Here mechanical properties of face
cubic centered colloidal crystals obtained out of equilibrium by solvent
evaporation of coated Au nanocrystals suspension, called supracrystals,
are reported as a function ligand chain length (n) and interparticle edge-to-edge distance within the supracrystals
(δpp) for two nanocrystal sizes (d). Young’s modulus (E*) and hardness (H) are independent of δpp and of the supracrystal
morphology. Both E* and H are in
the range of few tenths of a MPa to a few GPa. Tuning of δpp by 50% is achieved by controlling the solvent vapor pressure
(P
t) during the evaporation process. For
any nanocrystal size, at P
t = 0, E* and H values markedly increase with
increasing n from 12 to 14. At P
t = 39% and 75%, such dependency disappears. This trend
differs from classical nanocomposite materials and is attributed to
a change in the conformation of flexible ligands with n and to free thiol-containing molecules trapped in the supracrystal
lattices.
The electro-mechanical sensing properties of freestanding monolayered membranes of dodecanethiol coated 7 nm gold nanoparticles (NPs) are investigated using AFM force spectroscopy and conductive AFM simultaneously. The electrical resistance of the NP membranes increases sensitively with the point-load force applied in the center of the membranes using an AFM tip. Numerical simulations of electronic conduction in a hexagonally close-packed two-dimensional (2D) array of NPs under point load-deformation are carried out on the basis of electronic transport measurements at low temperatures and strain modeling of the NP membranes by finite element analysis. These simulations, supporting AFM-based electro-mechanical measurements, attribute the high strain sensitivity of the monolayered NP membranes to the exponential dependence of the tunnel electron transport in 2D NP arrays on the strain-induced length variation of the interparticle junctions. This work thus evidences a new class of highly sensitive nano-electro-mechanical systems based on freestanding monolayered gold NP membranes.
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