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Photophysics and Photochemistry of Ⅱ-Ⅵ Semiconductor Nanocrystals


Colloidally synthesized, 0D semiconductor nanocrystals, also referred to as Quantum Dots (QDs), are promising light-harvesting materials which have found increasing use in photovoltaic devices, LEDs and other optoelectronic devices. In the past two decades, they have been extensively studied in terms of their photophysical and photochemical properties. One of the most prominent features of this class of material is their size tunable optical properties, which can be explained by the quantum size effect, whereby the bandgap of a semiconductor is pushed to a larger value as the exciton experiences increased confinement. In the strong confinement regime, the degeneracy of the hole states gives rise to the fine structure of the band-edge exciton, which helps explain their unusually long radiative lifetimes (~1μs at 10 K) relative to the bulk exciton recombination time (1ns), detailed in Chapter 1.

In Chapter 2, the size dependence of the radiative lifetimes of CdSe quantum dots is illustrated and compared to the calculated results using the Einstein coefficients. The quantum yield of CdSe particles is strongly related to the ratio of radiative and nonradiative recombination processes. This affects the size dependence of radiative lifetimes, and is the reason for a large discrepancy in the literature. In this section, we compare the results obtained for particles synthesized using a standard method and those obtained from an optimized synthesis. To correctly calculate the radiative lifetime of CdSe particles, it is found that three factors need to be considered: size-dependent integrated extinction coefficient, the bandedge frequency and the Boltzmann fraction of population of dark versus bright states within the lowest energy exciton fine structure.

In Chapter 3, the mechanism of two-photon darkening is elucidated. When the CdSe particles are subjected to high power density irradiation, they demonstrate a prompt photoluminescence depletion, followed by a recovery in polar solvents, on the time scale of tens of minutes. The mechanism proposed is that a significant fraction of biexcitons is generated under intense irradiation, which can then undergo fast Auger recombination by transferring the recombination energy of the exciton to the remaining hole and excite it higher into the valence band. The excited hole can either relax back to the top of the valence band or tunnel out to the surface and ionize a ligand, specifically, trioctylphosphine. In the latter case, the ionization forms QD-/TOP+, which has a certain probability to dissociate into QD- and TOP+. The resulting QD- is dark, because it is negatively charged and unligated, which is attributed to the prompt photoluminescence (PL) depletion. The depletion of PL is reversible, when certain passivating ligands are present in solution, and indeed it is the charge neutralization and ligand reattachment/reorganization that determines the time scale of the PL recovery. The details are given in Chapter 3.

In Chapter 4, we provide an examination of the excited hole photochemistry of surface charged CdSe/CdS particles, under a low power irradiation regime. Surface charging is a common phenomenon for particles having relatively high quantum yields, in which the electrons have an equilibrium between the valence band and thermally accessible surface empty orbitals. The surface charged particles, though being overall neutral, have an extra hole in the valence band and a positive trion will subsequently form once these particles are subjected to low power irradiation. Similar as the Auger dynamics for the biexciton described in Chapter 3, the Auger recombination within the positive trion will also generate an excited hole, which again can ionize a surface-attached ligand, oleylamine in this case. As a result of this ionization, the resulting L+ can ligate with neutral and bright QDs, causing delayed and reversible photodarkening of the particles. The role of the excited hole in trion and biexciton photochemistry is also discussed.

Finally, in Chapter 5 the correlation between the selenium – oxygen (Se-O) bond stretch infrared feature and the extent of surface charging is discussed. The Se-O bond is formed through oxidation of occupied P orbitals on the surface chalcogenide atoms, and as such shows a strong dependence on the surface stoichiometry and ligation. Surface charging, discussed in Chapter 4, involves the vacant surface P orbitals. Therefore, the extent of surface charging is affected by the density of surface unoccupied orbitals, which also has a strong surface stoichiometry and ligand dependence. The filled and empty surface P orbitals both depend on the surface stoichiometry and the extent of ligation and are therefore correlated. Specifically, we find a linear dependence between the fraction of surface charging and the intensity of the 800 cm-1 IR feature.

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