期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 132, 期 10, 页码 3344-3354出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja907511r
关键词
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资金
- NSF [CHE-0911080, DMR-0701462, DMR-0084173]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [0911080] Funding Source: National Science Foundation
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.
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