The ability to tune the band-gap energies of semiconductor quantum dots, nanoplatelets, and quantum wires, their significant absorption cross sections, and high photoluminescence quantum yields make these nanostructures promising moieties for use in optoelectronic devices, solar concentrators, chemical sensors, and biological labels. The variable dynamics of the electron-hole pairs that occur within semiconductor nanostructures, however, can complicate the utility of these devices. The variability of the dynamics is born from the different paths accessible for the charge carriers to undergo. In this work, three pathways are proposed to be of primary consequence, namely, electronic intraband relaxation, coupling to surface-mediated processes, and tunneling to the external environment. The relative dominance of these paths will vary from sample to sample. More importantly, within a sample, the contributions of the available pathways are found to change with changes in excitation energy.
To this end, I investigated the dependence of the ensemble photoluminescence: PL) quantum yields: QYs) on excitation energy for numerous semiconductor nanoparticles with quantum confinement in varying dimensions. A strong dependence of the PL QY on excitation energy is observed in quantum dots: QDs), nanoplatelets: NPLs), and quantum wires: QWs). The highest PL QYs are within the first 300 meV above the band edge, and there is a severe drop in the PL QY towards the highest excitation energies investigated, ~3.1 eV. These high PL QYs are 91 % for CdSe/ZnS QDs, 24 % in CdSe NPLs, which are dispersed in toluene and 25 % in CdTe/CdS QWs, which is dispersed in TOP. These values drop to 12, 8, and 8 % by 3.1 eV, respectively. There are some recognized trends to the shape of this dependency. Local minima in PL QY values occur when intraband relaxation is restricted and ligand or surface mediated transitions are available. These variations in PL QY are reduced when a shell is added to produce a type-I heterostructure. This trend is realized in both QDs and QWs. However, QWs are more weakly confined systems with large surface areas. Their saw-like densities of states that result from the long, unconfined dimension of the QWs and increased valence state mixing yields a higher density of states which leads to a smoother PL QY dependence of the excitation energy. The minimal undulations in the PL QYs that do still exist in these QWs, are further minimized with the addition of a shell to create a type-I heterostructure. Conversely, the pseudo-2D confinement and atomic flatness of NPLs results in narrow, discrete bands of states separated by large energies, ~ 200 meV. This electronic structure restricts intraband relaxation and promotes coupling to other pathways that sponsor non-radiative recombination even more efficiently than QDs. In all samples, exciting with high energies severely diminishes PL QYs as these energies generate highly excited charge-carriers that can access solvent/environmental pathways.
Available at: http://works.bepress.com/jessica_m_hoy/1/