(735f) Engineering the Light-Matter Interactions of Ultrasmall CdSe Quantum Dots Via Modification of Surface Species for Light Harvesting Applications

The highly tunable nature of the electronic band structure of quantum dots (QDs) is promising for many reasons, among them engineered light harvesting materials for solar energy capture. For a given composition, alteration of the size of the QD alone will change the light absorbed and emitted by the QD. Upon entering the ultrasmall regime, sub 2 nm radii, CdSe QDs have been shown to emit white light, i.e. all wavelengths of visible light, upon exposure to UV light. Two main theories have arisen to explain this phenomenon: 1) a variety of surface traps yielding various energies of emitted wavelengths, or 2) a constantly fluctuating band gap due to a high degree of disorder on the surface. This second theory is known as fluxionality. In this research, the applicability of the two theories is compared based upon alteration of the ultrasmall CdSe band gap via modification of the surface. At this size, approximately 90% of the atoms in the QD are on the surface, thereby allowing for significant control over the QD behavior via design of different surface conditions. The conditions that we have implemented include placing the QD in mediums with a range of dielectric constants, addition of capping ligands, and addition of shell materials. Techniques used to probe the nature of the light-matter interactions of these systems include time correlated single photon counting (TCSPC), ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) for crystal structure information, and linear sweep voltammetry for device performance information of these QDs in preliminary solar cells. Continuing work will include simulations of the QDs in the various environments to garner better understanding of the alterations of the electronic band gap based on the conditions imposed.