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Figure 8. Si-Ge and Si-Ge-Si core-shell nanowires. (a) Cross-section elemental mapping indicating a 21-nm-diameter Si core (blue circles), 10-nm Ge core-shell (red circles), and <1-nm interface. Inset, TEM image of the corresponding Si-Ge core-shell nanowire. (b) Highresolution TEM image of a representative crystalline nanowire core and shell from the same synthesis as the wire in (a). Scale bar is 5 nm. (c) Cross-section elemental mapping of a double-shell structure with an i-Si core (diameter, 20 nm), i-Ge shell (thickness 30 nm), and pSi out-shell (4 nm). Si is blue circles and Ge is red circles. Reprinted with permission from [143], L. J. Lauhon et al., Nature 420, 57 (2002) © 2002, Macmillan Magazines Ltd.

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Distance (nm)

Figure 8. Si-Ge and Si-Ge-Si core-shell nanowires. (a) Cross-section elemental mapping indicating a 21-nm-diameter Si core (blue circles), 10-nm Ge core-shell (red circles), and <1-nm interface. Inset, TEM image of the corresponding Si-Ge core-shell nanowire. (b) Highresolution TEM image of a representative crystalline nanowire core and shell from the same synthesis as the wire in (a). Scale bar is 5 nm. (c) Cross-section elemental mapping of a double-shell structure with an i-Si core (diameter, 20 nm), i-Ge shell (thickness 30 nm), and pSi out-shell (4 nm). Si is blue circles and Ge is red circles. Reprinted with permission from [143], L. J. Lauhon et al., Nature 420, 57 (2002) © 2002, Macmillan Magazines Ltd.

preferentially deposit CdS near the core since its lattice matches better to CdSe than ZnS does. This layer of CdS mediates the growth of the more highly strained ZnS. The shell growth is uniform and epitaxial, completely coating the CdSe core, although it may have defects present due to the lower growth temperature. After an irreversible photochemical annealing process, the core-shell nanorods have increased quantum efficiencies and are stable in air under visible or UV excitation [144].

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