Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA.
J Am Chem Soc. 2010 Mar 17;132(10):3344-54. doi: 10.1021/ja907511r.
Evidence of size-dependent reconstruction in quantum dots leading to changes in bonding is observed through analysis of the (77)Se{(1)H} cross-polarization magic angle spinning and (77)Se spin-echo solid-state NMR for Cd(77)Se quantum dots. The CP-MAS and spin-echo data indicate discrete surface and core (77)Se sites exist with the QD, in which the surface is comprised of numerous reconstructed lattice planes. Due to the nearly 100% enrichment level for (77)Se, efficient spin coupling is observed between the surface (77)Se and sublayer (77)Se sites due to spin diffusion in the Cd(77)Se quantum dots. The observed chemical shift for the discrete (77)Se 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.
通过对 Cd(77)Se 量子点的 (77)Se{(1)H}交叉极化魔角旋转和 (77)Se 自旋回波固态 NMR 的分析，观察到了量子点中与键合变化相关的尺寸依赖重构的证据。CP-MAS 和自旋回波数据表明，QD 中存在离散的表面和核心 (77)Se 位，其中表面由许多重构的晶格平面组成。由于 (77)Se 的几乎 100%富集水平，在 Cd(77)Se 量子点中，由于自旋扩散，表面 (77)Se 和亚层 (77)Se 位之间观察到有效的自旋耦合。离散 (77)Se 位的观察到的化学位移可以通过 Ramsey 表达式与有效质量近似相关联，表明化学位移随尺寸的变化与 1/r(2)的尺寸依赖性有关，而化学位移与逆能带隙的关系呈线性关系。离散位的 NMR 位移的相关性允许对重构 QD 中键合性质的尺寸依赖性变化进行价键理论解释。NMR 结果提供了一个结构模型，其中在直径小于 4nm 时发生全局重构，而在直径大于 4nm 时发生明显的自限重构过程。
