Basic Science of Nanomaterials (Ch. 11) High surface/bulk ratio Catalysis Reactivity Melting point...
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Transcript of Basic Science of Nanomaterials (Ch. 11) High surface/bulk ratio Catalysis Reactivity Melting point...
Basic Science of Nanomaterials (Ch. 11)
High surface/bulk ratio• Catalysis• Reactivity• Melting point• Vapor pressure• Ostwald ripening
Finite size effects• Quantum confinement• High probability of defect- free crystals• Mesoscopic phenomena
Semiconductor nanocrystal synthesis
• High temperature, non-aqueous solvents• Separate nucleation and growth steps• Size focusing during crystal growth• Capped nanocrystals have few or no defects
Emission spectra of several sizes of (CdSe)ZnS core-shell quantum dots
M. Bawendi, et al., J. Phys. Chem. B 1999, 101, 9463.
Semiconductor Core-Shell Quantum Dots
• Perfect quantum well structure leads to bright luminescence• Nanocrystal size determines band gap
CdSe/CdS core-shell particlesExcitons confined to core
Defect-free nanocrystalshigh fluorescence quantum yield
A. P. Alivisatos et al., Science 1998, 281, 2013.
Dual fluorescence labeling of actin filaments and fibroblasts
2.1 - 4.6 nm
Cd
Se
Shape Control of Semiconductor Nanocrystals
L. Manna, E. C. Scher, A. P. Alivisatos, JACS 122, 12700 (2000)
• Nanocrystal habit controlled by surfactant composition
• Multiple injection/growth cycles develop specific crystal faces
CdSe Tetrapods
L. Manna, E. C. Scher, A. P. Alivisatos, JACS 122, 12700 (2000)
Y. Wu, P. Yang, J. Am. Chem. Soc., 123, 3165, 2001
Vapor-liquid-solid (VLS) growth of semiconductor nanocrystals
Single crystal nanowiresCatalyst nanoparticle controls diameterLength determined by reaction time
Compositionally Modulated Semiconductor Nanocrystals
Totem pole and core-shell structuresMultiple bandgaps and p-n junctions
Lieber, et al., Nature 415, 617 (2002); Nature 420, 57 (2002).Yang, et al., NanoLett 2, 83 (2002); Buhro et al., J. Am. Chem. Soc. 123, 4502 (2001).
Surface energy and physical/chemical properties
Surface atoms have
higher energy than bulk atoms
(dangling bonds)
How does nanocrystal size affect:
•Melting point ?•Vapor pressure ?•Chemical reactivity ?
Ag nanocrystal melting point
S. A. Little et al., Appl. Phys. Lett. 2012, 100, 051107
Nanoparticle melting