Structure-Dependent Optical Properties and Electronic Relaxation Dynamics of Colloidal Nanoparticles

This dissertation presents structure-specific descriptions of optical properties and electronic energy relaxation dynamics of structure precise monolayer-protected gold clusters (MPCs) and narrow size distributed semiconductor nanocrystals (NCs). Femtosecond pump-probe transient extinction spectroscopy has been conducted on MPCs to determine the size at which the transition from non-metallic to metallic electronic behavior occurs. Excitation-pulse-energy-dependent measurements confirmed Au144(pMBA)60 (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron-phonon coupling constant of (1.63 ± 0.25) × 1016 W m-3 K-1 after a series of gold monolayer protected clusters (MPCs) whose sizes ranged from 1.5 nm to 2.4 nm were studied. Smaller, non-metallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum-confined electronic states. Volume-dependent electronic relaxation dynamics for < 1.8 nm MPCs were attributed to an electron-phonon bottleneck, which arose from a combination of large energy differences between electronic states and phonon frequencies and spatial separation of photo-excited electrons and holes. Then, electronic energy relaxation of Au144(SR)60q ligand-protected nanoclusters, where SR = SC6H13 and q = -1, 0, +1 and +2, was examined using femtosecond time-resolved transient extinction spectroscopy. The observed differential transient spectra contained three distinct components: 1) transient bleaches at 525 nm and 600 nm, 2) broad visible excited-state absorption (ESA), and 3) stimulated emission (SE) at 670 nm. The bleach recovery kinetics depended upon the excitation pulse energy and were thus attributed to electron-phonon coupling typical of metallic nanostructures. The prominent bleach at 525 nm was assigned to a core-localized plasmon resonance (CLPR). ESA decay kinetics were oxidation-state dependent and could be described using a metal-sphere charging model. The dynamics, emission energy, and intensity of the SE peak exhibited dielectric-dependent responses indicative of Superatom charge transfer states. Based on these data, the Au144(SR)60 system is one of the smallest-known nanocluster to exhibit quantifiable electron dynamics and optical properties characteristic of metals. To understand the origin of near infrared (NIR) photoluminescence (PL) of Superatomic nanoclusters, electronic energy relaxation dynamics of Au25(SR)18-1 were studied. The transient spectra were composed of three distinct components: 1) spectrally broad excited-state absorption (ESA); 2) stimulated emission and 3) ground-state bleaching. Strong solvent dielectric dependencies of the component 2 relaxation rates and photoluminescence emission implicated metal-to-ligand charge transfer mechanisms in mediating electronic energy relaxation dynamics of MPCs. A full picture of electronic relaxation pathways in Au25(SR)18 has been proposed. After initial excitation from ligand bands into high-energy excited states, rapid internal conversion is followed by non-radiative relaxation into charge transfer states localized on MPC protecting ligands. NIR photoluminescence is attributed to the radiative transition from charge transfer states to ground states. All these data suggested that protecting ligand structure and the surrounding environment could modify nanoclusters-to-ligand charge transfer in MPCs. Finally, charge carrier relaxation dynamics of electronically excited CdSe and CdSe/CdS core/shell nanocrystals (NCs) were studied using femtosecond time-resolved transient absorption spectroscopy, employing both visible and NIR probe laser pulses. Following 400-nm excitation, the combination of visible and NIR laser probe pulses were used to determine the influence of surface passivation on electronic relaxation dynamics for nanocrystals overcoated with either organic ligands or inorganic semiconductors. In particular, low-energy NIR photons were used to isolate transient absorption signals due to either electron and hole intraband transitions. Four relaxation components were detected for CdSe NCs passivated by organic molecules: 1) picosecond hole relaxation; 2) electron deep trapping; 3) electron surface trapping; and 4) exciton radiative recombination. Based on TA data collected over a broad energy range, electron deep trapping at Se2- sites was suppressed for CdSe NCs passivated by inorganic (CdS) semiconducting materials. By comparing the time-dependent transient absorption data of a series of CdSe/CdS NCs with different shell thicknesses, evidence for the transition from Type-I to quasi Type-II NCs was obtained. These data illustrate the sensitivity of femtosecond time-resolved transient absorption measurements carried out over visible and near infrared probe energies for determining the influence of nanocrystal structure on electronic relaxation dynamics.