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The current model for understanding trapping of charge carriers to the surface of semiconductor nanocrystals is inconsistent with experimental evidence indicating that carriers can thermally de-trap from surface sites. A proper understanding of the microscopic details of charge trapping would guide chemical design of the nanocrystal surface for applications such as charge transport, sensing, or photochemistry. This thesis presents a model of surface charge trapping in which transitions to surface state are governed by rates derived from semiclassical electron-transfer theory. In this picture, trapping to the surface induces a strong polarization in the nanocrystal, resulting in a trapped state with strong electron-phonon coupling via the Frölich mechanism. This trapped state then emits over a broad energy range due to a Franck-Condon vibronic progression. This model is shown to be consistent with the temperature-dependence of core and surface emission as well as the spectral properties of surface emission. The strong coupling of the surface state is validated by independent experiments, and the model is shown to hold promise for explaining the experimental data regarding the trapping of hot (excess energy) carriers.iii RésuméLe modèle prévalent concernant le piégeage des porteurs de charges à la surface de nanocrystaux semi-conducteurs est inconsistant avec certains résultats expérimentaux indiquant que les charges peuvent subir une relaxation thermique depuis les états de surfaces. Une bonne compréhension des aspects microscopiques du processus de piégeage des porteurs de charges permettrait de guider le design de la surface des nanocrystaux en vue d'applications comme le transport de charges, la détection ou la photo-chimie.Ce travail de doctorat propose un modèle du piégeage des charges où les taux de transition vers des états de surface sont basés sur la théorie semi-classique du transfert d'électrons. Dans ce modèle, le piégeage à la surface crée une forte polarisation dans le nanocrystal, ce qui résulte en un état piégé avec un grand couplage électron-phonon via le mécanisme de Fröhlich. Cet état piégé émet dans une large bande spectrale due à la progression vibronique de Franck-Condon. Le modèle proposé est consistent avec la dépendance en température du spectre d'émission du centre et de la surface ainsi que des propriétés spectrales d'émission de la surface. Le couplage fort de l'état de surface est validé par des expériences indépendantes et il est montré que le modèle est prometteur pour l'analyse d'expériences sur le piégeage des porteurs de charges chauds (ayant un excès d'énergie).iv

Small CdSe semiconductor nanocrystals with diameters below 2 nm are thought to emit white light due to random surface defects which result in a broad distribution of midgap emitting states, thereby preventing rational design of small nanocrystal white light emitters. We perform temperature dependent photoluminescence experiments before and after ligand exchange and electron transfer simulations to reveal a very simple microscopic picture of the origin of the white light. These experiments and simulations reveal that these small nanocrystals can be physically modeled in precisely the same way as normal-sized semiconductor nanocrystals; differences in their emission spectra arise from their surface thermodynamics. The white light emission is thus a consequence of the thermodynamic relationship between a core excitonic state and an optically bright surface state with good quantum yield. By virtue of this understanding of the surface and the manner in which it is coupled to the core excitonic states of these nanocrystals, we show both chemical and thermodynamic control of the photoluminescence spectra. We find that using both temperature and appropriate choice in ligands, one can rationally control the spectra so as to engineer the surface to target color rendering coordinates for displays and white light emitters.

The excitonics of semiconductor nanocrystals (NC) depend upon temperature in a complex manner due to the interplay between the kinetics of hot exciton relaxation/trapping and the thermodynamics leading to cold exciton recombination. We apply a semiclassical electron transfer model of surface trapping to temperature-dependent absorption and emission data to elucidate a microscopic picture of the factors which govern the fate of hot and cold excitons. The linear absorption spectra reveal a unique temperature-dependence to the energies of higher excitonic states, while oscillator strength is shown to be temperature invariant. We identify the phonon based origin to the anomalous low temperature peak energy trend in photoluminescence (PL) spectra. PL intensities, PL lifetimes, and absorption spectra are used to demonstrate that variation of quantum yield with temperatures arises from the thermally controlled fraction of NC which emit, rather than from an activated nonradiative pathway common to all NCs. Experimental quantum yield spectra are shown for several NCs and we perform a much-needed analysis of the role of surface PL in quantum yield. Finally, we show that a semiclassical electron transfer model including hot excitonic effects can explain experimental quantum yield spectra and suggests how to probe kinetic trapping processes via simple steady-state spectroscopy.