Ligand Surface Density Decreases with Quantum Rod Aspect Ratio

The relationship between phosphonic acid (PA) binding strength, surface coverage, and quantum rod or dot aspect ratio was studied. Seven nanocrystals with an aspect ratio (l/w) between 1.0 and 11.7, including five CdSe with l/w between 1.0 and 5.2 and two CdSe/CdS with dot-in-rod and rod-in-rod microstructures with l/w between 7.5 and 11.7, were synthesized. The nature of phosphonic acid binding was studied in situ using one-dimensional ¹H nuclear magnetic resonance (NMR) spectroscopy as well as two-dimensional diffusion ordered spectroscopy (DOSY) and relaxation ordered spectroscopy (ROSY). The DOSY results were used to separate strongly binding phosphonic acid signatures from weakly binding ones, thus allowing for estimates of ligand densities (LDs) to be calculated, which showed a significant drop from LD = 2.21 nm^-2 for spherical CdSe with l/w = 1.0 to LD = 0.23 nm^-2 for CdSe/CdS with l/w = 11.7. Interestingly, intermediate l/w = 1.7 showed a higher LD of 4.81 nm^-2. The results suggest a wrapping or relatively flat binding by phosphonic acids on the sides of the rods, leading to decent surface coverage for low aspect ratios but lower LD for more elongated rods. This model was further confirmed using ROSY, which revealed similar relaxation times (T₁) for each proton of the phosphonic acids on the rods, indicating similar ligand-quantum rod (QR) interface.