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 Angstrom diameter A g and Au nanocrystals is investigated by examining surface-area isothe rms of Langmuir monolayers and transmission electron micrographs of La ngmuir-Blodgett (LB) films. The effects of temperature, organic passiv ant chain length, and nanocrystal size and composition are studied. Th ree distinct types of phase behavior are observed and may be classifie d 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 Angstrom(3), the phase diagram is dominated by extended, low-d imensional structures that, at high pressures, compress into a two-dim ensional foamlike phase. This behavior is rationalized as originating from the interpenetration of the ligand shells of adjacent particles. For V-e < 350 Angstrom(3), dispersion attractions between the metal co res dominate particle condensation. For 350 Angstrom(3) > V-e > 150 An gstrom(3), the particles condense to form closest packed structures, w hich, for sufficiently narrow particle size distributions, are charact erized by crystalline phases. For V-e approximate to 30 Angstrom(3), t he particles irreversibly aggregate into structures similar to those e xpected from a diffusion-limited-aggregation (DLA) model. Optical prop erties of certain LB films of the closest packed phases are reported.