Size- and Site-Dependent Reconstruction in CdSe QDs Evidenced by 77Se{1H} CP-MAS NMR Spectroscopy

Evidence of size-dependent reconstruction in quantum dots leading to changes in bonding is observed through analysis of the Se-77{H-1} cross-polarization magic angle spinning and Se-77 spin echo solid-state NMR for (CdSe)-Se-77 quantum dots. The CP-MAS and spin echo data indicate discrete surface and core Se-77 sites exist with the QD, in which the surface is comprised of numerous reconstructed lattice planes. Due to the nearly 100% enrichment level for Se-77, efficient spin coupling is observed between the surface Se-77 and sublayer Se-77 sites due to spin diffusion in the (CdSe)-Se-77 quantum dots. The observed chemical shift for the discrete Se-77 sites can be correlated to the effective mass approximation via the Ramsey expression, indicating a 1/r(2) size dependence for the change in chemical shift with size, while a plot of chemical shift versus the inverse band gap is linear. The correlation of NMR shift for the discrete sites allows a valence bond theory interpretation of the size-dependent changes in bonding character within the reconstructed QD. The NMR results provide a structural model for the QDs in which global reconstruction occurs below 4 nm in diameter, while an apparent self-limiting reconstruction process occurs above 4 nm.