PREPARATION AND ULTRAFAST OPTICAL CHARACTERIZATION OF METAL AND SEMICONDUCTOR COLLOIDAL NANO-PARTICLES

The ultrafast dynamics of photoinduced electrons in several metal and semiconductor colloidal nanoparticle systems are characterized using f emtosecond laser spectroscopy. Various preparation methods are used an d, in several cases, modified for making particles with long-term stab ility and narrow and controllable size distributions. The particle siz e and size distribution are determined using transmission electron mic roscopy and electronic absorption spectroscopy. For aqueous gold and s ilver colloids, spatial size confinement is found to cause substantial ly slower electronic relaxation due to reduction of non-equilibrium el ectron transport and weaker electron-phonon coupling. In gold colloids , photoejection of electrons into the liquid is observed, which is att ributed to a two-photon enhanced ionization process. The effect of sur factant on the electron dynamics in CdS colloids is examined and found to be significant, substantiating, the notion that electrons are domi nantly trapped at the liquid-solid interface. In Ru3+-doped TiO2 collo ids, the electronic decay is found to be as fast as or even faster tha n in undoped TiO2 and other semiconductor colloids such as CdS, sugges ting that ion doping of large bandgap semiconductor colloids is not ne cessarily effective in lengthening the electron lifetime. In almost al l cases studied, the majority of the photoinduced electrons are found to decay within a few tens of picoseconds due to non-radiative relaxat ion. The results are discussed in the context of the potential applica tions of metal and semiconductor nano-particles in areas including pho tocatalysis and photoelectrochemistry.