Impact of Structure and Surface Chemistry on Optical Properties and Trap States in Semiconductor Quantum Dot Systems*

Doped and un-doped semiconductor quantum dots (QD) are an emerging material of interest with applications for photovoltaics and photodetectors. They are solution-processed and their electro-optical properties are tunable by size, structure, doping character and, due to their high surface-to-volume, chemical environment as controlled by capping ligands. This work examines the interplay of these factors to understand the impact on absorption and trap states of QD thin films. Crystalline germanium QD were synthesized using long-chain oleylamine ligands. QD uncapped and capped with oleylamine and dodecanethiol ligands were studied with extended x-ray absorption fine structure (EXAFS) which yielded detailed structural information about small changes in bond length across ligand systems, doping concentrations and QD size. These findings were correlated with absorption and trap state measurements taken with photothermal deflection spectroscopy (PDS), a system that eliminates scattering and transmission effects by measuring absorption through heat effects.