Study Of Structural,Optical,Dielectric And Magnetic Properties Of SnO₂-based Nanoparticles

Among all the metal oxide semiconductors SnO₂ is one of the prominent wide band gap metal oxide semiconductor nanostructure. It is an n-type semiconductor and its n-type conductivity is due to presence of large number of oxygen vacancies in its rutile tetragonal structure. It has a direct band gap of 3.6 eV at 300 K. Its high conductivity and high transparency makes it one of the most ideal candidate having applications in the fields of microelectronics, optoelectronic and spintronic devices. The increasing interest in SnO₂ nanocrystalline materials is large surface to volume ratio and quantum confinement effect. One of the methods to modify its properties is doping. Introduction of suitable dopants will result in modification of microstructure and defect chemistry leading to variation of electrical, optical and magnetic properties. AC measurements are important means to study the dynamic properties （conductance, capacitance and dielectric loss） of the materials, providing full information about the interior of the material in relatively low conductivity and help to distinguish between localized and free band conduction. Large defect density （oxygen vacancies） results to enhance the photoluminescence emission intensities and magnetic properties of SnO₂ nanoparticles. The present thesis contains the synthesis and characterization of SnO₂ based dilute magnetic semiconductor nanostructures. The structural, optical, electrical and magnetic properties of SnO₂ nanoparticles were studied. All the undoped and doped SnO₂ nanoparticles are synthesized by chemical precipitation route.In undoped and doped SnO₂ with increasing calcination temperature particle size increases with enhancement in grain growth and crystalline quality. At low calcination temperatures these nanoparticles are in oxygen poor state revealing large concentration of oxygen vacancies. As the calcination temperature increases the oxygen content increases and due to complete oxidation of Sn²⁺ to Sn⁴⁺ the crystalline quality increases. At low calcination temperatures, emission intensities are extended to visible light due to large defect density but as the calcination temperature is elevated the emission intensities decreases due to decrease in defect density and increase in particle size. Band gap energies of all particles are larger than the bulk band gap value of SnO₂ （3.6 eV） and there is a slight increase in band gap of the samples as the calcination temperature is increased, which are due to annealing temperature distortion. Dielectric properties are enhanced due to smaller sized nanoparticles （large number of grain boundaries and oxygen vacancies） at low calcination temperatures. With increasing calcination temperature dielectric constant reduced due to increasing particle sizes having reduction in electric polarization due to reduction of poorly conducting grain boundaries and oxygen vacancies. Dielectric dispersion behavior, relaxation peaks and enhancement in electrical conductivity at higher frequencies are observed. The observed dispersion behavior and enhancement in electrical conductivity are due to Maxwell Wagner interfacial polarization and hopping of charge carriers between Sn²⁺/Sn⁴⁺ ions. Also the increased electrical conductivity in samples calcined higher temperatures is due to growth of well conducting grains volume, which reduces number of poorly conducting grain boundaries. Room temperature ferromagnetism with reduction in saturation magnetization is observed with elevation in calcination temperature, which is due to reduction in oxygen vacancies at higher calcination temperatures.In case of Zn doped SnO₂ nanoparticles with increasing concentration of dopant not only particle sizes but oxygen content also reduces revealing large number of oxygen vacancies in smaller nanoparticles. Incorporation of Zn+ in SnO₂ lattice to replaced Sn⁴⁺ is accompanied by oxygen vacancies. Emission intensities are enhanced and extended to visible light due to presence of large number of oxygen vacancies with increasing amount of Zn incorporation in SnO₂ lattice. The determined band gap values for all doped samples are larger than the bulk SnO₂ （3.6 eV）. The increase in band gap is mainly due to small sized nanoparticles and Burstein Moss （BM） effect. Dielectric properties are enhanced with increasing dopant （Zn） concentration due to large number of oxygen vacancies and smaller sized nanoparticles having large number of poorly conducting grain boundaries. The sharp increase in electrical conductivity at higher frequencies and dielectric dispersion behavior is due to increase in charge density and hopping process.In （Zn,Co） co-doped SnO₂ and （Zn, Cu） co-doped SnO₂ nanoparticles incorporation of Zn²⁺, Co²⁺ and Cu²⁺ are accompanied by introduction of oxygen vacancies. The increasing dopant concentration in SnO₂ resulted in smaller nanoparticles and reduction in oxygen content revealing large number of oxygen vacancies. The determined large band gap values as compared to bulk SnO₂ （3.6 eV） are due to confinement in small sized nanoparticles and with increasing content of dopants. The increase in band gap is mainly due to small sized nanoparticles and Burstein Moss （BM） effect. Emission intensities in visible region of light are enhanced due to small sized nanoparticles having large number of oxygen vacancies with increasing amount of dopants. The large number of poorly conducting grain boundries in small sized nanoparticles causes enhancement in dielectric constant and relaxation peaks. Whereas, the increased charge density and hoping process at higher frequencies can results a sharp increase in the electric conductivity. Room temperature ferromagnetism with enhancement in saturation magnetization is observed because of oxygen vacancies introduced by increasing amount of dopants.The enhanced dielectric constant, electric conductivity, visible light emission and room temperature ferromagnetism make all these nanoparticles very useful in the field of microelectronics, optoelectronics and spintronics.