Surface effects on the ultrafast electronic relaxation of some semiconductor and metallic nanoparticles

My research has focused on understanding the surface effects on the optical and electronic properties of some metallic and semiconductor nanomaterials. When the particle sizes are on the nanometer length scale, a large fraction of atoms in the particles are on the surface. The bonding of the surface atoms being unsaturated could cause trapping and introduce defects that interact with the excited electrons. The effect of the surface on the optical and electronic properties of some semiconductor and metallic nanoparticles is investigated. When the size and shape of nanomaterials change, both the electron density of the excited electrons on the surface and the electronic structure change. Therefore, it becomes important to understand how these changes affect the electronic motion in the particles in order to exploit their full potential in a variety of applications.; A very critical part of the research presented in this thesis has been the nanomaterial synthesis. Chapter two discusses different methods used to make some metallic and semiconductor nanoparticles of different sizes and shapes. Learning different techniques is found to be very important in designing new synthetic methods to make desired materials. Making CdSe nanorods 40 nm in length and having the diameter smaller than the Bohr radius (quantum confinement limit) has important accomplishments of the present work.; The synthesis of copper nanoparticles of different sizes ranging from 10 nm to 60 nm in diameter is also described. This was the first synthesis to achieve isolated copper nanoparticles in this size range using photochemical methods. It was necessary to have control over the size in order to study size dependent ultrafast electron relaxation dynamics.; Synthesis of high quality, monodispersed CdS nanoparticles is also described. The quantum yield for CdS nanoparticles was found to be very good without passivation by a higher band gap material. Optical gain was observed in this material in solution at room temperature.; Chapter three examines the effect of changing the size and shape of CdSe nanoparticles on the rate of thermalization of hot electrons. It was believed that electron relaxation in semiconductor nanoparticles would be very long due to quantum confinement, leading to the phonon bottle neck effect. This was not found to be the case experimentally and was thought to be due to the fact that electrons relax from upper states by giving off energy to the holes. We also found that electron thermalization gets slower when the size of spherical particles is increased or when the shape is changed into rod. Using femtosecond transient spectroscopy, chapter three also shows that electron coupling to the surface state is a viable mechanism for promoting rapid relaxation rates in practical type particles synthesized by bottom up method in colloidal solutions. This mechanism shows that upper state electrons relax faster than electrons in lower excited state due to a higher density of electrons.; Chapter four shows that the probability of electrons to become trapped increases with increasing excitation energy by monitoring the steady state fluorescence. This indicates that electron trapping is more efficient from upper excited states. It is shown that by passivating the particles with ZnS, one can minimize surface traps and hence increase fluorescence quantum yield. The effect of shape on the fluorescence lifetime is also investigated and there is shown to be a sharp change in the lifetime when the particle's aspect ratio becomes larger than 1.35. These results are in agreement with the previous theoretical calculations that energy state crossover at this aspect ratio making the transition allowed along the z-axis.; Chapter five discusses the effect of electron-surface phonon coupling on the relaxation dynamics copper nanoparticles of different sizes. For the first time, the size dependence of electron-surface phonon relaxation in such a system was observed. F...