Structure and characterization of passivated inorganic nanocrystals and three dimensional nanocrystal arrays

Self-Assembled crystals composed of passivated metal nanocrystals exhibit novel optical properties for possible use in wavelength conversion in all optical devices and use in pressure sensor gauges. Silver nanocrystals of approximate core diameter of 4.5 nm were produced in the aerosol phase and passivated with organic alkanethiol molecules. Dissolved in non-polar toluene solvent, the nanocrystals spontaneously “crystallize” into periodic ordered structures upon evaporation of the nanocrystal solution on an ultra-thin carbon substrate. Transmission electron microscopy imaging and diffraction were used to characterize the individual nanocrystals and their self-assembled structures. The “supercrystals” formed have crystal structures analogous to atomic crystals and exhibit face-centered-cubic and hexagonal-close-packed structures. The material tethering the nanocrystal cores into place in the periodic assembly consists of the thiol passivation molecules interpenetrating between nanocrystal cores and shows unusually high temperature stability up to 975K. It is shown that these crystals grow analogously to atomic crystals through evaporation of a solvent and exhibit very faceted, triangular shapes with edges on the micron scale but of a very thin, platelet-like nature. The growth is presumed to occur at the liquid-air interface at the surface of the evaporating nanocrystal solution drop and growing laterally. Variation of the internanocrystal core spacing for self-assembled monolayers of these nanocrystals was accomplished through changing the length of thiol surfactant used to passivate the nanocrystals.