their component nanostructures. As heterostructures provide a platform to establish electronic communication between neighbouring components via interface. It is desirable to have heterostructure with coherent interfaces to improve their efficiency. Coupling different nanocrystals to form multicomponent heterostructures can introduce multifunctionality in the system. Solution-based synthesis techniques stand out as versatile techniques to precisely tune the size, shape, composition of colloidal inorganic nanocrystals. A variety of colloidal nanocrystals ranging from metals and semiconductor to oxides have been synthesized successfully by solution-phase methods. By a judicious selection of surfactant, solvent and reaction parameters such as temperature and pressure, it is possible to control the thermodynamics and kinetics of the reaction. A fair understanding of nucleation and growth is also essential to engineer the nanocrystals. The development of robust synthetic technique for semiconductor superlattices (which is defined as periodic heterostructure of two different materials with respect to their band gap, crystallochemistry or composition), can be described as a renaissance in the field of semiconductors/thermoelectrics. A broad class of heterostructures based on II-IV and III-V semiconductors such as transition metal-phosphide, arsenide and chalcogenide have been explored widely due to their interesting electrical and optical properties. Among these semiconductors, Te finds wide application in the field of optoelectronics and thermo-electrics and its derivatives like lead telluride and bismuth telluride are used as state-of-the-art thermoelectric materials. Theoretical studies show, coupling these tellurides in 1D could increase the thermoelectric efficiency (ZT) due to the periodic potential, but a general and simple scheme to achieve the same with coherent interface is still challenging. Heterostructures of different architecture and dimensionality have been realized primarily by vapor-phase methods. In the scope of this thesis, semiconducting superlattice structure and various metallic heterostructures have been obtained via simple solution-phase method. Further, formation mechanism of these complex heterostructures have been explored by detailed electron microscopic investigation. In summary, solution phase synthesis method to design semiconductor superlattice and metallic nanoscale heterostructure are discussed and their formation mechanism is studied using electron microscopy. The insight gained from this work can help to design various complex heterostructures with added functionalities