Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

We investigate the size-dependent optical absorption coefficients of CdSe nanocrystals at both the band-edge and high within the absorption profile. The absorption properties in both of these regions must be selfconsistent to ensure accuracy of the measured coefficients. By combining transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy, we map out the optical absorption properties and establish reliable size-dependent band-edge calibration curves. The measured absorption properties are compared to a simple 0D confinement model, to classical theory based on light absorption by small particles in a dielectric medium and to state-of-the-art atomistic semiempirical pseudopotential modeling. The applicability of these newly established calibration curves is demonstrated by analyzing the nucleation and growth kinetics of CdSe nanocrystals in solution.

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Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remains unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe and CdTe) to molecular acceptors (anthraquinone, methylviologen and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ~ 0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model, in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Frank-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes.

Using atomistic, semiempirical pseudopotential calculations, we show that if one assumes the simplest form of a surface state in a CdSe nanocrystal--an unpassivated surface anion site--one can explain theoretically several puzzling aspects regarding the observed temperature dependence of the radiative decay of excitons. In particular, our calculations show that the presence of surface states leads to a mixing of the dark and bright exciton states, resulting in a decrease of 3 orders of magnitude of the dark-exciton radiative lifetime. This result explains the persistence of the zero-phonon emission line at low temperature, for which thermal population of higher-energy bright-exciton states is negligible. Thus, we suggest that surface states are the controlling factor of dark-exciton radiative recombination in currently synthesized colloidal CdSe nanocrystals.