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