Spectroscopy, photo-physics, and time resolved exciton dynamics of GaSe quantum dots

Author(s): Mirafzal, Hoda | Advisor(s): Kelley, David F. | Abstract: The first part of this dissertation discusses GaSe monomer and aggregated particles. GaSe nano-disks have been prepared by several different synthetic methods. A study on the effect of various ligations suggests that well-aggregated stable particles are ligated by tight-binding alkyl phosphonic acid anhydrides. Addition of dodecyl aldehyde to particles that are primarily ligated by trioctylphosphine and trioctylphosphine oxide results in strongly coupled aggregates that cause a large red shift of the absorption spectrum (1600 cm-1) and the reversal of singlet and triplet states. This spin reversal results in changes in time-resolved anisotropy and a dramatic decrease in radiative lifetime. The quantum yield of particles increases from 4.7% in monomers to 61% in strongly coupled aggregates. GaSe aggregates can be mixed with a smectic-A phase liquid crystal, LC (4-octyl, 4'-cyanobiphenyl), where the liquid crystal forces the particles to form long stacks that are in line with the director axis of the LC. This only happens when the synthesized GaSe particles are extremely well-aggregated. The second part of this dissertation discusses the synthesis and exciton dynamics of various morphologies of CdTe/CdSe nano-heterostructures. Highly luminescent CdTe spherical nanoparticles with an average size of 3.4 nm are synthesized using a novel synthetic method that uses Octadecylphosphonic acid in the Te precursor. These particles can have a quantum yield of up to 90%. Core/shell and dot/tetrapod CdTe/CdSe heterostructures synthesized from these Te cores are used to study the biexciton Auger dynamics and the electron cooling rates in these structures by means of femtosecond transient absorption measurements. An effective mass approximation (EMA) is used to model the exciton dynamics, specifically Auger times, in these particles. Calculations of the electron and hole wavefunctions using the EMA model predict electron and hole overlap and radiative lifetimes that match those of the experimental data. A better agreement between the experimental and calculated data is observed if compression effects, resulting from depositing a smaller-lattice shell onto a larger-lattice core, are considered. The analysis shows that as thicker Se shells are deposited, both the Auger and electron cooling processes are progressively suppressed, as expected. Calculations show that the Auger time is a strong function of, and thus directly proportional to the coulombic interaction energy between the electron and the hole.