Excited State Dynamics of Dyes Bound to Nanocrystals

Abstract

Dye-sensitized nanoparticle (NP) or dye-NP systems consist of dye molecules bound to the surface of wide-bandgap semiconductor metal oxide NPs. The dye molecules absorb light, and the excited dye molecules then transfer an electron to the conduction band (CB) of the NP. These dye-NP systems are used for applications such as solar cells and photocatalysis. This thesis aims to enhance understanding of the fundamental processes governing dye-NP systems, such as electron transfer between dyes and NPs and the organization and aggregation of dye molecules on the NP surface. To this end, colloidal nanocrystals (NCs), which are crystalline particles dispersed in a solvent medium, are used. These colloidal NCs present a more homogenous environment for dye-binding, and this reduced heterogeneity can make it easier to elucidate the molecular excited-state dynamics. In addition, they offer the ability to control surface coverage of the NCs by changing the dye:NC ratio. Chapter 1 presents the relevant background, and Chapter 2 details the experimental techniques and setups used. Chapter 3 studies electron transfer from dye molecules to Indium oxide (In2O3) NCs. In2O3 has a low CB minimum, potentially enabling it to access low-lying excited states. Dye molecules previously studied with ZnO NCs were used to gain a fundamental understanding of electron injection dynamics to In2O3 and compare it with ZnO NCs. Chapter 4 studies the aggregation of a perylene diimide (PDI) derivative bound to ZnO NCs. The surface coverage of the system was altered by changing the PDI:ZnO NC ratio to understand the organization of the molecules on the surface as a function of surface loading. Chapter 5 studies the arrangement of dye molecules on the NC surface, which is essential for designing efficient systems for processes like singlet fission (SF), which involves multiple chromophores. Using a combination of Monte-Carlo simulations and spectroscopic techniques, Förster resonance energy transfer (FRET) between donor and acceptor dye molecules bound to NCs was studied to gain insights into the distribution of the molecules on the NC surface. Chapter 6 studies the impact of n-doping on electron transfer dynamics in dye-NC systems. Electron transfer from dye molecules used in Chapter 3 to Tin (Sn)-doped Indium oxide (ITO) NCs was studied to examine the impact of doping on electron transfer from dyes to NCs and back electron transfer from NCs to dyes.