Quantum Dot Ligand Decomposition and Its Impact on Exciton Delocalization

Semiconductor quantum dot films are attractive optoelectronic materials as their optical properties can be tuned by varying quantum dot size, composition, and surface chemistry. To this end, treating CdSe quantum dot films with phenyldithiocarbamate (PDTC) ligands is known to redshift quantum dot absorption features, which has been attributed to electronic hybridization between PDTC’s valence orbitals and states within CdSe’s valence band. However, PDTC has also been suggested to decompose following attachment to CdSe, leading to formation of a partial CdS shell that can also shift quantum dot absorption bands. Here, by combining steady-state spectroscopic and structural measurements, we track how CdSe quantum dot films chemically and structurally evolve at different stages during PDTC ligand functionalization. Films treated for short times with a polar, protic exchange solution containing PDTC show spectral signatures that signal the attachment of intact bound PDTC ligands. However, prolonged exposure to this solution gives rise to spectroscopic and structural features that signal partial ligand decomposition and film compaction. These treatment-time-dependent changes modify the energetic landscape of the films as we find PDTC decomposition promotes exciton trapping and accelerated decay. Our work shows that care must be taken when modifying quantum dot solids via ligand exchange as unintended side reactions can degrade favorable electronic properties if not carefully controlled.