Quantum Dot Dispersion in Block Copolymer Matrices

Abstract

Quantum dots (QDs) have demonstrated viability for a wide set of applications ranging from bioimaging to electronics. Their unique size-tunable band gaps and accessibility for roll-to-roll processing via solution synthesis makes them promising candidates in many of these areas. Several of these applications benefit from carefully manipulated spacing in QD films, often achieved through integration of QDs in a polymer matrix. Although many studies have achieved varying degrees of success in dispersing QDs in polymer matrices, there remains much to be understood about the path dependency of QD integration and how QDs may be integrated into sphere-forming polymer matrices. In this work, CdSe QDs were synthesized via a hot injection technique in an air-free environment. These QDs were fabricated in a range of sizes based on the reaction time and were evaluated for their crystal structure, absorbance, fluorescence, ligand coverage, and dispersion in various solvents. The resulting QDs feature a wurtzite crystal structure and exhibit narrow absorbance and emission peaks. The QDs are well stabilized in nonpolar solvents like hexane and toluene via trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) ligands. In the first approach towards QD integration in polymer matrices, the native TOPO ligands were exchanged for a poly(ethylene glycol) ligand functionalized with a thiol end group. The resulting QDs were qualitatively and quantitatively analyzed to determine the ligand density on the QD surface and the QD dispersion in different solvents. After ligand exchange, the QDs were no longer dispersible in nonpolar solvents like hexane but formed stable dispersions in solvents like tetrahydrofuran, chloroform, and water. Upon ligand exchange, 50-85% of the original ligands were removed and an average of 2-4 poly(ethylene glycol) ligands were installed on each QD surface. The extent of QD dispersion in various homopolymers was evaluated using transmission electron microscopy (TEM). CdSe QDs were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate phase separation of the polymer and the QDs where the spacing between QDs does not change upon addition of the polymer. In a separate study, CdSe QDs were mixed with poly(butadiene) in n-hexane and dropcast into thin films for TEM. These films demonstrated an increase in spacing between the QDs of roughly 2 nm. However, the majority of the polymer does not end up in the space between QDs and is phase separated from the QD crystal phase. Finally, CdSe QDs after ligand exchange with PEG were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate some mixing between the QDs and the polymer. Finally, the extent of QD dispersion in various diblock copolymers was evaluated using small angle x-ray scattering (SAXS), TEM, and fluorescence measurements. CdSe QDs were mixed with a lamellae-forming poly(ethylene-b-cyclohexylethylene) in benzene and dried and heated to T ≈ 200°C and then cooled to 140°C to induce polymer ordering. The resulting solid composites exhibited aggregates of QDs via SAXS and TEM measurements. In a separate study, CdSe QDs were mixed with a body-centered cubic forming poly(styrene-b-butadiene) in benzene. Two types of samples were prepared: one formed from drying the polymer and QDs from the solvent and subsequently heating to 140°C, and one formed from dropcasting the dispersion of polymer and QD from the solvent. The resulting solid composite prepared with temperature processing was microtomed and exhibited QD aggregation in TEM. The dropcast sample exhibited phase separation of the QDs and the polymer. Finally, CdSe QDs were mixed with a body-centered cubic poly(lactide-b-butadiene) in a mixture of n-hexane and chloroform. Micelles with the minority poly(butadiene) block on the outside were formed in similar mixtures of n-hexane and chloroform and observed via dynamic light scattering. QDs were added to the polymer in a mixture of n-hexane and chloroform after micellization (at higher hexane concentrations) and before micellization (at lower hexane concentrations); in the latter case, hexane was added to induce polymer micellization after the addition of QDs. These dispersions were dropcast into films for TEM, which revealed similar film structures for both samples where QDs appeared at the interstices between roughly spherical shapes; these shapes were attributed to the formation of polymer microemulsions during the drying process. The effect of chloroform and n-hexane content of the solvent mixture on the composite formation was tested for various mixtures of n-hexane and chloroform. TEM of QD and polymer samples prepared from these solvent mixtures showed phase separation of QDs and polymer at low n-hexane concentrations and the polymer microemulsion structure for higher n-hexane concentrations. Although this work focused on the integration of CdSe QDs, the insights demonstrated here should prove relevant for other nanoparticle-composite systems.