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 77Se{1H} cross-polarization magic angle spinning and 77Se spin−echo solid-state NMR for Cd77Se quantum dots. The CP-MAS and spin−echo data indicate discrete surface and core 77Se sites exist with the QD, in which the surface is comprised of numerous reconstructed lattice planes. Due to the nearly 100% enrichment level for 77Se, efficient spin coupling is observed between the surface 77Se and sublayer 77Se sites due to spin diffusion in the Cd77Se quantum dots. The observed chemical shift for the discrete 77Se sites can be correlated to the effective mass approximation via the Ramsey expression, indicating a 1/r2 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...