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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 defectfree 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 Semiconductor Core-Shell Quantum Dots • Perfect quantum well structure leads to bright luminescence • Nanocrystal size determines band gap Emission spectra of several sizes of (CdSe)ZnS core-shell quantum dots M. Bawendi, et al., J. Phys. Chem. B 1999, 101, 9463. 2.1 - 4.6 nm CdSe/CdS core-shell particles Excitons confined to core Defect-free nanocrystals high fluorescence quantum yield Dual fluorescence labeling of actin filaments and fibroblasts A. P. Alivisatos et al., Science 1998, 281, 2013. Shape Control of Semiconductor Nanocrystals • Nanocrystal habit controlled by surfactant composition Cd Se • Multiple injection/growth cycles develop specific crystal faces L. Manna, E. C. Scher, A. P. Alivisatos, JACS 122, 12700 (2000) CdSe Tetrapods L. Manna, E. C. Scher, A. P. Alivisatos, JACS 122, 12700 (2000) Vapor-liquid-solid (VLS) growth of semiconductor nanocrystals Single crystal nanowires Catalyst nanoparticle controls diameter Length determined by reaction time Y. Wu, P. Yang, J. Am. Chem. Soc., 123, 3165, 2001 Compositionally Modulated Semiconductor Nanocrystals Totem pole and core-shell structures Multiple 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 ? Melting point of silver nanoparticles Nanoparticle melting S. A. Little et al., Appl. Phys. Lett. 2012, 100, 051107 Ag nanocrystal melting point S. A. Little et al., Appl. Phys. Lett. 2012, 100, 051107 Fig 3. (Color online) Melting point of silver nanoparticles plotted versus the average particle radius, estimated as one-half the physical thickness. The dashed line indicates the melting point of bulk silver. The solid line is the fit of Eq. (3), yielding the surface energy density.