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

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• 作者：Derek D. Lovingood ; Randall Achey ; Anant K. Paravastu ; Geoffrey F. Strouse
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：March 17, 2010
• 年：2010
• 卷：132
• 期：10
• 页码：3344-3354
• 全文大小：445K
• 年卷期：v.132,no.10(March 17, 2010)
• ISSN：1520-5126

文摘

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 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.