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Exciton Dynamics in II-VI Semiconductor Nanoparticles

  • Author(s): Morgan, David Patrick
  • Advisor(s): Kelley, David F
  • et al.
Abstract

Colloidal II-VI quantum dots (QD) possess tunable optoelectronic properties that make them well-suited in applications as luminescent materials. However, their performance is often limited by nonradiative exciton quenching processes typically involving interaction of the exciton with surface states, crystalline defects, or other charge carriers. It is the purpose of this dissertation to identify and understand these decay mechanisms at a fundamental level. Two of the dominant processes, Auger recombination (AR) and hole trapping (HT), are explored from several angles. Biexciton lifetimes of alloyed Cd1-xZnxSe go through a minimum at small mole fractions of Cd (x = 0.1 – 0.4), and this behavior is attributed to partial hole localization in Cd-rich regions of the particle. For small x, the biexciton times are nonexponential, whereas the pure materials and alloys with x ≥ ~ 0.5 show single exponential decays. A model of the alloy valence band potential is used to approximate hole density gradients (taken to be proportional to AR rates) for different alloy compositions, and the model reproduces both the faster and broader distribution of rates in the small Cd concentration regime.

The positive trion excitonic state is selectively prepared in CdSe and CdSeCdS core/shell particles by manipulating the surface chemistry. Comparison of the TA and PL decays shows that the trion lowest exciton bleach magnitude is ~ 1.5x greater than the exciton on account of the differences in fine structure. Stimulated emission is increased because all of the trion fine structure levels have fully allowed transitions to the ground state, while the trion excited state absorbance is weaker since the additional hole decreases the degeneracy of the transitions between the first and second multiexciton states, i.e. valence band state filling. Trion PL is blueshifted ~ 6 nm from the excitonic PL in the core/shells and ~ 0 - 1 nm in the cores, closely matching the sum of the additional e-h stabilization and h-h repulsion terms calculated in the effective mass approximation using first-order perturbation theory.

The effects of Ag doping on the photophysical properties of CdSe and CdSe/CdS quantum dots (QDs) are studied in terms of surface composition. TBP-ligated samples that suffer from hole trapping show a rise then fall of PLQY with increasing [Ag+], while higher QY particles passivated with TBP and amine or a thin CdS shell decrease upon doping. The initial increase is assigned to the passivation of pre-existing surface hole traps by interaction of interstitial Ag+ with surface Se2- ions, while the subsequent decrease is due to the introduction of substitutional Ag+, which act as a new source of hole traps. Well-passivated samples don’t have high energy filled surface states, so the dopants remain in the +1 oxidation state, act only as hole traps, and lower QYs. Both CdSe and CdSeCdS particles ligated with amine but no phosphine have Fermi levels within kT of the VB, giving rise to the positive trion through thermal population of surface states. In this case, some fraction of the Ag+ is reduced to Ag by the amine, which then shuts off trion Auger recombination by interacting with empty Se(0) orbitals.

Ligation of CdSe QDs with alkylamines raises QYs by decreasing the extent of hole trapping. This cannot be explained by a simple MO theory type interaction between the filled Se2- and the N lone pair of the amine, so a new passivation mechanism is proposed that considers the electrostatics of L-type ligand binding. Electron donation by the amine creates a surface dipole layer that raises the VB and CB levels of the particle having little effect on the surface state energetics, thereby lowering -ΔG for hole trapping. The mechanism is corroborated with the observed increase in CB electron transfer rates to methylviologen and decreased hole transfer rates to 4-methylbenzenethiol upon treatment with the amine.

Transient absorption (TA) spectra of CdSe nanoplatelets (NPL) are analyzed in terms of the exciton fine structure and population of low-lying lateral excited states. Treatment with an external hole trap decreases the NPL HH bleach, while the analogous spherical particles show no spectral manifestations of hole trapping in TA measurements. The differences are attributed to the strong z-quantum confinement breaking the LH/HH degeneracy as well to the reduced e-h exchange interaction in NPL. Because the X  XX triplet-triplet transition proceeds from the LH and is forbidden in the HH band, splitting off the LH results in reduced PA and increased bleach of the NPL lowest exciton band. The weak e-h exchange interaction allows significant population of the bright state and hence more stimulated emission in NPL compared with QD.

The relative bleach magnitudes of the same 4.5 ML nanoplatelets are measured by transient absorption spectroscopy and are found to be much less than those of their spherical counterparts. Analysis of the relative bleach magnitudes (change in optical density at the HH exciton normalized to the absorbance) shows that absorption of a photon bleaches ~ 10-25 % of the GSA, with the relative bleach getting smaller with increasing platelet lateral dimensions. This suggests that the size of the exciton is smaller than the platelet and is independent of the platelet areas. This is an unsurprising result given that the bulk Bohr radii of CdSe excitons is about 5 nm , that the NPL Bohr radii should be significantly less than those seen in bulk due to the nearby layer of organic, low dielectric constant material, and because the platelet dimensions are greater than the Bohr radii. So the lack of significant quantum confinement along x and y causes the excitonic areas to be platelet-size independent. This consideration is taken into account when explaining the reported size independence of the radiative lifetime, which we measure to be about 5 ns for lateral areas between 85 and 320 nm2. Taking into account the singlet-triplet splitting, the less than unity e-h overlap integral, and the fraction of the platelet area occupied by an exciton allows near-quantitative reproduction of the radiative lifetimes using the integrated extinction coefficients as in the Einstein A&B coefficient treatment of radiative rates.

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