Metallic and semiconductor nanoparticles: Synthesis, characterization and femtosecond laser spectroscopic studies.

摘要

The optical properties and the femtosecond relaxation dynamics of metallic and semiconductor nanoparticles and their dependence on the particle shapes were studied. Metallic and semiconductor nanoparticles were synthesized and characterized using optical absorption and emission spectroscopy as well as the Transmission Electron Microscopy (TEM). The ultrafast relaxation dynamics and the transient absorption spectra have been monitored using femtosecond pump-probe experiment.; Regarding the shape dependent optical properties of metallic nanoparticles, gold nanospheres show surface plasmon absorption at 520 nm, however gold nanorods show two plasmon absorption bands due to the oscillation of the surface electrons along the transverse and the longitudinal modes. New absorption features appear, which are assigned to quadrupole resonance absorption along the short axis perpendicular to the triangular or hexagonal nanoplatelets. One of the most unique properties of gold nanorods is that they fluoresce with quantum yield enhancement one million times than the gold bulk due to the local field effect created by the longitudinal surface plasmon absorption (lightning nanorod effect).; To study the effect of the surrounding matrix on the electron-phonon and phononphonon ultrafast relaxation dynamics of gold particles, the relaxation dynamics of the gold particles embedded in hydrogel and in organic gels have been compared. A large decrease in the phonon-phonon relaxation time was observed in the organic gel due to their lower thermal conductivity compared to water. The chemical interaction between the solvent molecules trapped inside the gel network and the gold particles surface enhances the rate of the heat transfer between the particles and the bath, giving rise a faster rate of relaxation.; Shape dependent optical and ultrafast electron dynamics of semiconductor CdSe nanocrystals was studied. The femtosecond transient absorption spectra of CdSe nanorods show more spectral features than the dots of comparable diameter. This is due to breaking the near degeneracy in the spherical dots by elongation of the unique c-axis in the CdSe nanorods. The population and relaxation dynamics of the excited states in CdSe nanorods of different aspect ratios are studied using femtosecond transient absorption spectroscopy.