Slowing Hot Electron Cooling in CdSe Quantum Dots Using Electron-Rich Exciton-Delocalizing Ligands

Understanding hot carrier dynamics in semiconductor nanocrystals is an important research focus due to their applications in photonics and photovoltaic devices. In this report, we investigated the effects of surface-bound exciton-delocalizing ligands (EDLs) on the lifetimes of hot electrons in CdSe quantum dots (QDs). After treatment of CdSe with two different phenylithiocarbamates (PTCs), a class of EDLs, the depletion times of the band-edge exciton bleach were roughly equivalent as observed through ultrafast transient absorption spectroscopy. However, following the initial ultrafast depletion, the PTC-treated samples continued to deplete while the untreated CdSe began recovering. Inspection of other transient features - such as the 3(rd) exciton and hot biexciton - reveal a general trend in which the PTC-treated samples relax more slowly at short times (<10 ps) when compared with the untreated CdSe. At longer delay times, in the range of nanoseconds, the CdSe+CF3OPTC loses nearly 80 % of its excited state populations, while the CdSe+MeOPTC loses only 20-40 %. We discuss the role that exciton delocalization plays in determining these observed rates as well as how they compare to previous studies. Kinetic differences between the two ligands are attributed to their electron donating/withdrawing abilities and coupling to the CdSe QD. Coherent vibrational wavepacket analysis supports this line of reasoning, showing increased coupling between the exciton and the longitudinal optical (LO) phonon due to increased Coulombic field strength around the hole and electron-donating MeOPTC. These results indicate that electron-rich PTCs are especially good candidates for use in QD devices that would make use of hot carriers.