Colloidal semiconductor nanocrystals (NCs) are a novel set of materials with unique properties. Due to their intrinsic high surface area to volume ratio, surfaces play a dominant role in many physical and chemical processes. Surface ligands — molecules that bind to the surface — is the essential component of nano-material synthesis, processing and application. My dissertation here is mainly focusing on two parts: (a) the characterization of surface ligand binding using fluorescence-based techniques and (b) the rational design and engineering of surface ligands for our targeted application: photon upconversion.
We start from developing a FRET based model sensing the initial few ligand binding events on the oleic-acid capped CdS NCs. The binding behavior can be described as an interaction between a ligand with single binding group and a substrate with multiple, identical binding sites. A modified Langmuir isotherm can be applied to obtain the average number of newly introduced ligand and its binding constant. With this model, we also demonstrate that inorganic NCs have distinct binding sites for different chemical species. This is evidenced by the results that only resonance energy transfer (RET) is observed for the carboxylic acid ligand, while both RET and charge transfer are observed for the amine ligand.
Then, the length and binding geometries of surface ligand are prudently selected to promote triplet energy transfer, which is the key step in the photon upconversion process. First, a series of anthracene transmitter ligands with variable-length aromatic oligo-p-phenylene and aliphatic linkage were employed. The triplet sensitization of anthracene transmitter molecules by CdSe NCs shows a strong distance dependence. Anthracene transmitter bound closest to the NC surface gave the highest quantum yield of 14.3% for the conversion of green to violet light, the current record for a hybrid platform. Later, a series of bidentate bis-pyridine anthracene isomers that differ in binding geometries were designed to find the best complementary fit to the NC surface. Among them, only the ligand with an intramolecular N-N distance of 8.2 Å had the best match to the surface of CdSe NCs, yielding a upconversion QY as high as 12.1±1.3%.
This work lays the foundation of incorporating the hybrid complexes as active materials in optoelectronic devices, such as solar cell, transistors and photodetectors, and also relate the performance of those devices to the structure at their atomic level.