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6 Assembly of nanocrystals

6 Assembly of nanocrystals - Lecture 6 Assembly of...

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Lecture 6 Jan 27, 2009 Assembly of Nanocrystals Announcement: - Project group has been assigned. Meet with your group mates ASAP. - Next project assignment due on Feb 12: submit a one-page summary of the topic you are exploring. Indicate you preference of oral presentation or written paper.
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Making colloidal crystals (synthetic opals) Anvar Zakhidov et al., Science 282 (1998) p. 897. This opal was grown by slow crystallization of a monodispersed SiO 2 colloid over a period of 10 months in a 1-m-long glass cylinder. The inverse opal or graphite shown below was made by chemical vapor deposition into an SiO 2 opal. After the graphite was formed, the SiO 2 was etched out with HF. SiO 2 Opal Graphite Inverse Opal
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Periodic arrays of nanocrystals Self-assembly of superlattices readily occurs when a dispersion of monodisperse nanocrystals is spread on a substrate and the carrier solvent is allowed to slowly evaporate. The spheres don’t touch each other because they are surrounded by organic surfactants like the ones shown to the right. Co nanocrystals
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Interactions between colloidal particles Jacob Israelachvili Intermolecular and Surface Forces p. 248. VDW interactions pull particles together. If the particles are charged, Double layer repulsion can keep them apart. In most cases, pH determines the charge on the surface. Salt can be used to screen Coulomb repulsion.
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Strong repulsion limit If the spheres are charged, then they repel each other. An ordered structure will form rapidly so that the distance between sphere can be maximized.
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