Electronic properties of supracrystals of Au nanocrystals: influence of thickness and nanocrystallinity.

Well-defined superlattices of colloidal nanocrystals, called supracrystals, are expected to have interesting physical properties. While the electronic properties of thin supracrystals have been extensively studied in the planar configuration, little is known about electron transport through micrometer-thick supracrystals. Here, we investigate the electronic properties of supracrystals made of Au nanocrystals with diameters of 5, 6, 7 and 8 nm using scanning tunneling microscopy/spectroscopy at low temperatures. The current-voltage characteristics show power-law dependences with exponents varying strongly with supracrystal thicknesses from 30 nm to a few microns. The crystallinity of these nanocrystals, called nanocrystallinity, is exclusively single domain for 5 nm nanocrystals and a mixture of single and polycrystalline phase for 6, 7 and 8 nm nanocrystals. We observed that supracrystals made of 5 nm nanocrystals have a different behavior than supracrystals made of 6, 7 and 8 nm nanocrystals and this might be related to the nanocrystallinity. These results help us to better understand the electron transport mechanism in such miniscule structures built from a bottom-up approach.