Surfactant-Directed
Fabrication of Supercrystals from
the Assembly of Polyhedral Au–Pd Core–Shell Nanocrystals
and Their Electrical and Optical Properties
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Abstract
Au–Pd
core–shell nanocrystals with cubic, truncated
cubic, cuboctahedral, truncated octahedral, and octahedral structures
have been employed to form micrometer-sized polyhedral supercrystals
by both the droplet evaporation method and novel surfactant diffusion
methods. Observation of cross-sectional samples indicates shape preservation
of interior nanocrystals within a supercrystal. Low-angle X-ray diffraction
techniques and electron microscopy have been used to confirm the presence
of surfactant between contacting nanocrystals. By diluting the nanocrystal
concentration or increasing the solution temperature, supercrystal
size can be tuned gradually to well below 1 μm using the surfactant
diffusion method. Rectangular supercrystal microbars were obtained
by increasing the amounts of cubic nanocrystals and surfactant used.
Au–Ag core–shell cubes and PbS cubes with sizes of 30–40
nm have also been fabricated into supercrystals, showing the generality
of the surfactant diffusion approach to form supercrystals with diverse
composition. Electrical conductivity measurements on single Au–Pd
supercrystals reveal loss of metallic conductivity due to the presence
of insulating surfactant. Cubic Au–Pd supercrystals show infrared
absorption at 3.2 μm due to extensive plasmon coupling. Mie-type
resonances centered at 9.8 μm for the Au–Pd supercrystals
disappear once the Pd shells are converted into PdH after hydrogen
absorption