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Published January 9, 1997 | public
Journal Article

Pressure/Temperature Phase Diagrams and Superlattices of Organically Functionalized Metal Nanocrystal Monolayers: The Influence of Particle Size, Size Distribution, and Surface Passivant


The phase behavior of organically passivated 20−75 Å diameter Ag and Au nanocrystals is investigated by examining surface−area isotherms of Langmuir monolayers and transmission electron micrographs of Langmuir−Blodgett (LB) films. The effects of temperature, organic passivant chain length, and nanocrystal size and composition are studied. Three distinct types of phase behavior are observed and may be classified in terms of the extra (conical) volume (V_e) available to the alkyl capping group as it extends from a nearly spherical metal core. For V_e > 350 Å^3, the phase diagram is dominated by extended, low-dimensional structures that, at high pressures, compress into a two-dimensional foamlike phase. This behavior is rationalized as originating from the interpenetration of the ligand shells of adjacent particles. For V_e < 350 Å^3, dispersion attractions between the metal cores dominate particle condensation. For 350 Å^3 > V_e > 150 Å^3, the particles condense to form closest packed structures, which, for sufficiently narrow particle size distributions, are characterized by crystalline phases. For V_e ≈ 30 Å^3, the particles irreversibly aggregate into structures similar to those expected from a diffusion-limited-aggregation (DLA) model. Optical properties of certain LB films of the closest packed phases are reported.

Additional Information

© 1997 American Chemical Society. Received 22 April 1996. Published online 9 January 1997. Published in print 1 January 1997. We would like to acknowledge many helpful discussions with the following people:  Prof. Andrea Liu, Dr. Joseph Shiang, Ms. Pam Ohara, and Prof. Bill Gelbart. Helpful insight into the Langmuir−Blodgett technique was gleaned from Mr. Gary Marshall and Dr. Jiyu Fang. Certain of the particles discussed here were prepared by Ms. Erica DeIonno. J.R.H. and D.V.L. acknowledge primary support from the NSF-NYI program and the David and Lucile Packard Foundation. C.M.K. acknowledges support from the NSF. Certain equipment used here was supported by the Office of Naval Research, Order No. N00014-95-F-0099, under the auspices of the ONR-funded Molecular Design Institute at UC Berkeley.

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