The Young’s modulus of three-dimensional self-assembled Ag nanocrystals, as so-called supracrystals, is correlated with the type of coating agent as well as the nanocrystal morphology. The Young’s moduli of supracrystals of icosahedral Ag nanocrystals are measured in the range of tens to hundreds of megapascals revealing an extremely soft mechanical behavior. The alkylamine molecules used as coating agents weakly interact with the Ag nanocrystal surface favoring translational and orientational ordering of atomic lattice planes of nanocrystals. In such experimental conditions, both the average distance between nanocrystals and the increase of the nanocrystal diameter control the measured Young modulus: It increases with decreasing the interparticle distance and increasing the nanocrystal diameter. When dodecylamine (C12NH2) molecules is replaced by dodecanthiol (C12SH), the orientational ordering between nanocrystals, produced from the same batch as C12NH2, disappears by inducing a drop in the Young modulus. This is attributed to formation of a “skin” at the nanocrystal surface causing a transition from shaped to spherical nanocrystals. Finally, by comparing with various studies performed in our group with Co and Au nanocrystals, we explain the formation of such extremely soft materials with Ag nanocrystals by both the strength of the binding between nanocrystals and coating agent and the ligand-ligand interactions.
Source : Coating agent-induced mechanical behavior of 3D self-assembled nanocrystals. A. Çolak, J. Wei, I. Arfaoui, M.P. Pileni PCPC 2017, 19, 23887